Molecular dynamics simulations of the formation of ethane clathrate hydrates

This study details molecular dynamics simulations for the formation of ethane clathrate hydrates. Ethane hydrate nucleation shares with methane hydrate nucleation the formation of coordinated guest clusters in water, resulting in structures such as guest-saturated pentagonal and hexagonal faces. Ethane hydrate also forms structure I, which consists of two 512 and six 51262 cages in its unit cell. Observations from the simulations reveal differences between methane and ethane in the nucleation process, including the types and occupancies of cages. While the formation of occupied 512 cages dominates methane hydrate nucleation, nucleation of ethane hydrates predominately exhibits the 4151062 cage as both the first and most common early cage type. These 4151062 cages do not occur in the crystal structure, and due to their oblong shape, restrict the orientation of the coordinated guest ethane molecule. The initial kinetically favored 4151062 cages are observed to undergo transformation into different cage types, such as the thermodynamically favored 51262 cage.