Effect of Methanol Concentration on CTAB Micellization and on the Formation of Surfactant-Templated Silica (STS)

We use light-scattering techniques to study the effects of methanol concentration on alkaline, cetyltrimethylammonium bromide (CTAB) water:methanol micellar solutions. We use X-ray diffraction, SEM, TEM, 29 Si NMR, and gas sorption measurements to study the structure, microstructure, and porosity of surfactant-templated silica (STS), synthesized by adding tetramethoxysilane (TMOS) to the above micellar solutions. The measured critical micelle concentration (cmc) for CTAB at 25 °C in a 0.22 M NaOH (pH 13.2) solvent increases from ∼1.3 × 10 - 3 M for r = 0% to ∼5.5 × 10 - 2 M for r = 60% (where r is the wt % methanol in the mixture) as the concentration of methanol increases. In turn, the long-range order of STS decreases as the methanol concentration increases. Ordered STS forms for 0 ≤ r < 60%, where the concentration of CTAB, c, is greater than cmc in the precursor solution; disordered STS (resembling wormlike micelles) forms for 60 ≤ r ≤ 90%, where c < cmc. For r > 90% transparent, amorphous chemical gels form. The presence of methanol leads to a uniform submicron microstructure as compared to faceted 1−10-μm particles with pure water. After template removal, apparent BET surface areas for STS can exceed 950 m 2 /g, and the void volume can exceed 0.6 cm 3 /g. Initially, there is a high fraction of uncondensed silica in the as-made product ( Q 3 / Q 4 ≈ 2.1), but after calcination a strong, bonded siloxane framework forms ( Q 3 / Q 4 ≈ 0.40).