Ebook: Water: Fundamentals as the Basis for Understanding the Environment and Promoting Technology

Water is fascinating in all its phases, forms and states of aggregation. It may look simple, just two hydrogen atoms and one oxygen atom, as we all learned in school, yet its many puzzling properties are still not completely understood. In particular, what is really missing is a microscopically based understanding of the origin of the many water anomalies. Although we know what happens, for instance, if water in a glass bottle is left in the freezer, we have imperfect understanding of the molecular origin of the anomalous density maximum that eventually determines the expansion of water and the breaking of the bottle.

The situation is even more intriguing and consequential, as it is fair to state that the possibility of life on our planet is strongly coupled to this particular anomalous behavior of water, namely the fact that ice, being less dense than water, floats in the oceans at the Earth's poles. This fact has far-reaching consequences. It plays a major role in determining the climate on our planet, it allows the diffusion of nutrients in the sea, and finally permits all human activities based on extracting resources from a liquid ocean, to name but a few. We could then easily resonate with Paracelsus who, in the 16th century, said “water was the matrix of the world and of all its creatures”. Five centuries later, we certainly know quite a bit more about water than Paracelsus, but our understanding of water's properties, both as a pure substance and as a solvent, is still far from being complete.

Our proposal to organize a school on the complexity of water in the beautiful setting of Villa Monastero in Varenna was, therefore, aimed at presenting to interested students several examples of ongoing research on water. Our intention was to offer a glimpse on the many questions that remain to be understood about this molecule. Distinguished scientist were selected to lecture on what we think is the present state of the art, along with current controversies, pertaining to our knowledge on water. The list of topics is by no means exhaustive or complete, and probably reflects more our interests and tastes, but we believe that it is nevertheless wide enough to provide a solid basis to young researchers in the field.

Among the “hot topics” that are at present vigorously debated in the literature is liquid water polymorphism. While polymorphism is a well-known phenomenon for solids, conventional wisdom has until recently asserted that a pure substance can have only one liquid phase. Evidence for liquid polymorphism and the associated liquid-liquid coexistence line was discussed during the first lecture, by H. E. Stanley. This topic is of particular importance, as the existence of an end point of the liquid-liquid coexistence line, water's hypothesized second critical point, might explain many of water's anomalous properties, such as the pronounced increase in the isothermal compressibility and the isobaric heat capacity upon cooling.

The interesting possibility that low-temperature vitreous water phases, differing in density, may be linked to similarly different liquid phases at higher temperature was reported by T. Loerting. Between these low temperature glassy phases and room temperature water, there is the strange realm of supercooled water. What is it? Under carefully controlled experimental conditions, water can remain as a liquid below the melting temperature. Under these circumstances, water is metastable with respect to ice, and it is called supercooled water. In this realm, all the oddities of water, in addition to the above-mentioned density anomaly, become more pronounced. Methods to study the structural properties of supercooled water and their differences with respect to those of ice were presented by A. K. Soper in his lecture. C. A. Angell and A. Nilsson discussed water properties in the fascinating realm of supercooled conditions, including the especially challenging region known as “no man's land”. The investigation of this metastable region, where water is too cold not to freeze upon cooling from the liquid side, and too warm not to crystallize upon heating from the vitreous state, requires new strategies, such as those presented by M. A. Ricci and R. Saykally. These are of particular relevance, as the location of the hypothesized second critical point is believed to lie in “no man's land”.

The study of the properties of water as a solvent is another important topic. Examples range from water in the atmosphere, as discussed in the lecture by T. Koop, to water at the interface of biomolecules, as addressed in the lecture by F. Mallamace. The study of water-biomolecule interactions brings us back to Paracelsus' statement and to the question of whether water is indeed a unique and universal matrix of life, or whether on the contrary it is just the one that happens to pertain to our planet. Fundamental to that question is the role that water plays in sustaining the biochemistry of the cell. It has become increasingly clear over the past two decades that water is not simply “life's solvent” but is indeed a matrix more akin to the one Paracelsus envisaged: a substance that actively engages and interacts with biomolecules in complex, subtle, and essential ways.

A reliable test for the successful outcome of an event like the one we have organized is whether in, say, 20 years, significant advances will have occurred in our understanding of water that can be traced to the ideas discussed in the Varenna school. Obviously (and unfortunately!) we have no crystal ball, but we can make a list of the required ingredients and hope for the better. This list surely includes the beauty of the location, the curiosity and interest of the students triggered by the lectures, and, last but not least, the overall atmosphere encouraging interaction, discussion, and friendship. In Varenna we had all of the above in spades.

In this elementary course, we will introduce some of the 69 documented anomalies of the most complex of liquids —water— focusing on recent progress in understanding these anomalies by combining information provided by recent spectroscopy experiments and simulations on water in bulk, nanoconfined and biological environments. We will interpret evidence from recent experiments designed to test the hypothesis that liquid water has behavior consistent with the hypothesized “liquid polymorphism” in that water might exist in two different phases. We will also discuss recent work on nanoconfined water anomalies as well as the apparently related, and highly unusual, behavior of water in biological environments. Finally, we will discuss how the general concept of liquid polymorphism is proving useful in understanding anomalies in other liquids, such as silicon, silica, and carbon, as well as metallic glasses, which have in common that they are characterized by two characteristic length scales in their interactions.

