Electrical Double Layer Structuring of Alkali Metal Bromide Solutions at the Mica-Water Interface

Abstract

The electrical double layer (EDL) is an assembly of structured ions and solvent molecules at a solid-liquid boundary resulting from the screening of surface charge by an electrolytic medium. In natural systems, the EDL mediates crucial (bio)geochemical reactions at mineral-water interfaces, affecting the development of soils through associations with organic matter, the fate and transport of environmental contaminants, and microbial community composition. Traditional models of the EDL are predicated upon the assumption that the surface is in contact with dilute aqueous phase, neglecting the Coulombic forces generated by ion-ion interactions. To refine conceptualizations of the high-salinity EDL structure, we conduct molecular dynamics (MD) simulations and X-ray reflectivity (XRR) experiments which evaluate the distribution of alkali metal bromide electrolytes (Na-, K-, Rb-, and CsBr) at the muscovite mica (001)-water interface. We employ a novel set of MD simulation force field parameters that are optimized to describe the physicochemical properties of concentrated solutions by scaling down the effective charge of ions. Concurrently, we utilize an element-specific resonant anomalous XRR method to directly visualize Rb\(^{+}\) and Br\(^{-}\) structuring at the mica-water interface. Scaled-charge-parameterized MD simulations predict the reversal of negative surface charge 2-6 Å from the interface by a layer of cation outer-sphere surface complexes in high salinity systems. This overcharging phenomenon has been experimentally observed, and necessitates the adaptation of non-classical EDL models for describing concentrated solutions. Advances in understandings of the high-salinity EDL structure has implications for the development of modern environmental technology to support desalination efforts and geological carbon sequestration in hypersaline brines.