Understanding the Aqueous Self-Assembly of Raw Sophorolipid Mixtures Using a Statistical Approach Applied to Synchrotron Small Angle X-Ray Scattering (SAXS): A Focus on pH, Dilution, Aging, and the Underestimated Role of Residual Fatty Acids

Abstract

The aqueous self-assembly of raw sophorolipid mixtures has been studied by small angle x-ray scattering (SAXS) to understand the impact of the composition variability and stability of the samples on formulation properties. The sophorolipid mixtures are mainly composed of acidic and lactonic C18:1-cis mono and diacetylated sophorolipids with a small fraction of non-acetylated compounds and congeners with different chain length and unsaturation. The mixture is also composed of fatty acids, acetic acid, salts and yeast remaining from the fermentation process. Two samples, with distinct residual fatty acids content were selected for this work and the self-assembly of these mixtures in water was studied under various conditions (pH, dilution, aging and fatty acids removal) to understand which parameter is critical on the phase and morphology and size of self-assembled aggregates. The micellar phase of sophorolipids can be modeled with a core-shell prolate ellipsoid form factor from which core and shell size, as well as anisotropy can be extracted. This study highlights the unexpected strong impact of fatty acids. Above pH 5.5, the presence of fatty acids (≥ 9 wt%) has an impact on the micellar radius, +40% larger compared to a fatty-acid free sophorolipid. At pH ≤ 5.5, combined with high content of fatty acids, a phase transition is observed from spheres to elongated micelles. An aging study (3 weeks), performed on sophorolipid mixtures show no evolution of the micellar phase and structural parameters during storage at high temperature (60°C).