Introduction
Nanoparticle dispersions are a series of suspensions of nanoparticles in water or organic solvents. Dispersions generally offer phase stability, dimensional stability and hardness, which can be used to enhancing properties of the nano particles and control their functions as well. Nanoparticles in dispersions can sometimes settle upon storage, in which case they can be mixed before use. Some surface-functionalized nanoparticles can be soluble in water or organic solvents. These solutions wouldn’t settle or exhibit phase separation if they have been properly stored.
Nanoparticles have different surface structures and surface interactions from those of the sub-micron sized particles; in particular, they show a much stronger aggregation tendency. For applications in functional materials and products, it is important to develop techniques of controlling the dispersion and aggregation of nanoparticles.
Fig 1 Nanoparticle dispersions
Improve the Dispersion Stability
Surface modification of nanoparticles is one of the most common methods to improve the dispersion stability of nanoparticles. It requires a design of the surface structure based on the type of nanoparticles and the liquid media.
Adsorption of polymeric dispersants: Adsorption of a polymeric dispersant on nanoparticles is one of the simplest surface modification techniques. When dispersing hydrophilic nanoparticles in aqueous media or in organic solvents with high polarities, anionic or cationic polymer dispersants are widely used to generate the steric repulsive force originating from the polymer chains and to increase the surface charge. For example, a suspension of TiO2 nanoparticles was modified with a surfactant ensure the complete dispersion of nanoparticles, with high solid contents, in various organic solvents (Fig 2).
Fig 2 (a) An anionic surfactant designed to have a hydrophobic alkyl chain, hydrophilic PEG chain and a polymerizable group; (b) The suspension of TiO2 nanoparticles.
(Sci. Technol. Adv. Mater., 2010, 11, 044304)
Chemical modification of the surface: Chemical modification of the particle surface is also a useful technique to improve the stability of nanoparticles in various liquid media. Silane coupling agents, which have alkoxy groups and organic functional groups, are used to modify the oxide nanoparticle.
Physical dispersion method of nanoparticles: Some physical methods such as ultrasonic irradiation and mechanical milling can be utilized for redispersion of dry powder aggregated at the (nearly) primary particle.
