In recent years, owing to the awareness on environmental hazards in the wide use of organic solvents for metal ion extraction, selective extraction and concentration of toxic and precious metals from aqueous streams and liquid wastes by new smart technologies has been the object of intense studies. Between these new strategies, micellar media were found to have significant advantages over a two-phase water-organic solvent system for the extraction process.1,2 In addition of lower environmental impact, the micelles are distributed uniformly through the medium as very small (nano-sized) aggregates, therefore avoiding mixing problems due to unhomogeneity. In the presence of a negatively charged surface, such as is provided by the head groups of the surfactant, the metal ion will be electrostatically attracted to the micelle surface and the extractant will concentrate in the hydrophobic core of the micelle. Under these conditions the extent of extraction will be enhanced as the result of a local concentration effect on both reactants.A study on the kinetics and thermodynamics of the reaction between Ga(III) and ligand 4-(2-pyridylazo)resorcinol (PAR) has been done in sodium dodecyl sulphate (SDS) surfactant solutions, by means of the stopped-flow technique and by UV-vis spectroscopy. A rate promotion effect is found for the forward reaction and for the equilibrium constant in the presence of micelles. The maximum catalysis effect is found when the surfactant concentration is just above the cmc (critical micellar concentration), followed by a decrease at higher SDS concentrations. The kinetic constant of complex formation in SDS solutions at pH=4 is much lower that the one measured at pH=6. In fact, under the latter pH conditions, the neutral PAR species prevails. Such a neutral molecule will have lower tendency to condensate on the negative micelle surface than the protonated H3PAR+ species. Further mechanistic details will be discussed.
|Titolo:||Kinetic and Thermodynamic Studies on Ga3+/PAR Complex Formation in SDS Surfactant Solutions|
|Anno del prodotto:||2004|
|Appare nelle tipologie:||4.2 Abstract in Atti di convegno|