Salinity-Induced Suppression of Interfacial Activity and Anisotropic Water Diffusion in Nanoscale Slits: Insights from Molecular Dynamics Simulations

Understanding the coupled effects of salinity and nanoconfinement on surfactant behavior is paramount for optimizing enhanced oil recovery (EOR) in deep-sea and tight reservoirs. Herein, we employ all-atom molecular dynamics simulations to investigate the interfacial structure and dynamics of a betaine-type zwitterionic surfactant at an oil-water interface confined within a 10 nm slit, under varying NaCl concentrations (0.5, 1.0, 2.0, and 3.0 M) mimicking real deepwater tight-reservoir conditions, with oil confined in nanoscale rock pores. Our results reveal that the surfactant significantly thickens the interface and reduces water diffusivity in both parallel and perpendicular directions. Increasing the salinity partially suppresses the surfactant-induced interfacial broadening. The interfacial thickness exhibits a nonmonotonic dependence on NaCl concentration, showing a reproducible local minimum at 0.5 M and only modest variations across 1.0-3.0 M, while remaining substantially larger than the surfactant-free reference interface. This suppression is attributed to strong electrostatic interactions between the ions and the surfactant headgroups. Concurrently, the orientational order parameter of the surfactant tails decreases with increasing salinity. Crucially, water diffusion exhibits marked anisotropy: lateral diffusion is strongly hindered by high salinity, with a decreasing diffusion rate for higher salinity. In contrast, vertical diffusion remains largely insensitive to salinity with its magnitude primarily governed by geometric confinement, highlighting the dominant role of geometric confinement over solute effects in the normal direction. These findings provide atomic-level insights into the failure mechanisms of surfactant flooding in high-salinity reservoirs and design principles for next-generation EOR agents that can withstand the dual challenges of ion screening and nanoconfinement.