Thermodynamic Scaling of the Viscosity of Van Der Waals, H-Bonded, and Ionic Liquids

TL;DR

This study demonstrates that the viscosity of various liquids, including van der Waals, hydrogen-bonded, and ionic liquids, can be scaled using a thermodynamic parameter TV^g, revealing insights into intermolecular forces.

Contribution

It extends the thermodynamic scaling of relaxation times to viscosity across different liquids, linking the scaling exponent to intermolecular force strength.

Findings

Scaling works across supercooled and high-temperature regimes.

g decreases from van der Waals to ionic liquids.

H-bonded liquids show deviations due to structural changes.

Abstract

Viscosities and their temperature, T, and volume, V, dependences are reported for 7 molecular liquids and polymers. In combination with literature viscosity data for 5 other liquids, we show that the superpositioning of relaxation times for various glass-forming materials when expressed as a function of TV^g, where the exponent g is a material constant, can be extended to the viscosity. The latter is usually measured to higher temperatures than the corresponding relaxation times, demonstrating the validity of the thermodynamic scaling throughout the supercooled and higher T regimes. The value of g for a given liquid principally reflects the magnitude of the intermolecular forces (e.g., steepness of the repulsive potential); thus, we find decreasing g in going from van der Waals fluids to ionic liquids. For strongly H-bonded materials, such as low molecular weight polypropylene glycol and water, the superpositioning fails, due to the non-trivial change of chemical structure (degree of H-bonding) with thermodynamic conditions.