Phase behaviour of oleanolic acid, pure and mixed with stearic acid: Interactions and crystallinity

The phase behaviour of pure oleanolic acid (OLA) and in mixtures with stearic acid (SA) was characterized by differential scanning calorimetry (DSC), X-ray powder diffraction (XRD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and nuclear magnetic resonance (NMR). The crystalline OLA as received (OLAar) becomes amorphous after being dissolved in chloroform and vacuum-dried at 50 °C (OLA50). Upon heating, both forms transform to the needle shape crystalline form (OLA220). Dimerization through H-bonding between COOH groups was detected both in OLAar and OLA220. Dimers are stronger in OLA220, where H-bonding also involves the alcohol groups and plays a role in the crystalline organization. A eutectic type phase diagram was established for mixtures, with the eutectic composition close to pure SA. Mixtures rich in SA are miscible in the liquid and in the amorphous solid states, where the presence of SA–OLA co-dimers, formed through H-bonding between carboxyl groups, was detected. Miscibility and SA crystallinity decrease drastically with the OLA content.