Electrochemistry in AOT Reverse Micelles. A Powerful Technique to Characterize Organized Media

PATRICIA G. MOLINA; JUANA J. SILBER; CORREA N.M*; LEONIDES SERENO

JOURNAL OF PHYSICAL CHEMISTRY C

American Chemical Soc.

Lugar: Washington DC, USA; Año: 2007 vol. 111 p. 4269 - 4276

1932-7447

A detailed study of the electrochemical behavior of the [Fe(CN)6]-4/3 redox couple was performed in water/AOT/n-heptane reverse micelles using a Pt microelectrode according to the high resistivity of the media. The results of varying the surfactant concentration at W0 =[H2O]/[AOT]= 10 and constant [Fe(CN)6]-4 concentration show an anomalous response of the limiting current value, iL. Despite the invariant reverse micelles size the iL value, expected to be constant, increases with the AOT concentration. These interesting results are explained as due to the ion pairing that [Fe(CN)6]-4 may undergo with K+ counterions giving less charged species that tend to reside at the AOT reverse micelles interface and need more energy to undergo the oxidation process. The association process is favored when the molecular probe occupation number is greater than 2 because the low water availability for ion solvation. As the water content increases, the ion solvation increases and reduces de degree of association. The free [Fe(CN)6]-4 ion resides in the water pool where the electrochemical response is close to the one obtained in bulk water for the non associated species. Absorption spectroscopy experiments confirm these assumptions. As AOT reverse micelles can be considered monodisperse systems which make the entrance of the molecules or ions inside the reverse micelles a random effect and [Fe(CN)6]-4/3 couple resides exclusively in their water pool due to Coulombic repulsion with the surfactant headgroups we propose a model based in the Poisson distribution to account for different properties. The model and the measurements allow us to determine: i) the real concentration of the electroactive specie, the one that it is not associated with the K+ counterions; ii) the AOT reverse micelle diffusion coefficient and iii) the micellar hydrodynamic radius. The values are in very good agreement with results obtained by other techniques which clearly show that electrochemical measurements can be a powerful alternative approach to investigate organized media such as reverse micelles.