An equation of state (EoS) suitable for describing associating fluids is presented. The equation combines the simplicity of a cubic equation of state (the Soave−Redlich−Kwong), which is used for the physical part and the theoretical background of the perturbation theory employed for the chemical (or association) part. The resulting EoS (Cubic Plus Association) is not cubic with respect to volume and contains five pure compound parameters which are determined using vapor pressures and saturated liquid densities. Excellent correlations of both vapor pressures and saturated liquid volumes are obtained for primary-alcohols (from methanol up to 1-tridecanol), phenol, tert-butyl alcohol, triethylene glycol, and water. Moreover, excellent prediction of saturated liquid volumes may be obtained from parameters which have been estimated by regressing only vapor pressures. Finally, we suggest a method for reducing the number of adjustable parameters for alcohols to three while maintaining the good correlation of vapor pressures and saturated liquid volumes. We investigate the possibility of using the homomorph approach for estimating the EoS parameters and explain the problems observed. The estimated pure compound parameters have been tested in the prediction of second virial coefficients with satisfactory results.

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The cubic-plus-association (CPA) equation of state (EoS) is applied in this study to binary aqueous mixtures containing hydrocarbons. The CPA EoS combines the Soave−Redlich−Kwong (SRK) cubic equation of state for the physical part and perturbation theory for the chemical (association) part. Rigorous expressions for the contribution of the association term to the pressure and to the chemical potential, which do not include any derivatives of the mole fraction of molecules i not bonded at site A (X A i ), are presented. Three different association models for water have been considered depending on the number of hydrogen bonding sites per water molecule:  the two-, three-, and four-site models. Successful correlation of both vapor pressures and saturated liquid volumes is obtained with all three models. However, satisfactory correlation results of the mutual solubilities of water/aliphatic hydrocarbon systems are obtained only with the four-site model using a single interaction parameter (k ij ) in the attractive term of the EoS. A generalized expression of k ij as a function of the molecular weight of the members of the homologous series is presented, something that allows CPA to be used as a predictive tool. Very satisfactory prediction results are obtained, comparable to the correlation ones of the SAFT EoS for the water solubility in the hydrocarbon-rich phase and orders of magnitude better for the hydrocarbon solubility in the water-rich phase. Satisfactory predictions are also obtained for the vapor-phase compositions and the three-phase equilibrium pressures.