Properties of Interfaces

Abstract

A phase modulated ellipsometer has been used to study adsorption at a vertical binary liquid/pyrex surface. When the ellipsometer was used in conjunction with a computer program which could calculate the reflection coefficients off any predefined dielectric profile it proved to be a very effective tool for surface studies. A pyrex surface was characterised first, and found to have a 10Å chemisorbed water layer on it which persisted even when the pyrex surface was in contact with various liquids, For very polar liquids, such as aniline, preferential alignment was found to occur at the pyrex surface giving rise to an anisotropic dielectric layer. A procedure was determined which gave a measure of the anisotropy; it was demonstrated on the aniline/pyrex surface. Critical mixtures of the binary liquids cyclohexane/methanol and cyclohexane/aniline were studied in contact with pyrex. The height and temperature dependence was deduced above and below the critical temperature (Tc). For the cyclohexane/methanol-pyrex system cyclohexane was depleted from the surface. Above Tc the adsorption followed a non-classical mean field profile. For the cyclohexane/aniline-pyrex system alignment of aniline at the pyrex surface below Tc was important. Above Tc complicated oscillations as a function of height and temperature were found for the amplitude ratio rp/rs, for the Brewster angle and for the relative intensity; these last observations are unexplained. A phenomenological theory of wetting has been developed, based on two premises. 1. The theory can be developed from equality of the chemical potentials. 2. Thickness and area variations at the wetting surface should be treated equally. Excellent quantitative agreement is obtained with the multitransitional observations of Beaglehole at the liquid/vapour surface of the binary liquid mixture cyclohexane/methanol plus a small quantity of water. The agreement only depends upon the weak conjecture that the hydrodynamic term reorders the relative stability of the solutions but does not affect their magnitudes. This theory raises questions as to the applicability of current mean field theory to liquid/vapour surfaces. Mean field theory does not incorporate capillary wave fluctuations and their correct incorporation is believed to significantly alter the behaviour.