Interfacial Properties of the Hexane + Carbon Dioxide + Brine System in the Presence of Hydrophilic Silica

Molecular dynamics simulations were performed to understand the interfacial properties of brine (up to 5.4 mol/kg NaCl) and brine + silica systems in the presence of CO2, hexane, and their equimolar mixture under geological conditions. Simulation results of brine + CO2, brine + hexane, and brine + CO2 + hexane systems agree reasonably well with the theoretical results predicted using the density gradient theory based on the cubic-plus-association equation of state (with Debye–Hückel electrostatic term). In all these systems, the interfacial tension (IFT) increases linearly with increasing NaCl concentration. Here, simulated slopes of the NaCl concentration dependence of IFT are about 1.99 mN/(m mol kg–1), under all conditions. We observe a negative surface excess for NaCl, which may explain the increase in IFT with increasing NaCl concentration. The contact angle (CA) of H2O + CO2 + silica and brine + CO2 + silica systems increases with pressure and decreases with temperature. However, the CA of H2O + hexane + silica and brine + hexane + silica systems is nearly independent of temperature and pressure. These CAs are not significantly affected by the presence of CO2. An important result is that in all investigated systems, the CA increases with increasing salt content. Our simulated CA is in the ranges of 51.4–95.0°, 69.1–86.0°, and 72.0–87.9° for brine + CO2 + silica, brine + hexane + silica, and brine + CO2 + hexane + silica systems, respectively. The density profiles indicate that the positively charged hydrogen atom of the surface hydroxyl group attracts Cl– ions to the surface. In all investigated systems, the adhesion tensions decrease with increasing NaCl concentration.