colloid

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As colloids (from ancient Greek κόλλα kolla " glue " and εἶδος eidos "shape, appearance"), particles or droplets which have in the dispersion medium are dispersed (solid, gas or liquid). The size of the individual particles is typically in the nanometer or micrometer range. If they are mobile (for example in a liquid dispersion medium), they show Brownian motion .

Colloidal suspensions are of great importance in the food and cosmetics industries and in basic research, especially in statistical physics . The colloid chemistry is the area of chemistry that deals with their material properties.

Definitions and forms

Most colloids are assumed to be emulsions or suspensions of liquids or solids in a liquid. In principle, both the disperse phase and the dispersion medium can assume any of the three aggregate states, i.e.:

A list of examples of colloid systems is more expedient than a rigid, already fuzzy definition.

Disperse phase Dispersion medium Name colloid Examples
liquid liquid Emulsions , microemulsions Milk , blood , mayonnaise , cosmetics
gas liquid Microfoam Whipped cream
Solid liquid Sol , also: colloidal suspension, colloidal solution Ink , mud , coffee , colloidal gold, or colloidal silver
liquid gas Aerosol fog
Solid gas smoke
liquid Solid
gas Solid Foam , milk quartz
Solid Solid Special composite materials , opal glass

Due to their particle size, colloidal solutions stand between real solutions (molecularly disperse) and suspensions (roughly disperse).

  • Pastes have a high concentration of the disperse phase, so that there is no or very little flowability
  • Instead of individual particles, gels usually have long-chain macromolecules, as in jelly or glue .
  • Liquid crystals are colloids that form ordered structures in a liquid.
  • Aerosols are colloidal dispersions in gases such as smoke and mist .

Disperse systems with approximately the same particle size are referred to as monodisperse or isodisperse, those with different particle sizes as polydisperse. If the disperse phase and the dispersant can be clearly distinguished, it is a matter of “simple colloids”. If they form intertwined networks without a clear assignment, they are “network colloids”.

The order of 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 thin-plate clay mineral and also forms a colloidal system. This also applies to fiber-like or network-like structures that have colloidal dimensions in two spatial directions. Colloids do not necessarily have to consist of individual particles. The lower limit of about one nanometer is more striking, as there is a fairly uniform transition to the properties of molecularly disperse systems.

History and origin of the term

Colloids were already being used when there was no knowledge of their systematics. The scientific discussion of colloids has only recently been recorded, when the previously limited technical possibilities for a targeted, reproducible production of well-defined colloids improved.

Forms of colloidal gold were already known to the alchemists and Pierre Joseph Macquer suspected in 1744 that this could be a fine distribution of gold in a dispersion. Selmi carried out his first empirical studies in 1845, followed by Michael Faraday's experiments with colloidal gold in 1856 .

The British physicist Thomas Graham introduced the English term "colloid" in 1861, which he derived from the Greek word for glue. He used it to subdivide substances into “crystalloid” and “colloidal” substances based on their diffusion behavior through porous membranes. However, Graham's criteria were not effective. What he called colloidal was not a chemical property but a state of fine physical subdivision of certain samples. Since the beginning of the twentieth century, the term has been used in the sense of the modern definition. In 1922, Wolfgang Ostwald founded the Colloid Society for the care and promotion of 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 studied by Richard Zsigmondy (Nobel Prize 1925) and his colleagues.

properties

Macroscopically

A slight opalescence in colloidal silicon dioxide (hydrodynamic diameter: 92.7 nm).

Because of their comparatively large interfaces in relation to their volume , the effects of surface chemistry play a special role for colloids. Colloids also usually have the Tyndall effect . Due to light scattering at the interfaces, even dispersions of transparent phases appear milky or cloudy ( opalescence can also occur) as long as the refractive indices are not exactly the same.

Microscopic

The following interactions can occur between colloidal particles.

Hard bullet repulsion
This is the simplest interaction. Hard colloid particles cannot deform or penetrate each other when they collide. This seems trivial, but the “hard ball” model, in which there is no other interaction, is one of the most important systems in statistical physics. The associated potential is infinite for distances smaller than the particle diameter and zero otherwise.
Electrostatic interaction
Electrically charged colloid particles of the same name repel each other. The interaction potential has the form . If the colloids have different charges, they form an ionic model system (some particles attract each other, others repel each other). Ions present in the liquid (as in dissolved salts) can more or less shield the electrostatic potential .
Van der Waals forces
are caused by fluctuating dipoles, among other things. The Hamaker constant describes the Van der Waals interaction of two macroscopic bodies. It follows from Hamaker's constant that the Van der Waals interaction is minimized if the refractive index of the liquid and the particles are made more similar.
Entropic interaction
Even smaller particles or polymer chains that are in the liquid in addition to the actual colloids exert an effective osmotic pressure and compress the colloids. This can be clearly understood in such a way that the total entropy of the system increases when the large particles move closer together, since the small particles then have more space available.

Importance and uses

chemistry

The colloid chemistry examines the characteristics dispersed colloidal systems and is an independent field of physical chemistry .

physics

Colloid suspensions are important model systems to check predictions of statistical thermodynamics or to simulate atomic solid-state processes.

  • The interactions between individual colloid particles can be adjusted by selecting the particles, treating their surface and the composition of the liquid. The strength and range of the interaction can be set separately and thus different potential curves can be modeled. The particles behave like atoms of a metal or like those of an ionic system and form corresponding crystals. With sufficient concentration, crystallization occurs even with non-interacting particles (“hard spheres”), which paradoxically has entropic causes.
  • Colloids are roughly 1000 to 10,000 times larger than atoms. Therefore, they are much easier to observe and with much less experimental effort ( dynamic light scattering or confocal microscopy ).
  • Their movement is significantly slower than that of atoms. This allows the observation of processes such as crystallization , which take place too quickly in atomic systems.

process technology

Due to the fine distribution of one phase in the other, colloidal systems have an extremely large interface in relation to their volume. This is used wherever interface effects are important, such as in drying technology or in the reaction of two immiscible liquids.

Soil science

In soil science, the size spectrum of colloids is expanded to two micrometers . This classification includes the soil-relevant clay fraction , as soil particles with a diameter of up to 2 μm have colloidal properties. This is due, among other things, to the leaf-shaped habit of clay particles. In this case, properties that occur due to the mass of the particles are less than the properties of the large specific surface .

medicine

In infusion therapy , colloidal infusion solutions are used, which stabilize or increase the volume in the blood vessels . They contain colloidal macromolecules such as carbohydrates ( hydroxyethyl starch , dextrans ) or proteins ( gelatin or human albumin ).

See also

literature

Individual evidence

  1. M. Daoud, CE Williams (Ed.): Soft Matter Physics. Springer, Berlin and Heidelberg 1999, ISBN 3-540-64852-6 .
  2. Günter Jakob Lauth, Jürgen Kowalczyk: Introduction to the physics and chemistry of interfaces and colloids . Springer, Berlin, Heidelberg, ISBN 978-3-662-47017-6 , pp. 354 f ., doi : 10.1007 / 978-3-662-47018-3_10 .
  3. V. Prasad, D. Semwogerere, E. Weeks: Confocal microscopy of colloids. In: Journal of Physics: Condensed Matter. Volume 19, 2007, p. 113102, doi: 10.1088 / 0953-8984 / 19/11/113102 (PDF) .