Effects of the polymorphic transformation of glutaric acid particles on their deliquescence and hygroscopic properties

The effects of the polymorphism of glutaric acid (GA) particles on their deliquescence properties at room temperature were studied. The polymorphic states of GA, including the metastable α-form and the thermodynamically stable ß-form, were characterized by Raman spectral measurements. The deliquescence of GA particles was also studied by Raman and optical observations. It was found that the α-form deliquesced at 85-86% relative humidity (RH), whereas some α-form transformed to the ß-form rather than completing full deliquescence. The ß-form absorbed water slowly at 87-89% RH, which was higher than the deliquescence relative humidity (DRH) of the α-form. The water-uptake process was not completed in 5 h, but the particles deliquesced quickly (< 15 min) when the RH was increased to about 90%, indicating that mass transfer was delayed in the ß-form and that a higher RH of 90% was needed to cross the kinetic barrier. We showed that polymorphic transformation alters the deliquescence properties of GA particles, which might explain the discrepancies in DRH values of GA particles (83-90%) reported in the literature. This transformation should be taken into account in future laboratory experiments and aerosol thermodynamic models. © 2010 American Chemical Society.