Surface energetics of the wetting of a hydrophobic powder

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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 (ΔiG) were negative. The corresponding enthalpies of immersion (ΔiH) were measured calorimetrically and found to be endothermic. The entropies of immersion (ΔiS), 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 ΔiH of ethanol + water mixtures to the pure water values afforded θ≈ 100°, a small positive ΔiG value and an appreciable negative ΔiH value which was compensated by a negative ΔiS 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.

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