DEACTIVATION OF SILICA SURFACES WITH A SILANOL-TERMINATED POLYSILOXANE - STRUCTURAL CHARACTERIZATION BY INVERSE GAS-CHROMATOGRAPHY AND SOLID-STATE NMR

Retention gaps deactivated with Silicone OV-1701-OH show good chromato graphic performance and remarkable stability against water induced sta tionary phase degradation. In an attempt to better understand the fund amentals of the deactivation process using silanol terminated polysilo xanes, a fumed silica was deactivated with Silicone OV-1701-OH. In con trast to fused silica capillaries, fumed silica (Aerosil A-200) can be studied by Si-29 cross-polarization magic-angle-spinning (CP MAS) NMR , thus serving as a model substrate for fused silica. Retention data f rom inverse gas chromatography at infinite dilution and Si-29 CP MAS N MR data of five Aerosil phases, differing in residual silanol surface concentration, are correlated with the aim of validating this approach for stationary phase characterization. A comparatively detailed model of the deactivating polymer layer that explains the observed adsorpti on activities is deduced. Surface silanols are shown to play a key rol e in the formation of the polymer layer, the structure of which is of primary importance for the adsorption behavior after deactivation. Con trary to common belief, the absolute silanol surface concentration aft er deactivation is only of secondary importance for the overall adsorp tion activity. High silanol surface concentrations enhance degradation of the polysiloxane chains into small cyclic fragments as well as sub sequent adsorption and immobilization to the silica substrate surface. The mobility of linear polysiloxane chains in the kHz regime (as dete rmined by NMR cross-polarization dynamics) appears to determine the ex tent to which the residual silanols are accessible for analytes. It is therefore anticipated that there is an optimum silanol surface concen tration of fused silica surfaces to be deactivated with silanol termin ated polysiloxanes: it should be large enough to adsorb polymer fragme nts, but not too large to avoid excessive residual silanol activity.