(698i) Predicting Contact Angle in Oil/Brine/Calcite Systems Using Molecular Simulations

The contact angle is commonly used to characterize the wettability of a solid surface. It reflects the spreading tendency of a liquid over the solid surface (here calcite) in the presence of another immiscible liquid (here a model oil). In this study, we employed molecular dynamics simulations to determine the solid/liquid and liquid/liquid interfacial tensions in an oil/brine/calcite system. We applied the test area method to calculate the interfacial free energies and then used these quantities in Young’s equation to obtain the contact angle. Two different salts (sodium chloride and calcium chloride) at 5 different concentrations between 0.145 M and 4.98 M were studied. The oil phase was modeled as a 14%:86% (by weight) mixture of toluene and n-dodecane. Molecular dynamics simulations were also performed with a model in which a mixture of oil and brine is trapped in a calcite nano-pore, and the contact angle was determined by tracing the oil/brine interface profile.The wettability alteration from less to more water-wet scenarios as a function of ion types and concentrations were captured by both models. The radial distribution function of the ions shows that at low concentration these ions preferentially accumulate at the interface, whereas at high concentration they tend to stay in the bulk. Based on our findings, the accumulation of ions at the interface increases the interactions between the brine and the solid, leading to a high interfacial energy which subsequently alters the wettability scenario from less to more water wet.