# Presentation: 114755

## Diffusion of Surfactant Micelles and ShapeFluctuations of MicroemulsionsStudied by Neutron Spin Echo (NSE)

*Gaithersburg, MD 20899*
- E-mail:
- dbossev@nist.gov
- Internet:
- http://www.ncnr.nist.gov/

**Diffusion of Surfactant Micelles and Shape****Fluctuations of Microemulsions****Studied by Neutron Spin Echo**** (NSE)**

**Dobrin P. Bossev**

## NSE is a “dynamic” method

- •  NSE is a new method at NIST. The idea has come to Mezei in 1972 at a red traffic light at the corner of Alagút street in Budapest
- •  NSE is complementary to SANS. Using NSE we can measure the dynamics of the scattering system:
**SANS ******** static picture           NSE ******** dynamic picture**
- •  NSE is a quasielastic method – small deviation from the elastic scattering
**SANS ******** elastic scattering               NSE ******** quasielastic scattering**
- • The NIST spectrometer is best used for measuring coherent diffusive or dispersionless excitations in the range of 0.01 to 200 nanoseconds. Bridges the gap in time scale between conventional inelastic neutron scattering and dynamic light scattering. Yields the intermediate scattering function *I*(*q*,*t*) as a result.

## Content

**•  ****Why NSE?**
**•  ****Experimental system**
**•  ****NSE method**
**•  ****Summary**

**Content**

## Surfactant aggregates in solution

- Hydrophobic tail
- CH3-CH2-CH2- ...
- CF3-CF2-CF2- ...

- Hydrophilic head
- SO3-Na+
- NH3+Cl-
- -(OCH2CH2)n-

**Surfactant molecule**

- Oils and water do not mix!
- Why?
- Water is a polar liquid, ** = 81
-  Oils are non polar, ** ~ 2
- (** - dielectric const.)

**Spherical micelles**

**When surfactants are dissolved in water they:**
- - reduce the surface tension because they are adsorbed on the surfaces
-  form variety of aggregates – micelles, lamellae, bicelles, vesicles, etc

**H****2****O**

**Surfactant**

**Cylindrical micelles**

**Lamellae**

- micelles are small,
- ~ 4 nm in diameter

**H****2****O**

**Vesicles**

## Micellar properties

**Micellar properties**

- Oils and water do not mix?!? The surfactants help them mix.

**Micelle**

**H****2****O**

**Surfactant**

**Microemulsion droplet**

- Surfactant

- Solubilized organic
- molecules (oil)

- microemulsion droplets
- are bigger, dia. 4 - 50 nm

**H****2****O**

- When surfactants are dissolved in oils they form “inverse” micelles, lamellae, etc

- Surfactant

**Inverse micelle**

- n-decane
- C10H22

**Inverse microemulsion droplet**

- H2O

- n-decane
- C10H22

- Surfactant

**oil**

## Applications

**Surfactants are very useful:**
**• **To reduce the interfacial tension between oils and water
**• **To solubilize oils in water (and water in oils)
**• **To stabilize liquid films and to produce foams
**• **To stabilize emulsions
**• **To modify surfaces and interparticle interactions
**• **To facilitate spreading of liquids on surfaces (wetting)
**• **Other applications
**Surfactants in our daily life:**
**• **Food – mayonnaise, ice cream, milk, ...
**• **Industry – lubricants, stabilizers, emulsifiers, foamers, detergents, soaps ...
**• **Medicine – drugs, bio applications, ...
**• **Cosmetics – healthcare products ...
**• **Agriculture – aerosols, fertilizers ...
**• **Many other ...

## Properties of the surfactant film

**Properties of the surfactant film**

- H2O

- n-decane
- C10H22

- Surfactant

**Properties of the surfactant film change with:**
- • Molecular structure
- • Additives
- • Ionic strength
- • Co-surfactant
- • Temperature, pressure etc.

**Surfactant film**

**Why are the microemulsions so interesting:**
- • Thermodynamically stable, isotropic, and optically transparent solutions
- • *R* ~ 2 – 50 nm (good scatterers)
- • The curvature of the surfactant film can be controlled

**Properties of the surfactant film:**
- • Interfacial tension
- • Lateral elasticity
- • Spontaneous curvature
- • Bending elasticity
- • Saddle splay elasticity

## Why NSE?

**Why NSE?**

- H2O

- C10H22

- Surfactant

**Surfactant film**

**Diffusion:**
- • NMR
- • Dynamic light scattering
- (*t* scale > 100 ns)

- Shape fluctuations are in very short time and length scales!

