Micellization in binary biosurfactant/synthetic surfactant systems: Effects of temperature and hydrophobic group structure of alkyl benzenesulfonate

Micellization mechanism of biosurfactant/synthetic surfactant systems is essential in predicting their industrial applications. Biosurfactant, [Glu1,Asp5]-C15 surfactin, mixing with three isomers of sodium cetyl benzenesulfonate (SCBS) with benzenesulfonate attached at the 2nd, 5th, 8th carbon on the alkyl chain (abbreviated as 2-SCBS, 5-SCBS, and 8-SCBS), constituted the binary biosurfactant/synthetic surfactant systems in this work. Effects of mixing ratios, temperature, and hydrophobic structures of SCBS on micellization process were investigated by surface tensiometry, steady state fluorescence measurements, dynamic light scattering, and small angle neutron scattering. The results showed that at 60 ℃ synergism in micellization occurred in surfactin/2-SCBS and surfactin/5-SCBS binary systems, while antagonism appeared in surfactin/8-SCBS binary systems. Molecular interaction in mixed micelle at 60 ℃ turned from strong attractive to strong repulsive with the benzenesulfonate moving from the end to the middle of the alkyl chain. Surfactin exhibited weak interaction with both 5-SCBS and 8-SCBS at 25 ℃. Mixed surfactin/2-SCBS micelles were formed in spherical cylinders with more hydrophobic core and within 66 Å. Mixed surfactin/5-SCBS micelles and mixed surfactin/8-SCBS micelles showed similar morphology evolutions at 25 ℃, long elliptical cylinders transformed into small spherical cylinder by reducing both length and radius ratio of cross section with increasing surfactin mole fraction.