Dielectric Relaxation Processes in Ethanol/Water Mixtures

We have determined the complex dielectric spectra of ethanol/water mixtures at 25 °C for the nine molar fractions of ethanol, XEA = 0.04, 0.08, 0.11, 0.18, 0.3, 0.5, 0.7, 0.9, and 1.0, in the frequency range 0.1 ≤ ν/GHz ≤ 89 using TDR in 0.1 ≤ ν/GHz ≤ 25 and waveguide interferometers in 13 ≤ ν/GHz ≤ 89. At 0.3 ≤ XEA ≤ 1.0, a three-step relaxation model turns out to be most appropriate. Besides a Cole−Cole relaxation for the dominating low-frequency process (j = 1), assigned to the cooperative dynamics of the H-bond system, which exhibits a pronounced increase of its relaxation time, τ1, when going from XEA = 0 to 1, two additional Debye terms (j = 2 and 3) with the relaxation times of τ2 ≈ 10 ps and τ3 ≈ 1−2 ps are required to reproduce the high-frequency part of the spectrum. In view of the well-established relaxation mechanisms of pure liquids, these high-frequency processes can be validly assigned to the motion of singly H-bonded ethanol monomers at the ends of the chain structure (j = 2) and the flipping motion of free OH (j = 3), respectively. The unusual increase of the amplitude Δε2 with decreasing XEA in ∼0.5 ≤ XEA ≤ 1.0 suggests insertion of water molecules into the zigzag structure of winding H-bonded ethanol chains resulting in a reduction of the average chain length and an increase of the number of end-standing ethanol molecules that can contribute to the τ2-mode. At XEA < 0.3, τ1 rapidly approaches τ2 and Δε2 → 0, so that the intermediate ethanol monomer process (j = 2) becomes inseparable while the fast switching process with τ3 ≈ 1 ps can always be resolved. The analysis of the effective dipolar correlation factor, geff, revealed that the parallel arrangement of dipole vectors of ethanol molecules is fairly disturbed by the presence of a small amount of water. Water has a strong perturbation effect on the ethanol hydrogen-bonding chain structure in the ethanol-rich region of 0.3 ≤ XEA ≤ 1.0.