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 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 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 τ2-mode. At X EA < 0.3, τ 1 rapidly approaches τ 2 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, 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.

Near-Field Radar Microwave Imaging as an Add-on Modality to Mammography

“…To select the coupling liquid, the dielectric relaxation of many liquids, such as [92][93][94][95][96][97][98][99][100][101][102], were considered. John D. et al used multi-step techniques to approximate the average dielectric properties of the breast tissue and reported a minimum and maximum of about 26.5 and 27 for the relative permittivity in 1-3 GHz, respectively [103].…”

Complex Permittivity Measurement of High Loss Liquids and its Application to Wine Analysis

“…Numerous techniques for complex permittivity determination of high loss liquids are applied at microwave frequencies [Sato & Buchner, 2004;Masaki at al., 2007;Agilent 85070E, 2008;Buckmaster et al, 1985;Hu, et al, 1994;Cherpak et al, 2004]. Among of them it is well known that both resonator [Afsar & Ding, 2001] and waveguide methods are used for the complex permittivity measurement of high loss liquids.…”

“…Dissolved molecules in water can change the relaxation time both due to coupling of part of free water molecules (hydration) and due to the rebuilding of hydrogen bonds in water clusters. The latter mechanism is common for water -ethanol mixtures [Sato & Buchner, 2004]. Owing to mentioned reasons of complex permittivity measurement of water solutions is sensitive to the variation of their chemical composition at Ka band.…”

Dielectric measurements of nanoliter liquids with a photonic crystal resonator at terahertz frequencies

“…It has previously been observed that the permittivity of ethanol-water solutions in the THz band do not follow ideal mixing laws 13 and instead we utilize published results 14 for their permittivity at these molar concentrations. Figure 3(a) shows the measured change in resonant frequency, with respect to air, against 0 for the ethanol-water solution.…”

“…It can be seen that the method produces reasonably accurate values close to other reported values in the literature. 14,17,18 A potentially significant source of error in these measurements is the thermal drift of the resonance, due to the temperature dependent permittivity of the silicon constituent of the PCR, 19 however, this effect can be minimized by making the reference and sample measurements as close together as possible. All measurements were undertaken at an ambient temperature of 296 K. Additionally, the semi-empirical model has an inherent inaccuracy compared to the calibration measurements that is worst for the case of high permittivity liquids, giving maximum errors of 4% and 14% in the real and imaginary parts of the relative permittivity, respectively, over the measured permittivity range.…”