POLYDISPERSITY IN COLLOIDS - IMPLICATIONS TO STATIC STRUCTURE AND SCATTERING

Theoretical and experimental analyses of the structure of colloidal dispersions have traditionally relied on model systems of monodisperse particles. However, numerous theoretical studies aimed at incorporating the effects of size-polydispersity, charge-polydispersity, and polydispersity in attractive interactions on the equilibrium structure of dispersions of spherical particles have appeared in recent years. These studies can be divided into three different groups according to the nature of the interaction forces considered, namely, dispersions of neutral hard spheres, sticky hard spheres, and charged particles. Here, we review these studies and examine the major results and conclusions. Previously unreported new results obtained by us for dispersions of charged spheres in high-ionic-strength media are also included. The theoretical techniques for computing partial structure factors and average structure factors for the above-mentioned interaction potentials are summarized, and analytical expressions are presented wherever appropriate. Representative examples of applications of each class of interaction potentials identified above are presented in order to illustrate the use of the theoretical tools in interpreting the scattering data. The common features of the average structure factors for the above model potentials are emphasized, and issues such as the importance of correctly modeling intraparticle scattering properties of the particles and transformation of observed intensities to average structure factors are illustrated. An examination of the so-called decoupling approximation (which seeks to separate the intraparticle scattering effects from interparticle interaction effects) is also presented wherever the required data are available.