Transcript Document

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.
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.
II. METHODS
Classical all-atom MD simulations are
performed in the isothermal-isobaric (NPT)
ensemble using the GROMACS 3.3.3.
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.
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.
Structure characteristic angle q and
potential energy of confined water against E
at T=200 K are given in Fig. 4.
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).
connect with a continuous transition and
the solid-solid transition from polygonal to
(7, 3) + 1L INTs is terminated by a critical
point.
Z-component of dipole moment (Dz) per
water molecule for different E at 200 K (Fig.
5) is calculated. It reveals that this solidsolid transition mainly results from the
interplay between strong intermolecular
hydrogen bonding and collective water
dipole orientation along the electric field.
Fig. 8 Calculated phase diagram (T-E plane).
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.
V. CONCLUSIONS
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.
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.
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
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 solidsolid 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
1. Koga, K. et al. Nature 2001, 412, 802.
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.