Wet-chemical analysis of surface concentration of accessible groups on different amino-functionalized mesoporous SBA-15 silicas

Surface functionalization is one of the key steps toward the utilization of mesoporous materials in different applications. In this study, we characterized amino-functionalized mesoporous SBA-15 silicas that have been prepared by different means; co-condensation, post-grafting of amino-silanes, and surface hyperbranching polymerization of poly(ethylene imine). Special focus is put on the accessibility of the introduced function. The materials are thoroughly characterized both by structural and by surface chemical means. We generally observe a good agreement between the C-value derived from BET surface area analysis, the number of accessible amine groups determined by quantitative imine reaction in solution, and zeta-potential measurements. Furthermore, indirect information about differences in the number of amine groups present on the outside surface of the particles can also be obtained. Our results clearly show that there are large differences between the availability of the amine function for materials functionalized by different methods and that the differences calculated as the number of accessible amine groups per unit area can be as large as 20-fold. The co-condensation route is the least efficient surface functionalization method under the studied conditions, and we actually observed a decrease not only in the relative but also the absolute number of accessible amine groups per unit area with an increasing amino-silane molar fraction for the co-condensed materials, which indicates that the commonly used gravimetrical means for determining the extent of true surface functionalization may give rise to large errors. Surface polymerization leads to the highest number of accessible amine groups, but often, the first polymerization cycle leads to polymer formation in micropores in the mesopore walls, which renders these amine groups inaccessible. These results demonstrate that the real surface composition of surface-functionalized mesoporous materials may have very little in common with the schematic representations often found in the literature.