Effect of wall-solution van der Waals interaction and surface charge density on slip boundary conditions of aqueous electrolyte solution

Molecular dynamics simulations are employed to investigate the Poiseuille flow of NaCl aqueous solution through nanochannels with generic solid walls, with a specific focus on the coupled effects of wall-solution van der Waals interaction (WSI_VdW) energy and surface charge density (δ) on interfacial slip behavior. The results reveal that the contact density (ρc) exhibits a non-monotonic dependence on the δ, characterized by an initial decrease followed by an increase at sufficiently large δ. At low δ, both the fluid velocity and slip length (Ls) are significantly larger under low WSI_VdW energy than under high WSI_VdW energy. This disparity, however, diminishes rapidly with increasing δ. Beyond a critical δ value, the velocity and Ls exhibit minimal variation with further increases in δ and become insensitive to the WSI_VdW energy. The underlying mechanism is elucidated by analyzing the normalized in-plane structure factor S(G1)/S(0). Furthermore, the correlation between slip length and S(G1)/S(0), ρc, and δ is established. It is demonstrated that across a wide range of WSI_VdW energies and δ values, Ls can be accurately described by a single-variable function dependent on S(G1)/S(0), ρc, and δ. This study provides valuable theoretical insights for modeling interfacial slip boundary conditions in electrolyte flows through charged nanochannels.