R. Pottel

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.

The dielectric permittivity spectrum of aqueous colloidal phospholipid solutions between 1 kHz and 60 GHz.

The dielectric permittivity spectrum between 1 kHz and 60 GHz of aqueous colloidal solutions of predominantly zwitterionic phospholipids is presented from results of previous and recent measurements. It shows three dispersion/loss regions around 22 GHz. 80 MHz and below 40 MHz (30 degrees C) which are attributed to rotational diffusion of the water molecules and of the zwitterionic phosphorylcholine groups, and to limited translational diffusion of ionic lipid molecules and/or its counterions. respectively. Merely a few mole percent of ionic lipids cause comparatively large dielectric dispersion. Ignoring the fact that such impurities may be present in zwitterionic phospholipid compounds, which have not been especially purified, this has led to misinterpretation of the dielectric spectrum in the past. An approximate quantitative description of the measured spectra is given for vesicle solutions with only very small additional low-molecular-weight salt content. It reproduces the sensitive dependence of the ionic lipid-induced dielectric dispersion (step height and frequency) on various parameters: phospholipid vesicle size. ionic lipid content, as well as the self-diffusion coefficient of the ionic lipid molecules and of its counterions, moving within the phospholipid bilayers or on their surface, respectively.

Dielectric relaxation of hydrogen bonded liquids: Mixtures of monohydric alcohols with n-alkanes

The complex dielectric spectra for mixtures of unbranched monohydric alcohols with n-alkanes have been measured between 1 MHz and 18 GHz at various mole fractions xCnOH of the alcohols (0.2⩽xCnOH⩽1). Within the frequency range of measurements the spectra exhibit two dispersion/loss regions which can be analytically well represented by a Davidson–Cole-type and a Debye-type relaxation term, respectively. The relaxation parameters are discussed in terms of a wait-and-switch model of relaxation, considering also the effect of the nonpolar hydrocarbon groups in the enthalpy of activation of the dielectric relaxation process. This model allows for a consistent description of the dielectric properties of the alcohol/n-alkane mixtures which, depending on the relative length of the hydrocarbon chains, divide into two groups. Structural features of the alcohol/n-alkane mixtures are proposed to account for the characteristic behavior of both groups of liquids as a function of alcohol content.

Ultrasonic absorption and sound velocity of dimethyl sulfoxide/water mixtures in the complete composition range

Abstract At 25°C the ultrasonic absorption coefficient and the sound velocity of dimethyl sulfoxide/water mixtures have been measured for various mole fractions between 0 and 1. The measurements of the absorption coefficient have been performed as a function of frequency between 200 kHz and 2.75 GHz. The sound velocity has been determined by three methods at different frequencies of the above range. At given mole fraction the absorption per wavelength increases linearly with frequency and the sound velocity is constant. Thus no relaxation process is found which could be attributed to the suggested formation of stoichiometrically well-defined dimethyl sulfoxide/water complexes. The volume viscosity and the adiabatic compressibility are derived from the measured quantities. The dependence upon the mixture composition is discussed together with other ones to show that there are special mixture properties at mole fractions around 30 % dimethyl sulfoxide. The mixtures, however, seem to be homogeneous hydrogen-bonded networks rather than being composed of definite molecular complexes.

Dielectric relaxation and molecular motions in C14-lecithin-water systems

Abstract The dependence of the complex permittivity on the frequency has been measured between 105 and 6 × 1010 Hz for aqueous solutions of dimyristoylphosphatidylcholine at several temperatures around the crystalline/liquid-crystalline phase transition temperature of the samples. To the observed data is fitted a sum of Cole-Cole functions and also a model relaxation function to yield various relaxation parameters. The variation of these parameters with temperature is discussed. A noteworthy result is that there exists a pronounced cooperativity effect in the diffusive motions of the phosphorylcholine groups at the bilayer surface and that the mobility of the cationic trimethylammonium head group is dramatically smaller than with lysolecithin micelles in aqueous solutions. As another remarkable result the hydration water relaxation time appears to be distinctly smaller than the reorientation time of the molecules in the pure solvent at the same temperature.

