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, where as a positive W corresponds to oil lenses that do not wet water. A simple c alculation scheme is presented in this paper, based on a combination of an approximation of the Dzyaloshinskii-Lifshitz-Pitaevskii (DLP) theory and ei ther an adequate equation of state or refractive index measurements, to pre dict the wetting properties as a function of thermodynamic variables such a s temperature, pressure and fluid composition. This paper also presents ell ipsometry measurements of the wetting behavior of a propane/n-hexane binary system on water at 20 degrees C under varying pressure (controlled by the injection of the gaseous propane component into the ellipsometry cell). The se measurements indicate that this system displays the same wetting behavio r and goes through the same sequence of wetting transitions under increasin g pressures as those encountered for pure n-alkanes (e.g., n-pentane or n-h exane) under increasing temperatures. Namely, we observed upon increasing t he pressure (or the propane content in the system) a transition between the usual partial wetting state (at low pressures), and the so-called pseudo-p artial wetting state, in which oil lenses coexist with an oil film with thi ckness approximate to 100 Angstrom. A continuous (or critical) transition b etween this pseudo-partial wetting state and complete wetting occurs for a higher pressure corresponding to the vanishing of the Hamaker constant W. O ur expectation is that most condensates and volatile oils (or at least thos e mostly composed of alkanes) spread on brines under reservoir (high pressu res and temperature) conditions. (C) 1999 Elsevier Science B.V. All rights reserved.