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

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. •The characteristic acoustic impedance of mixtures was discussed.•Correct formula for the thermodynamic ideal mixture was suggested.•The ideal impedance differs from that predicted from the additivity rule.•The new approach leads to reliable conclusions about molecular interactions.