Introduction to Surfaces and Colloids - Surface and Colloid Chemistry: Principles and Applications - page 8

small. Instability will be observed if the ratio of the barrier height to k B T is around
1-2 units.
The idea that two species (solid-solid) should interact with one another, so that
their mutual potential energy can be represented by some function of the distance
between them, has been described in the literature. Furthermore, colloidal particles
frequently adsorb (and even absorb) ions from their dispersing medium (such as in
groundwater treatment and purification). Sorption that is much stronger than what
would be expected from dispersion forces is called chemisorption , a process that is
of both chemical and physical interest. For example, in a recent report, it was men-
tioned that finely divided iron particles could lead to enhanced photosynthesis in
oceans (resulting in the binding of large amounts of CO 2 ). This could lead to control
of the global warming effect.
Emulsions: As one knows from experience, the fact that oil and water do not mix
suggests that these systems are dependent on the oil-water interface. The liquid 1 -
liquid 2 (oil-water) interface is found in many systems, and the most important is the
world of emulsions .
The trick in using emulsions is that both water and oil (the latter is insoluble in
water) can be applied simultaneously. Further, other molecules may be included that
may be soluble in either phase (water or oil). This obviously leads to the observation
of thousands of applications of emulsions. It is very important to mention here that,
actually, nature uses this trick in most of the major biological fluids. The most strik-
ing example is milk. The emulsion chemistry of milk has been found to be the most
complex, and still not very well investigated.
In fact, the state obtained by mixing oil and water is an important example of
interfacial behavior of liquid 1 -liquid 2 . Emulsions of oil-water systems are useful
in many aspects of daily life, such as milk, foods, paint, oil recovery, pharmaceuti-
cal, and cosmetics. The colloidal chemistry of milk makes it the most complicated
naturally made product.
If olive oil is mixed with water and shaken, the following is obtained:
About 1 mm diameter oil drops are formed.
After a few minutes, the oil drops merge together and two layers are again
formed.
However, if suitable substances are added that change surface forces, the olive oil
drops formed can be very small (in the micrometer range).
These considerations are important in regard to different systems such as paints,
cements, adhesives, photographic products, water purification, sewage disposal,
emulsions, chromatography, oil recovery, paper and print industry, microelectronics,
soaps and detergents, catalysts, and biological systems (such as cell, virus). In some
oil-surfactant-water-diverse components, liquid crystal (LC) phases (lyotropic LC)
are observed. These are indeed the basic building blocks in many applications of
emulsions in technology.