An extensive set of molecular dynamics simulations of 1m NaCl solutions has been carried out on a system of 2127 SPC/E water molecules and 40 Na+ and 40 Cl- in the NVT ensemble using effective pair potentials. Full Ewald summation was applied for long range forces. Simulated state points are 300, 370, 420, 470, 520, 590, 650, 700 and 770 K at 1.02 g/ccm and 420, 520, 590 and 650 K at approximately saturated vapour pressure conditions.
The simulation results have been interpreted with respect to structural properties, dynamics of the exchange of water molecules between the hydration shell environment and bulk water and solute speciation as a function of temperature and density.
One of the most striking structural features is the decrease of the average Na-O distance in the solution with increasing temperature. From 300 K to 650 K the maximum of the first peak in the Na-O radial distribution function shifts from 0.226 to 0.223 nm. An overall broadening of this peak shifts its left flank by about 0.007 nm to shorter distancies. At the same time the average hydration number decreases.
These phenomena are probably reflecting the much enhanced dynamics of water exchange between the first hydration shell and bulk water. The average residence time for a water molecule in the first hydration shell of a Na ion decreases exponentially with temperature from about 30 ps at 300 K to 1-2 ps at 650 K and higher temperatures. Residence times for water molecules in the first shell around Cl ions decrease from about 20 ps to 1-2 ps.
Combining these observations leads to a first hypothesis which relates the decrease in the hydration number to the statistics of the hydration water exchange. Simply speaking, due to the fast exchange the reduction of the hydration number reflects the increasing contributions from more and more exchange events per unit time. A smaller hydration number as well as the higher kinetic energy of solvent molecules allows for shorter ion-water distances. This picture might be the molecular explanation for the need of the
g-function fitting parameter in the HKF model of aqueous species (e.g. Tanger and Helgeson, 1988) as well as the recent experimental observation of a contraction of the first hydration shell with increasing temperature using EXAFS methods (Seward and Henderson, 1995).
In the Molecular Dynamics simulations, a strong dependence of solute speciation on temperature and density was observed, thus supporting the predictions of Oelkers and Helgeson (1993) on the existence of cluster ions at elevated temperatures. Unfortunately, the potentials used in the simulations tend to overemphasize ion association. Therefore, no quantitative picture of solute speciation can yet be given. Clusters as big as Na4Cl5- where observed as short-lived entities. Not surprisingly, lifetimes of solute clusters increase with decreasing cluster size.
In order to learn more about the molecular mechanisms responsible for salt effects on isotope fractionation in aqueous solutions (Driesner and Seward, 1996), quantum mechanical calculations of various hydration complexes have been carried out. Typically, density functional theory calculations using the Becke3LYP functional and - for very small hydration complexes - ab initio calculations on the MP2/6-31G** level were applied. A first model of isotope salt effects integrating results from both the MD simulations and the quantum mechanical calculations is currently being developed.
Driesner, T. & Seward, M., J. Conf. Abs. 1, 144 (1996).
Oelkers, E. & Helgeson, H.C., Science 261, 888-891 (1993).
Seward, T.M. & Henderson, C.M.B., Terra Abstr. 1/1995 70 (1995).
Tanger, J.C. & Helgeson, H.C., Amer. J. Sci. 288 19-98 (1988).