The study of water confined in complex systems in solid or gel phases
and/or in contact with macromolecules is relevant to many important
processes ranging from industrial applications such as catalysis and
soil chemistry, to biological processes such as protein folding or ionic transport
in membranes. Thermodynamics, phase behaviour and the molecular mobility
of water have been observed to change upon confinement depending on
the properties of the substrate. In particular, polar substrates
perturb the hydrogen bond network of water, inducing large
changes in the properties upon freezing.
Understanding how the connected random
hydrogen bond network of bulk water
is modified when water is confined in small cavities
inside a substrate material is very important
for studies of stability and the enzymatic activity of proteins, oil recovery
or heterogeneous catalysis,
where water–substrate interactions play a fundamental role.
The modifications of the short-range
order in the liquid depend on the nature of the water–substrate interaction,
hydrophilic or hydrophobic, as well as on its spatial range and on the
geometry of the substrate.
Despite extensive study, both experimentally and by
computer simulation, there remain a number of open problems.
In the many experimental studies of confined water,
those performed on water in Vycor are of particular interest
for computer simulation and theoretical studies
since Vycor is a porous silica glass
characterized by a quite sharp distribution of
pore sizes and a strong capability to absorb water.
It can be considered as a good candidate for
studying the general behaviour of water in hydrophilic nanopores.
But there there have been a number of studies of water confined
in more complex substrates, where the interpretation of
experiments and computer simulation is more difficult,
such as in zeolites or in aerogels or
in contact with membranes.
Of the many problems to consider we can mention the
study of supercooled water.
It is particularly important to understand
whether the glass transition temperature could be experimentally accessible
for confined water. In this respect the modifications induced by the
confinement on the
dynamics of water on supercooling are of extreme interest and
a number of experimental and computer simulation studies have been
devoted in recent years to this topic.
This special section contains papers from different groups
which have contributed with
various experimental and computer simulation techniques
to the progress made in the study of water in confined geometry.
I thank all of the authors for their stimulating
contributions.
I am very pleased in particular that Sow-Hsin Chen agreed to contribute
since he has done pioneering experimental work
on the dynamical properties of
confined water upon supercooling, and he is still very active in the field.
The work presented by the group of J Swenson concerns
also the glass transition of confined water.
The Messina group (Crupi et al) is very active
in the study of dynamical properties of confined water
and they present their results on water in zeolites.
From the experimental side there is also a contribution from
J Dore's group, one of the first to perform neutron scattering
studies on confined water. The work of J Klein looks at the
mobility of water molecules confined in subnanometre films.
Important contributions on the computer simulation side come
from the Geiger group (Brovchenko et al). They performed
very accurate simulations of water in nanopores, exploring a
large portion of the phase space. Puibasset et al
were able to build a very realistic model to
simulate water inside Vycor. Zangi et al review the extensive
work performed on confined water. Jedlovszky is an expert
on the model potential for water and studied how the hydrogen
bond network of water can be modified by the presence
of an interface.
The special issue is intended to stimulate interest and future work
on this important subject.
