Surface energetics of the wetting of a hydrophobic powder

Abstract

The contact angles, θ, of pure liquids and of ethanol + water mixtures on griseofulvin powders have been determined from the rates of liquid permeation through powder beds. The associated free energies of adhesion and immersion (Δ_iG) were negative. The corresponding enthalpies of immersion (Δ_iH) were measured calorimetrically and found to be endothermic. The entropies of immersion (Δ_iS), derived from the above results, were positive. Ethanol molecules were shown to be absorbed preferentially from ethanol + water mixtures. The variation of cos θ and of the corresponding thermodynamic quantities with mol fraction of ethanol in ethanol + water mixtures is novel and correlates directly with the known thermodynamic properties of the liqud phase and are interpreted in terms of the intermolecular interactions and the degree of order within the liquid and at the solid–liquid interface. Extrapolation of cos θ and Δ_iH of ethanol + water mixtures to the pure water values afforded θ≈ 100°, a small positive Δ_iG value and an appreciable negative Δ_iH value which was compensated by a negative Δ_iS value. The corresponding values for wetting by hydrocarbons were of opposite sign. The thermodynamic data indicate that griseofulvin is a member of a new class of surface for which the present models for calculating the solid–vapour interfacial free energy and its polar and dispersion components do not apply.