INTERACTIONS OF AMINOSILANE WITH ALUMINA AND SILICA SUBSTRATES DEPOSITED FROM NONAQUEOUS AND AQUEOUS-MEDIA

N-15 (Nitrogen-15) NMR studies of the widely-used coupling agent 3-ami nopropyltriethoxysilane (APS) deposited onto native-oxide alumina and silica in submonolayers from nonaqueous and aqueous solutions reveal a t least four different APS nitrogen environments. When APS is deposite d on alumina from the nonaqueous solvents IPA (isopropanol) and NMP (N -methylpyrrolidone), three nitrogen environments, which are believed t o be the amine, 'closed form' and unique form, are present. However, w hen APS is deposited on alumina from water, only two nitrogen environm ents, which are believed to be the amine cation and unique form, are s een. When aqueous APS is deposited on silica, the amine, amine cation and closed form are all present together, but low sensitivity makes it difficult to detect the unique form, if present. The unique nitrogen environment shows a nitrogen that appears to be a rigid and protonated form with considerable electronic deshielding relative to the amine/a mine cation, possibly from an oxygen bond with a surface hydroxyl or f rom a nitrogen bond with another APS. At the most, 10% of the APS-on-a lumina nitrogen show this unique resonance which is sensitive to surfa ce effects and may be actively involved in adhesion. N-15 NMR of aqueo us APS supports a nitrogen with considerable cation 'character'. Both the 5-or 6-member ring form with intramolecular stabilization (Pluedde mann's 'closed' form) and the zwitterion form with intermolecular stab ilization (a dimer with a 7-member ring, for instance) are supported. Comparisons of the APS nitrogen states, in nonaqueous solution and on the surface after deposition from that solution, show that a change in the oxidation state of the nitrogen is likely to occur. Such a phenom enon is consistent with a dynamic interaction of APS with the surface such that either the amine is oxidized to a cation or is reduced from the cation to the amine. The current research supports an APS-surface interaction such as the 'flip' model proposed by Linde.