Colloidal Systems - Surface and Colloid Chemistry: Principles and Applications - page 141

7 Colloidal Systems

7.1 IntroductIon
In this chapter, the very widespread industrial application of colloid chemistry
will be described. Surprisingly enough, mankind has been aware of colloids for
many thousands of years. The old Egyptian and Mayan civilizations, without
cement, used their knowledge about adhesion (between blocks of stones) when
building pyramids.
Our lives are a continuous encounter with solid particles of different sizes, rang-
ing from stones on a beach, sand particles, or dust floating around in the air, without
our being especially aware that there exists a special relation between particle size
(surface area) and their characteristics. The rather small particles in the range of
size from 50 Å to 50 μm are called colloids . The most obvious difference is appar-
ent between sand particles and dust particles. It is fascinating to observe how dust
or other fine particles remain in suspension in the air. In the 19th century, it was
observed under the microscope (Brown) that small microscopic particle suspended
in water made some erratic movements (as if hit by some other neighboring mol-
ecules). Since then this has been called Brownian motion . This erratic motion arises
from the kinetic movement of the surrounding water molecules. Thus, colloidal par-
ticles would remain suspended in solution through Brownian motion only if the grav-
ity forces did not drag these to the bottom (or top).
If sand is tossed into the air, the particles fall to the earth rather quickly. On the
other hand, in the case of the talcum particles, these stay floating in the air for a long
time. These differences will be described later. The size of particles may be consid-
ered from the following data:

Colloidal dispersions 10 nm-10 μ

Mist/fog 0.1 μ-10 μ

Pollen/bacteria 0.1 μ-10 μ

Oil in smoke/exhaust 1 μ-100 μ

Virus 10 nm-10 μ

Polymers/macromolecules 0.1 nm-100 nm

Micelles 0.1 nm-10 nm

Vesicles 1 μ-1000 μ

The stability of colloidal systems is subject to the state of their configuration, very
roughly comparable to a bucket that is stable when standing up but if tilted beyond a
certain angle, topples and comes to rest on its side (Figure 7.1).

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