Bioinspired NanoscaleWater Channel and its Potential Applications - Bioinspiration: From Nano to Micro Scales - page 27

Fig. 1.29 ( a ) Averaged water flow ( black ) and net flux ( red ) through the channel with respect to
ı, together with those for the system without an external charge ( dashed lines ). ı is the distance
of the imposed charge to the centerline minus the radius of the single-walled carbon nanotube. ( b )
Va l u e s o f P WC and P WW , with respect to ı. P WC is the electrostatic potential between the water
molecule in the middle of the nanotube with the imposed charge, and P WW is the average value of
the electrostatic potential between the water molecule and one of its neighboring water molecule
(reprinted from [ 22 ]. Copyright 2007 National Academy of Science, USA)
Electrical field can also be used to control the channels. Electrical field can be
initiated by an external charge. As is shown in Fig. 1.29 , we have introduced a
positive charge of quantity 1.0 e on the plane of the graphite that divides the full
space into two parts [ 22 ]. Similar to the case under the deformation of the SWNT,
the average flow and net flux across the channel are approximately equal to those
in the system without any charge until a value of ı C D 0.85 A. Here, the distance of
the imposed charge to the centerline minus the radius of the SWNT is also denoted
by ı. Both the average flow and net flux decrease monotonically and sharply as ı
further decreases. At ı D 0, the channel is almost closed with the average flow below
1.0 ns 1 .
These behaviors result from the competitions between the water-charge interac-
tion and the water-water interaction (mainly from the hydrogen bonds). As is shown
in Fig. 1.2 , all the water molecules inside the channel have orientation concerted,
along or opposite to the nanotube. This orientation distribution is not easy to be
changed by an external charge outside the nanotube due to the strong hydrogen
bonds between the neighboring bonds. Only when the charge is close enough so
that the interaction of the water and the charge is comparable to a hydrogen bond,
the water chain inside the nanotube shows considerable change. In Fig. 1.29 ,we
show the average value of the electrostatic potential between the water molecule
in the middle of the nanotube with the imposed charge, P WC , together with the
average value of the electrostatic potential between the water molecule and one of
its neighboring water molecule. It is clear that at ı ı C , P WW D P WC .Further
approach of the charge makes the interaction between the water and the charge
stronger, and the orientation of the water molecule facing the charge changes its