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 + (II 2+ ), 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 II 2+ 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 Å2/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 II 2+ 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 II 2+ 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.

Interfacial Properties of Unsymmetrical Bolaform Amphiphiles with One Ionic and One Nonionic Head Group

“…This result suggests that the hydroxyl group of HTAB does associate with the aqueous subphase and thereby forces HTAB into a looped conformation (Fig. 2) (22)(23)(24). We note that a number of past studies of symmetric α, ω-alkanediols with hydrocarbon chains containing 16-30 carbons have shown that α,ω-alkanediols with long chains tend to form multilayer structures at the interface (25)(26)(27).…”

“…The chemical potential was evaluated from a model of the Gibbs free energy of each monolayer. The model that we use for the Gibbs free energy is based on a model used previously by us to describe the surface properties of aqueous solutions of a ferrocenyl surfactant with the structure Fc(CH 2 ) 11 N + (CH 3 ) 3 (23). The model assumes five principal contributions to the Gibbs free energy of a monolayer of surfactant,…”

“…1) as well as a molecular thermodynamic model of Gibbs monolayers of the redox-active surfactant Fc(CH 2 ) 11 N + (CH 3 ) 3 , where Fc is ferrocene (23). The relevant conclusions extracted from these past studies are threefold.…”

Electrochemical Sensing of Anions and Heparin by an Alkyl-chain Ferrocene Cationic Surfactant

“…Gallardo et al concluded that the adsorbed alkyl chain of FcUTMA was bent at the air-water interface. 12,13 This abnormal adsorbed state was attributed to the chemical structure of FcUTMA, which has an ammonium ion and ferrocene group positioned at either end of the molecule. The anion sensing ability of ACFcCSs was reported by Reynes et al, [14][15][16] who investigated inorganic anions and ATP in predominantly organic media.…”

Spontaneous Emulsification Produced by Chemical Reactions

“…Compounds with ferrocenyl groups, for instance, have been investigated by several groups (13)(14)(15)(16)(17). In one case the surface tension of (11-ferrocenylundecyl)trimethylammonium bromide was measured under various conditions (18). In the reduced state where the ferrocenyl group is uncharged, the compound is surface active at low concentrations and forms micelles at concentrations above about 0.1 mM.…”