PNNL

Abstract

Mobile ions at mineral-water interfaces can respond to an applied electric field, adopting a new distribution effectively representing a polarization of the electrical double layer. When the field is released, ions relax to their distribution at equilibrium. In both cases the dynamics are characteristic of the interface. However, current models of electrokinetic phenomena are not sufficiently robust to accurately predict collective ion dynamics at structurally and chemically specific mineral-water interfaces. In this study we use electrostatic force microscopy (EFM) to investigate the dynamics of ion relaxation at hydrated calcite (104) surfaces at controlled relative humidity (RH). Electrically biased probes are used to polarize distributions of calcium and carbonate ions intrinsic to this interface over a range of RH values. Polarization kinetics are tracked by monitoring the tip-sample force gradient during charging, and EFM imaging is used to characterize the spatial relaxation dynamics after the applied field is released. Electrostatic finite element modeling of the sample/probe system across length-scales from nanometers to millimeters reproduces the observed stretched exponential charging response. Together, the results allow us to estimate the ion diffusivities at the interface across a wide range of RH values. These diffusivities increase by roughly five orders of magnitude as the RH is increased from 5% to 90%, highlighting the critical role of adsorbed water for surface ion solvation that enables ion mobility.