Phase behavior of ternary mannosylerythritol lipid/water/oil systems

Mannosylerythritol lipids (MELs) are glycolipid biosurfactants (BS) abundantly produced from renewable resources by yeast strains of the genus Pseudozyma. In this study, the ternary phase behaviors of two types of MELs, i.e. MEL-A and MEL-B, mixed with water and oil were investigated at 25 °C based on polarized optical microscopy and small-angle X-ray scattering (SAXS). When n-decane was used as an oil phase, diacetylated MEL-A formed single-phase water-in-oil (W/O) microemulsion in a remarkably large region. MEL-A, with a negative spontaneous curvature, also formed sponge (L3), reverse bicontinuous cubic (V2), and lamellar (Lα) phases. Meanwhile, monoacetylated MEL-B, with the opposite configuration of the erythritol moiety, gave single-phase bicontinuous microemulsion and showed a triangular phase diagram dominated by the Lα phase, suggesting that MEL-B has an almost zero spontaneous curvature. Moreover, we succeeded in preparation of oil-in-liquid crystal (O/LC) emulsion in the biphasic Lα + O region of the MEL-B/water/n-decane system. The obtained gel-like emulsion was stable for at least 1 month. These results clearly demonstrated that the difference in the number of acetyl group on the headgroup and/or the chirality of the erythritol moiety drastically changed the phase behavior of MELs. Accordingly, these MELs would be quite distinctive from conventional BS hitherto reported, and would have great potential for the preparation of microemulsion and LC-based emulsion.