Summary The reaction of Cab-0-Sil, a highly dispersed vitreous quartz, with hexamethyldisilazane (HMDS) was studied in the temperature range 38O-50O0C, using ,’Si solid state NMR and other techniques. Such studies are of importance in view of deactivation procedures of fused silica at high temperatures in capillary GC. The commonly accepted reaction equation: predominates only below ca. 400“C.Above ca. 400°C the intermediate cleavage product Me,SiNH, reacts with surface silanot groups to form iSiOSiMe,NH, + CH,. At higher temperatures these groups may ultimately form (3iO),SiNH, groups (analogously to the formation of bi-and tridentate linkages starting from :SiOSiMe, groups), but (3SiO),SiNH, groups are also directly formed at lower temperatures, simultaneously with the iSiOSiMe3 groups, probably by reaction with siloxane bridges: The reactions of the trimethylsilylamine part of HMDS with Cab-0-Sil were confirmed by an independent series of silylations using N,N-dimethyltrimethylsilylamine instead of HDMS. The presence of amino groups as ESiNH, was confirmed by FT-IR. This may be one of the reasons why very high temperature silylation with disilazanes does not provide a satisfactory deactivation of fused silica GC columns: the active 5iOH groups are “replaced” by active ZSiNH, groups. However, such a material may be of interest in LC. Silylation with silazanes at ca. 350°C in humid atmosphere andlor after extensive hydroxylation (leaching) of the surface should yield sufficiently deactivated surfaces with a rather well defined surface structure. (Me,Si),NH + 2 iSiOH + 2 ESiOSiMe, + NH,
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