Driving ion transfer across the toluene (trihexyl(tetradecyl)-phosphonium tris(pentafluoroethyl)trifluoro-phosphate) | water (electrolyte) interface with the MnTPP(III/II) redox system

Wojciech Adamiak 1Masanori Satoh 2Galyna Shul 1Ewa A. Rozniecka 1Jingyuan Chen 2Frank Marken 3Marcin Opallo 1

1. Polish Academy of Sciences, Institute of Physical Chemistry, Kasprzaka 44/52, Warszawa 01-224, Poland
2. University of Fukui (U. Fukui), 3-9-1 Bunkyo, Fukui 910-8507, Japan
3. University of Bath, Department of Chemistry, Claverton Down, Bath BA2-7AY, United Kingdom

Abstract

Recently the ionic liquids have been applied as electrochemical solvents [1,2]. However, reports of electrochemical studies involving ionic liquid/water interface are scarce. Some of them utilize three phase boundary formed at the electrode/ionic liquid/water interface [3,4]. Typically in such a system an electric charge is generated by electrode reaction of the redox probe within ionic liquid deposited on the electrode surface. The electrode is immersed in the electrolyte aqueous solution. To keep electroneutrality within the organic phase the electrode reaction is associated with ion transfer across liquid/liquid interface [4]. The use of weakly coordinating solvents with low dielectric permittivity (e.g. toluene) ensures relatively good solubility of hydrophobic redox probes (e.g. porphyrinato metal complexes). Recently the solution of phosphonium-phosphate ionic liquid (cph.aph) in toluene as the suitable electrolyte system has been proposed [5]. Herein, the deposit of cph.aph supported toluene containing manganese(III) porphyrine (MnTPP) chloride as the redox probe was examined in terms of ion transfer across liquid/liquid interface. The effect of cph.aph concentration on UV-VIS spectra of MnTPP-toluene solution was examined. They are clearly affected by addition of the ionic liquid and observed changes may be considered as an evidence for axial ligation of ionic liquid anion to the porphyrine molecule. Cyclic and differential pulse voltammetry experiments were carried out at glassy carbon electrode modified with organic solution and immersed in aqueous phase. Reversible anion transfer occurs and it has been found that the peak potentials depend on the nature and concentration of the anion present in the aqueous phase. Analysis of data indicates that after immersion of the electrode into aqueous solution spontaneous anion exchange across liquid/liquid interface and equilibrium occurs. Then the Mn(III/II)TPP electro-reduction brings about anion transfer between organic and aqueous phase. Hydrophobic anions are transferred at more negative potentials and only for strongly hydrophilic anions (F-, SO42-) a change in mechanism are observed. The addition of cph.aph. shifts peak potentials towards more negative values. This effect may be assigned to stabilization of Mn(III)TPP+X- (X = PF6-ClO4-, SCN-, NO3-, Br-) ion pairs in toluene by the ionic liquid.

[1] M.C. Buzzeo, R.G. Evans, R.G. Compton, Chem. Phys. Chem. 5 (2004) 1106.

[2] J. Zhang, A.M. Bond, Analyst 130 (2005) 1132.

[3] N. Nishi, S. Imakura, T. Kakiuchi, Anal. Chem. 78 (2006) 2726.

[4] J. Niedziolka, E. Rozniecka, J. Stafiej, J. Siriex-Plenet, L. Gaillon, D. Di Caprio, M. Opallo, Chem. Commun. (2005) 2954.

[5] N.W. Duffy, A.M. Bond, Electrochem. Commun. 8 (2006) 892.

 

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Presentation: Poster at SMCBS'2007 International Workshop, by Wojciech Adamiak
See On-line Journal of SMCBS'2007 International Workshop

Submitted: 2007-08-31 12:02
Revised:   2009-06-07 00:44