Journal
JOURNAL OF OLEO SCIENCE
Volume 63, Issue 1, Pages 67-73Publisher
JAPAN OIL CHEMISTS SOC
DOI: 10.5650/jos.ess13047
Keywords
biosurfactant; sophorolipid; Stamerella bombicola; atomic force microscopy; Langmuir monolayer
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The synergic effect and miscibility of the lactonic and acidic forms of sophorolipids (SLs) produced by Stannerella bombicola NBRC 10243 were first evaluated through atomic force microscopy (AFM), together with the Langmuir monolayer technique. The pi-A isotherm of a pure lactonic sophorolipid (LS) monolayer mostly exhibited a liquid expanded monolayer, while that of an acidic sophorolipid (AS) monolayer showed a liquid condensed monolayer, suggesting that the lactonization of SL-s makes the molecules more bulky and prevents them from adopting a close-packed arrangement. Plots of the mean area per molecule of mixed LS/AS monolayers gave positive deviations from the ideal curves, implying that the LS and AS molecules are miscible. Interestingly, the positive deviation of excess area (A(ex)) from ideality was most significant at a mole fraction (X-AS) of 0.3, which closely resembles the composition of the LS/AS mixture secreted by S. bombicola in culture. The AFM images of mixed LS/AS monolayers transferred at 20 mN/m revealed no phase-separated microdomain structures, but rather showed small protruding objects for all compositions, indicating that LS and AS are partially miscible, as predicted by the positive deviations from the ideal curves. Cross-section analysis of the AFM images indicated that the observed protruding objects are AS-rich monolayers formed on the LS/AS monolayer. Our results clearly demonstrate that AFM combined with the Langmuir technique is useful for the exploration of the miscibility and synergic effects of microbial products.
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