Thermostability of Condensed-phase Amines in the Presence of Sulfuric and Oxalic Acids

Grace Teall, CHONG QIU, University of New Haven

     Abstract Number: 375
     Working Group: Aerosol Chemistry

Abstract
Reduced Nitrogen Compounds such as ammonia and amines are widely observed in the ambient particulate matters and may neutralize inorganic and organic acids in the particle phase to increase the aerosol pH. The physical and chemical properties of the resulting salts, especially those associated with amines, remain largely unknown, creating barriers on better understanding the formation and transformation of ambient aerosol, especially in marine, agricultural and some industrial environments with significant amine emissions. Previous studies presented complex behaviors of particle-phase amine salts under various conditions, casting doubts on the true chemical identities and compositions of aerosol samples containing amines.

In this study, a molecular-level, bottom-up approach was used to investigate the thermostability of the salts of selected alkylamines (methylamine, MA; dimethylamine, DMA; and trimethylamine, TMA) with sulfuric and oxalic acids (as representative inorganic and organic diacids in ambient particles, respectively). The sulfate and oxalate salts of the three amines have been chemically synthesized, and their chemical identities verified. Thermal Gravimetric Analyzer (TGA) was used to determine the thermal decomposition profile of milligram scale salt samples in their dry state and in aqueous solutions. Considering the curvature effect, the results on amine sulfates appeared to agree well with those from aerosol-based measurements. Oxalate salts of the three amines demonstrated significantly lower decomposition temperatures than those of the respective sulfate salts. The stability of sulfate and oxalate salts of these amines depends strongly on their chemical structures and phase state. For example, MA and DMA oxalate salts are thermally stable both in dry state and in aqueous solution; but it was not possible to isolate TMA oxalate from its aqueous solution due to its thermal instability and subsequent decomposition into TMA hydrogen oxalate. Our results suggested that the diacid salts of DMA will consistently decompose at higher temperatures than those of MA and TMA salts, yet the presence of condensed water may affect its %mass remaining after thermal heating. Our lab measurements may help explain some of the field observations and provide fundamental property data for further modeling, computation and simulation.