Adsorption Density Calculations from in Situ FT-IR/IRS Data at Dilute Surfactant Concentrations

Abstract

Adsorption densities of the surfactant oleate at the surface of a fluorite crystal have been calculated from in situ FT-IR internal reflection spectroscopic (1RS) data at very low surfactant concentrations. For the first time, a complete adsorption isotherm (5 × 10⁻⁷ to 5 × 10⁻⁴ M) has been calculated from the in situ spectral data with the use of internal reflection theory. It was found that monolayer coverage occurred at very low surfactant concentrations (−5 M), while at higher bulk concentrations, adsorption consists of multilayers of what appears to be surface-precipitated calcium dioleate. Calculated absorption densities by this in situ method agree quite well with those found previously by more traditional experimental techniques. These results provide considerable confidence that quantitative in situ FT-IR/IRS analysis of surfactant adsorption at the surface of reactive internal reflection elements is possible.