Interfacial properties of the hexane + carbon dioxide + water system in the presence of hydrophilic silica.

Abstract

Molecular dynamics simulations were conducted to study the interfacial behavior of the CO2 + H2O and hexane + CO2 + H2O systems in the presence of hydrophilic silica at geological conditions. Simulation results for the CO2 + H2O and hexane + CO2 + H2O systems are in reasonable agreement with the theoretical predictions based on the density functional theory. In general, the interfacial tension (IFT) of the CO2 + H2O system exponentially (linearly) decreased with increasing pressure (temperature). The IFTs of the hexane + CO2 + H2O (two-phase) system decreased with the increasing mole fraction of CO2 in the hexane/CO2-rich phase xCO2 . Here, the negative surface excesses of hexane lead to a general increase in the IFTs with increasing pressure. The effect of pressure on these IFTs decreased with increasing xCO2 due to the positive surface excesses of carbon dioxide. The simulated water contact angles of the CO2 + H2O + silica system fall in the range from 43.8° to 76.0°, which is in reasonable agreement with the experimental results. These contact angles increased with pressure and decreased with temperature. Here, the adhesion tensions are influenced by the variations in fluid-fluid IFT and contact angle. The simulated water contact angles of the hexane + H2O + silica system fall in the range from 58.0° to 77.0° and are not much affected by the addition of CO2. These contact angles increased with pressure, and the pressure effect was less pronounced at lower temperatures. Here, the adhesion tensions are mostly influenced by variations in the fluid-fluid IFTs. In all studied cases, CO2 molecules could penetrate into the interfacial region between the water droplet and the silica surface.