In situ Detection of Adsorbates at Silica/Solution Interfaces by Fourier Transform Infrared Attenuated Total Reflection Spectroscopy Using a Silica-Coated Internal Reflection Element

Abstract

A silica-coated ZnSe internal reflection element (IRE) was used to acquire attenuated total reflection (ATR) Fourier transform infrared (FT-IR) spectra of ethyl acetate at an n-heptane/silica surface. A thin silica film was deposited onto the IRE by withdrawing the substrate from a suspension of fumed silica; upon drying, a durable, porous SiO₂ layer is formed on the IRE that is stable in contact with aprotic solvents. The layer thickness is targeted to be comparable to the depth of penetration of infrared radiation beyond the IRE/film interface. These films were used to investigate the adsorption of ethyl acetate onto silica from n-heptane. A multidimensional least-squares fit of the spectroscopic data reveals multiple surface-site energies on the silica surface, as reflected in both the solution concentration dependence of ethyl acetate accumulation on the surface and the frequencies of vibrational modes that shift upon adsorption.