Sound attenuation near the demixing point of binary liquids: interplay of critical dynamics and noncritical kinetics

Abstract

The nature and origin of sound attenuation due to critical fluctuations near the liquid consolute point are discussed. Starting from basic principles, the background of critical phenomena is reviewed and the conceptions of theoretical approaches to describe the critical contributions to the propagation of sound are analysed. Experimental broadband spectra of suitable binary systems are evaluated jointly with results from quasi-elastic light scattering, shear viscosity and heat capacity measurements to verify or disprove theoretical predictions. It is shown that spectra of systems without or with only small-amplitude ultrasonic contribution from noncritical relaxation processes can be represented by theory with the asymptotic high-frequency sonic attenuation coefficient as a simple adjustable parameter. As a result, sonic spectra of more complex systems, exhibiting significant contributions from noncritical ultrasonic relaxations, are discussed assuming the critical part to be known from theory and auxiliary data. This modus operandi allows for a clear extraction of parameters relevant to the noncritical elementary processes in liquid mixtures, such as conformational changes, protolysis and hydrolysis reactions, monomer exchange from micelles and rotational isomerizations of membrane molecules. The influence of the critical dynamics on the noncritical kinetics is disclosed for some topical examples. Adiabatic couplingconstant 25 4. Mixtures featuring noncritical relaxation phenomena 26 4.1. Elementary reactions 26 4.2. Micelle formation 29 4.3. Membrane domain structure fluctuations and axial diffusion 31 5. Conclusions and perspectives 33 Acknowledgments 34 References 34

Finite-size scaling study of dynamic critical phenomena in a vapor-liquid transition

“…These numbers are obtained via the dynamic renormalization group and mode-coupling theoretical calculations and found to be in agreement with experiments [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. Like the static case, the dynamic exponents are also not all independent of each other, they follow certain scaling relations.…”

“…Here note that the theoretical numbers for x ζ for vapor-liquid and liquidliquid transitions are slightly different [23,24]. This difference is within the error bars of computation via molecular dynamics.…”

Does the Viscosity Exponent Derive from Ultrasonic Attenuation Spectra?

“…The latter binary system had indeed been measured down to 180 kHz only. In our analysis, however, the adverse effect of a higher low-frequency limit of the sonic attenuation data will be partly compensated by the extremely small amplitudes [3] The low-frequency sonic attenuation data of the nitrobenzene-n-hexane critical mixture had been measured with the aid of a statistical reverberation method [22], for which uncertainties α/α = 0.1(0.05 MHz ≤ ν ≤ 0.1 MHz) and α/α = 0.07(0.1 MHz ≤ ν ≤ 1 MHz) had been given [30]. The low-frequency n-pentanolnitromethane data were obtained from cavity resonator measurements [24], using a plano-concave cylindrical cell to reduce disturbances from mechanical stress at varying temperature [31].…”

“…In the frequency-normalized format, the α/ν 2 data decrease monotonically with frequency ν to approach a limiting high-frequency value B . Obviously, however, this decrease in α/ν 2 cannot be adequately represented by a Bhattacharjee-Ferrell relaxation function ("BF" [3,32]),…”

“…Effectively masking the individual interactions of the fluids, critical fluctuations induce universal characteristics in thermodynamic and transport properties. Universality, involving scaling and power laws, has attracted much attention in the past decades and is still today a topic of lively scientific debate [1][2][3].…”