Unlocking the secrets of salt crystal formation at the nanoscale

In nature and technology, crystallization plays a crucial role, from the formation of snowflakes and pharmaceuticals to the creation of sophisticated batteries and desalination membranes. Despite its importance, crystallization at the nanoscale is not much understood, primarily because it is very challenging to observe the process directly at this scale. My research overcomes this hurdle by employing cutting-edge calculation methods, allowing us to visualize atomic interactions with unprecedented details.

My research published in Chemical Science revealed new details about how salt crystals form in small nanometer-sized spaces.

This study used sophisticated molecular dynamics simulations enhanced by cutting-edge machine learning techniques to investigate how sodium chloride (NaCl), a common table salt, crystallizes when it is trapped between two graphene sheets separated by just a few billionths of a metre. These extreme conditions, known as nano-facilities, dramatically alter molecular behavior compared to everyday conditions.

Understanding how crystallization occurs in a nano-comprised space opens the door to precisely control the crystal structure and properties. These discoveries could lead to innovative advancements in nanotechnology, energy materials and chemical engineering.

This study revealed several notable findings. Most notably, we observed that confinement generally makes solid salt crystals more stable and significantly increases the melting point compared to salts in bulk water. This stability was intricately dependent on the exact spacing between graphene sheets. At some confinement levels, unusual crystal structures emerged, containing salt-based salt forms that were generally stable only at much lower temperatures.

Further analysis using advanced machine learning approaches provided insight into the driving forces behind these anomalous crystallization behaviors. The team adopted a thermodynamic explanatory representation of state prediction information bottlenecks and AI and other black box paradigm technologies to determine key reaction pathways and uncover the molecular determinants essential for crystal formation under confinement.

Simulations demonstrated that the process of crystallization under these nanoconditions involves carefully organized interactions between ions, water molecules, and their confined surfaces. Importantly, the team has identified the removal of ions, particularly water molecules that directly surround the chloride ions, playing a pivotal role. This water removal, coupled with unique dielectric behavior under extreme confinement, amplifies the Coulomb forces between the ions, supporting the formation of solid salt structures.

This basic study has extensive implications. By accurately understanding the conditions that prioritize a particular crystal structure, scientists can better control processes critical to advanced technical applications. For example, enhanced knowledge of nano-combined crystallization mechanisms could lead to better strategies for improving the efficiency and stability of electrochemical energy storage devices and to better strategies for water purification via advanced desalination membranes.

Furthermore, this study introduced a general computational framework that combines enhanced sampling molecular dynamics with machine learning analysis. This can be widely applied to other complex chemical and physical processes at the nanoscale. This methodology could reveal new behavior in limited systems, which are central to a variety of fields, such as energy storage, catalysts, and pharmaceutical manufacturing.

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Ruiyu Wang is a postdoctoral researcher at the University of Maryland College Park University and specializes in molecular dynamics simulation, enhanced sampling, and machine learning. His current research focuses on the nucleation and phase transition of aqueous solutions under special environmental conditions, with potential applications in energy science. Leuille received her PhD. We studied water structure, dynamics and topology at the water/solid interface from Temple University. His doctoral studies investigated how ion adsorption and surface charging affect the properties of aqueous interfaces.