Unusual, pH-Induced, Self-Assembly Of Sophorolipid Biosurfactants

Abstract

International audienceAn increasing need exists for simple, bioderived, nontoxic, and up-scalable compounds with stimuli-responsive properties. Acidic sophorolipids (SL) are glucose-based biosurfactants derived from the yeast broth of Candida bombicola (teleomorph: Starmerella bombicola). The specific design of this molecule, a sophorose head with a free end-COOH group at the end of the alkyl chain, makes it a potentially interesting pH-responsive compound. We have specifically investigated this assumption using a combination of small angle neutron scattering (SANS), transmission electron microscopy under cryogenic conditions (Cryo-TEM), and nuclear magnetic resonance (NMR) techniques and found a strong dependence of SL self-assembly on the degree of ionization, α, of the COOH group at concentration values as low as 5 and 0.5 wt %. At least three regimes can be identified where the supramolecular behavior of SL is unexpectedly different: (1) at low α values, self-assembly is driven by concentration, C, and micelles are mainly identified as nonionic objects whose curvature decreases (sphere-to-rod) with C; (2) at mid α values, the formation of COO– groups introduces negative charges at the micellar surface inducing an increase in curvature (rod-to-sphere transition). Repulsive electrostatic long-range interactions appear at this stage. In both regimes 1 and 2, the cross-section radius of the micelles is below 25 Å. This behavior is concentration independent. (3) At α = 1, individual micelles seem to favor the formation of large netlike tubular aggregates whose size is above 100 nm. Such a complex behavior is very unique as it is generally not observed for common alkyl-based surfactants in concentration ranges below 5–10 wt %

Surface-induced assembly of sophorolipids

“…It was interesting to note that at basic pH, structure of an APG film seemed to be more densely packed (about 16 200 patches per mm 2 ) than at acidic pH (about 4000 patches per mm 2 at pH 4), but the overall surface organization was the same at all pH values. These two tests performed on control molecules confirmed that the evolution of sophorolipids self-assembly onto silicon wafer arises from the pH-dependence of self-assembly properties of this molecule, 56 and that these were not artifacts generated by possibly different wettability of silicon under different pH conditions.…”

“…They have been recovered using method 1 as reported in a previous study by Baccile et al 65 Their purity has been evaluated at about 90% of both terminal and subterminal C18:1 congener and their equilibrium state in water is micellar as described earlier. 56,62,66 Sodium dodecyl sulfate (SDS) has been purchased from Aldrich and used as received. C10-Alkylpolyglycoside (APG, commercial name: Oramix) was provided by SEPPIC (France).…”

“…56,[60][61][62] For pH o 5, sophorolipids form neutral spheroidal micelles, 61 while for 5 o pH o 7, micelles interact with one another due to negative charge coming from the partial deprotonation of the carboxylic acid. 61,62 Finally, for pH 4 8, small spheroidal micelles and platelets to some extent are detected using combined SAXS and cryo-TEM.…”

“…96,97 Given the fact that silica had the lowest PZC, its surface could reasonably be considered to be rich in SiO À groups (enhancing the g À S component of surface energy) at different extent according to the solution pH. At pH 4, the sophorolipids solutions form micelles that are not charged 56 and silica can be considered as poorly charged, while at pH 6, the sophorolipids solutions are characterized by increasingly charged micelles undergoing electrostatic repulsion, 56,62 whereas silica can be practically considered to be fully charged. Moreover, in case of TiO 2 , given the higher IEP, much difference in terms of surface charge between pH 4 and pH 6 must not be expected.…”

“…1), and in particular their pH-responsive self-assembly have been shown earlier. [55][56][57] It has also been established that the structure of the molecules greatly influenced the self-assembly. 58 Acidic monounsaturated sophorolipids form ellipsoidal micelles in water in a very broad pH range (from pH 11 to pH 3), 59 although at basic pH, formation of nanoplatelets is also observed.…”