Nanoconfined water under electric field at constant chemical potential undergoes electrostriction

Abstract

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.

Blue Energy and Desalination with Nanoporous Carbon Electrodes: Capacitance from Molecular Simulations to Continuous Models

“…However, the water density inside the pores (see Table III) is in fact larger than in the bulk. Such an increase may be due to several factors, including confinement, which perturbs the structure of the fluid, in particular, the ability to form hydrogen bonds, or electrostriction in the presence of the local electric fields inside the electrode [85]. We also note that there is an asymmetry between the electrodes, with a slightly larger water density in the positive electrode correlated with a TABLE II.…”

Wetting transparency of graphene in water

“…10, Fig 9 in Ref. 16). In these cases, polarization of confined water by applied field induces longer-ranged electrostatic attractions on field-free bath water.…”

“…26 In addition to its reliability in describing the interfacial and dielectric properties of water, [27][28][29][30][31][32] it conveniently allows a direct comparison with previous works. 2,5,16,19,25,[33][34][35] Our results presented in Table I are obtained using conventional 12-6 LJ potential between water oxygens and carbon atoms, σ CO = 3.19 Å and ε CO = 0.4389 kJ mol −1 . According to case 12 of Table II in Ref.…”

“…(8)- (12), leading to smaller θ . In case of butylated graphane, 16 the distance between hydration layers is considerably increased because of additional length of butyl chains. From the simulated density profiles of butylated graphane in water, we estimate d ∼ 12 Å, as opposed to ∼7 Å for graphene.…”

“…Wetting from both sides has typically been considered in molecular dynamics (MD) simulations, where graphene lattice has been used in modeling hydrophobic confinements. [7][8][9][10][11][12][13][14][15][16] For graphene walls wetted from the outer side of the confinement, the effective water wall affinity and wettability are increased, compared to that of pristine graphene and a subtle change of wettability can make a significant difference. 17 In a practical simulation, a change in the effective contact angle can be absorbed in the empirical force field; however, this may necessitate ad hoc reparameterizations for the same material, or has to be kept in mind in interpreting each specific situation.…”