LANGMUIR-BLODGETT

TROUGHS

OPERATING MANUAL

6th EDITION
Welcome

ii

The information contained within this manual is for guidance only in the use of the product.

Since the product is continually being developed and improved, the information is subject

to change without notice and though given in good faith, may contain errors or omissions.

Nima Technology welcomes any suggestions for further development or improvement of

the product or this manual.

All suggestions and enquiries should be addressed to:

Nima Technology Ltd

The Science Park

Coventry CV4 7EZ

England

Tel: (+44) (247) 641 9457

Fax: (+44) (247) 669 2511

e-mail: info@nima.co.uk

Website: http://www.nima.co.uk

First edition 1989

Second edition 1991 (revised and updated)

Third edition 1992 (revised and updated), 2nd printing 1993

Fourth edition (revised and updated) 1995, 2nd printing 1998

Fifth edition (revised and updated) 1999

Sixth edition (revised and updated) 2001, 2nd printing 2002

3rd printing updated for WIN XP, 2004

Software Generation 5.16 for WIN95 - XP

Authors:

P. Martin and M. Szablewski

Cranfield University

Milton Keynes, England

Editor: Frank Grunfeld, Nima Technology Ltd

Copyright © 1989, 1991, 1992, 1993, 1995, 1998, 1999, 2001, 2002, 2004 Nima Technology Ltd.

MS-DOS and Windows are a registered trademark of Microsoft Corporation.

IBM is a registered trademark of International Business Machines Corporation.

PageMaker is a registered trademark of Adobe Aldus Corporation.

LabView is a registered trademark of National Instruments Corporation.
iii

Welcome

WELCOME

Thank you for buying a Langmuir trough from Nima Technology. Our troughs are very

versatile, precision scientific instruments. They can accomplish a wide range of tasks -

from the analysis of lipids and surfactants at the liquid-gas interface to the fabrication of

ordered multilayers for applications in molecular electronics.

With proper care and attention, your trough will be running for many years; and the uses

to which it is put then will be as relevant as they are today.

This manual is divided into several chapters and incorporates everything you need to know

on how to get started in Langmuir-Blodgett (LB) films and using Nima troughs.

There is a 'Langmuir-Blodgettry' chapter which gives a thorough grounding in the

principles involved. Then there is a self-contained 'Tutorial' which takes about a day to

work through, designed to familiarise you with the basic commands of the trough and gain

a practical grounding in LB films. Further understanding of Langmuir films can be gained

by working through the series of experiments contained in the 'Experiments' chapter. A

full description of all the trough commands and functions is given in the 'Software' chapter.

Finally a full technical description of how to set up the motors and sensors is given in the

'Technical Appendix'.

Should any questions arise, please do not hesitate to contact Nima Technology who will

be delighted to help in any way - info@nima.co.uk

Licence Statement

Purchase of a Nima LB trough gives the buyer unlimited licence to copy and disseminate

both information contained in this manual and the software for running the instruments,

provided this is for educational purposes only.

Guarantee

Your Nima LB trough is covered by our unconditional guarantee. This means that if your

trough does not work, we will repair it free of charge, no matter whatever cause, including

accidental damage by students. We do however ask that you pay for the freight charges

to and from our workshop in Coventry.

For legal reasons, this guarantee is limited to 3 years from date of purchase.

This guarantee does not, however, apply to peripheral equipment, such as PCs, surface

potential sensors & pH meters, made by other manufacturers and consumables such as

platinum plates and rings.
Welcome

iv

CONTENTS

GETTING STARTED .......................................................................... 1

Introduction ............................................................................................................. 2

Unpacking ............................................................................................................... 3

Hardware setup ...................................................................................................... 4

Software installation .............................................................................................. 8

Operation................................................................................................................. 11

Surfactant check..................................................................................................... 14

LANGMUIR-BLODGETTRY .............................................................17

Langmuir films ....................................................................................................... 18

The Langmuir-Blodgett technique........................................................................ 19

Langmuir films - theory ......................................................................................... 23

Langmuir films - practice ...................................................................................... 28

LB film deposition onto a substrate ..................................................................... 31

Laboratory Requirements ..................................................................................... 35

Bibliography ............................................................................................................ 41

TUTORIAL .........................................................................................47

Preparations ............................................................................................................ 48

Producing an isotherm .......................................................................................... 50

Depositing a multilayer LB film ............................................................................ 56

Deposition onto hydrophilic substrates .............................................................. 61

Interpreting transfer ratios .................................................................................... 63

Operating the trough in 'coupled barrier' mode ................................................. 65

Operating the trough in alternate layer mode ..................................................... 66

Further Work & Conclusion .................................................................................. 69

EXPERIMENTS .................................................................................71

Preparations ............................................................................................................ 72

A typical pressure-area isotherm ......................................................................... 75

Experiment 1 - Presence of a monolayer............................................................ 76

Experiment 2 - Molecular flow .............................................................................. 78

Experiment 3 - Area of a molecule ....................................................................... 82

Experiemnt 4 - Collapse of a fatty acid ................................................................ 83

Experiment 5 - Structural changes ....................................................................... 85

Experiment 6 - Acid to salt conversion ................................................................ 87

Experiment 7 - Cholesterol films .......................................................................... 89

Experiment 8 - CMC determination ...................................................................... 91
v

Welcome

SOFTWARE .......................................................................................93

Introduction ............................................................................................................. 94

Setup Menu ............................................................................................................. 98

Monolayer Menu ..................................................................................................... 100

Dipper Menu ............................................................................................................ 102

Calibrations Menu .................................................................................................. 106

TECHNICAL APPENDIX ................................................................. 117

Frequently asked questions .................................................................................. 118

Barrier mechanisms .............................................................................................. 125

Dipper mechanisms ............................................................................................... 128

Pressure sensor ..................................................................................................... 131

Temperature sensor............................................................................................... 133

Magnetic stirrer ...................................................................................................... 133

Surface potential sensor........................................................................................ 134

Cable connections .................................................................................................. 137

Synthesis of ferric stearate ................................................................................... 138

INDEX ..............................................................................................139
GETTING STARTED
Getting Started

2

Introduction

This section describes how to unpack and set up your LB trough. Information is also given

about how to install the software.

Before unpacking your trough, it is a good idea to check that you have everything that you

will need to hand. The following list itemises the minimum laboratory accessories that are

required for efficient operation.

Laboratory Requirements

Bench Space At least 2m in a clean, draught and dust free environment.

Pump Running water and a sink are required for connection of the aspirator

pump provided - alternatively a vacuum pump and flask can be used.

Water A supply of pure (deionised and organic free) water is required for

troughs. Recommended manufacturers include Elga, Millipore and

Barnstead.

Balance To be capable of measuring accuracies of at least 0.1mg.

Sundries Polythene powder-free gloves, surfactant-free, lint-free tissues

(supplied), cleaning solvents (e.g. chloroform), small volumetric flasks

(10ml).

PC A Pentium PC running WIN95 or higher. The Nima software occupies

less than 10MB of disc space. A serial or USB port is essential to

communicate with the serial interface.
Getting Started

3

Unpacking

Before unpacking, make a visual check of the packaging. Make a note of any obvious

damage. During unpacking, look at each item carefully, identify it (most items are labelled)

and check that it looks undamaged.

In the event of damage, notify Nima Technology and the freight agent immediately. It is

a good idea to take pictures of the damaged packaging and goods at this stage - this will

help any insurance claims and speed replacement of damaged items.

You should find some or all (depending on what type of trough you have) of the items below

in the crate your instrument was shipped in:

*Langmuir trough

*Dipper mechanism (optional - conventional and/or alternate layer) for dipping troughs

*Pressure sensor(s) (2 for alternate layer troughs, Models 1222D2)

*Interface unit, serial cable and mains lead

*1 CD with the operating software (or 2 diskettes)

*Monolayer kit

- plastic box with small compartments containing the following:

tweezers, pointed

tweezers, flat (for dipper trough only)

10, 50 or 100μl syringe (depending on size of trough)

spatula

balance pan, pan carrier & 100mg calibration weight

bag of hooks

bag of paper Wilhelmy plates

miniature tools

chromed aspirator pump

PTFE nozzle for aspirator pump

*Box of 'Kimtech' tissues - type 75512

*Packet of polythene, powder-free gloves,

*Rubber tubing for aspirator pump.

*This manual!

Take care when unpacking your Nima LB trough - it is a precision piece of

machinery made by craftsmen. Take exceptional care when unpacking the pressure

sensor, as this is the most delicate part of the system.
Getting Started

4

Hardware setup

Locate a quiet, clean corner of your lab, free from air currents. You will need about 2m of

bench space and a comfortable chair to sit on. These measurements need plenty of

patience and an uncomfortable student is a sloppy researcher.

Access to running water (to connect the aspirator) is recommended. Alternatively, using

a vacuum source and funnel is also recommended.

Please identify your trough from the diagrams below and set it up as shown:

Model 102M / 112D

Pressure sensor mounted on edge of trough (A) or on separate stand (B)

2 mechanically coupled barriers

Sapphire window (removable with locking key)

Tubing connectors for temperature control

A Figure 1.1: Model 102M B

Figure 1.2: Model 112D
Getting Started

5

Figure 1.3: Model 311D Figure 1.4: Model 312D

Figure 1.5: Changing / replacing removable PTFE top

Shown here: switching between Model 301S and 311D

Model 311D / 312D

311D: Pressure sensor and dipper mounted at one end of the trough

Single barrier compressing towards one end of trough

312D: Pressure sensor and dipper mounted at centre of the trough

Two barriers compressing symmetrically towards centre of trough

With or without acrylic cover depending on application

Most troughs have removable tops for easy cleaning.

These just lift off:
Getting Started

6

Model 611D

Trough with 1 or 2 barriers

Pressure sensor

Dipper and well all at one end or in

centre

Hinged cover

Tubing connectors for temperature

control

Model 601A

Pressure sensor at one end of the

trough

1 barrier

Acrylic, hinged cover

Tubing connectors for temperature

control

Model 702BAM

Figure 1.6 Model 601A Figure 1.7: Model 611D

Figure 1.8: Model 702BAM
Getting Started

7

Model 1212D1 / 1222D2 / 1232D1D2

Nomenclature

1222D1

function : D1 = conventional LB

D2 = alternate layer LB

Number of barriers

dipper type 0, 1 ,2 or 3

area x 100 cm2

2 compartment trough with 2 pressure sensors

Alternate layer dipper mechanism (A)

or conventional dipper (B)

2 hinged covers

2 temperature control circuits.

B

A

Figure 1.10: Model 1222D2

Figure 1.9: Model 1212D1
Getting Started

8

Levelling & temperature control

Nima troughs are levelled by means of the four adjustable legs which support the trough.

For temperature control, you can connect suitable tubing to the troughs now or later. The

troughs come with standard fittings which can be unscrewed if you prefer your own.

Pressure sensors

The pressure sensors on troughs use a Wilhelmy plate (in this case, a small length of

chromatography paper - Whatmans Chr1) to measure the surface pressure (reduction in

surface tension).

For each pressure sensor, take 2 or 3 'S' shaped hooks from the monolayer kit and attach

them in 'daisy chain' fashion to the small wire protruding from the pressure sensor. Take

one of the pre-cut Wilhelmy plates provided, thread one of the hooks through the hole

punched in it, and attach it to the lowest hook already suspended from the pressure sensor.

Interface Unit

The unit is supplied with a universal mains adaptor for 100 - 240V, 50/60Hz. Connect the

three pin mains lead into the mains adaptor and the 6mm jack into the 'DC in' socket of the

interface. Do not switch on yet.

Connect the round 5 pin DIN plugs to the back of the interface unit. These are marked 'BARA' for barrier A, 'BAR-B' for barrier B (if you have one), 'PA' for pressure sensor A, 'PB' for

pressure sensor B (or tensiometer), 'LIN-DIP' for a conventional linear dipper and 'ALT-DIP'

for an alternate dipper.

Attach the serial cable to the interface unit (IU4) and to the serial port of the PC - the interface

end is a 9 pin 'D'. The PC end is also 9 pin D and a 25-9 way converter is also supplied.

If you are using a mouse on a serial port, you must use a different port for the interface.

However, beware(!) because some mouse software drivers clash with the trough serial

port driver. Check in your WIN Control Panel under:

System / Device Manager / Ports / Resources / Interrupts

Please also see our help site at:

www.nima.co.uk/help
Getting Started

9

Software Installation

Notation and computer literacy

A certain familiarity with computers and Windows is assumed. You must know the correct

way to insert a CD into your personal computer (PC) and you must understand what a file

is and what a directory is. You must also be able to 'Run' programs in Windows. It is also

helpful if you know how to change icons.

If you are not happy with these ideas, we suggest that you consult the manuals of

your computer and software vendor. There are probably also a wide range of help

manuals available at your university bookshop.

PC requirements

You need to have a Pentium PC running Windows 95 or later.

Communications

Your PC has to have a serial port or USB port to communicate with the Nima interface. If

you want to use a USB port, you will need a USB to serial port converter. Please make sure

that you use the one compatible with your Windows version.

Display

Please set the Display to a minimum of 800 x 600 pixels. This can be changed under:

Start / Settings / Control Panel / Display / Display Modes

Fonts

The Nima software uses Microsoft Sans Serif fonts, as our software is compatible with all

versions of Windows from 95 to XP you may need to load the specific MS Sans Serif font

onto your PC. Copies are on the Nima CD.

Software organisation

There is one main program which controls all the functions of the instrument:

Nima516.exe where '5' is for the 5th generation and the 16 is the version

number. This is the standard software for Langmuir troughs.

Then there is

Nima516.ini which contains essential data for the graphics screen and fonts

There are also files containing data which the main program needs before it will run. These

initially hold standard data which you can alter as you go. These are:

install.dat This contains all the installation data for your trough.

sensors.cal This contains all the calibration data for your sensors.

standard.con This contains a standard set of operating conditions.
Getting Started

10

Installation from CD

Insert the CD into your CD drive. You computer may detect that a CD has been inserted

and autorun the program. If it does not autorun then select ‘Run’ from Windows Start Menu.

Enter ‘D:\Launcher.exe’ in the Open box where ‘D’ is the letter assigned to you CD drive.

When the Launcher program loads click the top button to install the Nima software. The

installation wizard will prompt you to choose a directory to install into - the default is ‘c:\nima’

.You can choose a different directory at this stage. Once you have chosen a directory click

‘Finish’ and the wizard will complete the installation.

Once the installation is complete your calibration files are copied from the CD into the

directory you chose. This may leave a DOS window open, if it does you may safely close

this window. The launcher software also allows you to view a copy of this manual on the

CD, it also provides a link to our website. Once you have finished with the launcher program

click the cross in the top right of the window to close it.

A shortcut to the Nima516.exe will be displayed in a Nima group file window. If you want to

use the 'Nima' trough icon, use the right hand mouse button to click on the shortcut to

change its properties and select the 'trough.ico' icon.

And finally there is:

serpdrv This is the serial port driver which allows communication between

your PC and the interface unit (IU).

All these files must be located in the same directory. The default directory is C:\nima, but

any other directory can be used.

Figure 1.11: Launcher window
Getting Started

11

Installation from disk

Older models came with two floppy disks, if you have been supplied with these please

follow the instructions below:

Insert disk 1 into drive a and select Run from Windows Start Panel. Enter a:\setup.exe in

the Open box and the software will prompt you to load into c:\nima .You can choose a

different directory at this stage. Then the Install Wizard will take over - just click on the 'finish'

button to start (Microsoft Logic!).

If you have problems, such as 'unable to find disk 2', please copy the 3 files: 'setup.exe',

'data.001' and 'data.002' into the 'c:\windows\temp' directory and run 'setup.exe' from this

directory, using the 'Run' command.

Installation from the website - www.nima.co.uk

Click on the 'Software' button in the margin on the left side of our home page. The

instructions for downloading are given on the screen. Very briefly, these consist of:

1. Download - just click on 'download' and the 'zipped' file (2MB) is loaded onto your PC.

2. Unzip - using WinZip, unzip the file into its 3 constituents.

 A link to www.winzip.com is given, if you don't have this program.

3. Setup - as above - 'Installing from disk'.

Running

Double click on Nima516.exe to run it (or on its shortcut if you have placed it on your

desktop). 'OK' the introduction window and a set of pressure-area axes will be displayed

with a menu bar along the top, as in the figure below. Note that an isotherm is displayed only

if the interface is disabled.

Figure 1.12: Main screen of Nima software
Getting Started

12

Setting up the Interface

Before anything else, please click on 'File' in the menu bar and 'Setup' to enter the 'Setup

Menu'.

Select the port you think you are using (if in doubt, start with port 1) and enable the interface.

If the V2 light on the interface illuminates, then communications have been established

successfully. If nothing happens, change the port number to '2'. Usually, PCs use port 1

or 2 for the external serial port connections. If you still have trouble you can try port 3 or 4,

but it is unlikely they are being used. If you are using an USB adaptor, the serial port numer

is often higher, say 5 or 6.

If you continue to have problems, please check that there is not a clash with other serial

devices, such as mouse drivers and networks. You can look at your port settings in the WIN

Control Panel under:

System/Device Manager/ Ports

Once communications have been established, click on the 'ACCEPT' button and the

calibration file is updated automatically, so that the next time you launch the program, the

correct serial port will be addressed immediately.

Setting up for 2 operating modes or multiple users

If you have a trough like the 632D1D2, then it is possible to run this in 2 modes - either with

the conventional dipper D1L or with the alternate layer dipper D2.

The simplest way to switch between operating modes in the software is to install both

versions of the software (which are supplied) into different directories. For example, you

can make a directory called 'ConLB' for the conventional dipper and a directory called

'AltLB' for the alternate layer software.

If you then make shortcuts for the two different Nima516.exe files, you can run one or the

other at the click of your mouse. Do not run both simultaneously, as the serial port drivers

will clash - unless, of course, you have 2 serial interfaces running 2 troughs.

Software Updates

Updates are regularly made available on our website - free of charge.

The update will prompt you to install it into a different directory - 'c:\Program Files\Nima542'

- but any directory will do, as long as it's not the old one!

Then copy the 3 calibration files 'install.dat', 'sensors.cal' & 'standard.con' from your

exisiting 'Nima' directory to the new one. This will save having to re-calibrate all the motors

and sensors.
Getting Started

13

Operation

There are 'one click' buttons for basic functions like 'opening' and 'closing' the barrier.

These are:

O Open barrier

C Close barrier

D Dipper down

U Dipper up

STOP Stop all motors

CLEAR Clear data from memory

F Dipper forwards (alternate layer troughs only)

B Dipper backwards (alternate layer troughs only)

Other commands are embedded in the menus along the top of the screen:

File (f1) - this gives access to the Save, Load and Setup Menu. Also to Exit the program.

Graph (f2) - the data can be displayed as a pressure-area isotherm, pressure & area

v time, immersion depth-area or transfer ratios.. Select the appropriate display from

the Graph Menu. Axes are auto-scaled but may be manually re-scaled by right clicking

anywhere on the graph and then typing over axes labels.

Barrier Menu (f3) Selects the barrier mode you require: Choose between Monolayer

Menu, Open, Close, Isotherm or Pressure (Pi) Control, Isotherm cycle, Coupled

Mode and Area Control.

Dipper Menu (f4) - Selects the dipper mode you require - Choose between Dipper Menu,

Up, Down (Forwards or Backwards for alternate layer dipper), Programmed Dip and

Creep Up or Down

Run time variables (f7 - separate box at top left of screen) - allows important values

to be changed before or during a run. Choose between Target Pressure, Feedback

Gain, Immerse To, Raise To, Cycle Max Area and Cycle Min Area.

The STOP button can also be accessed from the 'ESC' key.

Typical operating sequences for taking a pressure-area isotherm and depositing an LB

multi-layer are given overleaf.
Getting Started

14

A typical pressure-area isotherm

The following steps are recommended to take an isotherm:

1. Clean the trough thoroughly with a Kimwipe (soaked in chloroform). Ensure you are

wearing the polythene gloves provided and working in a well ventilated area.

2. Make up a solution of 1mg/ml arachidic acid in chloroform.

3. Fill the trough with clean water.

4. Close the barrier(s) by clicking on 'C' - or the 'home' button of your keyboard.

- the barrier(s) will move to their 'close-to' position.

5. Aspirate the surface of the water enclosed by the barrier(s).

- keep your eye on the pressure reading - this will drop as the water level reduces

and is cleared of surfactants.

6. Open the barrier(s) by clicking on 'O' - or the 'end' button of your keyboard.

-the barriers will move to their 'open-to' position.

7. Zero the pressure sensor by clicking on 'Z'

- the current surface tension (~72.8 mN/m) will be defined as zero surface pressure

i.e. 0 mN/m.

8. Take an isotherm without molecules by clicking on 'Barrier' and 'Isotherm'

- the barriers will close and the isotherm will be displayed on the screen.

If it the trough is clean, the curve will be horizontal along the X-axis. If there are

surfactants on the surface, a small rise in pressure will be observed at smaller

areas.

Please note that the axes are set to 'auto-scaling' and adapt themselves to the data

recorded, so at first the data will appear greatly magnified.

9. Clean the surface again with the aspirator.

10. Zero the pressure sensor.

Whenever some water has been sucked out of the trough, it will have to be rezeroed.

11. Open the barriers and spread the solution from the micro-litre syringe.

Wait for the pressure to return to 0mN/m - this indicates that the solvent has

evaporated. If the pressure does not return to zero, you have spread too much and

will have to remove some monolayer with the pump.

12. Take the pressure-area isotherm

 - you should now see a good pressure area isotherm of arachidic acid on water.

For an example of such an isotherm, please see the graph in Figure 1.6.

13 Stop the barrier by clicking on 'STOP' - or the 'Esc' key on your keyboard.

If you do not, the barrier will continue until it passes its 'close to' area and then

a message saying 'barrier too closed' will appear. The barrier will then be stopped

automatically.
Getting Started

15

A typical multi-layer deposition (conventional dipper)

1. Follow the instructions for the pressure-area isotherm first (previous page).

This will ensure that the trough is clean and ready to go.

2. Make up a solution of 10-4M ZnSO4 from the chemicals supplied with the trough.

Make enough to fill your trough about twice.

3. Prepare a glass slide by rubbing it with the ferric stearate supplied.

This will make it hydrophobic.

4. Move the dipper up by clicking on 'U' - or the 'PgUp' button of your keyboard.

The dipper will come to rest at its top position.

5. Insert the slide between the jaws of the dipper clamp.

6. Go the Dipper Menu ('Dipper' on menu bar).

7. Select the appropriate values for your deposition.

Typical values are: Dipper Speed: 20 mm/min

Target pressure: 25 mN/m

Top End: 5 mm

Bottom End: 20 mm

Number of Layers: 10

8. Spread the monolayer and select 'Pressure Control' from the Barrier menu.

The film will be compressed to its target pressure. If pressure control is unstable,

reduce the 'Feedback Gain' value (Variables menu - top left of screen). A value of

1.0 is good for almost all conditions.

9. Select 'Program Dip' from the Dipper menu

The dipper will be run down and up under the selected conditions.

10. When the required number of layers have been deposited, the dipper will stop.

11. Data can be viewed also in the (pressure & area) -time and dipper - area axes,

selected from the Graph Menu.

Saving / Loading (File Menu - f1)

Saves and loads data as ASCII files with *.txt extension. These can be imported into most

spreadsheets.

Copying, pasting & printing

Copy to pasteboard by hitting the 'ALT-Print Screen' keys on your PC. Paste into other

applications (e.g. Word, PageMaker) - the graph will be pasted as it appears on the screen.

