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

We report the development of a molecular-thermodynamic model for Gibbs monolayers formed from the redox-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. Langmuir 1996, 12, 4116-4124) which demonstrated electrochemical oxidation of II+ to II2+ to lead to large and reversible changes in the excess surface concentrations and surface tensions of aqueous solutions of this redox-active surfactant. The results of the model 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 similar to 49 mN/m at a limiting surface area of 85 Angstrom(2)/molecule (in 0.1 M Li2SO4). The underlying cause of the reduction in surface tension is 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 the surface 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 the solution and an increase in surface tension from 49 to 72 mN/m. The process of desorption is driven by an oxidation-induced decrease in the hydrophobic driving force for self-association of the surfactants as well as an electrostatic repulsion between adsorbed surfactants. In contrast, at concentrations of II+ that substantially exceed its CMC, oxidation of II+ to II2+ drives the disruption of micelles to monomers in the bulk solution, thus increasing the chemical potential and excess surface concentration of surfactant: the oxidation-induced increase in excess surface concentration of surfactant leads to a decrease in surface tension. These results, when combined, provide principles for the design of redox-active surfactants.