Field-exposed water in a nanopore: liquid or vapour?

Abstract

Field-exposed water behaviour in hydrophobic confinement confirms classical electrostriction in nanoscale channels and nanoporous materials. 2 AbstractWe 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.

Nanoconfined water under electric field at constant chemical potential undergoes electrostriction

“…26,27 Our model calculations show that water uptake in a confinement consistently increases with the strength of local electric field, conforming to the classical electrostriction behavior in agreement with previous Monte Carlo studies. 6,7,13 The improvements we have made in modeling the interface between the field-exposed and unperturbed regions can explain the differences between our findings and the system behavior observed at equivalent conditions in an earlier MD approach. 5…”

“…3 (bottom) conforms to this picture. For present fields, the alignment of water molecules grows essentially linearly with the field, cos θ z ∝ |E z | 13,64 (see also Fig. 5) and U E ∝ E 2 z .…”

“…13 and 14, different conditions can lead to qualitatively different responses of bulk and confined water to the field. Specifically, the field enhances the density 6 and stabilizes the liquid phase 13,14 if the confinement is open to a field-free bath supplying extra molecules to preserve constant chemical potential. In other scenarios, the field can reduce the density 12,15 and increase the volatility 12 of confined water.…”

“…14,18 Clearly, electric control of nanowetting is most effective when the field is applied in the confinement without perturbing the surrounding reservoir. This setup is easily amenable 4,6,7,13 to Monte Carlo approaches, e.g., the Grand Canonical (GCMC) or Gibbs Ensemble (GMC) simulations, were nonphysical molecule exchanges enable modeling of the field-exposed confinement separately from the bulk phase. In Molecular Dynamics (MD), on the other hand, transfer of molecules between the two coexisting phases involves both regions in direct physical contact.…”

Vapor-liquid coexistence of the Stockmayer fluid in nonuniform external fields

“…Clearly, in real systems there exists an interfacial region at the pore edges where the field is nonuniform. The field effect in this type of system has been studied for one component fluids [10,11] where it was found to gradually increase the fluid density within the pore moderately. A stronger field effect was found by Brunet et al [12,13] who studied mixtures.…”