This paper has been submitted at Nov 4th, 1996, for publication in a
scientific journal and is now subject to the reviewing process.
You may ask for a hardcopy with the complete manuscript, which contains all figures and all
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This is the 3rd revised version.
Dr. Bernhard Wessling
Zipperling Kessler / Ormecon Chemie
D-22949 Ammersbek
1.3 Examples of practically important colloids
Colloidal systems can be found everywhere in the natural and technical world. When you complain about the weather - fog is an aerosol; you want to drink fresh milk - an emulsion; butter, mayonnaise, creams, or the pumping of oil out of the earth with the help of water - again emulsions; photographic films, paints, toothpaste, mud, sauce béarnaise, hot chocolate, asphalt, blood - sols; pearls, pigmented ceramics or plastics, dry coatings, bones, muscles - solid dispersions; glue, and jam - gels; soap water in dish-washers, washing machines and for cleaning windows - association colloids.
Their character - stability, digestibility, film forming properties, viscous and elastic properties and more - are solely depending on their colloidal state and structure.
It would be very inappropriate to say that it doesn't matter whether we are dealing with a solution of enzymes, some other proteins, salts and iron, or with a highly structured colloidal system which we call "a cell", "a muscle". An enzyme would not work in the way we know and need it, if it were truly soluble in water or blood.
Titanium dioxide, TiO2, would not be white and strongly opaque in dispersions in paints or plastics, if it were not used in that specific colloidal particle size (0.3 µm) as it is. Above this, it is a weakly opaque filler with very low hiding power. Below this, it is a nicely transparent UV absorber, very hard to disperse in plastics. What a disadvantage, if TiO2 were soluble in water or organic solvents used in paint manufacturing. This would change the particle size during drying of the coating where the dissolved TiO2 would precipitate. Also, if it would be soluble in different solvents to a different extent, then the use of various organic solvents or water would be limited due to poor reproducibility of the paint performance.
Paints would not form those even, dense and glossy (or dull, as you like) coating if the various necessary components (like anti-corrosion pigments) would be soluble in the resin or - even worse - in the solvent only. The rheological properties of the colloidal system allows to apply and dry the paints in a way that the most useful form of finish is resulting.
Soap containing water would not wash the textiles, clean the dish or the window (or the frying pan with the baked sauce béarnaise when cooking too long), if soap would be soluble in water; nor would water - if soluble in oil - be able to help us pump the oil out of almost empty oil caverns, which is only possible because of using very specific detergents for making a three-phase colloidal system.
I want to assume now that the reader will agree that it is very important to distinguish between solutions and dispersions, as well for practical as for scientific reasons. The basic difference between the 2 systems is, that colloidal particles have a substantial amount of their molecules at their surface, which no other system has. The surface and the interface is responsible for all differences between dispersions and solutions. This doesn't make the one or the other system "better", "preferred" or "more interesting", just only different.
The science of solutions does not require study of their interfacial phenomena - there are none. Here, study of chain conformation is of great relevance. A scientific description of dispersions, on the other side, without accounting for their interfacial interactions would be completely insufficient, as the interfaces play a dominant role.
