Water clathrate structures

FIGURE 2.5 Formation of a clathrate structure by water molecules surrouudiug a hydrophobic solute. 

The spontaneous separation of oil and water, a familiar observation in everyday life, is due to the energetically unfavorable formation of clathrate structures. When a mixture of water and oil is firmly shaken, lots of tiny oil drops form to begin with, but these quickly coalesce spontaneously to form larger drops—the two phases separate. A larger drop has a smaller surface area than several small drops with the same volume. Separation therefore reduces the area of surface contact between the water and the oil, and consequently also the extent of clathrate formation. The AS for this process... 

Sorensen, C.M., Clathrate Structures and the Anomalies of Supercooled Water, in Proc. 12th Int. Conf. on Prop, of Water and Steam, Orlando, FL, September (1994). 

In a review of the thermodynamics of water, Franks and Reid (1973) showed that the optimum molecular size range for maximum solubility was similar to hydrate stability. Franks and Reid noted, this is not intended to imply that long-lived clathrate structures exist in solution—only that the stabilization of the water structure by the apolar solutes resembles the stabilization of water in a clathrate lattice. Glew (1962) noted that, within experimental error, the heat of solution for ten hydrate formers (including methane, ethane, propane, and hydrogen sulfide) was the same as the heat of hydrate formation from gas and ice, thereby suggesting the coordination of the aqueous solute with surrounding water molecules. 

Fig. 17. Model of the clathrate structure of the water in clathrates I. The centre of the hole is occupied by hydrophobic guest molecules in gas-hydrates (model of the iceberg formation in aqueous solution )...

Gas hydrate inhibitors. Gas hydrates, solid water clathrates containing small hydrocarbons, are problematic for oil and gas production because they can precipitate and cause line blockage. Simple cationic surfactants containing at least two butyl groups were previously developed to inhibit formation of gas hydrate precipitates in gas production lines [87]. However, similar to the situation with cationic drag reduction additives, poor toxicity profiles prevent widespread commercial acceptance. Ester quaternaries with structures somewhat similar to those used in fabric care have been claimed as hydrate inhibitors [88 ]. Additionally, certain alkylether quaternary compounds, e.g. C12-C14 alkyl polyethoxy oxypropyl tributyl ammonium bromide, were shown to have hydrate inhibition properties [89]. 

If apolar hydration is characterized by the conditions that AG° > 0, TAS < 0 and AH < 0, then a process which minimizes exposure of apolar groups to water should be a thermodynamically favoured process. Then if two apolar groups of either the same or different molecules come together in water, AS for this process will be positive because some of the structured water is released into the bulk solvent. Such association is called hydrophobic, hydrophobic bonding or hydrophobic interaction (Kauzmann, 1959). The term bond is probably inappropriate because the association is due to entropy rather than to enthalpy effects, a consequence of the disruption of the clathrate structure around the apolar solute (Jolicoeur and Friedman, 1974). Despite the general acceptance of the concept of hydrophobic association, there are different approaches to the problem of understanding this phenomenon. 

Water clusters containing simple ions are another area of current experimental and theoretical interest. Accordingly, they are also the subject of EA studies. Chaudhury et al. [113] have used EA methods on empirical potentials to obtain optimized structures of halide ions in water clusters, which they then subjected to AMI calculations for simulation of spectra. EA applications to alkali cations in TIP4P water clusters [114,115] have led to explanations of experimental mass-spectroscopic signatures of these systems, in particular the lack of magic numbers for the sodium case and some of the typical magic numbers of the potassium and cesium cases, and the role of dodecahedral clathrate structures in these species. 

A consideration of thermodynamic properties of the aqueous solution of rare gases and hydrocarbons led to the iceberg model for water structure around nonpolar molecules [139], which later had to be abandoned (see Part IV, Chap. 23.4). The gas hydrate clathrate structures described in Part IV, Chap. 21 provided... 

Here the layers contain only water molecules which form antidromic pentagons, quadrilaterals and homodromic hexagons (Fig. 21.10). Clathrate or semi-clathrate structures have been postulated for choline chloride hydrate, (H3Q3N+CH2CH2OH 2H20 CP, on the basis of similarities in the solid-state infrared spectra [162], but this has not been confirmed by crystal structure analysis. 

Site III). Each pseudo unit cell contains 27 water molecules, and 20 of these form a pentagonal dodecahedron that fines the a-cage (Figure 17). This water structure is found in some water clathrates and in LTA surrounds the central Na ion with its attendant three water molecules. It is cation water assemblages like this that are hkely to be the precursors for the formation of zeolite frameworks in synthesis gels (see Section. 4.2.3). 

The polynuclear anions are usually chlorocuprate(I) complexes, but compound K[Cu2(CN)a] HgO has been identified in the phase diagram of the CuCN-KCN-HaO system 334), The anion s structure is made up of [CuaiCNlg] sheets with the lattice water clathrated in (OuCN)e rings 93). 

Data on the separation of thietane by gas chromatography have been reported along with data on other sulfur compounds. Thin-layer chromatographic separation of thietane from 1,2-dithiolane and 1,2.3-trithiane works well on silica gel. Thietane forms a hydrate (dec 11.7°) with water. Its clathrate structure was investigated by use of the nmr line shapes of the deuterium oxide hydrate. ... 

A bewildering variety of reduced and mixed-valence poly vanadates has been reported (Table 5). Several of these can be viewed as clathrates that encapsulate a guest anion or water molecule within a hollow shell (entries 2, 4, 8, 10-14). Technically, some of the entries in Table 5 should be regarded as heteropolyanions, but it is more convenient to discuss them here, in view of their structural similarities. The guest anions in the clathrate structures have been regarded as templates that control the overall structure shape, and these structural data have been successfully used to generate force-field... 

Figure 20.2. Structure of water clathrates. When water molecules interact with non-water substances, water molecules form acage-like structure to surround them. Depending on the size of the substance, water molecules pack in different ways to solubilize or to form an interface with the materials.

Monomolecular surface films change the structure of the uppermost water layer within a thickness of around some micrometers to possibly some hundred micrometers (Huhnerfuss and Alpers 1983, Huhnerfuss 1986). For example, ice-like clathrate structures are induced by OLA films in a water layer of d < 190 pm. Furthermore, the surface potential of pure water of about - 180 mV becomes positive and may approach values of > 400 mV (Gericke and Huhnerfuss 1989), and the dilational surface viscosity is drastically increased (Huhnerfuss 1985). The relaxation time for disturbances of the surface film order TCOmp attains values of around 10 to 20 min, which are about 1013 times larger than the relaxation time xs of the water molecules (Huhnerfuss and Alpers 1983). 

Fig. 12.15. Clinographic projection showing the arrangement of some of the water molecules in the clathrate structure of chlorine hydrate, Cl2.7fH20.

See also Hydrogen Bonds, Dielectric Constant, Covalent Bonds vs Non-Covalent Forces, Clathrate Structure of Water... 

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