Print out in the usual way.
Getting Started

16

Surfactant Check List

Making Langmuir-Blodgett films is easy on your Nima LB trough - provided that you know

how to keep surface active agents (surfactants) at bay. The following list should help

eliminate any contamination.

Water source - use a commercial de-ioniser such as those made by Vivendi (ELGA

UHQII) or Millipore (Milli-Q). These units are capable of producing 18M-ohm.cm water with

organic content of less than 20ppb. They have 4 stages of purification, reverse osmosis

and organic removal. If you use distilled water, please be aware that conductivity won't be

much better than 1M-ohm.cm. This will affect your work if you are working with, for

example, fatty acids where the mineral ions in the distilled water will combine with any

dissociated fatty acids.

Water storage - only store water in good quality borosilicate glass bottles with ground

glass stoppers. Ideally, the water should not be stored for more than 30 minutes, as after

this time significant quantities of mineral ions will leach out of the glass.

Never use plastic bottles or even 'teflon' bottles, as these can contain surfactants which

will seriously contaminate your surface. Don't even use plastic stoppers on glass bottles.

We recommend a 2 litre round-bottomed, long-necked pyrex glass bottle with glass

stopper for your trough work.

Solvents - alway buy small volumes of the best quality solvents. Free radicals can form

in the solvent after 3 - 6 months (depending on temperature and light exposure), so store

them in the fridge. If free radicals do form, they will cut your long chain molecules into

smaller pieces and you will get some very strange isotherms.

Always dispense the solvent into an intermediate glass bottle, to prevent back-contamination

of your source bottle. Keep all glassware exclusively for Langmuir film work and never use

plastic bottles or stoppers. And never, ever parafilm!

Always use a balance to measure weights of solvents - and work out the volume from the

solvent density. Make a note of the weight before storing your solution in the fridge and

measure it again before the next time you use it - in this way you can work out the

concentration despite any solvent evaporation.

Trough cleaning - always use the Kimwipe tissues (type 7105, or Kimtech type 75512

supplied with the trough - or type EX-L No 34256 in the USA) soaked in chloroform wearing

polythene gloves (also supplied). Other tissues and gloves may contain surfactants which

will make the trough dirtier than when you started.

Make sure you are working in a well ventilated room and ask your laboratory supervisor

about safety regulations and toxic solvents. Keep a separate bottle of chloroform for

cleaning, to prevent back-contamination of the source bottle.

Soak the Kimwipe with the chloroform and gently, but firmly, wipe down the trough's PTFE

surface and barriers. Take extra care with the edges, as any aggregates will conglomerate

there. The cleanliness of the trough can be checked visually by filling it with water and then

using the aspirator pump to empty it again. Hold the pump nozzle at one corner of the trough

and watch the water pull away from the PTFE. A good indication of cleanliness is if the water

pulls away cleanly from the PTFE without leaving any droplets behind.

If a simple wipe with a Kimwipe does not clean it well enough, stand the trough in a fume
Getting Started

cupboard and fill it with chloroform. Allow it to stand for about 10mins and give all inside

edges a good scrub with a 'cotton bud' (the little sticks used for cleaning out your ears).

Then remove the chloroform, taking care to dispose of it safely, and rinse the trough several

times with pure water. Note that the chloroform musn't be allowed to evaporate away,

otherwise the dissolved contaminant will be left in the trough.

If you are using materials that are insoluble in chloroform, use other suitable solvents to

wipe the trough down. If there are no suitable solvents, then use a detergent (see below).

Never use strong acids or alkalis and never experiment for the sake of it - it is very easy to

contaminate a trough and very difficult to eliminate unknown contaminants.

Detergent Cleaning: If there is no suitable solvent available for your trough contamination,

it will have to be washed with detergent. Commercially-available detergents include

Decon-90, Helmanex and Tikopur. These need to be used sparingly and the trough has be

thoroughly rinsed (10 times with warm water) afterwards. Full details are available in the

'Langmuir-Blodgettry' section of this manual.

Remember to use the detergent sparingly and to rinse the trough thoroughly afterwards!

Conclusion

By now you should have assembled your Nima LB trough, carried out some simple checks

to establish that it is operational and seen a glimpse of the cleanliness requirements.

The following chapters of this manual will give a thorough introduction to the theory and

practice of Langmuir-Blodgett films.

16a
LANGMUIR

BLODGETTRY
Langmuir-Blodgettry

18

Introduction

This section deals with the theory and practice of Langmuir Blodgettry. It covers the

materials used - solvents, water, chemicals and substrate - and their preparation as well

as cleanliness and safety. It also provides a comprehensive account of the practice of

Langmuir Blodgettry - its history, the subphase, spreading, isotherms and dipping.

Langmuir films

Langmuir films consist of surface active agents or 'surfactants' trapped at the interface

between two dissimilar phases, either liquid-liquid or liquid-gas.

Surfactants are molecules which are amphiphilic, that is molecules which are composed

of a hydrophilic part and a hydrophobic part. Hydrophilic groups consist of groups such as

carboxylic acid, sulphates, amines and alcohols. These are all attracted to polar media

such as water and the forces acting upon them are predominantly coulomb type (1/r2).

Hydrophobic (or oleophilic) groups such as a hydrocarbon chain, fats and lipids are much

less (if at all) water soluble and the forces acting upon them are predominantly van der

Waal's type (1/r12 and 1/r6

). Amphiphilic molecules are trapped at the interface because

they posses these two very different types of bonding within the one molecular structure.

When surfactants, dissolved in a non-aqueous volatile solvent, are introduced onto a polar

liquid surface, the solvent will evaporate leaving the surfactants oriented at the liquid-gas

interface. The hydrophilic 'head' groups pull the molecule into the bulk of the water and the

hydrophobic 'tail' groups point into the air. A surface monolayer will only be achieved if the

amphiphatic balance of the molecule is correct; that is the balance between hydrophobic

and hydrophilic parts. If the hydrophobic 'tail' group is too short (not hydrophobic enough)

the molecule will be dragged into the water and will dissolve while if there is no hydrophilic

part, the molecules may form thicker multilayer films on the surface or even evaporate.

Sweeping a barrier over the water surface causes the molecules to come closer together

and eventually to form a compressed, ordered monolayer - much like forcing together balls

on a billiard or pool table. The film produced by such a method is known as a Langmuir film.

Figure 3.1: Expanded and compressed monolayers on a water surface
Langmuir-Blodgettry

19

The Langmuir-Blodgett technique

Langmuir-Blodgett films consist of mono-molecular layers stacked sequentially onto a

solid substrate. A solid substrate is lowered into the water, breaking through the Langmuir

film and, provided that certain criteria have been met, the Langmuir film attaches itself to

the substrate, coating it in a mono-molecular layer.

Once the first layer has been deposited, further layers will be deposited on each

subsequent pass of the substrate through the air-water interface. Multilayers can therefore

be deposited to produce a film, the thickness of which is the product of the individual

molecular chain length and the number of times that the substrate has crossed the airwater interface.

Figure 3.2: Deposition of the first monolayer onto a hydrophobic substrate.

Figure 3.3: The substrate reverses (a) and the second layer is deposited (b).
Langmuir-Blodgettry

20

History and development

The effects of oils on water surfaces have been known for many centuries. Babylonians

in the eighteenth century BC have been recorded as practising divinity by observing the

effects of oils spread on water. The Japanese printing art of sumi-nagashi (black-ink-letflow) is one of the first technical applications of floating monolayers. A suspension of submicron carbon particles and proteins are spread on a water surface and subsequently

covered with gelatin to create patterns which can then be transferred onto a sheet of paper

lowered onto the surface. The traditional craft of 'marbling' also works on a similar principle

- paint is dispersed on an aqueous surface and a piece of paper is drawn up out of the water

to be covered by the pretty patterns created.

Scientific interest

The first scientific interest in mono-molecular layers was shown by Benjamin Franklin

(during a visit to London to discuss certain colonial matters) when he spread oils onto the

pond at Clapham Common and observed that the one teaspoon full of oil which he used

had a calming influence over half an acre of water. Taking '1 teaspoonful' to be 5ml, this

would mean that a film of thickness 2.5 x 10-7m (about 100 layers) was covering the 'half

an acre' (2,000m2). Franklin reported his findings to the Royal Society of London in 1774:

Figure 3.4: Part of Benjamin Franklin’s original paper
Langmuir-Blodgettry

21

Modern investigations of molecular films began with Agnes Pockels experimenting with a

very simple trough in her kitchen. Her ideas, expressed as letters to Lord Rayleigh, were

published in 'Nature' in 1891. She realised the importance of cleanliness and developed

many of the techniques which are regarded as standard today. Her experiments included

the first containment of a monolayer by means of a barrier.

Further investigations carried out by Rayleigh, Devaux and Hardy confirmed the monomolecular nature of such films. However it was Irving Langmuir who contributed the

greatest advances, building on many of the ideas of Pockels and others to publish 'The

Constitution and Fundamental Properties of Solids and Liquids' in 1917 which described

the 'new' surface effects. Langmuir studied the pressure-area relationship of molecules

on an aqueous surface. The areas occupied by molecules such as acids, alcohols and

esters were found to be independent of the hydrocarbon chain length, thus showing that

only the hydrophilic head groups were immersed in the subphase.

Katherine Blodgett, who worked with Langmuir on the properties of floating monolayers,

developed the technique of transferring the films onto solid substrates and hence building

up multilayer films. Her results were published in 1934 and 1935, culminating in a paper with

Langmuir in 1937 and a paper on anti-reflection coatings in 1939 (see the 'Tutorial' chapter).

Langmuir-Blodgett films have been used in some practical devices. The first, developed

by Blodgett, were the above mentioned anti-reflection coatings, and step thickness

gauges.

The recent past and the future

Interest in Langmuir-Blodgett films subsided with the outbreak of the Second World War,

and remained low until the 1960’s when Kuhn and Möbius showed how monolayers could

be used to construct precise supermolecular structures. They used the Langmuir-Blodgett

technique to demonstrate the fluorescence and quenching of dye molecules attached to

fully saturated fatty acids under ultraviolet light. This work and the publication of Gaines'

'Insoluble Monolayers at Liquid Gas Interfaces' in 1966 initiated a revival of interest in the

field.

Many possible applications of Langmuir-Blodgett films in the fields of electronics, optics,

molecular electronics and biotechnology are currently being investigated by research

groups throughout world. Some examples are:

Biology: Cure for 'blue baby' syndrome. Some babies are born without the ability to breathe.

This is because their lungs cannot absorb oxygen just after birth. Surfactants are being

developed to be sprayed into the lungs to absorb oxygen. Once this process is stimulated,

it is self regenerating. Surfactants are being designed by testing Langmuir monolayers for

this oxygen absorbing ability.

Chemistry: Sensors - to have chemical sensors (such as gas detectors) it is best to have

a large surface to bulk ratio. This gives higher signals for lower concentrations of gas and

can improve reversibility of the detection process. LB layers are ideal candidates.

Physics: Optical waveguides and devices are being developed for computing and

communications applications. The capability of fabricating dimensions to mono-molecular

tolerances and their ability to incorporate different molecular architecture give LB films

some unique advantages in this field.
Langmuir-Blodgettry

22

The original troughs

Trough design has changed markedly since the early waxed troughs used by Langmuir,

Schaefer and Blodgett. These were ordinary baking trays (purchased at the local hardware

shop) that were waxed to make them hydrophobic. They were filled to the brim with water

- this allowed for easy cleaning of the surface by simply sweeping a bar along the ridges

formed by the trough walls. Films were confined by floating a length of waxed silk on the

water surface thus dividing the surface into two compartments. One of the compartments

was filled with piston oil of known surface pressure which compressed the monolayer,

spread in the other compartment, to a pressure which would remain constant throughout

the experiment. Contamination of the monolayer under study and the preparation of the

waxed silk were the major problems. This prompted the development of new barrier

systems which would eliminate such problems.

Gaines gives an example of a rectangular trough with a boom which moves along the top

of the trough walls (the trough being filled to the very brim) by means of suspended weights

attached to the barrier through a pulley system. There was no cross-contamination

problem as no piston oil was involved but the gravity driven barrier did not allow easy

measurement of the surface pressure-area characteristics. Although all these troughs

required considerable experimental skills to operate successfully, Blodgett was able to

build up thousands of layers at speeds of up to twenty layers per minute using such

apparatus.

Figure 3.5: Langmuir’s film balance

Reprinted with kind permission from the Journal of the American Chemical

Society, 1917, Vol 39, pg 1869, Copyright 1917, The American Chemical

Society.
Langmuir-Blodgettry

23

Langmuir films - theory

Spreading

The monolayer is formed on the surface of the subphase by dissolving the amphiphilic

substance in a suitable solvent - see also the section on 'Solvents' later in this chapter -

and spreading the resulting solution dropwise on to the water surface.

The spreading solution is best applied to one point of the surface by allowing small drops

to fall from the syringe while it is held a few millimetres above the sub-phase. Each drop

should be allowed to evaporate before the next is applied. An alternative procedure that is

used for molecules known to form more rigid films, involves distributing the drops equally

over the entire surface of the sub-phase.

The process of spreading can occur in a number of ways. Usually a thin film is formed by

the spreading solution while the Langmuir film forming material attains its favoured

configuration at the air-water interface and the solvent evaporates, leaving behind only the

monolayer. In some cases the deposited droplet does not spread over the subphase but

instead a thin film of dilute solution of monolayer-forming material spreads from its edges;

as solvent evaporates, more film spreads from the droplet. As most solvents used are

slightly soluble in water, depletion of the spreading solvent will not be due exclusively to

evaporation but also to some dissolution of it into the subphase.

Spontaneous spreading will continue until the surface pressure of the monolayer is equal

to the 'equilibrium spreading pressure'. At this point, all the available surface is covered in

monolayer and any further droplets of solution remain embedded in the monolayer as

floating lenses, as no more spreading out can take place. Any undispersed material must

be removed otherwise it may be deposited as bulk aggregates on substrates dipped into

the monolayer.

Isotherms

Molecules in a solution are subject to attractive forces; in the bulk of the solution these

forces are equal. However, at a surface or interface the forces are unequal and the net

effect is to pull the peripheral molecules into the bulk of the solution. This effect gives rise

to surface tension.

The surface tension can be defined as 'the work required to expand the surface isothermally

by unit area' . The tendency of surface-active molecules to accumulate at interfaces

favours expansion of the interface and hence lowers the surface tension. Such behaviour

makes it possible to monitor the surface pressure as a function of the area occupied per

molecule provided that the number of molecules deposited on the surface is known.
Langmuir-Blodgettry

24

The characteristics of a monolayer on the water surface are studied by measuring the

changes in surface tension upon compressing the monolayer. The reduction of surface

tension is known as the surface pressure. Pressure readings are made by means of

'Wilhelmy' plates attached to a microbalance (see 'surface pressure measurement', later

in this chapter). The plot of surface pressure versus area occupied per molecule is known

as a 'pressure-area isotherm' - isotherm because compression takes place at constant

temperature. The shape of the isotherm is characteristic of the molecules making up the

film and hence provides a two-dimensional 'fingerprint'.

Forces at the subphase surface

The forces affecting the polar head groups submerged in the aqueous subphase are ionic

and proportional to 1/r2 ('r' being the intermolecular separation) whilst the forces between

the hydrocarbon chains are due to van der Waal’s interactions and proportional to 1/r6 and

1/r12. Hence the interactions in the subphase are of longer range than those in the

superphase. The isotherm can usually be seen to consist of three distinct regions:

Figure 3.6: A stearic acid isotherm on pure water
Langmuir-Blodgettry

25

After initial spreading onto the subphase, no external pressure is applied to the monolayer

and the molecules behave as a two-dimensional gas, which can be described by:

πA = KT

where π is the surface pressure, A the molecular area, K the Boltzmann constant and T

is the thermodynamic temperature. On compression of the monolayer, some ordering of

the film takes place and it behaves as would be expected of a two-dimensional liquid.

With continued closing of the barrier, the increase in pressure causes additional ordering,

the monolayer behaving as a quasi-solid. This solid state is characterized by a steep and

usually linear relationship between surface pressure and molecular area.

Eventually the collapse pressure, πc, is reached at which the film irretrievably loses its

mono-molecular form. The forces exerted upon it become too strong for confinement in

two dimensions and molecules are ejected out of the monolayer plane into either the

subphase (more hydrophilic molecules) or the superphase (more hydrophobic molecules).

However, collapse is not uniform across the monolayer but is usually initiated near the

leading edge of the barrier or at discontinuities in the trough - such as corners or the

Wilhelmy plate. Usually a 'collapsed' film will consist of large areas of uncollapsed

monolayer interspersed with 'mountain ridges' where monolayers have been crushed

together to form bulky aggregates. This is rather like mountain formation at the collision of

the earth's tectonic plates, but on a vastly different scale.

The collapse pressure can be defined as the maximum to which a monolayer can be

compressed without the detectable expulsion of molecules from the Langmuir film. It

depends upon the details of the experimental procedure used, i.e. the chemical

substances used, the temperature, the rate at which the film is compressed and

procedures to which the film has been subjected. The value of the collapse pressure for

a simple saturated fatty acid can be in excess of 50mN/m which is equivalent to about 200

atmospheres if extrapolated to three dimensions.

Quantitative information can be obtained on the molecular dimensions and shape of the

molecule under study. When the monolayer is in the two-dimensional 'solid' or 'liquid

condensed' phase the molecules are relatively well oriented and closely packed and the

zero-pressure molecular area(A0) can be obtained by extrapolating the slope of the 'solid'

phase to zero pressure - the point at which this line crosses the x-axis is the hypothetical

area occupied by one molecule in the condensed phase at zero pressure (see Figure 3.5).

For a fully saturated alkanoic acid such as stearic acid the molecular area determined in

such a manner is 22-25 A2

. Such an area corresponds to the cross-sectional area of a

hydrocarbon chain, suggesting that the compressed monolayer consists of close packed

vertically orientated chains. The Nima software calculates this zero-pressure molecular

area(A0) for you - as described in the 'Tutorial', later in this manual.

It is important to study the isotherm of the monolayer of the film forming material as a prerequisite to the determination of dipping characteristics. Information can be obtained as to

the way in which the molecules pack at the interface and the stability of the compressed

layer at high pressures. Improved quality can be obtained by 'annealing' the film, that is by

compressing (never going above πc) then expanding a number of times. Such a procedure

encourages more efficient packing and usually a shift to lower molecular areas is

observed. Recent work suggests that the Langmuir film exists in a number of different liquid

crystal phases on the water surface, depending on the film temperature and pressure.
Langmuir-Blodgettry

26

Measurement of surface tension

Surface pressure and surface tension

When a monolayer is compressed on the water surface, the instantaneous surface

tension (STinst ) of that surface is reduced - this reduction being known as the surface

pressure (SP or π ). ST and SP have the same units (mN/m) and magnitude, but the SP

increases as the ST decreases:

Hence: STinst + SP = constant

- the constant being the absolute ST of the liquid - 72.8mN/m for water at STP (298K, 1

atmosphere)

STinst SP, π STabs of water

clean water surface 72.8 0.0 72.8 mN/m

compressed monolayer 22.8 50.0 72.8 mN/m

Surface pressure (or surface tension) of the water surface is measured by immersing an

object (typically a plate, ring or cylinder) acrosss the air/water interface and measuring the

force experienced by this object.

Consider a plate suspended across the air-water interface:

Figure 3.7: The Wilhelmy Plate

thickness, t

width, w

contact angle, θ

perimeter = 2.(w+t) air

water

immersed 

depth, d
Langmuir-Blodgettry

27

The forces upon the plate are gravity and surface tension acting downwards into the water,

and buoyancy due to displaced water acting upwards.

If the plate has dimensions l x w x t (length, width, thickness) and a density, ρ, and is

immersed in water to a depth, d, then the net force downwards, F, is described by the

equation:

Force = (ρPlwt).g - (ρLdwt).g + 2.(w+t).(ST).cosθ Eq.1

(Force = weight - upthrust + surface tension )

where ρP is the density of the plate, ρL the density of the liquid, ST the surface tension of

the liquid, θ the contact angle of liquid to plate and g the acceleration due to gravity.

Before making any measurements, the pressure reading is zeroed, thereby eliminating the

weight term from Eq.1:

Force = - (ρLdwt).g + 2.(w+t).(ST).cosθ Eq.2

 (Force = - upthrust + surface tension)

The upthrust term is eliminated from Equation 2 as the plate is always kept at a constant

level by the balance, no matter what the surface tension:

Force = 2.(w+t).(ST).cosθ Eq.3

Finally, use of paper plates ensures a contact angle of liquid to plate of 0° and the

expression reduces to:

Force = 2.(w+t).(ST) Eq.4

Hence for a plate, the surface tension is:

STplate = Force / 2.(width + thickness) Eq.5

where surface tension is in mN/m, force in mN and perimeter in metres.

Generally therefore, the surface tension is given by:

ST = Force / perimeter Eq.6

Hence for a 10.25mm wide, 0.25mm thick paper plate, a 100mg weight will give a reading

of 46.7mN/m. This is useful figure when you are checking your pressure sensor.

Calculation for a ring

For a ring the surface tension is given by:

STring = Force / (πD + π(D-2d) ) Eq.7

where 'D' is the outer diameter of the ring and 'd' is the diameter of the wire in metres.

Surface tension is in mN/m and force is in mN.

However, a correction factor is also required - this is explained in more detail in the Nima

Tensiometer manual

Note that: 1 dyne/cm = 1 mN/m.
Langmuir-Blodgettry

28

Langmuir films - practice

Materials suited to the LB technique

The LB technique, by definition, requires amphiphilic molecules to be trapped at the

interface of two phases (either liquid-liquid or liquid-gas). For the molecules to be trapped

at the interface, the 'amphiphatic balance' i.e. the balance between the hydrophilic and

hydrophobic constituents within the same molecule must be correct for the two phases

concerned. For instance, the amphiphatic balance of a molecule may be correct for an airwater interface but not for an water-hexane interface. Hence, when choosing materials,

this factor has to be taken into account.

In practice, monolayer and deposition criteria are not so exacting, as many researchers

have produced excellent multilayer films using the LB technique with molecules that have

only a very small hydrophilic component and that literally float on the water surface. Strictly

speaking though, these are not LB films.

In general, materials such as long chain fatty acids and alcohols are used as film forming

materials. The hydrocarbon part -CH2- of the molecule is responsible for the material's

repulsion of water while the polar -COOH or -OH group has sufficient affinity for water to

anchor the molecule in the aqueous subphase.

At an air water interface such amphiphilic molecules orient themselves with their hydrocarbon

chains upwards in the air and their hydrophilic head groups downwards into the aqueous

phase:

Figure 3.8: Fully saturated fatty acid (stearic acid). 'Fully saturated' signifies that

all the available carbon bonds have been taken up by hydrogens
Langmuir-Blodgettry

29

The materials which have been studied extensively as floating monolayers are the alkanoic

acids and their salts and long chain alcohols. The length of the hydrocarbon chain is critical

and its effect on the surface tension of the subphase is given by Traube’s rule, an

approximation which states that for a given homologous series of amphiphiles the

concentration required for an equal lowering of surface tension in dilute solution decreases

by a factor of about three for each additional CH2 group.

