Ordered Mesoporous Silica Templated by Nonionic Fluorinated Liquid Crystals

Here, for the first time, liquid crystals of fluorinated surfactants have been used as templates for the preparation of mesostructured silica, We have focused our study on the hexagonal liquid crystal phase of the C8F17C2H4(OC2H4)(9)OH/water system. The different synthesis parameters such as pH, surfactant/silica precursor ratio, and surfactant concentration have been optimized. The recovered materials have been characterized by SAXS measurements, nitrogen adsorption-desorption, and transmission electron microscopy. Results clearly evidence that the pH and the kind of acid play a crucial role in the synthesis of well-ordered materials. As a matter of fact, by using HCl as acid if the pH value is increased above 2, the regular channel array is lost and the recovered materials exhibit a randomly oriented channel system. Moreover, in the synthesis conditions used, due to the counter effect, HCl should be employed rather than H2SO4. Another parameter that affects the structure of the recovered material is the surfactant/silica ratio. Indeed, the mesopore ordering is detected only for ratios located in the range between 0.119 and 0.175. An increase of the surfactant/silica molar ratio results in materials with a disordered channel array. Finally, the structural parameters of the mesoporous materials have been correlated to those of the liquid crystal phase used as the fingerprint for their synthesis. At high surfactant concentration, the pore diameter matches well with the hydrophobic diameter of the cylinders of the H, phase.