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 co ncentration on alkaline, cetyltrimethylammonium bromide (CTAB) water:m ethanol micellar solutions. We use X-ray diffraction, SEM, TEM, Si-29 NMR, and gas sorption measurements to study the structure, microstruct ure, 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 degrees C in a 0.22 M NaOH (pH 13.2) solvent increases from similar to 1.3 x 1 0(-3) M for r = 0% to similar to 5.5 x 10(-2) M for r = 60% (where r i s the wt % methanol in the mixture) as the concentration of methanol i ncreases. In turn, the long-range order of STS decreases as the methan ol concentration increases. Ordered STS forms for 0 less than or equal to r < 60%, where the concentration of CTAB, c, is greater than cmc i n the precursor solution; disordered STS (resembling wormlike micelles ) forms for 60 less than or equal to r less than or equal to 90%, wher e c < cmc. For r > 90% transparent, amorphous chemical gels form. The presence of methanol leads to a uniform submicron microstructure as co mpared to faceted 1-10-mu m 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 fr action of uncondensed silica in the as-made product (Q(3)/Q(4), approx imate to 2.1), but after calcination a strong, bonded siloxane framewo rk forms (Q(3)/Q(4) approximate to 0.40).