On the Structure Selectivity of Clathrate Hydrates

We propose a strategic method to search for a new clathrate hydrate by investigating the selectivity of its crystalline structure, which has been expected to depend mostly on the guest species and less on the thermodynamic conditions. Relative stability among various morphologies is examined in terms of the chemical potential of water of clathrate hydrates in equilibrium with guest gases. This evaluation is performed by calculating the free energy of cage occupancy according to the standard statistical mechanical theory with the aid of the Yarmolyuk and Kripyakevich's rule on the Frank-Kasper type alloys to estimate the numbers of cages of various types. Thus, a comprehensive interpretation of the selectivity of crystalline structures is successfully made. We explain why two major structures are stable in most of the thermodynamic conditions and establish a relation between polymorphism of clathrate hydrate and the guest size and other parameters, thereby suggesting a way to find a new clathrate hydrate by appropriate choice of the guest species and/or the thermodynamic properties. It is found that there is a small room in the above parameter space for the other structure than the major two to be the most stable, including a new structure. In addition, simple but thorough elucidation is given for preferential formation of TS-I structure in bromine hydrate.