T. Driesner et Pt. Cummings, Molecular simulation of the temperature- and density-dependence of ionic hydration in aqueous SrCl2 solutions using rigid and flexible water models, J CHEM PHYS, 111(11), 1999, pp. 5141-5149
Molecular dynamics simulations of aqueous SrCl2 solutions have been perform
ed with two flexible water models [the Bopp-Jancso-Heinzinger (BJH) and mod
ified Toukan-Rahman simple point charge model (SPC-mTR)] as well as the rig
id simple point charge (SPC) model. Recent extended x-ray absorption fine s
tructure spectroscopy (EXAFS) studies of Sr2+ hydration reported a decrease
of the average distance between Sr2+ and water molecules in the first hydr
ation shell with increasing temperature. The available Sr2+-water potential
for rigid SPC water and its variants is not able to reproduce this hydrati
on 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 ex
pansion of the first hydration shell of the Cl- ion with increasing tempera
ture. 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 comp
arison of Cl- coordination numbers at different temperatures and densities
difficult. We demonstrate that although the coordination number as determin
ed from nearest neighbor hydrogen atoms (as preferred by neutron diffractio
n 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 wi
th the available experimental data. Hence, care has to be taken when discus
sing trends in hydration "strength" with temperature and density. (C) 1999
American Institute of Physics. [S0021-9606(99)52135-1].