Oil–Water Interfacial Interaction Study between Ionic Liquid and Saturated Hydrocarbon by Interferometry and Calculation Insights

Ionic liquids (ILs) serve as effective oil-displacing agents during oil exploitation, and elucidating their interaction mechanisms at the oil–water interface is pivotal for enhanced oil recovery (EOR). In this work, the oil–water interfacial interaction between Cl-imidazolium ILs and C18-saturated hydrocarbon (C18-SH) were systematically studied from TIR-interferometry experimental and theoretical insights. Interfacial mass results revealed thermodynamic models of Freundlich-multilayer for [C12MIm]Cl but Langmuir-monolayer for [C16MIm]Cl. The interaction spontaneity was indicated as [C12MIm]Cl < [C16MIm]Cl with free energy of ΔG[C16MIm]Cl = – 9.36 kJ mol-1< ΔG[C12MIm]Cl = – 0.49 kJ mol–1. Interfacial thickness results revealed diffusion-arrangement dynamic processes and arrangement-limiting step for both [C12MIm]Cl and [C16MIm]Cl. The interaction speed was indicated as [C12MIm]Cl < [C16MIm]Cl with rate constants of |karr([C12MIm]Cl)| = 0.04 s-1< |karr([C16MIm]Cl)| = 0.07 s–1. The density functional theory (DFT) calculation obtained the energy gaps of ΔEH–L[C12MIm]Cl = 1.497 eV > ΔEH–L[C16MIm]Cl = 1.415 eV from LUMO–HOMO distributions. Meanwhile, MDS results indicated that [C16MIm]Cl exhibits superior interaction stability than [C12MIm]Cl with lower interaction energy of ΔEint([C16MIm]Cl) = −156.20 kJ mol-1< ΔEint([C12MIm]Cl) = −110.21 kJ mol–1 and longer cover distance of doil–water ([C16MIm]Cl) = 9.85 Å > doil–water([C12MIm]Cl) = 9.07 Å. As a result, the mechanism model of “multihemimicelle packing” was illustrated for [C12MIm]Cl interaction and “monomicelle coverage” for [C16MIm]Cl interaction. This work systematically clarified the micromacro interaction performance of ILs at oil–water interface, which effectively reveal the interaction mechanisms and provide help for ILs applications in EOR.