Clathrate hydrates prediction

In an oligonucleotide-drug hydrate complex, the appearance of a clathrate hydrate-like water structure prompt a molecular dynamics simulation (40). Again the results were only partially successful, prompting the statement, "The predictive value of simulation for use in analysis and interpretation of crystal hydrates remains to be established." However, recent molecular dynamics calculations have been more successful in simulating the water structure in Ae host lattice of a-cyclodextrin and P-cyclodextrin in the crystal structures of these hydrates (41.42). 

In the protosolar nebula, H2O ice condenses at temperatures lower than 150K. Upon lurther lowering of the temperature, part of the H2O ice transforms into NH3 hydrate and CH4 clathrate hydrate. These are the compositions of ice predicted by the equilibrium condensation tlieory. It should be pointed out, however, tliat tlie equilibrium might not actually be realized at low temperatures. Thus, Uie ice composition predicted by the equilibrium condensation theory may not be tlie actual composition, but should be regarded as a model composition. 

Answers to such difficult questions can be found in applied thermodynamics - in terms of measured, macroscopic values of pressures, temperatures, compositions, volumes, enthalpies, etc. This chapter provides an overview of natural gas clathrate hydrates - structures, phase diagrams, and thermodynamic predictions/measurements that guide our understanding in dealing with such questions. The hydrate historical perspective provides an example of how knowledge advances in a technical field. At the conclusion of the chapter, future thermodynamic challenges are presented. 

CLATHRATE HYDRATE EORMATION AND GROWTH EXPERIMENTAL OBSERVATIONS VERSUS PREDICTED BEHAVIOUR... 

Classical Descriptions of Inclusion Compounds, p. 253 Classification and Nomenclature of Supramolecular Compounds, p. 261 Clathrate Hydrates, p. 274 Concepts in Crystal Engineering, p. 519 Crystal Structure Prediction, p. 371 Cyclodextrins, p. 398... 

In 1958, van der Waals and Platteenw developed a statistical mechanical theory for predicting the stability region of the clathrate hydrate phase with different guest molecules under different temperature and pressure conditions. The van der Waals-Platteeuw (vdWP) theory is based on the thermodynamic condition of equilibrium between a hydrate phase water/ice ()8) phase and i guest species encapsulated in the hydrate at the phase boundary ... 

The strength of the vdWP theory is its ease of use and the speed with which predictions of the clathrate hydrate phase diagram can be generated. If the scope of validity of this theory is recognized, it can be a valuable tool in the hand of researchers in predicting hydrate stability conditions. 

Thermodynamic properties of clathrate hydrates have been calculated by the van der Waals and Platteeuw (vdWP) theory [16]. It has been applied to predicting... 

The fact that both heats of formation and equilibrium pressures of the hydrates of spherical molecules correctly follow from one model must mean that the L-J-D theory gives a good account of the entropy associated with the motions of these solutes in the cavities of a clathrate. That the heat of formation of ethane hydrate is predicted correctly, whereas the theoretical value of its vapor pressure is too low, is a further indication that the latter discrepancy must be ascribed to hindered rotation of the ethane molecules in their cavities. 

Nevertheless, as in many previous observations, the clathrate formation by dipolar host compounds could not have been predicted in advance. In fact, there are no channels in the crystal structures of hydrated moxnidazole hydrochloride (closely related species to furaltadone hydrochloride) and of hydrated furaltadone base (Fig. 13)37). Rather, the latter two structures are best described as solvates with the H20 molecules contained in local voids between adjacent moieties of the host. 

The same phenomenology must be important locally on Earth, too, where thick evaporite deposits of hydrated salts and local thick beds of methane clathrate in permafrost or seafloor sediments should influence the thermal environment of the crust. The predicted control on the crust s thermal state by hydrate deposits should have consequences for the localization of hydrothermal springs around and within evaporite basins, hydrothermal metamorphism... 

As well, since the hydrates are clathrates, and as such, non-stoichiometric, the cage occupancies must be measured to define the system and to provide important insights into how well the various modeling parameters used to predict the structure, phase equilibria and composition actually fit experimental data Fortunately, instrumental methods that operate on a molecular scale often can be used to obtain both bulk properties as well as structural information. ... 

---