Influence of solution conditions and mineral surface structure on the formation of oleate adsorption layers on fluorite

Adsorption of oleate on two slab samples of fluorite obtained from two different sources was studied under acidic (pH 6) as well as basic (pH 10) conditions and with different pre-treatments using infrared external reflection spectroscopy. The ratio of intensities of two most favorable calcium oleate conformations on the fluorite surface, characterized by absorbance bands at 1536 and 1576 cm−1 corresponding to bidentate and unidentate configuration, respectively, varied significantly with adsorption/desorption treatment indicating sensitivity of the structure of the adsorbed layers of oleate to experimental conditions and the nature of the mineral surface. The alterations in the surface as a consequence of various pre-treatments were found to influence the structure of the adsorbed layer as observed by changes in the absorbance bands in the corresponding infrared spectra. The most stable surface structure is the compact well-organized monolayer of oleate adsorbed on fluorite. Our molecular modeling computations of oleate adsorption on fluorite {111} surface indicated bidentate configuration to be most favorable. The orientation of the optimized oleate molecule (angles) thus calculated also matched well with experiments. The optimized unidentate fluorite–oleate surface complex was also found to have comparable energy even though relatively lower than the bidentate configuration.