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 f orm the basis of a procedure to create gradients in surfactant-based p roperties 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-activ e species at rates competitive with their electrochemical formation pe rmits the creation of surfactant molecules with lifetimes that can be manipulated from 10(-2) to 10(3) s. Electrochemical reduction of a fer rocenium precursor (I2+) to a ferrocenyl surfactant (11-ferrocenylunde cyl)trimethylammonium bromide (Fc(CH2N+(CH3)(3) Br-; Pc = [eta(5)-C5H5 ]Fe[eta(5)-C5H5]) (I+) at a Pt electrode protruding from the surface o f an aqueous solution caused the motion of fluid away from the electro de under the influence of a gradient in the surface tension of the sol ution (Marangoni phenomenon). The velocity of the solution increased a s 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 wh en the concentration of ferrocenium precursor was 0.6 mM; the change i n 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 di splaced back and forth across the surface of an aqueous solution by us ing two working electrodes. The lifetimes of ferrocenyl surfactants I formed electrochemically were manipulated from similar to 10(-2) to 1 0(3) s by changing the concentration of oxidizing agent (Fe3+) dissolv ed within the bulk of the aqueous solution; the Marangoni flow of flui d was observed to result from a balance in the rates of electrochemica l 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 d etrimental.