Solidified Methane Storage Using an Efficient Class of Anionic Surfactants under Dynamic and Static Conditions: An Experimental and Computational Investigation

Rapid hydrate formation and high storage capacity are two key parameters in commercializing hydrate-based solidified natural gas technology. Surfactants are known as the most effective additives to accelerate the kinetics of gas hydrate formation under different conditions. This research introduces an efficient class of anionic surfactants - carboxyl-sulfonated surfactant (CSS) - as gas hydrate promoters for the first time. To that end, CSSs with different alkyl chain lengths were synthesized, and their promotion effect on methane hydrate formation was evaluated in static and dynamic conditions. Those CSSs with 10 carbon atoms provided a maximum conversion of 88.4% at 500 ppm, higher than that of sodium dodecyl sulfate (87.9%). However, those with 12 carbon atoms were selected as the optimal promoter in terms of kinetic constants. The results of a high-pressure autoclave experiments revealed that the hydrophilic-hydrophobic balance of CSSs strongly affected their promotional activity. CSSs with a short alkyl chain had lower promotion efficiency as they could not increase the solubility of gas in water, especially under static conditions. Additionally, molecular dynamic simulations showed that the length of the alkyl chain directly influences the performance of promoter molecules because a longer hydrophobic tail can attract more methane molecules from the solution to supply them for the growing hydrate surface. Moreover, the CSSs with 12 carbon atoms aggregated into a stable micelle during hydrate formation according to the hydrophobic interaction, which attracted methane molecules around its hydrophobic tail and enhanced the hydrate growth rate. These findings can be useful in developing novel surfactants for energy-efficient methane storage in hydrates.