Ultrasonic relaxation properties of alcohol-water mixtures and related systems.

The excess ultrasonic absorption exhibited by alcohol - water mixtures reflects, and in principle leads to an understanding of, the interaction of water with hydrophobic groups. The structure of water and the properties of hydrophobic interactions are there fore briefly reviewed, followed by a more extended discussion of relevant properties of alcohol - water mixtures. The apparatus, technique of operation and analysis of results are described. The interaction of sound with binary liquid mixtures is discussed in terms of (a) perturbing stoichiometric chemical equilibria, and (b) interacting with non-stoichiometric concentrations, and a survey is made of previous applications of these ideas to suvh mixtures. The water-rich, plateau', region defined by ultrasonic absorption, over which absorption is effectively independent of alcohol concentration, is investigated for a number of alcohol - water systems The extent of this region is correlated with the stereochemistry of the alcohol and with a volume limitation by water. The excess absorption is linked with incipient phase-separation, using as probe a partially-miscible ternary system. Analysis of the frequency-dependence of absorption in each alcohol - water system produces two relaxation frequencies, each effectively independent of alcohol and each exhibiting a minimum near the highly sound-absorbing composition. These features are consistent with the interpretation that the process perturbed is a hydrophobic association of alcohol, rate-controlled by water. Concentration-fluctuation theories of frequency-dependence could be applied satisfactorily only to the partially-miscible ternary system mentioned above. 1,3-Dioxolane-water mixtures produce excess ultrasonic absorption, with features reminiscent both of alcohol- and acetonitrilc-water mixtures. Urea-water mixtures appear to show a low-frequency relaxation process. Amide- water mixtures, including urea-water mixtures, possess minima in sound absorption as a function of composition. Alkyl-substited amides also exhibit maxima, the intensities of which reflect the size of the alkyl substituent, as with alcohol-water mixtures. Investigation of alcohol-acetonitrile-water ternary systems shows that the processes which produce sound absorption in alcohol-water and acetonitrile-water mixtures are dissimilar, and incompatible in the ternary system. Finally, relative partial molar volumes in a number alcohol-water systems have been computed from literature density data, leading to correlations with maximum density effects.