Real-Time Interferometry Study and Molecular Simulation of Adsorption Interaction for Imidazolium Ionic Liquid at Alkane Oil–Water Interface

Ionic liquids (ILs) can act as oil-displacing agents to enhance oil recovery (EOR), and understanding the interaction mechanism of ILs at the oil-water interface is therefore crucial for oil extraction. In this work, the experimental technology of surface interferometry was developed for real-time study of the macroscopic interaction between imidazolium ILs ([C₁₂mim]Br and [C₁₆mim]Br) and octadecane oil. By the interfacial thickness result, kinetic analysis revealed the nonequilibrium dynamic process of diffusion and arrangement for [C₁₂mim]Br interaction but another rearrangement for ([C₁₆mim]Br interaction. The arrangement process was indicated as the limiting step, and the rate order was [C₁₂mim]Br < [C₁₆mim]Br with constant value of |k_{arr([C12mim]Br)}| = 0.03 < |k_{arr([C16mim]Br)}| = 0.08. By the interfacial mass result, thermodynamic analysis revealed a Freundlich multilayer for [C₁₂mim]Br but a Langmuir monolayer for [C₁₆mim]Br. The interaction spontaneity was indicated as [C₁₆mim]Br > [C₁₂mim]Br with free energy of ΔG_[C16mim]Br = -10.67 < ΔG_[C12mim]Br = -0.47 kJ mol^-1. In addition, molecular dynamics simulation (MDS) was employed to identify the microscopic configurations and energies for the interfacial interaction. The interaction stability was revealed to [C₁₆mim]Br > [C₁₂mim]Br with an energy change of ΔE_{int([C16mim]Br)} = -147.06 kJ mol^-1 < ΔE_{int([C12mim]Br)} = -118.85 kJ mol^-1 and an overlapped distance of d_{oil-water(3[C16mim]Br)} = 11.17 Å > d_{oil-water(3[C12mim]Br)} = 9.51 Å> d_oil-water = 8.88 Å. The interaction mechanism was illustrated by "multilayer hemimicelle stacking" model for [C₁₂mim]Br but "monolayer micelle spreading" for [C₁₆mim]Br. This study establishes a systematic platform for investigating the interaction mechanisms and performance of ionic liquids (ILs) at oil-water interfaces, offering critical insights for reservoir engineering and EOR applications.