Abstract
Dielectric behavior of electrolyte aqueous solutions with various concentrations in a cylindrical nanopore of MCM 41 silica has been investigated. The effect of confinement is investigated by using isothermal-isosurface-isobaric statistical ensemble, which has proved to be an effective alternative to the Grand Canonical Monte Carlo (GCMC) simulation method. Several single-salt solutions have been considered (e.g., NaCl, NaI, BaCl2, MgCl2) in order to investigate the effect of ion polarizability, ion size, and ion charge. The effect of salt concentration has also been addressed by considering NaCl solutions at different concentrations (i.e., 0.1 mol/L, 0.5 mol/L, and 1 mol/L). The motivation in performing this integrated set of simulations is to provide deep insight into the dielectric exclusion in NF theory that plays a significant role in separation processes. It was shown that the dielectric constant increased when ions were added to water inside the nanopore (with respect to the dielectric constant of confined pure water) unlike what was obtained in the bulk phase and this phenomenon was even more pronounced for electrolytes with divalent ions (MgCl2 and BaCl2). Therefore, our simulations indicate opposite effects of ions on the dielectric constant of free (bulk) and nanoconfined aqueous solutions.
Highlights
In the nanometric electrolyte solutions, the confined solution contacts with the inner wall of the cavity across a large area and the interfacial solution is affected by the induced electric polarization which determines the intensity of solution-mediated intermolecular forces [1,2]
Confined water molecules exhibit a faster relaxation in the presence of salt which might partly explain the results obtained in Figures 6 and 7, i.e., the increase in the dielectric constant inside pores for electrolyte solutions with respect to confined pure water
The striking result is that our molecular simulations predict an increase in the dielectric constant when ions are added to water inside the nanopore unlike what was obtained in the bulk phase and this phenomenon is even more pronounced for electrolytes with divalent ions
Summary
In the nanometric electrolyte solutions, the confined solution contacts with the inner wall of the cavity across a large area and the interfacial solution is affected by the induced electric polarization which determines the intensity of solution-mediated intermolecular forces [1,2]. Varieties of geometries have been constituted to calculate the local dielectric constant of aqueous solution within cylindrical, spherical, and flat plate confined mediums. Toward this end, Ghoufi et al has predicted an anomalous variation of dielectric constant of pure water confined in cylindrical and spherical nanoconfinement, respectively [29,33]. We have studied the dielectric characteristics of aqueous electrolyte solutions confined in a cylindrical nanopore of MCM 41 silica (see Figure 1) by means of Grand Canonical Monte Carlo (GCMC) simulations with an Isothermal-Isosurface-Isobaric statistical ensemble [35].
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