Anomalous behaviour of transport properties in a supercooled water-glycerol mixture

TL;DR

This study uses molecular dynamics simulations to explore how glycerol affects water's transport properties at low temperatures, revealing anomalous behaviors and a crossover in dynamics that depend on glycerol concentration.

Contribution

It uncovers a novel dynamic crossover in water-glycerol mixtures at low temperatures, highlighting the complex interplay between water and glycerol dynamics.

Findings

Violations of the Stokes-Einstein relation in low-temperature regimes.

A crossover in diffusion, viscosity, and hydrogen-bond dynamics at low glycerol concentrations.

Distinct behaviors of water and glycerol dynamics depending on concentration.

Abstract

Glycerol acts as a natural cryoprotectant by depressing the temperature of ice nucleation and slowing down the dynamics of water mixtures. In this work we characterize dynamics -- diffusion, viscosity, and hydrogen-bond dynamics -- as well as density anomaly and structure of water mixtures with 1\% to 50\% w/w glycerol at low temperatures via molecular dynamics simulations using all-atom and coarse-grained models. Simulations reveal distinct violations of the Stokes-Einsten relation in the low temperature regime for water and glycerol. Deviations are positive for water at all concentrations, and positive for glycerol in very dilute solutions but turning negative in concentrated ones. The all-atom and coarse-grained models reveal an unexpected crossover in the dynamics of the 1% and 10 % w/w glycerol at the lowest simulated temperatures. This crossover manifests in the diffusion coefficients of water and glycerol, as well as in the viscosity and lifetime of hydrogen-bonds in water. We interpret that the crossover originates on the opposing dependence with glycerol concentration of the two factors controlling the solution's slow-down: the increase in tetrahedrally coordinated water and the dynamics and clustering of the glycerol molecules. We anticipate that this dynamic crossover will also occur for solution of water with other polyols.