Co(II) Extraction by sodium di(2-ethylhexyl) phosphate at the water/toluene interface: phase structure and aggregation in relation to the kinetics of complexation
The kinetics of liquid-liquid extraction o aqueous Co(II) by toluenic NaD2EHP and the structural and aggregative properties of the organic phase O, a middle surfactant-rich phase B and the aqueous phase W were studied for the non-stirred system H2O/toluene/NaD2EHP/CoBr2/KBr. Interfacial turbulences also appeared in this system.
Two kinds of toluenic NaD2EHP solutions, different in their viscous behaviour, could be distinguished according to the hydration degree of the surfactant. Dry or poorly hydrated NaD2EHP gives molecular solutions of monomeric species if dissolved at more than 70°C or polymeric species if dissolved at c.a. 30°C. Hydrated NaD2EHP forms microemulsions. The complex formed is either octahedral with two H2O molecules or tetrahedral. It can also form a viscous phase denser than water.
NaD2EHP enters the aqueous phase giving rise to two kinds of emulsions (A and B). The mmore surfactant-rich layer B lies below the W/O interface. The kinetic study shows that reaction takes place both at the macroscopic W/O interface and at the droplets surfaces of phases A and B. The first reaction site prevails at high cobalt concentrations and the second at low cobalt concentrations.
Key wordsCo-extraction aqueous solutions Na-di(2-ethylhexyl)-phosphate toluenic solutions hydration degree differant complexes
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