A NEW APPROACH TO THE SILICA-GEL SURFACE - CHARACTERIZATION OF DIFFERENT SURFACE REGIONS BY SI-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) groups which cannot be described with a single set of T1ρH values. Quantitative analyses of such silicas with CP excitation require CP curves as standards. For these samples THSi values measured with pulse excitation differ from those obtained by CP excitation. T1ρH values depend on the contact time, selected for the experiment. Moreover, multiexponential T1Si 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 T1Si. Explanations for CP behavior and T1Si 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. © 1990 American Chemical Society.