Browsing by Author "Behrends, Ralph"

A method for measuring the complex shear viscosity spectra of low viscosity liquids in the frequency range 6-120 MHz is described. Details of the resonator cell, based on a thickness shear (AT-cut) quartz resonator as central device, and of the RF bridge circuit for the compensation of the unloaded capacitance of the resonator are presented. A measuring and data-evaluation procedure that allows the careful consideration of spurious higher order vibration modes and also of intrinsic cell losses is discussed. Sources of experimental error are shown and the usability of the method is illustrated by reference measurements. The complex viscosity spectrum of n-dodecanol is displayed and its relaxation frequency is compared with the value resulting from broad-band acoustical attenuation spectrometry.

In this tutorial article methods of acoustic relaxation measurements of liquids are reviewed. The present performance of various techniques is described and desirable trends in future developments are shown. An account is given on standards in the optical monitoring of acoustic waves (Brillouin scattering), on time domain (jump) techniques, as well as on frequency domain (continuous and pulse modulated ultrasonic wave) methods. The merits and limitations of the different techniques of measurement, covering the frequency range from 10 kHz up to nearly 10 GHz now, corresponding with relaxation times between about 0.1 ms and 100 ps, are discussed. Efforts to further enhance the experimental accuracy and to reduce the required sample volume are indicated. (C) 2001 Elsevier Science B.V. All rights reserved.

The density, sound velocity, and ultrasonic attenuation coefficient of aqueous solutions of urea were measured at temperatures between 10 degreesC and 35 degreesC and at concentrations ranging from 0 to 9 mol/L (equivalent to21.7 mol %) urea. The data are correlated using simple empirical expressions, and the isentropic compressibilities are calculated. These urea solutions provide a favorable set of reference data for acoustical measurements of liquids with sound velocity between 1500 m(.)s(-1) and 1700 m(.)s(-1).

Broadband acoustical spectrometry data for three types of unimolecular reactions in liquids are discussed: outer sphere complex - contact ion pair isomerization, monosaccharide chair-chair conformational changes, and propyl formate structural isomerization. The temperature dependent relaxation times and amplitudes are discussed in terms of a thermally activated relaxation scheme to yield the thermodynamic and kinetic parameters of the elementary chemical equilibrium. Limitations in the determination of these parameters are indicated, (C) 2003 Elsevier B.V. All rights reserved.

Broadband acoustical spectrometry (10 kHz less than or equal to v less than or equal to 5 GHz) of aqueous solutions of monosaccharides at thermal equilibrium are reported as well as time-resolved acoustical attenuation and optical activity measurements of saccharide solutions during approach of their tautomer equilibrium from nonequilibrium concentrations. The existence of the chair-chair conformational flexibility is shown. Its presence depends on the anomeric form, reflecting the balance of axial and equatorial hydroxy groups of the saccharide ring, as predicted theoretically. For the widely used D-fructose, the time constant of the anomer equilibrium (0.05 min(-1), 20 degreesC) has been determined acoustically. Relaxation time and activation enthalpy of the ring conformational isomerization are 1 x 10(-6)s and 42 kJ/mol, respectively, at 25 degreesC.

Broadband ultrasonic attenuation spectra (100 kHz to 2 GHz) of aqueous solutions of vesicles from 1,2-dimyristoyl-L-3-phosphatidylcholine, with different amounts of cholesterol admixed, have been measured at temperatures between 20 and 28 degrees C. The spectra have been evaluated in terms of suitable relaxation functions. They are discussed in view of the effect of cholesterol on the membrane behavior around the gel-fluid phase transition temperature T-m. In addition to a frequency-independent asymptotic high-frequency term, all spectra reveal a critical term and a Debye-type relaxation term with relaxation time around 0.5 ns. The former is evaluated in the light of the Bhattacharjee-Ferrell dynamic scaling theory. It is,assigned to the critical domain structure fluctuations of the membranes. Critical slowing of fluctuations is demonstrated Also shown are relations of the critical amplitudes to thermodynamic parameters. The Debye term reflects the rotational isomerization of the phospholipid alkyl chains. The relaxation time of isomerization reveals a significant steplike change at T-m. At moderate cholesterol content an additional Debye relaxation term exists. It is assigned to the axial diffusion of the membrane molecules. Because it likewise shows effects of slowing near T-m, the diffusion appears to be coupled to the domain structure fluctuations. A further relaxation term at small cholesterol concentration is assumed to be due to small-range shape fluctuations of vesicles near the phase transition temperature.

