Phase Transformation Kinetics of Suberic Acid Polymorphs and Amorphous Dispersions Formed from Monodisperse Droplet Evaporation

Recognizing the importance of the crystallization of organic molecules is important in many industries, ranging from pharmaceuticals to food products. These molecules have particle properties that are required for a specific product to have the desired characteristics. These properties include size, morphology which is the particle shape, and internal structure which is the arrangement of molecules within the particle. They can affect bioavailability, solubility, and ease of processing. The internal structure of a solid can be either crystalline (where there is a regular pattern to the molecules in the solid) or amorphous (where there is no long-range pattern). Polymorphs are different crystalline arrangements of a specific molecule. At any given temperature and pressure there is one polymorph that is thermodynamically stable, while other polymorphs are metastable. Finally, an amorphous form is typically the least stable in comparison to all available crystalline forms. Since amorphous particles have a less stable internal structure, they are able to break down easily and are therefore more soluble, which is often desired in the pharmaceutical industry.

In this work, I studied the crystallization and phase transformation kinetics of particles formed via monodisperse droplet evaporation of suberic acid solutions. A mixture of suberic acid and a polymer additive: polyvinylpyrrolidone (PVP) produces particles ranging from crystalline (formed from pure suberic acid) to partially and fully amorphous (both formed with the presence of PVP), depending on the suberic acid:PVP ratio. X-Ray Diffraction (XRD) data shows the transitioning of an initially fully amorphous suberic acid/PVP sample to one that is partly crystalline. Suberic acid has two published polymorphs: alpha and beta and evidence of a third polymorph, gamma, was recently discovered via production in a Vibrating Orifice Aerosol Generator (VOAG). A sample formed from pure suberic acid in isopropyl alcohol showed initial evidence of a mixture of alpha and gamma polymorphs, however over time the metastable gamma form showed evidence of transitioning to the stable alpha form.