ON THE DYNAMICS AND STRUCTURE OF CHARGE-STABILIZED SUSPENSIONS

Charge-stabilized suspensions of spherically shaped particles show a v ariety of interesting properties. As a consequence, there has been con siderable interest in the investigation of the microstructure and the dynamic properties of well characterized model systems, such as polyst yrene spheres dispersed in water and charged silica spheres dispersed in an organic solvent. Among various experimental techniques, static a nd dynamic light scattering have been the major tools for the characte rization of colloidal suspensions. Whereas there is an essentially qua ntitative understanding of monodisperse suspensions, only very recentl y good progress was achieved, both theoretically and experimentally, i n understanding certain properties of colloidal mixtures and intrinsic ally polydisperse one-component suspensions. Colloidal mixtures show a dditional phenomena, e.g., a variety of microstructures and phase beha viour, tracer-diffusion and interdiffusion, which do not exist in mono disperse systems. This article offers a survey on our current knowledg e of the dynamics and statics of charge-stabilized suspensions in the fluid phase, with emphasis on the authors own work. It further contain s a summary of basic concepts, and of analytical and numerical methods , which are relevant for the theoretical description of charge-stabili zed suspensions. Special effort is made to point out the salient diffe rences between colloidal mixtures and monodisperse suspensions. The ca lculated or computer-simulated quantities characterizing the suspensio ns are compared, whenever available, with the results of light scatter ing experiments. The article is divided into two major parts. The firs t part (chapters 2 and 3) is concerned with static properties. It incl udes a discussion of the origin of the repulsive and attractive forces between charged colloidal particles, the concepts of the effective ch arge and global correlation functions, the influence of the finite siz e of the counterions on the microstructure of concentrated ionic micel lar solutions, and the extension of the rescaled mean spherical approx imation to colloidal mixtures. The central issue of the first part is, however, the theoretical modeling of intrinsic polydispersity, and th e calculation of static structure factors and radial distribution func tions by various integral equation methods. The relative accuracy of t hese methods is assessed from the comparisons with computer simulation s and light scattering results. The main body of this article is conta ined in the second part (chapter 4), which is concerned with the dynam ics of charge-stabilized suspensions. A thorough discussion of the var ious levels of description of the suspension dynamics is given in term s of the time scales characterizing various relaxation processes assoc iated with the colloidal particles and the molecules of the host fluid . The description of the dynamics of the colloidal particles, based on the generalized Smoluchowski equation, is justified for the time inte rval accessible in dynamic light scattering experiments. A summary of general properties of the generalized Smoluchowski equation is provide d, and various ordering relations for diffusion coefficients are prese nted. The combined influence of the electrostatic and solvent mediated hydrodynamic interactions on the short-time dynamics of monodisperse and polydisperse charge-stabilized suspensions is investigated in grea t detail. It is shown that the effect of hydrodynamic interaction is s trongly enhanced by the presence of long-ranged electrostatic repulsio n, and its influence is more pronounced for collective diffusion than for short-time self-diffusion. The additional influence of polydispers ity is found to be quite significant. Finally, a thorough study of tra cer-diffusion in charge-stabilized suspensions is presented. Mean squa re displacements and long-time tracer-diffusion coefficients are calcu lated with two alternative approximations, i.e., a mode-coupling schem e and a single relaxation time ansatz. The range of validity of these approximations is assessed by numerous comparisons with Brownian dynam ics simulation results, and with large-wavenumber dynamic light scatte ring and forced-Rayleigh data. It is observed that tracer-diffusion is quite sensitive to the amount of intrinsic polydispersity.