The specific heat C-p at constant pressure, the shear viscosity eta(s), and the mutual diffusion coefficient D of the 2,6-dimethylpyridine-water mixture of critical composition have been measured in the homogeneous phase at various temperatures near the lower critical demixing temperature T-c. The amplitude of the fluctuation correlation length xi(0)=(0.198 +/- 0.004) nm has been derived from a combined evaluation of the eta(s) and D data. This value is in reasonable agreement with the one obtained from the amplitude A(+)=(0.26 +/- 0.01) J/(g K) of the critical term in the specific heat, using the two-scale-factor universality relation. Within the limits of error the relaxation rate Gamma of order parameter fluctuations follows power law with the theoretical universal exponent and with the amplitude Gamma=(25 +/- 1)x10(9) s(-1). No indications of interferences of the critical fluctuations with other elementary chemical reactions have been found. A noteworthy result is the agreement of the background viscosity eta(b), resulting from the treatment of eta(s) and D data, with the viscosity eta(s)(nu=0) extrapolated from high-frequency viscosity data. The latter have been measured in the frequency range of 5-130 MHz using a novel shear impedance spectrometer.

Ultrasonic attenuation spectra, the shear viscosity, and the mutual diffusion coefficient of the n-pentanol-nitromethane mixture of critical composition have been measured at different temperatures near the critical temperature. The noncritical background contribution, proportional to frequency, to the acoustical attenuation-per-wavelength spectra has been determined and subtracted from the total attenuation to yield the critical contribution. When plotted versus the reduced frequency, with the relaxation rate of order-parameter fluctuations from the shear viscosity and diffusion coefficient measurements, the critical part in the sonic attenuation coefficient displays a scaling function which nicely fits to the data for the critical system 3-methylpentane-nitromethane and also to the empirical scaling function of the Bhattacharjee-Ferrell dynamic scaling theory. The scaled half-attenuation frequency follows from the experimental data as Omega(1/2)(emp)=1.8+/-0.1. The relaxation rate of order-parameter fluctuation shows power-law behavior with the theoretically predicted universal exponent and the extraordinary high amplitude Gamma(o)=(187+/-2)x10(9) s(-1). The amount of the adiabatic coupling constant \g\=0.03, as estimated from the amplitude of the critical contribution to the acoustical spectra, is unusually small. (C) 2004 American Institute of Physics.

Using the equal volume criterion and also the pseudospinodal conception the critical demixing point of the triethylene glycol monoheptyl ether/water system (C7E3/H2O) has been determined as Y-crit=0.1 and T-crit=296.46 K (Y, mass fraction of surfactant). From density measurements the critical micelle concentration (cmc) followed as Y-cmc=0.007 at 288.15 K and Y-cmc=0.0066 at 298.15 K. The (static) shear viscosity eta(s) and the mutual diffusion coefficient D of the C7E3/H2O mixture of critical composition have been evaluated to yield their singular and background parts. From a combined treatment of both quantities the relaxation rate Gamma of order parameter fluctuations has been derived. Gamma follows power law with universal critical exponent and amplitude Gamma(0)=3.1x10(9) s(-1). Broadband ultrasonic spectra of C7E3/H2O mixtures exhibit a noncritical relaxation, reflecting the monomer exchange between micelles and the suspending phase, and a critical term due to concentration fluctuations. The former is subject to a relaxation time distribution that broadens when approaching the critical temperature. The latter can be well represented with the aid of the dynamic scaling model by Bhattacharjee and Ferrell (BF) [Phys. Rev. A. 31, 1788 (1985)]. The half-attenuation frequency in the scaling function of the latter model is noticeably smaller (Omega(BF)(1/2)approximate to 1) than the theoretically predicted value Omega(BF)(1/2)=2.1. This result has been taken as an indication of a coupling between the fluctuations in the local concentration and the kinetics of micelle formation, in correspondence with the idea of a fluctuation controlled monomer exchange [T. Telgmann and U. Kaatze, Langmuir 18, 3068 (2002)]. (c) 2006 American Institute of Physics.

Ultrasonic attenuation spectra (100 kHz to 2 GHz) and complex dielectric spectra (300 kHz to 40 GHz) of aqueous solutions of 1,2-dimyristoyl-L-3-phosphatidylcholine vesicles are reported and are discussed in view of their behavior near the main phase transition of the lipid. The relaxation terms in the spectra are assigned to the domain structure fluctuations of the membranes, the structural isomerization of alkyl chains, the axial diffusion of lipid molecules within the membrane, and the reorientational motions of the zwitterionic phospholipid headgroups. The relaxation times of the alkyl chain isomerization and of the headgroup motions on the bilayer surfaces show a steplike change at the transition temperature, T-m. The axial diffusion and the domain fluctuations exhibit substantial effects of slowing near T-m as characteristic for critically demixing liquids at their consolute point.

