On the pressure dependence of the thermodynamical scaling exponent gamma

TL;DR

This paper introduces a simple method to determine how the thermodynamical scaling exponent gamma varies with pressure in supercooled liquids, revealing that gamma decreases with pressure but remains above 4, impacting the understanding of intermolecular potentials.

Contribution

The paper presents a new method to assess the pressure dependence of gamma using only the pressure dependence of the glass transition and the equation of state.

Findings

Gamma decreases with increasing pressure in studied liquids.

Gamma remains always larger than 4.

Liquids with gamma near 4 show smaller pressure dependence.

Abstract

Since its initial discovery more than fifteen years ago, the thermodynamical scaling of the dynamics of supercooled liquids has been used to provide many new important insights in the physics of liquids, particularly on the link between dynamics and intermolecular potential. A question that has long been discussed is whether the scaling exponent gamma is a constant or it depends on pressure. Here we offer a simple method to determine the pressure dependence of gamma using only the pressure dependence of the glass transition and the equation of state. Using this new method we find that for the six non-associated liquids investigated gamma always decreases with increasing pressure. Importantly in all cases the value of gammaremains always larger than 4. Liquids having gamma closer to 4 at low pressure show a smaller change in gamma with pressure. We argue that this result has very important consequences for the experimental determination of the functional form of the repulsive part of the potential in liquids.