Supercritical density fluctuations and structural heterogeneity in supercooled water-glycerol microdroplets

TL;DR

This study investigates how supercooled water-glycerol microdroplets exhibit increased density fluctuations and structural heterogeneity due to water's liquid-liquid phase transition, with implications for cryogenic applications.

Contribution

It provides experimental and simulation evidence of density fluctuation behavior in supercooled glycerol-water solutions near the LLPT, revealing how glycerol shifts the temperature of maximum compressibility.

Findings

Density fluctuations increase upon cooling.

Glycerol shifts the maximum compressibility temperature.

Supercooled solutions exhibit structural heterogeneity.

Abstract

Recent experiments and theoretical studies strongly indicate that water exhibits a liquid-liquid phase transition (LLPT) in the supercooled domain. An open question is how the LLPT of water can affect the properties of aqueous solutions. Here, we study the structural and thermodynamic properties of supercooled glycerol-water microdroplets at dilute conditions ($\chi_g=3.2~\%$ glycerol mole fraction). The combination of rapid evaporative cooling with ultrafast X-ray scattering allows us to outrun crystallization and gain access to the deeply supercooled regime down to $T=229.3$ K. We find that the density fluctuations of the glycerol-water solution or, equivalently, its isothermal compressibility, $\kappa_T$, increases upon cooling. This is confirmed by molecular dynamics simulations, which indicate that the presence of glycerol shifts the temperature of maximum $\kappa_T$ from $T=230$ K in pure water down to $T=223$ K in the solution. Our findings elucidate the interplay between the complex behavior of water, including its LLPT, and the properties of aqueous solutions at low temperatures, which can have practical consequences in cryogenic biological applications and cryopreservation techniques.