Solution chemistry in adsorption layer formation of oleate on fluorite

Adsorption of oleate on fluorite was studied by the infrared reflectance te chnique (DRIFT) and the solution depletion method. There is a good agreemen t between spectroscopic data collected directly from the mineral surface an d adsorption isotherms obtained by the depletion method only if the formati on in solution of microcrystals and calcium-sodium oleate micelles are take n under consideration (true adsorption isotherm). Application of centrifuga tion to separate the fine particles of fluorite from oleate solution causes the calcium oleate microcrystals and calcium-sodium oleate micelles, forme d in solution at higher oleate concentration, to be counted as the oleate m olecules adsorbed on the mineral because they are large enough to be centri fuged. This results in significant discrepancies observed between the surfa ce coverages determined spectroscopically and by the solution depletion met hod. The formation of calcium oleate microcrystals and calcium-sodium oleat e micelles in solution takes place at a residual oleate concentration about five times higher than that calculated from the solubility product of calc ium oleate. In very diluted solutions oleate ions adsorbed on the surface o f fluorite particles form at submonolayer coverage a disordered structure a nd 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 co ncentration in solution the adsorbed amount of oleate on the fluorite surfa ce reaches about a two-monolayer coverage and then decreases to a submonola yer level. Significant decrease of activity of oleate ions in solution at h igher initial concentration are caused by their participation in the format ion in solution of the calcium oleate microcrystals and mixed calcium-sodiu m oleate micelles. The sur face hydrophobic adsorption layer of oleate on f luorite contains water molecules which stabilize the well-organized structu re. Compared to apatite, the monolayer structure of oleate produced on fluo rite is much more organized (rigid) and more stable, showing the preferenti al formation of hydrophobic layers on fluorite rather than on apatite if th e two minerals are present in the same oleate solution. (C) 2000 Academic P ress.