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