Systematic investigation on the abnormal surface and interfacial activity of fatty acid ionic liquids

Abstract Ionic liquids are regarded as the promising and efficient additives to use in various aspects of petroleum industry, especially in chemical EOR. In this investigation, three novel fatty acid ionic liquids (FAILs), methyltrioctylammonium octanoate ([N8881][C8:0]), methyltrioctylammonium laurate ([N8881][C12:0]) and methyltrioctylammonium palmitate ([N8881][C16:0]), were selected and evaluated for their performances in terms of surface/interfacial activity, temperature and salt resistances. The three FAILs could cause remarkable reduction of surface tension to ~30mN/m in their aqueous solutions, and the [N8881][C8:0] with the shortest anionic alkane chain exhibited the greatest surface activity among three FAILs. Then the size-match effect and intermolecular interaction were employed to explain the abnormal variations, which were confirmed by the control experiments using mixed surfactants and the molecular dynamics simulation. It was found that the binding energy in [N8881][C8:0] (-28.961 kcal/mol) was larger than that in [N8881][C16:0] (-22.942 kcal/mol), which should be responsible for their different surface activity. Moreover, the crude oil/water interfacial tension (IFT) could be decreased to the much level (4.9×10-3 mN/m and 1.7×10-2 mN/m) even at high-temperature conditions (80 °C) and at high-salinity conditions (100000 mg/L), indicating the FAILs might be the ideal candidate for the EOR used surfactants with great temperature and salt tolerances. Meanwhile, the variations of IFT results further verified our proposed mechanism.