From: tomas_e@RELAX.Physchem.KTH.SE (Tomas Eriksson)
Subject: Re: Surfactants?
Reply-To: tomas_e@RELAX.Physchem.KTH.SE
Organization: Royal Institute of Technology, Stockholm
Date: Wed, 30 Sep 1992 19:36:28 GMT

In article <1992Sep28.183729.13216@col.hp.com>, chrism@col.hp.com (Chris Magnuson) writes:
>  My gen chem book is a bit vague on the topic of surfactants.  It does 
>say that surfactants reduce tension between molecules (lower viscosity) 
                                                        ^^^^^^^^^^^^^^^
>but it does NOT say how this is accomplished.  How are surfactants
>composed chemically that they can do this?

If your chemistry textbook say exactly what you wrote, the author 
doesn't know what he's talking about. (Unfortunately, this seems to be 
the case with a few textbook authors, and a majority of student-guide-
with-answers-to-questions-to-accompany-a-textbook authors...)

Let's assume that we have an aqueous solution with a little surfactant 
in it. A liquid prefers to minimize the size of it's surface. (That's why 
drops are roughly spherical; a sphere has the lowest surface-to-volume 
ratio.) To increase the size of the surface you need to supply energy 
(do work). The energy needed to create unit area of new surface is the 
surface tension. For pure water at room temperature this is about 
72 mJ/m^2 = 72 mN/m. This may seem very little, but it can have quite 
dramatical effect. (The surface tension of water is btw rather high - 
but not as impressive as those of liquid metals, such as Hg at room 
temperature.) A surfactant is a SURFace ACTive AgeNT. This means that 
the concentration of surfactant is much higher at the surface, i.e. at 
the aqueous solution/air interface than somewhere in the middle of the 
solution. This in turn lowers the surface tension; it's no longer as 
energetically "expensive" to form the surface, since there's a lot of 
surfactant molecules there (which prefer the surface), and not so many
water molecules. Surfactant molecules prefer the surface because they 
are amphiphilic. This means that different parts of the molecules have 
different preferences as to what kind of medium they like to be in, i.e. 
one hydrophilic (water-loving) part, and one hydrophobic (water-hating) 
part. The latter may be called lipophilic (oil-loving) instead.

A typical surfactant could be sodium stearate, i.e. soap:
								       -  +
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-COO Na

\-----------------------------------------------------------------/  ^
 The C17H35 part is long and hydrophobic			     |
								     |
The Na+ is dissociated in solution, and the carboxylic group with ---+ 
associated hydration water is the hydrophilic group

In solution, the long hydrocarbon chains of those stearate ions present 
at the air/water interface will be directed towards the air side of the 
interface, while the carboxylate groups will still be in the aqueous 
phase.

Viscosity is a different story, and at this level I think it's wise not 
to confuse things too much by discussing the correlation between 
viscosity and chemistry. But generally, by putting large molecules in 
water, you would expect the viscosity to increase.

Tomas
PhD student
-------------------------------------------------------------------------------
Tomas Eriksson                                          tomas_e@physchem.kth.se
            Surface Force Group, Department of Physical Chemistry,
               Royal Institute of Technology, Stockholm, Sweden