Kinetics of interaction of 3-aminopropyltriethoxysilane with silica gel using elemental analysis and 29Si NMR spectra

Three silica gel sample systems, modified with 3-aminopropyltriethoxy silane (APTS), were prepared by sequentially sampling the reaction mixture at various time intervals. The concentrations of 3-aminopropylsilyl groups (APS) bound on the silica surface were determined by elemental analysis. For the same sample systems, 29Si NMR intensities of an (-O)4Si species belonging only to the silica gel particles and corrected by a cross-polarization correction factor were also measured. Both the APS-concentrations and the corrected 29Si NMR intensities depended upon reaction time, reflecting the rate of the APTS-silica gel reaction. Kinetic analysis of these data was made by use of the Gauss-Newton method, and the overall reaction was found to consist of three reaction processes (an initial fast reaction, a slower second reaction and a much slower third reaction). In particular, the conversion of (-O)3SiOH to (-O)4Si is predominant in the second reaction process and the pore size of a silica gel particle affects the reaction mechanism.