PSEUDOTERNARY MIXTURES

The present paper deals with binary mixtures, in which one component exists in two or more different species, which are in equilibrium with each other1. It is shown that this equilibrium constant, if expressed in mole fractions, depends generally on the concentration of the other component. This dependence is especially great if there are strong intermolecular effects between the species. For estimations of the activity coefficients of the species, it is necessary to introduce only species with a defined arrangement of atoms. General points of view are discussed for a calculation of the activity coefficients of the species and their relative proportion in mixtures. Examples of binary systems are investigated in order to show the influence of the equilibrium between the species and its change with concentration on the thermodynamic properties of the mixtures. The examples deal with the conformational equilibrium of the dihalogenoethanes, the dimerisation equilibrium of acetic acid, and the associative behaviour of phenol in mixtures with nonpolar solvents. The treatment of the dihalogenoethane mixtures is an extension of the ideas of Neckel and Volk2; it is based on the cell model of ternary mixtures3. Here the entropic effects due to the change of the conformational equilibrium are small. A comparison with different properties of mixtures allows qualitative conclusions with respect to dipole-dipole orientation energies and interaction energies between nonideal dipoles and nonpolar molecules. The dimerisation equilibrium of acetic acid changes very markedly with the concentration of the other component. The thermodynamic properties of the mixtures with carbon tetrachloride, benzene, and water can be explained by means of few parameters, which allow for a strong attractive interaction between monomer and dimer and for plausible interactions between the species and the solvent. Finally the mixture phenol+carbon tetrachloride is treated on the assumption of a continuous association of the phenol as well as a cyclic trimerisation. The calculation of the thermodynamic properties is given on basis of this model; the best values of the parameters will be discussed in future papers, where the thermodynamic, dielectric and n.m.r.-properties of various phenol systems shall be compared. © 1969 Springer-Verlag.