Water-ethanol mixtures at different compositions and temperatures. A dieletric relaxation study

At eight temperatures T between 0 and 60 degrees C and at five mole fractions x(e) of ethanol (0 < x(e) less than or equal to 1) the complex (electric) permittivity of ethanol/water mixtures has been measured as a function of frequency nu between 1 MHz and 24 GHz. At 25 degrees C the ethanol permittivities are completed by literature data for the frequency range 200 MHz to 90 GHz. The spectra for ethanol and for the ethanol/water mixtures are compared to permittivity spectra for water which, at some temperatures, are available up to 900 GHz. All spectra of the ethanol/water system can be well represented by the assumption of two relaxation regions. The relaxation time tau(1) of the dominating relaxation process varies between 4 ps (x(e) = 0, 60 degrees C) and 310 ps (x(e) = 1, 0 degrees C); The relaxation time tau(2) of the second relaxation process is smaller. Evaluation of the extrapolated low frequency ("static") permittivity yields a minium in the effective dipole orientation correlation of the ethanol/water system at 0.2 less than or equal to x(e) less than or equal to 0.4. In this composition range, other parameters also exhibit extrema, indicating a microheterogeneous structure of the mixtures and the existence of precritical concentration fluctuations. Interesting, the activation enthalpy Delta H1(double dagger) and entropy Delta S-1(double dagger) of the dominating dielectric relaxation process also display a distinct maximum at around x(e) = 0.22. These activation quantities have been obtained from Eyring plots of the relaxation time tau(1) at different mixture compositions. The relaxation parameters of the ethanol/water system are discussed in terms of a wait-and-switch model of dipole reorientation.