Introduction

Polymers spread at the interface between two fluid media are of importance in a number of areas, liquid— liquid extraction, stabilisation of emulsions and as model membrane systems being some examples. By far the most extensive body of work reported has been for cases in which one of the fluid media is air, the liquid phase generally being water. Polymers spread at the air/water interface can be viewed as pseudo-two-dimensional systems; thus there is fundamental interest in ascertaining whether theory can be adapted to explain the observations. One practical advantage of spread polymer films is that film compression leads to an increase in surface concentration, so it is relatively easy to change this quantity experimentally.

Surface pressure isotherms: classification, theory and scaling laws

The surface pressure (π) is the difference in surface tension between that of the pure sub-phase liquid surface and that when the surface is covered by a film. Surface pressure isotherms for materials of low relative molecular mass are values of π plotted as a function of the area of surface available per molecule. For spread films of materials of low relative molecular mass, the surface pressure isotherm may display a number of transitions as the area available is decreased.