Cloud point curve of nonionic surfactant related to the structures of mesoporous materials

We have investigated the phase behavior of a fluorinated surfactant R7F(EO)7 in water. The cloud point is situated at 19 °C for 2 wt% of surfactant. Using this surfactant, mesoporous materials have been synthesized with micellar solution prepared either at 10 °C (below the cloud point) or at 40 °C (above the cloud point). Results show that whatever the syntheses conditions, only wormhole-like structure is recovered. The effect of perfluorodecalin addition on the fluorinated surfactant/water system was also investigated. Swollen micelles, microemulsion, and lamellar (Lα)(Lα) liquid crystals were identified. When perfluorodecalin is added, the cloud point is shifted toward higher temperature. As regards the mesoporous syntheses, perfluorodecalin plays a dual role. First, incorporation of perfluorodecalin leads to the formation of well ordered materials. Secondly, the pore size enlargement occurs when perfluorodecalin is added. Our results evidence that the ratio between the volume of the hydrophilic headgroup (VH)(VH) and the hydrophobic part (VL)(VL) of the surfactant is not an efficiency parameter to explain the ordering improvement of mesoporous materials and that we should rather consider the existence of the cloud point curve, which disturbs the cooperative templating mechanism (CTM).

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