Non-linear ultrasonics to determine molecular properties of pure liquids

Wave propagation of ultrasound through condensed media is fundamentally non-linear in nature. The extent of non-linearity is related to the ratio of the third to second virial coefficients (BA) of the equation of state of the medium. This ratio can be directly measured by determining the rate of change in sound speed with pressure. Several simple relations are developed that relate acoustic non-linearity to the molecular properties, namely internal pressure, free energy of binding, the effective van der Waals' constants, the translational diffusion coefficient, and the rotational correlation time. Using these relationships and the measured the acoustic non-linearity parameter and the sound speed, the molecular properties are determined for the liquids: water, alcohols and fluorocarbons. In all cases the values obtained from ultrasonic measurements compare closely with the values reported in the literature based on alternative measurements. In conclusion, the non-linear propagation of ultrasound offers an independent means of determining the molecular properties of condensed media.