Microscopic influence mechanisms of tetra-n-butylammonium bromide on hydrate growth and stability in saline environment

Marine CO₂ sequestration in hydrate form offers a promising solution to mitigate the escalating greenhouse effect. Numerous experimental studies have proven the efficacy of TBAB as a hydrate promoter even in saline environment, while its underlying microscopic mechanism remains unclear. This study employs molecular dynamics simulation to investigate the effect of TBAB with varying concentrations on CO₂ hydrate crystal stability and growth in saline environment under different thermodynamic conditions. A comprehensive analysis of system state, molecular distribution, intermolecular interactions, and molecular mobility is conducted. It is found that additional TBAB reduces the mobility of CO₂, Na⁺ and Cl⁻ ions near the hydrate crystal, thereby promoting hydrate crystal formation and limiting ionic attack on hydrate crystal. Meanwhile, the adsorption layer of TBA⁺ ions at the hydrate crystal surface protects the hydrate crystal from attack by inorganic ions via electrostatic interactions. Furthermore, the adsorbed TBA⁺ ions facilitate the formation of more stable semiclathrate hydrate. This study reveals a compelling microscopic mechanism for the promotion effect of TBAB on hydrate growth and stability in a saline environment, suggesting its potential for carbon sequestration and informing future additive design.