Effect of Sodium Sulfate, Ammonium Chloride, Ammonium Nitrate, and Salt Mixtures on Aqueous Phase Partitioning of Organic Compounds

Abstract

Dissolved inorganic salts influence the partitioning of organic compounds into the aqueous phase. This influence is especially significant in atmospheric aerosol, which usually contains large amounts of ions, including sodium, ammonium, chloride, sulfate, and nitrate. However, empirical data on this salt effect are very sparse. Here, the partitioning of numerous organic compounds into solutions of Na₂SO₄, NH₄Cl, and NH₄NO₃ was measured and compared with existing data for NaCl and (NH₄)₂SO₄. Salt mixtures were also tested to establish whether the salt effect is additive. In general, the salt effect showed a decreasing trend of Na₂SO₄ > (NH)₂SO₄ > NaCl > NH₄Cl > NH₄NO₃ for the studied organic compounds, implying the following relative strength of the salt effect of individual anions: SO₄^2- > Cl^- > NO₃^- and of cations: Na⁺ > NH₄⁺. The salt effect of different salts is moderately correlated. Predictive models for the salt effect were developed based on the experimental data. The experimental data indicate that the salt effect of mixtures may not be entirely additive. However, the deviation from additivity, if it exists, is small. Data of very high quality are required to establish whether the effect of constituent ions or salts is additive or not.