Colloidal dispersions are ubiquitous in everyday life. Examples include ink, paints, milk, lubricants or fog, and technological applications are common in the cosmetic, food, oil and pharmaceutical industries. The dispersions are essentially two-phase systems, involving mesoscopic solid or liquid particles, with typical sizes in the range 10-10{sup}3 nm, suspended in a liquid or a gas(aerosols) of much smaller molecules, often improperly referred to as the "solvent". Restriction will be made here to solid, spherical colloidal particles, like polymethylmethacrylate (PMMA) particles, sterically stabilized by adsorbed polymer "brushes", dispersed in an organic solvent. The large asymmetry of such binary "mixtures" of mesoscopic and microscopic particles poses a severe challenge to a statistical description of static and dynamic properties. Clearly, some coarse-graining is warranted. As regards the static properties (structure, phase behavior, etc.), this may be achieved by integrating out the microscopic degrees of freedom associated with the solvent molecules and any additional component, like ions or polymer coils; this procedure leads to effective (solvent-averaged) interactions between colloidal particles, which have an entropic contribution, and are state-dependent[1].
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