Discovery and recovery of delta p-aminobenzoic acid

Abstract

We explore the polymorphism of p-aminobenzoic acid (pABA) under high-pressure conditions. We have been able to isolate a new high-pressure form (δ-pABA) at pressures exceeding 0.3 GPa from three different pressure-transmitting media, water, water : ethanol and pure ethanol. We explore the compression behaviour of α-pABA in each of these media using neutron powder diffraction and observe that the transition is kinetically hindered using the aqueous ethanol and ethanol solutions compared with the pure aqueous medium. δ-pABA is sufficiently stable to be recovered to ambient pressure to enable its characterisation via X-ray powder diffraction and differential scanning calorimetry. At ambient pressure we have observed that δ-pABA converts into α-pABA on heating beyond 70 °C.