# Unravelling the Origin of Water’s Thermal Conductivity Maximum: Compressibility, Tetrahedrality and Nuclear Quantum Effects

## TL;DR

This paper explains why water has a maximum thermal conductivity at certain temperatures, linking it to molecular arrangements and quantum effects.

## Contribution

The study identifies the thermodynamic and microscopic origins of water's thermal conductivity maximum.

## Key findings

- The thermal conductivity maximum in water is due to nuclear quantum effects and molecular arrangements.
- The TCM is not unique to water but occurs in other tetrahedral liquids with intermediate tetrahedrality.
- High and low density liquid states coexist in water, influencing its thermal properties.

## Abstract

Water is arguably
the most important liquid on Earth. Consequently,
its anomalous properties have been intensely investigated for over
50 years. However, water’s thermal conductivity maximum (TCM)
remains hitherto unexplained. Beyond its substantial fundamental interest,
this problem is critical because many natural (e.g., climate regulation),
industrial and chemical processes in which water appears as solvent
at near-standard conditions correspond to the anomalous heat transport
regime of water. We use all-atom and minimal coarse-grained models
to isolate the TCM’s thermodynamic fingerprint, and subsequently
demonstrate its thermodynamic and microscopic origin: (1) the depopulation
of librational modes due to nuclear quantum effects and (2) the balance
of two interconverting molecular arrangements, the high density and
low density liquid states, that coexist in water. We systematically
investigate tetrahedral liquids modeled with Stillinger-Weber potentials,
which allows the interpolation between simple liquids and low coordination
materials such as carbon. We show that the TCM is not exclusive to
water, but an anomalous behavior shared by pure liquids with intermediate
tetrahedrality. Our work provides a thermodynamic explanation for
the TCM of water and tetrahedral liquids in general.