Relative permittivity and dielectric relaxation in aqueous alcohol solutions

In this work, our objective was to study the hydrogen bonding of alcohols in aqueous solutions and alcohol dipole relaxation by measuring the complex permittivity. Alcohols chosen for study were methanol, ethanol and propanol over the full composition range and from 297–363 K. Measurements were made with a frequency domain technique from 100 MHZ to 20 GHz. All of the alcohol–water mixtures studied had a negative excess permittivity that was skewed towards higher water mole fractions. The composition dependence of the dielectric relaxation showed remarkable behavior. At water concentrations of xwater<0.2 there was little change in the alcohol relaxation time constant, whereas at xwater>0.85 or so, the relaxation time had a sharp slope. Our data show that there is a distinct curvature between the pure component time constants, which indicates a gradual, nonlinear change of the relaxation time with composition. From the dielectric data, it would seem that there are three subtle but distinct regions of hydrogen-bonding environments.