Colloid

As colloids (from Greek κόλλα, Kolla, " glue " and εἶδος, eidos, " form, appearance " ) are particles or droplets which have in the dispersion medium (solid, gas or liquid) are finely dispersed. The single colloid is typically between one and 500 nanometers in size. If they are movable ( such as in a liquid dispersion medium ), so show colloids, usually Brownian motion.

Colloidal suspensions are of great importance in the food and cosmetic industry and in basic research, particularly in statistical physics. Colloid chemistry is the area of ​​chemistry that deals with its material properties.

• 4.1 Chemistry
• 4.2 Physics
• 4.3 Process Engineering
• 4.4 Soil Science

Definitions and forms

Usually, it is assumed for colloids that are emulsions or suspensions of droplets or particles in a liquid. In principle, both the dispersed phase and the dispersion medium is a solid, a liquid or a gas may be. Leading goal as a rigid, anyway only fuzzy definition here is a listing of examples of colloidal systems.

• Milk, blood, mayonnaise, cosmetics (emulsions - droplets in liquid)
• Fog ( droplets in the gas )
• Smoke ( particles in the gas )
• Foam, milky quartz (gas in solids )
• Whipped cream (gas in liquid)
• Ink, mud, coffee, colloidal gold or colloidal silver (solid particles in liquid, even colloidal suspensions )
• Special composite materials, opal (solid particles in solids )

Colloidal solutions are between true solutions ( molecularly ) and suspensions ( roughly dispersible).

• Pastes have a high concentration of the dispersed phase, so that no or only a very low fluidity present
• Gels have instead of individual particles mostly long-chain macromolecules like jelly or glue.
• Liquid crystals are colloids that form ordered structures in a liquid.
• Aerosols are colloidal dispersions of gases, such as smoke and fog.

Disperse systems with approximately the same particle size are as monodisperse or isodispers, those designated with different particle size as polydisperse. Are dispersed phase and dispersion medium to distinguish with certainty, it is "simple colloids ". Do they form interlocking networks without a clear assignment there are " network - colloids ".

The magnitude of colloids can only refer to one dimension, so that one can differentiate in the structure of colloids. Kaolinite is an example of a very dünnplattiges clay mineral and also forms a colloidal system. This also applies to fibrous or network-like structures which have colloidal dimensions in two spatial directions. Colloids do not necessarily consist of individual particles. Striking is the lower limit of about one nanometer, as it is here at a fairly uniform transition to the molecular properties of disperse systems.

History and Origin of the term

Colloids have been used when there was still no knowledge of their systematics. The scientific examination of colloids is only recorded in recent times, as improved the earlier limited technical features for effective, reproducible preparation of well-defined colloids.

Already the alchemists were forms of colloidal gold known and Pierre Joseph Macquer suspected in 1744 that this might not be a fine distribution of gold particles in a dispersion. First empirical investigations led Selmi in 1845 by 1856 followed the attempts Michael Faraday with colloidal gold.

The British physicist Thomas Graham led in 1861 to the English term " colloid " one, which he derived from the Greek word for glue. He used it to due to their diffusion behavior to divide substances through membranes in " crystalloid " and " colloidal " substances. However, Graham's criteria were not effective. What he referred to as colloidal, was not a chemical property, but a state of fine subdivision of certain physical samples. Since the beginning of the twentieth century, the term is used in the sense of the modern definition. In 1922, the colloid company was founded to foster and promote the colloid science in Leipzig, which still exists today.

A kinetic theory for colloidal systems was first created by Marian Smoluchowski. The chemistry of colloids and their properties were particularly (1925 Nobel Prize ) and his colleagues examined by Richard ZSIGMONDY.

Properties

Macroscopic

Because of their relatively large compared with the volume boundary surfaces play effects of surface chemistry of colloids a special role. Colloids have also usually on the Tyndall effect. By light scattering at the interfaces of transparent dispersions themselves appear milky or cloudy phases, as long as the refractive indices are not exactly the same.

Microscopic

The following interactions may occur between colloidal particles.

Importance and applications

Chemistry

Colloid chemistry that studies the properties of dispersed colloidal systems, is an independent field of physical chemistry.

Physics

Colloidal suspensions are important model systems to verify predictions of statistical thermodynamics or simulate atomic solids processes.

• The interactions between individual colloidal particles can be adjusted by selection of the particles, their surface treatment and composition of the liquid. You can adjust the strength and range of the interaction separately and used to model various potential profiles. The particles behave like atoms of a metal or as an ionic system and form corresponding crystals. With sufficient concentration, crystallization occurs even in non-interacting particles on (" hard balls " ), which paradoxically has entropic reasons.
• Colloids are about a factor of greater than 1000 to 10,000 atoms. Therefore, they are much easier to observe, and with significantly less experimental effort ( confocal microscopy or dynamic light scattering ).
• Their movement is significantly slower than that of atoms. This allows the observation of processes such as crystallization, which run too fast in atomic systems.

Process engineering

Colloidal disperse systems have the fine distribution of one phase in the other in proportion to their volume enormously large interfacial area. This is utilized wherever interfacial effects are important, such as in the drying technique, or by the reaction of two immiscible liquids.

Soil Science

In soil science, the size spectrum of the colloids is extended two microns. This classification includes the clay fraction of soil science relevant, since soil particles having a diameter to about 2 microns colloidal properties. This is partly the consequence of the sheet- like habit of clay particles. Here come properties that occur due to the mass of the particles over the properties of the large specific surface, back.