Non-critical Fluctuations of Liquids: Cinderella of Ultrasonic Spectroscopy?

Broadband ultrasonic spectra (100 kHz to 4.6 GHz) of aqueous solutions of organic solutes are discussed in the light of non-critical fluctuations of local concentrations. A unifying model, combining aspects of previous theories by Romanov and Solov'ev, Montrose and Litovitz, and Endo, is delineated and results from a description of spectra in terms of that model are discussed briefly. Special attention is given to aqueous solutions of some series of solutes with varying lengths and differing steric arrangements of hydrophobic groups. It is shown that, within the series of solutes with unbranched alkyl groups, the length of alkyl groups controls the correlation length of fluctuations, whereas the nature of the hydrophilic group is of minor importance in this context. Solutes with distributed hydrophobic groups reveal smaller correlation lengths. Also presented is a recent thermodynamic theory of fluctuations in surfactant solutions below the critical micelle concentration (cmc). Application of that theory shows that the predicted ultrasonic spectral function can be fitted as well to experimental spectra as that of the unifying model. Some features of the thermodynamic theory are outlined, among them the finding that the correlation length is predicted to display its maximum at a concentration $$c_{\mathrm{c}}$$ below the cmc. The concentration difference, $${\text {cmc}}-c_{\mathrm{c}}$$ , is related to parameters of the free energy of the system. [ABSTRACT FROM AUTHOR]