Water clathrate

Another type of hydrogen bonding of oxetane is clathrate formation with water. Clathrate compositions of 1 6.5 and 1 17 moles of water have been observed <72CR(C)(274)1108). Oxetane has been found to be more effective than other cyclic ethers in solvating uranyl hexafluoroacetonylacetate (81IC1415). 

For a monograph on water clathrates, see Berecz Balla-Achs Gas Hydrates, Elsevier New York, 1983. For reviews, see Jeffrey, in Atwood Davies, MacNicol, Ref. 60, vol. I, pp. 135-190 Cady J. Chem. Educ. 1983, 60, 915-918 Byk Fomina Russ. Chem. Rev. 1968, 37, 469-491. 

Clathrate hydrates are inclusion compounds formed by the enclosure of a small guest molecule within a hydrogen bonded cage of solid-state water. Clathrate hydrates are co-crystals and are thus distinct from ice, which is made of pure water, and hence can have different physical properties to ice such as a different melting point. The classic example of a clathrate hydrate is the burning snowball of methane clathrate hydrate. The combustion of the methane in the clathrate is self-sustaining, Figure 7.1. Many... 

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]. 

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). 

Figure 17 The water clathrate in the large cage of hydrated zeolite A. It is composed of 20 H2O molecules arranged in a pentagonal dodecahedron...

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). 

A quantitative study of the rate of product formation as a function of exposure time and pressure showed that HjS and CH4 arise from secondary reactions in the photolysis, viz., sensitization of the substrate by an electronically or vlbrationally excited molecule of dimethyl disulfide the unit efficiency of the primary step was verified. Thiyl radicals were also observed by EPR and uv measurements on photol-yzed CHaSH in the solid state , in water clathrate matrices and in flash photolysis experiments . Recent evidence points to the formation of hot H-atoms in the primary process. The Hg( Pi)-sensitized decomposition of CH3SH also leads to the formation of CH3S + H but there is a minor (ca 10%) contribution from C-S cleavage, viz. 

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.

Until 1962 only physical inclusion compounds were known. Argon, krypton, and xenon form cage or clathrate compounds with water (clathrate hydrates) and with some organics such as quinol. The host molecules are arranged in such a way that they form cavities that can physically trap the noble gas atoms, referred to as guests. The noble gas will be released upon dissolution or melting of the host lattice. 

Extremely fast decomposition of tetraalkylammonium salts in freshly prepared aqueous alkaline solutions weis observed upon immediate heating of the solutions, whereas no decomposition was observed in aged solutions. This is of importance when zeolite synthesis under fast heating conditions such as microwaves is performed. The decomposition reaction is initially first order in the hydroxide concentration and comes to completion due to OH-consumption. The results are discussed in terms of the formation of water-clathrate-like structures around the alkylcunmonium groups upon ageing of the starting solutions. 

Figure 4 shows two frameworks with direct, cormected nodes. The first one has a connectivity of 4 and corresponds to the stracure of [Si46] K6.8,which is isotype to a water clathrate. Hie Si-Si bonds are covalent and the distance Si-Si is =2.4 A, that gives a fiee aperture of about 2.4 A through die 6-membered silicon rings. The second corresponds to the structure of [65] Ca boron has a cormectivity of 5 in the framework and calcium can be considered as a pest at die center of the cubic unit-cell. Synthesis of such compounds would deserve studies using more bulky pests( 6.2.)... 

It is well-established that tetraalkylammonium salts and other nonpolar solutes exert a strong structure-stabilizing effect on the water lattice [Z09], documented for example in the slowing down of water rotational [Z08] and translational motion [ZIO]. Another important aspect on the interaction between tetraalkylammonium halides and water is that they form crystalline hydrates of high stability. In these the hydrogen-bonded water molecules form large clathrates in which the hydrophobic cations are enclosed [211]. One remarkable observation is that the halide ions may replace water molecules in the water clathrate lattice [312]. 

The disappearance of the silicate feature near heliocentric distances of 2 AU correlates v/ell with the expected stability of water clathrate grains at this distance (Sekanina 1973). However, the albedo of the grains did not change significantly at this distance in Comet Uest, indicating that water ice nay act as a glue that holds the grains in clumps until they are well removed from the nucleus. 

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