92168.pdf

Liquid-gas and liquid-liquid interfaces 103

substrate) and temperature play an important part in determining the

nature of the film. Monolayers can be roughly classified as:

1. Condensed (solid) films, in which the molecules are closely packed
   and steeply orientated towards the surface.


2. Films which are still coherent but occupy a much larger area than
   condensed films. They have no real three-dimensional equivalent,
   since they act as highly compressible liquids. A number of distinct
   types of these expanded films have been recognised^21 , the most
   important being the liquid-expanded state, but these will not be
   considered in detail.


3. Gaseous or vapour films, in which the molecules are separate and
   move about the surface independently, the surface pressure being
   exerted on the barriers containing the film by a series of collisions.

Gaseous films

The principal requirements for an ideal gaseous film are that the
constituent molecules must be of negligible size with no lateral
adhesion between them. Such a film would obey an ideal two-
dimensional gas equation, 7r/4 = kT, i.e. the IT- A curve would be a
rectangular hyperbola. This ideal state of affairs is, of course,
unrealisable but is approximated to by a number of insoluble films,
especially at high areas and low surface pressures. Monolayers of
soluble material are normally gaseous. If a surfactant solution is
sufficiently dilute to allow solute-solute interactions at the surface to
be neglected, the lowering of surface tension will be approximately
linear with concentration - i.e.

y - y 0 - be (where b is a constant)

Therefore

TT ~ be

and dy/dc - -b

Substituting in the Gibbs equation

kT Ac