Thermodynamics and Nucleation Kinetics of m-Aminobenzoic Acid Polymorphs

The polymorphism of m-aminobenzoic acid has been investigated. Two polymorphs have been identified and characterized by X-ray powder diffraction (XRPD), Fourier transform IR (FTIR), microscopy, and thermal analysis. The melting properties and isobaric heat capacities of both polymorphs have been determined calorimetrically, and the solubility of each polymorph in several solvents at different temperatures has been determined gravimetrically. The solid-state activity (i.e., the Gibbs free energy of fusion) of each polymorph has been determined through a comprehensive thermodynamic analysis based on experimental data. It is found that the polymorphs are enantiotropically related, with a stability transition temperature of 156.1 °0C. The published crystal structure belongs to the polymorph that is metastable at room temperature. Energytemperature diagrams of both polymorphs have been established by determining the free energy, enthalpy, and entropy of fusion as a function of temperature. A total of 300 cooling crystallizations have been carried out at constant cooling rate using different saturation temperatures and solvents, and the visible onset of primary nucleation was recorded. The results show that for this substance the polymorph that will nucleate depends chiefly on the solvent. In water and methanol solutions, the stable form I was obtained in all experiments, whereas in acetonitrile, a majority of nucleation experiments resulted in the isolation of the metastable form II. It is shown how this can be rationalized by analysis of solubility, solution speciation, and nucleation relationships. The importance of carrying out multiple experiments at identical conditions in nucleation studies of polymorphic systems is demonstrated.QC 20101029. Uppdaterad från Manuskript till Artikel (20101029).