Nanoparticles

Nanoparticles (sized between 1 and 100 nanometers) are considered a bridge between bulk materials and atomic or molecular structures. A bulk material has constant physical properties regardless of its size, but often this is not the case at the nanoscale.  

One of the important characteristics of nanoparticles is their large surface area-to-volume ratio (compared to bulk materials). This property can be illustrated by a simple thought experiment. Consider a cube with a 1 meter edge length. If you cut it into two pieces, then you would create 2 additional surfaces each with the same area as one of the 6 original faces. While the total volume is unchanged by the cut, the surface area has increased by 33%. Repeat this exercise until all particles reach a size of approximately one nanometer. The result is a group of small particles that has the same volume as the original cube but has a surface area that is a billion times higher. There are nanomaterials that have a surface area equivalent to a football field for just a few grams of material.

Large surface area makes some nanoparticles highly soluble in liquids. This behavior is important for 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). An increase in surface area generally leads to an increase in reactivity. 

Particle size can also affect the color of the material, with applications in the production of inks 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. 

Once nanoparticles are produced, it is desirable that they maintain their small dimensions. However, in many cases, agglomeration of small particles produces bigger clump of material. When this happens, many of the advantages of using nanoparticles are lost.  Scientists have found ways to prevent particle agglomeration by using surfactants (special surface coatings) that reduce the bonding forces between particles.