U. Kaatze

The Dielectric Properties of Water at Microwave Frequencies

— the simple Debye function is appropriate to describe the relaxational behaviour ofwater at microwave frequencies. In the far infrared, however, a discrepancy between the values predicted by extrapolation of the relaxation function and the measured quantities is found. The parameters of the Debye relaxation function as obtained by a nonlinear regression analysis are presented for thirteen temperatures and are briefly discussed.

Broad-band ultrasonic measurement techniques for liquids

Various methods for broad-band ultrasonic absorption and velocity measurements in liquids, using CW and pulsed signals, are reviewed. Several different ultrasonic spectrometric techniques are required to cover the wide range of liquid wavelength and attenuation values that exist in the kilohertz to gigahertz frequency range. Advantages and problems of various ultrasonic and related methods are discussed. Sonic cells for liquids and electronic equipment for spectral measurements are presented. Together with applications in relaxation spectrometry, examples of broad-band ultrasonic absorption spectra, utilizing different CW resonator and pulse-modulated travelling wave techniques, are presented.

The dielectric properties of water in its different states of interaction

A tutorial on dielectric (relaxation) spectrometry of liquids is given in this article. Some methods of measuring complex (electric) permittivity spectra are briefly described. Results for water are presented and related to characteristic properties of the liquid structure and to models of the molecular dynamics, particularly as resulting from computer simulation studies. Dielectric spectra for aqueous solutions of low weight electrolytes, polyelectrolytes, small molecules, and polymers are discussed to illustrate effects of kinetic depolarization, structure saturation, as well as positive, negative, and hydrophobic hydration. Reference is also made to fluctuations in the hydrogen bond network of mixtures of water with liquids that are completely miscible with this unique solvent.

Broadband dielectric spectrometry of liquids and biosystems

This review provides a short introduction into the basic principles of dielectric spectroscopy and it covers modern methods in use to measure the dielectric properties of liquid samples over the frequency range from about 10?6 Hz to 1012 Hz. The approaches described include frequency domain and time domain techniques, low frequency, high frequency and microwave measurement systems, as well as broadband and spot-frequency modes of operation. Reflection, transmission and resonant methods are presented, sketches of suitable sensors and block diagrams of measurement set-ups are shown. Accuracy aspects and the merits and limitations of the different techniques of measurement are discussed.

Hydrogen network fluctuations and dielectric spectrometry of liquids

Complex dielectric spectra of associating liquids are presented and are discussed considering their relaxation properties. Particular emphasis is given to water and aqueous solutions and to alcohols and mixtures of alcohols with dipolar and non-polar solvents. The relaxation properties of the associating systems are examined in the light of a waitand-switch model of dielectric relaxation in which the relaxation time is governed by the period for which a given ensemble of hydrogen bond partners within the hydrogen network has to wait until favorable conditions for a reorientation of a molecular permanent dipole exist. These conditions are provided by an additional molecule or group which firstly tends to lower the potential energy barrier for reorientation and which secondly, at the same time, offers a site for the formation of a new hydrogen bond. Some experimental findings are discussed in terms of the wait-and-switch model, among them the reduced dielectric relaxation time of water under hydrostatic pressure, effects of negative and hydrophobic hydration of ions, and the influence that water, on the one hand, and n-alkanes, on the other hand, act on the relaxation of alcohols.

Reference liquids for the calibration of dielectric sensors and measurement instruments

A comparative study of microwave complex dielectric permittivity data from different laboratories is provided for liquids. Four liquids—water, cyclohexane, methanol and dimethyl sulphoxide—are recommended as reference materials because their parameter sets from different measurements result in almost identical predictions of their dielectric properties in the frequency range up to 10 GHz. Within the limits of experimental error this agreement includes that of the extrapolated low frequency permittivity with the recently determined static permittivity of the liquids. Parameters for water are given for the frequency range 0–60 °C. For cyclohexane, which does not display relaxation behaviour up to the submillimetre frequency range, the frequency-independent permittivity is represented between 10 and 50 °C. Relaxation and static permittivity data for methanol and dimethyl sulphoxide are presented at 25 °C.

Techniques for measuring the microwave dielectric properties of materials

This review gives a brief introduction to the principles of microwave measurements of the complex permittivity of materials. The fundamentals of reflection, transmission and resonant methods are summarized and examples of sample cell design and measurement systems are presented for each category. The frequency domain and the fast response time domain methods are discussed in view of their merits and limitations. Accuracy aspects and reference materials for calibration procedures are mentioned briefly. Selected examples of applications indicate the diverse usability spectrum of microwave dielectric measurements in basic research and in the characterization of materials in manufacturing processes as well as monitoring and control routines.

Ultrasonic Broadband Spectrometry of Liquids A Research Tool in Pure and Applied Chemistry and Chemical Physics

This review is a survey of the many scientific applications of ultrasonic broadbandspectrometry (absorption and velocity measurements with coherent sound waves)in liquids and liquid systems, covering, at present, a frequency range from nearly10 kHz to 10 GHz. Ultrasonic spectrometry has proved to be an almost universalresearch tool in many laboratories, one that is useful for investigation of variouschemical, biochemical, and physicochemical systems. Sound waves traversingliquids induce periodic perturbations in pressure and temperature, which can shiftequilibria, resulting in characteristic sound absorption and velocity dispersionspectra. An analysis of such spectra yields valuable information about thermodynamic and kinetic parameters of the particular system that is often difficult toobtain by other methods. Since such periodic perturbations imposed on the systemare incremental in nearly all cases, the system can be studied under equilibriumconditions. All nonlinear effects (heating, nonconstant fluid compressibility, andothers) are negligible, permitting, for instance, the application of linearized rateequations. In this review, various examples of measured broadband spectra arepresented. Related elementary processes are discussed. Among these are ionicand molecular reactions, including mechanisms of association and complexation,proton transfer, solvation, isomerization, interconversion, side-group rotation,hydrogen-bonding, as well as stacking processes and micelle formation. Specialattention will be given to the extensive research on chemical relaxation.Fundamental early and recent publications are cited and discussed. Many referencesare included with particular emphasis on less well known research and publicationsfrom countries of the former USSR. This review aims at a demonstration of thewidespread applications of modern ultrasonic techniques in many fields ofliquid-state research.

Dielectric relaxation spectroscopy of liquids: frequency domain and time domain experimental methods

Starting from basic principles of dielectric relaxation measurements, an account is given on the present state of the frequency and time domain methods. The relative merits and limitations of the different techniques of measurement are discussed.

Cholesterol-Induced Variations in the Volume and Enthalpy Fluctuations of Lipid Bilayers

The sound velocity and density of suspensions of large unilamellar liposomes from dimyristoylphosphatidylcholine with admixed cholesterol have been measured as a function of temperature around the chain melting temperature of the phospholipid. The cholesterol-to-phospholipid molar ratio xc has been varied over a wide range (0 </= xc </= 0.5). The temperature dependence of the sound velocity number, of the apparent specific partial volume of the phospholipid, and of the apparent specific adiabatic compressibility have been derived from the measured data. These data are particularly discussed with respect to the volume fluctuations within the samples. A theoretical relation between the compressibility and the excess heat capacity of the bilayer system has been derived. Comparison of the compressibilities (and sound velocity numbers) with heat capacity traces display the close correlation between these quantities for bilayer systems. This correlation appears to be very useful as it allows some of the mechanical properties of membrane systems to be calculated from the specific heat capacity data and vice versa.