“…[47][48][49][50][51][52] The former assignment is based on an idea of Tvaraška and Bleha 53 and is an obvious assumption here because the ␦ term is only found for carbohydrates possessing an exocyclic-CH 2 OH group. 25,26 Additionally, the relaxation rate ␦ Ϫ1 is independent of the carbohydrate concentration, thus pointing at a unimolecular chemical equilibrium.…”
Acoustical absorption spectra between 10 kHz and 2 GHz are reported for various monosaccharides in water. With the exception of solutions of methyl--D-arabinopyranoside ͑0.5 mol/l͒ the spectra reveal absorption with relaxation characteristics in excess to the asymptotic high frequency absorption term. Up to three relaxation terms per spectrum emerge within the measuring frequency range. Regression analysis of the measured spectra in terms of a suitable analytical spectral function yields five relaxation regimes with relaxation times on the order of 1 s, 100 ns, 10 ns, 1 ns, or 100 ps, respectively. These relaxation regimes are assigned to the chair-chair ring inversion, two modes of pseudorotation, an exocyclic side group isomerization and a molecular association mechanism. Particular emphasis is given to the ring inversion which is additionally verified by time resolved measurements of nonequilibrium tautomer systems, utilizing the coupling of the inversion to the carbohydrate mutarotation. Further evidence is derived from measurements of solutions of D-fructose in mixtures of ethanol and water.
“…[47][48][49][50][51][52] The former assignment is based on an idea of Tvaraška and Bleha 53 and is an obvious assumption here because the ␦ term is only found for carbohydrates possessing an exocyclic-CH 2 OH group. 25,26 Additionally, the relaxation rate ␦ Ϫ1 is independent of the carbohydrate concentration, thus pointing at a unimolecular chemical equilibrium.…”
Acoustical absorption spectra between 10 kHz and 2 GHz are reported for various monosaccharides in water. With the exception of solutions of methyl--D-arabinopyranoside ͑0.5 mol/l͒ the spectra reveal absorption with relaxation characteristics in excess to the asymptotic high frequency absorption term. Up to three relaxation terms per spectrum emerge within the measuring frequency range. Regression analysis of the measured spectra in terms of a suitable analytical spectral function yields five relaxation regimes with relaxation times on the order of 1 s, 100 ns, 10 ns, 1 ns, or 100 ps, respectively. These relaxation regimes are assigned to the chair-chair ring inversion, two modes of pseudorotation, an exocyclic side group isomerization and a molecular association mechanism. Particular emphasis is given to the ring inversion which is additionally verified by time resolved measurements of nonequilibrium tautomer systems, utilizing the coupling of the inversion to the carbohydrate mutarotation. Further evidence is derived from measurements of solutions of D-fructose in mixtures of ethanol and water.
“…This situation, and also the need for the somewhat artificial l m parameter in the original Romanov−Solov'ev theory, has prompted us to rederive a theoretical model of concentration fluctuations and of their effects in the ultrasonic attenuation spectra of binary liquids. Here we present a comprehensive treatment, taking into account the merits of the previous theories. ,− It turned out that this model allows for a uniform description of all broadband ultrasonic spectra under investigation. − Applying the comprehensive model, we also found unexpected correlations between the tendency of an aqueous system to form a microinhomogeneous structure and the hydrophobic properties of the second constituent of the mixture. These correlations are also briefly discussed in this article.…”
Section: Introductionmentioning
confidence: 97%
“…In conformity with the assumption of a diffusive process, these data in part of the frequency range decrease rather proportionally to ω -1/2 , thus indicating that the cause of the sonic relaxation is not a stoichiometrically well defined chemical equilibrium but rather a concentration fluctuation mechanism. For this reason, additionally given in Figure is the graph of the term which is based on the prominent theory of noncritical concentration fluctuations, the Romanov−Solov'ev model. − Written in the format of eq 3, A RS is a frequency-independent amplitude and I RS is a scaling function. The integral I RS can be given in an explicit form: 1 Ultrasonic excess attenuation per wavelength, (αλ) exc , at 25 °C displayed as a function of frequency ν for a mixture of water and 2-(2-butoxyethoxy)ethanol (C 4 E 2 ) with mole fraction x = 0.04 of C 4 E 2 .…”
Section: Introductionmentioning
confidence: 99%
“…For this reason, additionally given in Figure 1 is the graph of the term R D (ν) ) which is based on the prominent theory of noncritical concentration fluctuations, the Romanov-Solov'ev model. [15][16][17][18] Written in the format of eq 3, A RS is a frequency-independent amplitude and I RS is a scaling function. The integral I RS can be given in an explicit form:…”
This work is based on the experimental evidence from ultrasonic absorption spectra, measured between about
100 kHz and 5 GHz for some series of liquid mixtures with water as a constituent. Since none of the existing
models of noncritical concentration fluctuations appropriately applies for the shape of the existing set of
spectra, a comprehensive theoretical model has been derived which includes the favorable characteristics of
previous theories. In this model, fluctuations in concentration are assumed to equalize not only by diffusion
but, in parallel, also by a specific rate process. In addition, allowance is made for spatial correlations in the
concentration fluctuations. Discussion of the measured ultrasonic spectra in terms of the comprehensive model
evidences the effect from the hydrophobic properties of the nonaqueous constituent of the mixtures upon the
microheterogeneous structure of the liquid.
“…Ultrasonic Absorption Spectrometry. Acoustical absorption spectra of liquids are usually described by two terms where B ‘= ( B / c s ) represents the frequency independent part in the total absorption coefficient α and α exc denotes the excess contributions that may be due to chemical relaxations, concentration fluctuations ,,, as well as dissipation mechanisms related to scattering . Here, ν denotes the frequency and c s the sound velocity of the liquid.…”
At 25 °C the ultrasonic absorption coefficient of aqueous
solutions of triethylenediamine (TED) has been
measured as a function of frequency ν (200 kHz ≤ ν ≤ 4.6 GHz) and
solute concentration c (0.0025 mol/L
≤ c ≤ 2.56 mol/L). One solution has also been studied
at 15 °C and 35 °C. To look for the dependence
upon the pH some spectra have been recorded with NaOH or HCl added.
The measured spectra have been
analyzed in terms of a model relaxation function containing a Debye
relaxation term and a term that represents
noncritical fluctuations in the concentration. The Debye term is
related to the protolysis of the TED molecule.
Depending on the pH either the first or the second step of
protonization of TED is reflected by the relaxation
process. The forward and reverse reaction rates and the isentropic
reaction volume have been determined
from the relaxation parameters and it has been found that the amplitude
and relaxation time of the Debye
term can be described by one set of activity coefficients for the first
protolysis reaction. The noncritical
concentration fluctuations are analytically represented by an extended
version of the Romanov−Solov'ev
theory that also considers spatial correlations. At c
> 0.5 mol/L a correlation length of about 4 Å is
found.
The finding of TED association at an elevated solute content is
consistent with previous results from dielectric
spectrometry.
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