Aqueous electrolytes confined within functionalized silica nanopores

Abstract

Molecular dynamics simulations have been carried out to investigate structural and dynamical characteristics of NaCl aqueous solutions confined within silica nanopores in contact with a "bulk-like" reservoir. Two types of pores, with diameters intermediate between 20 Å and 37.5 Å, were investigated: The first one corresponded to hydrophobic cavities, in which the prevailing wall-solution interactions were of the Lennard-Jones type. In addition, we also examined the behavior of solutions trapped within hydrophilic cavities, in which a set of unsaturated O-sites at the wall were transformed in polar silanol Si-OH groups. In all cases, the overall concentrations of the trapped electrolytes exhibited important reductions that, in the case of the narrowest pores, attained 50% of the bulk value. Local concentrations within the pores also showed important fluctuations. In hydrophobic cavities, the close vicinity of the pore wall was coated exclusively by the solvent, whereas in hydrophilic pores, selective adsorption of Na + ions was also observed. Mass and charge transport were also investigated. Individual diffusion coefficients did not present large modifications from what is perceived in the bulk; contrasting, the electrical conductivity exhibited important reductions. The qualitative differences are rationalized in terms of simple geometrical considerations.

Electrokinetic flow of an aqueous electrolyte in amorphous silica nanotubes

“…The structure of water and aqueous solutions in hydrophilic confinements of interest in energy applications has been a popular subject for molecular dynamics simulations for a long time. While we cannot give an exhaustive review, some relevant examples include the work of Leung and others on salt water in model silica pores 3,4 , as well as the studies of Argyris et al 5 and Videla et al 6 . Ho et al considered the effect of varying silica protonation states on the structure of the confined aqueous solution 7 .…”

Water confinement in nanoporous silica materials

“…16 Consequently, in recent years molecular simulations have been employed to investigate structural, dynamics, and dielectric properties of water confined in silica nanopores of various sizes, geometries, and hydrophilic/hydrophobic features. 2,3,[16][17][18][19][20][21][22][23][24] However, most of these studies only reported results for aqueous solutions confined inside uncharged systems. In contrast, only few works have focused on the properties of aqueous solutions confined into charged silica nanopores.…”

Simulations of Nanoseparated Charged Surfaces Reveal Charge-Induced Water Reorientation and Nonadditivity of Hydration and Mean-Field Electrostatic Repulsion

“…Water-explicit simulations of charged surfaces in principle include all these effects and should thus allow for a crucial test of the assumption of additive hydration and PB pressures, in particular since the surface charge and its location are precisely known. Previous simulations reported ion density profiles between nanometer-separated charged surfaces [10][11][12][13]25 .…”