Calculation of the surface pressure and the contact energy in the adsorption of alkanes on water

Abstract

In an attempt to complete the theoretical description of the complex wetting of alkanes on aqueous substrates, the surface pressure in the adsorption of pentane, hexane, and heptane on water is calculated as a function of the bulk gas pressure using a two-dimensional van der Waals equation in conjunction with Hirasaki’s expression for the Henry’s law constant. The contact energy as defined within Cahn theory is subsequently deduced using Dobbs’ modified version of this theory and employed to calculate the wetting behavior of the above-mentioned alkanes on water. The experimentally observed sequence of two changes in the wetting state, namely a first-order transition between a microscopically thin and a mesoscopically thick film, followed by a continuous divergence of the film thickness, is found within the theory as well. While the critical wetting transition is brought about by long-range forces and can be described by Dzyaloshinskii–Lifshitz–Pitaevskii theory, the first-order thin–thick transition involves the short-range interactions between substrate and adsorbate. In the absence of a complete theory for the latter, they are accounted for in an effective way in the present approach, which relies only on the macroscopic bulk properties of the different media involved and which leads to satisfactory results for the transition temperatures.