Salt and Water Uptake in Nanoconfinement under Applied Electric Field: An Open Ensemble Monte Carlo Study

Abstract

Permeation of electrolytes in nanoporous materials underlies many applications in energy and materials technologies. Wetting of apolar nanopores can be enhanced by electric field, attracting water and ions from unperturbed electrolyte bath. We study absorption of water and NaCl in the pores by Expanded Ensemble Grand Canonical Monte Carlo simulation, which implements particle insertions and deletions through incremental changes in particles' coupling with the system. We determine the uptake of water and ions in the pores, and concomitant changes in pore thermodynamics, as functions of field strength in the pore and salinity in the external bath. Pressure increase and reduction of wetting free energy, σ, in the pore intensify near-quadratically with the field. Surprisingly, the influence of bulk salinity on σ can change qualitatively with pore width and field strength. Conforming to Gibbs adsorption isotherm, narrow pores with salt molality below that of the bath experience an increase in σ with rising bulk salinity. The field can change salt depletion to excess and consequently reverse the salinity dependence of wetting free energy from increasing to declining function of bulk molality. Field polarity continues to play a role, leading to asymmetric wettability at opposing walls as we previously observed in the absence of ions.

Dynamic Response in Nanoelectrowetting on a Dielectric

“…In both systems, the capacitance features an approximately linear increase with A sl within the range covered in the simulations. The linear dependence reflects the predominant contribution of the first hydration layer to the total polarization of the nanoscale aqueous phase, consistent with observations in nanofilms. , The slopes are equal in both systems. Approximating this derivative in eq by overestimates the predicted increase in cos θ c by a factor of 2–3 (see Figure S1 in the Supporting Information).…”

“…The linear dependence reflects the predominant contribution of the first hydration layer to the total polarization of the nanoscale aqueous phase, consistent with observations in nanofilms. 37,38 The slopes from Figure 5 reduces the deviation (dashed lines in Figure 3) but does not capture the observed trend toward the saturation. The differences between simulated cos θ and predictions of eq 1 are rationalized by partial distortion of the droplet's hemispherical shape and the neglect of the influence of the electric field on the surface tension, γ lv , 16 of the droplets.…”

Electrokinetic flow of an aqueous electrolyte in amorphous silica nanotubes

“…It should be noted that in experiments salt concentration in the nanopores can deviate from that of the bulk solution 61 although the extent of exclusion is hard to quantify in simulations, even at equilibrium conditions 19,20 . One can hence expect not only considerable electrostatic but also detectable composition gradients at nanopore ends present in practical devices 62 .…”

“…First, we consider a more realistic silica pore composed of amorphous silica instead of crystalline silica. Second, in addition to fixed charge force fields 17,18 , which we have previously used in Grand Canonical Monte Carlo simulations of the pore/salt solution partitioning 19,20 , we also examine polarizable models using Drude oscillators 21,22 to allow internal charge redistribution of water molecules and ions. Explicit polarizability may be important for describing the interfacial behaviour of the fluid more ac-curately, in particular the surface preference of large anions such as Cl − , even though more recent suggestions indicate that polarizability alone is not responsible for the surface preference 23,24 .…”

High-Pressure Infiltration–Expulsion of Aqueous NaCl in Planar Hydrophobic Nanopores

“…which relates surface adsorption to the changes in interfacial tension. 40,69 Above, Γ(μ i ,P bulk ) represents the surface excesses of species i with the specified chemical potential and bulk pressure. Large surface deficits of ions in the narrow pore, i.e., strongly negative Γ(μ i ,P bulk ), imply a significant increase of σ upon increasing the bulk salinity.…”