The synthesis of the silica particles functionalized with carboxyl
groups was carried out in a two-step sequence according to a procedure
described in the literature [35]. To a suspension of Aerosil
200 silica (2.50 g) in toluene (250 mL) was added triethoxysilylbutyronitrile
(25.00 g). The reaction mixture was refluxed for
18 h. After cooling to room temperature, half of the solvent was
removed under reduced pressure and the resulting suspension
was filtered off. The solid was successively washed with toluene
(3 mL × 50 mL), absolute ethanol (3 mL × 50 mL), diethyl ether
(3 L × 50 L) and dried under vacuo over P2O5. This yielded 3.45 g
nitrile-functionalized silica SiO2(CN). A suspension of SiO2(CN)
(3.15 g) in 50% sulfuric acid (100 mL) was heated at reflux for 3 h.
After cooling to room temperature, distilled water (100 mL) was
added and the suspension filtered off. The product SiO2(CO2H)
was thoroughly rinsed with water until washings were neutral.
The solid was then air-dried followed by drying in vacuo over P2O5
yielding 3.20 g of carboxylic acid-functionalized silica.