Issue 25, 2013

Thermodynamics and nucleation of the enantiotropic compound p-aminobenzoic acid


In this work, the thermodynamic interrelationship of the two known polymorphs of p-aminobenzoic acid has been explored, and primary nucleation in different organic solvents investigated. The solubility of both polymorphs in several solvents at different temperatures has been determined and the isobaric solid-state heat capacities have been measured by DSC. The transition temperature below which form α is metastable is estimated to be 16 °C by interpolation of solubility data and the melting temperature of form β is estimated to be 140 °C by extrapolation of solubility data. Using experimental calorimetry and solubility data the thermodynamic stability relationship between the two polymorphs has been estimated at room temperature to the melting point. At the transition temperature, the estimated enthalpy difference between the polymorphs is 2.84 kJ mol−1 and the entropy difference is 9.80 J mol−1 K−1. At the estimated melting point of form β the difference in Gibbs free energy and enthalpy is 1.6 kJ mol−1 and 5.0 kJ mol−1, respectively. It is found that the entropic contribution to the free energy difference is relatively high, which explains the unusually low transition temperature. A total of 330 nucleation experiments have been performed, with constant cooling rate in three different solvents and with different saturation temperatures, and multiple experiments have been carried out for each set of conditions in order to obtain statistically significant results. All performed experiments resulted in the crystallization of the high-temperature stable α-polymorph, which is kinetically favoured under all evaluated experimental conditions. The thermodynamic driving force required for nucleation is found to depend chiefly on the solvent, and to be inversely correlated to both solvent polarity and to solubility.

Graphical abstract: Thermodynamics and nucleation of the enantiotropic compound p-aminobenzoic acid

Article information

Article type
07 Dec 2012
05 Feb 2013
First published
07 Feb 2013
This article is Open Access
Creative Commons BY license

CrystEngComm, 2013,15, 5020-5031

Thermodynamics and nucleation of the enantiotropic compound p-aminobenzoic acid

M. Svärd, F. L. Nordström, E. Hoffmann, B. Aziz and Å. C. Rasmuson, CrystEngComm, 2013, 15, 5020 DOI: 10.1039/C3CE26984A

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