Theoretical insights into the structural characteristics and inhibition mechanisms of quaternary ammonium salt and imidazolium-based ionic liquid: DFT and MD simulations

Abstract

Quaternary ammonium and imidazolium-based corrosion inhibitors have attracted extensive research interest due to their effectiveness. However, despite extensive investigations into the correlation between functional groups and inhibitor adsorption modes, the microscopic mechanisms underlying the corrosion inhibition on metal surfaces remain inadequately understood. This study investigates the inhibition performance of an acidic imidazolium ionic liquid (IBIL) and a quaternary ammonium derivative (QAS), each containing a ten-carbon alkyl tail, using quantum chemical calculations and molecular dynamics (MD) simulations to predict their inhibition efficiency and adsorption behavior, based on DFT and MD simulations. The results indicate that the inhibition efficiency follows the order: IBIL > QAS for the Fe(100) surface in CO 2 -saturated 1 wt% NaCl solutions, consistent with previous experimental data. Different molecular structures of inhibitors lead to distinct adsorption behaviors on the steel surface. QAS adheres via a direct electron adsorption model, while IBIL adopts a cyclic electron adsorption model, involving electron donation and acceptance from different functional groups. This study not only enhances the understanding of inhibition mechanism but also provides valuable insights for the rational design and industrial application of high-performance inhibitors. IBIL and QAS with varying alkyl (R) groups effectively inhibit carbon steel corrosion in CO 2 -saturated 1 wt%NaCl solution, with inhibition efficiency following the order: QAS < IBIL. The molecular structures of the inhibitors significantly influence their adsorption modes on the steel surface. QAS adopts a direct electron adsorption model, while IBIL involves cyclic electron adsorption, with donation and acceptance occurring from different groups. Quantum chemical calculations reveal that IBIL exhibits superior inhibition efficiency compared to QAS, regardless of IBIL's acidity, which consistent with experimental observations. The inhibition performance of an inhibitor should be evaluated based on multiple factors. MD simulations reveal that the nearly parallel alignment of R groups with the Fe(110) surface, along with the physical shielding effect of the alkyl chains, further enhances the inhibition efficiency. The more negative adsorption energies correlate with improved inhibition performance, consistent with experimental results, providing valuable insights for designing optimized inhibitors. • TSILs interact with Fe via cyclic electron donation and back-donation, while QAS does so via intra-group interactions. • IBIL is predicted to show superior inhibition efficiency over QAS, consistent with experimental results. • The π-bond in QAS and -SO 3 H group in IBIL exhibit negative electrostatic potentials, enhancing electron donation and inhibition. • R groups of QAS and IBIL align nearly parallel to Fe surface, with adsorption energy validating their inhibition efficiency.

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