Tuning the Molecular Structure of Cationic Gemini Surfactants to Enhance Hydrate Growth for CO ₂ Sequestration

CO2 emissions are a major contributor to global warming, prompting significant interest in carbon capture and storage technology. Among the emerging strategies, sequestration of CO2 in the form of gas hydrates in the subsurface has attracted considerable interest as a promising route for long-term storage. Despite its potential, slow growth kinetics and a low gas storage capacity inhibit practical deployment. Therefore, several CO2 hydrate kinetic promoters have been investigated to address these challenges. However, conventional surfactants often generate excessive, stable, and potentially toxic foams, hindering process scale-up. To address these limitations, the present study explores the synthesis and application of biodegradable, cost-effective cationic Gemini surfactants featuring two distinct spacer groups with varying chain lengths. The chemical structures of these surfactants were confirmed by using 1H NMR, 13C NMR, Fourier-transform infrared spectroscopy, and ESI-HRMS. CO2 hydrate formation was systematically examined under isochoric, isothermal conditions with pure CO2 gas as the hydrate former and aqueous solutions of the Gemini surfactants at 0.1 and 0.5 wt %. Vital kinetic metrics were estimated. The outcomes were compared with those of cetyltrimethylammonium bromide, a cationic surfactant considered as a benchmark promoter. Overall, the Gemini surfactants with six and eight carbon atoms in the alkyl chains promoted CO2 hydrate growth more effectively than Gemini surfactants having ten carbon atoms in the alkyl chain. The promotion was found to be more pronounced with surfactants having an oxygen atom in the spacer group. Notably, the gas uptake at 0.1 wt % of Gemini surfactant concentration was comparable to that observed at 0.5 wt %, implying economic benefit of the additives and a potential for scale-up. Moreover, a multistage kinetic model was fitted to the water-to-hydrate conversion data to provide valuable insights into the factors governing hydrate growth, viz., the variation of rate constants related to primary and growth stages and water-to-hydrate conversions at the completion of the primary growth stage with the alkyl chain lengths of surfactants.