Effect of ion hydration on the first-order transition in the sequential wetting of hexane on brine

Abstract

In recent experiments, a sequence of changes in the wetting state (`wetting transitions') has been observed upon increasing the temperature in systems consisting of pentane on pure water and of hexane on brine. This sequence of two transitions is brought about by an interplay of short-range and long-range interactions between substrate and adsorbate. In this work, we argue that the short-range interaction (contact energy) between hexane and pure water remains unchanged due to the formation of a depletion layer (a thin `layer' of pure water which is completely devoid of ions) at the surface of the electrolyte and that the presence of the salt manifests itself only in a modification of the long-range interaction between substrate and adsorbate. In a five-layer calculation considering brine, water, the first layer of adsorbed hexane molecules, liquid hexane, and vapor, we determine the new long-range interaction of brine with the adsorbate {\em across} the water `layer'. According to the recent theory of the excess surface tension of an electrolyte by Levin and Flores-Mena, this water `layer' is of constant, i.e.\ salt-concentration independent, thickness $δ$, with $δ$ being the hydrodynamic radius of the ions in water. Our results are in good agreement with the experimental ones.