Facilitated ion-transfer of alkaline-earth metal cations by naphtho-15-crown-5 across the water vertical bar 1,2-dichloroethane interface: voltammetric and electrospray ionization mass spectrometric studies

The facilitated ion-transfer processes of Ca (2) (+), Sr2 +, and Ba (2 +) c ations from an aqueous phase to a 1,2-dichloroethane (DCE) phase by naphtho -15-crown-5 (N15C5) were studied by cyclic voltammetry. When the metal ion concentrations in the aqueous phase were much higher than the N15C5 concent rations in the organic phase clear voltammograms were observed: the peak cu rrents were controlled by the diffusion of N15C5 in the DCE phase. The diff usion coefficient or N15C5 in the DCE phase was evaluated to be 5.3 x 10 (- ) (6) CM2 s (-1). The analyses of these voltammograms revealed that the tra nsfer of Cal + cation was facilitated due to the formation of 1:2 complex o f cation to ligand ratio, Ca(N15C5)(2)(2+) and Sr 2 (+) and Ba (2+) cations were facilitated due to the formation of 1:1 complexes, Sr(N15C5)(2+) and Ba(N15C5)(2+), respectively. The stoichiometries of the complexes were also studied by electrospray ionization mass spectrometry (ESI-MS). Both 1:1 an d 1:2 complexes were observed at low concentration of N15C5 for the Ca2+ -N 15C5 system. At high concentration of N15C5, only 1:2 complex was observed. In the cases of Sr2+ -N15C5 and Ba2+ -N15C5 systems, no signal was observe d at low concentration of N15C5 whereas, at high concentration of N15C5, bo th 1:1 and 1:2 complexes were observed for the Sr2+ -Nl5C5 system and only 1:1 complex was observed for the Ba 2+ -N15C5 system. Comparing the intensi ties in the ESI-MS spectra, the stabilities of different complexes in the g as phase were discussed. (C) 2001 Published by Elsevier Ltd.