A Molecular-Thermodynamic Model for Gibbs Monolayers Formed from Redox-Active Surfactants at the Surfaces of Aqueous Solutions: Redox-Induced Changes in Surface Tension

Abstract

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 ∼49 mN/m at a limiting surface area of 85 A2/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 classic...