Out-of-plane permittivity of nanoconfined geofluids: a molecular dynamics study

In fluid-driven mineral replacement reactions, reaction-induced porosity generation is one of the most important mechanisms that provide fluid pathways into the reaction front. Our multi-scale electron microscopic imaging techniques show that majority of these generated pores are at the nanometer scale. Due to the large surface-to-volume ratios, fluids confined in this nanoporous rock network demonstrate new properties. For example, the permittivity of nanoconfined water is no longer a scalar quantity like bulk and shows a tensorial behavior because of the anisotropy of the system perpendicular to the wall direction. In the present study, we used molecular dynamics (MD) simulations to investigate the out-of-plane permittivity of water confined between two mineral slabs at elevated pressures and temperatures. We explored the rock material effect by simulating calcite, brucite, and quartz as confining minerals. In all cases, with the reduction of the width of the nanochannel the out-of-plane permittivity of water declines. Thus, in smaller nanopores (φ<10 nm) there is a significant difference between the permittivity of nanoconfined and bulk water that can have a profound impact on mineral solubility, aqueous speciation, and fluid transport properties in nanoporous metamorphic rocks. Although a rise in the out-of-plane permittivity is seen through the decreasing temperature or increasing pressure, its value in all cases is much lower than the bulk water dielectric constant in similar conditions.