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

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.