The ability to understand how water molecules can be confined on the nanoscale has gathered attention recently. This is especially true for understanding the phase behavior between water and ice and the thermodynamic principles behind them.
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A team of Researchers from the US and China have now encapsulated water molecules under a graphene cover to study the structural and thermodynamic behaviors of the different nanoscale water phases.
The knowledge of how ordered and disordered hydrogen bond mechanisms form networks on the nanoscale has been an unsolved puzzle and has so far produced a lack of understanding towards the structural behaviors of nanoscale water and ice. Whilst Researchers have a good idea of how bulk water behaves, especially under varying environmental conditions, the characterization and understanding of how nano-confined water behaves is a recent research drive.
Interesting properties and phenomenon occur under nano-confinement and can vary considerably against their bulk counterparts. In a finite system, it is known that a group of water molecules can modulate the hydrogen bond network, define their thermodynamics and produce structural ordering through anisotropic pressure affecting the water condensation process.
The Researchers used a series of molecular dynamics (MD) calculations to predict the structural and thermodynamic behaviors of ordered and disordered water molecules confined on the nanoscale by a graphene cover.
Graphene was chosen as the encapsulating material due to its structural ordering containing wrinkles and ripples that allow for efficient nano-confinement. The encapsulation of materials using graphene has also been proposed as a method to produce reactive cells which could explore the molecular processes within trapped molecules, liquids and solids.
Throughout their simulations, the Researchers used a combination of large-scale atomic/molecular massively parallel simulators (LAMMPS), all-atom optimized potentials for liquid simulations (OPLS-AA), extended simple point charge models (SPC/E), particle-particle particle-mesh algorithms (PPPM) and SHAKE algorithms. The Researchers also controlled the environmental conditions, namely the temperature, with Berendsen and Nosé-hoover thermostats. The Researchers simulated various environments between 200 and 1600 Kelvin (K).
The Researchers also used a combination of 2D half-cylindrical models and 3D hemispheric models to simulate the graphene sheet and encapsulated water system. Within the system, the researchers directly deposited graphene onto a substrate containing a pre-equilibrated water droplet.
The Researchers found multi-layered structures containing up to 1000 water molecules could be confined under the graphene sheet and were stabilized by spatial confinement and an induced pressure resulting from interfacial adhesion.
The Researchers also observed monolayer encapsulations which formed a 2D crystalline lattice with defects. The formation of these structures arose from the spatial confinement and are a product of the epitaxial template offered by the graphene sheet and in-plane ordering.
The analysis of the free energy of the system showed that structural orders with a low entropy were compensated by a high energy of formation from the pressurized confinement. The order-to-disorder transition for the condensed phase was found to be between 480 and 490 K. Around the transition, the researchers also observed a sharp reduction in the number of hydrogen bonds and an increase in the entropy of the system.
The structural transition of the water molecules was found to be solid-to-fluid in nature. This was shown through a fast diffusion of encapsulated water with an anomalous temperature dependence. The transition was also characterized and proven through a discontinuous change in the entropy and structure of the hydrogen bond network.
The Researchers have provided a fundamental understanding of the nano-confinement and encapsulation dynamics of these unique water phases. The wide temperature range tested offers an interesting and novel platform for the experimental testing of water in a confined environment.
The research also lends itself to helping produce pressurized cells, nano-devices and nano-confined reactive cells. The trapping and confinement of water between 2D materials is a useful approach in the production of novel devices, and the implementation of graphene offers potential applications surrounding the coating and trapping of volatile molecules and in microscopy studies.
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Source
“Structures and thermodynamics of water encapsulated by graphene”- Jiao S., et al, Scientific Reports, 2017
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