Nanoconfined water under electric field at constant chemical potential undergoes electrostriction

Electric control of nanopore permeation by water and solutions enables gating in membrane ion channels and can be exploited for transient surface tuning of rugged substrates, to regulate capillary permeability in nanofluidics, and to facilitate energy absorption in porous hydrophobic media. Studies of capillary effects, enhanced by miniaturization, present experimental challenges in the nanoscale regime thus making molecular simulations an important complement to direct measurement. In a molecular dynamics (MD) simulation, exchange of water between the pores and environment requires modeling of coexisting confined and bulk phases, with confined water under the field maintaining equilibrium with the unperturbed environment. In the present article, we discuss viable methodologies for MD sampling in the above class of systems, subject to size-constraints and uncertainties of the barostat function under confinement and nonuniform-field effects. Smooth electric field variation is shown to avoid the inconsistencies of MD integration under abruptly varied field and related ambiguities of conventional barostatting in a strongly nonuniform interfacial system. When using a proper representation of the field at the border region of the confined water, we demonstrate a consistent increase in electrostriction as a function of the field strength inside the pore open to a field-free aqueous environment.