Ion association in binary mixtures of water-CO2 in supercritical conditions through classical molecular dynamics simulations

Abstract

The structural and thermodynamic properties of Na+ - Cl− ion-pair association in water-CO2 binary mixtures in supercritical conditions for infinitely dilute solutions are studied using constrained molecular dynamics simulations over a wide range of compositions. It is found that solvation structure varies dramatically with the solvent composition. Contact ion pairs (CIPs) are found to be more stable than all other configurations as seen from the potentials of mean force (PMFs). PMFs of the NaCl ion pair in pure CO2 look almost like the pair potential between the ion pair. Stabilities of CIPs increase with increase in the mole fraction of CO2. An increment in the average number of hydrogen bonds with an increase in the mole fraction of H2O in the bulk as well as in the solvation shell of the ions is observed. Ion-pair association in aqueous CO2 mixtures in supercritical conditions is found to be endothermic and driven by entropy. Preferential solvation analysis shows that both Na+ and Cl− ions are preferentially solvated by water and even a small percentage of water in the mixture prevents CO2 molecules from entering the first solvation shell of ions due to the strong hydrophilicity of the ions.