Electrochemical methods were combined with redox-active surfactants to actively control the motions and positions of aqueous and organic liquids on millimeter and smaller scales. Surfactant species generated at one electrode and consumed at another were used to manipulate the magnitude and direction of spatial gradients in surface tension and guide droplets of organic liquids through simple fluidic networks. Solid microparticles could be transported across unconfined surfaces. Electrochemical control of the position of surface-active species within aqueous films of liquid supported on homogeneous surfaces was used to direct these films into periodic arrays of droplets with deterministic shapes and sizes.

We report the use of redox-active surfactants Fc(CH2)nN + (CH3)3‚Br -, where Fc ) ferrocene ) [η 5 -C5H5]-Fe[η 5 -C5H5] and n ) 8, 11, or 15, in a study of principles for active control of interfacial properties of aqueous solutions. By comparing the surface activity of Fc(CH2)11N + (CH3)3 with that of HO(CH2)11N + -(CH3)3 and CH3(CH2)11N + (CH3)3, we demonstrate that Fc(CH2)nN + (CH3)3 behave as unsymmetrical bolaform surfactants with one ionic "head" group (N + (CH3)3) and one nonionic "head" group (Fc), whereas ferrocenyl surfactants, when oxidized to Fc + (CH2)nN + (CH3)3, have properties of symmetrical bolaform surfactants such as N + (CH3)3(CH2)15N + (CH3)3. Oxidation of Fc(CH2)nN + (CH3)3 to Fc + (CH2)nN + (CH3)3 leads to changes in interfacial properties through three mechanisms, at least. First, near their critical micellar concentrations (cmc's), oxidation caused desorption of monolayers of Fc(CH2)8N + (CH3)3 and Fc(CH2)11N + (CH3)3 from the surfaces of their aqueous solutions, thereby recovering the surface tension of the aqueous electrolyte. We measured changes in surface tension as large as 23 mN/m. Second, at concentrations greater than the cmc, oxidation of Fc(CH2)11N + (CH3)3 caused little, if any, change in the excess surface concentration of surfactant. The accompanying increase in the density of charge within the monolayer did, however, cause a decrease in surface tension of 6 mN/m. Third, oxidation of Fc(CH2)15N + (CH3)3 caused its monolayers at the surface of water to change from condensed states to expanded ones at constant surface pressure. We infer the phase transition within the monolayer to be driven by changes in conformation that accompany the transfer of ferrocene, upon oxidation, from the outer region (side in contact with air) of the condensed monolayer into contact with the aqueous subphase.

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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.

Electrochemical methods in combination with redox-active surfactants form the basis of a procedure to create gradients in surfactant-based properties of solutions. The gradients are created by the formation of surface-active species at controlled rates in spatially localized (