Dissolved Ionic Salts under Nanoconfinement

Abstract

Since the first isolation of graphene barely twenty years ago, quasi two-dimensional structures have been investigated in great detail as their properties such as their relative permittivity and heat conductivity vary greatly from the macroscopic three dimensional (bulk) case. More recently, dilute electrolyte solutions confined in nanoscale slits have been observed to form clusters at ambient conditions and a theoretical framework for describing this phenomenon based on a quasi two-dimensional Coulombic interaction (Q2D) has been developed. Using classical molecular dynamics simulations (MD), we found that sodium chloride (NaCl) as well as a generic, divalent salt dissolved in water showed that the electric current per ion is not strongly affected by the applied electric field but decreases with increasing ionic density, due to increased cluster formation. For a range of salts, including LiCl, NaCl, CaSO4 and CaCl2, analysis of the potentials of mean forces (PMF) shows that dissolved cations and anions in nanoslits attract each other more strongly than in the bulk case. Comparing the PMFs with the Q2D calculations, we find that they match well in the long range limit. However, at distances of a few angstroms, we show that Q2D fully neglects excluded volume effects and that its predictions for highly charged ions (valence Z=2) deviate from the effective interactions extracted by our MD simulations.