Analysis of the Effects of Silica Surface Structure on the Local Diffusion and Hydrogen Bonding of Water via Molecular Dynamics.

Water near solid-liquid interfaces is well known to exhibit structural and dynamical properties distinct from those of the bulk, owing to strong interactions with the solid surface. While water dynamics near the interface have been quantified, a direct atomistic link between spatially heterogeneous surface-water hydrogen bonding and local diffusion remains elusive. Here, using classical molecular dynamics simulations of water confined by silica, we examine how surface topology and silanol distributions modulate the local structure and diffusion of water. To characterize the structure of water near the interface, water density profiles and spatial density maps were analyzed. The results show that multiple adsorbed water layers form on crystalline silica surfaces, whereas no clearly defined adsorbed water layers are observed on amorphous silica surfaces. The diffusion properties of water were evaluated by calculating the local self-diffusion coefficient in the direction parallel to the interface. The results indicate that the local self-diffusion coefficient decreases as the distance from the wall decreases. To further clarify the diffusion behavior of water near the interface, we introduced a spatially resolved local diffusivity (LD) metric. The LD analysis reveals a pronounced suppression of water mobility that strongly and negatively correlates with the local probability density of silanol-water hydrogen bonds (R ≈ -0.8 for the crystalline system and R ≈ -0.6 for the amorphous system).