The micellar diffusion coefficient, Dmic , is a fundamental property of surfactant aggregates in water. Its value sheds light on the changes in micellar structure as a function of experimental variables. From Dmic , the micellar hydrodynamic radius, RH , hence aggregation number, Nagg , can be calculated. We employed voltammetry with platinum disc microelectrodes (Autolab PGSTAT 30 potentiostat; 25 �C) to determine Dmic  in water for a series of surface-active ionic liquids, SAILs, 1-R-3-methylimidazolium chloride; R = C12 , C14 , and C16 . We employed ferrocene as electroactive probe, due to its very low solubility in water (9.8 x 10-6  mol/L), and high affinity for the micelle (association constant with CTABr = 1738). Consequently the contribution of the free, i.e., unbound probe can be neglected. We have calculated Nagg  as a function of the length of (R) by the following sequence: (i) The limiting currents for the reversible electron transfer to the micelle-bound probe were calculated at different [SAIL] and extrapolated to the critical micelle concentration (determined by conductivity) to give Dmic ; (ii) RH  was calculated from Dmic  by using the Stokes-Einstein equation; (iii) Nagg  was calculated from RH , based on the monomer volume and spherical micellar geometry. We calculated the following Nagg : 48, 79, 105 for R = C12 , C14 , and C16 , respectively; these agree with the values calculated from light-scattering, 50, 76, 96, respectively (Galgano, El Seoud, J. Colloid Interface Sci. 2011, 361, 186). This application of microelectrodes has the merit of simplicity and low cost; no additional electrolyte is needed, e.g., as in light scattering            We thank FAPESP and CNPq for financial support and fellowships to D. Valencia, P. Galgano, M. Bertotti, and O. El Seoud.

Electrochemical behavior of micelle-solubilized ferrocene: A simple procedure to calculate the aggregation numbers of micellized cationic surface-active ionic liquids

247th National Meeting (2014)

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