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| - Effect of Electrolyte Concentration on Interfacial and BulkSolution Properties of Ferrocenyl Surfactants withAnionic Headgroups
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| - We report the effect of electrolyte concentration on the surface and bulk solution properties of a redox-active ferrocenyl surfactant, Fc(CH2)11SO3-Na+, where Fc is ferrocene. Electrochemical oxidation of theelectrically neutral Fc moiety to the ferrocenium cation reversibly transforms the anionic surfactant(Fc(CH2)11SO3-) into a zwitterionic species (Fc+(CH2)11SO3-). Whereas the effect of an increase in electrolyteconcentration on the surface activity of Fc(CH2)11SO3- and Fc+(CH2)11SO3- is similar to that for classicalanionic (surface tension decreases) and zwitterionic surfactants (surface tension invariant), respectively,the bulk solution behavior is not. We measured oxidation of Fc(CH2)11SO3- to Fc+(CH2)11SO3- to lead tothe spontaneous transformation of either monomers or globular micelles into vesicle-like assemblies(Rg/Rh = 1.0 ± 0.3) with hydrodynamic diameters of 90 ± 10 nm in 1 mM Li2SO4. We observed the additionof salt to Fc+(CH2)11SO3- to lead to a modest decrease in the size of the vesicular aggregates (to 60 ± 7nm at 100 mM Li2SO4) followed by formation of a precipitate (0.7 M). This result contrasts with past studiesof mixtures of anionic and cationic surfactants which have reported the addition of high concentrationsof salt to lead to the conversion of vesicular solutions into micellar ones. This difference in phase sequencelikely reflects constraints on the ionic composition of assemblies formed from Fc+(CH2)11SO3- that do notexist when using mixtures of anionic and cationic surfactants. This paper also reports briefly on the effectof a change in oxidation state on dynamic surface tensions of solutions of this class of surfactant. Theseresults suggest that the kinetics of adsorption of Fc+(CH2)11SO3- plays an important role in regulatingthe time dependence of the surface tension.
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