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

Abstract

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.

Microscopic Mechanisms for the Dynamic Wetting of a Heavy Oil Mixture on a Rough Silica Surface

“…However, the oil-wetting mode transition was not observed in our previous study where only saturates were present on the rough silica surface. 13 The above findings suggested that it was the asphaltenes that led to the wettability alternation on the rough surface. To confirm this, we removed the asphaltene molecules during the model construction and ran the simulation following the same procedures.…”

“…To quantify the penetration capability of each oil fraction on the silica surface, the amount of oil molecules entering the grooves was characterized by counting the number of oil molecules with half of the atoms inside silica grooves. 13 The interaction energies were calculated using the standard modules of Gromacs. The self-diffusion coefficient was computed using the Einstein equation.…”

“…The apparent contact angle was calculated by the radius and height of the oil droplet on the silica surface, which was determined by the relative concentration of oil molecules along the X and Z directions (see details in Figure S3). To quantify the penetration capability of each oil fraction on the silica surface, the amount of oil molecules entering the grooves was characterized by counting the number of oil molecules with half of the atoms inside silica grooves . The interaction energies were calculated using the standard modules of Gromacs.…”

“…11 The effects of surface topography on the driving forces for the conformational change of hexadecane on alumina surfaces were reported by Xie et al 12 Most recently, we also modeled the dispersion, immersion, and extraction of dodecane on silica surfaces with varying roughness. 13 There are still numerous issues to address before accurately describing the dynamic wetting and adsorption of oil on rough minerals. For example, the heavy and polar fractions of crude oil (e.g., asphaltenes and resins) were not taken into account in the above studies.…”

“…The wetting transition rate of an oily fluid on a rough surface was calculated using the forward flux sampling method by Savoy and Escobedo . The effects of surface topography on the driving forces for the conformational change of hexadecane on alumina surfaces were reported by Xie et al Most recently, we also modeled the dispersion, immersion, and extraction of dodecane on silica surfaces with varying roughness …”