The thermal expansion coefficients of the alpha and beta polymorphic forms of p-aminobenzoic acid in relation to their bulk crystal chemistry

Abstract

The thermal expansion behaviour of the alpha and beta polymorphs of para-aminobenzoic acid are presented and discussed in terms of the bulk crystal chemistry and the associated strengths of the constituent intermolecular synthons for these two materials. Analysis of temperature dependant powder diffraction data over the temperature range 298.15–403.15 K facilitates calculation of the linear thermal expansion coefficients: α_a = 8.36 × 10⁻⁰⁶ K⁻¹, α_b = 94.5 × 10⁻⁰⁶ K⁻¹ and α_c = 9.91 × 10⁻⁰⁶ K⁻¹ for the alpha polymorph and α_a = 21.5 × 10⁻⁰⁶ K⁻¹, α_b = 48.5 × 10⁻⁰⁶ K⁻¹ and α_c = 2.22 × 10⁻⁰⁶ K⁻¹ for the beta polymorph. The exceptionally large increase in the thermal expansion of the b axis for the alpha form reflects the weak dispersive interactions which propagate along this axis. In contrast, the a and c axes contain relatively strong hydrogen bonds which stabilise the lattice and limit thermal expansion. The thermal expansion of the beta form reflects the more isotropic nature of the intermolecular synthons for this polymorph in comparison to the alpha form. The thermal expansion of the b axis of the beta form is larger than that of the a and c axes but to a much lesser extent than that observed for the alpha form. This is rationalised through identification of a hydrogen bonding component which contributes to the stabilisation of the b axis in comparison to the almost fully dispersive nature found in the alpha structure.