Dielectric relaxation of lower alcohols in the whole fluid phase

Dielectric relaxation of ethanol and 1-propanol have been measured in a wide fluid phase including the supercritical condition for the first time. The results of the static permittivity epsilon(0) and the dielectric relaxation time tau(D) are presented in the temperature and pressure range up to 670 K and 30 MPa. Kirkwood's g-factor deduced from epsilon(0) suggests that the static orientational correlation of dipoles becomes prominent at densities above similar to2d(c), d(c) being the critical density. We divide the fluid phase into four regions and discuss the dielectric relaxation mechanism by extending our model previously applied to water [K Okada , J. Chem. Phys. 110, 3026 (1999)]. In the vapor and low-density liquid, the dielectric relaxation is governed by binary collision of molecules. In the high-temperature liquid, the molecules that escape from the hydrogen bond (HB) network also contribute to the relaxation, and the HB breaking is promoted by thermal excitation of the intermolecular stretching modes. In the low-temperature liquid, the escape time is strongly enhanced relative to the HB lifetime. In the glass transition region, the HB breaking is induced in a more cooperative way than the intermolecular stretching vibrations. The present interpretation gives a reasonable explanation to three relaxation times that were obtained by precise measurements near room temperature, and also provide a microscopic basis for the glass transition scenario by Hansen [J. Chem. Phys. 107, 1086 (1997)]. (C) 2003 American Institute of Physics.