Ultrasonic relaxation studies of aqueous alcohols and related systems.

Mixtures of non-electrolytes and water are frequently used as solvent media for chemical investigations. Many of these mixtures have sound absorption characteristics which indicate the presence of complex equilibria, which are often reflected in derived thermodynamic quantities. The present thesis deals with mixtures of butanol and water in particular. The relevant theory of ultrasonic relaxation is developed, and is frequently used in the ensuing discussion. A review is made of the properties of alcohol-water mixtures together with previous sound absorption investigations of these systems. Theories developed to explain the latter are reviewed and discussed. The equipment and technique involved in measuring sound absorption using the pulse technique is described. Investigations of the sound absorption properties of butanol-water mixtures at a fixed frequency as a function of solvent composition has revealed the existence of an initial "plateau" region. This is shown to indicate that in this region the solution exists in the form of a liquid clathrate. In the region of the P.S.A.C. the experimental data has been satisfactorily analysed in terms of three relaxation processes. The processes at higher frequencies are concentration dependent and have been interpreted in terms of the formation and dissociation of hydrophobic dimers and tetramers. The effect of urea on the concentration independent low relaxation frequency indicates that this is due to co-operative disruption of local water structure when hydrophobic interactions occur. The effect of temperature on the relaxation spectrum of a butanol-water mixture at X2=0"05, indicates the existence of a thermal anomaly over the region 40 ~ 60 °C, which is in agreement with spectroscopic investigations. For the diethylamine-water system the "plateau" region is absent. Instead, in dilute solutions of diethylamine a single concentration dependent relaxation frequency is observed, which has been associated with the existence of an acid-base equilibrium, Finally ultrasonic relaxation studies of alcoholic solutions of lithium chloride are reported. Here two relaxation processes are observed at concentrations 0.1 molar but only one relaxation process at lower concentrations. It is suggested that the results indicate the existence in solution of two types of ion-pair differing only in the configurations of their solvation sheaths.