Nanoparticle dispersions: a colloid and polymer solution perspective

For most solid nanoparticles there are no true solvents in the sense that a powder or crystal of these nanoparticles would spontaneously dissolve when immersed in them. There are exceptions but these typically involve unusual solvents such as super acids or chemical modification of the particles to make particles and solvent compatible. Conventional fluids, including water, are generally poor solvents or dispersants and in them the nanoparticles need to be stabilised against aggregation. Indeed, nanoparticles dispersed or dissolved in a liquid behave very much like polymers and colloidal particles do. The properties of such dispersions can thus be understood in terms of what is known about the behaviour of colloids and polymer solutions. Important aspects are Van der Waals and Coulomb interactions, steric interactions, the impact of depletion agents, phase separation and the tendency of elongated colloidal particles and stiff polymers to form nematic and other types of liquid-crystalline phase. For this book a question of particular interest is how the nanoparticles behave if they are present in a liquid crystalline host fluid, and what kind of medium-induced interaction operates between these particles. However, most types of interaction are also present in isotropic host uids, so the attention of this chapter will primarily be directed towards conventional dispersions. I shall give an overview of the physico-chemical principles most relevant to understanding the behaviour of fluid dispersions and solutions of nanoparticles, using spherical, cylindrical and at, plate-like nanoparticles as illustrative examples.