What do you need Colloids for?

What are Colloids?

The phrase "colloid" (Greek for glue-like) - somewhat misleading from today's point of view - was introduced by Graham in 1861. Graham postulated the size of these particles to be in the range from one nanometer to one micrometer (10-9-10-6 m). This range is still given today to define colloidal particles.

position colloids
Figure 1: position colloids occupy between molecular and solid-state dimensions

In Figure 1, this range is illustrated to show the position colloids occupy between molecular and solid-state dimensions. Thus, colloidal sciences deal with particles that combine molecular and solid-state properties.

Colloids are no chemical class of substances; we are dealing rather with a certain state of matter, like the condensed states of the solid or liquid. What is so special about this state is that macroscopic properties are greatly affected by size in these dimensions.

For more than 150 years, chemists have been exploring the colloidal state of matter, characterised by Ostwald as a "world of neglected dimensions". Selmi described aqueous dispersions of silver chloride, sulfur and Prussian blue in 1845. Soon afterwards, Faraday examined gold sols and deduced that this (colloidal) state of matter must be thermodynamically instable and that stabilisation must be a kinetic phenomena. Some of the dispersions prepared by Faraday are still on display in the British Museum today.

In the beginning of the twentieth century, colloidal science established a connection between (preparative) chemistry and (theoretical) physics. Einstein discovered the connection between Brownian motion and the diffusion coefficient while Perrin used this relationship to calculate Avogadro's number. Ever since, colloidal science has linked various disciplines of science, such as biology and the materials sciences, to name but two.

What is special about Colloids?

Concerning their physicochemical properties, colloids cannot be consistently described if understood only as solid-state matter. They are subject to the "quantum size effect", which renders classical physics inappropriate and makes aspects of quantum mechanics neccessary to describe these particles.

band gap
Figure 2: dependence of a quantum-sized semiconductor's band gap on particle size

An example of such properties is shown in Figure 2, which shows the dependence of a quantum-sized semiconductor's band gap on particle size. It is larger than the discrete spectroscopic transitions of the molecular semiconductor but larger than that in the bulk material. Variation of n, the number of molecules associated within the particle, sensitively governs the spectrocopic properties of the particle, an effect which can be observed with the naked eye.

cadmium phosphide
Figure 3: semiconductor cadmium phosphide with different colors depending of particle size

This effect is shown in Figure 3, where the semiconductor cadmium phosphide, which appears black as a bulk material, can take all colors, even white, when particle size - and thus the band gap - is decreased. In this manner, spectrocopic properties can be tuned by adjusting particle size.

cadmium sulfide dispersions
Figure 4: fluorescent colour of cadmium sulfide dispersions

Another example is also shown in Figure 4. Here, the fluorescent colour of cadmium sulfide dispersions is shifted from blue-green to red by partially precipitating a less-soluble compound, in this case mercury sulfide, onto the particle surface. In this way, electronic properties of colloids can be specifically influenced.

Interactions at the particles' interfaces are much more pronounced compared to bulk systems, as the surface-to-volume ratio is much larger in colloids. Volume is connected to mass and the quantity of material, so it appears reasonable to substitute the definition of colloids above by something like:

Colloids are particles where the energy is strongly governed by surface effects.

What are applications of Colloids?

Modern colloid science not only helps us to understand processes in nature; there are many and various technical applications. Colloids have had a considerable impact on the development of new dyes and laquers, for high performance materials and in pharmacy. The quantum size effect and its influence on photocatalytic, photochemical, photovoltaic and nonlinear optical properties open up a tremendous range of interesting applications.

There is already a large variety of products today that make use of colloidal properties. The range of applications covers detergents, laquers, dyes, advanced inks and pharmaceutical products, as well as inorganic additives, organic pigments, and polymers - colloidal systems are applied in all sectors of modern chemistry, constantly generating new products with enhanced (and often tunable) properties.

Nanolytics offers its services to companies that deal with colloidal systems and need specific experimental or theoretical support concerning the special properties of these systems arising from the quantum dominated nature of colloids. We specialise in colloid analysis and on the relevance of nanoscopic properties towards macroscopic features. We have high-end colloid analytical equipment in our lab, where the extremely powerful - but not easily accessible - method of Analytical Ultracentrifugation plays a central part.