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. In this sequentialwetting scenario, there occurs a first-order transition from a partial-wetting state, in which only a microscopically thin film of adsorbate is present on the substrate, to a 'frustrated complete wetting state' characterized by a mesoscopically, but not yet macroscopically thick wetting film. At higher temperatures, one observes a continuous divergence of the film thickness and finally, at the critical-wetting temperature, the complete-wetting state, featuring a macroscopic film thickness, is reached. This sequence of two transitions is brought about by an interplay of short-range and long-range interactions between substrate and adsorbate. The critical wetting transition is controlled by the longrange forces and is, thus, found by determining where the Hamaker constant, as calculated from a Dzyaloshinskii-Lifshitz-Pitaevskii-type theory, changes sign. The first-order transition involves both short-range and long-range forces and is, therefore, more difficult to locate. While the pentane/water system is well understood in this respect by now, a detailed theoretical description of the hexane/brine system is hampered by the a priori unknown modification of the interactions between substrate and adsorbate upon the addition of salt. 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 * E-mail: volker.weiss@fys.kuleuven.ac.be 1 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 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. Once this radius has been determined, the first-order transition temperatures can be calculated from the dielectric properties of the five media. Our results for these temperatures are in good agreement with the experimental ones.

Wetting of a solid substrate by a “civilized” model of ionic solutions

“…A phenomenological interpretation of experimental ellipsometry data on wetting of hexane on brine reveals ionic effects of the aqueous substrate on the interfacial phase transitions, which can be explained in terms of the interplay between long-range dispersion forces and the screened Coulombic interactions generated by the substrate. 11 In the present paper we address the problem of the influence of dissolved ions on the wetting transitions of a model polar solvent in contact with a charged planar substrate. Apart from their obvious entropic contribution to the surface free energy of the solution, the electrostatic interactions between the ions and the polar molecules, and with the charged substrate, are expected to significantly affect the adsorption profile of the solution through the formation of electric double layers at the wall/liquid and liquid/vapor interfaces.…”

Nonclassical Surface Nucleation of 6CB at the Air–Liquid Interface of a 6CB Oil-in-Water Nanoemulsion

“…As far as we are aware, the coalescence of a nano-oil droplet with the air–water surface from a nanoemulsion has never been considered as an initiator of surface nucleation. Instead, prior experimental ,,, and theoretical − studies have primarily dealt with wetting transitions in oil–water systems , and how wetting transitions are influence by the presence of salt ,− , or how salt in aqueous solutions influences the interfacial tension, bulk phase diagram, , or surface phase diagram …”