Molecular Insights into the Concentration Dependent\nPromotion Effect of Tetrabutylammonium Bromide on Hydrate Growth:\nA Molecular Dynamics Simulation Study

Tetrabutylammonium\nbromide (TBAB) has been proven to improve the\ngrowth of hydrate via experimental methods, which may be attributed\nto its different concentrations. In this study, the molecular dynamics\n(MD) simulation is employed to investigate the concentration dependent\npromotion effect of TBAB on the growth of CO<sub>2</sub> hydrate.\nThe tetrahedral order parameter, number of cages, hydrate crystal\ngrowth trajectories, significant microconfigurations, and distribution\nof CO<sub>2</sub> and TBAB are analyzed in detail. It is found that\nthe promotion effect of TBAB is more prominent at low concentrations\n(5 and 10 wt %) than that at high concentrations (15 and 20 wt %).\nDuring the growth of hydrate crystal, tetrabutylammonium (TBA<sup>+</sup>) ions adsorb on the hydrate crystal and serve as guest molecules\nto form TBA<sup>+</sup> semiclathrate hydrate cages. Then, the TBA<sup>+</sup> semiclathrate hydrate cages undergo a self-adjustment process\nand induce the generation of CO<sub>2</sub> hydrate cages. At high\nconcentrations, the great accumulation of TBA<sup>+</sup> at the hydrate–liquid\ninterface disturbs the effective adsorption and self-adjustment processes,\nand the tightly packed arrangement of TBA<sup>+</sup> at the gas–liquid\ninterface partially inhibits the mass transfer of CO<sub>2</sub>.\nThis study provides visible mechanisms of the concentration dependent\npromotion effect of TBAB from the microscopic level, which complements\nthe vacancy in experimental studies.