Mechanisms of Pore Size Control in MSU-X Mesoporous Silica

Mesoporous silicas of the MSU-X type, made with nonionic polyethylenoxide-based (PEO) surfactants, were synthesized according to a two-step process that gives intermediary stable hybrid micelles. The influence of the various synthesis parameters, such as the silica concentration, the addition of a swelling agent (TMB: trimethyl-1,3,5 benzene), the condensation catalyst (NaF), and the temperature of silica condensation, was systematically explored, especially their influence on the material pore size. Surprisingly, changing the silica over surfactant ratio does not modify the silica wall thickness but the other parameters have a strong effect on the final pore size. Considering the model of hybrid micelles that was previously reported (Boissière et al. Chem. Mater. 2001, 13, 3580), we explained the effect of the different parameters by a classical swelling effect of the hydrophobic micelle core for TMB, whereas the modulation of the hydrophilicity of the PEO chains (for temperature) or that of the silica oligomers (for NaF) describes correctly the variations observed. Finally, the cumulative effect of these parameters is demonstrated and we show that materials with narrow pore size distributions tuned between 2.5 and 5 nm can be obtained without changing the surfactant nor adding any swelling agent, by varying only the synthesis parameters.