Molecular dynamics simulation of limiting conductances for LiCl, NaBr, and CsBr in supercritical water

We report molecular dynamics simulations of LiCl, NaBr, and CsBr in supercritical water in order to explain the experimental observations of the limiting conductances as a function of the density of water at supercritical state points. As was the case in our previous work on NaCl in supercritical water [Lee et al., Chem. Phys. Lett. 293, 289 (1998)], we find that the experimental trends in the limiting conductances as a function of water density are reproduced in our simulations-a clear change of slope from the assumed linear dependence of limiting conductances of LiCl, NaBr, and CsCl on the water density. We also found that the effect of the number of hydration water molecules around ions dominates in the higher-density region while the interaction strength between the ions and the hydration water molecules (as measured by the potential energy per hydration water molecule) dominates in the lower-density region. In the case of Cs+ and Br-, however, the latter factor in the lower-density region is not as dominant as in the cases of Na+ and Cl- since a clear difference between the potential energy per hydration water molecule at densities above and below 0.45 g/cm(3) was not clearly observed in the cases. In the case of Li+, the interaction between the ions and the hydration water in the lower-density region is almost a nonfactor since the potential energy per hydration water molecule is monotonically decreased with decreasing water density, which is consistent with the linear increase of the limiting conductance for the Li+ ion with decreasing water density. (C) 2000 American Institute of Physics. [S0021-9606(00)50701-6].