H3O+Cl- Pair Association in Steam and Highly Compressible Aqueous Environments

An extensive molecular-based simulation effort is conducted to analyze ion-pairing behavior in near-critical dilute HCl aqueous solutions along three near- (super- and sub-) critical isotherms over a wide density range, that is, from steam-like to liquid-like environments. The study encompasses the determination of the ion-pair association constant via potential of mean force calculations and the resulting degree of ion-pair association. The main goal behind this effort is to address a few relevant questions regarding the thermodynamic and corresponding microscopic behavior of the ion-pair formation in steamlike and highly compressible environments for which experimental data are extremely difficult to obtain accurately and in the process to illustrate the “uneventful” behavior of the association constant along the critical isotherm of the solvent. Direct comparison is made between simulation, theoretical developments, and experiment to identify potential pitfalls in the interpretation of experimental data and subsequent macroscopic modeling.