Structure and Dynamics of Water and Ions at Quartz (101) and (001) Surfaces under Applied Electric Fields from Molecular Simulations

Abstract

Electrical double layer (EDL) models are commonly adopted as a framework for understanding electrokinetic properties at mineral-fluid interfaces but the dynamics of ion and water mobilities are typically not well known. Extending the previous work performed at equilibrium conditions, here it is examined how applied electric fields induce mobilities of monovalent and divalent ions at hydroxylated quartz (001) and (101) interfaces with various electrolyte solutions (NaCl, KCl, and CaCl₂). The simulations reveal how the diffusion coefficients depend on the orientation and magnitude of the applied electric field, with a particularly strong effect for fields applied parallel to the quartz surfaces. While the effect in perpendicular applied fields is more subtle, nonetheless the disruption of the water in the first layers at the surface with corresponding effects on wettability is observed. The details of EDL ion drift mobilities are found to be strongly correlated to the silanol density and crystallographic orientation at the interface. The findings shed light on the complex interplay between local and external forces affecting how these interfaces respond in applied field applications that include electrical impedance spectroscopy, electroosmotic flow, and ζ -potential measurements.