A new automated waveguide system for the precise measurement of complex permittivity of low-to-high-loss liquids at microwave frequencies

An automated measuring system is described which enables the complex microwave permittivity of liquids to be determined at particular frequencies. A special cell design offers the possibility of precisely measuring a range of liquids from low to high loss. Constructional details of the sample cell are presented together with a system diagram of the microwave apparatus and a description of the analysis of primary data. Possible experimental errors are considered and results for measurements in the XN- and Ku-waveguide bands (5.3-8.4 and 12.5-18 GHz) are discussed.

On a hydration model utilized in the discussion of dielectric spectra of aqueous solutions

Abstract A solution model is discussed which allows the microwave part of the permittivity spectrum of aqueous solutions to be related to characteristics of the hydration water. The parameters, which can be derived from measured dielectric spectra thereby are the hydration water relaxation time, the number of hydration water molecules per molecule of solute, the static orientational polarizability of the hydration water, and a quantity, which refers to the distribution of hydration water relaxation times. The (continuum) model, appropriate for solutions of (nearly) spherically shaped solute particles, has regard to internal electric fields resulting from polarization charges at interfaces. Possible errors in the parameter values are indicated, which may arise if the internal fields are only incompletely taken into account. Previously measured spectra for a series of aqueous solutions of 1,4-diazabicyclo[2,2,2]octane have been evaluated on the basis of the present model. The results for these (favourable) solutions are presented to show, that the found dependence of the parameter values on solute concentration is consistent with the idea of the proposed hydration model.

Dielectric studies on water in solutions of purified lecithin vesicles

Abstract The complex permittivity of sonicated aqueous solutions of purified dimyristoylphosphatidylcholine has been measured as a function of frequency between 3 kHz and 40 GHz. The dielectric spectrum of the samples shows two dispersion/absorption regions, one centered at about 80 MHz the other at about 20.GHz (30°C). Otherwise than in previous studies no additional dispersion/absorption process has been found at frequencies below 10 MHz. The complex dielectric spectrum of the samples is discussed with respect to the dynamical state of solvent water in solutions of single-bilayer vesicles. The main relaxation time of the solvent water, τ 1 ((2 πτ 1 ) −1 ≈ 20 GHz), is smaller than that of pure water, τ W , at the same temperature. This effect results from the action of internal depolarizing fields which obviously overcompensate and enhancement of τ 1 due to specific solute/solvent interactions (hydration) as had been previously found with micellar solutions of lysolecithins. It cannot be excluded, that some solvent water shows unusual dynamical behaviour. If there exists a substantial amount of such motionally perturbed water, however, it must be characterized by a relaxation time close to that of the phosphorylcholine zwitterions, τ 2 ((2 πτ 2 ) −1 ≈ 80 MHz).

A Comparison of Dielectric Measurement Methods for Liquids in the Frequency Range 1 GHz to 4 THz

From measurements on hyperpure and commercially pure low- to high-loss liquids over a wide spectral range and using a variety of experimental equipment, we have been able for the first time to quantify both the systematic and the random uncertainties to which extra-high-frequency dielectric measurements are subject. From our measurements it has become possible to specify certain liquids and solids as standard reference materials for the calibration of high-frequency dielectric measuring equipment.

Dielectric properties of organic solute/water mixtures. Hydrophobic hydration and relaxation: In honour of Professor Hermann Gerhard Hertz

Abstract Results from measurements of the dielectric spectrum of organic solute/water mixtures are summarized. A survey of hydrophobic hydration effects as emerging from the experimental data is given. Recent ideas of the mechanism of dielectric relaxation in hydrogen bonded liquids are discussed. Comparison is made with some aspects resulting from computer simulation studies.