Despite its immense abilities to quantify many aspects of ion-ion and ion-solvent interactions, dielectric relaxation spectroscopy (DRS) has long been neglected as a tool for the investigation of the structure and dynamics of electrolyte solutions. The reasons for this are briefly discussed and it is shown that many of the difficulties associated with this technique have been overcome in recent years by technological developments. Representative applications of DRS to the investigation of ion solvation and ion association in electrolyte solutions of chemical, industrial, geochemical and biological interest, including room temperature ionic liquids and polyelectrolyte systems, are discussed. The advantages of linking DRS measurements to information obtained from other experimental techniques and from computer simulations are highlighted.

Interactions and dynamics in electrolyte solutions by dielectric spectroscopy

This report summarises the results of a recent programme of dielectric reference liquid measurements at National Physical Laboratory (NPL). It is intended that full details of the methods of measurement will be published in the scientific literature in the near future. Comprehensive tables of dielectric relaxation parameters for high purity methanol, ethanol, propan-1-ol, propan-2-ol, butan-1-ol, dimethyl sulphoxide and ethanediol are presented over a range of temperatures (in most cases at 5o intervals in the range 10 - 50 oC). These were derived from measurements at NPL between the years 1997 and 2000 at frequencies extending up to 5 GHz. The relaxation parameters are generally those of the single or double Debye models, which are found to offer a good fit to the measured data in nearly all cases. In addition, static permittivity data are presented for pure water, acetone, silicone oil and cyclohexane. All of the data presented is traceable to UK National Standards. Advisory notes on the use of the tables are provided.

Tables of the complex permittivity of dielectric reference liquids at frequencies up to 5 GHz.

The dielectric constants (relative permittivities) of water, methanol, ethanol, butanol and acetone were measured at 91.3 kPa and (283.15 and 293.15) K and are reported here. The dielectric constants were determined by using a new setup based on a low-pass filter. The obtained dielectric constant values are compared with those reported in the literature, and are consistent with those reported in the literature. The obtained dielectric constant data were also compared with those calculated by the Kirkwood model. The comparisons indicated that Kirkwood model can be successfully used for calculation of dielectric constants of the pure fluids.

Dielectric Constants of Water, Methanol, Ethanol, Butanol and Acetone: Measurement and Computational Study

The requirements for dielectric measurements on polar liquids lie largely in two areas. First there is scientific interest in revealing the structure of and interactions between the molecules - this can be studied through dielectric spectroscopy. Secondly, polar liquids are widely used as dielectric reference and tissue equivalent materials for biomedical studies and for mobile telecommunications, health and safety related measurements. This review discusses these roles for polar liquids and surveys the techniques available for the measurement of their complex permittivity at RF and Microwave frequencies. One aim of the review is to guide researchers and metrologists in the choice of measurement methods and in their optimization. Particular emphasis is placed on the importance of traceability in these measurements to international standards

A review of RF and microwave techniques for dielectric measurements on polar liquids

Dielectric spectra were measured for eight, mostly imidazolium-based, room temperature ionic liquids (RTILs) over a wide range of frequencies (0.2≤ν/GHz≤89) and temperatures (5≤θ/°C≤65). Detailed analysis of the spectra shows that the dominant low frequency process centred at ca. 0.06 to 10 GHz (depending on the salt and the temperature) is better described using a symmetrically broadened Cole-Cole model rather than the asymmetric Cole-Davidson models used previously. Evaluation of the temperature dependence of the static permittivities, effective dipole moments, volumes of rotation, activation energies, and relaxation times derived from the dielectric data indicates that the low frequency process cannot be solely due to rotational diffusion of the dipolar imidazolium cations, as has been thought, but must also include other contributions, probably from cooperative motions. Analysis of the Debye process observed at higher frequencies for these RTILs is not undertaken because it overlaps with even faster processes that lie outside the range of the present instrumentation.

Temperature dependence of the dielectric properties and dynamics of ionic liquids.

Coaxial sensors are used in applications that require accurate and traceable measurements of complex permittivity. For example, coaxial sensors are often used for measurement of the complex permittivity of tissue equivalent materials (TEMs) used in specific absorption rate (SAR) measurements of exposure to RF fields. It is therefore important that well-founded metrological techniques for their use are developed and published. Although there are many published papers on coaxial sensors, few discuss the experimental techniques required to obtain the most accurate results. In this paper experimental approaches for obtaining the most accurate measurements are described. Common pitfalls with the technique are discussed. A Monte Carlo modelling (MCM) technique is used to provide estimates of uncertainty which are compared to those of measurements on reference liquids made with 3.5 mm, 7 mm and 15.1 mm diameter sensors. The MCM technique allows uncertainties to be estimated when measuring dielectrics for which there are no reference materials that have comparable properties, for example for TEMs at frequencies below 100 MHz.

https://iopscience.iop.org/article/10.1088/0957-0233/18/5/026/pdf

Dielectric metrology with coaxial sensors

Measurements of the static permittivity of pure samples of eight polar liquids—acetone, butan-1-ol, dimethyl sulphoxide, ethanediol, ethanol, methanol, propan-1-ol and water—and two non-polar liquids—cyclohexane and silicone oil—were measured in a shielded micrometer-driven, parallel-plate admittance cell at temperatures from 5 to 50 °C. The experimental method, which requires measurements of capacitance using a four-terminal-pair impedance analyser, is described in detail. The full measurement results have been tabulated separately in a National Physical Laboratory report. The present paper provides a comparative study of these latest results with previously published data and also describes the details of the uncertainty analyses employed. Admittance cell measurement techniques are also briefly reviewed to put this work into context.

https://iopscience.iop.org/article/10.1088/0957-0233/16/7/013/pdf

R N Clarke

Papers

Dielectric metrology with coaxial sensors

Traceable measurements of the static permittivity of dielectric reference liquids over the temperature range 5–50 °C