Hydrogen-bonded waters

The dissolution of polar molecules in water is favored by dipole—dipole interactions. The solvation of the polar molecules stabilizes them in solution. Nonpolar molecules are soluble in water only with difficulty because the relatively high energy cost associated with dismpting and reforming the hydrogen-bonded water is unfavorable to the former occurring. 

More complicated and less known than the structure of pure water is the structure of aqueous solutions. In all cases, the structure of water is changed, more or less, by dissolved substances. A quantitative measure for the influence of solutes on the structure of water was given in 1933 by Bernal and Fowler 23), introducing the terminus structure temperature, Tsl . This is the temperature at which any property of pure water has the same value as the solution at 20 °C. If a solute increases Tst, the number of hydrogen bonded water molecules is decreased and therefore it is called a water structure breaker . Vice versa, a Tsl decreasing solute is called a water structure maker . Concomitantly the mobility of water molecules becomes higher or lower, respectively. 

Fig. 1. The structure of gas hydrates containing a hydrogen-bonded framework of 46 water molecules. Twenty molecules, arranged at the comers of a pentagonal dodecahedron, form a hydrogen-bonded complex about the comers of the unit cube, and another 20 form a similar complex, differently oriented, about the centre of the cube. In addition there are six hydrogen-bonded water molecules, one of which is shown in the bottom face of the cube. In the proposed structure for water additional water molecules, not forming hydrogen bonds, occupy the centres of the dodecahedra, and...

Fig. 35.—(a) Stereo view of about a turn of the 3-fold double helix of potassium gellan (41). The two chains are drawn in open and filled bonds for distinction. Both intra- and inter-chain hydrogen bonds stabilize the helix. The vertical line is the helix axis. Octahedrally coordinated potassium ions (crossed circles) and triply hydrogen-bonded water molecules (open circles) located above the ions are integral components of the structure of 41. 

Hydrophobic interactions of this kind have been assumed to originate because the attempt to dissolve the hydrocarbon component causes the development of cage structures of hydrogen-bonded water molecules around the non-polar solute. This increase in the regularity of the solvent would result in an overall reduction in entropy of the system, and therefore is not favoured. Hydrophobic effects of this kind are significant in solutions of all water-soluble polymers except poly(acrylic acid) and poly(acrylamide), where large heats of solution of the polar groups swamp the effect. 

The formation of clusters of hydrogen-bonded water molecules around different sizes of apolar molecules... 

Gas hydrates are non-stoichiometric crystals formed by the enclosure of molecules like methane, carbon dioxide and hydrogen sulfide inside cages formed by hydrogen-bonded water molecules. There are more than 100 compounds (guests) that can combine with water (host) and form hydrates. Formation of gas hydrates is a problem in oil and gas operations because it causes plugging of the pipelines and other facilities. On the other hand natural methane hydrate exists in vast quantities in the earth s crust and is regarded as a future energy resource. 

Suhai128 investigated water dimer and an infinite chain of hydrogen-bonded water molecules by means of the DFT and post-Hartree-Fock calculations. For the infinite system, the DFT(BLYP), MP2, and MP4 binding energies were within 0.2 kcal/mol, whereas the corresponding interatomic distances were within 0.04 A. A similar agreement was reported for water dimer. 

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