Voltammetric behaviour of ferrocene in olive oils mixed with a phosphonium-based ionic liquid

https://doi.org/10.1016/j.jelechem.2014.08.001Get rights and content

Highlights

  • Voltammetry is performed in olive oils mixed with a phosphonium-based ionic liquid.

  • Measurements are made in the oil/RTIL mixtures using ferrocene as probe molecule.

  • Platinum disk microelectrodes are employed for cyclic voltammetric measurements.

  • Under these experimental conditions, mixed radial-planar diffusion regimes occur.

  • Diffusion coefficient values for Fc and Fc+ reflect the viscosity of oil and RTIL media.

Abstract

In this paper the mass transport characteristics of both the room temperature ionic liquid (RTIL) trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide ([P14,6,6,6]+ [NTf2]) and the RTIL mixed with an olive oil sample were investigated by voltammetry, using ferrocene as the probe molecule. The RTIL was used as electrolyte to increase the conductivity of the vegetable oil. To avoid problems related to ohmic drop, the measurements were taken with platinum microdisk electrodes, 10–12.5 μm radius. Cyclic voltammetric measurements at different scan rates were performed over the range 1–200 mV s−1, while the concentration of ferrocene was varied over the range 2.5–10 mM. The results obtained indicated that, under these conditions, a mixed radial-planar diffusion regime applied. Diffusion coefficient values for Fc in both pure RTIL and various oil/RTIL mixtures were evaluated by using the experimental peak current against square root of scan rate plots fitted to the theoretical relationship that applies for microdisk electrodes under a mixed diffusion regime. Digital simulation was employed to support the interpretation of the experimental voltammograms and also to obtain the diffusion coefficient of Fc+ in both RTIL and oil/RTIL media. Diffusion coefficient values of Fc and Fc+ depended on the sample matrix and essentially reflected the viscosity of the investigated media. The results obtained here can be useful to model voltammetric measurements of analytes in viscous and low conducting vegetable oil matrices.

Introduction

Voltammetric measurements in non-polar solvents have traditionally been impeded by the lack of readily available, soluble and well-dissociated supporting electrolytes needed to enhance their low conductivity [1], [2], [3]. In the past, the use of mixed non-polar and polar solvents, containing conventional supporting electrolytes, has allowed performing electrochemical measurements of compounds soluble only in non-polar media [4], [5], [6]. More recently, the extension of voltammetric methods into low-dielectric media has been made possible by the use of room temperature ionic liquids (RTILs) [3], [7], [8], [9], [10]. These can be employed in bulk, as media to dissolve non-polar compounds, or added to the non-polar solvents as electrolytes to enhance their conductivity [3], [7], [10]. In the latter context, advantageous applications of RTILs have also been reported for enhancing the conductivity of natural liquid matrices, such as edible vegetable oils, which are normally inaccessible to direct voltammetric measurements [11], [12]. In particular, it has been shown that the addition of tetraalkylphosphonium-based RTILs to maize and olive oil samples has allowed obtaining meaningful cyclic voltammetric responses at a platinum microdisk electrode, which has been exploited as an “electronic tongue” for differentiating oil samples according to their quality and geographical origin [11], or for verifying the authenticity of Italian PDO (protected designation of origin) olive oils [12]. These goals have been achieved by using a “blind analysis” strategy, based on the registration of current–potential profiles, recorded over a wide potential range in the oil/RTIL samples, and their treatment with chemometric exploratory and class-modelling techniques [11], [12].

The possibility of employing oil/RTIL mixtures to detect by voltammetry specific analytes in edible oils is also of interest. In this case, gathering information about mass transport characteristics of the media is an important step [13]. With this in mind, in this work we report on the voltammetric behaviour of ferrocene, taken as a probe molecule, in olive oil/RTIL mixtures. Because of its hydrophobic character, in fact, ferrocene can mime possible analytes (i.e. natural components or contaminants) of interest in edible oils. The RTIL employed here is trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide ([P14,6,6,6]+ [NTf2]) which has proven to solubilise in vegetable oils over a wide concentration range [11]. Because of the high viscosity of the media investigated, and to avoid problems related to ohmic drop, measurements were taken with a Pt microdisk electrodes [14], [15], [16].

Section snippets

Reagents and samples

[P14,6,6,6]+ [NTf2] was obtained from Merck (high purity grade  99%). Acetonitrile, ferrocene (Fc) and LiClO4 were analytical-reagent grade products and were purchased from Sigma–Aldrich (Milan, Italy). All chemicals were used as supplied by the manufacturers.

As for the RTIL, according to the commercial label, the water content was lower than 100 ppm. To avoid any further contamination with water, the RTIL was kept in a dry box until use. An extra-virgin olive oil sample, coming from Liguria, a

Voltammetric measurements in the pure RTIL

Preliminarily, CVs at different scan rates were carried out in the pure RTIL containing ferrocene at different concentrations. Fig. 1 shows a series of CVs recorded over the range of scan rates 1–200 mV s−1 in a 10 mM ferrocene/[P14,6,6,6]+ [NTf2] solution. As is evident, at 1 mV s−1, an almost sigmoidal shaped wave was obtained, indicating that radial diffusion is prevailing. Increasing the scan rate, the waves became progressively more peak-shaped in both forward and backward scans, in accordance

Conclusions

In the present study we have shown that the use of [P14,6,6,6]+ [NTf2] as supporting electrolyte in olive oils enables voltammetry to be undertaken with platinum microdisk electrodes in the low-conducting natural matrix containing an hydrophobic species such as ferrocene. A range of olive oil/RTIL mixtures with composition varying between 15% and 50% (w/w) have been prepared and investigated by cyclic voltammetry at different scan rates. It has been verified that the presence of the RTIL in

Conflict of interest

There is no conflict of interest.

Acknowledgments

Financial support from the Italian Ministry of Education, University and Research (MIUR) Rome, is gratefully acknowledged (PRIN 2010-11, prot. 2010AXENJ8). We also thank Dr. Luigi Falciola and Dr. Marco Ortenzi from the Department of Chemistry, University of Milan, for kindly performing viscosity measurements of the samples.

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