Effect of Curvature on the Packing and Ordering of Organosilane Monolayers Supported on Solids

Abstract

This work investigated the effects of the nanometer-sized curvature of the substrate on the structure of organic monolayers supported on dispersed and porous solids. A series of covalently attached monolayers (CAMs) was prepared via solution-phase reactions of CnH2n+1Si(CH3)2N(CH3)2 (n = 1–30) with well-defined silicas having (1) convex (fumed silicas) and (2) concave (SBA-15 silicas) surfaces and (3) silicas with nearly flat surfaces (large-pore silica gels). According to chemical analysis and Fourier transform IR, the grafting density and molecular ordering [assessed from the νa(CH2)] in CAMs were largely determined by the steric effects: the concave surfaces hindered while the convex surfaces assisted the formation of closely packed and highly ordered CAMs. The low-temperature adsorption of nitrogen was studied, and the energies of adsorption interactions of CAMs were evaluated using the C constant of the Brunauer−Emmett−Teller equation. For CAMs of intermediate and long alkyl chains (n > 8), the grafting density increased, the molecular ordering improved, and the energy of adsorption interactions decreased in the following range of substrates: concave surface−flat surface−convex surface.