At six temperatures T between 10 and 50 degrees C and at mole fractions x(g) of glycerol (0 < x(g)<= 0.9) the complex (electric) permittivity epsilon(nu) of glycerol/water mixtures has been measured as a function of frequency nu between 1 MHz and 40 GHz. The spectra of the glycerol/water mixtures can be well represented by a Davidson-Cole [J. Chem. Phys. 18, 1417 (1950)] relaxation function that reveals an unsymmetric relaxation time distribution. The effective dipole orientation correlation factor derived from the static permittivity displays an unspectacular behavior upon mixture composition. The dielectric relaxation time reveals a simple relation to the shear viscosity of the mixtures, but both quantities are not proportional to one another. The relaxation times at high temperatures nicely complement previously determined low temperature data, following a Vogel-Fulcher-Tammann-Hesse [Z. Phys. 22, 645 (1925); J. Am. Chem. Ceram. Soc. 8, 339 (1923); Z. Anorg. Allg. Chem. 156, 245 (1926)] (VFTH) temperature dependence. When the Eyring behavior is assumed a limiting high temperature form of the VFTH relation, enthalpy, and entropy of activation values are found which adopt significantly higher values in the glycerol rich mixtures than in the water rich liquids. The relaxation time distribution parameter at high water content indicates a dynamically heterogeneous structure of the liquids. Likely there exist glycerol rich and water rich microphases. (c) 2006 American Institute of Physics.

The water content is of considerable significance for the physical and chemical properties of many materials. The correct knowledge of the water content is therefore important in many areas of human activities, particularly in agriculture and food production, as well as in numerous branches of industry, Largely independent of special features of a sample, electromagnetic wave interactions with the dipolar water enable the non-destructive and, in most cases, non-invasive determination of the water content. In order to reach an adequate accuracy in the measurements the probes have to be carefully designed, matched to the systems under study. For this reason knowledge of the dielectric spectra of the aqueous phase is a prerequisite. In this article examples of contributions to the spectra are presented in order to show effects originating from relevant parameters like frequency, temperature, and pressure, and also from various solutes.

Acoustical attenuation spectrometry, dynamic light scattering, shear viscosity, density, and heat capacity measurements of the methanol/n-hexane mixture of critical composition have been performed. The critical part in the sonic attenuation coefficients nicely fits to the empirical scaling function of the Bhattacharjee-Ferrell [Phys. Rev. A 24, 1643 (1981)] dynamic scaling model if the theoretically predicted scaled half-attenuation frequency Omega(BF)(1/2)=2.1 is used. The relaxation rates of order parameter fluctuations, as resulting from the acoustical spectra, within the limits of experimental error agree with those from a combined evaluation of the light scattering and shear viscosity measurements. Both series of data display power law with amplitude Gamma(0)=44x10(9) s(-1). The amplitude of the fluctuation correlation length follows as xi(0)=0.33 nm from the light scattering data and as xi(0)=0.32 nm from the amplitude of the singular part of the heat capacity if the two-scale factor universality relation is used. The adiabatic coupling constant g=0.11 results from the amplitude of the critical contribution to the acoustical spectrum near the critical point, in conformity with g=0.12 as following from the variation of the critical temperature with pressure along the critical line and the thermal expansion coefficient. (c) 2006 American Institute of Physics.

A thickness shear quartz resonator technique is described to measure the shear viscosity of low-viscosity liquids in the frequency range from 6 MHz to 130 MHz. Examples of shear-viscosity spectra in that frequency range are presented to show that various molecular processes are accompanied by shear-viscosity relaxation. Among these processes are conformational variations of alkyl chains, with relaxation times tau(eta) of about 0.3 ns for n-pentadecane and n-hexadecane at 25 degrees C. These variations can be well represented in terms of a torsional oscillator model. Also featured briefly are shear-viscosity relaxations associated with fluctuations of hydrogen-bonded clusters in alcohols, for which tau(eta) values between 0.3 ns (n-hexanol) and 1.5 ns (n-dodecanol) have been found at 25 degrees C. In addition, the special suitability of high-frequency shear-viscosity spectroscopy to the study of critically demixing mixtures is demonstrated by some illustrative examples. Due to slowing, critical fluctuations do not contribute to the shear viscosity at sufficiently high frequencies of measurements so that the non-critical background viscosity eta(bg) of critical systems can be directly determined from high-frequency shear-viscosity spectroscopy. Relaxations in eta(bg) appear also in the shear-viscosity spectra with, for example, tau(eta) approximate to 2 ns for the critical triethylamine-water binary mixture at temperatures between 10 degrees C and 18 degrees C. Such relaxations noticeably influence the relaxation rate of order parameter fluctuations. They may be also the reason for the need of a special mesoscopic viscosity when mutual diffusion coefficients of critical polymer solutions are discussed in terms of mode-coupling theory.

