Phase Behavior and Phase Structure of a Cationic Fluorosurfactant in Water

The phase behavior of the cationic fluorocarbon surfactant 1,1,2,2-tetrahydroperfluorodecylpyridinium chloride in water has been studied using small-angle X-ray and 2 H NMR spectroscopy as the main techniques. The following phase sequence was found: micellar (L 1 ), hexagonal (H I ), centered rectangular (R; spacegroup c 2 mm ) and centered trigonal (T, spacegroup R 3̄ m ) with increasing surfactant concentration. At higher concentrations evidence for random mesh phase and a lamellar phase was found, at high temperatures a bicontinuous cubic phase was found. Compared to a cationic hydrocarbon surfactant of similar hydrophobicity, the occurrence of the intermediate phases is more extensive in the phase diagram, and the fluorocarbon surfactant prefers to form intermediate phases instead of a cubic phase. The surface curvature of the aggregates changes from highly curved cylindrical structures of the hexagonal phase to the planar bilayer structure of the lamellar phase over a sequence of geometries that allow the average of the mean curvature to change gradually as water content is decreased.