Solution chemistry in adsorption layer formation of oleate on fluorite

Adsorption of oleate on fluorite was studied by the infrared reflectance technique (DRIFT) and the solution depletion method. There is a good agreement between spectroscopic data collected directly from the mineral surface and adsorption isotherms obtained by the depletion method only if the formation in solution of microcrystals and calcium-sodium oleate micelles are taken under consideration (true adsorption isotherm). Application of centrifugation to separate the fine particles of fluorite from oleate solution causes the calcium oleate microcrystals and calcium-sodium oleate micelles, formed in solution at higher oleate concentration, to be counted as the oleate molecules adsorbed on the mineral because they are large enough to be centrifuged. This results in significant discrepancies observed between the surface coverages determined spectroscopically and by the solution depletion method. The formation of calcium oleate microcrystals and calcium-sodium oleate micelles in solution takes place at a residual oleate concentration about five times higher than that calculated from the solubility product of calcium oleate. In very diluted solutions oleate ions adsorbed on the surface of fluorite particles form at submonolayer coverage a disordered structure and then well-organized monolayers with a rigid aliphatic chain conformation. At the same time, two preferential conformations of the carboxylate group, unidentate and bidentate, are observed. With an increase in the oleate concentration in solution the adsorbed amount of oleate on the fluorite surface reaches about a two-monolayer coverage and then decreases to a submonolayer level. Significant decrease of activity of oleate ions in solution at higher initial concentration are caused by their participation in the formation in solution of the calcium oleate microcrystals and mixed calcium-sodium oleate micelles. The sur face hydrophobic adsorption layer of oleate on fluorite contains water molecules which stabilize the well-organized structure. Compared to apatite, the monolayer structure of oleate produced on fluorite is much more organized (rigid) and more stable, showing the preferential formation of hydrophobic layers on fluorite rather than on apatite if the two minerals are present in the same oleate solution. (C) 2000 Academic Press.