Spectroscopic analysis of interactions between alkylated silanes and alumina nanoporous membranes

Transport across alumina nanoporous membranes can be altered via surface attachment of alkylated trimethoxysilane compounds. The mechanism of attachment has been previously assumed to be monolayer silane coverage through full chemisorption regardless of reaction conditions. This chemisorption arises via covalent Si-O-Al bond formation resulting from condensation between the three putative silanols (due to hydrolysis of the three Si-OCH(3) bonds) and hydroxides present on the alumina surface. If this model was correct, methanol would be produced in large quantities in the reaction solution, and the methoxy moieties would no longer be present on the silane molecule. The results presented in this paper utilized FT-IR and both solution and solid-state NMR to examine the chemical nature of octadecyltrimethoxysilane (ODTMS) present on the alumina surface. The FT-IR results confirm the presence of the silane on the membrane. The (1)H solution NMR results indicate small but detectable methanol production during attachment. The solid-state NMR results demonstrate that the methoxy proton NMR integrated peak intensities remain in nearly the same ratios present in the free silane, concluding that the majority of methoxy groups are intact while the silane is attached to the membrane surface. These three results suggest that monolayer surface coverage and chemisorption through full covalent bonding is not the primary means of attachment for ODTMS on the surface of alumina nanomembranes under these reaction conditions.