How the "great secrets of water" solve "

In authoritative scientific journals, several articles have recently published at once, explaining the "abnormal" properties of water. “Anomalous” are not in the sense of “memory” and other, alas, popular delusions about water. Just in many cases, water does not behave like other liquids. Without a number of its “abnormal” properties, life would not be possible or take a completely different look. So, the ice, as you know, does not sink in the water, while the vast majority of other materials are drowned in liquid phases (gallium can be called as exceptions). All this allows fish and other water creatures to experience the winter under an ice crust. Typically, the bodies expand when heated, and compressed when cooling, but something curtains occurs with water. If, when cooling from a boil temperature to +4 ºС, it, as it should be, is compressed (the density increases, reaching a maximum), then when cooling below +4 ºС, it, on the contrary, expands (the density decreases). The water has high heat capacity, so it slowly accumulates heat and slowly gives it, significantly affecting the climate, making it softer. Moreover, the heat capacity itself grows when cooling below +35 ºС. Most of these properties are explained by the ability of water molecules to form four hydrogen bonds. Such connections arise when a large electrical atom (like oxygen of one water molecule) attracts hydrogen of another molecule. Inside the molecule, hydrogen H and oxygen o are connected by covalent bonds - they are divided by non -fan electrons, creating a common electronic pair. Hydrogen bond by about one tenth is covalent in nature, but basically it occurs due to electrostatic interactions between hydrogen and electroid atom.

Typically, the high heat capacity of water is explained by the need to break hydrogen bonds to turn into steam. Physicists Anders Nielson and Lars Peterson from Stockholm University explain everything at a deeper level - the emergence of local heterogeneities in the water.

In January, they published a review in Nature Communications [1], summing up the achievements of water research over the past few years. Water on a large scale is homogeneous, but is it homogeneous at nano -level, if you consider it not statically, but dynamically - tracking the fluctuations between different types of structures forming? This question was primarily asked by scientists. And it turns out that the water up to a boil is locally heterogeneous, in it, as short -lived fluctuations, the structures of low -tight water (NPV) can form, in which molecules prefer to gather in clusters, weakly tied to each other, and more homogeneous highly dense water (HSV). One of the most ambitious goals of many water studies is to search for the second critical point, the “critical point“ liquid - liquid ””, where the line between the described two types of water will disappear. Recall that at the usual critical point (about +374 ºС at a pressure of 22 MPa), the differences between the gaseous and liquid phases disappear.

In their review, scientists built the most complete phase diagram of water at the moment, taking into account the possible second critical point. If you cool the water below 0 ºС - make it hypothermia, then the fluctuations between the types of water will increase, ending in their merger at this point. Presumably, it is located in the area of ​​–45 ºС, but it is possible to cool water only up to –42 ºС when it begins to freeze spontaneously. The temperature is even lower, somewhere –196 ºС, you can get ultra-suffering cooling, a million degrees per second, but it is extremely difficult to conduct such studies due to instant freezing of water. The line of the phase transition of the HSV and NPV coincides with the Vidom line, on which the fluctuations of both types of water reach the maximum. The line should end at the second critical point.

Highly water exists precisely from the side of the species line, which meets the usual temperatures for the environment. This fact explains why such a structure is often interspersed with low -bearing water clusters, and is observed in experiments. When cooling, the water approaches the line of the species, fluctuations increase, which is why we have called “abnormal” properties.

On the other hand, the zone of high fluctuations reaches a temperature of +47 ºС, at which the compressibility of water is minimal. That is, all living organisms, except for particularly heat -loving creatures, are in the “abnormal” area of ​​the phase diagram, which had a significant impact on the development of life.

Attention in the review was also paid to the work of 2014 [2], the authors of which reached the water temperature 5 ºС below the previous record of –42 ºС. True, only for micron drops and only for a few moments, for which they managed to remove the diffraction picture of the structure. For water cooling, a linear source of coherent light was used (Linac Coherent Light Source - LCLS) - a laser working on hard X -ray radiation, one of the most powerful in the world. The experiment showed that at this temperature, the main composition is just low -densed water, close in structure to ice.

Immediately after the review in the same magazine, an article by Daniel Elton and Marivi Fernandez-Serg from the University of Stonuni Brook [3], who showed that water under normal conditions, was much more similar to the ice than was considered. It is possible that this means a larger fraction of NPV. Scientists suggested that peaks from the infrared spectrum of water, which should not be in theory, create phonons. Actually, a phonon is a concept from a solid physics, a quantum of fluctuations in the lattice. In the model of researchers, water molecules form long chains fastened with hydrogen bonds in which transverse phonons can spread, creating the very abnormal peaks. It turned out like biting ice in the water.

In another important work [4], the methods of molecular dynamics were studied, on the contrary, supercritical water - taken under conditions above a critical point. Such a substance has a high density close to ordinary water, low viscosity, it has no surface tension. Supercritical water is also divided into highly dense, showing more “liquid” properties, and low -tight with more “gas” properties, separated by the Vidome line. H2O molecules are assembled into clusters unrelated with each other with hydrogen bonds, and the density depends on the size of the clusters. An important consequence of work is the construction of the theoretical teragerz spectrum, which the experimenters can focus on. There are many complex tasks in water research - studying the degree of its heterogeneity, the sharpness of the border between fluctuating areas, determining the second critical point, researching behavior at temperatures below –42 ºС - however, the coordinated interaction of experimental, theoretical and computational techniques that have now developed, gives hope for the solution of many of them.

Oleg Fairy,
MIPTIA graduate student, employee of the Laboratory of Computer Design of Materials

1. Nilsson A., Petterson LGM The Structural Origin of Anomalous Properties of Liquid Water // Nature Communications (2015) www.nature.com/ncomms/2015/151208/ncomms9998/full/ncomms9998.html

2. Sellberg ja et al. Ultrafast X-Ray Probing of Water Structure Below The Homogeneus Ice Nucleation Temperature. Nature (2014) - www.nature.com/nature/journal/v510/n7505/abs/nature13266.html

3.lton DC, Fernández-Serra M. Hydrogen-Bond Network of Water Supports Propagating Optical Phonon-Like Modes. Nature Communications (2016) - www.nature.com/ncomms/2016/160104/ncomms10193/Full/ncomms10193.html

4. Śmiechowski M., Schran S., Forbert H, Marx D. Correlated Particle Motion and Thz Spectral Response of Supercritical Water. PHYS. Rev. Lett. (2016) - http://journals.aps.org/prl/abstract/10.1103/physrevlett.16.027801