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 (I2+) to a ferrocenyl surfactant (11-ferrocenylundecyl)trimethylammonium bromide (Fc(CH2N+(CH3)(3) Br-; Pc = [eta(5)-C5H5]Fe[eta(5)-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 I2+ 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 similar to 10(-2) to 10(3) 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.