Dynamics of Water Monolayers Confined by Chemically Heterogeneous Surfaces: Observation of Surface-Induced Anisotropic Diffusion

Water present in confining geometries plays key roles in many systems of scientific and technological relevance. Prominent examples are living cells and nanofluidic devices. Despite its importance, a complete understanding of the dynamics of water in nanoscale confinement remains elusive. In this work, we use molecular dynamics (MD) simulation to investigate the diffusive dynamics of water monolayers confined in chemically heterogeneous silica slit pores. The effect of chemical heterogeneity is systematically investigated through the fraction f_SiOH of randomly distributed surface sites that possess hydroxyl functional groups. Partial hydroxylation results in heterogeneous surfaces comprising nanoscale hydrophobic and hydrophilic regions. We find that the in-plane diffusivity of water increases monotonically with f_SiOH; at low surface hydroxylation (f_SiOH ≤ 50%), slow water dynamics arise due to the formation of icelike structures in the hydrophobic regions, while at f_SiOH ≥ 75%, surface-water H-bonds in the hydrophilic regions result in faster dynamics. We show that surface patterning with ordered hydrophobic and hydrophilic "stripes" can be used to induce one-dimensional diffusion, with water diffusing through the slit pore preferentially along the direction of the hydrophilic surface patterns.