Nuclear magnetic resonance analysis of silica gel surfaces modified with mixed, amine-containing ligands

Different approaches for quantitative analysis by 29Si and 13C CP/MAS nuclear magnetic resonance (NMR) of silica gel chemically modified by a mixture of long and short chain amines, –(O)3Si(CH2)3N(CH3)2(CH2)13CH3 and –(O)3Si(CH2)3N(CH3)3, are compared to elemental analysis. Unlike 29Si NMR, variable contact time data are necessary for accurate quantitative analysis by 13C NMR. Surprisingly, spectral overlap does not interfere with this approach. Surfaces prepared from reaction mixtures that consisted of 67 and 33% (v/v) long chain are found to actually contain 37 and 16% long chain amines, respectively. The mixed phase surfaces have more extensive cross-linking and fewer unreacted hydroxyls than single phase surfaces.