Electric Field Effects on Water and Ion Structure and Diffusion at the Orthoclase (001)–Water Interface

Abstract

Understanding the electrochemical properties of mineral−water interfaces tends to rely upon electrical double layer (EDL) models, but these models are based on the assumption that electrostatic equilibrium is constantly maintained. In reality, interfacial reactions, ion diffusion, and their electrochemical signatures are based in nonequilibrium conditions of locally or globally imbalanced electrical fields where current EDL models have limited purview. Here, we performed molecular dynamics (MD) simulations of the orthoclase (001) surface in contact with a 1 M NaCl aqueous solution under various electric fields, to explore the interplay between EDL structure and dynamics when perturbed by electric fields of different directions and strengths, by confinement, and by different distributions of structural surface charge. The simulations showed that confinement between two opposing (001) surfaces led to the development of an induced field when the applied field was perpendicular to the surfaces and, as a result, to ionic diffusion coefficients that were independent of electric field strength. In contrast, when the applied field was parallel to the surfaces, confinement resulted in ionic diffusion coefficients that were more strongly dependent on the magnitude of the electric field than in bulk water. Differences in the density and distribution of aluminol groups on the two surfaces had a significant impact on how the interfacial structure and dynamics varied in the presence of an electric field. Notably, these differences resulted in an electro-osmotic flow with opposite directions at the two surfaces under parallel applied electric field. Overall, the MD simulations highlighted the importance of considering atomic-level structure and heterogeneities when developing models of the electrochemical properties of mineral−water interfaces.

Quantifying the Impact of Electric Fields on the Local Structure and Migration of Potassium Ions at the Orthoclase (001) Surface

“…Here, we report a DFT-based modeling study of electric field effects on potassium cation (K + ) diffusion dynamics along vacuum and hydrated surfaces of the common potassium feldspar mineral orthoclase (KAlSi 3 O 8 ). By focusing exclusively on the Stern layer, this work is complementary to recent molecular dynamics simulations performed by Kerisit et al in which field-driven ion mobilities at the stable orthoclase (001) facet in contact with a 1 M NaCl aqueous solution were examined. While the faster mobilities of fully hydrated outer-sphere Na + /K + ions are more consistent with those that would typify cations in the diffuse layer, the slower process of field-driven displacement and diffusion of partially hydrated Stern layer cations along the surface was not specifically addressed, despite its potential importance to storage of electrical polarization energy and the subsequent relaxation response of the EDL when the field is removed.…”

“…Computational molecular simulations have begun to provide insight into electric field effects on EDL ions at mineral/water interfaces. Simulations based on classical force field molecular dynamics (MD) have proven especially valuable given their ability to efficiently model water and ion dynamics at nanometer scales under applied electric fields, including for quartz, , clays, and feldspar surfaces. , However, these simulations typically rely on less-expensive non-polarizable, non-reactive force fields to build a response to applied electric fields. They furthermore cannot account for the electronic structure at the mineral/water interface.…”

Terahertz electric field induced melting and transport of monolayer water confined in double-walled carbon nanotubes

“…We divide our results into two parts, and in the first part we change the field strength from 0.1 to 2.0 V nm À1 at selected frequencies; then, we vary the frequency from 0 to 30 THz at selected field strengths. Similar to most of the existing simulation studies, [26][27][28][29][30][31] the electric field is applied to the whole system, and the electric shielding effect of CNTs is not considered. This is because in classical MD simulations it is still difficult to consider the electric shielding effect of CNTs.…”

“…This is because in classical MD simulations it is still difficult to consider the electric shielding effect of CNTs. Actually, in most of the existing simulation studies, [26][27][28][29][30][31] the CNTs are modeled by uncharged Lennard-Jones (LJ) particles for nanofluidic transport, where the metallic properties are ignored. For a more accurate calculation, one may use the DFT method, 32 but the simulation system size will be very limited.…”

“…15,26 MD simulations demonstrated that the applied electric field shows anisotropy for the structure and dynamics of ion hydration shells. 27,28 Moreover, the electric fields can enhance thermophoresis and alter the configuration of interfacial water molecules, changing them from a disordered state to an ordered network structure. 29 Similar to static electric fields, terahertz electric fields also can effectively regulate the structure and dynamics of confined water molecules.…”