Investigation of the Structure and Dynamics of Surfactant Molecules in Mesophase Silicates Using Solid-State ¹³C NMR

The structure and dynamics of surfactant molecule reorganization in mesophase silicates have been investigated using variable-temperature 13 C solid-state nuclear magnetic resonance (NMR) spectroscopy. Functional groups and side groups of the surfactant CTAC (cetyltrimethylammonium chloride) were identified from high-resolution 13 C MAS (magic angle spinning) NMR spectra obtained using high-power 1 H decoupling. We also obtained information on surfactant organization and relaxation in mesophase silicates using a combination of NMR line-shape and relaxation-time analyses with variable-temperature NMR. The behavior of the surfactant in the ordered mesophase silicate was compared with that of the surfactant solution (CTAC−water) and that of the surfactant in the disordered silicate, which was precipitated in solution during an early stage of the reaction. The electrostatic binding between the electropositive end of the surfactant and the silicate substrate causes a ∼1 ppm downfield shift for the NMR resonance associated with the methyl groups next to the head group and substantial broadening for the peak corresponding to the methylene group adjacent to the head group. The splitting of the resonance associated with the N -methyl groups suggests that the methyl groups next to the head group lose their stereochemical symmetry due to the intermolecular interaction in the ordered mesophase silicates. Each segment of the surfactant associated with an ordered silicate is less mobile than the corresponding segment associated with a disordered silicate precursor. For both ordered and disordered silicates, the methylene group adjacent to the head group exhibits a marked lack of motion relative to other segments of the surfactant. Variable-temperature NMR studies show motional narrowing as temperature increases. The NMR results obtained from this study provide insight into the formation mechanism of mesophase materials.