Dynamics of fluorite-oleate interactions

The nature, structure, and kinetics of formation of the adsorbed layers of oleate on fluorite in basic solutions have been examined by means of infrar ed external reflection spectroscopy. The adsorption of oleate on fluorite w as found to be a very dynamic process which fails to reach equilibrium, Dif ferent steady states can be achieved. The surface composition and structure depend on competitive processes taking place at the interface, such as flu orite dissolution and interaction with water, and adsorption of oleate ions and other oleate aggregates from solution. Fluorite immersed in oleate bas ic solution shows surface phenomena dependent on an initial oleate concentr ation if other solution conditions (pH, agitation, etc.) are constant. Thre e characteristic regions of adsorption can be distinguished. In diluted sol utions, below 10(-5) M, a steady state is reached. The amount of calcium ol eate surface species does not exceed 0.3 of a statistical monolayer. At con centrations between 10(-5) and 10(-4) M the most dynamic interactions betwe en fluorite and oleate aqueous solutions were observed. The adsorbed amount shows a maximum. The level of the maximum (up to 20 statistical monolayers ) and the adsorption kinetics are strongly related to the initial oleate co ncentration. There is clear evidence that the thicker coverage is produced by a nucleation and growth mechanism. First nuclei are formed on the fluori te surface through tridimensional condensation before the monolayer is comp leted. Dissolution of the produced surface calcium oleate with an increase of adsorption time is caused by the Ostwald ripening that transfers the cal cium and oleate from the fluorite surface to solution, where fine particles of calcium oleate are formed. At higher than 10(-4) M oleate concentration the total adsorbed amount of oleate does not exceed a monolayer coverage a fter a long adsorption time. A steady state is reached, and micelles are fo rmed in solution, at concentrations lower than the cmc of sodium oleate due to the adsorption of calcium ions on the micelles, lowering significantly the calcium and oleate concentrations in solution. This prevents the format ion of thick tridimensional patches on the fluorite surface. Adsorption kin etics is also very sensitive to other changes, such as solution hydrodynami cs (agitation) or addition of external calcium lions or/and solid calcium o leate! to the oleate solution.