CECAM - Modeling and Simulation of Water at Interfaces from Ambient to Supercooled Conditions.Modeling and Simulation of Water at Interfaces from Ambient to Supercooled Conditions.

This workshop aims to enhance communication among the different communities involved in the study of water when it is confined, in contact with different solutes and/or with biological molecules. In this respect the computer simulation methodology seems at the moment the more appropriate to open the possibility of a more general understanding of the connection between the behaviour of water, and in particular confined water, and the properties of substrates, solutes and biological systems. In computer simulation, in fact, it is possible to take under control different microscopic parameters and study more in details the behaviour of systems at the interfaces. In the same time there is the possibility of looking at both static and dynamical properties upon changing the hydration level and/or the temperature. The understanding of the behaviour of water in contact with different substrates and/or in solutions is of crucial importance for a wide range of applications and studies of fundamental problems in physics, chemistry, biochemistry. There are a number of relevant topics, that can be mentioned. For instance the mobility of water and hydrated ions in channels or micropores is important in biological systems and in many systems of interest to chemistry and chemical engineering. It is well known that the presence of water plays a fundamental role in the function of the biological macromolecules [1]. The comprehension of the mechanism of solubility would be of great relevance in the studies of many phenomena like mineral dissolution, biomolecular aggregation [2]. How ions affect hydration and hydrophobic interactions is strictly related to the problem of the Hofmeister series. It has also been observed that water in the vicinity of substrates has a low temperature behaviour that differs from that of bulk water. In certain geometrical confinements the water molecules are unable to form a crystalline structure so water remains liquid below 273K. Studies on water confined in different substrates have shown a very rich scenario where the approach to the glass transition and the observation of different polymorphs are easier [3, 4]. Experiments on protein dynamics have evidenced the presence of a dynamical transition as a hydrated protein is heated from low temperature above about 220 K. This temperature is approximately close to the one at which the crossover from Arrhenius to non-Arrhenius would take place in the hydration water, according to recent experimental work on water at contact with lysozime, DNA and RNA [5]. In the studies of water as a solvent or confined in environments the large variety of phenomena involved, the great range of temperatures to cover, the differences in the type of substrates make difficult to have a clear understanding of the physical mechanisms and parameters which control the behaviour of water in the different cases. Anyway there are key issues that need to be addressed. How the hydrogen bond network is affected by confinement or solutes? When the hydrophobic units are small enough it is expected that water maintains its hydrogen-bond structure in spite of the perturbation of the solute. Instead it has been argued that large hydrophobic species could induce dewetting phenomena at the origin of the attraction between hydrophobic macromolecules [6]. For what concerns ions in water the traditional idea of explaining the Hofmeister series with the presence of structure making and structure breaking type of ions has become to be challenged by experiments and computer simulations. Not different are the questions when water is confined in hydrophobic or hydrophilic media: does water behave in the same way or does the water-confining medium interaction change its behaviour [7,8]? It is well known that water is characterized by its anomalies [9]: do the anomalies of water change for the presence of substrates? How relevant are those anomalies for the behaviour of the different substrates? Confined water can be experimentally studied also in the supercooled state upon approaching the glass transition temperature and in particular in this framework a crossover from fragile to strong behaviour has been found, as mentioned above, possibly related to the existence of a second critical point [4]. In bulk water this crossover has been related to the anomalies [10]. The first aim of the workshop is to exchange ideas between theorists working with computer simulations on the properties of water in various environments with different methodologies. The common motivation would be the understanding of the mutual influence on the static and dynamical properties of water and substrates. During the workshop it would be important to identify the main open problems, the possible unifying concepts and to discuss the more appropriate methods to be used in the different cases. An example is the possible interplay between ab initio and classical computer simulation. The final aim is to establish the state of the art and to identify the possible future lines of research in this field to answer questions like the ones proposed above.

Modeling and Simulation of Water at Interfaces from Ambient to Supercooled Conditions.

Location: CECAM-HQ-EPFL, Lausanne, Switzerland

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