In kinetic theory, the mean free path is defined as the mean distance travelled by a molecule between collision with any other molecule. For a dilute gas composed of hard spherical molecules of kinds A and B, the mean time τAB between successive collisions of a given A molecule with B molecules is given by
Here, dAB is the mean diameter of molecules A and B, is the number density of B molecules, and cAB is the mean relative speed of molecules A and B. According to the Maxwell-Boltzmann velocity distribution law, molecular velocities are not correlated and cAB is therefore given in terms of the mean speeds of the individual molecules by cAB = where, for molecules of type i with mass mi, ci = (8kT/πmi)1/2, where k is Boltzmann's constant and T is the absolute temperature.
The mean free path lAB travelled by a given A molecule between successive collisions with B molecules is simply cA τAB or
Real molecules interact through intermolecular forces which vary smoothly with distance. Consequently, there is no unique counterpart of the hard-sphere collision cross-section and no unique definition of what constitutes a collision. Often, dAB is replaced by the separation σAB at which the A-B intermolecular potential energy crosses zero. Alternatively, dAB may be determined from viscosity data by comparison with the theoretical predictions for hard spheres [Kennard (1938)]. These formulas are easily specialized to the case where A and B are identical and, if d is estimated from viscosity data, then
where η is viscosity and ρ density.
Kennard, E. H. (1938) Kinetic Theory of Gases. McGraw-Hill. New York.