About 4 gms of single walled carbon nanotubes have the same surface area as a football field.
One of the important characteristics of nanoparticles is their large ratio of surface area to volume (compared to bulk materials). This property can be illustrated by the following thought experiment: consider a cube of edge W=1 meter, and cut it into two pieces, thereby exposing additional 'faces' of the material. That is, new visible or usable areas are added while the total volume remains the same. Repeat this exercise until all particles reach a size of approximately one nanometer. The result is a group of small particles with enormous surface surface area which occupy the same volume we started with. There are nanomaterials which exhibit a surface area equivalent to a football field for just a few grams of weight.
Large surface area makes some nanoparticles highly soluble in liquids. This property found applications in paints, pigments, medicine pills, and cosmetics. A 30 nanometer iron particle has 5% of its atoms on the surface while the remaining reside inside. However, a 3 nm particle has 50% of its atoms on the surface. The atoms on the surface, which are not bonded on one side, are far more active than the atoms residing inside (which are bonded all around, thus they are "satisfied"). An increase in surface area therefore leads to an increase in reactivity.
Model of gold particles.
Particle size can also affect the color of the material, with applications in the production of inks, for example for glossy magazines where color schemes are of great interest. Large surface areas are also attractive for a process called adsorption. Adsorption occurs when a gas or liquid solution accumulates on the surface of a solid or a liquid (adsorbent), forming a film of molecules or atoms. Large surface areas increase adsorption. This process is useful in applications such as catalytic converters, water purification devices, waste recovery technologies, and manufacturing of deodorants, perfumes, varnishes and adhesives.
This image illustrates how surface area increases as the size is reduced. Image Credit: NASA Ames Center for Nanotechnology