The characteristic acoustic impedance of the thermodynamic ideal system and its “excess” for a non-ideal mixture

Abstract

The characteristic acoustic impedance is sometimes discussed in terms of molecular interactions in liquid mixtures. Commonly, the mole-fraction-weighted average of the characteristic impedances of the pure components of the mixture serves as a reference function for the impedance deviation. The latter is sometimes called an “excess”. Such deviations, however, lack physical meaning. In the paper, the thermodynamic correct formulae for the characteristic impedance of the thermodynamic ideal mixture were reported and discussed. In these formulae, the isobaric thermal expansibilities and the heat capacities of pure components are considered. Four examples illustrate substantial differences between the specific impedance deviations calculated in the conventional and thermodynamic correct manner.