Journal of Chromatography, 406 (1987) 317-324 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands CHROMSYMP. 1247 ACCESSIBILITY OF IRREGULAR (FRACTAL) SURFACES OF POROUS SILICAS TO SILYLATION REACTIONS DINA .FARIN and DAVID AVNIR* Department of Organic Chemistry and The Fritz Haber Research Center for Molecular Dynamics, The Hebrew University zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA of Jerusalem. Jerusalem 91904 (Israel) SUMMARY The effect of surface irregularity on parameters such as surface concentration, area per ligand and interligand distance is discussed with special emphasis on sily- lation reactions of silica. For the proper evaluation of these parameters, it is suggested that either the effective area for the ligand (and not the nitrogen-BET value) or the fractal (adsorptive or reactive) dimension of the surface be used as a basic parameter. Experimental examples were analysed according to this approach. EFFECT OF SURFACE IRREGULARITY ON THE ACCESSIBILITY TO SILYLATING REAC- TIONS Grafting of surfaces is a key process in the preparation of chromatographic materialslV4, given the ability to fine-tune the type of surface-adsorbate interactions by a suitable choice of the derivatizing agents. It is well established that the perform- ance of the chromatographic material is determined not only by the nature of the phase-reversing ligand, but also by other material properties, the most important of which are the packing density of the ligands on the surface, the surface concentration or coverage, the area per ligand and the proportion of surface sites that undergo reaction. For silica in its varions forms all of the above-mentioned parameters are determined by the availability of silanols to silylation reactions and by their distri- bution (the surface heterogeneity) 5,6. There are three main factors that determine the accessibility of silanols to reaction. The first one is relatively simple to evaluate -the “umbrella” effect of the ligand. Most ligands have cross-sectional areas that shield more than the silanol(s) with which they react. The second factor is the surface ir- regularity, tortuosity and connectivity and the degree to which those pore and surface features shield the silanols from an incoming reagent molecule, Most of the discussion in this report is devoted to this factor. The third parameter is surface heterogeneity, from the point of view both of the possible clustering of reactive sites and the dif- ferences in reactivities of silanols due to inductive effects of the surroundings (neigh- bouring impurities or, again, the degree of near-by clustering). As will be argued below, the second and the third parameters are closely related. A routine practice in many chromatographic studies is the use of the nitro- 0021-9673/87/$.03.50 0 1987 Elsevier Science Publishers B.V.