Are polar liquids less simple?

TL;DR

This study investigates the relationship between potential energy-virial correlations and dynamic properties in liquids, revealing that strong correlations are characteristic of simple van der Waals liquids but break down in polar and hydrogen-bonded liquids, which deviate from inverse power law interactions.

Contribution

It demonstrates that strong U-W correlations are not necessary for dynamic properties like density scaling, especially in polar and hydrogen-bonded liquids, highlighting the role of intermolecular potential forms.

Findings

Simple van der Waals liquids show strong U-W correlations and expected dynamic behavior.

Polar liquids exhibit weaker correlations but still conform to dynamic properties.

Hydrogen-bonded liquids do not show strong correlations or dynamic property adherence, due to deviations from inverse power law potentials.

Abstract

Strong correlation between equilibrium fluctuations of the potential energy, U, and the virial, W, is a characteristic of a liquid that implies the presence of certain dynamic properties, such as density scaling of the relaxation times and isochronal superpositioning of the relaxation function. In this work we employ molecular dynamics simulations (mds) on methanol and two variations, lacking hydrogen bonds and a dipole moment, to assess the connection between the correlation of U and W and these dynamic properties. We show, in accord with prior results of others [T.S. Ingebrigtsen, T.B. Schroder, J.C. Dyre, Phys. Rev. X 2, 011011 (2012).], that simple van der Waals liquids exhibit both strong correlations and the expected dynamic behavior. However, for polar liquids this correspondence breaks down - weaker correlation between U and W is not associated with worse conformance to density scaling or isochronal superpositioning. The reason for this is that strong correlation between U and W only requires their proportionality, whereas the expected dynamic behavior depends primarily on constancy of the proportionality constant for all state points. For hydrogen-bonded liquids, neither strong correlation nor adherence to the dynamic properties is observed; however, this nonconformance is not directly related to the concentration of hydrogen bonds, but rather to the greater deviation of the intermolecular potential from an inverse power law (IPL). Only (hypothetical) liquids having interactions governed strictly by an IPL are perfectly correlating and exhibit the consequent dynamic properties over all thermodynamic conditions.