Molecular velocity auto-correlation of simple liquids observed by NMR MGSE method

TL;DR

This paper uses NMR MGSE to analyze molecular velocity auto-correlation spectra in simple liquids, revealing different decay behaviors and heterogeneity effects in polar and non-polar liquids.

Contribution

It introduces a novel NMR MGSE approach to study molecular velocity auto-correlation spectra, distinguishing between decay models and heterogeneity effects in liquids.

Findings

Non-polar toluene shows a $t^{-3/2}$ decay tail.

Polar liquids exhibit spectra consistent with trapped particle diffusion.

Heterogeneity causes non-exponential echo decay at short times.

Abstract

The velocity auto-correlation spectra of simple liquids obtained by the NMR method of modulated gradient spin echo show features in the low frequency range up to a few kHz, which can be explained reasonably well by a $t^{-3/2}$ long time tail decay only for non-polar liquid toluene, while the spectra of polar liquids, such as ethanol, water and glycerol, are more congruent with the model of diffusion of particles temporarily trapped in potential wells created by their neighbors. As the method provides the spectrum averaged over ensemble of particle trajectories, the initial non-exponential decay of spin echoes is attributed to a spatial heterogeneity of molecular motion in a bulk of liquid, reflected in distribution of the echo decays for short trajectories. While at longer time intervals, and thus with longer trajectories, heterogeneity is averaged out, giving rise to a spectrum which is explained as a combination of molecular self-diffusion and eddy diffusion within the vortexes of hydrodynamic fluctuations.