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ScienceWeek
CHEMISTRY: ON COLLOID INTERACTIONS
"Kolloid" was the ancient Greek word for glue. In modern terminology, a colloid is simply a suspension of small particles in a medium. Milk is a colloid because it is a suspension of milk fat globules in water, and so is paint, a suspension of solid pigment granules in oil or water. Cigarette smoke is a colloid because it is a suspension of ash particles in air (smoke will appear white if the particles are large enough to reflect light from their surfaces, and bluish if the particles are so small that all they can do is scatter the light). Blood, a suspension of living cells in serum, is also a colloid, together with a host of other materials vital to life.
The defining feature of colloids is that the particles are small and consequently the total surface area is huge. The surface area of the milk fat globules in a pint of homogenized milk, for example, is around two hundred square meters -- about the floor area of an average house. Forces between the surfaces of the particles thus dominate the behavior of a colloidal suspension. These forces (called surface forces) arise largely from the single layer of molecules at the surface of each particle. Normal red cells, for example, repel and slide past their neighbors because their surfaces are coated with a layer of negatively charged sugar molecules. Milk fat globules stay apart for a similar reason, except that in this case the protective molecules are natural milk proteins.
In the absence of the repulsive forces provided by protective layers, similar particles will stick together because they are pulled towards each other by a universal attractive force of electrical origin called the Van der Waals force, which increases rapidly in strength as the particles get closer to one another. This can cause problems. If milk fat globules stick together, for example, the milk separates into a layer of cream above and a layer of water below. If red blood cells stick together, they can no longer squeeze through tiny capillaries, and the capillaries become blocked. This happens to people suffering from sickle-cell anemia, one of the "molecular" diseases in which the alteration of one small group of atoms in the hemoglobin molecule causes that molecule to adopt a different shape, leading in turn to the deformation of the whole red cell. The molecular changes also make the outside of the cell slightly sticky, so that instead of repelling and sliding past its neighbors, it adheres to them, especially in tight corners such as those in joints, where the cells clump together, blocking blood flow and causing excruciating pain.
Particles such as milk fat globules or red blood cells will only stay apart if the repulsive force is sufficient to overcome the attractive Van der Waals force at some stage in the approach. If the repulsive force is not present naturally, it can be added. One of the principal ways of doing this for suspensions of hydrophobic particles in water (e.g., grime in bathwater) is to add detergent molecules. The hydrophobic tails of the detergent molecules anchor themselves to the particle surfaces, thus hiding both from the surrounding water, while the electrically charged heads of the detergent molecules form a protective outer layer that repels other, similarly coated particles and keeps them in a loose suspension that can easily be rinsed away instead of collecting as a sticky layer on the sides of the bath.
The idea that a balance of attractive and repulsive forces controls colloid stability was developed independently in the early 1940s by two groups of scientists, Boris Deryagin and Lev Landau (1908-1969) in Russia and Evert Verwey (1905-1981) and Theo Overbeek in Holland. Both groups published their ideas after the Second World War and, after a brief battle over priority, the theory became democratically known as the Deryagin-Landau-Verwey-Overbeek theory, universally abbreviated to "DLVO".
Adapted from: Len Fisher: How to Dunk a Doughnut: The Science of Everyday Life. Arcade Publishing 2003, p.127. More information at: http://www.amazon.com/exec/obidos/ASIN/1559706805/scienceweek
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CHEMISTRY OF COLLOIDS
The following points are made by P-G. de Gennes (Nature 2001 412:385):
1) Chemists in the 19th century recognized the existence of systems with unusual behaviors: for example, hydrated alumina, starch, dextrin, and gelatin. Wolfgang Ostwald (1883-1943) grouped these substances into a separate category and coined the term "colloids" (meaning "glue-like"). Thomas Graham (1805-1869) had demonstrated that the constituent entities had low mobility, and he had been convinced they were aggregates of smaller molecules. Colloids could be metal particles, such as the gold colloids made by Michael Faraday (1791-1867); they could also be oxides or organic systems.
2) The early concept of colloids was vague, at first even counterproductive, and colloid science delayed the birth of polymer science. Rubber vulcanization, for example, was claimed not to be a chemical reaction (now called "crosslinking") but rather a modification of an aggregated state. Ultimately, due primarily to Hermann Staudinger (1881-1965), polymers were clearly identified and removed from the colloid group.
3) But colloids are clearly important: white paint, for example, is a colloid based on titanium oxide and water. Most foods, cosmetics, and inks consist of colloidal structures. Colloids comprise soap micelles, emulsions, suspensions of solids, (with sizes from microns to nanometers), and latex particles whose dimensions can be tuned in composition and size. A common feature of colloids is that the particles are floating in a solvent; at low enough concentrations, a colloid can flow freely, which is of considerable practical importance.
Nature http://www.nature.com/nature
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Notes:
A colloid is basically a system of particles 1 to 1000 nanometers in diameter dispersed in another phase, and such systems, particularly systems of electrically charged colloids, have important practical significance and are also of considerable theoretical interest. The existence of long-range attractive (as opposed to the expected repulsive) electrostatic forces between particles of like charge is one of the current major controversies of colloid science. The established classical theory of colloidal interactions predicts that an isolated pair of like-charged colloidal spheres in an electrolyte should experience a purely repulsive screened electrostatic (Coulombic) interaction. Direct measurements of such interactions have shown quantitative agreement with the classical theory, but recent experiments have provided evidence that the effective interparticle potential can have a long-range attractive component in more concentrated suspensions and for particles confined by charged glass walls. This long range attraction in concentrated systems is apparently due to multi-body interactions. Theoretical explanations have been proposed but remain the subject of controversy.
In this context, "screening" is a reduction of the effective electric field at a point, the reduction due to the space charge of ambient charged particles of sign opposite to the source of the field.
ScienceWeek http://scienceweek.com
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