Mesocrystal engineering using non-bonded interaction to obtain optically transparent mesoporous silica films and plates with uniform orientation

A sol-gel process which achieves self-organization between surfactant and silicate through van der Waals type, weak, multiple, non-bonded interaction has been developed in order to synthesize mesoporous silica in the form of optically transparent films, fibers and plates. The synthesis process consists of the following two steps: an organic silica precursor such as tetraethylorthosilicate is hydrolyzed and simultaneously condensed to a suitable degree of polymerization in non-aqueous solutions containing substoichiometric amounts of water, hydrochloric acid and surfactants. The solvent is subsequently evaporated so that the surfactant-silicate organization is achieved by the weak enthalpy effect without the interference of the mixing entropy effect. The synthesis can be controlled so that mesoporous channels with uniform diameter are either hexagonally packed, parallel to the flat external surface or randomly oriented, depending on the reactant ratios and degree of the silicate polymerization in the first step. The optically transparent mesoporous silica plates, which are crack-free up to centimeters in size and 0.5 mm in thickness, exhibit uniform birefringence throughout the entire plate like a single crystal. The mesoporous silica materials show possibilities of the application as advanced materials, direct measurement of transport properties through channels, investigation of the order-disorder effects, and spectroscopic investigation of adsorbed species without using the diffuse reflectance. (C) 1998 Elsevier Science B.V. All rights reserved.