Diffusion

Diffusion is a slow process-like when a smell spreads slowly through a room. The smell particles move away from where there are many of this particles and move where the are less particles-but remember,diffusoin is always slow. Diffusive transport takes place due to the random motion of the molecules, in the absence of any mean motion of the center of mass. This concept is easiest to understand by using a gas mixture in two bulbs separated by a tube. One of the bulbs contains the pure solvent A, while the other contains a mixture of A and a small amount of the solute B. It will be assumed, for simplicity, that A and B have equal molecular mass and diameter, and the initial pressures and temperatures in the two bulbs are equal. When the stop cork between the two is opened, there is no net transfer of mass between the two bulbs, since the pressures and temperatures are equal. However, there will be a transfer of the solute B from one bulb to the other until the concentrations in the two bulbs are equal.

Diffusion is a time-dependent process, constituted by random motion of given entities and causing the statistical distribution of these entities to spread in space. The concept of diffusion is tied to notion of mass transfer, driven by a concentration gradient, but diffusion can still occur when there is no concentration gradient (but there will be no net flux).

In molecular diffusion, the moving entities are small molecules. They move at random because they frequently collide. Diffusion is this thermal motion of all (liquid and gas) molecules at temperatures above absolute zero. Diffusion rate is a function of only temperature, and is not affected by concentration. Brownian motion is observed in molecules that are so large that they are not driven by their own thermal energy but by collisions with solvent particles. Electrons have higher diffusion constant than holes leading to fewer excess electrons at the center as compared to holes.

While Brownian motion of large molecules is observable under a microscope, small-molecule diffusion can only be probed in carefully controlled experimental conditions. Under normal conditions, molecular diffusion is relevant only on length scales between nanometer and millimeter. On larger length scales, transport in liquids and gases is normally due to another transport phenomenon, convection.

In contrast, heat conduction through solid media is an everyday occurrence (e.g. a metal spoon partly immersed in a hot liquid). This explains why the diffusion of heat was explained mathematically before the diffusion of mass.