The aggregation and micellization of ionic surfactants in aqueous solution detected using surface-confined redox and ion-pairing reactions

Abstract

This article describes the use of a redox-induced ion-pairing reaction confined to an electrode surface to detect the micellar aggregation of ionic surfactants in water. Cyclic voltammetry characterizes the Faradaic response of self-assembled monolayers (SAMs) of ferrocenyldodecanethiolate on gold in aqueous solutions of the sodium salts of the anionic surfactants n-decyl sulfate, n-decyl sulfonate, and taurodeoxycholate, the bromide salt of the cationic surfactant decyltrimethylammonium, and the sodium salt of the azo dye Orange II. These electrolytes present different aggregation behaviors in water. The cyclic voltammograms indicate that the surfactant and dye anions and the bromide counteranion of the cationic decyltrimethylammonium pair with the electrogenerated SAM-bound ferrocenium. The voltammetric signatures are sensitive to the concentration and aggregation state of the ions in solution. The concentration dependence of the apparent redox potential (E°’SAM) presents the same features observed by potentiometry using surfactant ion-selective or bromide electrodes, indicating that E°’SAM tracks the activity of the unaggregated or free surfactant, dye or bromide anions in solution. The formation of micellar aggregates above the critical micelle concentration (cmc) causes the free anion activity to deviate from the molar concentration of added surfactant, resulting in a break in the plot of E°’SAM versus the logarithm of the surfactant concentration. The quantity of surfactant anions associated to the oxidized SAM increases with the solute concentration and plateaus out at concentrations > cmc. These findings point to the potential application of ferrocene-terminated SAMs to the investigation of ionic micelle-forming systems and the determination of their cmc’s.