The Exceptional Properties of Liquid Water

These charges are so positioned in each H2O molecule that they reproduce its dipole aud quadrupole moments. However, this necessary simplification has a drawback— MD has an inherent inability to take into account the directionality of the H-bonds these water molecules establish, thus discarding one of the central properties of H-bonds. It is at the origin of the failure of MD approaches to reproduce the exceptional properties of liquid water described in Ch. 9. The same difficulty is encountered in describing the properties of the H-bond network developed by H2O molecules that are embedded in macromolecules. 

Water also possesses the property of dissolving a variety of substances and causing their ionic dissociation. There are very few other compounds which compare with water in this respect, with the exception perhaps of liquid HF, HGN and HCONH2, although these compounds have been far less investigated because of the experimental difficulties encountered in using them. 

Figure 9.9 Exceptional physical properties of liquid water (solid lines) temperature dependences (upper diagrams) of the density d (45) and isothermal compressibility Xt (adapted from Refs. (45 7)) pressure dependences (lower drawings) of the shear viscosity 7] at various temperatures (adapted from Ref. (48)) and of the isothermal diffusion coefficient Z) at 0 (adapted from Ref. (49)). Dashed lines sketch typical dependences displayed by almost all other liquids. Note that at —15 °C no value is given for 17 at/ > 300MPa, because of a phase transition towards ice V (Figure 8.5).

In this chapter we did not investigate the exceptional biological properties of liquid water. The reason is that if it is well known that water is absolutely necessary for life to proceed, at least life as we know it on Earth, it is not evident that the presence of liquid water is a fundamental prerequisite for life to occur. The necessary condition for life to occur seems to be the presence of H2O molecules with their specific ability to build around them an exceptionally dense H-bond network. Even if liquid water is the basic and most simple system that provides such a dense H-bond network, it is not the only one, as dense but slightly different networks may also exist in more locally defined structures with H2O molecules. We examine this point in more details in Ch. 10. 

The exceptional chemical properties of liquid water are better understood at the molecular level, even if much work stiU remains to be done before precisely describing them. The role of HjO molecules is, however, not limited to liquid water. It is fundamental in biomedia, where the H-bond network developed by HjO molecules is somewhat less extended than in liquid water, due to the presence of other molecules, most of the time macromolecules that have densities of hydrophilic groups much smaller than HjO molecules. The capacity of even a small number of H2O molecules to develop such a nevertheless dense H-bond network... 

Notwithstanding their very low vapor pressure, their good thermal stability (for thermal decomposition temperatures of several ionic liquids, see [11, 12]) and their wide operating range, the key property of ionic liquids is the potential to tune their physical and chemical properties by variation of the nature of the anions and cations. An illustration of their versatility is given by their exceptional solubility characteristics, which make them good candidates for multiphasic reactions (see Section 5.3.4). Their miscibility with water, for example, depends not only on the hydrophobicity of the cation, but also on the nature of the anion and on the temperature. 

The physical properties of bismuth, summarized in Table 1, are characterized by a low melting point, a high density, and expansion on solidification. Thermochemical and thermodynamic data are summarized in Table 2. The solid metal floats on the liquid metal as ice floating on water. Gallium and antimony are the only other metals that expand on solidification. Bismuth is the most diamagnetic of the metals, and it is a poor electrical conductor. The thermal conductivity of bismuth is lower than that of any other metal except mercury. 

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