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

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.