Accessibility and shielding of silanol surface centers on porous silica beads; UV/vis absorption and fluorescence studies

The adsorption equilibria of proton accepting and donating analytes on porous silica beads applicable as stationary chromatographic phases were investigated by UV/vis absorption spectroscopy. Fluorescence spectroscopy was utilized to characterize the nature of the species formed at the silica surfaces after adsorption. In order to control the equilibria and states of adsorption the active silanol surface centers were partly shielded by adsorbed water or by two types of polymeric coatings, (i) polymerized 1,4-divinylbenzene (DVB) with loadings of 200 mg and 500 mg DVB/g silica, respectively, (ii) polymerized N,N′-diallyl-l-tartardiamide bis-(4-tertbutylbenzoate) (TBB) with a loading of 135 mg TBB/g silica. Acridine orange, 1,2,7,8-dibenzacridine, 3,4,5,6,-dibenzacridine, and lumichrome were used as fluorescent analytes with proton accepting or donating nitrogen centers. The fluorescence anisotropies show that the adsorbed species at the uncoated silica surface are highly immobilized. Coating considerably reduces the equilibrium constants of adsorption. polymerized N,N′-diallyl-l-tartardiamide bis-(4-tertbutylbenzoate) works much better than polymerized 1,4-divinylbenzene. In the latter case a large amount of polymer is necessary in order to produce a significant effect.