Study of the aggregation of Cyanex 272 and Ionquest 290 by Vapor Pressure Osmometry
Résumé
The extractant molecule bis-2,4,4-trimethylpentyl phosphoric acid (BTMPP), is the active component of Cyanex 272 and Ionquest 290. BTMPP is known to form aggregates, especially dimers, in nonpolar organic diluents. The aggregation results from strong intermolecular hydrogen bonding between the P=O and P-OH groups. The knowledge of this aggregation is important, as it influences solvent extraction equilibriums by reducing the amount of extractant molecules available for the extraction. In the present work, aggregation of BTMPP was studied by Vapor Pressure Osmometry (VPO). The average numbers of aggregation were calculated for several solutions containing various concentrations of extractant for both Cyanex 272 and Ionquest 290. The average numbers of aggregation are around 2 – 2.2 at 50°C, depending on the concentration of BTMPP. It was concluded that BTMPP forms monomers, dimers and trimers. Then the equilibrium constants of dimerization and trimerization were estimated by a numerical optimization procedure developed on Matlab. The logarithms of the constants of dimerization and trimerization are respectively estimated at 3.0±0.2 and 4.8±0.2 at 50°C (constants in L/mol and L²/mol² resp.). VPO measurements were also conducted at 43°C and 60°C. Only small differences were observed for the average numbers of aggregation and the equilibrium constants calculated at 43°C, 50°C and 60°C. It was concluded that the temperature dependence of the aggregation of BTMPP is low from 43°C to 60°C.
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