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

Citation
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
Citations number
47
Categorie Soggetti
Physical Chemistry/Chemical Physics
Journal title
JOURNAL OF CHEMICAL PHYSICS
ISSN journal
00219606 → ACNP
Volume
111
Issue
11
Year of publication
1999
Pages
5141 - 5149
Database
ISI
SICI code
0021-9606(19990915)111:11<5141:MSOTTA>2.0.ZU;2-4
Abstract
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].