INVESTIGATION OF THE STRUCTURE AND DYNAMICS OF SURFACTANT MOLECULES IN MESOPHASE SILICATES USING SOLID-STATE C-13 NMR

The structure and dynamics of surfactant molecule reorganization in me sophase silicates have been investigated using variable-temperature C- 13 solid-state nuclear magnetic resonance (NMR) spectroscopy. Function al groups and side groups of the surfactant CTAC (cetyltrimethylammoni um chloride) were identified from high-resolution C-13 MAS (magic angl e spinning) NMR spectra obtained using high-power H-1 decoupling. We a lso obtained information on surfactant organization and relaxation in mesophase silicates using a combination of NMR line-shape and relaxati on-time analyses with variable-temperature NMR. The behavior of the su rfactant in the ordered mesophase silicate was compared with that of t he surfactant solution (CTAC-water) and that of the surfactant in the disordered silicate, which was precipitated in solution during an earl y stage of the reaction. The electrostatic binding between the electro positive end of the surfactant and the silicate substrate causes a sim ilar to 1 ppm downfield shift for the NMR resonance associated with th e methyl groups next to the head group and substantial broadening for the peak corresponding to the methylene group adjacent to the head gro up. The splitting of the resonance associated with the N-methyl groups suggests that the methyl groups next to the head group lose their ste reochemical symmetry due to the intermolecular interaction in the orde red mesophase silicates. Each segment of the surfactant associated wit h an ordered silicate is less mobile than the corresponding segment as sociated with a disordered silicate precursor. For both ordered and di sordered silicates, the methylene group adjacent to the head group exh ibits a marked lack of motion relative to other segments of the surfac tant. Variable-temperature NMR studies show motional narrowing as temp erature increases. The NMR results obtained from this study provide in sight into the formation mechanism of mesophase materials.