Field exposed water in a nanopore: liquid or vapour?

TL;DR

This study uses molecular simulations to explore how electric fields influence water behavior in nanopores, revealing classical electrostriction effects and the thermodynamic mechanisms behind phase stability and transitions.

Contribution

It provides new insights into the thermodynamic causes of electric field-induced water expansion and contraction at the nanoscale, challenging recent theoretical predictions.

Findings

Electric fields can destabilize the aqueous liquid phase in confined water.

Classical electrostriction explains electrowetting behavior in nanopores.

Field-induced perturbations of water structure are relatively weak.

Abstract

We study the behavior of ambient temperature water under the combined effects of nanoscale confinement and applied electric field. Using molecular simulations we analyze the thermodynamic causes of field-induced expansion at some, and contraction at other conditions. Repulsion among parallel water dipoles and mild weakening of interactions between partially aligned water molecules prove sufficient to destabilize the aqueous liquid phase in isobaric systems in which all water molecules are permanently exposed to a uniform electric field. At the same time, simulations reveal comparatively weak field-induced perturbations of water structure upheld by flexible hydrogen bonding. In open systems with fixed chemical potential, these perturbations do not suffice to offset attraction of water into the field; additional water is typically driven from unperturbed bulk phase to the field-exposed region. In contrast to recent theoretical predictions in the literature, our analysis and simulations confirm that classical electrostriction characterizes usual electrowetting behavior in nanoscale channels and nanoporous materials.