Kinetics of complexation of lanthanide ions by 3-methyl-4-acyl-5-pyrazolone derivatives in micellar and microemulsion media

The stopped-flow technique has been used to investigate the kinetics of complexation of two metal ions representative of the lanthanide series (namely Eu(III) and Lu(III)) by hydrophobic pyrazolones, in micellar solutions of CTAB and in microemulsions of CTAB/butanol/water/heptane (or toluene). Two pyrazolone derivatives having very different pK's were concerned: 1-p-nitrophenyl-3-methyl-4-p-octylbenzoyl-5-pyrazolone (HNPMOBP) and 1-phenyl-3-methyl-4-lauroyl-5-pyrazolone (HPMLP). The influence of parameters such as ionic strength, pH, and temperature or metal ion concentration was studied. Depending on the conditions one or two kinetic processes were observed. The first process associated with the ligand/metal reaction was taken into account by a simple mechanism assuming that the reactive species is the anionic form of the ligand. The values of the apparent formation rate constants obtained in these microheterogeneous media were compared with values from the literature in homogeneous solutions. Complexation with Eu(III) was about an order of magnitude faster than with Lu(III), suggesting that it is controlled by the departure of water molecules from the metal solvation shell. The reaction was considerably faster with HNPMOBP compared to HPMLP. No significant partitioning effect could be demonstrated by adding microemulsified heptane or toluene.