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In this paper, the performance of three imidazolium-based ionic liquids (ILs) including 1-hexyl-3-methylimidazolium chloride ([HMIM][Cl] or IL6), 1-octyl-3-methylimidazolium chloride ([OMIM][Cl] or IL8), and 1dodecyl-3-methylimidazolium chloride ([DMIM][Cl] or IL12) in reducing the interfacial tension (IFT) between crude oil and IL solutions was analyzed for the first time under a wide range of salinities (1000 to 195 476 ppm) at a reservoir temperature of 80 °C. The purpose was to microscopically analyze the occurring phenomenon at the fluid−fluid interface to determine the mechanism leading to oil extraction and to address the existing ambiguities in the literature concerning the synergism between ILs and different types/concentrations of ions. The quality of the synthesized ILs and their accumulation at the crude oil/IL solution interface was analyzed via attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, the IFT between crude oil and IL solutions was measured by the pendant drop method, and the micelles' size distribution and molecular diffusivity of the ILs in the aqueous solution was measured by the dynamic light scattering (DLS) technique. On the basis of the ATR-FTIR and IFT results, the accumulation of the ILs molecules at the interface of the crude oil/IL solution was a function of alkyl chain length and the ionic strength (IS) of solution; the longer alkyl chain ILs (i.e., IL12 in this study) accumulated more at the interface of the solution with more IS, leading to a more IFT reduction. At a constant but low range of salinities, the longer alkyl chain IL exhibited a lower IFT value in the presence of diluted seawater (dSW) than the diluted formation water (dFW). This is because of the higher concentration of divalent cations (Mg 2+ and Ca 2+ ) in dSW than dFW and their interactions with resin and asphaltene molecules and the salting-in effect mechanism that overcomes the lower saponification ability of dSW than dFW.

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