Solubility and dielectric properties of benzoic acid in a binary solvent: Water-ethylene glycol

Abstract

The aim of this work was to study the mutual influence of permittivity and solubility of benzoic acid dissolved in water-ethylene glycol solutions. Dielectric measurements on water-ethylene glycol solutions, benzoic acid in solid state as well as in saturated water-ethylene glycol solutions have been performed in the low frequency range up to 1 MHz. The impedance analyser Hewlett-Packard HP 4284A and the commercial cells HP 16451B for solids and HP 16452A for liquids, were used for measurements. In this last case, a non commercial cylindrical cell has also been used. The water - ethylene glycol solution has been considered as an interesting model of a pharmaceutical solvent. Ethylene glycol, which plays a co-solvent role in benzoic acid saturated solutions with fractions ranging between 0 and 1, has been selected due to its moderate co-solvent strength. This condition merges from the comparison of its polar, dispersion and hydrogen bonds solubility parameters with those of the pure solute. Values of permittivity and conductivity of the different media have been obtained from measurements of impedance, Z, phase angle, φ, reactance, X, resistance, R, and capacity, Cp, in the low frequency range. For example, in the case of pure benzoic acid in solid state a permittivity value of 8.94 has been obtained. The behaviour of benzoic acid solubility as a function of permittivity at different frequencies and co-solvent fractions was finally analysed and in this way, the solubility — permittivity — polarity correlation has been studied.