Molecular dynamics simulation of limiting conductance for Na2+, Cl2-, Na-degrees, and Cl-degrees in supercritical water

We report results of molecular dynamics simulations of the limiting conduct ance of Na2+, Cl2-, Na degrees, and Cl degrees in supercritical water using the SPC/E model for water in conjuction with our previous study (Lee et al ., Chem. Phys. Lett. 293, 289 (1998)). The behavior of the limiting conduct ances of Na2+ and Cl2- in the whole range of water density shows almost the same trend as those of Na+ and Cl-, but the deviation from the assumed lin ear dependence of limiting conductances of Na2+ and Cl2- on the water densi ty is smaller than that of Na+ and Cl-. The ratio of the limiting conductan ce of the divalent ions to that of the corresponding monovalent ions over t he whole range of water density is almost constant. In the cases of Na+ and Cl2-, the dominating factor of the number of hydration water molecules aro und ions in the higher-density region and the dominating factor of the inte raction strength between the ions and the hydration water molecules in the lower-density region are also found as was the cases for Na- and Cl-. These factors, however, are not so strong as for the corresponding monovalent io ns because the change in the energetics, structure, and dynamics are very s mall mainly due to the strong Coulomb interaction of the divalent ions with the hydration water molecules. The diffusion coefficient of Na degrees and Cl degrees monotonically increases with decreasing water density over the whole range of water density. The increase of the diffusion coefficient wit h decreasing water density is attributed only to the dramatic decrease of t he hydration number of water in the first solvation shell around the unchar ged species. Among the two important competing factors in the limiting cond uctance of Na+ and Cl-, the effect of the number of hydration water molecul es around the uncharged species is the only existing factor over the whole range of water density since the interaction strength between the uncharged species and the hydration water molecules very small through the LJ intera ction. This result has confirmed the dominating factor of the number of hyd ration water molecules around ions in the higher-density region in the expl anation of the limiting conductance of Na+ and Cl- in supercritical water a t 673 K.