Water−Ethanol Mixtures at Different Compositions and Temperatures. A Dieletric Relaxation Study

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Abstract

At eight temperatures T between 0 and 60 °C and at five mole fractions xe of ethanol (0 < xe ≤ 1) the complex (electric) permittivity of ethanol/water mixtures has been measured as a function of frequency ν between 1 MHz and 24 GHz. At 25 °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 τ1 of the dominating relaxation process varies between 4 ps (xe = 0, 60 °C) and 310 ps (xe = 1, 0 °C). The relaxation time τ2 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 ≤ xe ≤ 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 ΔH1⧧ and entropy ΔS1⧧ of the dominating dielectric relaxation process also display a distinct maximum at around xe = 0.22. These activation quantities have been obtained from Eyring plots of the relaxation time τ1 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.

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