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Open Access Publications from the University of California

Ion association in aqueous solution


The properties of aqueous ionic solutions are determined by strong electrostatic interactions, which are influenced by polarizability and charge transfer interactions. Potential models which include polarizability and charge transfer have been developed for water and single ion properties. Here, the ion-ion interactions are optimized so that the osmotic pressure as a function of concentration is reproduced. Using the optimized potentials, the amount of ion pairing and larger cluster formation is calculated. For NaCl, NaI, and KCl, there is a small amount of pairing, with larger clusters present as well. For KI, there is much more pairing and much larger clusters are observed. The amount of pairing is consistent with the law of matching affinities, with the pairs that show the least pairing also being the most mis-matched in terms of size or solvation free energy. The charge transfered from the anions to water is more than from is transferred from the water to cations, so the water molecules acquire a negative charge, which increases with ion concentration.

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