Successive complex formation of multivalent ions with octaethylene glycol dodecyl ether at the nitrobenzene/water interface

Abstract

Transfer of Mg 2+ and La 3+ ions across the nitrobenzene (NB)/water (W) interface facilitated by octaethylene glycol monododecyl ether (C12E8) has been studied using cyclic voltammetry under conditions where the concentration of C12E8 in NB( b c C12E8 NB) is much smaller than the concentration of Mg 2+ or La 3+ ions in W. The shape of the voltammograms is markedly dependent on b c C12E8 NB. When b c C12E8 NB⩾ 1 mM, the voltammograms exhibit a clear shoulder on both forward and reverse scans of the potential. For La 3+ transfer, the convolution voltammograms exhibit two-step waves. The dependence of wave shape on ligand concentration indicates the successive formation of 1:2 (metal:ligand) and 1:1 complexes in the vicinity of the interface. The first wave is ascribed to the formation of the 1:2 (metal:ligand) complex, and the second wave to the 1:1 complex. This double wave merges into a single peak, when b c C12E8 NB </0.2 mM. From the midpoint potentials and the concentration dependence of the peak separation, stability constants for the 1:1 and 1:2 complexes in NB are determined to be log 10( K 1/M −1) = 8.8 and log 10( K 2/M −1) = 3.3 for Mg 2+ and log 10( K 1/M −1) = 11.0 and log 10( K 2/M −1) = 4.4 for La 3+, respectively.