Charge transfer between two immiscible electrolyte solutions part VIII. Transfer of alkali and alkaline earth-metal cations across the water/nitrobenzene interface facilitated by synthetic neutral ion carriers

Abstract

Facilitated transfer of proton, alkali and alkaline earth-metal cations across the water/nitrobenzene interface was observed in the presence of either N, N′-di[(11′-ethoxycarbonyl)undecyl]- N, N′,4,5-tetramethyl-3,6-dioxaoctanediamide (DODA) or 7,19-dibenzyl-2,3-dimethyl-7,19-diaza-1,4,10,13,16-pentaoxacycloheneicosane-6,20-dione (PEDA) in the nitrobenzene phase. These neutral synthetic compounds act as the ion carriers which, through the complex formation, mediate the translocation of the ion across the water/nitrobenzene interface. Using the convolution potential sweep voltammetry, the stoichiometry ( r:s) and the thermodynamic, transport as well as the kinetic parameters were evaluated for the single-step charge-transfer model: rM 2+ (w) + sL(n)=M r L s r2+ (n), where M 2+(w) is the metal cation with the charge number z in water and L(n) is the neutral ligand in nitrobenzene. The formation of 1 : 1 or 1 : 2 (cation to ligand) complexes is involved in the case of the monovalent or divalent cations respectively. The highest stability constant K in nitrobenzene was found for calcium ion: K= 5.8 × 10 21 M −2 or 2.0 × 10 18 M −2 for the acyclic (DODA) or the cyclic (PEDA) ligands respectively. The kinetic parameters were evaluated only for the transfer of the divalent metal cations, since the transfer of the monovalent cations is too fast. While the apparent charge-transfer coefficient is invariably very close to 0.5, the apparent rate constants at the formal potentials of the charge-transfer reactions differ considerably from each other and are sensitive to the nature of the ligand. For the acyclic ligand (DODA) the following sequence of the apparent rate constants was observed: Ca 2+⪢Ba 2+∼Sr 2+⪢Mg 2+, whereas for the cyclic ligand (PEDA) it was: Ba 2+⪢Sr 2+∼Mg 2+⪢Ca 2+. Among the factors underlying the kinetic behaviour there seem to be the double-layer effects, the internal as well as the solvent contributions to the activation energy.