1. Effect of Electric Field on the Behaviors of Phase and Phase Transition of Water Confined in Carbon Nanotube
Zhenyu Qian, Zhaoming Fu, and Guanghong Wei
Phys. Dept., Fudan Univ., Shanghai, People’s Republic of China
I. INTRODUCTION
Confined or interfacial water exists widely in nature and it is believed to play an important role in properties and functions of the entire systems. In this study, we have investigated the structure and the phase transition behaviors of water confined in SWCNT by performing MD simulations at atmospheric pressure and propose a rich phase diagram of confined water in the temperature-electric field plane.
II. METHODS
Classical all-atom MD simulations are performed in the isothermal-isobaric (NPT) ensemble using the GROMACS
III. RESULTS AND DISCUSSION
Water can freeze into three kinds of INTs (Fig. 1a-c) at T=200 K without external electric field. When exerting an electric field of E=2 V/nm along SWCNT’s axis, we find a new ice configuration (seeing Fig. 1d) combined with an outer helical (7, 3) ice tube and an inner helical nanoline.
We also find that as time goes on the three kinds of polygonal INTs can transform into each other in the absence of E or under weak electric field.
Populations of N at various T and E are calculated shown in Fig. 3. Temperature helps the structural transition from (6, 0) to (7, 0) INTs, because higher temperature gives water molecules higher kinetic energy to overcome the potential barrier against the (6, 0) configuration.
Structure characteristic angle q and potential energy of confined water against E at T=200 K are given in Fig. 4.
Z-component of dipole moment (Dz) per water molecule for different E at 200 K (Fig. 5) is calculated. It reveals that this solid-solid transition mainly results from the interplay between strong intermolecular hydrogen bonding and collective water dipole orientation along the electric field.
Electric field will both influence the phase behaviors of water confined in the SWCNT, and affect the phase transition properties. Fig. 6 shows potential energy of confined water against T under various E.
We also examine the ice configurations at lower and higher temperature and present the radial density profile in Fig. 7.
Calculated phase diagram for the model system is presented (Fig. 8). The first-order transition from solid to liquid phases may
connect with a continuous transition and the solid-solid transition from polygonal to (7, 3) + 1L INTs is terminated by a critical point.
V. CONCLUSIONS
We find a new ice phase (7, 3) + 1L that was not observed previously by merely freezing the water. In the low-T and low-E region, populations of polygonal INTs would vary as T and E change. The solid-solid transition occurs discontinuously with E in/decreased, accompanied with a marked hysteresis loop. The solid-liquid transition occurs as a first-order transition at lower E, connecting with a continuous transition at higher E.
REFERENCES
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2. Takaiwa, D. et al. Proceedings of the National Academy of Sciences 2008, 105, 39.
3. Han, S. et al. Nature Physics 2010, 6, 685.
Fig. 6 Potential energy of confined water (per water molecule) under various E.
Fig. 3 Population of structural characteristic number N at various T and E.
Fig. 7 Radial density profile of confined water under E=0 V/nm and E=2 V/nm.
Fig. 4 Analysis of structure transition of water confined in SWCNT by E-field. (a) Structural characteristic angle q and (b) potential energy of confined water (per water molecule).
Fig. 1 Snapshots of four different INTs at T=200 K (end view): (a-c) in the absence of E; (d) under E=2 V/nm.
Fig. 8 Calculated phase diagram (T-E plane).
Fig. 5 Analysis of z-component of dipole moment (Dz) per water molecule. (a) Time evolution of Dz; (b) Hysteresis loop of Dz.
Fig. 2 Structural characteristic number N of INTs as a function of time: (a) at T=200 K, E=0 V/nm; (b) at T=220 K, E=0.5 V/nm.