Surfactants are a ubiquitous class of chemicals in high demand for a range of industrial applications due to their ability to increase solubility, foaming capacity, and lower surface tension. Rhamnolipids are a class of biosurfactants of emerging importance produced by a variety of microorganisms. This work reports synthetic methods for the production of single-tailed decyl- and tetradecyl-rhamnolipids and their xylose analogues, xylolipids, that intend to improve the overall green metrics offered by this class of biosurfactants, as well as characterize their surfactant properties. The previously reported synthetic method by which single-tailed rhamnolipids may be obtained with high efficiency through a two-step process is further elucidated by substituting xylose for rhamnose. By utilizing a bacterially synthesized (R)-3-hydroxytetradecanoic acid, significant improvement in the overall green metrics is achieved as well as a simplification of the resulting product mixture. Finally, the use of a pH-modulated liquid-liquid extraction-based purification technique is used to isolate pure single-tailed rhamnolipids and xylolipids without the use of column chromatography. The products resulting from substitution of biosynthetic 3-(R)-hydroxytetradecanoic acid in place of a racemic synthetic counterpart were found to have comparable interfacial properties to those previously reported. The resulting R-diastereomers resulting from xylose and rhamnose conjugation were isolated and characterized using tensiometry at pH 4 and 8. Comparison of the interfacial properties reveals that at pH 4, biosynthetic-derived Xyl-(R)-C14 and racemic Xyl-C14 have comparable CMC values of 147 ± 16 and 114 ± 9 μM, respectively. This is mirrored by racemic Rha-C14 and biosynthetic Rha-(R)-C14 with values of 102 ± 1 and 162 ± 46 μM, respectively. However, under anionic conditions, there is a greater divergence in CMC values, with racemic Rha-C14 and Xyl-C14 exhibiting far lower CMC values of 523 ± 133 and 423 ± 93 μM, respectively, than their biosynthetic counterparts at 1.80 ± 0.28 and 1.63 ± 0.29 mM for Rha-(R)-C14 and Xyl-(R)-C14, respectively.

Synthesis and characterization of rhamnolipids and xylolipids using biosynthetic and synthetic b-hydroxy acids

ACS Spring 2022

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