Abstract
The formation of a clathrate hydrate crystal is characterized by several steps, each of them distinguished by a different structural arrangement and temporal duration. A precise definition of these different forms is a challenging task, because the entirety of the formation dynamics spans over a time interval ranging from few nanoseconds to several days. Computational methods are powerful and essential to define the nucleation step, but they fail in providing a reliable picture of the long-range order establishment. On the other side, the experimental methods employed in the study of the growth dynamics usually monitor the hydrate growth at the interface with the fluid and thus are limited by the diffusion of the guest molecules through the newly formed hydrate phase. This problem is overcome here by the confinement of an argon hydrate sample in a sapphire anvil cell, allowing monitoring of the melting and crystallization of hydrates under moderate pressures by FTIR and Raman spectroscopies. This approach, besides providing a spectroscopic characterization of this hydrate, allowed the time windows characteristic of the formation of a macroscopic amorphous phase to be identified, possibly coincident with the so-called blob, and its rapid evolution toward the achievement of the local structure. Long-range ordering takes place on a longer time scale, most of it is realized in few hours but still evolving for weeks. No hints for supporting the so-called memory effect are gained through this study.
Highlights
Clathrate hydrates are crystalline solids where polyhedral cages of hydrogen-bonded water molecules are stabilized by weak repulsive interactions between the guest molecules and the water molecules
The homogeneous nucleation of the hydrate phase from the fluid mixture is very unlikely with respect to heterogeneous nucleation, which initiates at the interface between the waterrich and the guest-rich phases
Nucleation is anticipated by an induction time necessary to reach the critical radii of the aggregates
Summary
Clathrate hydrates are crystalline solids where polyhedral cages of hydrogen-bonded water molecules are stabilized by weak repulsive interactions between the guest molecules and the water molecules. In the case of insoluble gases in water, the amount of guest molecules stored in these structures is even 3 orders of magnitude larger than that contained in the fluid phase from which it forms For this reason, the homogeneous nucleation of the hydrate phase from the fluid mixture is very unlikely with respect to heterogeneous nucleation, which initiates at the interface between the waterrich and the guest-rich phases. Understanding hydrate nucleation at a molecular level is challenging due to the small time/length scales of nucleation and to the stochastic nature of the process. Both these aspects dramatically limit the experimental studies on the subject. The induction time is strictly related to oversaturation and metastability of the solution, and since
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