At 25 °C the complex dielectric permittivity has been measured as a function of frequency ν (1 MHz ≤ ν ≤ 56 GHz) and mole fraction x (0 ≤ x ≤ 1) for mixtures of water with 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, and 1,7-heptanediol. Some parameters like the (extrapolated) static permittivity and the characteristic relaxation time can be directly taken from the spectra. But the spectra can be also analytically represented by semiempirical relaxation spectral functions. A function containing two relaxation terms, a high-frequency Cole−Cole term and a Debye term with smaller relaxation frequency, appears to be appropriate. The relaxation parameters of the water/diol mixtures have been evaluated and compared to other water/organic solvent systems to show that (i) a high degree of permanent electric dipole orientation correlation exists in the diols that is significantly reduced on addition of water, (ii) the characteristic relaxation time reflects a substantial effect of entropy in the mechanism of dielectric relaxation but exhibits also a significant difference from monohydric alcohol/water systems, and (iii) the subdivision of the spectra into two relaxation terms seems to be due to regions of different concentration of hydrogen-bonding sites within the mixtures.

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