Diffusiophoresis in Ionic Surfactant Gradients

Surfactants play a ubiquitous role in many areas of science and technology, and gradients often form-either spontaneously or intentionally-in a variety of nonequilibrium situations and processes. We visualize and measure the diffusiophoretic migration of latex colloids in response to gradients of cationic and anionic surfactants, both below and above the critical micelle concentration (cmc). Below the cmc, colloidal migration can be described using classic theories for diffusiophoresis under electrolyte gradients, although subtleties and distinctions do appear. Cationic surfactants adsorb onto anionic colloids, changing the surface charge and thus reversing the direction of diffusiophoretic migration. Above the cmc, diffusiophoretic mobilties decrease by orders of magnitude. We argue this to occur because charged monomers (rather than micelles) dominate colloidal diffusiophoresis. Because monomer concentrations remain essentially constant above the cmc, surfactant gradients imposed above the cmc result in very small monomer gradients-and, therefore, very weak diffusiophoresis. Our findings suggest conceptual strategies to understand diffusiophoresis in the presence of surfactants, as well as strategies to predict and design systems that harness them.