The effectiveness of functional groups attached to the hydrocarbon chains in providing

attraction to an aqueous subphase is summarised in the table below:

Table 3.1: Commonly used film forming materials

The types of materials listed above are those which have been extensively studied over the

years and are regarded as the classic Langmuir-Blodgett film forming materials. However,

current interest is primarily due to the introduction of 'active' molecules such as chromophores

or biologically functional molecules into monolayer films. Although traditionally long

hydrocarbon chains have been considered necessary, the incorporation of aromatic

moieties and unsaturated chains into the amphiphile tends to decrease the hydrocarbon

chain length necessary for deposition. For example, a substituted anthracene derivative

has been successfully deposited with only a butyl chain attached to the polyaromatic

system. In fact, with appropriate synthetic chemistry almost any active species can be

tailored into a form which will float as a monolayer.

The subphase

The subphase most used for the study of mono-molecular films is ultra pure water. Other

high surface-tension liquids which have been used as subphases include ethylene glycol,

glycerol and mercury.

Usually measures are taken to exclude any contaminants from the subphase as the

introduction of ions into pure water can dramatically affect the monolayer properties. For

example, the repulsion between adjacent ionised acid groups can be reduced by the

addition of divalent metal ions into the water. Introducing Cadmium (Cd2+) into the water

results in following reaction taking place:

No film Unstable films Stable film with C16 chain comC16 chain pounds dissolve

Hydrocarbon -C6

H4OCH3 -CH2OH -SO3

-

-CH2I -COOCH3 -COOH -OSO3

-

-CH2Br -CN -C6H4SO4

-

-CH2Cl -CONH2 -NR3

+

-NO3 -CH=NOH

-C6H4OH

-NHCONH2

-NHCOCH3
Langmuir-Blodgettry

30

 R R R R

 C C + Cd2+

(aq) C C

 O O- O O- O O O O

 Cd

Table 3.2: Reaction of Cd2+ with an alkanoic acid group

Care must be taken in making up the solutions - high quality reagents and ultra-pure water

must be used. Moreover, it should be remembered that the aqueous subphase consists

of a number of variables, not necessarily independent of one another. A change in one ionic

equilibrium will often affect several of the other equilibria within the subphase.

Changing the acidity and adding divalent ions to the subphase allows a considerable

degree of control over monolayer behaviour and the successful deposition of multilayers.

5H2O ⇔ 5[H+ + OH-] ⇔ [H9O4]

+ + OHCdCl2 ⇔ Cd2+ + 2ClNaOH ⇔ Na+ + OHHCl ⇔ H+ + ClTable 3.3(a) Typical ionic equilibria in the aqueous subphase:

Primary dissociations i.e. equilibria of ionic species introduced into the

subphase.

Cd2+ + 2OH- ⇔ Cd(OH)2

Na+ + Cl- ⇔ NaCl

Table 3.3(b) Typical ionic equilibria in the aqueous subphase:

Secondary dissociations i.e. equilibria set up between ions introduced from

differing sources. The addition of chloride ions to the subphase will reduce the

pH as this results in hydroxide ions being taken out of the subphase.

Buffers should only be used in biological applications and not for making LB films, as they

introduce new unknowns into the subphase.
Langmuir-Blodgettry

31

LB film deposition onto a substrate

Two types of surfaces can be coated by the LB technique - hydrophobic (water repelling)

and hydrophilic (water attracting). Deposition onto a hydrophobic surface always takes

place on the down path while for a hydrophilic surface, deposition always takes place on

the up path.

The surface of a substrate being coated with a LB film changes from hydrophobic to

hydrophilic with every layer. Thus when a hydrophobic surface is immersed and coated

with a monolayer, it will be completely hydrophilic while it is immersed. Then, when it is

withdrawn from the subphase and a second layer is deposited, the surface will be

hydrophobic again.

Figure 3.9: Deposition of the first 2 layers. The immersed surface of the substrate

becomes hydrophilic after deposition of the first layer and becomes hydrophobic again

after deposition of the 2nd layer. Note that the second layer does not start deposition until

the meniscus is 'flipped' over.

Hence, for deposition to occur, the prime criterion is that the curvature of the meniscus at

the liquid/substrate boundary and the direction of motion coincide.

It is easy to determine what type of surface you have - just immerse the substrate in the

subphase. If the meniscus curves down, the surface is hydrophobic - if it curves up then

the surface is hydrophilic.

A hydrophobic substrate will result in deposition on the first down pass, the hydrophobic

hydrocarbon tails adhering to the hydrophobic substrate. With a hydrophilic substrate, no

ordered deposition will occur until the first up-stroke because the hydrophobic tails are

repelled by the hydrophilic substrate as it is immersed in the water.

Therefore an even (2,4,6...) number of layers is always deposited onto a hydrophobic

surface and an odd (1,3,5...) number onto hydrophilic surfaces.

The surface quality and composition of the substrate controls the nature of subsequently

deposited layers. The deposition of the first layer is also of importance as this predetermines

the manner in which subsequent layers are laid down. For consistent film quality, the

monolayer must be kept at a constant pressure, even when transfer is actually occurring.
Langmuir-Blodgettry

32

Hydrophilic substrates

Deposition onto hydrophilic substrates proceeds in a similar manner, with the exception

that the first layer is not deposited until the first 'up' path, Figure 3.10.

This is a result of the water meniscus onto a hydrophilic surface being curved upwards and,

when the substrate is moved down into the subphase, the curvature of the meniscus and

the direction of motion do not coincide. Hence, molecules cannot be deposited.

Thus the first monolayer on a hydrophilic substrate is deposited on the first 'up' pass

through the monolayer, as the direction of the meniscus now favours the end of the

molecule attracted to the surface.

It is strongly recommended that, when using hydrophilic substrates, the substrate be

immersed in the subphase prior to spreading and compressing the monolayer. This will

any prevent molecular film aggregates depositing onto the hydrophilic surface during the

first down stroke - these have been observed to cause horizontal streaking - bands across

the substrate perpendicular to the dipping direction.

Figure 3.10: Wetting of a hydrophilic substrate and deposition of the first

monolayer onto a hydrophilic substrate

Transfer ratios & meniscus radius

An good measure of how well a film has transferred to a substrate is the transfer ratio, given

by:

TR = area of monolayer removed from subphase at constant pressure

area of substrate immersed in water

Such a ratio is usually measured for each substrate pass through the air-water interface.

However, it must be remembered that the measured transfer ratio is an aggregate value

over the entire immersed surface and often requires interpretation. It is possible, for

instance, to measure a TR of 0% if 100% deposition takes place on one face of the

substrate and 100% of film is peeled off the other face.

It is important to realise that for every layer after the first layer, deposition does not take place

until the meniscus has 'flipped over', as shown in Figure 3.9 (previous page). This is a result

of the fact that the monolayer has to turn itself 'on its head' to continue the deposition of the

second layer. This gives rise to the 'head to head and tail to tail' structure of the amphiphilic
Langmuir-Blodgettry

33

Langmuir-Blodgett film, Figure 3.11. If the meniscus radius is not taken into account, then

inaccuracies will occur in the transfer ratio calculation. For instance, if the meniscus radius

is 5mm and the length of the sample is 50mm, then a 10% error would occur if the area

calculation were made from the point of substrate movement reversal, rather than from the

point of the meniscus flipping over.

In the Nima software, the meniscus radius is taken into account in the transfer ratio

calculation and can be set in the Dipper Menu.

Deposition Types

Amphiphiles deposit in this 'head to head and tail to tail' (HHTT) fashion - on both the up and

down stroke of the substrate - resulting in 'Y-type' structures.

Y

Figure 3.11: Y-type deposition onto a hydrophilic substrate

There are two other deposition modes which have been observed - 'X' and 'Z' type. At

present there is no complete understanding of why some molecules dip in the X and Z

manner, as this goes contrary to the whole LB principle (although such phenomena can

be very desirable in the field of non-linear optics where such non-centrosymmetric

structures are required). Films which apparently deposit as X or Z-type have often been

found to be Y-type after X-ray analysis. Poor transfer ratios are not necessarily indicative

of X or Z-type deposition but rather may simply imply patchy film adhesion, the film

consisting of small areas of Y-type deposition interspersed with undefined regions where

film may have been deposited and peeled off repeatedly. If 'X' or 'Z' type deposition is

observed, great care must be used when interpreting results and all cleaning methods

thoroughly reviewed.

 X Z

 Figure 3.12: 'X' and 'Z' type deposition
Langmuir-Blodgettry

34

Alternate layer deposition

The Nima alternate layer dipper allows the fabrication of multilayers containing 2 different

film forming materials.

By depositing and compressing two different materials in compartments 'A' and 'B' and

programming the dipper to pass down through monolayer A and up through monolayer B,

a non-centro-symmetric multilayer structure with the sequence ABABAB... can be produced.

Such structures have been used to produce pyroelectric Langmuir-Blodgett films, using

a long chain fatty acid as one material and a long chain amine as the other. Alternate

monolayers have also been observed to exhibit second-order non-linear optical effects.

Organic materials with large non-linear susceptibility coefficients which would normally

stack in a centro-symmetric manner are incorporated into a non-centro-symmetric

structure by the use of passive spacer monolayers such as fatty acids. To achieve the

required structure, the spacer monolayers are alternated with the active monolayers.

Figure 3.13: Alternate layer deposition
Langmuir-Blodgettry

35

Laboratory Requirements

Safety

Before using any solvent, familiarise yourself with the relevant safety procedures. Solvents

must never be inhaled or touch your bare skin. Work in a fume cupboard when making up

the solutions. Similarly, when making up solutions of metal ions such as cadmium and

potassium, always wash your hands afterwards, to prevent accidental ingestion.

Check with your laboratory supervisor on safety regulations and correct disposal

after use. Remember you are responsible for your own and other's safety.

General

When studying mono-molecular layers which contain at most a few milligrams of material,

even a small amount of contaminant can cause serious errors.

The trough, substrates and other apparatus must be kept as clean as possible. Extraneous

greasy material concentrates at the air-water interface; the source of such contamination

is usually grease from fingers or hair, this being transferred to the subphase by supposedly

clean instruments which have been handled. Clean forceps or tweezers should always be

used when handling substrates and for all operations it is advisable to wear disposable,

powder-free polythene gloves. It is recommended that the syringes used for deposition are

cleaned in between use and that they are rinsed at least once with the solution to be

deposited. Clean laboratory coats should be worn, not only for protection but also to

prevent the deposition of fibres from clothing onto the subphase.

When not in use the trough should be kept empty of water, covered and out of direct

sunlight. If being moved, the pressure sensors and dipper mechanism should be removed

from the trough to prevent accidental damage.

Lab environment

As with all aspects of Langmuir-Blodgettry, cleanliness is of the utmost importance.

Airborne particulates tend to accumulate at the water surface. To combat this, the water

surface should be cleaned thoroughly using the water suction pump and the trough should

be situated in as clean and dust-free environment as possible. When the trough is filled with

water the clear covers should be in place over the water surface and dipper mechanism

to protect the trough from the vagaries of the atmosphere.

Particular care must be taken with Langmuir-Blodgett films which are to be used for optical

or electronic applications, as dust particles can become incorporated into the films and

give rise to light scattering, pinhole defects and ambiguous results. Some troughs are

housed in clean room facilities which maintain a positive pressure and filter the air intake

through micropore filters. Surfaces are usually sealed and temperature and humidity are

controlled. An isolated laminar flow cabinet can also be used as an alternative. However,

such facilities are not essential if care is taken to operate the trough with regular cleaning

in a relatively dust-free laboratory atmosphere. The room in which the trough is situated

must be adequately ventilated to ensure that solvent fumes do not accumulate in

dangerous quantities.
Langmuir-Blodgettry

36

Excesses of temperature and humidity should be avoided, as should locating the trough

in the close proximity to vibrating machinery as such vibrations can cause premature

collapse of monolayer films. An anti-vibration table is useful if problems with obvious ripples

from external disturbances can be discerned. However, small vibrations do not appear to

cause serious problems and have been known to enhance film quality by annealing the

surface monolayer.

Trough

The Nima troughs are fabricated from Polytetrafluoroethylene (PTFE), a material which will

not contaminate the subphase. The polymer is essentially chemically inert, will not leach

plasticiser and is the most hydrophobic polymer known. PTFE can be subjected to very

rigorous cleaning procedures.

In general, the best way to keep the trough clean is to use it regularly and to avoid

unnecessary collapse of mono-molecular films. The tissues used to wipe the trough must

be surfactant-free, such as the 'Kimwipes' supplied (type 7105 or 'Kimtechs' type 75512,

available from Kimberly-Clarke). Some other tissues, even if they are 'guaranteed 100%

clean' may contain amphiphiles that prevent formation of pure monolayers, as 'cleanliness'

has a different meaning in different environments. For example the Kimwipes would be

disastrous in a semi-conductor fabrication area, as they produce clouds of dust particles,

but they are ideal for LB films as they are surfactant free.

Before and after use, the trough should be thoroughly cleaned. Any leftover monolayer

material should be removed from the subphase surface before the trough is emptied. To

ensure that all parts of the trough are cleaned, the dipping head and barriers should be

removed first. The barriers can be removed and wiped with a solvent laden tissue.

Organic solvents are the best available degreasing agents and will work over a wide range

of solubilities. Hence, a thorough wipe with a chloroform laden Kimwipe cleans the PTFE

well. Care must be taken to wipe off all excess chloroform, as pools of CHCl3

 will evaporate

leaving surfactants behind. Take great care not inhale any chloroform and always

wear gloves. If your lab supervisor forbids the use of chloroform, use isopropyl

alcohol instead.

If you are using materials that are insoluble in chloroform, use other suitable solvents to

wipe the trough down. If there are no suitable solvents, then use a detergent (see next

page).

Never use strong acids or alkalis and never experiment for the sake of it - it is very

easy to contaminate a trough and very difficult to eliminate unknown contaminants.
Langmuir-Blodgettry

37

Detergent Cleaning

 If there is no suitable solvent available for your trough contamination, it will have to be

washed with detergent. Commercially-available detergents include Decon-90, Helmanex

and Tikopur. These need to be used sparingly and the trough has be thoroughly rinsed (10

times with warm water) afterwards.

Detergent cleaning is now recognized as the all-round best method for thorough trough

cleaning. It should be performed about once a week. Use only a detergent suitable for the

purpose. Do not use normal detergents. Nima recommends the use of the pure generic

detergent potassium octanoate.

The contaminants which are troublesome in an LB trough are the surface-active ones.

Most of them are only weakly surface active, not introduced on purpose, and present in only

microgram quantities. Normal detergents will remove contaminants, but being highly

surface active themselves, leave a trough useless for further work. The only detergents

which you can use to clean your trough are ones which can be completely rinsed away

afterwards.

The logic of detergent cleaning is that the micelles of the detergent present the contaminant

with an enormous area of surface with a hydrophilic-hydrophobic gradient. By the law of

mass action, any material with the slightest of tendencies to accumulate at the air-water

interface will also incorporate itself into the micelles, which are then removed. The

technique was first used over a decade ago in Switzerland and Germany and has proved

itself, in spite of its mildness, to be extremely effective against both low-molecular-weight

and polymeric contamination.

Commercially-available detergents capable of being rinsed completely from fluoropolymers

include Decon-90, Helmanex and Tikopur. Look for the mention “completely rinsable” on

the label. These detergents can be removed to deep sub-monolayer level with ten rinses

of water at 40°C .

We recommend the generic detergent potassium octanoate as being generally superior,

mainly because it can be removed to deep submonolayer level by four rinses of water at

room temperature. It is obtained most easily by neutralizing the free acid with potassium

hydroxide. Octanoic acid is liquid at room temperature and is obtainable from many

chemical suppliers at approximately $US 20 per liter. First dissolve the potassium

hydroxide carefully in the minimum quantity of hot water, then add the stoichiometric

quantity of acid and stir until thoroughly mixed. Finally, check the pH with a pH electrode or

indicator paper, and if this is greater than 9 reduce it by adding a little more acid. The pH

is not critical for cleaning the trough, but at a pH of 10 or greater the solution will attack the

aluminium parts.

Remember to use the detergent sparingly and to rinse the trough thoroughly

afterwards!
Langmuir-Blodgettry

38

Chemicals

The materials which are deposited should be as pure as possible. Very small quantities of

impurities, especially surface active contaminants, can radically alter the molecular area

calculated from the isotherm and affect the material’s film forming characteristics.

To avoid contamination, and so improve results, impurities should be restricted during

synthesis. Care should not only be taken with the purification of materials but also with the

solvents and starting materials used, which should be of the highest available purity.

Once prepared, materials should be carefully stored and handled to prevent hydrolysis,

oxidation or contamination. Ideally, samples should be kept in a clean, dry, sealable, inert

vessel, preferably made of glass as there can be contamination from plasticisers in plastic

materials. Non-contaminating methods should be devised for the weighing out and transfer

of materials. Care should be taken to ensure that spatulas are kept clean.

Water

The Nima troughs have different capacities between 40 and 1500ml. This small volume

reduces the demand of deionised water and the risk of contamination, and also eases

cleaning.

The water used in a Langmuir-Blodgett trough should be as pure as is possible. The

quantity of particles, ions, and surface-active materials should be kept to such a low level

that they will not significantly affect the surface properties of monolayer systems.

In the past, water was usually distilled a number of times, sometimes with an oxidative

stage using an alkaline permanganate solution. Nowadays, reverse osmosis, ultrafiltration, and ion exchange resins are commonly used. A charcoal filter is usually located

on the feed line to the still. For consistent results any of the commercially available

deionisers are recommended. (Manufacturers include Barnsted, Elga, Millipore & others.)

The water should be freshly made and discarded every time the molecule type is changed.

Storage should be in clean vessels which are inert; some plastic containers leach dialkyl

phtalate esters which are used as plasticisers in their manufacture.

The prime criterion for establishing the cleanliness of the subphase is a high value of

surface tension. The surface tension of pure, clean water at 293K is 72.8mN/m. A minimal

change should be observed on aging and insignificant changes should be seen on

compression of the surface. It should be noted that the value will decrease with an increase

in temperature. The surface tension of the water in the trough can easily be measured by

lifting the Wilhelmy plate out of the water after having zeroed the pressure sensor. To

protect the water surface from air borne particulates the transparent lid should be kept on

the trough whenever possible when the trough is full of water.
Langmuir-Blodgettry

39

Solvents

Some Langmuir-film forming materials, such as vegetable oils, spread spontaneously on

a water surface. However most materials must be deposited onto the subphase in a

solution of a volatile solvent. The solvent which is chosen is critical and will affect the film

forming properties of the solute. A number of factors must be considered:

Firstly a solvent must be chosen which has sufficient solvent power to dissolve a

substantial quantity of the material under investigation. A solution with much less than 0.1

mg/ml will require too large a volume to be deposited to form a reasonable area of

compressed monolayer.

The solvent must also be chemically inert with respect to the material under study and

should also be relatively pure. Small amounts of contaminants such as grease can be

removed by distilling the solvent in grease-free glass systems. The cleanliness of the

solvent can be verified by spreading the solvent neat on the subphase, allowing it to

evaporate and then running an isotherm of the remaining surface material. Any increase

in surface pressure observed will be due to surface active contaminants in the solvent

which need to be removed. This technique is known as 'blank spreading' and, as many

commercially available solvents contain film forming contaminants, is highly recommended.

The volatility of the solvent must also be considered - it must be such that the evaporation

time is short but not so short that the concentration of the solution cannot be determined

due to evaporation. The boiling point of the solvent should lie in the range 40-80o

C. Important

factors to consider are the hazards associated with organic solvents - when allowing the

solvent to evaporate the solvent vapours should not be inhaled.

Organic solvents which are very soluble in water should be avoided as they will tend to carry

amphiphilic material into the subphase and precipitate it out. If a water-miscible solvent is

used, a high concentration of the material to be studied must be used to counteract the

effect of the material being pulled into the subphase. Care must be taken when interpreting

molecular areas obtained with such solvents as they will usually be incorrect. If a material

is not soluble in non-polar, volatile spreading solvents, an acetone-hydrocarbon (e.g.

hexane) mixture can be used to increase the amount of solute which will dissolve without

introducing serious solubility problems.

The table below shows the properties of a few solvents which have been used for

depositing Langmuir film forming materials. Solubilities in water are at 25°C, in g/Kg.

Solvent Melting point Boiling point solubility in

 (°C) (°C) water

n hexane -94 69 0.01

Cyclohexane 6.5 81 0.07

Choloform -64 61 8

Diethyl ether -116 35 75

Acetone -93.4 56 ∞

Dichloromethane -98 40 1.3

Table 3.3: Properties of solvents
Langmuir-Blodgettry

40

Substrate

The quality of the first layer deposited onto the substrate determines the deposition

behaviour of subsequent layers - defects in the first layer can be passed through to and

observed in layers subsequently deposited. Because the substrate determines the nature

of the first layer deposited, it must be thoroughly cleaned.

At all stages of the cleaning procedure care should be taken not to touch the slides with

fingers as the skin grease contains a mixture of surface active biological lipids.

Substrates can be divided into two groups according to their surface - they can be either

hydrophilic or hydrophobic. A standard glass microscope slide or a polished silicon wafer

make excellent substrates for deposition of LB films. Both are normally hydrophilic, the

silicon always has an oxide layer after exposure to air.

Many substrate cleaning procedures are commonly employed. One which has been found

to work well is to wash the slides with detergent, then rinse in an ultrasonic bath in deionised

water and then with isopropanol. Instead of using an ultrasonic bath, Soxhlet cleaning can

be used. Argon plasma cleaning (100W incident power at 13.6MHz and 0.1 Torr for 2

minutes) just prior to deposition also gives an excellent surface.

Strong oxidising agents such as acidic peroxide or chromic acid are not recommended

- as they are a health hazard and require the substrate to be thoroughly rinsed post

immersion because they can introduce as many contaminants as they remove.

A hydrophilic glass surface can be prepared by soaking the clean slides overnight in a 2g/

dm3

 solution of sodium hydroxide. They are then rinsed with ultra-pure water and dried in

a stream of dry nitrogen.

A hydrophobic surface is obtained by standing the clean glass slide overnight in a fume

cupboard, in a beaker containing a few drops of hexamethyl disilazane and covered with

a watchglass. On evaporation, the hexamethyl disilazane condenses onto the clean slide,

rendering it hydrophobic. Evaporation of gold will also make a slide hydrophobic. Katherine

Blodgett also used the technique of rubbing in molten ferric stearate (see the 'Tutorial'

section) which works well and is not as hazardous to health as the other techniques.

Glass is not the only material which can be use as a substrate. Many other surfaces such

as fused silica and silicon wafers can also be used. When infra-red studies of LangmuirBlodgett multilayers are to be undertaken, calcium fluoride plates which are effectively

insoluble in water and transparent to infra-red radiation can be used. All these materials can

be prepared in the same manner as glass.

After preparation the slides should be stored in a clean dust-free sealable box.
Langmuir-Blodgettry

41

Bibliography - edited & updated by Jens-Peter Krause

This section contains a review of a number of books, journals, original papers and

conference proceedings as a guide to locating detailed information on the history,

practicalities and current research in the field of Langmuir-Blodgettry.

The list of references from books and journals to be found in this section has been revised

and expanded to some more general topics of surface science which might be important

for those working in the field of Lanmuir-Blodgettry. A section with original papers on

polymer and protein monolayer is now available. However, the list is by no means

comprehensive as the amount of available literature in the field of Langmuir-Blodgettry is

constantly and rapidly growing. It is included merely as a starting point for those beginning

to work with Langmuir and Langmuir-Blodgett films.

Books and review papers

General aspects on surface and colloid sciences

 1. Adamson, A.W., Physical chemistry of surfaces, 5th ed., Wiley-VCH, New York, 1990.

 2. Desjonqueres M.-C., Spanjaard D., .Concepts in surface physics, 2nd ed., Springer Verlag,

Berlin, 1996.

