The theory developed in our earlier papers is extended to predict dynamical and thermodynamic properties of clathrate structures by accounting for the possibility of multiple filling of cavities by guest molecules. The method is applied to the thermodynamic properties of argon and krypton hydrates, considering both structures I (sI) and II (sII), in which the small cages can be singly occupied and large cages of sII can be singly or doubly occupied. It was confirmed that the structure of the clathrate hydrate is determined by two main factors: intermolecular interaction between guest and host molecules and the configurational entropy. It is shown that for guests weakly interacting with water molecules, such as argon or krypton, the free energy of host lattices without the contribution of entropy is the main structure-determining factor for clathrate hydrates, and it is a cause of hydrate sII formation at low pressure with these guests. Explicit account of the entropy contribution in the Gibbs free energy allows one to determine the stability of hydrate phases and to estimate the line of structural transition from sII to sI in P-T plane. The structural transition between sII and sI in argon and krypton hydrates at high pressure is shown to be the consequence of increasing intermolecular interaction and the degree of occupancy of the large cavities.

Visual studies of the growth of methane hydrate at the interface of water with three kinds of methane-saturated crude oils were performed at supercooling ∼20 °C. The results were compared with the growth of methane hydrate at the interfaces of water–methane and water–methane-saturated decane or toluene. The average rates of hydrate film growth measured for the water–oil interfaces vary within the range of 0.8–1.0 mm s–1, which is somewhat lower than the average growth rate at the water–methane and water–decane (toluene) interfaces (1.6 mm s–1). It was found that in some cases hydrate nucleation proceeded not on the water–oil boundary but at the walls of the cell. Spontaneous intrusion of oil formations (drops, serpentlike formations) into the aqueous phase was observed in some experiments. These formations are likely to originate from oil catching by the bundles of needlelike hydrate crystals due to the capillary effect. Intense growth of the hydrate on the walls of the cell was observed for two kinds of crude oils. It was demonstrated that one of the factors blocking hydrate growth on the cell walls may be the presence of naphthenic acids (natural surface-active substances) in the oil phase.

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