Movement of particles from higher chemical potential to lower chemical potential (chemical potential can in most cases of diffusion be represented by a change in concentration). Diffusion, the spontaneous spreading of matter (particles), heat, or momentum, is one type of transport phenomena.

Because diffusion is thermally activated, coefficients for diffusion by a single mechanism at different temperatures may be described by an Arrhenius equation:

Data may be fit by a straight line on a graph of log D (m2/s) as a function of l/T (K-1). The resulting line has a slope of ΔH/(2.303R), where R is the gas constant (8.3143 J/mole K) and ΔH (J/mole) is the “activation energy.”

Some or all content above used with permission from J. H. Wittke.