Kinetics of the Aqueous-Ethanol Solution Mediated Transformation between the Beta and Alpha Polymorphs of p‑Aminobenzoic Acid

A quantitative kinetics analysis of the solution-mediated phase transformations in aqueous-ethanol between the enantiotropically related alpha and beta polymorphs of p-aminobenzoic acid (PABA) is presented. A new temperature-controlled transmission flow-cell design for use with a laboratory X-ray source which can be utilized for diffraction or scattering experiments is described. This transmission cell in combination with in situ slurry X-ray diffraction is utilized to provide a quantitative analysis of the percentage by mass of respective solid phases in flowing suspensions, coupled with solution concentration as measured with UV–vis spectroscopy. The separate phases are identified through individual fingerprint peaks, the (1 0 3) reflection at 2θ = 15.6° relating to the alpha phase and the (0 2 2) reflection at 2θ = 25.3° for the beta phase. Examination of the conversion of a slurry of beta PABA in a 70:30 water/ethanol solvent mixture to the alpha polymorph, at a temperature range of 24–30 °C reveals the rate of dissolution of the beta phase is consistent with zero order kinetics (rate constant k_dβ 6.80 × 10^–4 to 8.17 × 10^–5 wt % s^–1), while the growth of the alpha phase follows first order kinetics (k_gα 1.00 × 10^–3 to 2.50 × 10^–4 wt % s^–1). Extrapolation of the growth and dissolution rate constants yields a transformation temperature for the phase interconversion of 22–24 °C. Overall the data are consistent with a fast de-supersaturation of the solution concentration indicating a dissolution-controlled polymorphic transformation process.