Supercooled Water

Abstract

AbstractWater is the most important fluid in nature. The properties of water are of interest for different fields of research in physics, chemistry, biology and medicine. A large amount of experimental and theoretical work exists on pure water, water in solution and water at contact with different substrates. The water molecule has a very simple formula with covalent bonds and a permanent dipole moment. Molecules of water in a fluid phase or in a crystal are connected with hydrogen bonds. The hydrogen bond network determines the crystalline structure of ice. At melting the network, it is still present in the short range order of the liquid phase. The hydrogen bond is at the origin of all the properties of water. From a more fundamental point of view, water is interesting since it shows a great number of anomalies in different portions of its phase diagram. The most well-known anomaly of water is that ice has a density lower with respect to the coexisting liquid. The crystalline order makes possible to arrange the molecules in a larger volume with respect to the liquid phase. A precursor effect of the approach to freezing it is found in the anomalous behaviour of the density of liquid water. The liquid density increases with decreasing temperature, but around 4∘C at ambient pressure, the liquid density reaches a maximum, and then it starts to decrease. By varying the pressure, it is possible to trace a curve of the temperatures of maximum density. The most interesting and intriguing properties of water are observed under extreme conditions of the phase diagram. Anomalies are found particularly at high temperatures in the supercritical state and on the other side when the water is kept in the liquid state below the melting temperature. The supercooled region that can be explored in experiments, however, is limited by difficulties in avoiding crystallization. In this respect, due to the possibility to have very fast cooling rates, computer simulation has played a very important role in these studies. The behaviour of supercooled liquid water is a topic of a large amount of research both for experiments and theories. Here we will focus particularly on the recent study of the supercooled liquid water in approaching the glassy state. To explain the anomalous behaviour in that region of the phase space, it was formulated the hypothesis of the presence of a coexistence between two water forms in the liquid with a counterpart in the glassy states of amorphous ice. This idea opened a new possible interpretation of the phenomenology of water in all the thermodynamic space.KeywordsSupercooled and glassy waterLiquid-liquid critical pointWater as a two component liquidDynamics upon supercoolingFragile to strong crossover