Diffuse reflectance infrared Fourier transform spectral study of the interaction of 3-aminopropyltriethoxysilane on silica gel. Behavior of amino groups on the surface

Abstract

Three silica gel sample systems, modified with 3-aminopropyltriethoxysilane (APTS), were prepared by sequentially sampling the reaction mixture at various time intervals, and the diffuse reflectance infrared Fourier transform (DRIFT) spectra of these samples were measured in the regions 2700–3500 and 1300–2000 cm^-1. The IR bands observed at 1597 and 1629–1633 cm^-1 were assigned to the deformation modes of NH₂ and NH ⁺₃ groups, respectively. The intensities of these two bands are dependent on both the APTS concentration used in the preparation and the reaction time. The results are summarized as follows. For the sample systems in which smaller APTS concentration were used, most of the NH₂ groups of the aminopropyl segments are converted into the NH ⁺₃ groups on the surface, showing that the SiO^- H⁺ NH_2- type structure is predominantly stabilized on the surface of the silica gel. As the APTS concentration in the reaction mixture increases, the population of NH₂ groups in the silane layer coated onto the surface increases. Interpretation of the CH stretch region further suggests that cyclic structures may be formed on the surface as a consequence of the formation of NH ⁺₃ groups.