Phase diagram of the NaCl–water system from computer simulations

NaCl aqueous solutions are ubiquitous. They can crystallize into ice, NaCl, or NaCl · 2H2O depending on the temperature–concentration conditions. These crystallization transitions have important implications in geology, cryopreservation, or atmospheric science. Computer simulations can help understa...

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Bibliographic Details
Published in:The Journal of chemical physics 2022-02, Vol.156 (6), p.064505-064505
Main Authors: Bianco, V., Conde, M. M., Lamas, C. P., Noya, E. G., Sanz, E.
Format: Article
Language:English
Subjects:
Aqueous solutions
Crystallization
Melt temperature
Nucleation
Phase diagrams
Physics
Simulation
Sodium chloride
Solid phases
Temperature
Water chemistry
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Summary:NaCl aqueous solutions are ubiquitous. They can crystallize into ice, NaCl, or NaCl · 2H2O depending on the temperature–concentration conditions. These crystallization transitions have important implications in geology, cryopreservation, or atmospheric science. Computer simulations can help understand the crystallization of these solids, which requires a detailed knowledge of the equilibrium phase diagram. We use molecular simulations in which we put at contact the solution with the solid of interest to determine points of the solid–solution coexistence lines. We follow two different approaches, one in which we narrow down the melting temperature for a given concentration and the other in which we equilibrate the concentration for a given temperature, obtaining consistent results. The phase diagram thus calculated for the selected model (TIP4P/2005 for water molecules and Joung–Cheatham for the ions) correctly predicts coexistence between the solution and ice. We were only able to determine NaCl · 2H2O–solution coexistence points at higher temperatures and concentrations than in the experiment, so we could not establish a direct comparison in this case. On the other hand, the model underestimates the concentration of the solution in equilibrium with the NaCl solid. Our results, alongside other literature evidence, seem to indicate that ion–ion interactions are too strong in the model. Our work is a good starting point for the improvement of the potential model and for the study of the nucleation kinetics of the solid phases involved in the phase diagram.
ISSN:0021-9606
1089-7690
DOI:10.1063/5.0083371