Enhancing oil recovery by alkane-modified molybdenum disulfide nanosheets with the optimum alkyl chain length: The balance between dispersity and hydrophobicity to achieve high interfacial activity

Alkane-modified molybdenum disulfide (AM-MoS2) nanosheets have attracted widespread interest as oil displacement agents for enhancing oil recovery due to their unique oil-water interfacial properties. However, the relationship between the interfacial activity and alkyl chain structure of AM-MoS2 is not comprehensively understood. In this study, 3AM-MoS2 nanosheets were prepared with MoS2 and alkylamines to study the effect of alkyl chain length on the oil-water interfacial activity of AM-MoS2 nanosheets under different salinity conditions. The AM-MoS2 nanosheets prepared with octylamine, dodecylamine, and octadecylamine were referred to as AM8-MoS2, AM12-MoS2, and AM18-MoS2, respectively. AM18-MoS2, which possesses the longest alkyl chain among all the samples, exhibited the highest oil-water interface adsorption ratio within the experimental concentration and salinity ranges. However, the changes in oil-water interfacial tension (IFT) did not agree with the changes in the interface adsorption ratio of nanosheets. AM18-MoS2 exhibited the best interfacial activity, and the IFT was lower than that of MoS2, AM8-MoS2 and AM12-MoS2 when the salinity was no more than 5000 mg/L, while AM12-MoS2 and AM8-MoS2 exhibited the best interfacial activity and the lowest IFT when the salinity reached 10,000 mg/L and 20,000 mg/L, respectively. Considering that the interface adsorption ratio of AM18- MoS2 was higher than that of the other samples under all the experimental conditions, the change in IFT and interfacial activity could be attributed to the change in the morphology of the adsorption layer of nanosheets. The decreased charge repulsion and the increased hydrodynamic size of nanosheets, which resulted from increased salinity, had positive and negative effects, respectively, on the morphology of the adsorption layer. The competition between the two effects resulted in the inflection point in the relationship between the IFT and salinity of AM-MoS2. The AM-MoS2 nanosheets with the optimum alkyl chain length balanced the dispersity and hydrophobicity, which contributed to achieving the best interfacial activity at a certain salinity.