Clathrate hydrates with hydrogen-bonding guests

Abstract

Clathrate hydrates (CHs) are inclusion compounds in which “tetrahedrally” bonded H₂O forms a crystalline host lattice composed of a periodic array of cages. The structure is stabilized by guest particles which occupy the cages and interact with cage walls via van der Waals interactions. A host of atoms or small molecules can act as guests; here the focus is on guests that are capable of strong to intermediate H-bonding to water (small ethers, H₂S, etc.) but nevertheless “choose” this hydrate crystal form in which H-bonding is absent from the equilibrium crystal structure. These CHs can form by exposure of ice to guest molecules at temperatures as low as 100–150 K, at the (low) guest saturation pressure. This is in contrast to the “normal” CHs whose formation typically requires temperatures well above 200 K and at least moderate pressures. The experimental part of this study addresses formation kinetics of CHs with H-bonding guests, as well as transformation kinetics between different CH forms, studied by CH infrared spectroscopy. The accompanying computational study suggests that the unique properties of this family of CHs are due to exceptional richness of the host lattice in point defects, caused by defect stabilization by H-bonding of water to the guests.