77 research outputs found
A high frequency shear wave impedance spectrometer for low viscosity liquids
No full textA 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
Hydrogen bond fluctuations and dispersive interactions of alcohol/alkane mixtures. An ultrasonic relaxation study
No full textThe 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
Dielectric properties of water and aqueous solutions
No full textThe 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
Shear viscosity relaxation of a critical binary liquid
No full textTwo series of diffusion coefficients D are reported for the triethylamine-water binary critical mixture. One has been obtained from quasielastic light scattering measurements, the other one has been derived from broadband ultrasonic spectra, yielding the relaxation rate of order parameter fluctuations, and shear viscosity data. Using high frequency shear impedance spectrometry in the range 20-130 MHz, relaxations in the background part of the viscosity, resulting in viscoelastic mixture properties, have been found. Both series of D data agree either if a half-attenuation frequency distinctly smaller than the theoretical value Omega(1/2)=2.1 is used in the Bhattacharjee-Ferrell scaling function or if the viscosity extrapolated from the shear impedance measurements to low frequencies is applied to the Kawasaki-Ferrell relation. This extrapolated viscosity is smaller than the static shear viscosity measured with capillary viscosimeters
Molecular dynamics and conformational kinetics of mono- and disaccharides in aqueous solution
No full textAcoustical 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
High-Frequency Shear Viscosity of Low-Viscosity Liquids
No full textA 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
Structural isomerization and molecular motions of liquid n-alkanes. Ultrasonic and high-frequency shear viscosity relaxation
No full textBetween 20 and 120 MHz, the complex shear viscosity of n-tetradecane, n-pentadecane, n-hexadecane, and a n-eicosane/n-tetradecane mixture has been measured using a shear impedance spectrometer. The data are compared to ultrasonic absorption spectra of the liquids as measured between 1 MHz and 2 GHz. Relaxational behavior has been found. The extrapolated high-frequency shear viscosity eta(s)(infinity) is distinctly smaller than the static shear viscosity eta(s)(0). The shear viscosity relaxation time is discussed in light of literature data of the orientational correlation time from depolarized Rayleigh scattering and also of the collision time as resulting from dielectric spectrometry. The assumption appears to be likely that the shear viscosity relaxation is due to rotational isomerizations of the chain molecules. A damped torsional oscillator model has been used to evaluate the spectra. The reasonable number of three carbon atoms per oscillator unit results. The activation energies and enthalpies are derived from measurements at different temperatures
Hydrogen bonding and chain conformational isomerization of alcohols probed by ultrasonic absorption and shear impedance spectrometry
No full textThe 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
Ultrasonic and dielectric study of nonequilibrium monosaccharide solutions in water
No full textBased on broadband acoustical (10 kHzless than or equal tonuless than or equal to2 GHz) and dielectric (1 MHzless than or equal tonuless than or equal to40 GHz) spectrometry, time-resolved ultrasonic attenuation coefficient and static permittivity measurements have been performed on nonequilibrium tautomer solutions of D-arabinose and D-fructose in water. Via the chair-chair ring inversion the ultrasonic attenuation measurements display the decrease in the content of beta-arabinopyranoside and the increase of the alpha-fructopyranoside concentration during the establishment of the tautomer equilibrium. For the arabinose solutions, the mutarotation decay constant (m=(0.027 +/- 0.004) min(-1), 20 degreesC) from the ultrasonic measurements almost agrees with that from optical activity observations. For D-fructose the ultrasonic decay constant (m=(0.043 +/- 0.007) min(-1), 20 degreesC) is smaller than that from rotary polarization (m = 0.054 min(-1), 20 degreesC) and dielectric permittivity (m=(0.058 +/- 0.007) min(-1), 20 degreesC), likely because the latter methods probe parallel pathways in the tautomer equilibrium whereas the former one reflects only one pathway. (C) 2004 Elsevier B.V. All rights reserved
Acoustical spectrometry of unimolecular reactions in liquids
No full textBroadband 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
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