On the physical mechanisms underlying single molecule dynamics in simple liquids

Area of Science:

• Physical Chemistry
• Condensed Matter Physics
• Molecular Dynamics

Background:

• Understanding the molecular mechanisms governing viscosity and self-diffusion in simple liquids is essential for various scientific and engineering applications.
• Previous models often simplify the complex interplay of forces and dynamics at the single-molecule level.

Purpose of the Study:

• To elucidate the fundamental physical principles behind single-molecule-scale viscous forces and temperature-dependent self-diffusion in simple liquids.
• To investigate the role of electron cloud distortion and phonon dynamics in nonequilibrium molecular motion.

Main Methods:

• Theoretical physical arguments and analysis of published experimental data.
• Comparison of molecular dynamics simulations and experimental observations.

Main Results:

• Identified temperature-dependent London dispersion forces as the source of single-molecule-scale viscous forces.
• Linked viscosity decay with increasing temperature to electron cloud compression and reduced electron screening due to nuclear agitation.
• Determined that temperature-dependent self-diffusion is governed by specific low-frequency phonon modes.

Conclusions:

• Collision-induced electron cloud distortion is pivotal in single-molecule dynamics, enabling self-diffusive hops and viscous drag.
• The findings offer novel insights into the nonequilibrium molecular dynamics of nonpolar, nonmetallic liquids.