Dielectric Relaxation Processes in Ethanol/Water Mixtures

Abstract

We have determined the complex dielectric spectra of ethanol/water mixtures at 25 °C for the nine molar fractions of ethanol, X EA = 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 ≤ X EA ≤ 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, τ 1, when going from X EA = 0 to 1, two additional Debye terms ( j = 2 and 3) with the relaxation times of τ 2 ≈ 10 ps and τ 3 ≈ 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 Δε 2 with decreasing X EA in ∼0.5 ≤ X EA ≤ 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 τ 2 -mode. At X EA < 0.3, τ 1 rapidly approaches τ 2 and Δε 2 → 0, so that the intermediate ethanol monomer process ( j = 2) becomes inseparable while the fast switching process with τ 3 ≈ 1 ps can always be resolved. The analysis of the effective dipolar correlation factor, g eff, 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 ≤ X EA ≤ 1.0.