Clathrate hydrates characterization

Hester, K.C., Probing Hydrate Stability and Structural Characterization of both Natural and Synthetic Clathrate Hydrates, Ph.D. Thesis, Colorado School of Mines, Golden, CO (2007). 

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. 

The complete characterization of clathrate hydrates is a difficult task. The materials are crystalline however, they are nonstoichiometric. Sample handling must always be carried out where hydrate decomposition is negligible, for example, at low temperatures or at pressures where the hydrate is stable. 

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... 

In our study, the polarizability tensor of methane calculated from Gaussian 94 (22) is isotropic within the numerical accuracy of the calculation. Therefore the formula and calculations could be simplified. However, for most guest molecules in clathrate hydrates that are not as symmetric as methane, this derivation gives a general approach to characterize the induced dipole-dipole interaction. 

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. 

Gas hydrates constitute a class of solids in which small molecules occupy almost spherical holes in icelike lattices made up of hydrogen-bonded water molecules. This class of solids is known as clathrates. Technically speaking, clathrate compounds are characterized by the structural combinations of two substances that remain associated not through strong attractive forces, but because strong mutual binding of the molecules of one sort makes possible the firm enclosure of the other. 

Silicon and germanium have been found to form clathrate-type host lattices in which guest species are alkali atoms. The host lattices are exactly the same as those of type 1 and II hydrates "" (see 16.2.2). They are formed by atoms of only one kind, which are bonded together by strongly covalent forces, as in the Si or Ge diamond structures. The Si—Si or Ge—Ge bond lengths are of the same order of magnitude as in classical Si or Ge with the bond angles 109°28 which characterizes the tetrahedral sp hybridization of the carbon family. 

Many biomolecules are characterized by surfaces containing extended polar regions and also extended non-polar regions. A well-known example is provided by beta-amyloid - the well-known Alzheimer protein. It has extended hydrophobic regions separated by hydrophilic regions, as discussed in Chapter 7. The hydration of extended non-polar planar surfaces may involve novel structures that are orien-tationally inverted relative to clathrate-hke hydration shells, where unsatisfied HBs are directed towards the hydrophobic surface. We have discussed these two geometric arrangements in the appendix to this chapter (Appendix 8.A). 

The responsive behavior of ELRs has been defined as their ability to respond to external stimuli. This property is based on a molecular transition of the polymer chain in the presence of water at a temperature above a certain level, known as the Inverse Temperature Transition (ITT). This transition, whieh shares most of the properties of the lower critical solution temperature (LCST), although it also differs in some respects, particularly as regards the ordered state of the folded state, is clearly relevant for the application of new peptide-based polymers as molecular devices and biomaterials. Below a specific transition temperature (T,), the free polymer chains remain as disordered, random coils [20] that are fully hydrated in aqueous solution, mainly by hydrophobic hydration. This hydration is characterized by ordered, clathrate-like water structures somewhat similar to those described for crystalline gas hydrates [21, 22], although somewhat more heterogeneous and of varying perfection and stability [23], surrounding the apolar... 

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