Influence of methanol on the phase behavior of nonionic fluorinated surfactant: Relation to the structure of mesoporous silica materials

We have investigated the effect of methanol addition on the RF8(EO)9 and RF7(EO)8 surfactant-based systems. While upon the addition of methanol the L1 micellar phase grows, the direct hexagonal (H1) and the lamellar (Lα) liquid crystals progressively melt with the increase of alcohol content. Phase behavior and SAXS measurements proved that methanol molecules interact with the oxyethylene units of the surfactant. This involves a folding up of the hydrophobic chains in the liquid crystal phases. Moreover, for the RF7(EO)8 surfactant, the cloud point curve is shifted to high temperatures upon addition of alcohol. Starting from these systems, we have prepared mesoporous materials. Results show that due to the hydrogen bonds between the alcohol and the EO groups, the hexagonal structure of the mesostructured silica obtained from RF8(EO)9 is lost when the content of CH3OH is increased. In contrast, for the compounds prepared from the RF7(EO)8-based system, the pore ordering occurs in the presence of alcohol. This phenomenon has been related to the moving of the cloud point curve toward high temperatures with the addition of methanol. Our study reveals also that under our conditions the methanol released during the hydrolysis of the silica precursor does not affect the self-assembly mechanism in a positive or negative way.

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