Two measures of isochronal superposition

TL;DR

This paper introduces two quantitative measures to assess isochronal superposition in liquids, testing six substances and confirming that van der Waals liquids exhibit this property more strongly than hydrogen-bonded liquids, aligning with isomorph theory.

Contribution

It presents two new measures for quantifying isochronal superposition and applies them to six liquids, confirming theoretical predictions about their behavior.

Findings

Van der Waals liquids obey isochronal superposition more strongly.

Hydrogen-bonded liquids show less isochronal superposition.

Results support predictions of the isomorph theory.

Abstract

A liquid obeys isochronal superposition if its dynamics is invariant along the isochrones in the thermodynamic phase diagram (the curves of constant relaxation time). This paper introduces two quantitative measures of isochronal superposition. The measures are used to test the following six liquids for isochronal superposition: 1,2,6 hexanetriol, glycerol, polyphenyl ether, diethyl phthalate, tetramethyl tetraphenyl trisiloxane, and dibutyl phthalate. The latter four van der Waals liquids obey isochronal superposition to a higher degree than the two hydrogen-bonded liquids. This is a predic- tion of the isomorph theory, and it confirms findings by other groups.