Effect of Water Polarizability on the Properties of Solutions of Polyvalent Ions: Simulations of Aqueous Sodium Sulfate with Different Force Fields

We show that aqueous sodium sulfate solutions exhibit an unrealistically large degree of ion pairing and clustering when modeled using nonpolarizable force fields, with clusters resembling precipitate readily forming in a 0.5 m solution at ambient conditions. This aggregation behavior was found to be persistent in nonpolarizable water for a range of parameters of the sulfate anion. In contrast, a polarizable potential performs satisfactorily, producing a well dissolved salt with a degree of association that is consistent with activity data for real solutions. Most of this improvement is due to polarization of water molecules in the vicinity of the divalent sulfate anion, which enhances its solvation.