Page 107 - Introduction to Colloid and Surface Chemistry

Liquid-gas and liquid-liquid interfaces 97

molecule in thickness with the hydrophilic -COOH or -OH groups
orientated towards the water phase and the hydrophobic hydrocarbon
chains orientated away from the water phase.
These insoluble monomolecular films, or monolayers, represent an
extreme case in adsorption at liquid surfaces, as all the molecules in
question are concentrated in one molecular layer at the interface. In
this respect they lend themselves to direct study. In contrast to
monolayers which are formed by adsorption from solution, the
surface concentrations of insoluble films are known directly from
the amount of material spread and the area of the surface, recourse to
the Gibbs equation being unnecessary.
The molecules in a monomolecular film, especially at high surface
concentrations, are often arranged in a simple manner, and much can
be learned about the size, shape and orientation of the individual
molecules by studying various properties of the monolayer. Mono-
molecular films can exist in different, two-dimensional physical
states, depending mainly on the magnitude of the lateral adhesive
forces between the film molecules, in much the same way as three-
dimensional matter.

Experimental techniques for studying insoluble monolayers

Surface pressure

The surface pressure of a monolayer is the lowering of surface tension
due to the monolayer - i.e. it is the expanding pressure exerted by the
monolayer which opposes the normal contracting tension of the clean
interface, or

TT - JQ - y (4.14)

where y 0 is the tension of clean interface and y is the tension of
interface plus monolayer.
The variation of surface pressure with the area available to the
spread material is represented by a ir-A (force-area) curve. With a
little imagination, IT-A curves can be regarded as the two-
dimensional equivalent of the p-V curves for three dimensional
1
matter. (N.B. For a 1 nm thick film, a surface pressure of 1 mN m" is
6 2
equivalent to a bulk pressure of 10 N m~ , or ~ 10 atm.)
The Langmuir-Adam surface balance (or trough) uses a technique