Surface acidity and basicity of functionalized silica particles

Tetraethoxysilane has been co-hydrolyzed with functionalized organosilanes in a modified Stober process to produce silica particles with amino, carbox ylate or dihydroimidazole groups on the surface. The effects of reaction co nditions and the loading of the functionalized organosilane on particle siz e was examined by TEM. Fluorescence spectroscopy of the surface amino group s covalently modified with fluorescamine, and the surface carboxylate group s with 4-bromomethyl-6,7-dimethoxycoumarin, demonstrated that these functio nal groups were accessible for further reaction. Changes in surface acidity and basicity caused by the presence of functional groups (amine, dihydroim idazole, carboxylate) on the particle surface were determined using an indi cator titration technique. Particles with surface imidazole and amine group s and particles with surface carboxylate groups have enhanced basicity and acidity, respectively. Dihydroimidazole-modified silica had greater surface basicity than the amine-modified silica. The effect on basicity and acidit y increases as the amount of added functionalized silane increases. However , this increase is nonlinear with respect to the increase in added function alized silane. Particles with both surface dihydroimidazole and carboxylate groups demonstrated reduced surface basicity and acidity. (C) 1999 Elsevie r Science B.V. All rights reserved.