A brief outline of the formulas used in the calculations is given below. The calculations are based on the Lennard–Jones (6-12) potential:

.

In the above expression,

is called the collision diameter (a measure of the diameter of the molecule), and

is the maximum energy of attraction between a pair of molecules. The resulting working equation from Chapman–Enskog kinetic theory for estimating the product of the mixture molar density with binary diffusion coefficient

is

,

where

is the molar density of the binary mixture,

(g/mol) is the molecular weight of species

, and

is the binary collision diameter, which is estimated from collision diameter parameters

using the following mixing rule:

.

The quantity

is called the collision integral for diffusion and is a function of the reduced temperature

, defined as

.

In this expression,

is the Boltzmann constant and

is the characteristic energy appearing in the Lennard–Jones potential for the binary pair estimated using the mixing rule

,

where the units of

are kelvins. For ideal gases, we can estimate

as

. The resulting binary diffusivity is then given by

,

where

is in atm. In these calculations, a correlation for

as a function of the reduced temperature

is computed from data (table E.2 in [1]). The molecular parameters for the individual species are taken from table E.1 in [1].

[1] R. B. Bird, W. E. Stewart, and E. N. Lightfoot,

*Transport Phenomena*, 2nd ed., New York: John Wiley & Sons, 2002.