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MULTIDISCIPLINARY
Water science on the molecular scale: new
insights into the characteristics of water
Jun Hu1,∗ and Zexian Cao2
The highest good is like that of water. The goodness of water is that it benefits all the
creatures; yet itself does not scramble, but is content with the places that all men disdain.
It is this that makes water so near to Tao.
—Tao Te Ching
‘Hydrophobic water layer’: water does not wet
water. Water shows anomalous behavior on the
molecular scale.
WATER IS IMPORTANT
Water is a basic component of human existence, and people and ecosystems depend on it. It is one of the most fundamental requirements for the survival of
all living things. However, water is a finite resource that has quantitative limitations and qualitative vulnerability. Water shortage is a global concern owing to
increasing population, economic growth
and climate change. China is facing severe water scarcity. Some experts believe
that the water crisis may come before an
energy crisis in China. Energy and water
are inextricably and reciprocally linked;
the production of energy requires large
volumes of water (for example, in the
USA and China, electricity production
requires over 40% of all daily freshwater
withdrawals [1]), and both the treatment
and distribution of water depend upon
readily available, low-cost energy. More
recently, water management for unconventional shale gas extraction has dominated environmental debates surrounding the gas industry [2].
WATER SCIENCE ON THE
MOLECULAR SCALE
Water has long been an important
topic in science and technology. Water
has been intensely studied, and ‘water science’ refers to studies on water
and related issues. In the past, water
science was concerned with large volumes of water such as the flooding of
rivers, atmospheric water evaporation/
179
9. Shindell, DT, Lamarque, JF, Schulz, M et al. Atmos Chem Phys 2013; 13: 2939–74.
doi: 10.1093/nsr/nwu005
Advance access publication 14 May 2014
condensation, freezing/melting of seawater and physical/chemical properties of
bulk water in industrial processes. However, modern water science is also concerned with water on the molecular scale.
This is because modern science depends
on the understanding of matter at the
molecular level. Whether it is in the fields
of life sciences, materials science or environmental science, scientists realize that
answers to their questions are taking
place on the molecular scale.
From a molecular point of view, many
bulk properties of water are still not well
understood. ‘Water is simple but very
complex’, Professor Guozhen Yang said
in his opening speech at the Water Science Forum 2013, held in Beijing. It is
simple because everyone knows that water is H2 O and many scientists believe
that we understand water. However, we
know very little about it. Water is not
simply a molecule of H2 O, but a group
of H2 O molecules linked by hydrogen
bonds, and this constitutes the most mysterious matter in the world. Water has
many anomalous properties. For example, no other liquids are found simultaneously in all three phases: gas, liquid and
solid; water undergoes a negative thermal expansion below 4◦ C; water freezes
from the top surface; water O–H stretching vibrations last longer at high temperatures; hot water freezes more rapidly than
cold water; high surface tension and small
surface potential co-exist in water; and
anomalous magnetic and microwave radiation effects occur in water. Recently, it
has been revealed that water is more mysterious when confined to an interface, as
interfacial water has different properties
from the bulk state. Researchers have devoted much effort to understanding interfacial water properties; however, little
is known about water interfaces and the
180
PERSPECTIVES
National Science Review, 2014, Vol. 1, No. 2
structure of water when confined in small
spaces.
Recently, there has been a growing interest in water science owing to the great
progress in theoretical and experimental tools over the last decade, particularly
at the molecular scale. Third generation
synchrotron X-ray spectroscopy has revealed the atomic and molecular structures of liquid water and ice. Using scanning tunneling microscopy (STM), one
can see single water molecules and water clusters on metal surfaces in a vacuum at low temperature. High-resolution
images of the structure of water on
electrodes can be obtained routinely by
electrochemical STM. Surface force apparatus and atomic force microscopy
have been successfully used to measure the subtle interactions between two
solid surfaces immersed in water solutions. Sum-frequency generation probes
the structures of various water interfaces
at the molecular level as well as ultrafast surface dynamics. Infrared, THz and
neutron scattering spectroscopy sensitivities have been improved for obtaining a
wealth of information. Based on the data
obtained by the above advanced techniques and with the help of supercomputers, molecular dynamics and quantum
mechanics simulations are able to answer
some tough questions about anomalous
water behaviors.
