Estudo do comportamento eletroquímico do par redox [Fe(CN6)]3-/4- em microemulsões de biodiesel

Microemulsions (ME) have been widely used in electrochemical analysis due to the impossibility of doing this kind of measure directly in oil media due to its high viscosity and low conductivity. Although widely used, microemulsions are still poorly understand electrochemically. Microemulsions can be classified in three different types, oil in water (o/w), water in oil (w/o) and bicontinuous (phase transition) [1]. Thus, in this work, we report the importance of the knowledge of the electrochemical behavior of these biodiesel microemulsions, having as probe the [Fe(CN6)]3-/4- redox system (aqueous phase), soybean biodiesel (oil phase) and propan-1-ol (consolute). For the electrochemical measures, a conventional cell of three electrodes was used (glassy carbon as working electrode, platinum wire as an auxiliary electrode and an Ag/AgCl sat electrode as reference). A Potentiostat PGSTAT 30coupled with the software Nova 1.11 was used. Electrochemical characterizations were performed by cyclic voltammetry (CV) in the range of -0.2 to 0.5V at a scan rate 10 mV s-1 and chronoamperometry in the potential of 0.5V for 60s. The compositions of the microemulsions studied were: M1(37.5:60.0:2.5), M2 (30.0:62.5:7.5), M3 (22.5:62.5:15.0) and M4 (15.0:60.0:25.0) in %w/w of water +1.0 mol L-1KCl, propan-1-ol and soybean biodiesel, respectively. Through the study of the CVs it was possible to observe changes in the reversibility of the system for each of the ME studied. The studies of difusional coefficient (Do) was determined by chronoamperometry and the values found were: 5.6×10-8, 4.4×10-8, 6.0×10-8and 1.3×10-8cm2s-1for MEs 1, 2, 3 and 4, respectively. As can be observed, there was a decrease in the Do when compared with the Do of the probe in the aqueous homogeneous medium (~7.6×10-6cm2/s). This due to the higher viscosity of the medium. When a bicontinuous microstructure occurs, the molecules present in the ME present rapid diffusion, as observed in ME3 when compared to ME1 and ME2. Accordingly, the ME4 is w/o type, characterized by increasing the diffusion coefficient of the probe. Measurements of electrochemical impedance spectroscopy (EIS) were carried out in frequency range of 105Hz to 10-2Hz and amplitude of 10 mV, in the potential of 0.23 V. Analyzing the equivalent circuits, the ohmic resistance of the electrolytic solution (Rs), were 2.4 kΩ, 1.7 kΩ, 4.5 kΩ and 11.3 kΩ, respectively for ME1, ME2, ME3 and ME4. The exception was ME3, which, even with a higher amount of biodiesel, presented lower Rs, probably because it is a bicontinuous microemulsion, as reported in the study of the diffusion coefficient. These results become useful in understanding the electrochemical behavior of biodiesel MEs and are essential in choosing the most advantageous microstructure (o/w, bicontinuons phase and w/o) for the development of models for these systems in the analysis of compounds of interest in biodiesel.