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

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 10 3 s. Electrochemical reduction of a ferrocenium precursor ( I 2+ ) to a ferrocenyl surfactant (11-ferrocenylundecyl)trimethylammonium bromide (Fc(CH 2 ) 11 N + (CH 3 ) 3 Br -; Fc = [η 5 -C 5 H 5 ]Fe[η 5 -C 5 H 5 ]) ( 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 10 3 s by changing the concentration of oxidizing agent (Fe 3+ ) 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.