Role of quantum fluctuations in structural dynamics of liquids of light molecules

TL;DR

This study investigates how quantum effects like tunneling and zero-point energy influence the structural dynamics of light molecule liquids, revealing that quantum fluctuations significantly affect fragility and relaxation behavior, especially in confined water.

Contribution

It provides new insights into the role of quantum fluctuations in liquid dynamics and highlights differences between confined and bulk water regarding quantum effects.

Findings

Quantum fluctuations influence the fragility of light molecule liquids.

Confined water shows a weaker isotope effect than bulk water.

Quantum effects are highly sensitive to water's structure and barrier heights.

Abstract

A possible role of quantum effects, such as tunneling and zero-point energy, in the structural dynamics of supercooled liquids is studied by dielectric spectroscopy. Presented results demonstrate that the liquids, bulk 3-methylpentane 3MP and confined normal and deuterated water have low glass transition temperature and unusually low for their class of materials steepness of the temperature dependence of structural relaxation, or fragility. Although we do not find any signs of tunneling in structural relaxation of these liquids, their unusually low fragility can be well described by the influence of the quantum fluctuations. Confined water presents especially interesting case in comparison to the earlier data on bulk low-density amorphous and vapor deposited water. Confined water exhibits much weaker isotope effect than bulk water, although the effect is still significant. We show that it can be ascribed to the change of the energy barrier for relaxation due to a decrease in the zero-point energy upon D-H substitution. The observed difference in the behavior of confined and bulk water demonstrates high sensitivity of quantum effects to the barrier heights and structure of water. Moreover, these results demonstrate that extrapolation of confined water properties to the bulk water behavior is questionable.