On the physical mechanisms underlying single molecule dynamics in simple liquids

TL;DR

This paper proposes that single-molecule dynamics in simple liquids are governed by temperature-dependent London dispersion forces, electron cloud distortions, and phonon interactions, offering new insights into molecular behavior in nonpolar liquids.

Contribution

It introduces a physical mechanism linking electron cloud distortion and phonon interactions to single-molecule dynamics and viscosity in simple liquids, supported by experimental comparisons.

Findings

Viscous forces arise from temperature-dependent London dispersion forces.

Viscosity decay with temperature is due to electron cloud compression.

Self-diffusion is driven by low-frequency phonon modes.

Abstract

Physical arguments and comparisons with published experimental data suggest that in simple liquids: i) single-molecule-scale viscous forces are produced by temperature-dependent London dispersion forces, ii) viscosity decay with increasing temperature reflects electron cloud compression and attendant suppression of electron screening, produced by increased nuclear agitation, and iii) temperature-dependent self-diffusion is driven by a narrow band of phonon frequencies lying at the low-frequency end of the solid-state-like phonon spectrum. The results suggest that collision-induced electron cloud distortion plays a decisive role in single molecule dynamics: i) electron cloud compression produces short-lived repulsive states and single molecule, self-diffusive hops, while ii) shear-induced distortion generates viscosity and single-molecule-scale viscous drag. The results provide new insight into nonequilibrium molecular dynamics in nonpolar, nonmetallic liquids.