Polymorphism of Glutaric Acid Aerosols

Aerosols are very tiny particles (typically on the nano scale) that are present in the air. They are important to understand because of the sheer quantity of aerosols that are present in the atmosphere. Inorganic aerosols and their interactions are well-studied and understood but we still have a great deal of studying to do with regard to organic aerosols. Organic aerosols make up a large portion of the particles (20-50%) in the atmosphere by mass, a lot of these being dicarboxylic acids. Because of this, dicarboxylic acids are commonly studied in the aerosol field, with subjects of interest being their hygroscopicity and cloud condensation nuclei activity.

Glutaric acid, a water soluble dicarboxylic acid, is commonly used to model water activity in dicarboxylic acid aerosols. It exhibits dimorphism, which means that the molecule can form two different types of crystal structures. The two polymorphs of Glutaric Acid are known as the alpha and beta polymorphs. The alpha polymorph is in the metastable phase, meaning that it is less stable and after some time will revert back to the beta polymorph. The alpha polymorph is favored at higher temperatures and is more soluble than the beta phase, and has been produced under some very specific conditions. This is thought to have led to some variability with regard to the data collected in several experiments involving glutaric acid.

In this experiment, we investigated the formation of alpha glutaric acid polymorph in aerosols generated from an aqueous glutaric acid solution. The formation of both polymorphs from an aqueous solution has never been documented before. Understanding the conditions that lead to the formation of the alpha polymorph will be valuable when attempting to investigate glutaric acid as a dicarboxylic acid.