Ethane-1,2-diol–2-methoxyethanol solvent system. Dependence of the relative permittivity and refractive index on the temperature and composition of the binary mixture

Abstract

The extrathermodynamic parameters, relative permittivity (ε_r) and refractive index (n), have been measured for ethane-1,2-diol–2-methoxyethanol binary mixtures over the entire composition range, at various temperatures ranging between –10 and 80 °C. For this system, as far as for many solvent–cosolvent systems which are useful for analytical and electrochemical applications, the properties cited above are continuous but not linear functions of the binary composition.

Various equations have been applied in order to establish the best regression models for the relationship y=f(X), where X is the composition of the mixture. On the whole, minima occurred in the excess function (ε^E), showing the existence of various solvent–cosolvent complexes of different stoichiometric ratios (2 : 1, 1 : 1 or 1 : 2) and of different thermostabilities, which may be interpreted in terms of moderate interactions such as dipolarity, polarizability and hydrogen bonding between the mixed solvents. Among the above complexes, the ethane-1,2-diol–2-methoxyethanol 1 : 2 adduct is suggested to be the true ‘limiting mixture’, on the borderline between the types of behaviour typical of the two pure solvents. Furthermore, empirical patterns in the refractive index were used to scrutinize the most important mixing rules in order to determine the properties of the mixture on the basis of those of its pure components.