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

Abstract

The self-assembly of a non-ionic fluorinated surfactant C₆F₁₃CH₂(OC₂H₄)₃OH in water is investigated and compared with the system C₆F₁₃C₂H₄SC₂H₄(OC₂H₄)₂OH/water, which was previously studied. The same phases were identified, namely the sponge phase, the reverse bicontinuous cubic phase, the lamellar phase and the reverse micellar phase. These two systems mainly differ by 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 unit enables the formation of organized structures at higher water fractions, such as the sponge phase and the metastable lamellar phase. Both the structural study of the cubic and lamellar phases by small angle X-ray scattering and the analysis of Raman spectra show that an increase in the size of the polar head induces some conformational changes. In spite of these behaviour differences, some epitaxial relationships exist and clearly reveal whether the cubic phase is of the normal or inverse type.