Time-dependent friction on a charged tracer in a Brownian multicomponent plasma

Long-time self-diffusion of charged colloidal particles: electrokinetic and hydrodynamic interaction effects

The authors analyze the long-time self-diffusion of charge-stabilized colloidal macroions in nondilute suspensions using a mode-coupling scheme developed for multicomponent suspensions of interacting Brownian spheres. In this scheme, all ionic species, including counterions and electrolyte ions, are treated on an equal footing as charged hard spheres undergoing overdamped Brownian motion. Hydrodynamic interactions between all ions are accounted for on the far-field level. We show that the influence on the colloidal long-time self-diffusion coefficient arising from the relaxation of the microionic atmosphere surrounding the colloids, the so-called electrolyte friction effect, is usually insignificant in comparison with the friction contributions arising from direct and hydrodynamic interactions between the colloidal particles. This finding is true even for small colloid concentrations unless the mobility difference between colloidal particles and microions is not large. Furthermore, we observe an interesting nonmonotonic density dependence of the colloidal long-time self-diffusion coefficient in suspensions with low amount of added salt. We show that this unusual density dependence is due to colloid-colloid hydrodynamic interactions.

Influence of solvent granularity on the effective interaction between charged colloidal suspensions

We study the effect of solvent granularity on the effective force between two charged colloidal particles by computer simulations of the primitive model of strongly asymmetric electrolytes with an explicitly added hard-sphere solvent. Apart from molecular oscillating forces for nearly touching colloids that arise from solvent and counterion layering, the counterions are attracted towards the colloidal surfaces by solvent depletion providing a simple statistical description of hydration. This, in turn, has an important influence on the effective forces for larger distances which are considerably reduced as compared to the prediction based on the primitive model. When these forces are repulsive, the long-distance behavior can be described by an effective Yukawa pair potential with a solvent-renormalized charge. As a function of colloidal volume fraction and added salt concentration, this solvent-renormalized charge behaves qualitatively similar to that obtained via the Poisson-Boltzmann cell model, but there are quantitative differences. For divalent counterions and nanosized colloids, on the other hand, the hydration may lead to overscreened colloids with mutual attraction while the primitive model yields repulsive forces. All these new effects can be accounted for through a solvent-averaged primitive model (SPM) which is obtained from the full model by integrating out the solvent degrees of freedom. The SPM was used to access larger colloidal particles without simulating the solvent explicitly.

Polyion character of globular proteins detected by translational and rotational diffusion

Photon correlation spectroscopy measurements are performed on a solution of lysozyme, a small, highly charged protein at pH=4, in glycerol-buffer mixtures. From the correlation functions of the polarized and depolarized scattered intensity, we derive an effective protein radius by inverting the Stokes Einstein relation for either the translational or the rotational diffusion, as a function of the protein concentration 0< or =C< or =40 g/L. The translational radius decreases with the protein concentration and extrapolates at C approximately 0 to a value slightly larger than the mean rotational radius, which is independent of the protein concentration. By employing recent calculations on electrolyte friction effects (eletrolyte-protein interactions) on both the translational and rotational radius and by keeping also into account the protein-protein electrostatic interactions, we are able to account for the observed differences, suggesting also that lysozyme is more highly hydrated in glycerol mixtures than in the pure buffer. These results indicate that depolarized photon correlation spectroscopy measurements can be used as a valuable tool to detect small changes in the overall protein size.