Properties Of Matter Change At The Nanoscale

The properties of matter depend in part on size. The physical, chemical, and biological properties of matter generally differ at the nanoscale when compared to the larger quantities of the same material. This is due, in part, to the difference in surface area per unit of volume at the nanoscale.

Suppose we have a cube of salt. Its volume area is length times height times depth. Let's say that the block is 1 mm x 1 mm x 1 mm, which is 1 cubic millimeter altogether. (We're using millimeters (mm) instead of nanometers for simplicity.)

The cube has 6 square sides, each ofwhich is (1 mm x 1 mm) 1 square mm in area, so the whole cube has an area of 6 square mm. The ratio of the area (A) of the cube to the volume (V) of the cube is A divided by V (written "A/V") or a ratio of 6/1. Now, in contrast, suppose we have a bigger cube of salt whose edges are 2 mm long instead of 1 mm, so this cube has a volume (2 x 2 x 2) of 8 cubic mm. The area of each side of this salt cube is 2 mm x 2 mm, or 4 square mm, and the area of the cube's entire surface then is 6 x 4 square mm, or 24 square mm. So the area-to-volume ratio of the big block is 24 divided by 8, which equals an A/V ratio of 3/1. So, the bigger cube has a smaller A/V ratio than the smaller cube. This example demonstrates how smaller cubes will have greater ratios of surface area to volume. Table 2.2 summarizes this principle.

This table illustrates how as the sides of a cube get shorter, the volume of a cube gets smaller more quickly than its surface area, so the ratio A/V increases. The ratio A/V is twice as great for the cube of 1 mm as for the cube of2 mm, and three times as great as the cube of3 mm. This phenomenon happens on the nanoscale very significantly.

Table 2.2 Surface Area Per Unit of Volume

Length of Side of Volume of Cube

Surface Area of

Cube

54 24 6

Notice how the ratio of area to volume changes as the cube becomes smaller.

For a given substance, increasing the number of nanoscale particles also increases the proportion of atoms on the surface compared with the number of internal atoms. Atoms at the surface often behave differently from those located in the interior. Atoms at the surface have a higher energy state, which means they are more likely to react with particles of neighboring substances. The result is that chemical reactions can take place between atoms and molecules at surfaces acting as miniature chemical reactors.

By modifying materials at the nanoscale other properties such as magnetism, hardness, and electrical and heat conductivity can be changed substantially. These changes arise from confining electrons in nanometer-sized structures. As one example, electrons (subatomic particles) do not flow in streams as they do in ordinary electrical wires. At the nanoscale, electrons act like waves. When electrons act as waves, they can pass through insulation that blocks flowing electrons.

Another example of changes at the nanoscale is that some nano substances that should not dissolve in a liquid actually do dissolve. Some elements like gold also show change at the nanoscale. At the macro scale, gold metal is shiny yellow as you noticed in jewelry. If you break up the gold, to particle 100 nanometers wide, it still looks shiny yellow. But, when you break down the particle of gold to 30 nanometers across, the gold appears bright red. As the particle of gold gets even smaller than 30 nanometers, it looks purple and when it is a little bit smaller it will appear brownish in color. Color can change in some other substances, like gold, at the nanoscale. Physical properties including strength, crystal shape, solubility, thermal and electrical conductivity, and magnetic and electronic properties also change as the size decreases.

Let's look at another example of how a physical property changes at the nanometer scale. At the macro scale, a sheet of aluminum is harmless. However, when the particles of aluminum are cut down in size to 20 to 30 nanometers, the metal can explode.

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