Effects of fluorocarbon and hydrocarbon anionic micelles on the kinetics of water replacement in the palladium(II) aquocomplex of 1,1,7,7-tetraethyldiethylenetriamine by monoalkylthioureas

Abstract

The effect of sodium perfluorooctanoate (NaPFO), sodium perfluorononanoate (NaPFN) and sodium decyl sulfate (SDeS) micelles on the kinetics of water replacement in the cationic palladium(II) complex [Pd(Et₄dien)(H₂O)]²⁺, where Et₄dien = Et₂N(CH₂)₂NH(CH₂)₂NEt₂, by the nucleophiles methyl-, ethyl- and butylthiourea has been studied in aqueous acidic solutions at 25 °C and ionic strength 0.03 mol dm^–3(0.06 mol dm^–3 with SDeS). The micellar aggregates promote the substitution reactions reaching a maximum rate just beyond the critical micelle concentrations, which have been estimated experimentally by surface tension and conductivity measurements. The binding constants of the nucleophiles calculated from the present kinetic data, together with those obtained previously in the presence of sodium dodecyl sulfate (SDS), indicate that the capacity of the micelles to solubilize the thioureas increases in the order SDeS < SDS ≈ NaPFO < NaPFN. Moreover, for each surfactant used, solubilization is favoured by an increase in the hydrophobic character of the substrates. The estimated free energies of transfer of the thioureas from aqueous to micellar pseudo-phase show that the affinity of the hydrogenated micelles toward the substrate CH₂ group is larger than that of the fluorinated ones. The possible solubilization sites and reaction rates in the micellar pseudo-phases are also discussed.