Redox-Dependent Surface Tension and Surface Phase Transitions of a Ferrocenyl Surfactant:  Equilibrium and Dynamic Analyses with Fluorescence Images

The surface tension data for the surfactant Fc−CH 2 −N + (CH 3 ) 2 −(CH 2 ) 14 CH 3 −Br - (where Fc represents the ferrocenium cation) display the signatures of a surface gaseous−surface liquid expanded transition with binodals that depend on the oxidization state of the Fc headgroup. The phase transition is imaged using fluorescence microscopy. The equilibrium and dynamic surface tension data (obtained by the pendant bubble method) are analyzed in terms of an equilibrium model that accounts for surface phase transitions and electrostatic interactions and a dynamic model that accounts for the mass transfer kinetics of surfactant adsorption to the interface. The dynamic surface tension of the reduced form of the surfactant is controlled by bulk diffusion to the interface. The dynamic surface tension of the oxidized form of the surfactant is diffusion-controlled at dilute concentrations, but not at more elevated concentrations where adsorption−desorption barriers are important. The kinetic constants are determined from these data, yielding an adsorption kinetic constant β = 9.2 × 10 - 1 m 3 /(mol s) and a corresponding desorption kinetic constant α = 1.9 × 10 - 6 s -1 . This value for β is similar to those found for poly-ethoxylated surfactants, but the desorption kinetic constant is slower, indicating that desorption of this surfactant is highly hindered.