The Structure of Liquid Water

This chapter consists of a description of the structure of liquid water and the nature of ions in aqueous solution. The discussion is largely restricted to the interactions between monatomic ions with liquid water in which they become hydrated by acquiring a hydratiun sphere or shell-Additionally, a few diatomic and polyatomic anions are dealt with, including the important hydroxide ion. The hydration of ions derived from the s- and p-block elements of the Periodic Table, and the derivation of values of their enthalpies and entropies of liydralioii, are described in considerable detail. 

The normal form of ice has an open structure, as described in Chapter l. The oxygen atoms in the solid are surrounded by an approximately tetrahedral arrangement of four hydrogen atoms, two of which are covalently bonded to the oxygen atom the other two are hydrogen 

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Most nonpolar substances have very small water solubilities. Petroleum, a mixture of hydrocarbons, spreads out in a thin film on the surface of a body of water rather than dissolving. The mole fraction of pentane, CsH12, in a saturated water solution is only 0.0001. These low solubilities are readily understood in terms of the structure of liquid water, which you will recall (Chapter 9) is strongly hydrogen-bonded. Dissimilar intermolecular forces between C5H12 (dispersion) and H2O (H bonds) lead to low solubility. 

In this section, rather than give a detailed account of theories of the liquid state, a more qualitative approach is adopted. What follows includes first a description of the structure of ice then from that starting-point, ideas concerning the structure of liquid water are explained. 

Before considering the details of the structure of liquid water, it is important to define precisely what is meant by the term structure as applied to this liquid. If we start from ice I, in which molecules are vibrating about mean positions in a lattice, and apply heat, the molecules vibrate with greater energy. Gradually they become free to move from their original... 

Overall, the main conclusions that are to be drawn concerning the structure of liquid water are as follows. 

The structure of water in its liquid state is very complicated and is still a topic of current research. The structure of liquid water, with its molecules connected together by hydrogen bonds, gives rise to several anomalies when compared with other liquids.6... 

Kuharski, R.A. Rossky, P.J., Quantum mechanical contributions to the structure of liquid water, Chem. Phys. Lett. 1984, 103, 357-362... 

The structure of liquid water was described and compared to that of ice. The importance of hydrogen bonding was indicated. 

The problem of the structure of liquid water has attracted much attention, but as yet no completely satisfactory solution to it has been found. We shall postpone the discussion of this problem until after the description of the structure of certain crystalline hydrates of simple Bubstances. 

Many models for the structure of liquid water have been proposed. One of the most useful is the polyhedral model. 

In liquid water, the thermal motions of molecules are perpetual, and the relative positions of the molecules are changing all the time. Although the structure of liquid water has no definite pattern, the hydrogen bonds between molecules still exist in large numbers. Thus liquid water is a dynamic system in which the H2O molecules self-assemble in perfect, imperfect, isolated, linked and fused polyhedra (Fig. 16.3.3), among which the pentagonal dodecahedron takes precedence. 

Another ongoing debate is concerned with the structure of liquid water. In the conventional century-old picture, liquid water is simply an extended network where each water molecule is linked (or hydrogen-bonded) to four others in a tetrahedral pattern. Contrary to this, there are recent synchrotron X-ray results which suggest that many water molecules are linked to only two neighbors. Yet there are also more-recent X-ray data which support the original structure, and so this controversy is by no means resolved. Water clearly plays indispensable roles in essentially all fields of science, most notably in chemistry and the life... 

Unfortunately the descriptor bifurcated hydrogen bond was also used for the quite different configuration 2 in the very influential 1960 edition of The Hydrogen Bond [28]. Because of its possible relevance to the structure of liquid water, this configuration has received considerable theoretical attention [50, 51], although, in fact, it is rarely observed in the crystalline state, and is not found in any of the ice structures. 

The structure of liquid water has been studied both by Monte Carlo [355] and molecular dynamics [356]. In these calculations, hydrogen-bond potentials based on the water dimer model are unsatisfactory, and the simulations have to include the increased binding energy due to cooperativity to give acceptable thermodynamical results [357]. 

Swaminathan S, Beveridge DL (1977) A theoretical study of the structure of liquid water based on quasi-component distribution functions. J Am Chem Soc 99 8392-8398... 

The structure of liquid water has been the subject of much study, experimentally and theoretically. In addition to water dimers17 there are trimers18 and tetramers that have ring structures. The pentamer is also cyclic while the heptamer prefers a 3-dimensional structure. In the complex C6H6(H20)6, the water appears to have a structure containing four single and two double donor water molecules.19... 

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