The PCEAS model was used to study the liquid–solid and liquid–vapor phase transitions at constant pressure in systems containing glycols and glycol ethers. This method is based on minimizing the excess Gibbs energy over the solvation parameter, which takes into account the processes of association of molecules in various phases. To compute the diagrams, the data on enthalpy and phase transition temperatures of pure components are required, while the information about the interactions in the binary system is not necessary. We present analytical expressions for the enthalpy of vaporization and enthalpy of melting of glycols and glycol ethers obtained with the theory of similarity using molecular weight, critical temperature, temperature of the triple point, and the number of carbon atoms in the molecule as the parameters. In the absence of information about the critical temperature, the enthalpy of vaporization may be calculated using the boiling point value. It is shown that the prediction of the enthalpy of melting and enthalpy of vaporization allows us to calculate of the phase diagram, as well as the azeotropic and eutectic parameters in water–glycol ether and glycol ether–alkane systems.
modeling, glycol, glycol ether, PCEAS, enthalpy of vaporization, melting enthalpy, thermodynamic similarity, liquid–solid equilibrium, liquid–vapor equilibrium, eutectics, azeotrope
Glycols and glycol ethers are widely used as additives in food and cosmetics industries. Therefore, systems containing glycols and glycol ethers in mixtures with water, alkanes, and salts are actively studied [1–3]. These systems are the subjects of the current work because experimental and theoretical foundation for their application is lacking. There should be thermodynamic models that would be able to predict properties of pure components and phase equilibrium in systems containing glycols and glycol ethers with high accuracy.
For some of the members of the homology series, enthalpies of vaporization and melting are unknown or have not been measured with sufficient accuracy. In work  it has been noted that calculation of the parameters of eutectics and azeotrope mixtures used in technological processes of crystallization and rectification is complicated by the absence of accurate data on enthalpies of phase transitions of individual components.
Boiling temperature Tb and melting temperature Tm, pressure P, density ρ, enthalpy of vaporization ∆Hvap, and enthalpy of melting ∆Hm are determinative for computation of thermodynamic properties of pure substances and solutions. Prediction of the compound properties implies that the properties are determined basing on the compound structural formula. The theory connecting the structure of a molecule with its macroscopic parameters is not complete yet; therefore, it is necessary to summarize empirical data on the properties of various compounds. Method of thermodynamics similarity, which is a part of general similarity theory, forms theoretical basis for such a summary .
Enthalpy of vapor formation at normal temperature of boilingis called the enthalpy of vaporization. Solid–liquid and liquid–vapor phase transitions proceeding at normal temperature of melting are characterized by enthalpy of melting and enthalpy of vaporization. Methods of group contribution [6, 7] and quantum chemistry  are used for predictions.
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