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.
An example of such properties is shown in Figure 1, 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 spectroscopic properties of the particle, an effect which can be observed with the naked eye.
This effect is shown in Figure 2, 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.
Another example is also shown in Figure 3. 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 whoes energy is strongly governed by surface effects.