Wetting of hydrocarbon mixtures on water under varying pressure or composition

The wetting/spreading properties of oil on water in the presence of gas are related to the Hamaker constant W of the water/oil/gas system: a negative W corresponds to a stable thick (macroscopic) oil film wetting water, whereas a positive W corresponds to oil lenses that do not wet water. A simple calculation scheme is presented in this paper, based on a combination of an approximation of the Dzyaloshinskii–Lifshitz–Pitaevskii (DLP) theory and either an adequate equation of state or refractive index measurements, to predict the wetting properties as a function of thermodynamic variables such as temperature, pressure and fluid composition. This paper also presents ellipsometry measurements of the wetting behavior of a propane/n-hexane binary system on water at 20°C under varying pressure (controlled by the injection of the gaseous propane component into the ellipsometry cell). These measurements indicate that this system displays the same wetting behavior and goes through the same sequence of wetting transitions under increasing pressures as those encountered for pure n-alkanes (e.g., n-pentane or n-hexane) under increasing temperatures. Namely, we observed upon increasing the pressure (or the propane content in the system) a transition between the usual partial wetting state (at low pressures), and the so-called pseudo-partial wetting state, in which oil lenses coexist with an oil film with thickness ≈100 Å. A continuous (or critical) transition between this pseudo-partial wetting state and complete wetting occurs for a higher pressure corresponding to the vanishing of the Hamaker constant W. Our expectation is that most condensates and volatile oils (or at least those mostly composed of alkanes) spread on brines under reservoir (high pressures and temperature) conditions.