chapter 1 section 1.4

WHAT ARE COLLOIDS?

A colloidal dispersion consist of microscopic solid (or liquid) particles dispersed in a solvent [14]. The linear size of those particles is typically between 1 nm and . Small enough to exhibit Brownian motion caused by the surrounding solvent molecules but still much larger than those molecules. Biological complexes like viruses, proteins, micelles, vesicles consisting of membranes, blood cells but also synthetic polymers belong to this class of particles. Colloids are important in many industrial processes as well, as they occur in, for example, paints, cosmetics and foods. During the past decades, much effort has been devoted to the synthesis of well characterized colloidal particles that have a very narrow distribution in sizes and shapes. Such mono-disperse colloids can act as model systems to study the factors that determine the structure, dynamics and phase behavior of such systems [8]. There is a surprising analogy between the statistical behavior of such colloidal dispersions and that of simple atomic fluids. The statistical thermodynamic properties can be derived in the same way as for atomic systems, by treating the solvent as a continuous background that exerts fluctuating forces on the colloidal particles. These Brownian forces can be averaged and replaced by the so-called potential of mean force. Onsager [15] and McMillan and Mayer [16] showed that by using this effective potential as input the full statistical mechanical machinery developed for atomic systems can be used to describe the thermodynamic phase behavior of colloidal dispersions.