Are polar liquids less simple?

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.

14 pages, 8 figures

References in corpus (11)

• Pressure-energy correlations in liquids. IV. "Isomorphs" in liquid state diagrams
• "Isomorphs" in liquid state diagrams
• Thermodynamic Scaling of the Viscosity of Van Der Waals, H-Bonded, and Ionic Liquids
• Pressure-energy correlations in liquids. I. Results from computer simulations
• Thermodynamic scaling of diffusion in supercooled Lennard-Jones liquids
• What constitutes a simple liquid?
• On the density scaling of liquid dynamics
• Strongly correlating liquids and their isomorphs
• Connection between dynamics and thermodynamics of liquids on the melting line
• Theory of Relaxation Dynamics in Glass-Forming Hydrogen-Bonded Liquids
• Do all liquids become strongly correlating at high pressure?

Cited by in corpus (4)

• Dynamic Correlation Length Scales under Isochronal Conditions
• On the experimental determination of the repulsive component of the potential from high pressure measurements: what is special about twelve?
• Pressure densification of a simple liquid
• A test for the existence of isomorphs in glass-forming materials