Sound Velocities in Liquids Near Freezing: Dependence on the Interaction Potential and Correlations with Thermal Conductivity

We present systematic investigation of sound velocities in various fluids at the fluid–solid phase transition. First, theoretical estimates indicating that quasi-universal values of sound velocities at freezing can be expected are presented. Then, this prediction is verified on three model systems with quite different interactions (inverse power law, screened Coulomb, and Lennard-Jones pairwise potentials) and 15 real atomic and molecular liquids. It is documented that the ratio of the sound velocity to the thermal velocity tends to a quasi-universal value (cs/vT∼10) in many systems considered, but exceptions also exist. In particular, extremely soft interactions can result in indefinitely large ratios cs/vT. Complex hydrocarbon liquids also demonstrate high ratios cs/vT. On the other hand, liquids composed of light elements, such as hydrogen and neon, demonstrate lower ratios cs/vT. For model systems, we discuss relations between the thermodynamic sound velocity and instantaneous longitudinal, transverse, and bulk sound velocities. It is found that these relations are greatly affected by the potential softness. Finally, correlations between the thermal conductivity coefficient and the sound velocity are briefly discussed in the context of Bridgman's formula.