Electrochemistry in Reverse Microemulsions and the Concentration of the Electroactive Probe

Abstract

Introduction Reverse microemulsions are homogeneous dispersion of water droplets (1 to 100 nm diameter) in an oil, stabilized by a surfactant. In order to improve their stability, aliphatic alcohols (cosurfactants) are often added to the microemulsions1 . Among the different applications of the microemulsions, the use as a reaction media to synthesize nanoparticles has attracted attention in the last decade2 . Since the size and shape of nanoparticles are directly affected by those of the water droplets in the reverse microemulsions3 , the characterization of the reverse microemulsion is necessary. Among the techniques to characterize a reverse microemulsion, cyclic voltammetry has been proposed as an alternative. For this, it is considered that, in most of the cases, the electroactive probes are insoluble in the oil phase, and accordingly the current will depend on the concentration and the diffusion towards the electrode of the water droplets4 and on the concentration of the probe as well. It is important to point out that in order to obtain reliable results for other systems, the concomitant contribution of these factors must be comprehended. For this, a proposal is founded on: i) the concentration ratio water/surfactant determines the size, and hence the diffusion of the water droplets 5,6 ii) the occupancy number, defined as the number of probe units per water droplets in the reverse microemulsion7 . In this work, the effect that both variables have on the electrochemical behavior of reverse microemulsions is studied. The results suggest that cyclic voltammetry can be used as a characterization method of reverse microemulsions as long as these variables are considered. Methodology Hexadecyltrimethylammonium tosilate (CTAT), n-pentanol, hexanes, potassium ferricyanide and double distilled water were used as received. A platinum microelectrode (25 µm diameter) was used as working electrode, a platinum foil as counter-electrode and a platinum wire as pseudo-reference electrode. The cyclic voltammetries were performed in a potentiostat PGSTAT 128N. The sweep rate was varied between 2 and 5 mV/s. Results It has been proposed that the limiting current in a reverse microemulsion decreases as the water content increases, this is because the droplets are the carriers of the electroactive probe and as they grow they diffuse slower, therefore the electrical current declines as the water content increases. Contrary to the expected, the limiting current clearly has a positive slope, see figure 1, passing through a maximum that changes its location with the amount of the aqueous phase and the probe concentration. As reported in the literature, the parameter that links these two variables is known as the occupancy number, and its physical meaning is the average number of ions contained by each droplet. But in order to choose the best concentration of the probe to be used in the characterization of the reverse microemulsion it is necessary to understand the correlation between the water content, the probe concentration, the occupancy number and the limiting current. Conclusions The limiting current measured in a reverse microemulsion depends on the concomitant effect of the diffusion of the water droplets and the occupancy number. Figure 1. Limiting current as function of the water content for two different concentrations of potassium ferricyanide Bibliography (1) Moldes, Ó. A.; Morales, J.; Cid, A.; Astray, G.; Montoya, I. A.; Mejuto, J. C. Electrical Percolation of AOT-Based Microemulsions with N -Alcohols. J. Mol. 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Della; Ceglie, A. Microstructure and Dynamics of the Water-in-Oil CTAB / N-Pentanol / N-Hexane / Water Microemulsion : A Spectroscopic and Conductivity Study. J. Phys. Chem. 1996, 100 (8), 3190–3198.(7) Florez Tabares, J. S.; Correa, N. M.; Silber, J. J.; Sereno, L. E.; Molina, P. G. Droplet–droplet Interactions Investigated Using a Combination of Electrochemical and Dynamic Light Scattering Techniques. The Case of water/BHDC/benzene:n-Heptane System. Soft Matter 2015, 11 (15), 2952–2962. Figure 1