Attenuated total internal reflection Fourier transform infrared spectroscopy for in situ measurements of adsorption, transport, and binding kinetics at liquid/solid interfaces

Attenuated total internal reflection (ATR) Fourier transform infrared (FTIR) spectroscopy is developed as an analytical technique for studying adsorption and kinetic phenomena at the liquid/silica interface. Spectroscopic study of chemical processes at liquid/solid interfaces is challenging because of the large amount of solvent that is typically probed compared to the small amount of material interacting at the interface. To avoid this problem, a high surface area silica sol-gel film was dip coated onto a germanium (Ge) internal reflection element (IRE). This robust film provides sufficient surface area that adsorbate interactions dominate the vibrational spectra. The small depth of penetration of the IR radiation beyond the interface provided by the ATR method means solvent bands do not overwhelm the spectra. Nonlinear least squares modeling provides a novel method of correlating adsorption and kinetic parameters to vibrational spectra of species at the interface.;Adsorption studies were carried out for ethyl acetate and 2-propanol onto the silica interface from n-heptane. Ethyl acetate was found to adsorb to two distinct surface sties comprised of isolated silanols and surface water. Spectra of the ethyl acetate adsorbed to each site could be cleanly resolved. Adsorption of 2-propanol to the silica surface exhibited single adsorption site behavior. It is proposed that the surface heterogeneity is dependent on specific adsorbate/surface interactions. Displacement studies are also carried out to asses the nature of surface interactions in the presence of competing adsorbates.;Pyridine was also adsorbed to the silica surface from an n-heptane solution. It was also found to adsorb to two distinct surface sites as above and the spectra of the adsorbed pyridine could be resolved despite the close proximity, ∼2 cm−1, of the bands. A small amount of pyridine is found to be protonated at the interface. Absorbing the pyridine to a cyano-derivatized film reduces the adsorption constant of the stronger adsorption site but protonation of the pyridine still occurs.;Transport and surface binding kinetics of diphenyldimethylchlorosilane to the silica surface were also studied. The transport and binding processes could be separated by globally fitting spectral data acquired over several concentrations. Transport into the micropores of the sol-gel films was found to exhibit an exponential time response while the binding kinetics were best modeled with an irreversible Langmuir kinetic expression. The kinetics were also studied with triethylamine preadsorbed to the silica surface. The transport rate was found to slow significantly while the binding rate remained unaffected as compared to the untreated surface.