In situ studies of pyridine adsorption to bare and cyano-derivatized silica sol-gel films using attenuated-total-internal-reflection fourier-transform infrared spectroscopy

Adsorption of pyridine to silica sol-gel films has been characterized by using attenuated-total-internal-reflection Fourier-transform infrared spectroscopy (ATR-FTIR). By modeling the accumulation of pyridine in the film using least-squares optimization of simple isotherm models, the pure component spectra of adsorbed and solution-phase pyridine could be extracted from the data. Pyridine was found to adsorb to two distinct sites on the sol-gel surface. The stronger adsorption site was found to comprise isolated and weakly hydrogen-bonded silanols, while the weaker site appears to be primarily surface-bound water. Adsorbed pyridine is partially protonated at the surface, a 13% fraction at the stronger adsorption site and smaller fraction at the weaker site. Derivatizing the sol-gel surface with 3-cyanopropyidimethyl-chlorosilane decreases the total number of adsorption sites by 43%. Two distinct adsorption sites are still observed with the cyanopropyl-derivatized surface, but the stronger adsorption site is almost exclusively weakly hydrogen bonding silanols. The signal due to protonated pyridine is the same as was observed on the underivatized surface, but the percentage of pyridine that is protonated at the stronger site has increased to 22%. This result suggests that weakly hydrogen-bonded silanols are the sites of protonation.