A new approach to the silica gel surface : characterization of different surface regions by 29Si magic angle spinning NMR relaxation parameters and consequences for quantification of silica gels by NMR: characterization of different surface regions by silicon-29 magic angle spinning NMR relaxation parameters and consequences for quantification of silica gels by NMR

Native and some monofunctionally derivatized silica gels have been investigated by 29Si CP MAS NMR spectroscopy with pulse and with cross-polarization (CP) excitation. Contact time variation experiments for some native materials yield results for the siloxane (Q4)g roups which cannot be described with a single set of THsi and TlpHv alues. Quantitative analyses of such silicas with CP excitation require CP curves as standards. For these samples Tlsi values measured with pulse excitation differ from those obtained by CP excitation. TlpHv alues depend on the contact time, selected for the experiment. Moreover, multiexponential Tlsi analyses for Q3 and Q4 reveal that the longitudinal relaxation process of native silica gels generally can be separated into at least three different processes, each with its characteristic average value for Tlsi. Explanations for CP behavior and Tlsi inhomogeneities are offered in terms of different forms of coexistence in silica gels of relatively mobile amorphous regions. Implications of these results for quantification of surface groups of silicas with cross-polarization MAS NMR are discussed.