| Abstract
| - We report the development of a molecular-thermodynamic model for Gibbs monolayers formed from theredox-active surfactant (11-ferrocenylundecyl)trimethylammonium bromide (II+), or oxidized II+ (II2+),at the surfaces of aqueous solutions. This model provides an account of past experimental measurements(Gallardo, B. S.; Metcalfe, K. L.; Abbott, N. L. Langmuir1996, 12, 4116−4124) which demonstratedelectrochemical oxidation of II+ to II2+ to lead to large and reversible changes in the excess surfaceconcentrations and surface tensions of aqueous solutions of this redox-active surfactant. The results of themodel lead us to conclude that II+ assumes a looped conformation at the surfaces of aqueous solutions.This looped conformation lowers the surface tensions of aqueous solutions of II+ to ∼49 mN/m at a limitingsurface area of 85 Å2/molecule (in 0.1 M Li2SO4). The underlying cause of the reduction in surface tensionis not an electrostatic contribution to the surface pressure (as is the case with classical ionic surfactants)but rather an entropic contribution due to the constrained (looped) configuration of the surfactant at thesurface of the solution (chain packing). At concentrations around the critical micelle concentration (CMC)of II+ (0.1 mM), oxidation of II+ to II2+ results in the desorption of surfactant from the surface of thesolution and an increase in surface tension from 49 to 72 mN/m. The process of desorption is driven byan oxidation-induced decrease in the hydrophobic driving force for self-association of the surfactants aswell as an electrostatic repulsion between adsorbed surfactants. In contrast, at concentrations of II+ thatsubstantially exceed its CMC, oxidation of II+ to II2+ drives the disruption of micelles to monomers in thebulk solution, thus increasing the chemical potential and excess surface concentration of surfactant: theoxidation-induced increase in excess surface concentration of surfactant leads to a decrease in surfacetension. These results, when combined, provide principles for the design of redox-active surfactants.
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