Molecular Resolution into the Nucleation and Crystal Growth of Clathrate Hydrates Formed from Methane and Propane Mixtures

Type II clathrate hydrates are prevalent in numerous applications including gas production, gas storage, and seawater desalination. However, less focus has been given to understanding the molecular mechanism for the nucleation of type II clathrate hydrates, especially for mixed systems with dissimilar-sized molecules. Through a number of systematic molecular dynamic simulations for systems with different initial methane/propane content, we report here detailed molecular resolution into the mechanism of nucleation and crystal growth of these mixed clathrate structures. In addition to identifying the critical nucleus size for the nucleation, we determine the different uptake rates of methane and propane molecules into the solid phase, which were found to be uncorrelated between the solution composition of these species and the occupancy of the first cage to form or the occupancy in the growing solid clathrate phase. The evolution of the cages formed in the solid is tracked, and the coexistence of type I and type II structures is captured. The results show that the type I patterns appear earlier than type II even though thermodynamically the latter is preferred. The molecular details revealed here, along with new methods developed in this study, advance our understanding of the nucleation and crystal growth of clathrate structures, bringing new insights into the molecular events, structure, and ordering in the presence of molecules enclathrated in different-sized cages.

Automated summary

Through systematic molecular dynamic simulations, this study provides detailed insights into the nucleation and crystal growth mechanism of mixed type II clathrate structures, revealing uncorrelated uptake rates of methane and propane molecules in different solution compositions. The coexistence of type I and type II structures in the solid phase has been captured, indicating that type I patterns appear earlier than thermodynamically preferred type II structures.