Effects of salts and silica nanoparticles on oil-brine interfacial properties under hydrocarbon reservoir conditions: A molecular dynamics simulation study

Effect of the bare and functionalized single-wall carbon nanotubes on inhibition of asphaltene molecules aggregation: A molecular dynamic simulation

A comprehensive study has been carried out by utilizing the molecular dynamics technique in order to investigate the behavior of the N-(1-hexylheptyl)-N'-(2-phenylpropanoicacid)-perylene-3,4,9,10-tetracarboxylicbisimide (PAP) molecules in the n-heptane/toluene solution as well as the role of the bare and functionalized single-wall carbon nanotubes (SWCNTs) with the carboxyl groups (-COOH) on the aggregation of PAP molecules. It was found that the CNTs benefit two mechanism of steric hindrance and adsorbing the PAP molecules to suppress the affinity of PAP molecules to association. The results ascertain that the constant amount of the carboxyl groups acts more efficiently in restricting the growth of aggregate size if they are distributed on the surface of a larger CNT. Both of the increased nonbonding interactions between the functionalized CNTs and the PAP molecules, and the number of formed hydrogen bonds between them clearly proved the efficiency of the -COOH groups in improving the stability of PAP molecules. The strength of the adsorption free energies revealed that the PAP molecule shows more tendency to be adsorbed on the surface of CNT modified with carboxyl groups. Increasing the dosage of the -COOH groups on the surface of the CNTs with constant dimension causes an increment in the PAP molecules' solvent accessible surface area (SASA) value, indicating enhanced stability of the PAP molecules. Finally, the results would facilitate future studies on manipulating the asphaltene precipitation in the oil industry.

Molecular Dynamics Study of Silica Nanoparticles and CO2-Switchable Surfactants at an Oil-Water Interface

Adsorbing CO2-sensitive surfactants on the surface of nanoparticles is an important strategy for preparing stimuli-responsive Pickering emulsions. However, the microscopic mechanisms are still limited, owing to a lack of intuitive understanding at the molecular level on the interactions between nanoparticle and switchable surfactants at the oil-water interface. We employed the molecular dynamics (MD) simulations to explore the mechanism behind the reversible emulsification/demulsification of a Pickering emulsion stabilized by silica nanoparticles (NPs) and CO2-switchable surfactants, named N-(3-(dimethylamino)propyl)alkyl amide (CPMA). MD results show that the protonated surfactant CPMAH+ has strong hydrophilicity, forming an adsorption layer at the oil-water interface. The ionic surfactants can be tightly adsorbed on NP surface through electrostatic interactions. Thus, the formed colloid particle has both hydrophobic and hydrophilic properties, which is a key factor in stabilizing emulsion. When CPMAH+ molecules were deprotonated to CPMA, the hydration activity of the headgroups reduced greatly, inducing a mixture with oil molecules. There are still a certain number of CPMA molecules residing at the oil-water interface due to the hydrophilic amine groups. The results from repeated simulations show that NP can either stay in the water phase or locate at the interface. Even NP was finally adsorbed on the interface and combined with CPMA or oil molecules, the adsorption configuration of CPMA on the NP surface was essentially different from that of CPMAH+. The potential of mean force confirmed that the combination between NP and CPMA is quite unstable due to the disappearance of electrostatic attraction. Different binding configurations and stability between NP and CPMA or CPMAH+ were the fundamental reason for the reversible emulsification/demulsification of Pickering emulsion.