Thermodynamic properties of a square-well fluid in the liquid and vapor regions

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Abstract
The thermodynamic properties of a square‐well fluid are computed for temperatures below the critical temperature using a perturbation technique. Thermodynamic functions such as pressure, internal energy, and the Helmholtz function are tabulated and compared with computer simulation results. The critical point is found and the liquid–vapor phase diagram is obtained. The perturbation method uses high temperature parametric integral equation results as an original reference system and applies first order perturbation theory to the Helmholtz function. The pair potential (φ) is written as the sum of two parts, a reference potential (φr) and a perturbing potential (φp) and the separation is established by the relationship φr=αφ where α is a constant. This separation leads up to a particularly simple method of computing thermodynamic properties when the pressure and internal energy are known along one isotherm. In the case of the square‐well potential this approximation leads not only to simple equations but the results also appear to be reasonably accurate.