Dielectric properties of aqueous glycerol and a model relating these to the properties of water

Abstract

The complex permittivity of aqueous glycerol has been measured at several concentrations, at a frequency of 70 GHz and, by time domain spectroscopy, at frequencies from 0.01 to 10 GHz. The results are interpreted in terms of a single concentration-dependent Davidson–Cole dispersion. The distribution parameter β is constant at ca. 0.7 at glycerol mole fractions 1 to ca. 0.2, increasing gradually to unity as the solution is further diluted. The activation entropy rises gently up to mole fraction 0.2, then more steeply. The overall slow rise of free energy of activation results from the parallel rise of enthalpy and entropy. The single relaxation zone is taken to show that the relaxing element is common to solvent and solute. This conclusion leads to a formulation of the static permittivity of monohydric and polyhydric alcohol solutions in water and in non-polar solvents in terms of an effective concentration of an imaginary water-like reference substance having a dipole moment 8.33 × 10^–30 C m (or 2.5 D) at 20 °C. The model is applied to a number of solutes for which data in water and non-polar solvents are available and to 1,4-dioxane, for which the water equivalent is negative. The model can in principle be extended to other types of solute. The resulting dipole moments relative to the reference substance are not related explicitly to the size and structure of the solute molecule. Such a correlation can only be made with the aid of extraneous evidence such as that furnished by dielectric or ultrasonic dispersion.