A simulation study of homogeneous ice nucleation in supercooled salty water

TL;DR

This study uses computer simulations to analyze how salt affects the rate of homogeneous ice nucleation in supercooled water, revealing that salt increases interfacial free energy and slows nucleation despite higher thermodynamic driving force.

Contribution

It introduces a combined simulation approach to accurately predict the impact of salt on ice nucleation rates, aligning well with experimental data.

Findings

Salt decreases nucleation rate at given supercooling.

Interfacial free energy increases with salt concentration.

Model predictions agree with experimental measurements.

Abstract

We use computer simulations to investigate the effect of salt on homogeneous ice nucleation. The melting point of the employed solution model was obtained both by direct coexistence simulations and by thermodynamic integration from previous calculations of the water chemical potential. Using a Seeding approach, in which we simulate ice seeds embedded in a supercooled aqueous solution, we compute the nucleation rate as a function of temperature for a 1.85 NaCl mole per water kilogram solution at 1 bar. To improve the accuracy and reliability of our calculations we combine Seeding with the direct computation of the ice-solution interfacial free energy at coexistence using the Mold Integration method. We compare the results with previous simulation work on pure water to understand the effect caused by the solute. The model captures the experimental trend that the nucleation rate at a given supercooling decreases when adding salt. Despite the fact that the thermodynamic driving force for ice nucleation is higher for salty water for a given supercooling, the nucleation rate slows down with salt due to a significant increase of the ice-fluid interfacial free energy. The salty water model predicts an ice nucleation rate that is in good agreement with experimental measurements, bringing confidence in the predictive ability of the model.