† Corresponding author. E-mail:
Project supported by the National Natural Science Foundation of China (Grant No. 51271100), the National Key Research Program of China (Grant No. 2016YFB0300501), and the Taishan Scholar Construction Engineering.
Molecular dynamics (MD) simulations are performed to explore the layering structure and liquid–liquid transition of liquid water confined between two graphene sheets with a varied distance at different pressures. Both the size of nanoslit and pressure could cause the layering and liquid–liquid transition of the confined water. With increase of pressure and the nanoslit’s size, the confined water could have a more obvious layering. In addition, the neighboring water molecules firstly form chain structure, then will transform into square structure, and finally become triangle with increase of pressure. These results throw light on layering and liquid–liquid transition of water confined between two graphene sheets.
Water, the fountainhead of the cosmic inventory, is not only so indispensable to all living creatures that it renders its involvement in a vast number of processes which has important relevance to the survival of every organism such as respiration and the transportation of nutrients and metabolite, but also a topic perpetual interest due to its remarkable anomalous properties and important implications to natural science,[1] biological science,[2–6] material science,[7–11] etc. The bulk water has various solid structures including more than 15 crystalline phases and 3 amorphous phases. Because of the existence of rock, cell, microemulsion, ionic channel and interstellar, water is usually confined in many real situations, attracting a lot of attention devoted to structural and dynamical properties of water in restricted geometries.[12,13]
In the 1980s, Mishima et al.[14,15] adopted the experimental technique to reveal that a sudden change would happen between two amorphous phases of water and Poole et al.[16] via computer simulation forecast that liquid water might exist in a various of forms. Then, the transition between multiphase structures of the same liquid has attracted a keen interest. Recently, existing theories and present studies have confirmed that liquid–liquid transition could take place in supercooled water under certain conditions.[17] Up to now, liquid–liquid transition of supercooled water is usually caused by two factors: temperature and pressure.[18] Plenty of researches[19,20] have demonstrated that the supercooled water has a low-density liquid phase. Huang et al.[21] used small-angle x-ray scattering to verify the presence of density fluctuations in ambient water; this is retained with decreasing temperature while the magnitude is enhanced. Soper and Ricci[22] revealed that supercooled water has two different structures and both density and structure will change during the liquid–liquid transition. To sum up, previous studies almost focus on the liquid–liquid transition of supercooled water,[23,24] very few attention is paid to the liquid–liquid transition of the confined water.
During the last decade, simulations and experiments have been performed to study the water confined in various confined substrates, which have evidenced for confined water a rich variety of thermodynamics phenomena more puzzling than those known for bulk water.[25,26] In addition, phase behavior of water in confined space could be dramatically influenced by several factors, such as density,[27] pressure,[28,29] temperature,[30] and size of the confinedment.[31–33] Until now, despite plenty of progresses from both experimental techniques[34,35] and analytical theories[36–41] has been made in exploring the complex structures of water, it is still necessary to further probe it.
Here, we used MD simulation to explore the layering and liquid–liquid transition of confined water in nanoslit.
In this work, we employ MD simulation to investigate the structural evolution of water confined between two graphene sheets with a distance ranging from 8 Å to 14 Å (as shown in Fig.
![]() | Fig. 1. (color online) The initial model of simulation system. Red: oxygen atoms; blue: hydrogen atoms; gray: carbon atoms. |
All MD calculations are carried out by the package LAMMPS[42] in the constant number, pressure and temperature (NPT) ensemble, where the temperature is controlled by a Nose–Hoover thermostat. The interaction among water is described by the four-point TIP4P[43] rigid model. The SHAKE algorithm[44] is adopted to restrict the H–O distance and H–O–H angle. The adaptive intermolecular reactive empirical bond order (AIREBO) potential[45] is adopted to model the C–C interaction, and the interaction between water and carbon is described by the Lennard–Jones (LJ) potentail. The water molecules also interact via the LJ potential. The velocity Verlet algorithm[46] with a time step of 1.0 fs is chosen to calculate the time integration of Newton’s equation of the motion. We exert the constant pressure on water molecules, and the pressure is parallel to two graphene sheets varied from 0.505 GPa to 30.3 GPa. Moreover, all simulations are performed at a temperature of 300 K for 1 ns. To improve the computational efficiency, the substrates are fixed during the simulation process.
First, we explore the layering structure of confined water in various nanoslits. Figure
![]() | Fig. 2. (color online) Snapshots of liquid water with constant atoms between two graphene sheets with a distance ranging from 8 Å to 14 Å at various pressures ranging from 0.505 GPa to 30.3 GPa. |
Density distribution functions (DDFs) are plotted to further explore the combined effect of pressure and the distance of two graphene sheets on stratification phenomenon. For convenience, we consider the position of oxygen atom as the core of water molecule. Figures
![]() | Fig. 3. (color online) The DDFs of water molecules along z direction in different cases: (a) 8.0 Å; (b) 10.0 Å; (c) 12.0 Å; (d) 14.0 Å. |
Average density curves of these systems with change of pressure are shown in Fig.
From the previous section, the pressure affects the laying of confined water significantly. It is necessary to discuss whether pressure has an influence on inner structure of every layer. As for the liquid water confined between two graphene sheets with a distance of 8 Å, water molecules are divided into two layers along the z direction in all cases. Figure
![]() | Fig. 5. (color online) Snapshots of water molecules in one layer between two graphene sheets with a distance of 8 Å at various pressures ranging from 0.505 GPa to 30.3 GPa. |
It is found that the distance between two layers is not the same at different pressures, so DDFs are plotted to explore the influence of pressure on the distance of the two water layers. Figure
![]() | Fig. 6. (color online) Density distribution functions (DDFs) of water molecules along z direction at different pressure ranging from 0.505 GPa to 30.3 GPa. |
Figures
This study systematically investigates the layer structure and liquid–liquid transition of water confined between two graphene sheets by MD simulation. Not only the pressure but also the size of nanoslit could induce liquid–liquid transition, behaved as the change of layering, as well as the change of stacking pattern of water molecules in every layer. With increase of pressure and the nanoslit’s size, the layering in confined water becomes more and more obvious. Furthermore, by changing pressure, three structures of water confined between two graphene sheets are obtained. With increase of pressure, they firstly form chain structure, then transform into square structure, and finally become triangle. These findings could provide an opportunity for comprehensive understanding of layering and liquid–liquid transition of water in confined nanoslit.
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