 3. Evans D.F., Wennerström H., The colloidal domain: where physics, chemistry, biology and

technology meet, 2nd ed., Wiley-VCH, New York, 1999.

 4. Lyklema, J. Fundamentals of interface and colloid science, Academic Press, London, Vol. I.

Fundamentals: 1991, Vol. II. Solid-liquid interfaces:1995, Vol. III. Liquid-fluid interfaces: 2000

 5. Fleer G.J., Cohen Stuart M.A., Scheutjens J.M.H.M., Cosgrove T., Vincent B.,

Polymers at interfaces, Chapman&Hall, London, 1993.

 6. Jones R. A., Richards R.W., Polymers at surfaces and interfaces, Cambridge University

Press, Cambridge, 1999.

 7. Brezesinski G., Mögel H.-J., Grenzflächen und Kolloide: physikalisch-chemische Grundlagen,

Spektrum Akad. Verlag, Heidelberg, 1993.

 8. Theoretical approaches to surface structure, surface forces and monolayer rheology.

Israelachvili J. N., Intermolecular and surface forces, 2nd ed., Academic Press, London, 1992.

 9. Lawley K. P., Molecular surface interactions, Wiley-VCH, New York, 1989.

10. Dynarowicz P., Recent developments in the modelling of the monolayers structure at the

water/air interface, Adv. Colloid Interface Sci., 45, 215, 1993.

11. Dynarowicz-Latka P., Dhanabalan A., Jr. Oliveira O.N., Modern physicochemical research on

Langmuir monolayers, Adv. Colloid Interface Sci. 91, 221, 2001.

12. Davis, H. T. Statistical mechanics of phases, interfaces and thin films, Wiley-VCH, New York,

1996.

13. Ghezzi F., Earnshaw J.C., Finnis M., McCluney M., Pattern formation in colloidal monolayers

at the air-water interface, J. Colloid Interface Sci. 238, 433, 2001.
Langmuir-Blodgettry

42

Physical methods for characterising surfaces

1. O'Connor D. J., Sexton B. A., Smart R. St. C. (Eds.), Surface analysis methods in materials

science, Springer Verlag, Berlin, 1992.

2. Czanderna A. W. (Ed.), Methods of surface analysis, (Methods and Phenomena: their

Applications in Science and Technology ; 1), Elsevier, Amsterdam, 1989.

3. Walls J. M. (Ed.), Methods of surface analysis: techniques and applications, Cambridge Univ.

Press, Cambridge, 1989.

4. Woodruff D. P. ,Delchar T. A., Modern techniques of surface science, 2nd ed., Cambridge

Univ. Press, Cambridge, 1994.

Theory and practical considerations concerning the formation of LB

films

1. Pethica B.A., Experimental criteria for monolayer studies in relation to the formation of

Langmuir-Blodgett multilayers, Thin Solid Films 152, 3, 1987.

2. Mingins J. and Owens N.F., Experimental considerations in insoluble spread monolayers, Thin

Solid Films 152, 9., 1987.

3. Peterson I.R., Veale G. and Montgomery C.M., Preparation of oleophilic surfaces for LB

deposition, J. Colloid Interface Sci. 109, 527, 1986.

4. Roberts G.G., Molecular electronics using Langmuir-Blodgett films; Adv. Chemistry Series

218, 225, 1988

5. Peterson I.R., Langmuir-Blodgett films, Journal of Physics. D. Applied Physics, 23, 379, 1990.

6. Swalen J.D., Molecular Films, Annual Reviews in Materials Science 21, 373, 1991.

7. Kurihara K., Langmuir-Blodgett films in advanced molecular engineering, Colloids Surfaces

A: 123-124, 425, 1997.

8. Ulman A., An introduction to ultrathin organic films from Langmuir-Blodgett to self-assembly,

Academic Press, San Diego, 1991

9. Vandevyver M., Barraud A., Methods of characterization for Langmuir-Blodgett films, J. Mol.

Electronics 4, 207, 1988.

10. Welzel P.B., Weis I., Schwarz G., Sources of error in Langmuir trough measurements

Wilhelmy plate effects and surface curvature, Colloids Surfaces A: 144, 229, 1998.

11. Mingotaud A.-F., Mingotaud Ch., Patterson L.K., Handbook of monolayers, Academic Press,

San Diego, 1993.
Langmuir-Blodgettry

43

Thin films in science and industry

1. Dowson D., Priest M., Taylor C. M., Ehret P., Childs T. H. C., Dalmaz G., Berthier Y., Flamand

L., Georges J.-M. and Lubrecht A.A. (Eds.), Lubrication at the frontier: the role of the interface

and surface layers in the thin film and boundary regime, Elsevier, Amsterdam, 1999.

2. Kahovec J.(Ed.), Surface and interfacial phenomena in macromolecular systems WileyVCH, Weinheim, 1999.

3. Venables J. A., Introduction to surface and thin film processes, Cambridge University Press,

Cambridge:,2000.

4. Volkov, A.G. (Ed.), Liquid interfaces in chemical, biological, and pharmaceutical applications,

Marcel Dekker, New York, 2001.

Biological Systems

1. Baszkin A., Norde W. (Eds.), Physical chemistry of biological interfaces, Marcel Dekker, New

York, 2000.

2. Magdassi S. (Ed.), Surface activity of proteins: chemical and physicochemical modifications,

Marcel Dekker, New York, 1996..

3. MacRitchie F., Chemistry at interfaces, Academic Press, San Diego, 1990.

4. Lvov Y. and Möhwald G. (Eds.), Protein Architecture: Interfacing Molecular Assemblies and

Immobilization Biotechnology, Marcel Dekker, New York, 2000.

5. Malmsten, M. (Ed.), Biopolymers at interfaces, Marcel Dekker, New York 1998.

6. Bos M.A., van Vliet T., Interfacial rheological properties of adsorbed protein layers and

surfactants: a review, Adv. Colloid Interface Sci. 91, 437, 2001.

Original papers

Polymer monolayers and LB-films

1. Allain M., Benattar J.J., Rieutord F., Robin P., Surface study of langmuir-blodgett films by

electron microscopy and X-ray reflectivity, Europhys. Lett. 3, 309, 1987.

2. Cárdenas-Valera A.E., Bailey A.I.,, The interfacial rheological behaviour of monolayers of

PEO/PMMA graft copolymers spread at the air/water and oil/water interfaces, Colloids

Surfaces A: 79, 115, 1993.

3. Fleer G.J., Scheutjens J.M.H.M., Interaction between adsorbed layers of macromolecules, J.

Colloid Interface Sci., 111, 504, 1986.

4. Maruyama T., Fuller G., Frank C., Robertson C., Flow-induced molecular orientation of a

Langmuir film, Science 274, 233, 1996.

5. Oliveira O.N., Bonardi C., The surface potential of langmuir monolayers revisited, Langmuir

13, 5920, 1997.

Protein monolayers and LB-films

1. Picard G., Dynamic thin laminar flow method for making protein monolayers, Langmuir 13,

264, 1997.

2. Balla A., Razafindralambo H., Blecker Ch., Paqout M., Interfacial properties of gluten monolayers

spread on various chloride salt solutions: Effects of electrolytes, salt concentrations, and

temperature, J. Agric. Food Chem. 46, 3535, 1998.
Langmuir-Blodgettry

44

3. Douillard R., Aguié-Beghin V., Surface pressure of protein adsorption layers: application to

molecular mass and molecular area measurements, Colloids Surfaces A: 149, 285, 1999.

4. Dubreuil N., Alexandre S., Lair D., Sommer F., Duc T.-M., Valleton J.-M., Scanning force

microscopy of mixed fatty acid/protein Langmuir-Blodgett films transferred on muscovite, a

model of their organization, Colloids Surfaces B: 11, 95, 1998.

5. Innocente N., Blecker C., Deroanne C., Paquot M., Langmuir film balance study of the surface

properties of a soluble fraction of milk fat-globule membrane, J. Agric. Food Chem. 45, 1559,

1997.

6. Löscher F., Ruckstuhl T., Jaworek T., Wegner G., Seeger S., Immobilization of biomolecules

on Langmuir-Blodgett films of regenerative cellulose derivatives, Langmuir 14, 2786, 1998.

7. Murray B.S., Dynamics of proteins at air-water and oil-water interfaces using novel Langmuir

trough methods, Progr. Colloid Polym. Sci. 103, 41, 1997.

8. Krause J.-P., Schwenke K.D., Behaviour of a protein isolate from rapeseed (Brassica Napus)

and its main protein components - globulin and albumin - at air/solution and solid interfaces,

and in emulsions, Colloids Surfaces B: 21, 29, 2001.

Special LB editions & conference proceedings

Barlow W.A. (Ed.), 1980, Vol 68.

LB1: Roberts G.G. (Ed.), First international conference on LB, Durham, England;

Thin Solid Films, 1983, Vol 99.

LB2: Gaines G.L. Jr. (Ed.), Second international conference on LB, Schenectady, USA; 1985,

Thin Solid Films, Vols 132, 133 and 134.

Stroeve, P. and Franses (Eds.), Molecular engineering workshop, Davis, USA; 1987,

Thin Solid Films, Vol 152.

LB3: Möbius D., Barraud A. and Roberts G.G. (Eds.), Third international conference on LB,

Göttingen, Germany; 1988, Thin Solid Films, Vols 159 and 160.

LB4: Fukuda K. and Sugi M. (Ed.), 4th international conference on LB, Tsukuba, Japan; 1989,

Thin Solid Films, Vol 180.

LB5: Barraud A. (Ed), Fifth international conference on LB, Paris, France; 1991,

Thin Solid Films, Vols 210 and 211

LB6 Thin Solid Films vols 243 (1994) ....................... (1993: TroisRivieres)

LB7 Thin Solid Films vols 284/285 (1996) ............... (1995: Ancona)

ECOF6 J. Supramolecular Science vol 4 (1997) (1996 Sheffield)

LB8 Thin Solid Films vols 327-329 (1998) ............... (1997: Asilomar)

ECOF7 Materials Science and Engineering C vols C8-C9 (1999 Potsdam)

LB9 Colloids and Surfaces A * (in press 2001) ....... (2000: Potsdam)

ECOF 8 Lecce (2002)

LB10 Beijing 2003 (in press)

ECOF9 Valladolid (2004)
Langmuir-Blodgettry

45

Journals

Thin Solid Films - this journal publishes original research on thin-film synthesis, characterization,

and applications. Contents and abstracts are available online: http://www.elsevier.nl/inca/publications/

store/5/0/4/1/0/6/

Journal of Colloid and Interface Science - this journal publishes original research on fundamental

principles of colloid and interface science and application of these principles. Contents and abstracts

are available online: http://www.academic.press.com/jcis

Langmuir - this journal publishes original research on surface and colloid chemistry. Coverage

includes such topics as micelles, visicles, emulsions, gels, surfacants, colloids, crystal growth,

nucleation, liquid crystals, imaging spectroscopy, electro-chemistry, biological colloids & interfaces,

biopolymers, nanostructures, multicomponent systems, and materials. Contents and abstracts are

available online: http://pubs.acs.org/journals/langd5

Colloids and Surfaces A: Physicochemical and Engineering Aspects - this journal publishes

original research and review articles on the science of the fundamentals, engineering fundamentals,

and applications of colloidal and interfacial phenomena and processes.Contents and abstracts are

available online: http://www.elsevier.nl/locate/colsurfa.

Colloids Surfaces B: Biointerfaces - this journal publishes original research and review articles

on fundamental and applied research on colloid and interfacial phenomena in relation to systems of

biological origin. Contents and abstracts are available online: http://www.elsevier.nl/locate/colsurfb.

Other journals

In addition to the above journals, the Journal of the Agricultural and Food Chemisty, Advances in

Colloid and Interface Science, the Journal of Molecular Electronics (up to Dec 1991 Vol 7, No 3.),

Advanced Materials for Optics and Electronics (from Jan 1992) provide good background reading.

The Journal de Chemie Physique published a special issue relating to Langmuir-Blodgett films in

1988. This issue includes Langmuir -Blodgett film work in the fields of biology, non-linear optics and

molecular electronics amongst others.

Barraud A. (Director), 'The NATO Advanced Workshop on Langmuir-Blodgett films and related

structures', Journal de Chemie Physique 85, 1988.
Langmuir-Blodgettry

46

Historical

Benjamin Franklin's initial communication to the Royal Society of London in 1774 contains details

of his original monolayer experiments on the pond at Clapham and also describes the interesting

effects of monolayer films on supposedly dead flies!

Franklin B., Of the stilling of waves by means of oil, Philosophical Transactions of the Royal

Society 64, 445, 1774.

Wilhelmy first described the measurement of surface tension in:

L.Wilhelmy, Angew. Physik 119, 177, 1863.

Agnes Pockels' original letter to Lord Rayleigh was published in:

A.Pockels, Letter to the Editor, Nature 43, 437, 1891.

Langmuir's landmark paper was published in 1917:

'The Constitution and Fundamental Properties of Solids and Liquids'

 Irving Langmuir, J.Am.Chem.Soc 39, 1848, (1917).

A paper by Katherine Blodgett describes one of the first suggested practical and commercial

applications of Langmuir-Blodgett films, as anti-reflection coatings on glass (part of the

work from which is repeated in the 'Tutorial' section):

Blodgett K.B., Use of interference to extinguish reflection of light from glass; Physical

Review 55, 391, 1939.

A brief history of Langmuir-Blodgett films, containing many of the references to the original

work on Langmuir and Langmuir-Blodgett films can be found in the preface to Volume 99

of Thin Solid Films:

Gaines G.L. Jr., 'On the history of Langmuir-Blodgett films'; Thin Solid Films 99, pg ix, 198.

Interest was rekindled in Langmuir films by the elegant work of Kuhn & Möbius:

Kuhn,H. and Möbius,D., Systems of mono-molecular layers - assembling and physicochemical behaviour, Angewandte Chemie (Int'l Ed) 10, 629, 1971.

Throughout this manual, data is quoted from:

Tennent R.M (Ed.), Science Data Book, Oliver & Boyd, 1971.
TUTORIAL
Tutorial

48

Introduction

This tutorial is designed to teach you the basic commands of the Nima trough and the

essentials of Langmuir-Blodgettry by leading you through a series of examples. It is a good

idea to read the 'Langmuir-Blodgettry' chapter first. You will repeat work done originally by

Katharine Blodgett in 1938* and prepare a multilayer film of Cadmium arachidate which is

then skeletonised to display a series of interference colours. This technique was originally

used to prepare anti-reflection coatings for glass.

Follow the instructions given. The buttons to click are shown in the left hand column;

additional comments are shown in italics. If you get stuck, stop whatever is going on by

clicking on the STOP button. To see a short description, right hand click on an item. For

more detailed explanations of the functioning of the trough software, please refer to the

'Software' chapter of this manual.

Requirements

Before you begin, you will need:

. 1mg of arachidic acid to make into a monolayer

. About 100g of ferric stearate to make the substrate surface hydrophobic

. Glass slide (substrate) to deposit monolayers onto

. About 100mg of Cadmium chloride for the subphase

. Ethanol for skeletonising the LB film

. Chloroform for trough cleaning

. Ultra-pure deionised (DI) water for trough

. Kimwipe / Kimtech tissues (or similar surfactant free) for cleaning

Preparations

Solutions

You will deposit a mixture of arachidic acid and Cadmium arachidate onto your substrate.

The Cadmium arachidate is formed on the water when the arachidic acid is spread on a

Cadmium chloride subphase. The Cadmium ions bind to the (partly dissociated) arachidic

acid and form Cadmium arachidate. By chance, dissolved atmospheric carbon dioxide

gives rise to the required pH 5.5, at which about 50% of the fatty acid is dissociated. The

water used must be free from other contaminating metal ions.

* 'Use of interference to extinguish reflection of light from glass',

 K.B.Blodgett, Physical Review, volume 55, (1939) page 391.
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- Make up a solution of approximately 1mg/ml of arachidic acid in chloroform and note

down the exact concentration. A total volume of about 100μl of solution will be

needed, but it is recommended to make up 10ml for future use. Store this below

10°C to reduce evaporation. Use the glass bottle provided in the Monolayer Kit to

minimise contamination from surfactants.

- Make up a solution of about 10-4 M Cadmium chloride in deionised water in a clean

container - this can be made of borosilicate glass, as long as solution is not stored

in it overnight, as detectable quantities of mineral ions leach out of the glass after

about half an hour. Make up about 2 litres of solution so that the subphase can be

topped up from time to time. The molecular weight of Cadmium chloride (anhydrous)

is 183.3; 36mg will be needed to make up 2 litres of 10-4 M solution.

Trough

- Wipe over the trough surface, with a 'Kimwipe' (type 7105 or or Kimtech type 75512,

supplied with the trough - or type EX-L No 34256 in the USA) soaked in chloroform

wearing the polythene gloves (also supplied). Make sure you are working in a well

ventilated room and ask your laboratory supervisor about safety regulations and

toxic solvents.

- Wipe the other trough components (dipper mechanism, pressure sensor, acrylic

trough cover) with a Kimwipe soaked in alcohol to remove loose dust. Do NOT use

chloroform on plastics other than PTFE as it will dissolve them!

- Clean the syringe by sucking up and expelling a little solvent several times.

- Fill the trough with deionised water until it just brims over the top - about 2mm over

the rim of the trough.

Substrate

- Acquire a standard microscope glass slide.

- Clean the substrate initially by rubbing it vigorously with a cloth soaked in alcohol.

Rinse thoroughly with deionised water. Ultrasonic in deionised water.

- If a plasma etch unit is available expose the substrate to 2 minutes of Argon ion

plasma at about 0.1 Torr and 100W incident radiated power.

- Make the substrate surface hydrophobic by rubbing it with ferric stearate on a piece

of Kimwipe tissue until no visible trace of the chemical remains. Ensure that no

part of the surface is missed.

A small quantity of ferric stearate is provided in your Monolayer Kit - more can

be synthesised or acquired commercially - please see the Technical Appendix.
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50

Producing an isotherm

It is now time to run the software and turn the interface ON. Therefore, click on your trough

icon, or, if it is not your desktop, click on the Nima516.exe file in c:\Nima (or whichever

directory you have installed it into). The main screen of the software is illustrated below.

In order to produce an isotherm, the water subphase must first be clean. The molecules

have to be spread on the surface and the solvent is left to evaporate. They are then slowly

compressed and the resulting pressure-area isotherm displayed.

The following sections will lead you through this operation one step at a time. The software

commands are printed in bold down the left margin with an adjacent explanatory

paragraph.

Additional comments are printed in italics.

In case of emergencies, click on the STOP button or hit the Esc key!

Figure 3.1: Main Screen of Nima Software
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Subphase cleaning

Ensure that the trough has been cleaned and filled with pure DI water.

Attach a paper Wilhelmy plate to the pressure sensor with the 'S' shaped hooks

provided and lower it into the water, so that about 2mm of it is immersed.

O Open the barrier(s) by clicking on the 'O' button.

Z Zero the pressure sensor.

Check that a reading of about 70 mN/m is obtained when the Wilhelmy plate is lifted

out of the water. If it does not, check the change in surface pressure for the 100mg

calibration weight by adding the calibration pan and then the weight. This change

should be 46.7mN/m. If it does not, recalibrate the pressure sensor as described

in the 'Software' section. The reading may drift initially, this is because the paper

plate is still absorbing water.

Barrier Set the barrier speed to 100 cm2/min by entering '100' in the 'Barrier Speed' box.

Speed To save time, you can set it to larger values, say 250, or even 500 for larger troughs.

100

Barrier Select 'Isotherm' from the 'Barrier' dialog box to start the isotherm. The barrier will

Isotherm move at the given speed until either the space bar is pressed or the barrier travels

outside its 'allowed' position (this is set in the Monolayer Menu and prevents

accidental damage to the trough.)

Examine the isotherm produced. It should be almost flat except for a slight 'tail' at

small areas due to the presence of dirt on the water surface.

Turn on the aspirator pump and touch the pipette to the water surface to suck off

any floating material, such as dust and amphiphilic contaminants. If the pipette end

is positioned correctly, there will be a louder sucking noise as air is sucked up with

water. Move the head of the pipette around to collect all the material and clean for

about 30 seconds.

O Open the barrier(s) by clicking on 'O' .

Z Re-zero the pressure sensor.

This is necessary because the water level in the trough has now changed. Make

sure that the lower edge of the Wilhelmy plate is still touching the water surface.

Keep cleaning until the isotherm produced is completely flat. The change in surface

pressure on compression should be less than 0.5mN/m. For a quicker check on

cleanliness, simply close the barriers ('C') and watch the change in pressure

sensor reading.

If you have sucked off too much water, please refill the trough until the level is again

about 2mm above the rim of the PTFE. When refilling, always add the water behind

the barrier i.e. the other side from where the monolayer is to be spread. This will

ensure that any surfactants present in the water will rise to the surface in the unused

compart ment behind the barrier. Of course, the water should be clean, but you can't

be too careful in LB films!
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52

Monolayer Menu

Figure 3.2: Monolayer Menu

To ensure that the area axis (in A2 / molecule) is calibrated correctly, go to the Monolayer Menu

and enter the correct molecular weight, concentration and volume of your solution.

Barrier Select 'Monolayer Menu' from the 'Barrier' menu.

Mono Menu

MW Enter the molecular weight of your molecule.

Conc Enter the concentration of your solution.

Vol Enter the volume of your solution in μl.

The other variables can be left for now. They are fully explained in the

'Software' chapter.
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Spreading

Now that the conditions have been set, the molecules can be spread on the subphase.

- Rinse out the syringe by drawing up and expelling a little chloroform. Make sure that

you always expell excess chloroform into a designated 'chlorinated waste' bottle.

- Draw up 50μl of the arachidic acid solution into the syringe and deposit it drop by

drop onto the water from just above the surface.

There is no need to deposit on different places on the trough when using arachidic

acid - the molecules will spread out to cover the complete area available. With other

molecules, particularly those with large dipole moments, this may not be the case

and 'islands' can form.

- Leave the solvent to evaporate.

All the solvent must evaporate but Katherine Blodgett found that if it was given too

long, the deposited films were 'streaky'. The evaporation time does appear to have

some bearing on film quality - you may wish to devote your PhD to investigate this

unexplained observation! However, just wait until the pressure reading returns to

zero - that will do for now.

Compression

Barrier Set the barrier speed to 100 cm2/min by entering '100' in the 'Barrier Speed' box.

Speed This is a reasonable compromise between a good isotherm (i.e. undistorted by

100 dynamic effects) and time available. At this speed, the barrier takes about 5 minutes

to close for a standard size trough.

CLEAR Clear the memory and the graphs by clicking on 'CLEAR'. You will be asked to

confirm this clear, as once cleared, all the data is irretrievably lost!

This will clear the data stored when the subphase was being cleaned. It is good

practice to clear the data before taking an isotherm, to prevent the graph getting

cluttered up with unnecessary data from previous runs.

Graph Switch the area axis between absolute area (cm2) and area per-molecule (Å2) as

cm2<>A2 calculated from the data in the Monolayer Menu.

Barrier Start the compression by clicking on 'Isotherm' in the 'Barrier' menu.

Isotherm Watch the isotherm on the screen and compare it to Figure 3.3. It should show three

distinct regions - an initial flat region, corresponding to a 'gaseous' phase of the

molecules on the water, a gently rising liquid-expanded or 'liquid' phase and a

steeply rising, liquid-condensed or 'solid' phase.

STOP When the isotherm begins to collapse, click on STOP to stop the compression.

At this point, the steep upward rise of pressure will begin level off.

