Structural features of mixed monolayers of oleanolic acid and stearic acid

The features of oleanolic acid at interfaces and its effect on another component of the epicuticular wax are studied using a binary monolayer system consisting of oleanolic acid (OLA) and stearic acid (SA). A combination of lateral pressure studies, grazing incidence X-ray diffraction (GIXD), and specular X-ray reflectivity (SR) measurements, is proven to be optimal in this respect. In the case of the OLA monolayer, the LS phase exists in the whole accessible pressure region, whereas the SA monolayer develops the phase sequence from the nearest neighbor tilted L, phase to the next nearest neighbor tilted OV phase and, finally, to the hexagonal LS phase with nontilted molecules with increasing lateral pressure. The cross-section area of 44. 1 angstrom(2)center dot molecule(-1) obtained for the pure OLA monolayer from the position of the diffraction peak agrees well with the circular cross-section of the OLA molecular data. The thicknesses of the OLA monolayer determined by GIXD and XR coincide perfectly. The structure data obtained by GIXD and XR experiments, indicating a perpendicular orientation of the OLA molecules in the condensed monolayer phase in the whole lateral pressure region, are consistent with the calculated molecular data of OLA.The structural and phase properties of selected compositions of binary SA/OLA system are studied and result in a schematic phase diagram characterized by large miscibility gaps. The packing properties of both components after addition of minor amounts of the other component are completely different. The amount of OLA which can be integrated into the SA lattice is smaller than 10 mol %, whereas more than 10 mol % of SA can be incorporated into the OLA lattice. The addition of small amounts of OLA to the SA monolayer leads to a change in the phase sequence by disappearance of the OV phase, whereas the addition of small amounts SA to the OLA monolayer changes the packing density but does not change the phase structure.