Bioinspired NanoscaleWater Channel and its Potential Applications - Bioinspiration: From Nano to Micro Scales - page 3

Fig. 1.2 A snapshot of water confined in a (6,6) armchair single-walled carbon nanotube. The
red and gray balls represent the oxygen and hydrogen atoms, respectively. It can be seen clearly
that each oxygen atom is located in the right side of the two hydrogen atoms in the same water
molecule. The single-walled carbon nanotube is represented by gray lines

the most effective ones and can effectively shield the thermal noise and represent
the useful signal. In this chapter, we will review the recent progress obtained by
our group using MD simulations in the study of the water permeation across the
nanoscale channels inspired by the aquaporins.
Let us first review the basic properties of water in a classical view. As is shown
in Fig. 1.1 , a water molecule is composed of one oxygen atom and two hydrogen
atoms with the angle between OHO of about 104.52 o . The water molecule is polar
due to the residual charges on the oxygen atom and the hydrogen atoms. The most
important thing in the water molecules is the hydrogen bonds between neighboring
water molecules. Explicitly, an oxygen atom in one water molecule may have a
strong interaction with a hydrogen atom in another, which is called the existence of
a hydrogen bond. The strength of the hydrogen bonds, though much smaller than
that of the chemical bonds, can be more than tens of k B T . Thus, this bond usually
plays a very important role in the system full of thermal fluctuation. It is believed
that the anomalies of water mainly result from the existence of the hydrogen bonds.

1.1.1 Gating and Pumping of Nanoscale Channels

The water molecules are connected by one-dimensional hydrogen bonds as shown
in Fig. 1.2 when the water molecules exhibit single-filed structure in a nanochannel.
The motions of the water molecules are concerted transport through the channels.
Obviously, at least one hydrogen bond should be broken if we want to stop the water
flow across the nanotube. There are at least two methods to achieve this goal. One
is to deform the nanotube and the other is to use external electric field to attract the
water molecules.
As the model shown in Fig. 1.3 , a single-walled carbon nanotube (SWNT)
13.4 A in length and 8.1 A in diameter is embedded in a graphite sheet as a water
channel. The SWNT and the graphite are solvated in a box (3 3 4 nm) with water