While the water molecule is simple, its condensed-phase liquid behavior is so complex that no consensus description has emerged despite three centuries of effort. Here we identify features of its behavior that are the most peculiar, hence suggest ways forward. We examine the properties of water at the boundaries of common experience, including stable states at high pressure, the supercooled state at normal and elevated pressure, and the stretched (“negative pressure”) state out to the limits of mechanical stability. The familiar anomalies at moderate pressures (viscosity and density (TMD) behavior, etc.), are not explained by H-bond breaking, according to common bond-breaking criteria. A comparison of data on the TMD, at both positive and negative pressures, with the predictions of popular pair potential models, shows dramatic discrepancies appearing in the stretched liquid domain. This prompts questions on the second-critical-point (TC2) hypothesis that has been guiding much current thinking. We turn to related systems for guidance, reviewing a hierarchy of water-like anomalies. We conclude that water models are far from complete and that proper understanding of water will depend on the success in mastering the measurement of liquid behavior in the negative-pressure domain —which we discuss.

In this article, the multiple roles of water in the Earth's atmosphere are reviewed. In particular, the behavior of atmospheric water in its various physical states such as water vapor, liquid water, and crystalline ice are discussed. Special emphasis is given towards its pure metastable and stable phases at ambient conditions. Moreover, the occurrence of these condensed phases in various types of atmospheric clouds is described. The thermodynamic boundary conditions for the formation of liquid-water clouds and of ice clouds are delineated, and the kinetics of the related phase transitions is discussed. Finally, a kinetic state diagram of atmospheric water is presented.

Here we present a picture that combines discussions regarding the thermodynamic anomalies in ambient and supercooled water with recent interpretations of X-ray spectroscopy and scattering data of water. At ambient temperatures most molecules favor a closer packing than tetrahedral, with strongly distorted hydrogen bonds, which allows the quantized librational modes to be excited and contribute to the entropy, but with enthalpically favored tetrahedrally bonded water patches appearing as fluctuations, i.e. a competition between entropy and enthalpy. Upon cooling water the amount of molecules participating in tetrahedral structures and the size of the tetrahedral patches increase. The two local structures are connected to the liquid-liquid critical point hypothesis in supercooled water corresponding to high-density liquid (HDL) and low-density liquid (LDL). We demonstrate that the HDL local structure deviates from a tetrahedral coordination not only through a collapse of the 2nd shell but also through severe distortions around the 1st coordination shell.

The development of deep-UV second-harmonic generation spectroscopy (SHG) for measuring the strong charge transfer to solvent (CTTS) transitions characteristic of all stable aqueous anions has provided a powerful new probe of water interfaces. By employing suitable models, quantitative thermodynamic results have been obtained for a number of fundamental electrolytes, which are generally in good agreement with theoretical calculations. Details of the experiments and models are described and salient results supporting a novel mechanism for the selective adsorption of ions to the air/water interface are reviewed.

The past decade or so has witnessed a large number of articles about water structure. The most incisive experiments involve radiation with a wavelength compatible with the observed inter-molecular separations found in water, of order ~3 Å, in other words mostly <1 eV neutrons and >10 KeV X-rays. Because X-rays are scattered by electrons while neutrons are scattered by nuclei, the two probes give complementary information, which, when combined with the pronounced isotopic contrast available between deuterons and protons, enables experiments to be devised that allow the three site-site radial distribution functions for water, namely O-O, O-H and H-H, to be determined uniquely. In practice systematic effects in both neutron and X-ray experiments prevent this ideal being attained, so recourse is made to computer simulation to extract these distribution functions from the data. Here a flavour of Monte Carlo simulation called Empirical Potential Structure Refinement (EPSR for short) is used to devise an empirical intermolecular potential which attempts to drive the simulated radial distribution functions as close as possible to the data. New X-ray and neutron scattering data on water in the temperature range 280–365 K are presented for the first time, alongside a new analysis of some much older neutron data on ice I_h at 220 K. This temperature analysis, above and below the water freezing point of water, reveals some non-intuitive water properties in the liquid and solid states.

Water is one of the most abundant molecules on Earth, of paramount importance to our daily lives and is of great relevance in astrophysics. Nevertheless its physical and chemical properties, which are often called anomalous, are not fully understood by now. Investigations in recent decades have shown that water exists in many crystalline forms — a phenomenon known as “polymorphism” — and in three amorphous forms — a phenomenon known as “polyamorphism”. In this article we review the crystalline ice phases and outline possibilities for future experimental discoveries of ice polymorphs. We then provide an overview about the current knowledge on polyamorphism and finally go into more detail about the question whether or not the amorphous ices are linked by glass-to-liquid transitions to deeply supercooled liquids, which has been a major focus in our research group over the last years.

Studies of water in confined geometries are of great relevance, since most water on the Earth surface is confined, in rocks, cells, food and so on. It is however yet unknown to what extent present knowledge about bulk water can help in understanding confined water behavior and vice versa, and to what extent the physics and chemistry of the confining medium affect the properties of bulk water. In other words, if there are any characteristics common to all confined waters. Here we present a structural study of water under several quite distinct confining media, showing that independently of the nature and shape of the substrate, the first water layers in the vicinity of the substrate surface are denser than those far away from it and that the average water density under confinement is lower than in bulk at the same temperature and pressure conditions. On the other hand, the extent of the structural disturbance to water structure, due to confinement, seems the result of a complex interplay between interaction forces and size.

Nuclear Magnetic Resonance is a very powerful technique to study water and aqueous systems. In particular, it is a very local probe with atomistic sensibility. It takes advantage of different probe-heads and numerous pulse sequences to study the structural and dynamical properties of the system. In this work we highlight the application of several of these methodologies for the comprehension of important phenomena such as the dynamical crossover in supercooled water, the clustering dynamics of water/methanol solution and the folding/unfolding process of hydrated lysozyme. Both technical details and physical implications will be discussed.