Measurement of the (quasi)-self-diffusion coefficient of solutions of Brownian macroparticles

D s of a dilute suspension of 0.15 μ polystyrene spheres moving through concentrated suspensions of 0.038 μ polystyrene spheres was determined with light scattering spectroscopy. Solutions from which small ions had been rigorously excluded were compared with suspensions containing such ions. If the concentration dependence of Ds is Ds=D0 (1+λφ), φ being the volume fraction of 0.038 μ spheres, removal of the small ions makes λ more negative. This behavior of λ agrees qualitatively with a dynamic friction model for Ds, but is inconsistent even as to sign with calculations based on the generalized Smoluchowski equations, as recently summarized by Evans and James [J. Chem. Phys. 79, 5553 (1983)].