Investigation of equilibrium wetting films of n-alkanes on α-alumina

Abstract

A review is given of previous disjoining pressure investigations of liquid films on solids, and the advantages of combining such investigations with corresponding vapour adsorption studies are discussed. The relevant thermodynamic framework is presented in outline. Direct measurements of the disjoining pressure of stable wetting films of n-octane and n-decane on α-alumina are reported for films of from 20–80 nm in thickness. The results obtained, together with data for much thinner films derived from a previous gravimetric study of the vapour adsorption of n-decane on the oxidised surface of aluminium foil, are compared with the predictions of Lifshitz and London–Hamaker theories of dispersion forces. It is found that both theories correctly predict the disjoining pressure and differential free energy of formation of the films, provided the films exceed monolayer coverage, and provided due allowance is made for retardation if the films exceed a thickness of about 5 nm. In making these predictions, it is sufficient to assume that the films have molar volumes, molar entropies and dielectric properties identical with those of the parent bulk liquids.