Computational investigation of the influence of organic-aqueous interfaces on NaCl dissociation dynamics.

NaCl pairing and dissociation was investigated at the CCl(4)-water and 1,2-dichloroethane (DCE)-water interfaces, and compared with dissociation results in the bulk and at the air-water interface utilizing polarizable potentials. The transition path sampling methodology was used to calculate the rate constant for dissociation, while umbrella sampling was used to map out a free energy profile for NaCl dissociation. The results found that ion pairing was weakest at the organic-water interfaces, even weaker than in the water bulk. This is in contrast to what has been observed previously for the air-water interface, in which NaCl ion paring is stronger than in the bulk [C. D. Wick, J. Phys. Chem. C 113, 6356 (2009)]. A consequence of the weaker binding at the organic-water interfaces was that ion dissociation was faster than in the other systems studied. Interactions of the organic phase with the ions influenced the magnitude of the Cl(-) induced dipole moment, and at the organic-water interfaces, the average Cl(-) induced dipole was found to be lower than at the air-water interface, weakening interactions with Na(+). These weaker interactions were found to be responsible for the weaker ion pairing found at the organic-water interfaces.