Dispensing Surfactants from Electrodes:  Marangoni Phenomenon at the Surface of Aqueous Solutions of (11-Ferrocenylundecyl)trimethylammonium Bromide

Abstract

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 (<mm) regions defined by electrodes. Chemical removal of the surface-active species at rates competitive with their electrochemical formation permits the creation of surfactant molecules with lifetimes that can be manipulated from 10-2 to 103 s. Electrochemical reduction of a ferrocenium precursor (I 2+ ) to a ferrocenyl surfactant (11-ferrocenylundecyl)trimethylammonium bromide (Fc(CH2)11N+(CH3)3 Br-; Fc = [η5-C5H5]Fe[η5-C5H5]) (I + ) at a Pt electrode protruding from the surface of an aqueous solution caused the motion of fluid away from the electrode under the influence of a gradient in the surface tension of the solution (Marangoni phenomenon). The velocity of the solution increased as the potential applied to the electrode decreased from 0.1 V (vs SCE) to −0.2 V (vs SCE). The maximum velocity of the fluid was observed when the concentration of ferrocenium precursor was 0.6 mM; the change in dynamic surface tension of an aqueous solution upon reduction of I 2+ to I + was also maximal at a concentration of ca. 0.6 mM. Fluid was displaced back and forth across the surface of an aqueous solution by using two working electrodes. The lifetimes of ferrocenyl surfactants I + formed electrochemically were manipulated from ∼10-2 to 103 s by changing the concentration of oxidizing agent (Fe3+) dissolved within the bulk of the aqueous solution; the Marangoni flow of fluid was observed to result from a balance in the rates of electrochemical creation and chemical removal of I + at the surface of the solution. The principles reported herein make possible the use of surfactants in environments in which their sustained and/or widespread presence is detrimental.

Active control of surface properties and aggregation behavior in amino acid-based Gemini surfactant systems

“…Some light-active and redox-active surfactants have been successfully used to show the reversible changes in aggregation morphology, viscosity, microemulsion separation, and solubilization [20,21]. Besides, research on active control of the interfacial tension has also been reported for designed surfactants using triggers including ultraviolet irradiation [22,23], electrochemical oxidation [24,25] and chemodegradation [26,27]. In previous investigations, amphiphiles containing disulfide bonds have attracted a great deal of attention [28][29][30][31][32][33] mainly because of their ability to be reduced chemically, electrochemically, photochemically, and enzymatically [34].…”

Multiphase flow in microfluidic systems – Control and applications of droplets and interfaces

“…Interfacial forces can be modulated using different methods. Firstly, interfacial forces can be changed by adding surfactants [36,45,[72][73][74][75][76]. Secondly, the channel wall surface can be modified by growing self-assembled monolayers -silanes on glass or silicon surfaces and thiols on Au or Ag surfaces [77][78][79][80][81][82].…”

Colloidal chirality in wormlike micellar systems exclusively originated from achiral species: Role of secondary assembly and stimulus responsivity

“…Amphiphiles (surfactants) containing redox active group ferrocene could have charge/hydrophobicity changes, thus self-assembly transition, via controlling the oxidation state of ferrocene [18]. N. L. Abbott et al reported the electrochemical principles for active control of liquids on submillimeter scales using redox-active surfactants [19] and marangoni phenomenon by dispensing surfactants from electrodes [20]. G. W. Gokel has reported the first example of vesicle formation from redox-switched hydrophobic ferrocene derivatives [21], and sonication-induced redox-switched vesicles formed by ferrocene derivatives upon oxidation [22].…”