Physically, the monolayer can no longer support the force applied and the

molecules move over each other to form aggregates on the surface, or are

dissolved in the subphase. With highly coloured films, this is clearly visible.

Remember that a surface pressure of 50mN/m is equivalent to 200 atmospheres

pressure over the molecular length of 25Å !
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54

Barrier Change the barrier speed to -100 cm2/min. A negative speed corresponds to the

Speed barrier moving backwards.

-100

Barrier Expand the floating monolayer by clicking on 'Isotherm' again.

Isotherm The isotherm should have exactly the same shape as when the film was compressed

but will be shifted towards smaller areas (i.e. to the left of the graph) by an amount

equal to the area of collapsed film.

Barrier Clean off the surface. Use the suction pump with its teflon tip to clean the molecules

Pi control off the surface as previously described. 

Target Select 'Target Pressure' from the 'Variables' menu and adjust it to 25mN/m.

Pressure This will compress the barriers, keeping the pressure at 25mN/m as you remove

25 the molecules, ensuring that the monolayer is not collapsed. Cleaning shouldn’t

take more than a minute or so.

Well done - you have now taken your first pressure-area isotherm!

Figure 3.3: Typical isotherm of fatty acid

Saving and further analysis

File Save the isotherm, by clicking on 'Save' in the 'File' dialog box.

Save Enter a name (such as 'test1') and the data will be saved as a *.txt file (i.e. test1.txt)

along with its operating conditions which are saved as a *.con file. The data file

can be loaded into Excel (or other spreadsheets) and the data will be displayed in

columns, each headed by a label to signify what it is. It can be charted and pasted

into your reports or papers.

Graph Click on 'P&A-Time' on the 'Graph' menu to view two separate plots of pressureP&A-T time and area-time.
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Graph Click on 'Graph' and 'Change Axes A2<>cm2' to convert the area scale to Å2. The

A2<>cm2 conversion will use the data supplied by you in the Monolayer Menu.

Right click over the graph and de-select 'AutoScale X' - then use the pointer to mark

off an area of the isotherm such as the 'liquid condensed phase'. The selected area

will expand to fill the screen and the Area/molecule / Å2 will be displayed in the box

below the graph, Figure 3.4.

If you disable auto-scaling for both X and Y axes, then you can set the scaling of the

axes by writing over the minimum and maximum abscissa values. Or you can

select different areas of the graph with the 'X-Y scaling palette. Feel free to

experiment.

Cursor Click on the cursor control palette (bottom left of the graph).

control There is a white square which activates the cursor controls when clicked. You can

move the cursor anywhere on the screen and read off the X and Y co-ordinates.

Click on the little padlock icon and select 'lock on plot' to move the cursor along the

actual curve.

a) x-axis now in Å2/molecule b) expanded to read off

 - liquid condensed phase selected 'zero pressure molecular area'

Figure 3.4: Isotherm of fatty acid

Further investigations

Repeat the isotherm procedure two or three times, varying the amount of material used,

solvent evaporation time, compression speed etc. to try to improve upon your first isotherm

and get a feel for the technique.

You might like also to change the concentration of Cadmium chloride in the subphase. This

will alter the pressure at which the liquid-expanded changes to the liquid-condensed phase.

The greater the concentration of CdCl2 , the lower the pressure of the phase change. The

liquid expanded phase disappearing altogether at about 10-3M concentration.
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Depositing a multilayer LB film

Now that you have an isotherm of the material to be studied, it can be deposited with some

confidence. Arachidic acid / Cadmium arachidate forms good, optical quality Langmuir

films on the subphase and deposits easily at surface pressures of about 25mN/m (in the

liquid condensed phase).

For simplicity, the following sequence is for a conventional LB dipper:

- Prepare a subphase of about 10-4M CdCl2

 .

- Clean and check the water surface as before.

- Have the glass slide coated with ferric stearate ready.

U Move the dipper to its topmost position by clicking on 'U'.

Attach the glass slide to the clamp.

D Move the dipper down by clicking on 'D' until the lower edge of the slide is about

5mm above the water surface.

STOP Stop it by clicking on STOP

h Read off the dipper position reading - let's call this h mm.

Select 'Dip Top End' from the 'Variables' Menu (top right of screen) and enter h into

the box (top left of the screen).

The 'Variables' Menu contains a number of useful parameters that may be

changed in the main screen of the program before or during a run. Hence, it is not

necessary to stop the process and go to the Monolayer or Dipper Menu in the

middle of a run.

h+d Now decide how far you want to dip the substrate into the water - i.e. what length

of substrate you want to coat. Typically this may be 30mm for a standard dipper or

maybe10mm if you are using a mini dipper with a small slide.

If we call the immersion depth 'd', then enter 'h+d' into the 'Dip Bottom End' variable

box (again selected from the 'Variables' Menu).

Z Zero the pressure sensor by clicking on 'Z' .

Then spread the solution of molecules onto the subphase.

To avoid having to replenish the molecules on the subphase too often, it is useful

to putthe maximum possible number on each time. Spread the solution with the

syringe until the surface pressure displayed in the data margin to the left of the axes

just begins to rise. At this point, the surface is completely covered with molecules.

Target Select 'Target Pressure' from the 'Variables' Menu (top left of screen)

pressure Enter a target pressure of about 25mN/m in the 'Target Pressure' box.

25 The exact value will depend on the phase change between 'liquid expanded' and

'liquid condensed' phases. It is best to deposit just 2 or 3 mN/m above the phase

change, as the molecules will be tightly compressed, but small excursions in

pressure (caused by the action of the slide entering the water) will not give rise to

local collapse of the monolayer.
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Barrier Enable the pressure control by clicking on 'Pressure Control' in the 'Barrier'

Pi Control menu.

The barrier(s) will compress the monolayer to the target pressure according

to the relationship:

 barrier speed = δπ * (feedback gain)

where δπ is the difference between current pressure and target pressure and

feedback gain is a numerical value (also set in the 'Variables' Menu). Larger values

of gain increase the barrier speed and so reach the target pressure more quickly,

but may then overshoot. Too low a value means that target pressure is never

achieved. A critical value of 1.0 is recommended.

The barriers will slow down as the target pressure is reached. Wait until surface

pressure is within 1mN/m of its target.

Note that the maximum barrier speed under pressure control is limited by the value

given in the 'barrier speed' display box (at the centre left of the screen). This is useful

if you are using delicate molecules and don't want to compress them too fast.

Dipper Begin dipping.

Progrm Dip The dipper will move down through the water surface.

As the slide moves through the interface, the monolayer is transferred onto

the solid surface. The barriers will move inwards to maintain the target

pressure automatically.

Graph Select the 'Dipper-Area' graph from the 'Graph' dialog box.

Dip-Area A series of straight 'V' shaped lines will appear as the dipper is moved up and down

and the barrier moves in to compensate for removal of molecules from the water

surface.

Layers Adjust the 'Layer Required' box to 40, so that 40 layers (equivalent to a thickness

Req 40 of λ /2 are built up, as explained later under 'skeletonisation').

The dipper will continue to deposit layers on to the substrate until either all 40 layers have

been deposited or the area of monolayer is reduced to below the following:

 area < [ ('close-to' area + 2 * substrate area)]

You are then requested by the software to reopen the barriers and respread the molecules.

Spread the material, as before. If you have not collapsed the film, there is no need to clean

the surface. Then re-compress and restart the dipper.

Continue until all 40 layers have been deposited.

Points to consider during dipping

*Do not stop the dipping whilst a layer is in the process of being transferred - it will

reduce the quality of your LB film.

*Do not clear the memory whilst dipping as this will clear the record of the number of

layers. You can, of course, keep a separate tally, if you want to work with a clear screen

after every re-spreading.
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58

* Watch the meniscus of the subphase as deposition is occurring.

If the meniscus is uniform, the deposition likely to be is good; if it 'dances around',

the deposition is unlikely to be as good. This 'dancing' indicates imperfections on

the substrate surface.

* The dipping speed, target pressure, feedback gain and 'end' positions can all be

varied as the dipping is taking place, without stopping the program. The 'Variables'

Menu allows selection of the appropriate variable and the preset values are altered

using the 'up' and 'down' arrows next to the box.

Completion

Once all the layers have been deposited, the substrate can be removed and the data can

be saved and plotted out. A typical deposition isotherm is displayed below.

* Save the data ('File' dialog box and 'Save' with a user defined file name).

* Remove the substrate with its deposited Langmuir-Blodgett film. Be careful not to

touchthe surface.

* Store it in a cool, dry place.

* Clean the remaining monolayer off the subphase surface.

* Empty the trough.

* Clean the syringe.

* It is recommended to always empty the trough overnight. If you prefer to leave the

trough full of water to prevent dirt falling on the PTFE surface, please remove dipper

and pressure sensor(s) to prevent corrosion of electronic components.

Figure 3.5: Typical deposition pressure-area isotherm
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Further analysis

Graph Click on 'P&A-Time' on the 'Graph' menu to view two separate plots of pressureP&A-T time and area-time. This will show the 'steps' in area as the barrier is moved in to

compensate for the removal of monolayer onto the substrate, Figure 3.6 below.

Figure 3.6: Typical deposition (pressure & area) - time isotherm

Graph Click on 'Dipper-Area' on the 'Graph' menu to view the plot of area against dipper

Dipper position. If there was good deposition, this plot will be in a series or regular 'V' s. As

-Area the dipper is oscillated up and down regularly, the barrier moves in linearly to

compensate for the removal of monolayer, Figure 3.7 below.

Note that it takes a few mm worth of dipper travel at the start of every layer before

deposition recommences - this is caused by the reversal of the meniscus radius,

as explained previously in the 'Langmuir-Blodgettry' chapter of this manual.

Figure 3.7: Typical dipper position-area isotherm
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Transfer Click on 'Transfer ratio' on the 'Graph' menu to view the plot of transfer ratios (TRs).

Ratio Ideally, TR is always 100%, but rarely in practice! There are many reasons for this,

as explained in 'interpreting transfer ratios' overleaf.

Figure 3.8: Typical transfer ratio plot

Skeletonisation to produce an anti-reflective (AR)

coating

It is now time to 'skeletonise' the multilayer you have produced. Before skeletonisation, the

films refractive index is very similar to that of the substrate (about 1.50) making it almost

invisible. The film is skeletonised by dissolving away the arachidic acid, leaving only the

Cadmium arachidate. This reduces its refractive index and brings the film’s optical

thickness close to one quarter wavelength for blue light.

- Pour enough ethanol to cover the LB film into a beaker.

- Immerse the film in the ethanol for a number of seconds.

The exact time required varies between ten seconds and two minutes. When the

film is first immersed, it loses arachidic acid fairly quickly but the rate then slows.

Remove the film slowly to allow time for the surface to shed the alcohol.

- The colour becomes more vivid and shifts towards the red with longer soaking as

the film’s refractive decreases towards the extinction value, n = √nglass.

- Hold the dried slide in the reflection of a lamp. You will be amazed by the quality of

the AR coating!
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Deposition onto hydrophilic substrates

The previous section dealt with deposition onto an hydrophobic substrate - that is to say

the first layer was deposited on the 'down' path. Deposition can just as easily take place

onto an hydrophilic substrate. The only difference is that the first layer is on the first 'up' path.

Hence the substrate must first be lowered into the water.

It is recommended that the substrate is lowered into the water prior to spreading and

compressing the monolayer, as otherwise some irregular deposition may take place onto

the hydrophilic substrate as it is immersed through a compressed monolayer.

The software can be used to start the deposition below the water surface. This is done by

'fooling' the computer that the first layer has already been completed. Hence the number

of layers set will actually be (n + 1) where n is the actual number of layers to be deposited.

Thus even if only one layer is required, 2 layers must still be entered in the 'Layers Required'

box.

The following sequence should be followed for deposition onto an hydrophilic substrate:

Dipper Enter the Dipper Menu and set the dipping routine but remember that your first

Menu layer will be the second layer as far as the computer is concerned. Thus enter

(n+1) layers. Set the 'top' and 'bottom' positions for the substrate, as explained

in 'Setting the Dipping Conditions' earlier in this chapter.

C Close the barrier(s) and clean the surface.

O Open the barrier(s).

Then attach the substrate to the dipper and, still with the barrier(s) open, move

the dipper so that the substrate is immersed:

D Move the dipper down.

STOP Click on STOP to stop the dipper at the correct position.

Then spread and compress the monolayer in the usual way:

Barrier Start the compression under pressure control.

Pi Control

Dipper When the film has reached its target pressure, click on 'Program Dip' to start

Progrm Dip the dipper.

As the dipper is already below the surface of the water, it will assume that the

first layer has already been deposited and move to deposit the 2nd layer.

The dipping will then proceed in the usual way until the number of layers required have been

deposited.
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Interpreting transfer ratios

Ideally, each time the substrate crosses the air-water interface, the surface of the substrate

is covered completely in monolayer, giving a transfer ratio of 100%.

In practice, though, this is rarely achieved. There are several reasons for this:

1. Collapsing monolayer - unstable film

The monolayer may be unable to support the dipping ('target') pressure and may be

collapsing away happily to itself. If such a film is deposited, the transfer ratio will be more

than 100% as the collapse rate will be superimposed on the dip rate.

The term 'collapse' is used generally to describe a film that is unable to support a given

surface pressure. The reasons for the collapse may be that:

a) the molecules are too hydrophilic and are being dissolved in the subphase. This can

happen at pressure as low as 8mN/m. Molecules can dissolve in the subphase ahead of

the barrier and reappear behind the barrier, i.e. they can exist in an equilibrium state

between surfaces at different pressures and the subphase.

b) the molecules may simply be leaking under the barrier. Check that all PTFE surfaces

are clean and that the barriers are resting firmly on the trough.

c) the film pressure is simply too high for the material and it is forming 'mountain ranges'

on a molecular scale on the surface. Remember that, for a 25Å long molecule, a pressure

of 25mN/m is equivalent to 100 atmospheres in the plane of the film! These molecular

mountain ranges can be observed by the naked eye just ahead of the barrier.

To check for film stability, hold the film at the target pressure while under pressure control

and watch the area v time plot. If the area value is decreasing linearly, this indicates that

the film is unable to support the pressure or that the film is leaking under the barrier. If the

area value is decreasing asymptotically to a constant area value, this indicates that the

molecules are rearranging themselves (annealing) into a more energetically favourable

and stable order.

A stable film can be formed from arachidic acid on a 10-4M CdCl2

 subphase. The area

decrease for such a film has been measured at less than 0.01cm2/min at 30mN/m.*

2. Film depositing and peeling off again

Suppose that the film is stable and that the first layer has been successfully deposited onto

the substrate with a transfer ratio of 100%. If the substrate now crosses the air-water

interface for the 2nd layer and the film peels off because it has not sufficient attraction to

the first layer, a transfer ratio of -100% will be recorded! If this sequence is continued for

10 layers, the inexperienced observer may believe that 5 layers had been deposited when

actually none have.

Watch the meniscus between subphase and substrate carefully. If it is curved the opposite

direction to the direction, then the previously deposited monolayer is peeling off.

* 'A modular, multi-functional LB trough', F.Grunfeld, Rev of Scientific Instruments, vol 64 (2) Feb 1993
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3. Partial peeling off of monolayer

Extending on case (2) above where 100% of the film peeled off again, consider the case

where only 50% peels off. If on 50% of the area, the film peels off and on the other 50% there

is deposition, then to the outside observer it appears that the transfer ratio is 0%. This can

easily happen on substrates that have not been cleaned properly.

Great care must be taken to avoid such a situation. A good indicator is always the meniscus

of the subphase on the moving substrate. If the curvature of the meniscus is uniform and

follows the direction of dipping, this indicates good deposition. If the meniscus 'dances'

around i.e. its radius of curvature is constantly changing, the substrate and/or subphase

is probably very dirty.

Ensure that you do not report X or Z type deposition when you really have poor quality Y type!

Hence it always imperative that other evidence for the existence of the multilayer is

obtained. Note that some of the original X and Z type deposition observed was found to have

Y type structure, when checked for head group separation with X-ray diffraction (Gaines

- 'Insoluble monolayers'). The editor of this manual suspects that the some X and Z type

layers that have been reported in the literature are just plain bad Y type LB films!

4. Dip start and dip end positions

The setting of the dip start and dip end positions will also greatly influence the transfer ratio.

For conventional dippers (simple up and down motion), remember that even for ideal

deposition, the 2nd layer will not start being deposited until the meniscus has flipped over,

which can be up to 5mm worth of actual substrate movement. The 'meniscus radius'

setting in the Dipper Menu helps overcome this. If a radius of 5mm is chosen, then the TR

calculation is performed from 5mm after the 'dip start' position, thus eliminating this effect.

Figure 3.9: Substrate has to travel considerable distance, before meniscus flips over

and 2nd layer starts being deposited. This can introduce large errors into transfer ratio

calculation.

Hence, the transfer ratios will differ considerably if the dip start position is set above the

subphase surface or below it.

Similarly for alternate layer dipper mechanisms, the transfer ratios are critically affected by

the dip start and dip end of both sides A and B.
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6. Meniscus radius and transfer ratio calculations

The transfer ratio calculation is improved by including the 'meniscus radius'. This takes into

account the fact that the meniscus 'flips' over at the end of every layer before deposition

re-commences on the next layer.

A sketch of the important dimension is given below:

top end - typically 25mm

meniscus radius - typically 4mm

from end

immersed area deposited on

length

bottom end - typically 50mm

In the example above, the slide is set to oscillate between an immersion depth of 25 and

50mm, as measured from the top of the dipper.

If the meniscus radius is 4mm, then the actual area immersed is:

area immersed = 2 [width + thickness] x [ (50 - 25) - 4] mm2

= 2 [width + thickness] x [ (50 - 25) - 4] / 100 cm2

Hence for a standard 76 x 26 x 1 mm microscope glass slide, the (area immersed and

deposited on) is actually 11.34cm2, rather than 13.5cm2 - a reduction of over 10%.

Please note that the top end can be above the water surface. This is perfectly ok, as long

as only the immersed area of slide is given for 'substrate area' in the Dipper Menu.
65

 Tutorial

Good realistic plots for area/pressure v time and transfer ratios are given below:

a

b

 c d

 e

Figure 3.10: Area/pressure time plot and TRs of deposition with alternate layer dipper

a) compressing before dipping

b) first layer being deposited - a little too soon, as film is still annealing and

 TR looks too high

c) 2nd layer better

d) film still collapsing a little in between dips

e) film now appears stable

Transfer

 ratio, %

Time, minutes

Area, cm2 Pressure,

mN/m

No of layers
Tutorial

66

Operating a trough in 'coupled barrier' mode - option 'MC'

If you have 2 independent barriers, then the software may be used to move the monolayer

across the length of the trough using the 'coupled' mode. This is useful for scanning the

monolayer under a microscope or for the multi-compartment troughs, where the monolayer

can be moved over different subphases.

Typically, in this mode of operation, there are 2 compartments in the one trough which are

filled with differing subphases. The monolayer is spread and compressed in one

compartment and then transported into the second compartment, where some chemistry

can be done over a different subphase. If your trough also has a dipper, then the monolayer

can also be transferred to a solid substrate in the second compartment.

There are some extra function switches in this mode:

1. An 'A/B' switch to select between barrier 'A' and 'B'.

2. A 'SYM' button to switch between symmetrical compression (about the centre line

of the whole trough) and asymmetrical compression (within one compartment).

3. A 'coupled' mode in the 'Barrier' dialog box for moving the barriers in unison.

A typical run consists of:

1. Open barrier A by clicking on 'O' and making sure that the 'A/B' switch is set to 'A'.

2. Close barrier 'B' by switching over to 'B' and clicking on 'C'.

3 Click on 'STOP' when barrier B is over the compartment divider.

4. Spread the molecules in compartment A.

5. With the 'SYM' button set to 'NON SYM', select 'Isotherm' from the 'Barrier' dialog

box.

An isotherm will be made over compartment A.

6. 'STOP' before the monolayer collapses.

7. Select 'Coupled' from the Barrier dialog box.

The monolayer will be moved towards compartment B. Its speed is set as usual in

the 'Barr Speed' box and barrier B will be controlled so as to always keep the area

between the 2 barriers constant.

8. Click on STOP when the monolayer is over compartment B.

9. If you wish to dip, set up the dipper in the usual way and select 'Pi Control' from the

Barrier dialog box, making sure that the SYM button is still set to 'NON SYM'.

10. When the film is at target pressure, select Program Dip form the Dipper dialog box.

The monolayer will be deposited in the usual way.
67

 Tutorial

Operating a trough in alternate layer mode

Figure 3.11: Main Screen in Alternate Layer (AB) Mode

The main screen in the AB mode differs form the standard screen because there are, in

effect, 2 separate troughs. So there are 2 independent barriers and 2 separate pressure

sensors.

All the data and functions are available for both compartments. They are accessed by

clicking on the 'AB' switch (top left of the screen) to the compartment required. For

instance, if you want to zero pressure sensor A, switch to 'A' and click on 'Z'. If you want

to open barrier B, switch to 'B' and click on 'O'.

All the displayed values such as area, pressure and dipper position are grouped in 2

columns 'A' and 'B' and are displayed continuously.
Tutorial

68

A typical deposition run (alternate layer dipper)

1. Follow the instructions for the pressure-area isotherm first for compartment A.

Repeat for compartment B.

This will ensure that the trough is clean and ready to go.

2. Prepare a glass slide by rubbing it with the ferric stearate supplied.

This will make it hydrophobic.

3. Move the dipper to the top by clicking on 'F' (forwards) or 'B' (backwards).

The dipper will rotate continuously.

4. Click on 'STOP' when the dipper substrate carrier is at the top position (and the

carrier is horizontal).

5. Lay the slide on the substrate carrier, making sure it is held in place by the clips.

6. Go the Dipper Menu ('Dipper' in the menu bar).

7. Select the appropriate values for your deposition from the Dipper Menu below.

Typical values are displayed below.

8. Please see overleaf for an explanation of the dipper 'top end' & 'bottom end'

positions. These have to set to match the size of the substrate being coated.

9. Enter the sequence of 'A' and 'B' layers in the box and don't forget to enter the

corresponding 'Layers Required'.

The number counter allows you to check exactly which layer is which type.

Figure 3.12: Dipper Menu for Alternating Layer Dipper
69

 Tutorial

Alternate layer dipper 'top end' & 'bottom end' positions

substrate water

level

Top A Top B

Bottom A Bottom B

Figure 3.13: Alternate Layer Dipper 'Top' and 'Bottom' End Positions

The substrate is carried around the dipper - the arrows indicate the 'forwards' direction. The

critical positions are illustrated above.

'Top A' is just as the substrate is about to be immersed in side A - typically 75mm.

'Bottom A' is when it is just fully immersed in side A - typically 100mm.

'Bottom B' is just before it emerges on side B - typically 190mm

'Top B' is immediately after it has completely emerged on side B - typically 210mm

Typical values are for a standard 76 x 25mm glass microscope slide. Other size substrates

have different values, which are set before deposition commences.
Tutorial

70

10. Make sure that the dipper position is between '0mm' and 'top end A'.

If it is not, the layer counter may not start from zero.

11. Spread the monolayer in both compartments (as in the section 'taking an isotherm').

12. Select 'Pi Control' from the Barrier dialog box.

Both barriers will compress their respective monolayers.

13. Use the AB switch to view the compression in either compartment.

14. When both films are compressed to their target pressure, click on 'Program Dip'

(Dipper in the menu bar) to start the dipper in its pre-programmed sequence.

