Methods for generation of spatial gradients in concentration of monomeric surfactants and micelles in microfluidic...

We report methods suitable for use in microfluidic systems that permit the generation and manipulation of spatial gradients in concentrations of monomeric surfactants and micelles within aqueous solutions. The methods involve the use of the redox-active surfactant, (11-ferrocenylundecyl)trimethyl-ammonium bromide (FTMA) and build from past studies that have established that FTMA exhibits a critical micelle concentration of 0.1 mM (in 0.1 M Li2SO4), whereas oxidized FTMA remains dispersed in a monomeric state up to concentrations of at least 30 mM. Following the application of potentials of 0 V (vs Ag|AgCl; cathode) and +0.3 V (vs Ag|AgCl; anode) to electrodes separated by distances of 25-116 microm, we measured steady state currents of equal magnitude to be passed at each electrode within 1-20 s of the onset of the application of the potentials. We used dynamic light scattering and surface tension measurements to determine that oxidized and reduced FTMA do not measurably interact in solution and thus interpret the steady state currents, measured as a function of the concentration of FTMA added to the system and distance between the electrodes, within the framework of a simple model that assumes fast electrode kinetics, local micelle-monomer equilibrium within the bulk solution, and transport by diffusion only (no migration). Comparison of experimental measurements and model predictions reveals good overall agreement, consistent with the presence of one-dimensional gradients in concentrations of monomeric FTMA and micelles of FTMA in solution between the electrodes. The nature of the gradients can be manipulated by the potentials applied to the electrodes and can be used to achieve spatially localized populations of micelles in the system.