The main goal of this work is the molecular-based determination of the ion-pair association constant in model near-critical aqueous HCl solutions.  The radial profiles of the potential of mean force for the H₃O^+ÉCl^-  pair are determined by constraint molecular dynamics of infinitely dilute aqueous solutions along near-critical isotherms and from steam-like to liquid like densities.  The ion-pair association constant are consequently calculated and compared with those obtained from the most accurate recent electrical conductance experiments [Ho, P. C.; Palmer, D. A.; Gruszkiewicz, M. S. Journal of Physical Chemistry B 2001, 105, 1260-1266] at corresponding states.  Resulting equilibrium constants between contact and solvent-shared ion-pair configurations are then used to estimate the distribution between contact and water-mediated ion-pair configurations.  The adequacy of some simple fully electrostatic models to represent ion-pair association behavior in high temperature aqueous solutions is also discussed. 

This research was sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences under contract number DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC.