Molecular Insights into the Concentration Dependent Promotion Effect of Tetrabutylammonium Bromide on Hydrate Growth: A Molecular Dynamics Simulation Study

Tetrabutylammonium bromide (TBAB) has been proven to improve the growth of hydrate via experimental methods, which may be attributed to its different concentrations. In this study, the molecular dynamics (MD) simulation is employed to investigate the concentration dependent promotion effect of TBAB on the growth of CO2 hydrate. The tetrahedral order parameter, number of cages, hydrate crystal growth trajectories, significant microconfigurations, and distribution of CO2 and TBAB are analyzed in detail. It is found that the promotion effect of TBAB is more prominent at low concentrations (5 and 10 wt %) than that at high concentrations (15 and 20 wt %). During the growth of hydrate crystal, tetrabutylammonium (TBA+) ions adsorb on the hydrate crystal and serve as guest molecules to form TBA+ semiclathrate hydrate cages. Then, the TBA+ semiclathrate hydrate cages undergo a self-adjustment process and induce the generation of CO2 hydrate cages. At high concentrations, the great accumulation of TBA+ at the hydrate-liquid interface disturbs the effective adsorption and self-adjustment processes, and the tightly packed arrangement of TBA+ at the gas-liquid interface partially inhibits the mass transfer of CO2. This study provides visible mechanisms of the concentration dependent promotion effect of TBAB from the microscopic level, which complements the vacancy in experimental studies.