Molecular level separation of crude oil/water emulsion on carbon nanotube surface induced by weak interaction: a molecular dynamic simulation study

Abstract

Huge π-conjugated system of carbon nanotubes (CNTs) endows the two-dimensional carbon-based nanomaterials with special affinity towards aromatic organic compounds such as asphaltenes. Based on the special bonding structure, functional CNTs was designed and used for demulsification of crude oil/water emulsion and the treatment of emulsified oily wastewater in our previous study. Despite the macro bottle tests and micro-scale polarizing microscope were used to observe the separation process, the nanoscale or molecular scale demulsification mechanism was still not well understood. Therefore, molecular dynamics (MD) simulations were carried out to study the adsorption and separation behaviors of crude oil/water emulsion on the CNT surface. It was interesting to found that the oil and water was gradiently separated on the CNT surface due to their different spreading velocity resulted from the different affinity and interaction energy between various molecules (water, heptane and asphaltenes) and the CNT material. The demulsification mechanism of CNT was put forward as follows: when oil droplets approach the CNT surface, the water molecules rapidly spread and flow over the CNT surface, while the saturates slowly spread along CNT surface and the molecules with π-conjugated system (such as asphaltenes) were restricted due to their strong interaction with the CNT material. Because of their enormous difference of spreading velocity and time on CNT surface, the oil molecules were stayed and accumulated on CNT surface and thereby separated from the water molecules. The results obtained in this work provides a deep insight into the oil-water separation behaviors when using carbon-based nanomaterials to demulsify the crude oil-water emulsion and the treatment emulsified oily wastewater.

Graphical Abstract

Keywords:

• Carbon nanotube
• molecular dynamic simulation
• demulsification mechanism
• crude oil/water emulsion
• emulsified oily wastewater

Additional information

Funding

We acknowledge the financial support offered by the National Natural Science Foundation of China (Grant No. 51704042; 51574217), the Natural Science Foundation of Jiangxi University of Science and Technology (jxxjbs17042) and National College Students’ Innovation and Entrepreneurship Training Program (201810407001, 201810407003).