Environment is nothing but a scale bound experience. We all are familiar with the way we perceive the environment. Have you ever wondered how some other animal would experience it? Think about a cat, an ant, a worm, a bacterium or a virus. Suddenly world become a completely different experience when you look from a different scale. The same is true when it comes to materials. The environment that a material would experience and how it behave is strictly scale dependent.
Scale and the properties of matter
It’s a strange phenomina yet it is true. For an example, a very basic property like a color of a material are observed to change with the size of the material. Below shows three images of gold metal that is in three different particle sizes. First the bulk gold; bright, shiny, yellowish metal. If you lower the particle size of the bulk gold to micron sizes, the brightness dissapears. The gold becomes a reddish yellow powder. If you select to further lower the particle sizes, something amazing would happen. The yellow gold powder will turn in to a red, green or blue in color depending on the size of the particle.
Not only the color, many physical properties like boiling point, melting point, conductivity, density, etc also change. On the outset, this may not sound like much. But in reallity, this presents a very interesting opportunity for scientists to exploit. Since same material shows different properties at different particle sizes, we can now open door for many different applications for a single material. This is call “tuning of the properties”. That is by changing the particle size of the selected material, scientists can literally tune the property of interest until it exactly matches with the requirement.
Reasons for the behavior
There are reasons for this strange behavior of materials in nanoscale (1 nm to 100 nm). Comparatively larger objects that are in millimeter or micrometer sizes are governed by Newtonian laws. However, when you go down the scale and approach nanometer range, quantum effects takes over and start to dominate the environment. Hence, particularly in the range of 1 to 100 nm, properties like melting point, electrical conductivity, magnatic permeability and chemical reactivity changes significantly with the particle size.
Another major contributor to this extraordinary behavior of nanoparticles is the extreme surface to volume ratios these particles show. When you lower the particle size of a material, its surface to volume ratio is exponentially increased due to exposure of more and more inner atoms to the surface. This means, that if you get one gram of bulk material and one gram of nanomaterial and compare the surface area, nano particles would have thousands of times more surface area than the bulk material.
This is particularly important to chemical properties of a nanoparticle. At very small particle sizes like 1 nm, almost all the atoms of the particles are exposed to the outer surface. This has a substantial impact on the properties like reactivity, affinity and sorption abilities of the particle as these properties are mostly surface driven.
There can be certain situations where the particle itself is large yet the surface has nanosize pores that gives the material a larger boost in surface area. Activated carbon is a perfect example. These particles may not be essentially small, yet they show very high absorptive properties due to high surface area caused by nano pores.