Models for Bulk, Confined Water and Aqueous Solutions Upon Supercooling: State of the Art and Future Perspectives in Understanding Water Anomalies by Computer Simulations
CECAM-HQ-EPFL, Lausanne, Switzerland
It is now well established that water plays an important role in many processes in chemistry, chemical engineering, biology, geochemistry and many others technological applications.
In spite of the progress in experimental, computer simulation and theoretical methods there are still a number of open questions that are connected to the peculiar properties of bulk water and its anomalies.
The methods of computer simulation are very appropriate to investigate more in details the behaviour of water. In different fields of application water is at contact with different environments and the studies are focused on realistic models to catch the main features of water at contact with the different substrates. In statistical mechanical the problem of water is mainly studied for its peculiar properties in a more general framework also to explore the connection with analogous phenomena taking place in different systems
In a previous workshop in 2009 a number of relevant topics concerning dynamical and structural properties of water, water solutions and confined water were discussed.
The aim of the workshop was to exchange ideas between theorists working with computer simulations on the properties of water in various environments with different methodologies. During the workshop a number of open problems were identified and possible unifying concepts and appropriate models were discussed.
Due to the large variety of phenomena where water plays an important role it was observed in the final discussion that it would be useful to organize future meetings focused on more specific topics For this reason we submitted a proposal for a new workshop.
The present workshop will be devoted to the study of the anomalies of water upon supercooling. This problem is still attracting the attention of a large community of scientists and it was only partially discussed in the 2009 workshop.
Cold liquid water expands and becomes more compressible when cooled, at fixed pressure as the temperature decreases the density reaches a maximum and then decreases. At the temperature of maximum density (TMD) the thermal expansion changes sign. Upon supercooling it is experimentally observed a strong increase in response functions like isothermal compressibility and specific heat. Anomalies of water become more important below the melting point.
The role of the hydrogen bond network in the liquid phase and the anomalous behaviour of water approaching the melting and in the supercooled phases are under investigation since long time. Computer simulation gave a great contribution in the comprehension of this phenomenology.
Bulk water, if crystallization is prevented, can be supercooled in the liquid state below room temperature to the limiting temperature of homogeneous nucleation. It is experimentally possible to supercool liquid water down to TH= 232 K at 1 atm and to TH = 181 K at 2000 atm. These extreme conditions are not only obtained in laboratory, in nature water can exist in its liquid form at −20 ◦C in insects, −37 ◦C in clouds or −47 ◦C in plants.
Below the limiting temperatures the crystal phase occurs. Sixteen forms of crystal ices have been found. This shows that water has a tendency to polymorphism.
Polymorphism characterizes also the glassy water, since three forms of amorphous ice have been found experimentally, they are called high density amorphous (HDA), low density amorphous (LDA) and very high density amorphous (VHDA) ice. At ambient pressure glassy water can be formed with a rapid quench to 100 K. Glassy bulk water undergoes a glass transition upon heating to 136 K. Above 150 K cubic and then normal hexagonal ice is formed. There is a region, between 150 and 232 K at atmosferic pressure, called “no-man’s land_”, where bulk water cannot be studied in its supercooled liquid phase. At increasing pressure the limiting temperatures change but there is always a region where experiments on supercooled water are hampered by freezing occurring below T H. To explain the anomalies of water different scenario have been proposed, they all refer to the behaviour of water in the supercooled state. The different interpretations of the anomalies of water have become an important issue in the studies of phenomena like the onset of protein activity at low temperatures, cryonics, cryopreservation, cryostasis and cryobiology, as it has become clear that water plays a fundamental role,
In this workshop we intend to bring together some of the major experts of theoretical approaches and computer simulation of supercooled water in bulk, in solutions and in interaction with substrates and scientists interested to this field to exchanging and confronting ideas, following the lines sketched below.
All the anomalous behaviour of water is connected to the properties of the hydrogen bond (HB) network of water. In normal thermodynamical conditions the HB configuration is characterized by the well known tetrahedral arrangement of the molecules, as a consequence of the HB interaction. This local order can change with change in the temperature and with increasing compression. Different models are able to describe the microscopic changes in the structure of water, since they usually include the main features of the HB interaction. On the other hand they are based on different assumptions and approximations. Four different scenarios have been proposed until now in the interpretation of the anomalies of water:
A) The stability limit (SL) scenario is based on the behaviour of the liquid–gas spinodal prolonged upon supercooling.
B) The liquid–liquid critical point (LLCP) scenario hypothesizes the existence of a second critical point connected to a liquid-liquid transition in the no man’s land . The two liquid phases would be a low density liquid (LDL) and an high density liquid (HDL), corresponding to the LDA and HDA amorphous phases mentioned above.
C) The singularity-free scenario connects the anomalies upon supercooling to local density fluctuations with an interplay between the volume and the entropy behaviour, which explains the anomalous increase of the response functions without the appearance of singularities.
D) In the critical-point free scenario the liquid-liquid transition is hypothesized as a first order order–disorder transition.
These four scenarios predict fundamentally different behaviour, but the theories cannot be tested against experiments in bulk water since in the region of interest water undergoes the transition to the ice crystalline phase.
For this reason the interest has been recently focused on water in contact with different environments. It has been found that water adsorbed onto the surface of proteins or confined in nanopores freezes at much lower temperature than bulk water, with the possibility of studying liquid water in thermodynamical conditions where bulk water is solid.
A large amount of experimental work has been recently done on confined water to study dynamical properties of supercooled water. In particular it was observed a crossover at T around 225 K of the translational relaxation time from a non-Arrhenius behavior at high T to an Arrhenius behaviour at low T . This transition has been already predicted in a number of simulations of water models. These studies support the hypothesis of a second critical point or the singularity free scenario.
The liquid-liquid transition and related phenomena have been found also in other supercooled systems, like silica, and it is of great interest to explore the connections between the case of water and other systems that show an analogous behaviour.
Paola Gallo (Universitá degli Studi Roma Tre) - Organiser & speaker
Mauro Rovere (Roma Tre University) - Organiser