Molecular dynamics simulation study of ionic hydration and ion association in dilute and 1 molal aqueous sodium chloride solutions from ambient to supercritical conditions

The increasing demand for accurate equations of state of fluids under extreme conditions and the need for a detailed microscopic picture of aqueous fluids in some areas of geochemistry (e.g., mineral dissolution/precipitation kinetics) potentially make molecular dynamics (MD) simulations a powerful tool for theoretical geochemistry. We present MD simulations of infinitely dilute and 1 molal aqueous NaCl solutions that have been carried out in order to study the systematics of hydration and ion association over a wide range of conditions from ambient to supercritical and compare them to the available experimental data. In the dilute case, the hydration number of the Na+ ion remains essentially constant around 5.5 from ambient to supercritical temperatures when the density is kept constant at 1 g cm(-3) but decreases to below 5 along the liquid-vapor curve. In both cases, the average ion-first shell water distance decreases by about 0.03 Angstrom from ambient to near critical temperatures. The Cl- ion shows a slight expansion of the first hydration shell by about 0.02 Angstrom from ambient to near critical temperatures. The geometric definition of the first hydration shell becomes ambiguous due to a shift of the position of the first minimum of the Cl-O radial distribution function. In the case of the 1 molal solution, the contraction of the Na+ first hydration shell is similar to that in the dilute case whereas the hydration number decreases drastically from 4.9 to 2.8 due to strong ion association. The released waters are replaced on a near 1:1 basis by chloride ions. Polynuclear clusters as predicted by Oelkers and Helgeson (1993b) are observed in the high temperature systems. The hydration shell of the Cl--ion shows significant deviation from the behavior in dilute systems, that is, at near vapor saturated conditions, the expansion of the hydration shell is significantly larger (0.12 Angstrom from ambient to near critical temperatures). Due to a very large shift of the first minimum and second maximum of the Cl-O radial distribution function, the proper definition of the hydration number becomes even more ambiguous then in the dilute case. The results of the present study clearly show that current empirical approaches for modeling aqueous fluids using simple electrostriction concepts do not adequately mimic the properties of the actual microscopic structure at high temperatures. For example, the implementation of the g-function in the revised HKF model (Tanger and Helgeson, 1988) only partly reflects the actual changes of the hydration environment. Future successful equations of state will, therefore, have to take the fluid structure and dynamics on the molecular scale into account. The presence of polynuclear species in concentrated could be of importance for mineral dissolution kinetics. Copyright (C) 1998 Elsevier Science Ltd.