Organogels are useful as a template for the preparation of novel helical silica fibers

Sol-gel polymerization of tetraethoxysilane (TEOS) was carried out in the organogel phase consisting of cholesterol-based gelators (neutral 1 and cationic 2). TEOS polymerization in neutral 1 + MeCO₂H gel resulted in the conventional granular silica whereas that in cationic 2 + MeCO₂H gel yielded the novel mesoporous silica with a tubular structure. SEM and TEM observations and additive effects established that the organogel fibers act as a template in the TEOS polymerization process to yield the hollow silica fibers and that the electrostatic interaction plays a crucial role in adsorption of anionic oligomeric silica particles onto the cationic organogel fiber surface. Thus, the tubular structure is created after combustion of the gelators by calcination. Very interestingly, when TEOS polymerization was carried out in the 1 + 2 mixed organogel, fibrous silica with a right-handed "helical" structure was created. This phenomenon appeared only in the range of 2/(1 + 2) = 5-15 mol%. Since the higher-order helical structure is characteristic of supramolecular assemblies of "chiral" organic compounds, it is suggested that the chirality in the organogel fibers is successfully transcribed into the inorganic silica fibers. Thus, the concept presented in this paper describes a novel template effect and should be broadly applicable to the design of "supramolecular" silica materials useful for catalysts, memory storage, replication, etc.