Volatility of atmospherically relevant alkylaminium carboxylate salts

Heterogeneous neutralization reactions of ammonia and alkylamines with sulfuric acid play an important role in aerosol formation and particle growth. However, little is known about the physical and chemical properties of alkylaminium salts of organic acids. In this work we studied the thermal stability and volatility of alkylaminium carboxylate salts of short aliphatic alkylamines with monocarboxylic and dicarboxylic acids. The enthalpy of vaporization and saturation vapor pressure at 298 K were derived using the kinetic model of evaporation and the Clausius-Clapeyron relation. The vapor pressure of alkylaminium dicarboxylate salts is ∼10^-6 Pa, and the vaporization enthalpy ranges from 73 to 134 kJ mol^-1. Alkylaminium monocarboxylate salts show high thermal stability, and their thermograms do not follow our evaporation model. Hence, we inferred their vapor pressure from their thermograms as comparable to that of ammonium sulfate (∼10^-9 Pa). Further characterization showed that alkylaminium monocarboxylates are room temperature protic ionic liquids (RTPILs) that are more hygroscopic than ammonium sulfate (AS). We suggest that the irregular thermograms result from an incomplete neutralization reaction leading to a mixture of ionic and nonionic compounds. We conclude that these salts are expected to contribute to new particle formation and particle growth under ambient conditions and can significantly enhance the CCN activity of mixed particles in areas where SO₂ emissions are regulated.