Mechanisms of the memory effect of clathrate hydrates

Abstract

The memory effect in the nucleation of gas hydrates refers to the phenomenon that gas hydrates nucleate easier or faster in the water that has a history of gas hydrate formation than in fresh water that has no such history. Although several hypotheses have been proposed to explain the cause of the memory effect over the last several decades, it remains the longest-standing mystery in the nucleation of gas hydrates. Supersaturation of the guest gas has been considered to occur in a memory system, however, the extent of its impact on the memory effect has been unclear. In the current research, we used linear cooling ramp experiments under isobaric conditions to determine the nucleation curves of CO2 hydrate in quiescent water in a stainless-steel sample cell and in quiescent quasi-free water droplets supported by a bulk liquid of perfluoromethyldecalin contained in a Teflon sample cell. From systematically comparing the nucleation curves of both freshly prepared samples and samples with a history of CO2 hydrate formation in both the stainless-steel sample cell and the quasi-free water droplets, as well as the nucleation curves of methane – propane mixed gas hydrates previously reported, a coherent picture emerged. The impurity imprinting component, with the interfacial gaseous states as the entity of the “impurity”, and the guest supersaturation component, with or without nanobubbles in the bulk water, are comparable in size, albeit the former is the larger one.