Clathrate hydrates are thermodynamically stable under low-temperature and high-pressure conditions. It is well known that the formation of hydrate membrane at the interface between liquid CO_2 and water is difficult to form in the case of water temperature of over 5 Celsius unless a nucleation was forced. We report here some new evidence relating the mechanism of hydrate formation from the experiment using distilled water and natural waters including ion-exchanged water. The solution with hydrate clusters was prepared in advance by injecting a CO_2 droplet into the high-pressure system and forcing it to form hydrate on its surface. Dissolving additional CO_2 in the circulated high-pressure system controlled the concentration of guest molecule. Keeping the pressure constant at 40MPa, the temperature was changed from 4 to 10 Celsius. We measured the time of nucleation occurrence as a parameter of concentration of guest molecule, degree of sub cooling from the dissociation temperature of hydrate. In the case of distilled water, we observed no hydrate formation at above concentration range. This suggests that the cavity-like precursors derived from cluster structures in the solution are not always increased. On the other hand, in the case of ion-exchanged water, we observed the formation of hydrate at high concentrations. Furthermore, we investigated the nucleation behavior of hydrate in surface seawater sampled from the ocean and tap water. As a result, the induction time for the nucleation of hydrate took much longer unless some forcing condition was used. The required concentration to start the nucleation in natural waters was rather small compared with the case of distilled water. This means that the high concentration of CO_2 in natural waters and seawater promotes the hydrate nucleation. These results support the assumption that the concentration of CO_2 in seawater slows the dissolution of hydrate in which a released CO_2 droplet is soon covered with hydrate membrane.
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