Synthesis of surfactant-templated mesoporous materials from homogeneous solutions Page: 4 of 17
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requires about two orders of magnitude longer. The rate determining step for product for-
mation thus appears to be the hydrolysis of the alkoxide, as TEOS is known to hydrolyze
much more slowly than TMOS owing to its bulkier R-group.3
Reaction temperature significantly affects the hydrolysis rate of metal alkoxides.
We sought to predictably alter the mesophase reaction time by changing the synthesis tem-
perature. By using C16TABr with methanol or formamide as cosolvents we were able to
make periodic products in the range from 10 *C to 100 *C. At < 10 *C, C16TABr phase
To make products at < 10 *C, we used the sulfate salt of C16TA* (Ci6TAHSO4). The
C16TAHSO4 remains dissolved in solution to very low temperatures and allows periodic
products to be formed as low as -14 *C. The kinetics exhibit Arrhenius behavior from -14
*C T 100 *C ( Figure2).
Finally, the silica source influences the tertiary structure of the mesophase. The
rapid hydrolysis of the alkoxide generates hydrolyzed species throughout the reaction vol-
ume. As these species interact with the surfactants, the local concentration of each in-
creases and the assemblies begin to phase separate into microdomains. Invariably, phase
separation leads to physical gels composed of submicron crystallites.
At the time of phase separation, the surface of the silica/surfactant crystallites con-
tain a large number of reactive hydroxyl and methoxy groups (from 29Si NMR, Q3/Q4 = 2.1
after 5 minutes). Such species can readily chemically crosslink with the elimination of wa-
ter or methanol to form Si-O-Si bonds. We believe that the rapid formation of crystallites
throughout the reaction volume that have a high concentration of hydroxyl and methoxy
groups assists the formation of physical gels.
Figure 1. In situ X-ray diffraction of mesophases made at 25 *C from a 2 wt% C16TABr in 75/25
water/methanol solution. All samples gelled in 7 s. Aging times are shown on the right. The inset shows
part of the spectrum for the sample aged 1 minute.
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Anderson, M.T.; Martin, J.E.; Odinek, J. & Newcomer, P. Synthesis of surfactant-templated mesoporous materials from homogeneous solutions, article, August 1, 1995; Albuquerque, New Mexico. (digital.library.unt.edu/ark:/67531/metadc625238/m1/4/: accessed November 14, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.