Equation of State for Binary Fluid Mixtures

The compressibility factor PṼ/RT for a binary fluid mixture is obtained from tabulated values of a reduced equation of state for a general pure substance assuming the law of corresponding states for pure substances. The binary mixture is treated in zeroth order as a pseudo one-component substance obeying a Lennard-Jones 12–6 intermolecular potential with parameters so chosen that they reproduce the second virial co-efficient of the mixture. In first-order-of-perturbation theory, the difference between the actual intermolecular potentials (assuming all molecular sizes to be equal to that of the pseudo molecular size) and that of the pseudo one-component potential is introduced as a perturbation. The random-mixing assumption need not be made. Data for a total of 25 mixtures in 7 different binary systems are compared with the theory. The perturbation is seen to be unimportant. Theory holds to within about 15% if reduced densities less than 0.3 are considered.