Insights into the Oil Adsorption and Cyclodextrin Extraction Process on Rough Silica Surface by Molecular Dynamics Simulation

Molecular dynamics simulation was performed to investigate the adsorption and aqueous extraction of oil contaminants on silica surface with various roughness. The oil dispersion and immersion were characterized by molecular configuration, adsorption energy, and contact angle, while the oil detachment in water and cyclodextrin solution was evaluated by the overall extractability, extractability from the individual groove, and free energy analysis. Results demonstrated that the main resistance for oil release from the relatively shallow grooves was the strong intermolecular interactions among the oil molecules orderly stacked inside the grooves. It highlighted the role of cyclodextrin in-breaking through the energy barrier for pulling out one oil molecule from the tightly stacked oil structure, which resulted in the fast release of the remaining oil. Previous studies attributed the oil extraction to the sequestration of oil into the hydrophobic cavity of cyclodextrin, while our results demonstrated that such an inclusion process was critical in initially destroying the stable structure of the oil compacted inside the grooves but contributed little afterward. To the best of our knowledge, this was the first molecular-level study on the cyclodextrin-aided oil detachment from a mineral surface, which improves our understanding of the oil cleanup mechanisms during aqueous extraction.