Diffusion in Solutions of Micelles. What Does Dynamic Light Scattering Measure?

Dynamic light scattering (DLS) is routinely used to measure the diffusion of surfactant micelles. Theory and experiment suggest, however, that the microscopic concentration fluctuations monitored by DLS obey the same mutual diffusion equations that describe the decay of macroscopic concentration gradients. According to this interpretation, DLS provides mutual diffusion coefficients for the total surfactant components, including contributions from micelles, free surfactant monomers, and counterions. An attempt is made to decide the correct interpretation of DLS measurements by comparing DLS diffusion coefficients (D-DLS) with mutual diffusion coefficients measured by macroscopic gradient techniques for binary aqueous solutions of ionic and zwitterionic surfactants. Possible contributions to DDLS from free surfactant monomers are investigated by extending DLS measurements into the critical micelle (cmc) region where substantial portions of the surfactants diffuse as free monomers. The widely held assumption that D-DLS is the micelle diffusion coefficient is tested by comparing D-DLS with micelle diffusion coefficients measured unambiguously by NMR or Taylor dispersion techniques for solubilized trimethylsilane or decanol tracers. D-DLS is found to decrease sharply as the surfactant concentration is raised through the cmc, in agreement with the steep drop in the mutual diffusion coefficient caused by the association of free surfactant monomers. Above the cmc, D-DLS and the micelle and mutual diffusion coefficients are nearly identical for the zwitterionic surfactants. For the ionic surfactants, D-DLS and the mutual diffusion coefficients are several times larger than the micelle diffusion coefficients as a result of the diffusion of charged micelles with relatively mobile counterions to maintain electroneutrality. The results suggest that DLS diffusion coefficients are surfactant mutual diffusion coefficients.