Guest Partitioning in Carbon Monoxide Hydrate by Raman Spectroscopy

Gas hydrates are inclusion compounds composed of a H-bonded water network forming cages, inside of which gaseous (guest) molecules are encapsulated. Depending on the nature and partitioning of the guest molecules, various types of clathrate structures may be formed. In this work we have elucidated the guest partitioning of the CO hydrate, using high-resolution Raman microspectroscopy, and investigated the impact of pressure-temperature (P-T) conditions on this partitioning. For the first time, vibrational signatures of CO molecules encapsulated in a large cage and small cage are identified. It is also shown that the large cages of the CO hydrate have the ability to easily catch or release CO guest molecules, while the small cages remain singly occupied. Moreover, the study of the P-T dependence of the Raman signature demonstrates not only the CO stretching frequency dependence with the cage filling but also the tuning effect of the cage filling by the P-T conditions of treatment.