| Abstract
| - The concentration and Brønsted acidity of surface silanol groups on mesoporous silica (SBA-15) has been studiedby following the adsorption of benzylamine, BA, from water as a function of pH. The adsorbed amount of BA fromwater was compared to the maximum amount of BA that could be adsorbed from cyclohexane. Furthermore, the surfaceconcentration and acidity of carboxylic acid functions on surface-functionalized SBA-15 was also studied, whichallowed the relative surface concentration of remaining silanols to be obtained. Two types of silanols can be identified,where about 1/5 of the silanols have a pKa ≤ 2 and the remaining 4/5 of the silanols have a pKa of about 8.2. Accordingto the literature, these two types of silanols can be identified as Q3 and Q2 silanols, respectively, of which the Q3 silanolsare more acidic. For the surface-functionalized materials, pKa values close to their respective intrinsic values are foundfor the carboxylic acid functions. However, irrespective of the method of surface functionalization, 50% or more ofthe accessible surface groups are silanols. The results thus suggest that the effective charge density is largely controlledby the (de)protonation of the silanol groups even for surface-functionalized mesoporous silica, which render the surfacechemistry of the surface-functionalized silicas to be radically different from what simple schematic representationswould suggest. The results are suggested to be of importance for reaching a predictive level of understanding for thebehavior of mesoporous silica in a range of applications, as many of the foreseen applications for mesoporous silicainvolves water as the medium.
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