Interfacial energies of clean mica and of monomolecular films of fatty acids deposited on mica, in aqueous and non-aqueous media

Abstract

The influence of a variety of media on solid/fluid interfacial energies has been measured by a cleavage technique. The solid used was mica, chosen because of its near perfect cleavage and ideal bulk properties. Solid/vapour and solid/liquid interfacial energies, γ_SV and γ_SL, were measured by cleaving specimens in the form of strips, first, in an atmosphere of the vapour and then with the specimens completely immersed in the corresponding liquid. Samples of mica coated with monomolecular films of fatty acids were constructed in such a way that separation took place between the oppositely oriented films. Polar and non-polar liquids and vapours were used in these experiments. The results allow an investigation into the validity of Young's equation for contact angles which are zero or positive. For well-behaved systems in which no adsorption takes place during the cleavage, the relation is valid. For fatty acid films in an aqueous environment, it is necessary to introduce an additional term into the relation to preserve the equality, thus γ_SL=γ_SV–γ_LV cos θ+γ_H·. The energy γ_H has been associated with entropic effects occurring in the liquid phase. Its existence shows that there are measurable anisotropies in the water. The results can be explained if a single layer of the water in the immediate vicinity of the hydrophobic interface, is assumed to become preferentially oriented.

Model-building experiments have been used to estimate the number of water molecules involved in the interaction and a value of 213 cal/mol of oriented water was obtained for the energy involved. This allows one to predict values for the free energy of solution of small hydrophobic molecules in water, which are in good agreement with those of Frank and Evans. The effect disappears in a concentrated solution of urea indicating that the structure of the liquid near the interface is, in this case, effectively the same as it is in bulk, i.e., the urea has caused a breakdown in the ordered structure in the neighbourhood of the interface. This is consistent with current views of the denaturant action of urea on proteins and other bimolecules.