The deliquescence behaviour, solubilities, and densities of aqueous solutions of five methyl- and ethyl-aminium sulphate salts

We report measured solubilities of five aminium (i.e., monomethyl, dimethyl, trimethyl, diethyl, and triethyl) sulphate salts in water at 24 °C, and the densities of their aqueous solutions. Using these results, we have converted hygroscopic growth factors determined by Qiu and Zhang (Environ. Sci. Technol. 2012, 46, 4474–4480) to a moles of water per mole of solute basis, and obtained the relationships between concentration and equilibrium relative humidity (water activity) for solutions of the five salts. The results are compared with values predicted using the Extended Aerosol Inorganics Model (E-AIM) of Clegg and co-workers (J. Geophys. Res. 2002, 107, D14, Art. No. 4207). It is assumed in this model that ion and water activities in the solutions are the same as those for aqueous (NH4)2SO4 at the same molality. The experimental and modelled growth factors agree well in all cases, within the uncertainties of the data, which supports this assumption. Equations for the apparent molar volumes of the aminium sulphate salts in aqueous solutions are presented (based upon the measured densities and literature data), and also activity products of the salts in saturated aqueous solutions (based upon the measured solubilities and assumption of similarity with (NH4)2SO4). Simulations of the deliquescence curves of 1:1 and 1:9 mass ratio mixtures of monomethyl and dimethyl aminium sulphate with (NH4)2SO4 are shown to agree well with the measurements. The treatment of amines and aminium salts in the E-AIM model is described.