Deliquescence, efflorescence, and water activity in ammonium nitrate and mixed ammonium nitrate/succinic acid microparticles

We have investigated the thermodynamics and kinetics of ammonium nitrate/wa ter and mixed ammonium nitrate/succinic acid/water microparticles. The wate r activity of ammonium nitrate microparticles is determined as a function o f composition down to 12% relative humidity by accounting for the rapid eva poration of ammonia and nitric acid. Both the observed deliquescence and wa ter activities for ammonium nitrate/water microparticles are found to be in good agreement with predicted values. Supermicron ammonium nitrate particl es generated from high-purity methanol are found not to effloresce, but for m an anhydrous liquid state observed only in particle form. We also present data on the hydroscopicity and phase transitions of internally mixed ammon ium nitrate/succinic acid particles. A pronounced reduction in the particle growth factor at deliquescence is reported, indicating that the succinic a cid does not take up a significant amount of water. Additionally, the deliq uescence relative humidity is found to decrease slightly as the mass percen t of succinic acid is increased from 12.5 to 50%. Solid succinic acid in am monium nitrate drops acts to catalyze efflorescence of ammonium nitrate at high relative humidity. However, when the relative humidity is increased su fficiently to completely dissolve the succinic acid, no efflorescence is ob served. The resulting data are analyzed using classical nucleation theory t o derive the free energy barrier to nucleation, the critical supersaturatio n, the size of the critical nucleus, and the contact parameter between ammo nium nitrate crystal and the solid succinic acid.