Mt. Anderson et al., EFFECT OF METHANOL CONCENTRATION ON CTAB MICELLIZATION AND ON THE FORMATION OF SURFACTANT-TEMPLATED SILICA (STS), Chemistry of materials, 10(6), 1998, pp. 1490-1500
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).