The mixing rule with excess Gibbs energy was introduced into PR equation of state and UNIQUAC eq. was chosen as an expression of excess Gibbs energy. The interaction parameters of UNIQUAC were determined so as to reproduce vapor-liquid equilibrium data. As the result, the parameters of paraffin system are small enough to be neglected, but the parameters of other systems including polar components are large enough to act an important role in the fugacity coefficients of their mixtures.
In fact, vapor-liquid equilibria for binary paraffin, nitrogen-ethane, alcohol-water, acetone-water, and acetone-methanol systems could be successfully reproduced in the range of low to high pressure by the mixing rule with UNIQUAC.