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

We have carried out molecular dynamics simulations of NaCl solutions at roo m temperature (298 K) and at a supercritical temperature of 683 K using dis crete simple point charge (SPC or SPC/E) molecular models for the water sol vent. 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 funct ions were calculated and compared with corresponding functions for a contin uum model of the solvent also determined by computer simulation. Our studie s confirm the presence of significant amounts of ion pairing and clustering at supercritical conditions as seen in visualizations of the equilibrium c onfigurations of the solution. However, the degree of pairing and clusterin g of ions in supercritical solutions is significantly different for discret e and continuum representations of the solvent. Simulations of a 1 molal so lution of NaCl at 683 K, using a discrete molecular model for the solvent a t 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 small er clusters containing positive and negative charges are observed at 683 K when the electrolyte concentration is reduced to 0.5 molal at a solvent den sity 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 th e 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 e lectrolyte concentration. They are nearly equal to each other in the one mo lal 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 ove r an observed lifetime greater than 200 ps. (C) 2000 American Institute of Physics. [S0021-9606(00)50242-6].