In Situ Adsorption Studies at Silica/Solution Interfaces by Attenuated Total Internal Reflection Fourier Transform Infrared Spectroscopy:  Examination of Adsorption Models in Normal-Phase Liquid Chromatography

Abstract

ATR-FT-IR spectroscopy was employed to the study the adsorption of ethyl acetate and 2-propanol to the surface of thin silica sol-gel films in contact with n-heptane solutions. In situ vibrational spectra of silica-adsorbed species provided information regarding the mechanisms of solute retention and elution in normal-phase chromatography. Previous normal-phase chromatographic studies of ethyl acetate adsorption revealed nonlinear isotherms which were explained by both bilayer and adsorbate delocalization models. Infrared spectra of ethyl acetate at the silica surface versus concentration showed that nonlinear adsorption can be attributed to site heterogeneity, where adsorption to free silanols and surface-adsorbed water can be distinguished. Least-squares modeling of the data produced resolved spectra for the two sites and adsorption equilibrium constants that differed by about an order of magnitude. Adsorption of 2-propanol was best modeled by a single Langmuir isotherm showing no significant difference in adsorption energy for the two sites; 2-propanol was shown to easily displace ethyl acetate from the silica surface. Ethyl acetate could also displace 2-propanol from the silica, and least-squares modeling again revealed two-adsorbed-component spectra for ethyl acetate that were indistinguishable from spectra obtained when ethyl acetate adsorbed directly onto the surface.

Exploring catalytic solid/liquid interfaces by in situ attenuated total reflection infrared spectroscopy

“…In cases where it is not possible for a specific application to deposit a durable layer with this simple direct dropping technique, the powder particles can be attached to the IRE using a binder material which is coated first on the IRE. 27,28 Note that the catalyst immobilized on the IRE is often not present as a homogeneous layer perfectly in contact with the IRE, and consequently the evanescent field is not exactly created as described by theory (eqn (3)). The advantage of using a powder with small particles is to provide good sensitivity for spectra of adsorbed species as the surface area increases thereby, so does the number of molecules probed by the ATR evanescent wave compared to a thin film.…”

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