Dipole moments of isomeric alkoxyalcohols in cyclohexane. Comparison of Hedestrand and Fröhlich procedures with a new formula

Abstract

Limiting dipole moments of four isomeric alkoxyalcohols dissolved in cyclohexane at 298.15 K were determined from measurements of the relative permittivity of at least 17 dilute solutions up to solute mole fraction of 0.03. In addition, 5 to 7 data points were obtained up to a mole fraction of 0.1. A stepwise dilution device ensured dielectric measurements to be performed in highly dilute solutions with accurately determined concentrations. Densities of these solutions and refractive indices of the pure liquids were independently measured. Limiting dipole moments were calculated using Hedestrand's equation and an improved method of implementing Fröhlich's equation, which circumvents extrapolation difficulties referred to in the literature. A new formula, based on the one-liquid approach for extending the Onsager–Kirkwood–Fröhlich equation to liquid mixtures, is introduced and shown to yield a reliable and robust procedure for estimating dipole moments of polar molecules dissolved in non-polar solvents. Limiting dipole moment values for 2-tert-butoxyethanol (2.31 D), 1-propoxypropan-2-ol (2.14 D), 2-butoxyethanol (2.14 D) and 2-isobutoxyethanol (2.08 D) are recommended. The relative order of these values appears to determine the order of hydrophilicity of these four alkoxyalcohols as suggested by their recently reported limiting partial molar volumes and isentropic compressions in aqueous solutions.