Probing migration of ions at the oil-water interface

In low-salinity water flooding, the migration of ions near the interface, including the convection, diffusion, adsorption, desorption at the interface and the transfer across the interface, has significant influence on interfacial properties, such as the interfacial tension, viscosity and electric properties, accordingly affecting the stability of crude oil emulsions and the oil displacement. However, detailed microscopic knowledge of how ions migrate in the bulk phase or the interface is still scarce. Here, we systematically study the migration of monovalent ions near the decane-water interface under different ionic concentration, temperature and applied electric field strength by using molecular dynamics simulations. The results show that with ionic concentration increasing, interfacial tension increases positively. With temperature rising, interfacial tension decreases. The chloride ions which have weaker hydration possess higher diffusion rate. The ion-pair at the interface is the dominated factor for the transition of interfacial diffusion coefficients of ions, which has a significant impact on the interfacial thickness and the residence time of molecules in the interface, thus affecting the interfacial tension as well as other interfacial properties. The transfer of ions across the interface would happen only when the strength of applied electric field reaches a critical value. The transfer of ions across the interface is dominated by the hydrogen bonding and interaction between ions at the interface. The formation of water channel provides a passage for ion transfer through the oil phase. These results will be helpful to determine the optimal salt concentration in the enhanced oil recovery techniques.