PFG NMR Investigations of Tetraalkylammonium-Silica Mixtures

The results from PFG NMR studies of several tetraalkylammonium-silica mixtures are reported. pH data of these mixtures correlate well with previously reported literature. H-1 NMR shows line broadening and shifting of resonances for hydrogen atoms on the periphery of the TAA cations as the silica content is increased. Transverse relaxation measurements show a strong correlation with the pH data in that T-2 decreases strongly until the silica critical aggregation concentration (cac) is reached at which point the T-2 plateaus. These results point to organocation-silica interactions, which were quantified by using pulsed-field gradient NMR. The self-diffusion coefficient of a series of TAA cations in the presence of silica were measured, and a systematic decrease of the diffusion coefficient is observed as the cation/silica ratio decreases. Subsequent analysis gives confidence that the diffusion behavior of the TAA cations can be described as a two-state system, where cations are either free in solution or bound to the silica nanoparticles in the mixture. On the basis of this binding isotherms can be constructed and binding energies can be determined. These values are in the range of -10 to -14 kJ/mol, with tetramethylammonium on the high end and tetrapropylammonium cations on the low end. One conclusion drawn from this work is that the more hydrophilic TMA cations associate more strongly with the silica nanoparticles than TPA. The investigation shows that PFG NMR is a powerful tool for investigating these clear mixtures, and the results are discussed in the context of zeolite nucleation and growth.