Abstract

We have used COSMO-RS, a method combining quantum chemistry with statistical thermodynamics, to compute Setschenow constants (K_S) for a large array of organic solutes and salts. These comprise both atmospherically relevant solute-salt combinations, as well as systems for which experimental data are available. In agreement with previous studies on single salts, the Setschenow constants predicted by COSMO-RS (as implemented in the COSMOTherm program) are generally too large compared to experiments. COSMOTherm overpredicts salting out (positive K_S), and/or underpredicts salting in (negative K_S). For ammonium and sodium salts, K_S values are larger for oxalates and sulfates, and smaller for chlorides and bromides. For chloride and bromide salts, K_S values usually increase with decreasing size of the cation, along the series Pr₄N⁺ < Et₄N⁺ < Me₄N⁺ ≤ Na⁺ ≈ NH₄⁺. Of the atmospherically relevant systems studied, salting in is predicted only for oxalic acid in sodium and ammonium oxalate, and sodium sulfate, solutions. COSMOTherm was thus unable to replicate the experimentally observed salting in of glyoxal in sulfate solutions, likely due to the overestimation of salting out effects. By contrast, COSMOTherm does qualitatively predict the experimentally observed salting in of multiple organic solutes in solutions of alkylaminium salts.