Computer simulation studies of aqueous sodium chloride solutions at 298 K and 683 K

Abstract

We have carried out molecular dynamics simulations of NaCl solutions at room temperature (298 K) and at a supercritical temperature of 683 K using discrete simple point charge (SPC or SPC/E) molecular models for the water solvent. The solvent densities were 0.997 g cm−3 at 298 K and 0.35 g cm−3 and 0.175 g cm−3 at 683 K. The ion–ion and ion–solvent distribution functions were calculated and compared with corresponding functions for a continuum model of the solvent also determined by computer simulation. Our studies confirm the presence of significant amounts of ion pairing and clustering at supercritical conditions as seen in visualizations of the equilibrium configurations of the solution. However, the degree of pairing and clustering of ions in supercritical solutions is significantly different for discrete and continuum representations of the solvent. Simulations of a 1 molal solution of NaCl at 683 K, using a discrete molecular model for the solvent at a density of 0.35 g cm−3, show the presence of a single megacluster of 10 sodium and chloride ions in a system of 555 water molecules. Three smaller clusters containing positive and negative charges are observed at 683 K when the electrolyte concentration is reduced to 0.5 molal at a solvent density of 0.35 g cm−3 and also at a lower solvent density of 0.175 g cm−3. Molecular dynamics simulations of the velocity auto correlation functions of Na+ and Cl− ions have distinct forms related to the cluster to which the ion belongs. The diffusion coefficients of Na+ and Cl− ions, at infinite dilution, are larger at 683 K than at 298 K, and decrease with increasing electrolyte concentration. They are nearly equal to each other in the one molal solution at 683 K, which may correspond to a supersaturated solution in which the large cluster of sodium and chloride ions moves as an entity over an observed lifetime greater than 200 ps.