Kinetics of "extraction" of copper(II) by micelle-solubilized complexing agents of varying hydrophilic lipophilic balance. 1. Stopped-flow study

Micellar particles can solubilize lipophilic extractants similarly to the organic phase in classical biphasic extraction. A series of 6-(alkylamino)methyl-2-(hydroxymethyl)pyridines (C(n)NHMePy with n = 1, 4, 8, 10, 12, 14, and 16), good complexing agents for copper(II), has been used in this work to investigate the role of hydrophobic interactions on the kinetics of complexation in micellar media. Apparent rate constants for complex formation (k(f)app) and dissociation (k(d)app) obtained from stopped-flow experiments are reported in different micellar systems (CTAB, SDS, and C12EO6), showing important variations with the value of n. These results which mainly concerned the protonated form of the extractant molecules (pH 3.5) have permitted precise determination of the role played by the electrostatic interactions. k(f)app is found to decrease more than 20 times in CTAB and about 10 times in C12EO6 for n changing from 1 to 16. The influence of copper(II) and surfactant concentrations on the observed rate constants is interpreted in terms of partition coefficients which have been independently determined from dialysis experiments. The reaction is found to be insensitive to surfactant concentration for n = 1, 4, and 8 in CTAB and only for n = 1 and 4 for C12EO6. The activation parameters are determined for n = 4 and 14 in water, in CTAB, and in C12EO6. The effect of the addition of alcohol and oil to form different o/w microemulsions is finally reported.