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

Molecular dynamics simulations were performed to understandtheinterfacial properties of brine (up to 5.4 mol/kg NaCl) and brine+ silica systems in the presence of CO2, hexane, and theirequimolar mixture under geological conditions. Simulation resultsof brine + CO2, brine + hexane, and brine + CO2 + hexane systems agree reasonably well with the theoretical resultspredicted using the density gradient theory based on the cubic-plus-associationequation of state (with Debye-Hu''ckel electrostatic term).In all these systems, the interfacial tension (IFT) increases linearlywith increasing NaCl concentration. Here, simulated slopes of theNaCl concentration dependence of IFT are about 1.99 mN/(m mol kg(-1)), under all conditions. We observe a negative surfaceexcess for NaCl, which may explain the increase in IFT with increasingNaCl concentration. The contact angle (CA) of H2O + CO2 + silica and brine + CO2 + silica systems increaseswith pressure and decreases with temperature. However, the CA of H2O + hexane + silica and brine + hexane + silica systems isnearly independent of temperature and pressure. These CAs are notsignificantly affected by the presence of CO2. An importantresult is that in all investigated systems, the CA increases withincreasing salt content. Our simulated CA is in the ranges of 51.4-95.0 & DEG;,69.1-86.0 & DEG;, and 72.0-87.9 & DEG; for brine + CO2 + silica, brine + hexane + silica, and brine + CO2 + hexane + silica systems, respectively. The density profiles indicatethat the positively charged hydrogen atom of the surface hydroxylgroup attracts Cl- ions to the surface. In all investigatedsystems, the adhesion tensions decrease with increasing NaCl concentration.