The dipper will move at the 'Inter Dip Speed' between immersions and at 'Dip Speed

A' when depositing monolayer A and 'Dip Speed B' at side B.

The dipper will continue until the layers have been deposited or there are not enough

molecules remaining on the surface in which case a warning message 'barrier too closed!'

will appear. If this happens, please open the barrier(s) and spread more molecules.

View the data also in the area & pressure - time axes and the dipper - area axes.

Save in the usual way.

Further Work

Try depositing on both glass slides and silicon wafers. LB films on silicon wafers give vivid

interference colours. Try different numbers of layers and making 'stepped' layers. Vary the

thickness and skeletonisation time of films on glass to improve the anti-reflection coating.

Conclusion

The tutorial is now over. You have followed in the footsteps of one of the founders of the

Langmuir-Blodgett technique and replicated one of the first attempts at a commercial

application for a Langmuir-Blodgett film. You should have gained at least a basic knowledge

of Langmuir-Blodgettry and some of the features and commands available on the Nima

trough.

Many more details are given in the 'Software', 'Langmuir-Blodgettry' and 'Experiments'

chapters. We hope that you found this taste of the science of Langmuir-Blodgettry

enjoyable. If you have any problems or questions do not hesitate to get in touch with us at

the address below- we will be glad to help:

info@nima.co.uk

or visit our help site at:

www.nima.co.uk/help
	
	




	

	
	

	




	
	


	
	
	



	


	
		

	



	 
		
	


	

	
		
	





	
			


	
	
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SOFTWARE
Software

94

STOP BARRIER 

OPEN/CLOSE

MENU BAR

PRESSURE 

AREA 

INDICATOR

DIPPER 

INDICATOR

DELETE 

ALL DATA

DIPPER 

UP/DOWN

AXES SCALING CONTROL

VARIABLES 

MENU

Introduction

This chapter is designed for reference only and need not be read completely. For a step

by step guide of actual trough operation, please see the Tutorial chapter.

The operating software is organised as a graphics screen (the 'active screen') and a series

of menus. All the 'action' takes place in the graphics screen in one of several axes such

as 'pressure-area' as illustrated below.

Figure 5.1: Main Screen of Nima Software

The active screen

The active graphics screen appears at power up. The main part of the screen is taken up

by a graph. Then there is a menu bar running along the top with a series of 'drop down'

menus. In the top right hand corner the most recently loaded file name is displayed - initially

this is the 'standard' file. To the left of the graph is a margin displaying various sensor

readings, such as surface pressure and area.

Other functions are available from the 'menu bar' running along the top of the screen. These

menus are placed into logical groups such as 'File', 'Barrier', 'Dipper' etc.

Just below the menu bar are a series of large buttons. These are for often used functions

such as opening the barrier ('O') or moving the dipper up ('U') or down ('D'). The same

functions can be accessed via the menu bar. For instance to close the barrier, either click

on the 'C' button or click on the 'Barrier' menu and then 'Close'.

The Variables Menu in the box at the top left of the screen allows access to less often

changed variables, such as 'Target Pressure' or 'Cycle Max Area'. They can be changed

by clicking on their 'up' or 'down' arrows or typing in the new value.
 95

 Software

Menu Bar

'File' Menu

Load Loads previously stored data

Save Saves current data - give file name

Print Gives print instructions - ALT-PrtScreen

Setup Setup Menu - explained in detail later in this chapter

Signature Nima address - in case you want to get in touch

Exit Exit the program

'Graph' Menu

Pressure-Area Selects pressure-area isotherm axes

P&A - Time Selects pressure and area - time axes

Dipper-Area Selects dipper-area axes (for deposition troughs only)

Transfer Ratio Selects Transfer Ratio axes (for deposition troughs only)

Change Axes Changes area axes between cm2 and Å2/molecule

cm2<>Å2 (to values entered in Monolayer Menu)

Clear graphs Clears all data in memory and from graphs

'Barrier' Menu

Monolayer Menu Selects Monolayer Menu - explained in detail later in this chapter

Open Moves barrier(s) to 'open to' area

Close Moves barrier(s) to 'close to' area

Isotherm Starts barrier moving and data logging

Pressure control Compresses barrier(s) until target pressure is reached

Isocycle Starts barrier moving under iso-cycle (see Monolayer Menu)

Coupled Moves barriers in coupled mode (2B & MC option only)

Area control Compresses barrier(s) until target area is reached

Stop Stops barrier motor(s)

'Dipper' Menu

Dipper Menu Selects Dipper Menu - explained in detail later in this chapter

Dipper Up Moves dipper to top (or clockwise for alternate dipper)

Dipper Down Moves dipper to bottom (or anti-clockwise for alternate dipper)

Program Dip Starts pre-programmed dipping sequence

Dipper Stop Stops dipper motor

'Calibrations' Menu

Interface Test Runs interface diagnostic routine

Cali pressure sensor Self

Cali potential sensor evident

Cali temp sensor - all

Teach barrier pos explained

Cali barr speed later in

Cali dip speed this chapter!

Show cali data Displays current area and speed calibrations.

'Variables' Menu This menu changes the current variable available in the little box

at the top left of the screen. It is useful to be able to change these

'secondary' variables without having to stop and enter a full menu.
Software

96

Buttons

Keyboard

C Closes barrier(s) to the 'close-to' area defined in the Monolayer Menu. Home

O Opens barrier(s) to the 'open-to' area defined in the Monolayer Menu. End

STOP Stops all motors Esc

D Down (conventional dipper only) moves dipper to bottom. PgDn

U Up (conventional dipper only) moves dipper to top. PgUp

F Forwards (alternate dipper only) moves dipper anti-clockwise.

B Backwards (alternate dipper only) moves dipper clockwise.

CLEAR Clears all data in memory and from graphs.

A-B A-B switch (alternate and multi-compartment troughs only)

Switches action between A & B (when zeroing, opening/closing or displaying data).

SYM Symmetry button (2 independent barriers AND single compartment only)

Switches between symmetrical and non-symmetrical compression of the barriers.

Z Zeroes pressure sensor-if AB switch present, then Z will zero either sensor A or B.

Permanently Displayed Variables

Barrier Speed

Sets the barrier speed for isotherms. The range is approximately 5cm2/min to 1000cm2/

min depending on trough type and gearbox ratio. It can be negative for expansions,

annealing, hysteresis loops etc. The barrier speed can also be set in the Monolayer

Menu.

Cycles Required Done

Sets the number of isotherm cycles required (only when iso-cycle function is selected

in the Monolayer Menu). 'Done' displays the number already completed.

Dipper Speed up / down

Sets the dipper speed for depositions. The range is approximately 0.5mm/min to 50mm/

min depending on dipper type (and gearbox ratio). The dipper speed can also be set in

the Dipper Menu. For conventional dippers, separate 'up' and 'down' speeds are

available.

Layers Required Done

Sets the number of layers required when dipping - can also be set in the Dipper Menu.

'Done' displays the number already deposited.
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Variables Menu - selectedly displayed variables

These can be displayed by selecting the variables from the menu in the rectangular box at the top

left of the screen.

Target pressure

This is the pressure at which the monolayer will be compressed to when running

pressure control - can also be selected in the Monolayer and Dipper Menus.

Feedback gain

This is the speed of response under pressure control. The greater this number, the

faster the barrier will compress the monolayer, according to the following algorithm:

Barrier speed = [feedback gain] * [target pressure - actual pressure ]

If feedback gain is too large, the pressure control will oscillate out of control.

Recommended values are 1.0 - 2.0 depending on aspect ratio of trough (the longer

and slimmer the trough, the higher the gain).

Dip Top End

Sets the position to which the substrate will be raised (can also be set in the Dipper

Menu)

Dip Bottom End

Sets the position to which the substrate will be immersed (can also be set in the

Dipper Menu).

Cycle Max Area

The area to which the barrier(s) will be opened under iso-cycle control (but also

check the 'Cycle Min Pressure' in the Monolayer Menu).

Cycle Min Area

The area to which the barrier(s) will be closed under iso-cycle control (but also

check the 'Cycle Max Pressure' in the Monolayer Menu).

Target Area

This is the area at which the monolayer will be compressed to when running area

control - area control is selected in 'Barrier' in menu bar.
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Setup Menu

This is accessed from the 'File' Menu and allows you to configure the software to your

particular trough.

Figure 5.2: Setup Menu

Interface Enabled - YES if you want to control the trough.

NO if you want to run the software without the trough connected

(for example when evaluating data in your office)

Please note that on some PCs the software will run very slowly

when the interface is disabled AND there is no data loaded - so

the software always loads a default isotherm on power up when

the interface is disabled.

Serial port Selects the serial port on your PC that you have plugged the Nima

interface into.

Samples / Reading This allows the PC to average the data returning from the

interface, so the more samples the smoother the data, but the

slower the response. Recommended value: 5

Logging interval, secsThis is the time before the data is 'logged' into memory. If you are

doing a long run, say overnight, you may wish only to log once

every minute, so that your data files won't become too big.

Typical value: 0.5 seconds.

Set Up Langmuir Trough Enters trough set up menu (described overleaf).

Enable Calibrations Gives access to 'Calibrations' in the menu bar.
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Trough Setup Menu

Figure 5.3: Trough Setup Menu

Dipper YES if you have a dipper;

NO if you don't

Alternate layer dipper YES if you have an alternate layer dipper;

NO if you don't

2nd independent barr. YES if you have 2 barriers with 2 motors

NO if you have only 1 motor (even if it's driving 2 barriers)

2nd pressure sensor YES if you have an alternate layer trough

NO for single compartment troughs

Temperature sensor YES or NO

pH sensor YES or NO

Surface potential sensor YES or NO

Advanced Setup Menu For custom made troughs

To access this menu, download version 5.17 from our

website.
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Monolayer Menu

The Monolayer Menu is accessed from the 'Barrier' Drop Down Menu and is shown below:

Figure 5.4: Monolayer Menu

This screen allows you to set up your isotherm run to your preferences. For instance you

may wish to take a single isotherm or perform isotherm cycles. You may wish to move the

barrier at a constant speed or in a 'quasi-static' way.

Entry of the molecular weight of your molecules, the concentration and volume of solution

spread enables the software to calculate the absolute number of molecules spread on the

subphase and hence convert the area on the subphase surface to an area per molecule.

TROUGH FUNCTIONS

Barrier speed The barrier speed of a compression run when taking an isotherm. Note that

if you want to do a 'relaxation' (opening the barrier), please enter a negative

barrier speed in the active screen.

Target pressure This the pressure that the film will be compressed to under pressure control

('Pressure control' under 'Barrier' in the menu bar).

'Close to' area This is the minimum 'allowed' area. If your barrier moves to a smaller area,

it will be stopped and 'barrier too closed' will be displayed. Click on 'O' to reopen it.

'Open to' area This is the maximum 'allowed' area. If your barrier moves to a larger area,

it will be stopped and 'barrier too open' will be displayed. Simply click on 'C'

to close it.
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MOLECULAR PROPERTIES

Molecular weight Enter the MW of your molecule

Concentration, mg/ml Enter the concentration of your solution in mg/ml

Volume, μl Enter the volume of your solution in μl

Once the above data are entered, the computer can automatically label the area axes in

area per molecule. MW and concentration are only printed to 3 decimal places but can be

entered and are stored with greater accuracy.

A-B Switch between compartments (alternate layer mode only)

For an alternate layer trough (Model 622), molecule and solvent data are accessed one

compartment at a time. Use this switch to select between compartment 'A' and compartment

'B'.

ISOTHERM CYCLE FUNCTIONS

It is possible to program the barrier(s) to compress and expand the monolayer repeatedly

to observe hysteresis effects in isotherms. A choice of boundary conditions and the

number of cycles is given. It is possible to cycle between defined areas (in cm2) or defined

pressures (mN/m) or between one of each. Please note that the barrier will reverse on the

first parameter it meets, so you have to set both area and pressure parameters for the

successful functioning of this routine.

Example 1

If you wish to cycle through the 'liquid-expanded' phase of a fatty acid, set the limit for

minimum pressure to '0' mN/m and the limit for maximum pressure to about '30' mN/m.

The area limits should then be set at points well outside the areas corresponding to these

pressures, i.e. minimum area of 50 cm2 and a maximum area of , say, 500 cm2. (if you are

using a 600 cm2 trough).

Example 2

If you wish to cycle between the gaseous phase and the liquid condensed phase, typical

values would be:

Max area: 500 cm2

 (for a 600cm2 trough).

Minimum π: -2 mN/m - better than 0 mN/m, as the pressure reading can on occasions

drop below zero.

Min area: 50 cm2

 (or any low area value)

Maximum π: 45 mN/m- or whichever pressure you choose in the liquid condensed phase.

The barrier will then cycle between the max area and the max π limits.

Note - do not set the iso-cycle mode when dipping. Pressure will not be held

constant and monolayer transfer will be inhomogeneous.
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QUASI STATIC COMPRESSION

It is possible to make compression isotherms of monolayers 'quasi-statically' - that is to

move the barrier by a defined area, wait for a defined time and the move the barrier again.

This allows the monolayer to relax at each stage which is useful for stiff polymer films and

for collapsing films. It can also help to improve the determination of phase changes during

compression.

Select the size of area increment and the time interval between steps in this section. Then

click on 'Isotherm' under 'Barrier' in the menu bar to perform a quasi-static compression.

FILE Saving and loading

The operating conditions can be saved and recalled on their own. If a particular set of

conditions are frequently used, the 'standard' file can be overwritten to load these particular

conditions at power-up. Please note that only the operating conditions file are saved/loaded

here - no associated data files will be accessed.

Dipper Menu

The Dipper Menu is accessed from 'Dipper' in the menu bar and is shown below. This

screen allows you to set up your LB deposition to your preferences. For instance you may

wish to deposit a single layer or multiple layers. You may wish to move the dipper at a

particular speed and wait a certain time between layers.

Figure 5.5: Dipper Menu (for conventional dippers)
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Please note that the Dipper Menu for conventional LB dippers is shown here. The menu for

the alternating dipper is shown in the 'Tutorial' chapter.

Dipper speed The dipper speed is the speed with which the substrate crosses the

interface. Separate 'up' and 'down' speeds are available.

Target pressure This is the pressure that the film will be compressed to under pressure

control. When the monolayer is transferred to the substrate, the barrier will

be moved to keep the target pressure constant. This compensates for

removal of monolayer from the surface during dipping.

Top End This is the level that the substrate will be raised to.

Dipper position is measured in mm immersion from the top - so a dipper

position of 10mm means that the dipper is 10mm from the top. This is

explained diagrammatically overleaf.

Bottom End This is the level that the substrate will be immersed to.

Layers Required This is the number of layers that will be deposited.

Wait at Top This value determines how long the dipper will wait at the top of its cycle.

Wait at Bottom This value determines how long the dipper will wait at the bottom of its cycle.

Substrate Area This value is used to determine if, after completion of an 'up' stroke, there

is sufficient area left to deposit another 2 layers. If there is not, then 'Please

open barrier and respread...' is displayed.

It is also used in the transfer ratio calculation (as explained in the 'Langmuir-Blodgettry' section). The

substrate area is the total area immersed including both faces and the sides of your substrate (if

thick enough), less the meniscus radius.

Hence, for rectangular substrates:

substrate area = 2 * ((immersed length - meniscus radius) * (width + thickness))

Meniscus Radius This value improves the accuracy of transfer ratio calculations. It is

best to calculate the transfer ratio from the point at which the

meniscus has 'flipped over', rather than the dip start position, as

explained above and in the 'Langmuir-Blodgettry chapter.
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Setting 'top' and 'bottom' dipper positions - conventional dipper

The top and bottom positions are usually set so that the 'top' is where the substrate is

entering the water subphase and the 'bottom' is where it is immersed.

 substrate water

(dipper jaws

 not shown) level

Top Bottom

Figure 5.6: 'Top' and 'bottom' position for conventional dipper

Dipper Menu - alternate layer dipper

For alternate layer troughs the values of dip speed, target pressure and top & bottom ends are set

separately. So, separate values for side 'A' and 'B' are displayed.

The 'top' and 'bottom' positions are illustrated graphically overleaf.

In addition, the following values need to be set as well:

Inter Dip Speed

This is the speed the dipper will move at in between side 'A' and side 'B'. This saves time,

particularly when depositing large numbers of layers.

Alternate Layer Sequence

This is the sequence of layers deposited from side 'A' or side 'B'. Simply enter the

sequence you require in the box. The number control in the little box lets you check

exactly which number corresponds to which type of molecule.
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Alternate layer dipper 'top end' & 'bottom end' positions

substrate

water

level

 Top A Top B

Bottom A Bottom B

Figure 5.7: Alternate Layer Dipper 'Top' and 'Bottom' End Positions

The substrate is carried around the dipper - the arrows indicating the 'forwards' direction.

The critical positions are illustrated above.

'Top A' is just as the substrate is about to be immersed in side A - typically 75mm.

'Bottom A' is when it is just fully immersed in side A - typically 100mm.

'Bottom B' is just before it emerges on side B - typically 190mm

'Top B' is immediately after it has completely emerged on side B - typically 210mm

These typical values are for a standard 76 x 26mm glass microscope slide. Other size

substrates have different values, which are set before deposition commences.
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Calibrations Menu

All motors and sensors are calibrated at Nima before despatch. Once calibrated, it is

unlikely that the motors and potentiometers (position sensors) need to be calibrated again.

However, the full calibration proceedure is described here - just in case!

If you are updating your software, please load your new software into a new directory when

installing the new version and then copy the old calibration files ('sensors.cal', 'install.dat'

and 'standard.con') over to the new directory. This will save you having to recalibrate your

trough.

The following options are given in the 'Calibrations Menu':

Interface Test

Calibrate Pressure Sensor

Calibrate Potenial Sensor

Calibrate Temperature Sensor

Teach Barrier Positions

Calibrate Barrier Speed

Calibrate Dipper Speed

Show Calibration Data

In order to gain access to the Calibrations Menu, please enable it in the Setup Menu.

Interface Test

This displays the 'raw' data as sent by the interface to your PC. It is also possible to move

the motors 'manually' in this window. But take care - there are no safety stops and you

can crash your barrier / dipper and you may cause damage to the gearbox.

On entry, both interface and motors are disabled. Click on Enable Interface to start

reading the sensors. The INPUTS should be reading real data and the 'V2' light on the

interface should illuminate. If it does not, try changing the Serial Port No (bottom left of the

screen).

Typical AD values are: pressure sensor: 10,000 - 20,000

barrier: 15,000 - 25,000

dipper: 15,000 - 25,000

Click on MOTORS DISABLED to allow access to the slider controls on the right of the

screen.

The following values apply: 0 ∇ 4095

open STOP close

up STOP down

ALWAYS STOP THE MOTORS BEFORE THEY REACH THE END OF THEIR

TRAVEL!
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Calibrate Pressure Sensor

It is good practice to check the calibration of the pressure sensor(s) before a series of runs,

particularly if the data is to be published. It can be quickly checked by attaching the weighing

pan and checking the indicated reading in mN/m for the calibration weight in the active

screen - remember that for a 21.0mm perimeter Wilhelmy plate, 100mg is equivalent to -

46.7mN/m.

To calibrate, select 'calibrate pressure sensor' from the 'Calibrations' menu and follow the

instructions given on the screen. The readings of the pressure sensor are recorded with

the empty weighing pan and with the calibration weight in the pan. These are then

automatically stored on disc. The 100mg calibration weight (the small metal tab) and pan

are to be found in the monolayer kit. Ensure that the correct calibration weight and plate

perimeter are entered in the appropriate boxes. It is good practice to check the calibration

weight on a 6 figure balance.

Calibrate Surface Potential

The surface potential probe is plugged into the 'PB' socket at the back of the interface. The

surface potential option is enabled in the Setup Menu.

The software then guides you to bias the probe at two voltages 1.5V and 0.5V and the

equivalent reading of this stored. Please note that the absolute value of voltage is not so

important, as long as the difference is 1.0V. So you could choose 2.0 and 1.0V as well.

More details are given in the Technical Appendix.

Teach Barrier Positions

The 'minimum area' and 'maximum area' positions of the barrier have to be 'taught' to the

PC. This is done by 'manually' moving the barrier to the correct position and then confirming

the position and equivalent area of that position.

The barrier is moved with a slider control provided on the screen. There is also a STOP

/ CONTINUE button to instantly stop the barrier.

There are 4 different configurations for the various troughs made by Nima:

1. Standard - single barrier - Models 601A, 611M, 611D and 622(A&B separately)

2. 2 mechanically coupled barriers working symmetrically

- Models 601M, 601A/2B, 611M/2B, 611D/2B

3. 2 independent barriers working symmetrically but independently -Model 632/D1L

4. MC - 2 barriers with the ability to cross the centre line -Models 601/MC or 611/MC

These are explained in detail overleaf.
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Teach Barrier Positions (continued)

1. Single barrier

Min Max

Area Area

PS

Barrier in 'closed' position Barrier in 'open' position

Figure 5.8: Teaching areas for single barrier troughs

The minimum area is the area at which the barrier is almost completely 'closed' - that is

when it is nearest the pressure sensor ('PS' in the above diagram). It is good practice not

to run the barrier to the end, so that when the minimum area is calculated, the curvature

of the corners of the trough can be taken into consideration, thus giving a completely

accurate area calibration.

To calculate the area, simply take an accurate ruler and measure the length x width. Please

note that standard troughs have exact widths of whole numbers like 10 or 20cm. Most

troughs have rounded corners of curvature of radius of 1cm. So 1⁄2.(4 - πr2

) ( = 0.43cm2)

has to be deducted.

The maximum area is that area where the barrier is furthest away from the pressure sensor

and its trailing edge is just touching the edge of the trough. Again, measure the length x width

less the area of the rounded corners.

Please note in the case of 622 troughs with 2 barriers and 2 separate compartments, this

procedure is done twice, first for compartment A and then B (which has to use the same

areas).
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2. 2 barriers - symmetrical compression

PS

 centre line

Barrier A Barrier B

Barrier A centre line Barrier B

Figure 5.9: Teaching areas for troughs with 2 mechanically coupled barriers with

symmetrical compression.

The minimum area is the area at which the barrier is almost completely 'closed' - that is

when it is nearly at the centre line and almost touching the pressure sensor (PS).

To calculate the area, simply take an accurate ruler and measure the length x width. Please

note that standard troughs have exact widths of whole numbers like 10 or 20cm.

The maximum area is that area where the barriers is furthest apart and their trailing

(outside) edges are just touching the edge of the trough. Again, measure the length x width.
Software

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Teach Barrier Positions (continued)

3. 2 independent barriers

 PS

A

 centre line

Barr A open Barr A closed Barr B closed Barr B open

B

Min

 Area / 2

C

Max Area / 2

Figure 5.10:

Teaching areas for 2 independent barriers for symmetrical compression.

A: Typical minimum and maximum positions for both barriers.

B: Calculation of minimum area

C: Calculation of maximum area

- note that the shaded is only half the area!
 111

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The calibration procedure for 2 independent barriers is as follows:

1. Select 'Teach Barrier Position' from the Calibrations Menu and then click on 'A' for

barrier A.

2. Click on 'CONTINUE MOTOR' and let the barrier move to its 'closed' position, as

shown in Figure 5.10A.

3. Click on 'STOP MOTOR' when the barrier has reached its closed position. If

necessary adjust the position of the barrier by clicking on 'CONTINUE MOTOR'

and using the slider speed control to open or close it more slowly.

4. When it is at its 'closed' position, calculate the minimum area by measuring the area

enclosed, as shown in Figure 5.10B. Measure the distance from the centre line to

the leading edge of the barrier, as shown by the shaded area. Then double this area

and enter this value in the 'minimum area' box. You have to double the area, as it

is the area enclosed by the 2 symetrical barriers that is being measured.

