Measurements of Ultrasonic Absorption in Various Mixtures of Nitrobenzene

Abstract

The relaxation phenomena which, as now seems fairly well established, cause the difference between classical and experimental values of ultrasonic absorption coefficients in liquids, can be of various nature in the different liquids. It seems worthwhile to consider the case of binary systems in which it might be expected that relaxation phenomena of different nature could be present, depending on the characteristics of the components. We have studied seven binary mixtures having as one component nitrobenzene, whose molecules have a large dipole moment. The experimental results are here reported, measurements having been performed by an optical method. In the systems formed by an unassociated liquid having a large absorption coefficient (benzene, chloroform) and nitrobenzene, the curves of the absorption coefficient versus mol. fraction are like those obtained in the mixtures of an unassociated liquid with large absorption coefficient and an unassociated or associated liquid with a much smaller absorption coefficient. In this case the variation of the coefficient with concentration is caused, essentially, by the decrease of energy losses in the molecules of the first component, as a consequence of the higher efficiency of collisions between molecules on approaching the equilibrium distribution of energy among the internal degrees of freedom. In the systems of nitrobenzene and an alcohol (methyl, ethyl), on the other hand, it is to be expected that the relaxation phenomena are of structural nature. In this case the absorption coefficient, as a function of the mol. fraction, has a minimum at an intermediate concentration, just as it happens in some mixtures of water and alcohols.