This Demonstration considers three models for intermolecular potentials: square-well, Sutherland, and Lennard-Jones. Potentials for molecule (blue), molecule (green), and an mixture (orange) are shown. Use a slider to vary the binary interaction parameter , which represents how non-ideal the mixture is ( for the ideal case). The attractive-strength ratio represents the relative well depths of to . The diameter ratio represents the relative sizes of the molecules. The relative width is the ratio of attractive diameter to the repulsive diameter.

where is the intermolecular potential, is the distance between the molecule centers, is the molecular diameter, is the depth of the potential well, and is the well width for square-well potential.

The attractive strength parameter of the mixture is given by:

,

where is the binary interaction parameter, and and are the attractive strength parameters for molecules and .

A larger negative number for the binary interaction parameter represents stronger interactions between and relative to those between and or and . Analogously, a smaller positive number represents weaker interactions.

The molecular diameter parameter for the mixture is:

,

where and are the diameter parameters for molecules and .

A screencast video at [1] shows how to use this Demonstration.