## Experimental

**Experimental**

- Hydrophobic tail

- Hydrophilic head

**Surfactant molecule**

**AOT**

**Experiment I**
**Diffusion** of AOT micelles in C10D22
- (5.4 % vol. fraction)

**Experiment II**
**Shape fluctuations** in
- AOT/D2O/C10D22 microemulsion
- (5.4/4.6/90 % vol. fraction)

**Inverse microemulsion droplet**

- D2O

- C10D22

- AOT

**Inverse spherical micelle**

- C10D22

- ~ 25 AOT

- Translational
- diffusion

- Translational
- diffusion

- Shape
- fluctuations

## Shell contrast

- D2O
- (deuterated)

- C10D22
- (deuterated)

- AOT
- (hydrogenated)

**Experiment I  ****Experiment II**
- micelles     microemulsion
- Vol. fraction	0.054	0.1
- Avg. radius (Å)	15.9	34
- polydispersity	-	0.25
- SLD core (Å-2)	- 	6.410-6
- SLD shell (Å-2)	110-6	 1.610-6
- SLD solv. (Å -2)	6.510-6	6.510-6

- SLD (10-6 Å-2)
- --------------------------------------------
*n*-decane	-0.49
- H2O	-0.56
*d*-decane	  6.5
- D2O	  6.4

**Shell contrast**

- cross section (barn)
- H: 82.0	     D: 7.6

## Data analysis

## Summary of data analysis

**2 –  the damping frequency – **frequency of deformation**
- <|*a*|2> – mean square displacement of the 2-nd harmonic – **amplitude of deformation**
*p*2 – size polydispersity, measurable by SANS or DLS

## Why is NSE so “exotic”?

-  Shape fluctuations of lipid membranes and thin films
-  Intra-molecular and local segmental diffusion of proteins and polymers in solution
-  Intra- and inter- molecular dynamics of polymer melts and glasses
-  Other thermal fluctuations of soft matter
- •  The above phenomena produce energy transfer of *******E***** = 10****-5**** – 10****-2**** meV** (very small !!!)
- •  We need low energy neutrons. Cold neutrons: ** = 5 – 12 Å, *E* = 0.5 – 3.3 meV
- •  **The problem:** neutron beam wavelength spread
- ***/*** = 5 – 20%, *E/E *= 10 – 40%, *******E***** =  0.05 – 0.2 meV**
-     *******E***** =  0.05 – 0.2 meV**  **>>**  *******E***** = 10****-5**** – 10****-2**** meV**
- •  **The solution**: We need neutron precession in
- magnetic field. We are going to attach “internal”
- clock for each neutron. Thus, we can observe
- very small velocity changes of a neutron beam,
- regardless of the velocity spread

**Why is NSE so “exotic”?**

## Principle of NSE

**Principle of NSE**

***n***

***B***

***B***

**sample**

- Neutrons posses spin and magnetic moment. They precess in magnetic fields with the Larmor frequency that depends on the strangth of the magnetic field only. (*g* =1.83108 s-1T-1)

***B***

********L**

***S***

***N***

**!**

## Summary

- •  **NSE is a dynamic scattering method that ****yields the intermediate scattering function *****I*****(*****q*****,*****t*****). NSE has the highest energy and time resolution among the neutron scattering methods, which is achieved by using the neutron precession in magnetic fields as an “internal” clock. **
- •  **NSE is suitable for studies on:**
-  Brownian diffusion in micellar systems
-  Shape fluctuations of lipid membranes and thin films
-  Intra-molecular diffusion of proteins
-  Local segmental diffusion of polymers in solution
-  Intra- and inter- molecular dynamics of polymer melts and glasses
-  Other thermal fluctuations of soft matter etc (time scale: 0.01 – 200 ns)
- •  **Some limitations**:
-  The samples must produce strong scattering
-  Hydrogenated samples in deuterated matrix are the best choice
-  Samples must not be magnetic
-  The scattering should be in appropriate *Q*-range (0.02 < *Q *< 1.7 Å-1)

## Where to do NSE?

- •  There are NSE spectrometers in France, Germany and Japan
- •  NCNR is a user facility. Beam time proposals are accepted twice a year. Information is posted at: http://www.ncnr.nist.gov/
- •  Further reading on NSE: http://www.mrl.ucsb.edu/~pynn/

**Where to do NSE?**