5. Now click on 'ACCEPT' and then run the barrier to its 'open' position. Measure the

area as shown in Figure 5.10C from the centre line to the leading edge of the barrier.

Again, double it and enter that area into the 'maximum area' box.

6. Finally, perform the whole procedure for barrier B.

The only difference is that you will be given no choice of area to enter. So you have

to run barrier B to the same positions (but their mirror image of course) as barrier

A.
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Teach Barrier Positions (continued)

4. 2 barriers - crossing over for multiple compartment option

A

 centre line

Barr A open Barr B closed Barr A closed Barr B open

B

 Min

 Area / 2

 (negative)

C

Max Area / 2

Figure 5.11

Teaching areas for 2 independent barriers for multiple compartment operation.

A: Typical minimum and maximum positions for both barriers.

B: Calculation of minimum area

C: Calculation of maximum area

- note that the shaded is only half the area!

In this variation, the barriers can cross over the centre line so that the monolayer can be

compressed on one side and the transported over to the other. Hence the minimum area

is now across the centre line and is actually a negative number.
 113

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The calibration procedure for 2 independent barriers crossing over is as follows:

1. Run barrier B to its fully open position (in the active graphics screen). It is now out

of the way.

2. Select 'Teach Barrier Position' from the Calibrations Menu and then click on 'A' for

barrier A.

3. Click on 'CONTINUE MOTOR' and let the barrier move to its 'closed' position, as

shown in Figure 5.11. This will be well over the centre line. Make sure the barrier

does not hit barrier B or its belt drive.

4. Click on 'STOP MOTOR' when the barrier has reached its closed position. If

necessary adjust the position of the barrier by clicking on 'CONTINUE MOTOR'

and using the slider speed control to open or close it more slowly.

5. When it is at its 'closed' position, calculate the minimum area by measuring the area

enclosed, as shown in Figure 5.11B. Measure the distance from the centre line to

the leading edge of the barrier, as shown by the shaded area. Then double this area

and enter this value in the 'minimum area' box. You have to double the area, as it

is the area enclosed by two barriers that is being measured.

6. Now click on 'ACCEPT' and then run the barrier to its 'open' position. Measure the

area as shown in Figure 5.11C from the centre line to the leading edge of the barrier.

Again, double it and enter that area into the 'maximum area' box.

7. Finally, perform the whole procedure for barrier B.

The only difference is that you will be given no choice of area to enter. So you have

to run barrier B to the same positions (but their mirror image of course) as barrier

A.
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Calibrating barrier speed

1. Select 'Calibrate Barrier Speed' from 'Calibrations' on the Menu Bar.

2. Click on Start and the calibration will run automatically.

Please ensure that the barrier is physically moving when 'moving at the slowest speed'.

So, when the software displays 'Moving Barrier at Slowest Speed', check that you can see

the position indicator actually decrease (slowly!). If it's not changing, increase the 'Min

Barrier Speed' from the default value of 50, until you can see it change. For greatest

accuracy, run this routine twice, if you have changed the Min Speed during calibration, so

that this speed is measured over the full 60 second calibration time.

Calibrating dipper speed

1. Select 'Calibrate Dipper Speed' from 'Calibrations' on the Menu Bar.

2. Check that the correct dipper stroke and dip factor have been entered. Typical

values are given below and will be written in your calibration certificate:

Dipper type Stroke Dip factor

mini 25mm 8.28

standard 75mm 4.10

alternate 38mm 1.006

3. Click on Start and the calibration will run automatically.

Please ensure that the dipper is physically moving when 'moving at the slowest speed'.

So, when the software displays 'Moving Dipper at Slowest Speed', check that you can see

the position indicator actually decrease (slowly!). If it's not changing, increase the 'Min

Dipper Speed' from the default value of 40, until you can see it change. For greatest

accuracy, run this routine twice, if you have changed the Min Speed during calibration, so

that this speed is measured over the full 60 second calibration time.

Calibrate Temperature Sensor

The temperatuure sensor is plugged into the 'pH/Temp' socket at the back of the interface

and the temperature option is selected in the Setup Menu.

The software then guides you to immerse the sensor at two temperatures (approx 5° and

50°C) and the readings of these are stored.
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Software Updates

Updates are regularly made available on our website - free of charge.

Please follow the instructionns on the website - they:

Download the program from our website at: www.nima.co.uk/softwaree

Unzip the installation files.

The file unzips to smaller floppy sized files (‘setup.exe’, ‘data.001’, ‘data.002’ etc and

‘Nimahelp.txt’) that fit onto 2 floppy discs. So you can copy and install it on another PC.

Set up. Click on Setup.exe and the Nima software will be installed for you. You can install

it to any directory and there are no dynamic link libraries to worry about.

If you are upgrading, please install the new software into a new directory, for instance,

'c:\nimanew'.

Then copy the 3 calibration files 'install.dat', 'sensors.cal' & 'standard.con' from your

exisitng 'nima' directory to the new one. This will save having to re-calibrate all the motors

and sensors.

Conclusion

If you read the whole section at one sitting - well done!

Otherwise, just use this section as a reference if you ever get stuck.
TECHNICAL APPENDIX
Technical Appendix

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Introduction

A list of frequently asked questions is presented in this chapter. These questions were

prepared by listening to several generations of research students learning their Langmuir

films at Cranfield University, England.

This chapter also explains the detailed functioning of the various motor and sensor

assemblies. If a problem does ever occur, please don't hesitate to contact us on:

info@nima.co.uk

Frequently asked questions

General

What does 'Barrier A/B too open/closed' mean?

The barrier has moved outside its 'allowed' range. This range is given by the 'open-to' and

'close-to' areas, as set in the Monolayer Menu. Simply open or close the barrier to bring it

into range by typing 'O' or 'C' as appropriate.

Why does 'Pressure too low / too high' appear?

The value from the pressure sensor is outside its 'allowed' range of 0 - 2.5V. Usually, this

means that there is nothing hanging from the little suspension point. Just add the Wilhelmy

plate and you should be ok.

If this problem persists, check the 'Pressure Sensor' section, later in this chapter.

How full should the trough be?

The trough should be filled so that the water just brims over the top, say 2mm. The barriers

will work effectively over a wide range (at least ±2mm), because of the water’s surface

tension. A full trough allows plenty of leeway for cleaning without needing to top the

subphase up.

When I switch the trough off, do I need to leave it in any particular state?

No, it doesn’t matter how the trough is left, though it is good practice to leave it empty and

thoroughly clean, with the barriers open and the dipper at the top for future users.

How do I start?

To take a pressure-area isotherm, select the 'Barrier' Menu from the Menu Bar and click

on 'Isotherm'. The barrier(s) will move at the speed given in the 'Barrier Speed' control box,

on the left of the graph.

How do I escape from a command I don’t want to execute?

The 'ESC' key will STOP all motors moving. This has the same effect as clicking on the

'STOP' button.
 Technical Appendix

 119

How do I clear the screen?

Click on the 'CLEAR' button or select 'Clear Graph' from the 'Graph' Menu on the Menu Bar.

A confirm will be requested, as this will wipe out all the data from your memory and the

graphs.

How often is the surface pressure / all data sampled?

Surface pressure (and all sensor inputs) are continually sampled. Usually 5 readings are

taken and these are then averaged and displayed on the screen. The number of readings

can be changed in the 'Set up' Menu, selected from the 'File' Menu on the Menu Bar.

How do I configure the trough for multiple users?

Install the software as many times as there are users. During installation, you are given a

choice of directory to load the software to. (The default directory is c:\Nima). Enter the

directory name of your choice and then run the software from that directory. You can, of

course, create the associated short-cuts and place them on the main screen of WIN95.

How can I display a directory of stored isotherms?

Use the 'Load' or 'Save' commands (from the File menu) which displays the contents of

your working directory. Click on 'Cancel' if you don't want to load or save anything.

The interface is not responding

Examine the connections of the serial lead to interface unit and to the computer. The most

common reasons for the interface not working are:

1. Wrong serial port selected.

Please try another serial port - there are 4 to choose from com1..com4. Usually

com1or com2 are used. Use 'Interface Test' from the Calibrations Menu to select

the com port.

2. Serial port driver clash with mouse or network.

If communications are intermittent and/or sluggish, the interrupts may not be set

properly in your PC set up. Check the System/Device Manager/ Ports/ Resources

/ Interrupts in your WIN Control Panel.

Pressure sensors

Does the degree of waterlogging of the filter paper affect the pressure readings?

Yes, for a couple of minutes after the filter paper has been immersed, it will soak up water

and cause the pressure sensor reading to drift.

Does the length of filter paper used affect the pressure readings?

No, because the pressure sensors can be zeroed to offset the weight of the plate. The

change in force is all that is important.

How can I measure the pressure of a very stiff film?

This can be difficult with single barrier troughs.Try experimenting with the pressure sensor

position; putting pressure sensors on top of the barriers so that the measurement is made

at the point of compression prevents premature collapse of a rigid film. Of course, if you

have a 2 barrier trough, then the pressure sensor is mounted on the centre line of your

trough and this problem is eliminated.
Technical Appendix

120

Why are the Wilhelmy plates being dragged towards the trough edge?

This could be because the floating Langmuir film is very stiff or it could be because of static

charge on the PTFE or the acrylic cover. In the latter case, try an anti-static gun or spray.

Why does the pressure sensor reading appear to drift slowly when the trough is

first used?

For a couple of minutes after the filter paper has been immersed, it will soak up water and

cause the pressure sensor reading to drift.

Why doesn’t the pressure sensor reading change very much?

The Wilhelmy plate may have fallen off or may be out of the water. If you suspect a hardware

fault, suspend the empty weighing pan from the sensor, zero the display and add the

100mg calibration weight to the pan. The surface pressure should change by 46.7mN/m.

Isotherms

How can I plot a hysteresis loop?

There are 2 ways:

1. Compress as normal then use a negative barrier speed to run the expansion.

2. Select the isocycle function in the 'Barrier' Menu.

 - the 'isocycle' parameters are set up in the 'Monolayer Menu'.

My isotherm appears vastly expanded/compressed / has vanished

Not all the run appears on the screen

It is likely that the isotherm has been loaded without choosing the 'auto-scaling' of the axes

set to 'auto'. Right-click over the graph and select 'auto-scaling' for both X and Y axes.

If you're running in alternate mode - check that you're in the right compartment (A-B switch

at top left).

How do I get a pressure-time plot?

Select 'Pressure&Area - Time' from the 'Graph' Menu on the Menu Bar. Then set the barrier

speed to 0cm2

/min and click on 'Isotherm' on the Barrier Menu. The trough will be doing an

isotherm at zero barrier speed - i.e. a pressure-time plot.

Why do pressure-area isotherms occasionally have spikes?

This is probably because of external vibrations or air currents jolting the pressure sensors.

If spikes are visible on the screen, the Langmuir film will have been disturbed.

Why are pressure-area isotherms sometimes 'jumpy' / climbing in discrete steps?

The pressure sensor consists of a galvanometer movement. A suspension wire is

attached to the meter arm from which the Wilhelmy plate or balance pan hangs. The meter

arm or suspension wire can get slightly bent out of shape and catch on the inside of the

meter housing. The resulting friction causes jumpy rather than smooth changes in

recorded pressure. Open the cover plate and free the suspension wire with the tweezers

provided.
 Technical Appendix

 121

Why does the pressure reading not return to zero / drop below zero when the

barriers are opened after a compression?

The surface was probably not thoroughly cleaned before spreading or the monolayer has

collapsed.

Why is the area-per-molecule wrong?

There could be several reasons. The solvent used may be miscible with water, carrying

some material into the water with it, the chemical may be impure (high purity materials must

be used), the concentration and volume of solution need to be checked, the film may be

collapsing prematurely, etc. Each possible cause needs to be investigated thoroughly and

in a logical sequence.

Dipping

What pressure do I dip at?

Generally, it is best to dip at in the liquid condensed phase, just above the liquid expandedliquid condensed transition point. At this point, the film is packed almost as tightly as it can

be, yet minor excursions in pressure will not result in premature collapse. Deposition can

be monitored by watching the meniscus as the substrate crosses the interface - an even

meniscus usually implies good transfer. The film must not, obviously, collapse at any

stage.

Why are the transfer ratios wrong?

There are several possible reasons. If the substrate has not been properly cleaned and

treated prior to deposition, the film may be repeatedly deposited and peeled off the

substrate. If the 'dipstart' and 'dipend' positions or the area have been wrongly entered, the

computer will be working from incorrect data.

It may be that the barriers are not sufficiently clean so that material is escaping under them.

It may be that the monolayer is collapsing continually under pressure - check this on the

area-time axes, while holding it at the deposition pressure. With a conventional dipper

mechanism, the 'up' and 'down' deposition ratios will always be different, as the meniscus

will always take a finite distance to reverse directions.

See also 'Interpreting Transfer Ratios' in the 'Tutorial' section of this manual.

Why does the barrier continually move inwards with pressure control on?

The floating monolayer could be collapsing, the Wilhelmy plate may be out of the water or

material may be escaping under the barriers if these are not clean.

How can I prevent the barrier of an unused compartment of an alternate layer

trough drifting to become 'too closed'?

If during deposition only one compartment of an alternate layer trough is used, the barrier

for the other compartment can slowly drift over a period of time and become 'too open' or

'too closed'. This drift can be controlled by occasionally making the target pressure for the

unused compartment negative by adjusting it downwards using 'Target Pressure' controls

(top left of screen), accessed from the Variables Menu. Alternatively, the feedback gain can

be set to zero for the relevant compartment.
Technical Appendix

122

Why does the dipper position not go through '0' mm?

With, for example, a standard alternate layer dipper mechanism attached the dipper

position jumps from around 250mm to around 10mm. This is because the potentiometer

used to monitor the dipper position has a dead area of a few degrees around 'top dead

centre' (of the possible measuring systems, this method nevertheless has the fewest

drawbacks). A filter mechanism is built into the software to prevent this from confusing the

dipper control procedure.

Conclusion

We hope this has helped you with any questions or problems you may have had. The most

frequent problems we are asked about are the software settings. If you are in doubt, the

simplest thing to do is to re-install the software on your PC from the master diskette

supplied with your trough. This will have all the default settings and those specific to your

trough included.

Please don't hesitate to contact us at Nima, if you still have problems:

info@nima.co.uk
 Technical Appendix

 123

Barrier Mechanisms

Introduction

Barriers are driven along the trough by a toothed belt, as illustrated below:

spring loaded

arm

 barriers

pulley

wheel

 bolt

 for clamp

 clamps

 pulley wheel

Figure A1: Barrier Assembly. Troughs can have 1 or 2 barriers - 2 barriers shown here.

Each PTFE barrier is located on the Langmuir trough by a spring loaded arm. This arm can

be raised to allow the easy removal of the barrier from the trough for cleaning.

The arm runs on a precision linear rail and is driven along the trough by a toothed drive belt

mechanism. The barm is attached to the belt by means of a clamp, which is tightened by

a bolt.

The belt mechanism is moved by a precision dc motor/gearbox combination turning a

pulley wheel at one end of the belt. The barrier position is detected by a potentiometer

(called 'pot' in this manual) attached to the back side of the drive shaft. A second pulley

wheel, the 'idler pulley' is located at the other end of the belt - this is located in a slot and

allows the belt tension tobe adjusted.

This following pages are written to help you repair or replace any components in case of

problems.
Technical Appendix

124

Barriers - trouble-shooting

Symptoms

There are 2 major symptoms associated with a faulty barrier:

a) barrier not moving - either motor or gearbox faulty

b) barrier moving but area display incorrect - area not calibrated properly or potentiometer

faulty

However, before you take everything apart, it is worth checking some obvious things first:

Wiring & connections

If your barrier is completely 'dead', check that the cables are plugged into their correct

sockets in the interface unit - barrier A into 'Barrier A' and barrier B into 'Barrier B'.

Check the wires at the motor and potentiometer - sometimes these can come loose but

still be held in place by the protective rubber sleeving. Just give each wire a gentle tug, to

make sure that it is attached properly.

Mechanical action of barrier

Now check the mechanical action of the barrier. There is a hexagon bolt on the end of the

barrier, which tightens a small clamp onto the belt, Figure A1. When this is tightened, the

barrier moves along with the belt. When this is loose, the barrier can be pushed manually

and should run freely along the whole of its length. Please check that the barrier can do this.

If it is not running smoothly, please check for signs of scratching or pitting on the surface

of the barrier rail. Also check for any other obstructions interfering with the free movement

of the barrier.

Then move the barrier near the centre of its travel and tighten the locking bolt gently, so that

the belt is clamped firmly but not so tight that the belt is squashed out of shape.

Testing motor/gearbox

Symptoms of faulty motor/gearbox:

*Motor not responding under computer control - no noise, no movement

*Noise coming from motor but barrier not moving

Run the 'Interface Test' routine (under 'Calibrations' on the menu bar). The barriers are

controlled by slider controls on the right side of the screen. When the slider is in the middle,

the barriers are stopped. Moving them left and right makes the barriers move open or

closed. There is also a MOTORS ENABLED/DISABLED button which acts as an override

for emergency stops.

Locate the appropriate barrier control ('A' or 'B') on the top right of the screen and move the

slider to the right. The barrier should move towards its 'closed' position - the further you

move the slider control from the middle, the faster the barrier will move.
 Technical Appendix

 125

Take great care not to leave the barrier moving. As there are no safety controls in

this test routine, the barrier can crash into its end stop and may be damaged. Always

stop the barrier!

Click on MOTORS ENABLED to stop it again.

If the barrier is not moving at all and there is no noise coming from the motor, it may be

broken and will have to be replaced. Please note these motors are usually very reliable and

care should be taken that everything is plugged in properly, before taking it all apart.

If there is a noise coming from the motor but the barrier is not moving, then the gearbox may

be broken and will have to be replaced.

Replacement of the motor/gearbox is described overleaf.

Testing the barrier potentiometer ('pot')

Symptoms of faulty pot:

*Area-time plot is non-linear

*Barrier not moving, even though motor/gearbox is ok

*Pot difficult to turn by hand (after belt has been removed)

If the barrier is moving without problems but the area display is inaccurate, the barrier may

have slipped on the belt. So, please re-calibrate the areas first, as described in the Software

chapter of this manual.

If the areas have been calibrated properly and the area display is erratic, please take an

area-time plot of the barrier position. Select the area & pressure v time axes and click on

'Isotherm' under 'Barrier' in the menu bar to take an area time plot. If you do not see a straight

line of area-time, then the pot is faulty and will have to be replaced.

Replacement of the pot is described overleaf.
Technical Appendix

126

motor plinth

gearbox

spacer

 (must be orientated this way)

pulley wheel

slot

potentiometer

grub screw 'pot'

 pot locking nut

Replacement ot barrier motor/gearbox and potentiometer

Figure A2:

Exploded view of barrier assembly - showing motor/gearbox and potentiometer

1. Slacken the barrier clamp again and slip the belt out of the clamp.

2. Undo the grub screw of the pulleys (the little aluminium wheel with teeth for the belt)

attached to both the pot and the motor/gearbox. The grub screw is a little hexagon

socket screw that needs a 1.5mm hex key (supplied in our toolkit).

Note: if this grub screw is already loose, it may be that this is the problem. Tighten

it up and test the action of the barrier.

3. Using a 13mm (1⁄2 inch) spanner, gently loosen the locking nut that holds the pot to

the trough plinth (or the polished cover in a 601M). This will ease the tension in the

belt, as the pot is located in a slot, not a hole.

4. Gently pull the pulley wheels off the pot and gearbox shafts, so that the belt and

pulley wheels come away completely.
 Technical Appendix

 127

Replacement of motor/gearbox

1. Unscrew the 3 little screws holding the gearbox onto the trough plinth (or cover for

601M).

2. With the wires still attached to the motor, try to run it again. Is the output shaft

turning? If yes, try to stop it by hand - does the shaft stop turning? If yes, then maybe

the gears in the gearbox are slipping.

3. If only the gearbox is to be exchanged, then unscrew the 2 long screws that hold the

gearbox to the motor and replace with the new gearbox.

4. If the motor is to replaced, unsolder the wires from the motor, taking note of the

wiring configuration. This wiring configuration will depend on the gearbox and trough

type you have. But there are only 2 wires, so you can't go far wrong!

5. Replace the motor/gearbox assembly.

Replacement of pot

1. Completely unscrew the pot locking nut and remove the pot from the trough.

2. Disconnect the wires, making a note of the wiring configuration. This should be:

pin 1 red

pin 2 yellow

pin 3 blue

The red and blue wires may be switched over, depending on the direction of

movement of the barrier.

3. Replace the new pot, resolder the wires in the same way and fix it to the trough,

but don't yet tighten the pot locking nut completely.

Re-fitting the drive belt

1. With the pot locking nut still loose, attach the pulley wheels and belt as one unit,

making sure that the there is no undue force on the pot shaft. Too much force will

damage the bearing and cause the pot to jam up.

2. Now gently push the pot pulley wheel towards the end of the trough, so that some

tension is put on the belt - tight enough for the belt not to slip on the teeth of the

wheels, but not so tight that there is excessive force on the pot shaft. Then tighten

the pot locking nut.

3. Now, while the barrier belt clamp is loose and not driving the barrier, move the

barrier motor with the slider control until the barrier '16 bit decimal INPUT' reads

25,000 ± 100 units. Then manually push the barrier to its 'open' position and gently

tighten up the barrier clamp.

4. Re-calibrate the areas, as described in the Software chapter of this manual.
Technical Appendix

128

Dipper Mechanisms

There are 2 Nima dipper mechanisms - conventional and alternate. In this context,

'conventional' means that the substrate is oscillated vertically through the liquid-gas

interface while the alternate layer mechanism immerses the substrate through one

monolayer and withdraws it from another.

Both dippers come in 2 standard sizes:

D1L-25 miniature dipper for small substrates up to 25mm long.

D1L-75 standard conventional dipper for microscope slides and 3" (75mm) wafers.

D2 - standard alternate layer dipper for microscope slides.

D2-3" large alternate dipper for 3" (75mm) diameter wafers.

The 'L' in 'D1L' stands for linear, to differentiate it from the old 'D1S' dipper which was

sinusoidal. This is now out of production.

Setting the Linear Dipper, Model D1L

The standard conventional linear dipper, Model D1L, consists of a substrate clamp that is

magnetically attached to an arm which is driven up and down by a belt and pulley system.

Dipper Potentiometer assembly

 (expanded view)

 top spacer (1 of 3)

flexible coupling

 dipper body

gearbox shaft

grub screw

pot shaft

motor

 gear potentiometer - held

 box in place by 3 small screws

Motor/gearbox cover not shown

Figure A3: Illustration of Dipper, Model D1L-75

magnetic holder

for substrate clamp

(clamp not shown)
 Technical Appendix

 129

Setting the conventional dipper potentiometer position

1. First remove the round black cover by unscrewing the 2 screws..

2. Identify the flexible coupling which connects the pot to the gearbox, Figure A3. This

has 4 little grubscrews - 2 pinching on the pot shaft and 2 on the gearbox shaft.

3. Slacken the 2 gearbox shaft grub screws with the 1.5mm hexagon key provided.

If access to the grub screws is not possible, rotate the dipper until it is.

Run the 'Interface Test' routine (under 'Calibrations' on the menu bar). The dipper is

controlled by a slider control on the right side of the screen. When the slider is in the middle,

the dipper is stopped. Moving it left and right makes the dipper move up or down. There is

also a MOTORS ENABLED/DISABLED button which acts as an override for emergency

stops.

