Molecular simulation of the temperature- and density-dependence of ionic hydration in aqueous SrCl2 solutions using rigid and flexible water models

Molecular dynamics simulations of aqueous SrCl2 solutions have been performed with two flexible water models [the Bopp-Jancso-Heinzinger (BJH) and modified Toukan-Rahman simple point charge model (SPC-mTR)] as well as the rigid simple point charge (SPC) model. Recent extended x-ray absorption fine structure spectroscopy (EXAFS) studies of Sr2+ hydration reported a decrease of the average distance between Sr2+ and water molecules in the first hydration shell with increasing temperature. The available Sr2+-water potential for rigid SPC water and its variants is not able to reproduce this hydration shell contraction. Adding intramolecular flexibility in the form of the SPC-mTR potential only slightly improves the performance of the SPC model, while the BJH model performs significantly better. All models predict an expansion of the first hydration shell of the Cl- ion with increasing temperature. The degree of expansion is density and concentration dependent. Large shifts of the position of the first minimum in the g(ClO)(r) make the comparison of Cl- coordination numbers at different temperatures and densities difficult. We demonstrate that although the coordination number as determined from nearest neighbor hydrogen atoms (as preferred by neutron diffraction experimentalists) appears to decrease with increasing temperature, it is in fact increasing when the coordination number is properly defined as the number of nearest neighbor water molecules. When identical definitions for the hydration shells are used, the results for Cl- are in good agreement with the available experimental data. Hence, care has to be taken when discussing trends in hydration "strength" with temperature and density. (C) 1999 American Institute of Physics. [S0021-9606(99)52135-1].