Effect of the molecular structure of non-ionic fluorinated surfactants on their phase behaviour in water

The self-assembly of a non-ionic fluorinated surfactant C6F13CH2(OC2H4)(3)O H in water is investigated and compared with the system C6F13C2H,SC2H4(OC2H 4)(2)OH/water, which was previously studied. The same phases were identifie d, namely the sponge phase, the reverse bicontinuous cubic phase, the lamel lar phase and the reverse micellar phase. These two systems mainly differ b y the relative variations of the sponge and cubic domains. The substitution of a methylene unit by a sulfur-containing group enhances the stability of the cubic phase and the lengthening of the oxyethylene chain by only one u nit enables the formation of organized structures at higher water fractions , such as the sponge phase and the metastable lamellar phase. Both the stru ctural study of the cubic and lamellar phases by small angle X-ray scatteri ng and the analysis of Raman spectra show that an increase in the size of t he polar head induces some conformational changes. In spite of these behavi our differences, some epitaxial relationships exist and clearly reveal whet her the cubic phase is of the normal or inverse type.