Lyotropic Liquid-Crystalline Phases of Sophorolipid\nBiosurfactants

Abstract

Biological\namphiphiles derived from natural resources are presently\nbeing investigated in the hope that they will someday replace current\nsynthetic surfactants, which are known pollutants of soils and water\nresources. Sophorolipids constitute one of the main classes of glycosylated\nbiosurfactants that have attracted interest because they are synthesized\nby non-pathogenic yeasts from glucose and vegetable oils at high titers.\nIn this work, the self-assembly properties of several sophorolipids\nin water at high concentrations (20–80 wt %), a range so far\nmostly uncharted, have been investigated by polarized-light microscopy\nand X-ray scattering. Some of these compounds were found to show lyotropic\nliquid-crystalline behavior as they display lamellar or hexagonal\ncolumnar mesophases. X-ray scattering data shows that the structure\nof the lamellar phase is almost fully interdigitated, which is likely\ndue to the packing difference between the bulky hydrophilic tails\nand the more compact aliphatic chains. A tentative representation\nof the molecular organization of the columnar phase is also given.\nMoreover, some of these compounds display thermotropic liquid-crystalline\nbehavior, either pure or in aqueous mixtures. In addition, small domains\nof the lamellar phase can easily be aligned by applying onto them\na moderate a.c. electric field, which is a rather unusual feature\nfor lyotropic liquid crystals. Altogether, our work explored the self-assembly\nliquid-crystalline behavior of sophorolipids at high concentration,\nwhich could shed light on the conditions of their potential industrial\napplications as well as on their biological function.