A continuous interest in liquid modified electrodes is probably due to their unique property which is a well defined liquid-liquid interface. Typically, liquid deposits consist of water immiscible organic solvents in the form of droplet arrays [1], single droplets [2], or thin films [3], and the electrode is immersed in an aqueous electrolyte. In the majority of the studies, this organic liquid contains redox active molecules. The electron exchange between the redox probe and the electrode leads to a charge imbalance within the liquid deposit and the electrode reaction is followed by ion transfer across the liquid-liquid interface. In many cases these liquid modified electrodes exhibit ion-sensitive voltammetry, because ions have different affinities to the adjacent phases as defined by their standard transfer potentials.

In this work an electrochemical behavior of trihexyl(tetradecyl)phosphonium tris(pentafluoroethyl)trifluorophosphate ionic liquid (P_14,6,6,6FAP) modified electrode has been examined in terms of interfacial ion transfer. Manganese(III) tetraphenylporphyrin chloride (MnTPP⁺Cl^-) and ionic liquid supported toluene have been utilized as a redox probe and a water immiscible phase, respectively.

Electrochemical studies of ion transfer have been preceded by a single phase study of MnTPP⁺Cl^- in toluene-P_14,6,6,6FAP mixture in order to evaluate its interactions with this electrolyte. A blue shift of characteristic bands at UV-Vis spectra of the porphyrin have been observed as the concentration of P_14,6,6,6FAP was increased. Voltammetric studies and theoretical calculations of steady state currents revealed a decrease of porphyrin diffusion coefficient in the presence of P_14,6,6,6FAP. All these facts have been assigned to a significant Cl^- exchange for a large FAP^- anion at the axial position of the porphyrin complex.

Under biphasic conditions the electrode surface was modified by a ca. 30 um layer of the organic phase deposited by a solution-cast method. Voltammetric measurements revealed that MnTPP mid-peak potential depends on the nature (Fig.1.) and concentration of the aqueous electrolyte anion. A Nernst type equation has been derived to describe this dependence and anion-sensing mechanism has been evaluated. It has been proposed that heterogeneous electron transfer is followed by anion expulsion where the anion sensitivity results from lability of the axial coordination site of MnTPP⁺Cl^- and from spontaneous ion exchange (Fig.2.). This work can possibly contribute to application of toluene-based media in electroassisted ion extraction or in development of novel amperometric sensors.

References:

[1] F. Marken, R. D. Webster, S. D. Bull, S. G. Davies, J. Electroanal. Chem. 1997, 437, 209.

[2] F. Scholz, S. Komorsky-Lovric, M. Lovric, Electrochem. Commun. 2001, 3, 112.

[3] C. Shi, F. C. Anson, Anal. Chem. 1998, 70, 3114.

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