(308e) Simulations of Activity Coefficients and Solubilities for Aqueous Electrolytes: Why Are Our Models Off?

Aqueous electrolyte solutions play an important role in chemical engineering separations, and also in geochemical environments and for biophysics. The mean ionic activity coefficients quantify the deviation of salt chemical potential from ideal solution behavior; experimental measurements are available for many salts over broad ranges of concentration and temperature, but there have been practically no prior simulation results of these quantities, because of sampling difficulties for explicit-solvent electrolyte solutions. We have developed a new approach for determination of activity coefficients of aqueous electrolytes [1-2]. Common fixed-point-charge models for water and ions are unable to reproduce simultaneously activity coefficients and solubilities. Polarizable models [3] perform better, but still predict an incorrect temperature dependence of these properties. To complicate matters even more, direct coexistence simulations of salt solubilities systematically overpredict salt solubilities when compared to free-energy calculations using the same molecular models. Ongoing work in the group that aims to pinpoint the origin of these discrepancies will be described.

[1] Z. Mester and A. Z. Panagiotopoulos "Mean ionic activity coefficients in aqueous NaCl solutions from molecular dynamics simulations,� J. Chem. Phys. 142: 044507, 10 pp (2015). DOI: http://dx.doi.org/10.1063/1.4906320.

[2] Z. Mester and A. Z. Panagiotopoulos, "Temperature-Dependent Solubilities and Mean Ionic Activity Coefficients of Alkali Halides in Aqueous Solutions from Molecular Dynamics Simulations,� J. Chem. Phys. 143: 044505, 10 pp (2015). DOI: http://dx.doi.org/10.1063/1.4926840.

[3] H. Jiang, Z. Mester, O. A. Moultos, I. G. Economou, and A. Z. Panagiotopoulos, "Thermodynamic and Transport Properties of H2O+NaCl from Polarizable Force Fields," Chem. Theory Comput} 11: 3802-3810 (2015). DOI: http://pubs.acs.org/doi/abs/10.1021/acs.jctc.5b00421.