Abstract

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