Understanding the effect of moderate concentration SDS on CO2 hydrates growth in the presence of THF

Additives like Tetrahydrofuran (THF) and Sodium Dodecyl Sulfate (SDS) improve CO2 hydrates thermal stability and growth rate when used separately. It has been hypothesised that combining them could improve the kinetics of growth and the thermodynamic stability of CO2 hydrates. We exploit atomistic molecular dynamics simulation to investigate the combined impact of THF and SDS under different temperatures and concentrations. The simulation insights are verified experimentally using pendant drop tensiometry conducted at ambient pressures and high-pressure differential scanning calorimetry. Our simulations revealed that the combination of both additives is synergistic at low temperatures but antagonistic at temperatures above 274.1 K due to the aggregation SDS molecules induced by THF molecules. These aggregates effectively remove THF and CO2 from the hydrate-liquid interface, thereby reducing the driving force for hydrates growth. Experiments revealed that the critical micelle concentration of SDS in water decreases by 20% upon the addition of THF. Further experiments showed that only small amounts of SDS with THF is sufficient to increase the CO2 storage efficiency by over 40% compared to results obtained without promoters. These results provide microscopic insights into the mechanisms of THF and SDS promoters on CO2 hydrates, which allow for determining the optimal condition for hydrate growth.