| Résumé | We have used 1H magnetic resonance microimaging to probe both methane and carbon dioxide hydrate formation processes inside dispersed water droplets. When bulk techniques such as gas uptake measurements are used for determining the kinetics of hydrate formation, these show a gradual conversion to hydrate, suggesting a relatively homogeneous process that might be modeled using a set of intrinsic kinetic parameters. The spatially resolved microimaging measurements show that in fact the conversion to hydrate is quite inhomogeneous, some drops converting quickly, others requiring hours or days. This indicates that the observation of gradual conversion in bulk samples only arises as a result of averaging over many local environments. Quantitative measurements of kinetic processes in subvolumes of a larger sample suggests that the smaller the volume observed, the more inhomogeneous the process appears to be. When hydrate-coated water droplets in 3,5,5-trimethylpentane are converted to hydrate, there is evidence that nucleation can take place well away from the hydrate coating, with the hydrate sometimes growing in discrete steps before drops are completely converted. The results obtained indicate that in the quiescent systems studied here the definition of intrinsic kinetic parameters will be difficult, if possible at all, because of a stochastic component that competes with more gradual conversion processes. |
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