A molecular-thermodynamic model for Gibbs monolayers formed from redox-active surfactants at the surfaces of aqueous solutions: Redox-induced changesin surface tension

We report the development of a molecular-thermodynamic model for Gibbs mono layers formed from the redox-active surfactant (11-ferrocenylundecyl)trimet hylammonium bromide (II+), or oxidized II+ (II2+), at the surfaces of aqueo us solutions. This model provides an account of past experimental measureme nts (Gallardo, B. S.; Metcalfe, K. L.; Abbott, N. L. Langmuir 1996, 12, 411 6-4124) which demonstrated electrochemical oxidation of II+ to II2+ to lead to large and reversible changes in the excess surface concentrations and s urface tensions of aqueous solutions of this redox-active surfactant. The r esults of the model lead us to conclude that II+ assumes a looped conformat ion at the surfaces of aqueous solutions. This looped conformation lowers t he surface tensions of aqueous solutions of II+ to similar to 49 mN/m at a limiting surface area of 85 Angstrom(2)/molecule (in 0.1 M Li2SO4). The und erlying cause of the reduction in surface tension is not an electrostatic c ontribution to the surface pressure (as is the case with classical ionic su rfactants) but rather an entropic contribution due to the constrained (loop ed) configuration of the surfactant at the surface of the solution (chain p acking). At concentrations around the critical micelle concentration (CMC) of II+ (0.1 mM), oxidation of II+ to II2+ results in the desorption of surf actant from the surface of the solution and an increase in surface tension from 49 to 72 mN/m. The process of desorption is driven by an oxidation-ind uced decrease in the hydrophobic driving force for self-association of the surfactants as well as an electrostatic repulsion between adsorbed surfacta nts. In contrast, at concentrations of II+ that substantially exceed its CM C, oxidation of II+ to II2+ drives the disruption of micelles to monomers i n the bulk solution, thus increasing the chemical potential and excess surf ace concentration of surfactant: the oxidation-induced increase in excess s urface concentration of surfactant leads to a decrease in surface tension. These results, when combined, provide principles for the design of redox-ac tive surfactants.