The interface between a liquid and its vapour.

The concentration profile in a multilayer model of a system containing two immiscible fluid phases is given by the solution of a non-linear, second-order difference equation. The surface excess thermodynamic functions of the system can be evaluated only if this solution is known. Assuming that the solution is symmetric, attempts to obtain an exact solution using a method described by Ono were unsuccessful, but a numerical method is described which enables approximate solutions of any desired accuracy to be obtained. The possible existence of asymmetric solutions has been examined. Symmetric solutions have also been obtained by methods that do not require any explicit assumption of symmetry.