The relaxation behaviour of 1-hexanol/n-heptane and 1-dodecanol/n-tetradecane mixtures has been studied at some compositions using ultrasonic attenuation spectrometry in the frequency range 0.4-3000 MHz. All mixtures reveal a relaxation term due to hydrogen network fluctuations. It is discussed in conjunction with the principle dielectric relaxation of alcohol/alkane mixtures, indicating a dynamically micro-heterogeneous liquid structure. The spectra of the long-chain alcohol system display an additional relaxation due to the structural isomerisation of the hydrocarbon chains. In terms of a torsional oscillator model this relaxation reveals the effect of molecular ordering on the enthalpy of activation. (C) 2011 Elsevier B. V. All rights reserved.

The acoustical absorption spectra between 300 kHz and 3 GHz and the complex shear viscosity spectra between 6 and 120 MHz for some monohydric alcohols are reported. The acoustical spectra exhibit two relaxation regions, one located at frequencies around some hundred MHz and the other one around some GHz. The shear viscosity spectra reveal a relaxation process in conformity with the low-frequency acoustical relaxation. This relaxation is assigned to fluctuations in the structure of hydrogen bonded alcohol clusters. The high-frequency acoustical relaxation is discussed in terms of a damped torsional oscillator model of allkyl chain isomerization, corresponding with the rotational isomerization of n-alkanes. The high frequency (>5 GHz) shear viscosity of the alcohols is estimated and found in the same order (1 mPa s) as that for n-alkanes.

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 wait-and-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 tinder 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. (C) 2002 Elsevier Science B.V. All rights reserved.

Ultrasonic absorption spectra between 800 kHz and 2 GHz and sound velocities of aqueous solutions of calcium nitrate have been measured at different salt concentrations and temperatures as well as solutions of calcium chloride and magnesium nitrate at 25 degreesC. All solutions reveal a relaxation region with discrete relaxation time between 200 and 500 ps (nitrates, 25degrees) or at 100 ps (chloride, 25degrees). The amplitude of the relaxation term is substantially larger with the calcium nitrate system than with the other electrolyte solutions with idential salt concentration c. The relaxation time of Ca(NO3)(2) solutions is independent of concentration and the relaxation amplitude increases linearly with c, indicating a unimolecular reaction. The high-frequency ultrasonic relaxation is thus assumed to be due to the equilibrium between an outersphere ion complex and a contact ion pair.

For solutions of four saccharides in water with alkaline-earth chlorides added ultrasonic attenuation spectra between 100 kHz and 2 GHz are reported and compared to those for carbohydrate solutions without salt. Calcium chloride does not alter the relaxation times in the spectra of D-glucose and D(+)-maltose solutions, reflecting the exocyclic hydroxymethyl group rotation, a saccharide-saccharide association, and, with the disaccharide, also motions of both rings of a molecule relative to one another. The spectra of D-xylose and D-fructose solutions are substantially changed by the salts. With both saccharides an additional term with relaxation time around some nanoseconds exists which is assigned to a rearrangement of a carbohydrate-cation complex. Other relaxation terms of these saccharide solutions are also subject to noticeable changes by the salt, indicating specific carbohydrate-cation interactions. The ultrasonic spectra show that such interactions may exist also with carbohydrates which do not display the particular hydroxyl group sequences that are considered to promote complexation with cations. (C) 2004 American Institute of Physics.

Acoustical attenuation spectra between 10 kHz and 2 GHz, complex dielectric spectra between 300 kHz and 40 GHz, and time-resolved non-equilibrium measurements are reported for aqueous solutions of various mono- and disaccharides with and without 2:1 valent salts. The spectra reveal a variety of relaxation regimes with relaxation times between 1 mu s and 10 ps. In addition, the time-resolved observations enable the study of the mutarotation with relaxation times on the order of 10(3) s. Variation of the concentration and temperature as well as a careful choice of the saccharides allow a discussion of the relaxation processes in terms of a chair-chair ring inversion, two modes of pseudorotation, an exocyclic hydroxymethyl group rotation, a carbohydrate-carbohydrate association, and, in the disaccharide solutions, a rotation of the rings relative to another. Salt-containing solutions show also relaxation phenomena reflecting different steps cation-carbohydrate association and variations in the carbohydrate conformational isomerizations and associations due to interactions with cations.