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A dispersion is a system in which discrete particles of one material are dispersed in a continuous phase of another material. The two phases may be in the same or different states of matter. They are different from solutions, where dissolved molecules do not form a separate phase from the solute.
Dispersions are classified in a number of different ways, including how large the particles are in relation to the particles of the continuous phase, whether or not precipitation occurs, and the presence of Brownian motion. In general, dispersions of particles sufficiently large for sedimentation are called suspensions, while those of smaller particles are called colloids.
Degree of dispersion
The term dispersion also refers to the physical property of the degree to which particles clump together into agglomerates or aggregates. While the two terms are often used interchangeably, according to ISO nanotechnology definitions, an agglomerate is a reversible collection of particles weakly bound, for example by van der Waals forces or physical entanglement, whereas an aggregate is composed of irreversibly bonded or fused particles, for example through covalent bonds. A full quantification of dispersion would involve the size, shape, and number of particles in each agglomerate or aggregate, the strength of the interparticle forces, their overall structure, and their distribution within the system. However, the complexity is usually reduced by comparing the measured size distribution of "primary" particles to that of the agglomerates or aggregates.
Types of dispersions
|Dissolved or dispersed phase||Continuous medium||Solution: Homogeneous mixture: Dissolved phase < 1 nanometer||Colloid: Dispersed phase between 1 nanometer and 1 micrometer||Coarse dispersion (Suspension): Heterogeneous mixture: Dispersed phase > 1 micrometer|
|Gas||Gas||Gas mixture: air (oxygen and other gases in nitrogen)||None||None of them|
|Liquid||Gas||None||Aerosol: fog, mist, vapor, hair sprays||Aerosol|
|Solid||Gas||None||Solid aerosol: smoke, cloud, air particulates||Solid aerosol: dust|
|Gas||Liquid||Solution: oxygen in water||Foam: whipped cream, shaving cream||Foam|
|Liquid||Liquid||Solution: alcoholic beverages||Emulsion: miniemulsion, microemulsion||Emulsion: milk, mayonnaise, hand cream|
|Solid||Liquid||Solution: sugar in water||Sol: pigmented ink, blood||Suspension: mud (soil, clay or silt particles are suspended in water), chalk powder suspended in water|
|Gas||Solid||Solution: hydrogen in metals||Solid foam: aerogel, styrofoam, pumice||Foam: dry sponge|
|Liquid||Solid||Solution: amalgam (mercury in gold), hexane in paraffin wax||Gel: agar, gelatin, silicagel, opal||Wet sponge|
|Solid||Solid||Solution: alloys, plasticizers in plastics||Solid sol: cranberry glass||Gravel, granite|
Structure and properties
It is still[clarification needed] common belief that dispersions do not display any structure; i.e., the particles (or in case of emulsions: droplets) dispersed in the liquid or solid matrix (the "dispersion medium") are assumed to be statistically distributed. Therefore, for dispersions, usually percolation theory is assumed to appropriately describe their properties.
However, percolation theory can be applied only if the system it should describe is in or close to thermodynamic equilibrium. There are only very few studies about the structure of dispersions (emulsions), although they are plentiful in type and in use all over the world in innumerable applications (see below).
In the following, only such dispersions with a dispersed phase diameter of less than 1 µm will be discussed. To understand the formation and properties of such dispersions (incl emulsions), it must be considered that the dispersed phase exhibits a "surface", which is covered ("wet") by a different "surface" that, hence, are forming an interface (chemistry). Both surfaces have to be created (which requires a huge amount of energy), and the interfacial tension (difference of surface tension) is not compensating the energy input, if at all.
A review article describes various attempts to describe dispersions/emulsions. Dispersion is a process by which (in the case of solids' becoming dispersed in a liquid) agglomerated particles are separated from each other and a new interface, between an inner surface of the liquid dispersion medium and the surface of the particles to be dispersed, is generated. Dispersion is a much more complicated (and less-understood) process than most people[clarification needed] believe.
The above-cited review article also displays experimental evidence to support the fact that dispersions have a structure very much different from any kind of statistical distribution (which would be characteristics for a system in thermodynamic equilibrium), but in contrast very much showing structures similar to self-organisation, which can be described by non-equilibrium thermodynamics. This is the reason why some liquid dispersions turn to become gels or even solid at a concentration of a dispersed phase above a certain critical concentration (which is dependent on particle size and interfacial tension). Also, the sudden appearance of conductivity in a system of a dispersed conductive phase in an insulating matrix has been explained. The above-cited review article also introduces into some first complete non-equilibrium thermodynamics theory of dispersions.
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