Effect of polyol on the structure of nonionic surfactant reverse micelles in glycerol monoisostearate/decane systems

Using small-angle X-ray scattering (SAXS), effects of different polyols used as polar additives, glycerol (GC), ethylene glycol (EG), and 1,2-butanediol (1,2-BD), on the structure of nonionic surfactant glycerol monoisostearate (iso-C(18)G(1)) reverse micelles in decane have been investigated as a function of polyol concentration and temperature. The real space structural functions of the reverse micelles were obtained by generalized indirect Fourier transformation (GIFT) evaluation of the SAXS data, letting the form factor virtually model-free, and the results were complemented by conventional theoretical model fittings. The iso-C(18)G(1) forms spheroid type or slightly elongated prolate type micelles in n-decane. We have found that addition of these polyols causes two-dimensional (2-D) growth of the reverse micelles in a similar manner to that induced by added water, both the maximum length and micellar cross-section diameter continuously increasing with polyol concentration. EG is most effective to increase the micellar size. 1,2-BD, which is apparently more hydrophobic than EG, has a weakest effect on the micellar growth. Unexpectedly, GC exhibits a less pronounced effect than EG despite its stronger hydrophilicity, which may be related to the similarity of the molecular structure to the surfactant glycerol moiety. The data demonstrate that different polarities of additives can be an additional tunable parameter for controlling the structure of the reverse micelles. The results well complement our recent findings that slightly different properties of solvent oils can provide a significant effect on the reverse micellar structures in glycerol-based surfactant systems.