Move the slider to the right. The dipper will move towards its 'down' position - the further

you move the slider control from the middle, the faster the dipper will move.

Take great care not to leave the dipper moving. As there are no safety controls in

this test routine, the dipper can crash into its end stop and may be damaged. Always

stop the dipper!

4. In the 'Interface Test' program, slide the 'Dipper' control to the left to move the dipper

to the top.

5. When it reaches the top i.e. when dipper height = 75mm for a standard dipper or

25mm for a mini dipper, stop the motor by moving the slider to the centre.

6. If necessary, move the dipper down again by sliding the control a little to the right

and then back to the centre to stop it again. Repeat until the correct height is

reached.

7. Check that the grub screws of the other side of the flexible coupling are tightened

onto the pot shaft and rotate the whole assembly so that the '16 bit decimal INPUT'

on the screen reads 25,000 ± 100 units.

8. Re-tighten the grub screws on the flexible coupling and replace the outer cover.

9. Quit the Interface Test program and click on 'Calibrate Dipper Speed':

Set the stroke and factor as follows: Dipper Stroke Dip Factor

D1L-75 75mm 4.10

D1L-25 25mm 8.28

If in doubt, check your calibrations certificate for the stroke and factor or contact us

- we keep records of your settings.

12. Click on START and the dipper speed will be calibrated as normal.

The dip factor and stroke will be saved along with the speed calibration data.
Technical Appendix

130

Alternate Layer Dipper, Model D2

motor

housing

 gearbox

 housing

substrate

spring loaded seal

Figure A4: Alternate layer dipper, Model D2

The alternate dipper mechanism has its potentiometer in the smaller of the 2 stainless steel

housings. First, the dipper needs to be moved to its 'top' position:

1. Run the 'Interface Test' program.

2. Slide the dipper control to the left or right to move the dipper. It's not important which

way, as the substrate can go round indefinitely.

3. Stop when it reaches its top position by clicking on MOTOR ENABLED.

4. Undo the 2 screws holding the steel cover in place and remove the cover.

5. Loosen the 2 small screws holding the potentiometer in place.

6. Rotate the pot until its maximum value is displayed. In principle, this is 32,767, but

because of the 'dead band' of the pot, this is usually around 30,000. It is not important

if you have not adjusted it to the exact 'top dead centre' - as you will be setting the

'dip start' and 'dip end' position in the Dipper Menu anyway.

7. Tighten the three little screws and replace the cover.

Then its speed needs to be calibrated:

1. In the main graphics screen, click on 'Calibrate Dipper Speed' under 'Calibrations'

in the menu bar.

2. For the standard D2 dipper, set the stroke to 49.0mm and dip factor to 1.006.

3. Click on START and the dipper speed will be calibrated as normal.

The dip factor and stroke will be saved along with the speed calibration data.
 Technical Appendix

 131

VR3 VR2

 mini DIN

 socket

detector

 moving coil

 infra-red diode end-stop

± 2mm

make sure that

detector this wire is

dangling freely

extension wire

with

suspension point

Pressure sensor, PS4

This pressure sensor is the fourth generation of our sensors. It combines very high

accuracy with stability and reliability. It uses a taut-band meter movement which gives very

low friction. The design of the electronic circuit allows infinite loop gain giving zero

compliance i.e. whatever the load, the Wilhelmy plate remains at the same level. Hence

it is ideal for surface tension measurements.

However, as a result of the small forces being measured, some of the component parts

are delicate and can be bent out of shape. This technical note aims to help the user repair

the sensor without having to return it to the manufacturer.

Mechanical setup

Figure A5: Illustration of PS4 - cover removed.

The sensor should be set up as in the diagram above.

For proper operation the following points should be observed:

1. The moving coil must be able to move freely .

2. The extension wire must be dangling freely and must not be touching the inside of

the housing.

3. The moving coil should reach the horizontal position when the sensor is powered

up.

Electronic setup

There are 2 potentiometers to adjust:

VR2 offset of output signal - turn clockwise for increased output

VR3 gain of output signal - turn anti-clockwise for increased gain

When powering up, the coil should automatically reach its horizontal 'balance' position.

There is a wire end stop to prevent it moving by more than ±2mm from the horizontal, to

keep it in range.
Technical Appendix

132

If the meter does not come to its 'balance' position, push the diode a little towards the coil,

so that the optical signal (reflecting from the edge of the coil into the detectors) is a little

stronger. Then the electronics will take over.

VR2 (offset) and VR3 (gain) should be adjusted, so that the output level matches the

appropriate operating mode:

Suggested values are:

Force suspended from sensor Approximate output (Volts)

Tensiometers Troughs

Output with empty weighing pan: 0.2 0.8

Output with pan + 100mg: 0.6 1.6

Adjust the blue potentiometer labelled ‘VR2’ until 0.8V is displayed. Add the 100mg weight

and adjust VR3 until 1.6V is displayed. Remove the weight and check the output. Adjust

VR2 again if necessary. Repeat in an iterative manner until the required output voltages

have been achieved.

Completion

Finally, replace the cover and calibrate in the usual way. Note that the voltages need not

be set exactly, as the sensor will be calibrated anyway. It is also possible to reduce the gain

when wider plates are used or increase the gain for greater accuracy.

Pressure sensor out of range - if this message appears, it means that the pressure signal

has gone outside the AD range. Usually this is not serious, as it only happens when there

is no load on the sensor i.e. when the plate and hooks have been detached. To prevent this

occurring, simply suspend a paper plate from the pressure sensor.
 Technical Appendix

 133

Temperature sensor

Any commercially available temperature sensing system can be used with Nima troughs.

If you want to log the data on the PC, there is an input on the Nima interface. The only

criterion is that the output of your meter be in the range 0 - 2.5V.

The following 'pin-outs' apply on the 'pH/Temp' input on the Nima IU4 interface:

pin 1 temp

pin 2 ground

pin 3 pH

Magnetic Stirrer, Model MST

The magnetic stirrer consists of a steel pin encased in an PTFE disc which is located in

a 1mm deep recess in the trough. It is rotated by rotating a magnet below the trough with

a dc motor. Speed control is effected from a small control box with a single turn dial. The

wire from the motor is plugged into the control box using the 5 pin DIN plug labelled ‘MS’.

Power is supplied to the control box from the 9V encapsulated supply. Usually a little '1 into

2' box is supplied, so that the trough interface and the magnetic stirrer are supplied from

1 power supply.

 To operate, simply place the stirrer disc in its location on the trough and rotate the control

knob until the required speed is reached.
Technical Appendix

134

Surface potential sensor - Trek Electrometer 320

Introduction

If you have bought a Trek electrometer (either separately or through Nima), the following

section will show you how to install it.

guide

vibrating electrode

sensor head

raisable

platform

Knob to

raise or lower

sensor

Figure A6: Trek vibrating electrode mounted on Nima raisable platform

1. Carefully unpack the sensor and meter and set the sensor up as in the diagram

above.

Read the Trek manual thoroughly.

2. Ensure that the sensitivity switch on the front panel of the meter is set to '10V' and

the NULL VOLTAGE switch to '+'.

3. Ensure that the RESPONSE dial on the back of the meter is turned all the way to

'slow'.

4. Lower the sensor head to its bottom position by rotating the knob on the back of the

body, so that the vibrating electrode is about 1mm above the ground plate.

5. Connect the probe lead from the sensor to the large plastic socket on the back of

the meter and the ground lead from the ground plate to the GND socket on the back

of themeter

6. Turn the meter ON.

7. Adjust the NULL VOLTAGE dial until the displayed reading is 0.00 V.

If the dial comes to the end of its range before 0.00 V is reached, flip the Null Voltage

switch over to '-' and continue the adjustment.

8. The unit is now ready for use.
 Technical Appendix

 135

When taking readings, keep the vibrating electrode as close as possible to the test surface

without actually touching it. The RESPONSE switch on the back of the meter can be

adjusted to give optimal compromise between speed of response and noise of signal.

Never touch the vibrating electrode - it may have a high potential on it.

Checking absolute potential against a reference

Take the 1.5V battery and place it in series with the earth wire between the GND socket and

the ground electrode, as in the diagram in below:

Figure A7: Electrical connections to test the surface potential sensor

A 1.5V potential step has now been placed on the plate and this should be displayed on the

meter. Remember to check the difference in voltage - not the absolute value. Check the

battery voltage with a conventional meter and compare the reading on the control unit. The

change noted on the meter should be within 10mV of the displayed signal.

Using with the Langmuir trough

I it is now necessary to lay the ground plate into the subphase, as otherwise the high surface

energy of the PTFE trough will swamp the small electric field of the monolayer. The ground

plate also acts as a reference for the surface potential signal.

1. Place the stainless steel ground plate in the (empty) trough and plug the 2mm

'banana' connector provided on the earth lead to make good contact with the plate.

The socket on the plate connector consists of a 2mm stainless steel socket with

a PTFE skirt. The PTFE skirt prevents the connector from short-circuiting the

monolayer.

There are 2 possible positions for the connector in the ground plate, depending on the

geometry of the trough. A single barrier trough should use the end position, while a 2 barrier

trough should use the central position.

The other end of the ground lead has to be plugged into the 'GND' socket of the interface.

2. Connect the co-axial cable between the 'OUT' socket of the Trek meter and the 'PB'

socket' on the Nima interface.

3. Run the ‘Interface Test’ routine and check the ‘Pressure B’ reading. Adjust the

output voltage of the Trek with the voltage offset dial. Check that when the output

displayreads 0.0V on the display of the Trek, 0.0V ±0.1V is displayed by the

Sensor head Probe OUT GND

 Trek 320 Control Unit

~ 1.5V
Technical Appendix

136

Pressure Breading. Note that it is not important if the voltage displayed on the

screen and the Trek meter are not exactly the same - this is why you will be

calibrating the output precisely.

4. Go to the Setup Menu and enable the 'Surface Potential' option in 'Setup Langmuir

Trough'

5. Select the 'Calibrate potential sensor' from 'Calibrations' in the menu bar.

Follow the instructions on the screen: adjust the output on the Trek to read +0.5V,

click on the button to accept and then adjust it to read +1.5V and click again. The

software now knows what reading corresponds to a difference of 1V.

6. Isotherms of pressure and potential can now be captured, plotted and stored

simultaneously. Just click on 'Isotherm' to take the isotherm in the usual way.

Notes for best results

1. Compared to surface pressure measurements, changes in surface potential

readings are much more leisurely and need more experimental finesse. They

require the same kind of patience as do precise pH readings. Hence considerable

care during operation and interpretation is required.

2. It is easiest to make surface potential measurements of ionic solutions. Pure water

is a very good insulator and there are considerable higher stray (and variable)

capacitances on pure water subphases. Signal noise is proportional to the square

of ionic concentration.

3. Particular care must be made not to charge surfaces unnecessarily, as charges

will invariably leak to other surfaces near the probe and cause drift in the measured

readings. Ideally, antistatic protection should be worn (such as found in semiconductor fabrication areas) and antistatic mats should be placed around the trough bench.

In addition, a simple Faraday cage should be erected to prevent stray charges

interfering with measurements.

4. Place the vibrating plate of the probe about 2mm from the test surface. Should it

touchthe water, remove the droplet with the aspirator and start again. If the probe

is too far from the test surface, the signal will be too noisy for satisfactory

measurements. Don't be afraid to experiment - no damage will be done to the probe.
 Technical Appendix

 137

Device 601, 611 622 Alternate Conventional

Wire 622 side B side A Dipper Dipper D1L

Motor (+) green black green green

Motor (-) black green black black

Pot pin 1 or a red blue blue red

Pot pin 2 or b yellow yellow yellow yellow

Pot pin 3 or c blue red red blue

Pressure sensor PS4, mini DIN socket

Ground black (pin 2 on IU4 DIN socket)

-V white (pin 3 " " " )

Output red (pin 4 " " " )

+V yellow (pin 5 " " " )

Cable connections

5 pin DIN socket (240°)

 3 - blue

 black - 2 4 - yellow

 green - 1 5 - red

Viewed from the outside of the interface unit or from inside the DIN plug.

(Pin numbers are marked on both plugs and sockets)

The colours identify the five wires used inside the six-wire screened cable running to

each motor/sensor.

The connections of the wires to the LB trough devices to the DIN plug/socket pins are

shown below. The potentiometers have pins marked 1,2,3 or a,b,c.
Synthesis of ferric stearate

Ferric stearate is used to make the substrate surface hydrophobic (as described in the

Tutorial chapter). A small sample of this is supplied with your trough and it is also

commercially available*.

If you prefer to synthesise it in your own lab, the following process takes about an hour and

a half:

- Make up a solution containing 1g of sodium stearate in 50ml of ultra-pure water.

- When all the solid has dissolved, pour the solution into a clean burette.

- Make up a second solution containing 0.3g of ferric chloride (iron (III) chloride)

hexahydrate in 50ml of ultra-pure water.

- When all the ferric chloride has dissolved, pour the solution into a second clean

burette.

- Simultaneously add the solution from each burette to a beaker containing

approximately 10ml of pure water.

Stop every few millilitres to gently stir the contents of the beaker and to check

that the pH of the resultant solution does not vary from pH7. If the pH is too acidic

the product of the reaction will be white stearic acid; if it is too basic, the product will

be dark grey ferric hydroxide. The reaction taking place is:

3C17H35COONa+FeCl3

.6H2O

↓

Fe(C17H35COO)3

(s) +3NaCl+6H2O

- On addition of all the reagents, stir the solution gently and leave to stand for a

few minutes. The ferric stearate should precipitate out of the solution as a pale

brown solid.

- Pour off the excess liquid above the ferric stearate.

- Wash the product with pure water a couple of times.

- Leave the product to dry in air on a filter paper for about an hour.

The yield of ferric stearate product should be 1g.

* Commercially available from TCI Co Ltd, Tokyo, Japan. Fax (+81)(3)3270 8466
INDEX
A

Active screen 94

software 94

Alcohol 18

Alkanoic acid

dissociation 30

Alternate layer deposition 66

Alternate layer dipper 130

Alternate layer dipper enable 99

Alternate layer mode 101

Amines 18

Amphiphatic balance 28, 76

Amphiphiles 76

Amphiphilic 18

Anti-reflection coating 48, 60

Anti-vibration table 36

Arachidic acid 49, 56

Area

per molecule 100, 121

Area control 95, 97

Argon ion plasma 49 B

Babylonians 20

Bare skin 35

Barrier 11

hold time 100, 103

opening in software 94

speed 100, 103

Barrier mechanisms 123

Barrier menu 11

Barrier positions

calibration of 107

Barrier speed 96, 100, 114

calibration 114

setting 53

under pressure control 57

Barrier 'too open/closed' 118

Bench space 2

Biotechnology 21

Blank spreading 39

Blodgett 21

original paper 48

Boltzmann 25

Bottom end 103

Botttom position

setting dipper 104

Buttons

software 96

C

Cable connections 137

Cadmium

contamination 72

Cadmium arachidate 48

Cadmium chloride 48, 87

Calcium fluoride

For IR studies 40

Calibration

of barrier positions 107

of barrier speed 114

of dipper speed 114

of pressure sensor 107

of surface potential 107

weight 107

Calibration certificate 114

Calibrations Menu 95, 106

Carboxylic acid 18

Change axes

area units 95

Cholesterol

monolayer properties of 89

Chromic acid 40

Clapham Common 20

Cleaning 37, 73

aspirator / pump 51, 74

plasma etching 40

trough 36

water surface 51

Cleanliness 35, 72

for experiments 72

Clear graphs 95

Clear memory 11

Clear screen 119

Close to area 100

Coatings

Anti-reflection 21

Collapse

of monolayer 62

Collapse pressure 83

Com port 119

Compression 53. See also Isotherm

of monolayer 53

Concentration 101

Containers

For water 38

Contaminants 37

Contamination

surface active 38

Copyright 72

Coulomb 18

Coupled barrier operation 65

Cursor control 55

Cycle Max Area 97

Cycle Min Area 97

Cycles required 96 D

Decon-90 37

Deposition 13, 56, 57. See also Dipper

alternate layer 34

and peeling off 32, 62

first monolayer

onto hydrophilic substrates 32, 61, 62

onto hydrophobic substrates 19, 31

head to head and tail to tail 33

quality 58

Types: X, Y and Z 33

Detergent Cleaning 37
Devaux, H. 21

Dip Bottom End 97

Dip Top End 97

Dipper 11

adjustments 128

making LB film 56

mechanism

questions 122

moving in software 94

Dipper enable 99

Dipper mechanisms

setup 128

Dipper Menu 11, 95, 102

Dipper position 122

Dipper positions

setting top & bottom 104

Dipper speed 96, 103

calibration 114

Dipper start/end

positions 63

Dipper-Area graph 95

Dipping 121

Display directory 119

Disposal

solvents 35

toxic materials 72

Duplex film 76

E

Easy cleaning 54

Esc key 50

Exit software 95

F

Fats 18

Fatty acid

dissociation 30

saturated 83

unsaturated 85

Feedback gain 11, 97

Ferric stearate 49

hydrophobic surface 40

synthesis 138

File

loading and saving 54

File Menu 95

Film balance

cleaning 73

preparations 72

Film quality

streaky 53

Filter paper 119. See also Wilhelmy plate

Filters

for clean labs 35

First layer

on up stroke. See Deposition

Franklin, B. 20

Fume cupboard 72

G

Gaines, G.L. 21

Gas detectors 21

Gauges

Step-thickness 21

Gelatin 20

Gold

Hydrophobic surface 40

Graph

changing type 54

Graph Menu 95

Graphics

screen 94

Ground plate

for surface potential 135

Grunfeld, F 62

H

Hardy 21

Head to head and tail to tail 33

Helmanex 37

Help site - www.nima.co.uk/help 8

Hexamethyl disilazane 40

HHTT 33

Homologous series

of fatty acids 85

Horizontal streaking

during deposition 32

Hydrocarbon 18, 28

Hydrocarbon chain

area 82

isomers 85

length 29

minimum length 85

Hydrophilic 18, 28

molecules in subphase 62

surface preparation 40

Hydrophobic 18, 28

surface preparation 40

tail structure 85

Hysteresis loop 120

I

Ingestion 35

Installation 9

hardware 4

software 8

Interface 119

Interface Enabled 98

Interface Test

software 106

Interface unit

setting up 7

Interference colours 48

Ionic equilibria 30

Isotherm 12, 24, 50, 55, 82, 119, 120

arachidic acid 54

in tutorial 50

of similar fatty acids 85
Isotherm cycle 101 K

Kelvin probe

surface potential sensor 134

Kimwipes 12, 36, 49

USA 14, 49

Kuhn, H. 21 L

Laboratory

requirements 2

Langmuir, I. 21

Layers Required 96, 103

LB film

deposition 56

Levelling 7

Lipids 18

Liquid condensed phase 55, 56

Liquid crystal 25

Liquid expanded phase 55

Liquid-gas 18

Liquid-liquid 18

Load file 95

Loading

operating conditions 102

Loading a file 13, 54

Logging interval 98 M

Marbling 20

Materials

for experiments 72

Memory

clear 11

Meniscus 31, 32, 58, 76

during deposition 32, 58, 62

Meniscus radius 59, 63

Menu bar

software 94

Micelles 37

Micro-balance 131

Möbius, D. 21

Molecular

mountain ranges 62

Molecular area

determination 80

zero pressure 25

Molecular electronics 21

Molecular flow 78

Molecular weight 101

Monolayer

acid to salt conversion 87

annealing 36

cholesterol 89

collapse 25, 62, 77, 83

elasticity 77

flow 78

peeling off 62

physical prescence 76

reactions 87

spreading 76

stability 29, 62, 76

viscosity 79

Monolayer Menu 95, 100

Multi-compartment troughs 65

Multilayer 19, 56. See also Deposition

Multilayer deposition

in tutorial 56

Multiple users 10, 119 N

Nature 21

Non-centro-symmetric 34

Non-linear optical films 34 O

Oleophilic 18

Open to area 100

Operating Conditions Menu 52

Operating modes

multiple users 10

P

P&A-Time 54, 59, 60, 95

PC requirements 8

Peeling off

of monolayer 62

Peroxide 40

Phase changes

determination in monolayers 102

Piston oil 22

Plasma etch 49

Plotting an isotherm 54

Pockels, A. 21

Polar media 18

Port

com port 119

Position detection 130

Potassium octanoate 37

Potentiometer 123

dipper 129

Powder

monolayer flow 78

Preparations

for experiments 72

Pressure

equilibrium spreading 23

Pressure control 121

for dipping 57

setting 54

Pressure sensor 7, 118, 119

adjustment 131

calibration 107

questions 119

setup 131

zeroing 51, 74

Pressure sensor out or range 132

Pressure too low 118

Pressure-area graph 95
Pressure-area isotherm 12

Pressure-time 120

Print screen 95

Printing 13

Progrm Dip

software command 57

PS4 131

PTFE 73

Troughs 36

Pump 2

Pyroelectric films 34

Q

Quasi static compression 102

Questions 118

R

Rayleigh, Lord

John William Strutt 21

Refractive index

changing in LB films 60

Removable top 5

Repeated compressions 101. See also Isotherm

cycles

Requirements

for tutorial 48

laboratory 2, 35

Run time variables 11

S

Safety 35, 72, 87

Samples 119

Samples / Reading 98

Saving

operating conditions 102

Saving files 13, 95

Schaefer 22

Sensors 21

Serial port 7, 119

Serial port driver 8

Setup Menu 95, 98

Signature

software 95

Silk

waxed thread 22

Sodium hydroxide

substrate preparation 40

Software 8, 94, 106

active screen 94

buttons 96

commands in tutorial 50

menu bar 94

organisation 8

updating 10, 115

Solution

molecules in subphase 62

Solvents

boiling points 39

Soxhlet 40

Spreading 53, 76

Stop 11

Subphase

Cadmium ions in 29

cleaning 51, 74

metal ions in 87

solution of molecules 62

trivalent ions in 88

Substrate

cleaning 40

hydrophilic 31

preparation 49, 138

Substrate area

calculation of 103

Sumi-nagashi 20

Surface

active 18, 37

cleaning 62

with aspirator / pump 77

preparation 40, 138

plasma etch 49

pressure 23

pressure too high 62

Surface potential sensor 134

Surface tension 23

Surfactants 18, 37

Switching area axis

cm2 <> A2 53

T

Target area 95, 97

Target pressure 11, 97, 100, 103

setting for deposition 56

Teach barrier positions 107

Tectonic plate

monolayer model 77

Temperature control 7

Tikopur 37

Top End 103

Top position

setting dipper 104

Transfer ratio 32, 60, 63, 121

incorrect 121

Transfer ratios

interpreting 62

Traube’s rule 29

Trek electrometer 134

Trivalent metal ion

in subphase 88

Trouble-shooting

barriers 124

Trough

filling 118

preparation 49

storage 58, 73

transport 73

Trough top 5

U

Ultrasonic bath 40

Unpacking 3
Updating software 10, 115

V

Van der Waal 18, 24

Variables

permanently displayed 96

Variables Menu 56, 95

Volatility

of solvents 39

Volume 101

W

Wait interval

dipping 103

Water 2, 38

Waveguides 21

Website 9

Weight

calibration 107

Whatman's

Chromatography paper 26

Wilhelmy plate 24, 26, 107, 120

setting up 7

X

X type deposition 63

X-ray diffraction

head group spacing 63

Y

Y type deposition 33

Z

Z type deposition 33, 63

Zero

pressure sensor 51

Zero pressure

molecular area 55