Diffusion of atoms (green spheres shown above) and molecules occurs to different extents in different materials (represented by the red spheres). Diffusion is basically a random walk process of atoms and molecules, and as such it is strongly dependant of temperature and structure. As the temperature is increased and the atoms (molecules) vibrate more energetically, a small fraction of the atoms will relocate themselves in the lattice (or the amorphous material). Of course, the ability to diffuse depends not only on temperature, but also on how densely and tightly the atoms are bonded in position. The energy requirement for an atom (molecule) to change position is called the activation energy. This energy may be expressed as calories/mole.
As an example, imagine that a carbon atom (a small atom with a radius of 0.07 nm) sits interstitially among a number of iron atoms in a lattice. If the carbon atom has enough energy, it can move through the holes between the iron atoms to the next interstice when it vibrates in that direction. At 20°C there is only a small probability that it will have that much energy, but at higher temperatures, the probability increases.
The same happens when some plastic materials are stored in water. During such storage the rather small water molecules move in between the longer polymer molecules. When they move in between the polymer molecules, the latter separate and their van der Waal interaction decreases because of the increased separation. The decreased intermolecular interaction results in a material that has become plasticized.
From the above we can conclude that an increase in structure density aand charge density decreases the diffusion properties of a molecule diffusing through the surrounding structure.
E-mail: ksoderholm@dental.ufl.edu
© 1999, Karl-Johan M. Söderholm