Ultrasonic Absorption in Mixtures of Ethyl Alcohol and Water

Abstract

The pulse method has been used to measure the absorption of ultrasonic waves in mixtures of ethyl alcohol and water at the three frequencies 22 5, 37 5 and 52 5 Mc/s and at the temperatures 0?, 10? and 25? c The results confirm the findings of earlier workers that in both the pure liquid and the mixture there is more absorption than can be explained by the classical mechanisms of viscosity and heat conduction. When the composition of the mixture is varied a maximum of absorption is found for a mixture containing 20 to 30 molar per cent alcohol The magnitude of the absorption at the maximum increases rapidly as the temperature is lowered Over a range of compositions in the neighbourhood of the maximum the value of ?/f2 (? is the absorption coefficient and f the frequency) is found to decrease as f increases It is known that the extra absorption in the pure liquids can be explained in terms of a compressibility with a finite relaxation time, but this mechanism alone cannot account for the behaviour of the mixture It is suggested that here the additional absorption is due to interaction between the molecules of the two components, leading to the partial development of a structure or complex in the liquid with a composition of four molecules of water to one of alcohol The thermal properties of the mixture indicate that such interaction occurs, and the suggestion for the composition of the complex is supported by the fact that the freezing-point curve for the mixture shows that a structure with this composition is formed in the solid phase The extra absorption found by other workers in several similar mixtures can be explained, at least qualitatively, in the same way