Clathrate hydrates guest properties

Less common clathrate hydrates formed by compounds other than natural gas guests (such as Jeffrey s structures III-VII, structure T, complex layer structures) and high pressure hydrate phases are also briefly described to provide a comprehensive account of clathrate hydrate structural properties. 

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... 

In this study we performed experiments to investigate the incorporation rate of gas molecules in hydrates and the formation rate of clathrate hydrates from a liquid water phase in absence and in presence of a free gas phase. In case of a present free gas phase we observed the hydrate formation process in an optical cell. We also analysed the gas composition of the gas which was encased in the hydrate after the decomposition of the hydrates (Figure 2, Table 3). Due to the fact that the aim of this study was to investigate the influence of molecular properties of the guest molecules such as dimension (— diffusivity) and water solubility (—> concentration/fugacity) we focused our observation on the content of isomers which do have the same molecular weight but different molecular dimensions (see Table 2) and solubilities. 

In the last 30 years or so. clathrate science has blossomed. with the reports of many new guests, a new hydrate structural family, a complete revision of the stmcture-size relationships, new approaches for hydrate characterization. refined models for hydrate stability prediction, high-level calculations and computational work on hydrate physical properties, experimental work and models for kinetic processes, etc. 

We may evaluate thermodynamic stability under various conditions where a set of the independent thermodynamic variables is specified. Specification of the independent variables is equivalent to specification of ensemble. In any case, the number of water molecules is fixed to a constant value, and the temperature is also set to the constant one, T. The other mechanical conditions depend on the choice of the variables. The number of water molecules, irrespective of the choice of ensemble, is reserved for the extensive property to indicate the system size. Whatever the other properties are, they can be substituted for the (formal) intensive properties. According to the phase rule, the number of degrees of freedom for clathrate hydrate in equilibrium with the guest fluid consisting of a single component (A = 2,/jp = 2),//f = 2 + / c—r pis2. 

Hereafter in this section, the thermodynamic property, iig, is a mean value of the number of guest molecules in the clathrate hydrate and is identified with (ng) in the remaining parts. The system is now characterized by (K T, This... 

For a multiple-component clathrate hydrate where the number of molecules denoted by type a is highly affinitive but is fixed to n, the thermodynamic properties of the clathrate hydrate under deficiency of the molecules a are examined. The guest a, called promoter molecule, is a species that forms a clathrate hydrate at a given thermodynamic condition provided that enough amount of a is supplied. The other molecules are tentatively removed and the distribution of a among available cage types (the number of n j molecules) is determined so that Z" has the maximum, which is equivalent to Bragg-Williams model, and is written as... 

Owing to the assumptions in the vdWP theory, the chemical potential of water is divided into two parts one part is the chemical potential of empty lattice, /cj , which depends only on the crystal structure of the clathrate hydrate, and the other part is the chemical potential of cage occupancy, A/Cc. which depends on the properties of the guest molecule and the cage size. 

During the last decade there has been a rapid expansion in the scope of experimental studies on clathrate hydrates. In particular, differential scanning calorimetry studies have been used to obtain direct measurements of thermodynamic properties and accurate measurements of composition [14], and Xe NMR has been used as a direct probe of the environment of the guest molecules [15]. Measurements of the thermal conductivity have also attracted considerable interest [16]. Other related experiments include supersonic beam [6] and thin-film IR [17] studies, which have been used to study some of the processes that might contribute to nucleation of clathrate hydrates. These experiments are beginning to provide the sort of data base that is needed for a fundamental understanding of the behaviour of clathrate hydrates. 

The computer simulations presented above were designed to examine the cell theory of clathrate hydrates, and to indicate just why that theory breaks down in some circumstances. The simulations indicated that at least two difficulties can arise with respect to the fundamental assumptions guest-guest interactions can stabilise the clathrate significantly at high occupancies, and the properties of the water lattice can depend on the nature and number of molecules present. In addition, the usual methods of... 

An important advantage of the inclusion complexes of the cyclodextrins over those of other host compounds, particularly in regard to their use as models of enzyme-substrate complexes, is their ability to be formed in aqueous solution. In the case of clathrates, gas hydrates, and the inclusion complexes of such hosts as urea and deoxycholic acid, the cavity in which the guest molecule is situated is formed by the crystal lattice of the host. Thus, these inclusion complexes disintegrate when the crystal is dissolved. The cavity of the cyclodextrins, however, is a property of the size and shape of the molecule and hence it persists in solution. In fact, there is evidence that suggests that the ability of the cyclodextrins to form inclusion complexes is dependent on the presence of water. Once an inclusion complex has formed in solution, it can be crystallized however, in the solid state, additional cavities appear in the lattice, as in the case of the hosts previously mentioned, which enable the inclusion of further guest molecules. ... 

The name clathrate, from the Greek klathron KXaOpov, bolt or lock, since the volatile guest compound is locked into the crystal), was coined by Herbert M. Powell, who studied many of them. Examples of clathrates are provided by the gas hydrates, first identified by Sir Humphrey Davy, who prepared chlorine hydrate by bubbling chlorine into cool water. This hydrate was shown to have the chemical formula 8CI2 46H2O. The anesthetic properties of chloroform have been attributed to the formation of such gas hydrates in brain tissue. ... 

A remarkable variation of the type-I clathrate superstructure is associated with the formation of semiclathrates in the Ge-P-Q (Q = Se, Te) systems [34, 35]. The term semiclathrate was first introduced for hydrates of quaternary ammonium salts, which show general structural and physical properties of normal clathrates but in addition feature a single hydrogen bond between host and guest substructures [50]. Up to now quite a number of semiclathrates-hydrates have been reported in the literature [51] compared to a very limited number of inorganic semiclathrates, which are exclusively cationic semiclathrates. 

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