What is a Colloid?

"Ich weiß keinen Zweig der heutigen Naturwissenschaften, der derartig viele und verschiedenartige Interessenkreise berührt, wie die Kolloidchemie. Gewiß, auch Atomtheorie und Radioaktivität interessieren heute jeden intellektuell wachen Menschen. Aber dies sind geistige Delikatessen verglichen mit der Kolloidchemie, die für viele Theoretische und praktische Gebiete nötig ist heute wie das liebe Brot." Wolfgang Ostwald, 1922

In 1861 Thomas Graham coined the term colloid (which means "glue" in Greek) to describe Selmi's "pseudosolutions". The term emphasizes their low rate of diffusion and lack of crystallinity. Graham deduced that the low rate of diffusion implied that the particles were fairly large - at least 1nm in diameter in modern terms. On the other hand, the failure of particle sedimentation implied an upper size limit of 1 micrometer. Graham's definition of the range of particle sizes that characterize the colloidal domain is still widely used today.

Today colloid science is the study of systems involving small particles of one substance suspended in another. Suspensions in liquids form the basis of a wide variety of systems of scientific and technological importance, including paints, ceramics, cosmetics, agricultural sprays, detergents, soils, biological cells, and many food preparations.
Almost every technique and theoretical procedure of modern physics and chemistry has been and is being applied to the study of colloids so that even the specialist colloid chemist finds it difficult to remain au fait with the many ramifications of the subject.

Why are colloids so interesting and important? The answer is given by the following picture (unfortunately in German):

colloidal micelles

Classical chemistry and physics use relatively small molecules with a molecular weight up to 10000. Polymer chemistry is using molecules with a molecular weight three orders of magnitude higher.
On the other hand, solid state physics are interested in the properties of condensed matter.
Between these two scientific areas is the world of colloid chemistry. Colloid chemistry is closing the gap between molecular chemistry and solid state properties. Colloid chemists may use six orders of magnitude of molecular weight exclusively, including all the corresponding effects. Some people name this region as "mesoscopic", since the colloidal region is located in between well known scientific length scales.
The upper picture is very useful, since it shows a whole scientific research program. The special properties and possibilities of using colloids depend on their position in between large molecules and small solids.
For example, large surface areas are associated with the characteristic size of colloidal particles. It is an intrinsic property of all colloidal systems. For instance, a typical micellar solution containing 0.1M amphiphile has ca. 40000 sqr-meters of interfacial area per liter solution.

Over the past forty years, colloid science has undergone something of a revolution, transforming itself from little more than a collection of qualitative observations of the macroscopic behaviour of some complex systems into a discipline with a solid theoretical foundation. It can now boast a set of concepts which can go a long way towards providing an understanding of the many strange and interesting behaviour patterns exhibited by colloidal systems.

Future of colloids

Colloid chemists are starting make use of hierarchical building principles in order to synthesise materials with a hirachy of structure. It is possible to create new properties or property combinations by making use of the "molecular team formation" on different structure levels. Colloids are not only small particles, but they can consist of different functional and structural molecule types. Choosing the right architecture, these colloidal systems are able to solve problems that can not be solved by standard molecular systems.

This principle is not new. Nature has been using it for millions of years. An impressive example is the structure of our hair. Here, Ceratin molecules aggregate on many different aggregation levels until the final hair is formed. Natural fibres show surperior properties compared to man made fibres.
Another example is the our skin. There is no synthetic material that is so soft, elastic and strong as skin. It consits mostly of water, Collagen, Hyaluronic acid and Proteoglycan. These components form superstructures which result in the unique properties of our skin.

Such property combinations are not only useful for materials properties, they are also useful for electro-optic and transport properties of a material.
A lot of research groups are currently starting to synthesise composite materials of polymers and colloids. They are trying to introduce order into supramolecular systems by supramolecular order to the systems by selfassembly and modular synthesis. Colloid chemistry is one of the most promising scientific areas of our century.