MOLECULAR-DYNAMICS SIMULATION OF ELECTROLYTE-SOLUTIONS IN AMBIENT ANDSUPERCRITICAL WATER .2. RELATIVE ACIDITY OF HCL

Molecular dynamics-free energy perturbation simulations are used to st udy the acidity of HCl relative to water at conditions ranging from am bient density and temperature to several supercritical water states. A n increase in temperature from 25 degrees C to the critical temperatur e T-c (374 degrees C) along the coexistence curve reduces the differen ce between the pK(a) of HCl and pK(w) by a factor of 2. However, the c orresponding difference in free energies of dissociation remains const ant, even in supercritical water, along isochoric and isothermal (betw een 0.29 and 0.087 g/cm(3)) paths. This new observation is explained i n terms of the effect of temperature on the thermodynamic properties o f solvation of the various reacting species and is used to gain furthe r insight into the behavior of previous experimental data.