Searching for a Correlation between Interfacial Liquid Structure and Hydrodynamic Slip: A Case Study of Water/Silica...

Hydrodynamic slip at the solid-liquid interface is governed by molecular-scale interactions; however, establishing a direct correlation between interfacial liquid structure and slip behavior is challenging due to the difficulty of isolating nanoscale interfacial structures of water at the solid surface from the bulk water structure. Here, we investigate the relationship between hydrodynamic slip and interfacial water structure at the silica-water interface as a function of NaCl concentration. Vibrational sum-frequency generation (SFG) spectroscopy was used to probe interfacial water structure by deconvoluting Stern-layer and diffuse-layer contributions in the SFG response using the maximum entropy method, and liquid-cell atomic force microscopy was used to quantify the hydrodynamic slip length. Supported by molecular dynamics simulations that resolve molecular orientation and density profiles of water near the silica surface, we identify the distinct hydrogen-bonding configuration and orientation of interfacial water molecules. In the low ionic strength regime (≤10^-5 M NaCl), there are no significant changes in both SFG spectral features and slip length. In contrast, in the high ionic strength regime (>10^-5 M NaCl), significant changes in the water orientation in the Stern layer can be correlated with a reduction in slip length. These results indicate that hydrodynamic slip at hydrophilic, charged interfaces is governed not simply by ion presence but by ion-driven alteration of interfacial water structure.