Phase behaviour of oleanolic acid/stearyl stearate binary mixtures in bulk and at the air–water interface

The solid–liquid phase behaviour of oleanolic acid (OLA)/stearyl stearate (SS) was investigated by differential scanning calorimetry and polarizing optical microscopy. A eutectic type diagram, with the eutectic composition close to pure SS was obtained. Complementary studies by NMR, X-ray diffraction (XRD) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy were performed. A mutual influence was detected in mixtures: the low melting form of SS is favoured at low OLA molar fractions (XOLA) and spherulitic structures appear at high XOLA and high temperature. Additionally, H-bonding between OLA carbonyl groups increases in the presence of SS. The study of OLA/SS by the Langmuir method and Brewster angle microscopy revealed the organization at the air–water interface: OLA interacts with water in the first layer, while SS is completely segregated to the upper layer for XOLA > 0.3, and it distributes in the first and upper layers for XOLA < 0.3.