Electrochemical generation of gradients in surfactant concentration across microfluidic channels.

We report the generation and manipulation of spatial gradients in surfactant and micelle concentration across microfluidic channels by combining use of a redox-active surfactant with electrochemical methods. The approach is founded on the observation that 11-ferrocenylundecyltrimethylammonium bromide (FTMA) behaves as a surfactant in aqueous solution (e.g., self-assembles to form micelles at a critical concentration of 0.1 mM in aqueous 0.1 M Li(2)SO(4)) whereas oxidized FTMA remains dispersed in a monomeric state up to concentrations of at least 30 mM. By flowing aqueous FTMA solutions through microfluidic channels (width of 80 microm, depth of 72 microm, and length of 42 mm) and by applying potentials of 0 V (vs Ag|AgCl; cathode) and +0.3 V (vs Ag|AgCl; anode) to gold electrodes lining both side-walls of the microfluidic channels, we measured lateral gradients in concentration of oxidized FTMA and reduced FTMA to be generated across the microfluidic channels by splitting the exiting stream into four channels. These measurements revealed the lateral concentration profile of FTMA to be consistent with the presence of slowly diffusing micelles of FTMA in a spatially localized region near the cathode and monomeric FTMA only near the anode. The lateral concentration profiles of reduced and oxidized FTMA, and thus the patterning of micelles within the microfluidic channels, were manipulated via changes in the inlet FTMA concentration, potentials applied to the electrodes, and flow rate. These experimental measurements were compared to a simple model, which assumed fast electrode kinetics, lateral transport of FTMA by diffusion only (no migration), and local micelle-monomer equilibrium within the bulk solution. This comparison revealed qualitative but not quantitative agreement between model and experiment. Calculations of ionic conductivity and associated experimental measurements support the proposition that Ohmic resistance to the passage of current along the channel (between the working and the counter electrodes) contribute, in part, to the lack of quantitative agreement between the model and the measurements. The capability to generate and manipulate lateral concentration profiles of surfactants and micelles across microfluidic channels, as demonstrated by the results presented in this paper, offers the basis of new principles for continuous separation processes and microanalytical systems, and more broadly, new methods to generate gradients in concentration of analytes that interact with surfactants.