Formation of clathrate hydrates of water-soluble guest molecules

Clathrate hydrates of water-soluble guest molecules, such as ethylene oxide (EO) and tetrahydrofuran (THF), have been often investigated in experimental studies instead of gas hydrates because their dissociation temperatures are higher than the ice point under ambient pressure. We examine the formation mechanism of EO and THF hydrates using molecular dynamics simulations. The crystal growth rates are determined by the simulations of the hydrate/solution two-phase coexistence. It is found that the growth rate of EO hydrate is an order of magnitude higher than that of THF hydrate. The growth rates of THF hydrate largely deviate from the Wilson-Frenkel model, while the model well approximates the growth rates of EO hydrate, indicating that trapping of guest molecules on the hydrate surface, which causes the slowing of crystal growth of THF hydrate, is insignificant for EO hydrate. We also perform long-time simulations of aqueous EO and THF solutions to examine nucleation of clathrate hydrate. Spontaneous nucleation occurs only in the EO solution within the simulation time. Similar to previous studies on methane hydrate, the obtained solid structure exhibits no long-range order. It is found that the 5¹² hydrate cage, which is the most dominant cage type in the early stage of the nucleation of methane hydrate, is not a major cage type in the nucleation process of EO hydrate.