Guest-Guest Interactions and Co-Occupation by Distinct Guests in the Metastable State of Clathrate Hydrates

Abstract

The current knowledge of guest-guest interactions and co-occupation in clathrate hydrates is exclusive for the same guests (H-2 or N-2) at moderate pressure. Here, we introduce the unusual co-occupation of distinct guests in the metastable state of hydrates. With controlled hydrate fraction, particle size, and intensification of the sintering of SF6 hydrate particles formed from water and SF6 gas as a help gas, we observed an abnormal but unique synchronous behavior in Raman intensities of two guest molecules (SF6 and N-2/H-2) in hydrates consistently and repeatedly; over time, the scattering intensity for the guests (i) increases, (ii) decreases, and (iii) finally reaches the stable level. Without a concentration change of SF6, this abnormal behavior must arise from the possible changes in the scattering cross section of the molecules, suggesting that N-2/H-2 strongly interacts with SF6 in the large cages, resulting in a possible co-occupation during the metastable transition. These observations on the metastability of gas hydrate attest the importance of the sintering effect as a barrier to prevent fast gas diffusion for reaching equilibrium, which could have significant implication in increasing overall gas storage in clathrate hydrates.