Probe Diffusion Measurements of Polystyrene Latex Particles in Polyelectrolyte Solutions of Varying Ionic Strength

The diffusion of polystyrene latex particles in solutions of a poly(acrylic acid- co -acrylamide) polyelectrolyte as a function of both polyelectrolyte concentration and solution ionic strength has been investigated using dynamic light scattering. Two apparent diffusion coefficients were measured for the latex probe particles in the polyelectrolyte solutions. Designated as D fast and D slow, the fast diffusional mode increases to a maximum as a function of polyelectrolyte concentration, with the maximum increasing in magnitude as the ionic strength increases. D slow decreases as a function of polyelectrolyte concentration in low ionic strength solutions. However, D slow shows a slight increase with polyelectrolyte concentration in the higher ionic strength solutions. The results are discussed with reference to calculated correlation lengths for the polyelectrolyte solutions and are compared with the data obtained from polymer solutions measured using dynamic light scattering where similar trends have been observed. The latex spheres appear to probe the single chain motion, cooperative diffusion, and the macroscopic diffusional motions, self-diffusion, of the polyelectrolyte molecules.