Structure and Dynamics of Amino Functional Silanes Adsorbed on Silica Surfaces


Deuterated (aminopropyl)triethoxysilane (DAPES) and deuterated (aminobutyl)triethoxysilane (DABES) have been adsorbed on high-surface-area silica (Cab-O-Sil), dried, and probed with 2H (wide-line) and 29Si (CP/MAS) solid-state NMR techniques. When adsorbed, the DABES reacted to form siloxane linkages with the surface or other DABES molecules. The amount of DABES adsorbed on silica was dependent on the concentration of the silane coupling agent solution. The molecular motion of the adsorbed silane coupling agents on silica have been probed as a function of the amount and type of coupling agent. The slow-motional theory of Freed was used to simulate the resulting 2H spectra. The spectra were simulated by a superposition of a rigid and a mobile species. The C-D bonds of the mobile species undergo anisotropic diffusion (dz = 70-500 kHz and dxy = 10-100 kHz) plus faster two-site jumps about a diffusion axis tilted at 54.7° (rate = 1-2 MHz). When treated with a solution resulting in approximately monolayer coverage, the molecular motion of the coupling agent was the slowest. From treating solutions resulting in coverages above and below monolayer coverage, the spectra of the coupling agents on the surface were indicative of more mobile species. At the higher concentrations, the increase mobility a was similar to that of a polysilsesquioxane layer which was indirectly bonded to the surface was observed. Surfaces treated to be at submonolayer coverages were also less restricted. The 2H NMR spectra for DAPS and DABS on the silica surface also showed that the difference in the alkyl chain length of silane coupling agents did not significantly affect the molecular motions of silane when they were adsorbed on silica surfaces for these short (C3 and C4) chains. © 1991 American Chemical Society.



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© 1991 American Chemical Society (ACS), All rights reserved.