Most anomalous water behaviors can
be explained by its hydrogen-bonding
network. In water, the hydrogen atom attracts a neighboring oxygen atom from
another water molecule. This is the hydrogen bond, and its strength is about
23 kJ mol–1 , which is 5 times the average thermal collision fluctuation energy
at room temperature, and is far greater
than van der Waals interactions. Hydrogen bonds are 90% electrostatic and 10%
covalent; they are not too weak and not
too strong. Through hydrogen-bonding
networks water molecules form water
clusters. These water clusters are neither fully ordered nor fully disordered,
and fast dynamic changes occur; hence,
the structure of liquid water is complicated. Hydrogen-bonding networks are
interrupted by surface forces, resulting in
changes in the water structure near an in-
terface. That is why interfacial water has
different behaviors from its bulk state.
AN EMERGING FRONTIER
Some properties of water confined in
molecular scale dimensions play a crucial role in many important processes
and may offer new solutions to water
shortages. Water can flow through a hydrophobic nanotube at a fluid rate over
100 times that in bulk channels [3].
Since the pore sizes in membrane materials used for water treatment can be
fabricated on the nanoscale, this finding is important for the future development of membrane materials. Molecularly thin water films exist at air/solid interfaces and ordered structures such as
‘room temperature ice’ form [4,5]. More
surprisingly, some researchers reported
that water layers on hydrophilic surfaces sometimes behave like a hydrophobic surface that was dubbed ‘hydrophobic water’ [6]. These unusual properties
change the conventional concepts of wetting and friction, inspiring researchers
to design and fabricate new materials
and devices to manipulate water. These
molecular insights into water behavior
also influence processes in atmospheric
chemistry, soil evolution and rock erosion, and can provide new solutions to
many environmental problems and climate changes [7,8]. Traditionally, catalysis has been carried on in a vacuum. In
the 21st century, catalysis studies have
been started to be performed under atmospheric conditions, and scientists consider that surface and interfacial water
play an important role in catalysis [9].
Water in a cell has different properties
from that in a beaker because of crowding
and the confined environment. Recent
findings about the structures and dynamics of the water hydration layer around
proteins and DNA molecules enable a
better understanding of protein folding
and hence lead to better drug design. Water clusters participate in many biological interaction processes, and not only do
they affect biological structures but also
provide channels for electron and proton
transportation [10].
MODERN WATER SCIENCE
RESEARCH
Molecular level water science research
raises the hope to solve water-shortage
problems and provides many challenges
for scientists. When we investigate water
deeper, we find that water often causes
controversies in scientific communities.
In history, water’s complexity and oddness misled to scientists investigating
polywater and developing ‘water memory’ theories. Currently, in water science
research almost every important issue is
controversial. An important feature of the
hydrogen bond is that it is dynamic and
possesses direction. What kinds of structures a water molecule constitutes with
its neighboring molecules remains a mystery and raised hot debates. It is not clear
how hydrogen-bonding networks are disrupted if, for example, water molecules
are close to a surface, or if other molecules
are dissolved among them. The properties of water/air interfaces are currently
debated, and more recently, the observation of interfacial ‘nanobubbles’ with
long lifetimes have instilled new enigmas as thermodynamic equilibrium water/gas states are investigated. The mechanism of water splitting on a surface by
light, a very promising way to harvest energy from the sun, is still not clear.
‘The importance of water science depends on one’s viewpoint,’ Professor Y
Ron Shen said in his speech at the Water Science Forum. He indicated that water science on the molecular level is complex, requiring multidisciplinary knowledge. However, difficulties are encountered because of the lack of effective
molecular level techniques. Understanding the mysterious properties of water
at the molecular level needs theoretical
and experimental collaborations, and relating the results to effective ways to benefit society requires multidisciplinary research efforts. Molecular level water science research is an emerging discipline,
and Chinese scientists have the opportunity to work in multidisciplinary research
projects.
In summary, understanding water and
knowing how to manipulate water at the
molecular level will offer new solutions to
water scarcity problems and save China
PERSPECTIVES
and the world. Basic and applied water research is important to help build an environmentally sustainable world.
ACKNOWLEDGEMENTS
This paper is a collection of views presented at the
‘Water Science Forum, Beijing, June 4–5, 2013’. The
authors thank all the attendees, especially Professors Guozhen Yang and Y Ron Shen.
Jun Hu1,∗ and Zexian Cao2
1 Shanghai Institute of Applied Physics, Chinese
Academy of Sciences, China;
2 Institute of Physics, Chinese Academy of
Sciences, China
∗ Corresponding author.
E-mail: [email protected]
Hu and Cao
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Advance access publication 7 December 2013