Atomistic insights into the performance of thermodynamic inhibitors in the nucleation of methane hydrate

Abstract

Figure TOC Comparison of methane hydrate nucleation process (a) without or (b) with EG molecules. Critical nucleus size increase under the influence of Thermodynamic hydrate inhibitors. • Hydrophobic groups from the alcohols could approach the initial hydrate cages and destroy initial clusters. • The –OH groups were observed to form hydrogen bonds with the cage clusters and even initiate a new hydrate cage. • Alcohols can increase the critical nucleus size of gas hydrate. Thermodynamic inhibitors are intensively used in oil and gas pipelines to modify the formation conditions of gas hydrate, preventing the blockage of pipelines. However, their kinetic performance in hydrate nucleation is largely unclear. Herein, the molecular interactions of the small molecules of ethylene glycol and methanol with hydrate clusters were investigated. It was found that the hydrophobic groups from the glycols could approach the initial hydrate cages, thereby constricting the regional distribution of adjacent water molecules; thus, they functioned in the same way as the guest molecules that transiently helped to stabilize the water framework. However, these glycols would eventually dissolve into the solution to stay at the water/nanobubble interface; this would consequently destroy the initial cages upon losing the constraint from the surrounding hydrophobic molecules. In addition, the –OH groups were observed to form hydrogen bonds with the cage clusters and even initiate a new hydrate cage; however, they cannot stably reside there due to different atom coordination. Notably, these glycols can increase the critical nucleus size, which is considered the intrinsic mechanism of thermodynamic inhibition. Our results provide atomistic insights into the performance of glycols in the kinetic nucleation of gas hydrates and can help understand the interactions between guest and host molecules in the presence of hydrophobic and hydrophilic functional groups.