Molecular scale insights into CO2 hydrate growth regulated by surfactant

Abstract

Carbon dioxide hydrate is an efficient technology for carbon dioxide capture and storage, and its growth is a complex process involving multiple phases and components. Typically, the nucleation and growth of hydrates require the introduction of surfactants to improve interfacial compatibility. In this study, we designed and synthesized two new surfactants (QC₁₂PyBr and QAC₁₂Et₃Br), both deploying fluorescent group (quinoline) for the hydrophobic end. To enhance the weak interaction with H₂O molecules, pyridine and triethylamine are introduced as the hydrophilic end with QC₁₂PyBr and QAC₁₂Et₃Br respectively. This design endowed the surfactants with optical activity (UV illumination) while maintaining their original amphiphilic properties. The effects on hydrate growth are explored by molecular dynamics simulations, which provide molecular scale insights into CO₂ hydrate growth regulated by surfactant from the perspectives of CO₂ aggregation morphology, surfactant conformation, as well as weak intermolecular interactions, respectively. The results indicate that the spherical morphology with higher interface curvature of CO₂ aggregation is more favorable for hydrate growth. DFT calculations show that the differences in intermolecular interactions between hydrophilic groups and H₂O molecular lead to different surfactant spatial conformations at the gas–liquid interface. Additionally, a combination of simulation and experimental methods is employed to further verify the feasibility of the surfactants designed in this study, thus confirming that the synthesized surfactant possesses the ability to effectively regulate hydrate growth for CO₂ storage while providing a new method for monitoring hydrate film growth based on the fluorescence activity.