Corrosion Inhibition of S275JR Steel in Acid Chloride using Methyltrioctylammonium Chloride and Methyltriphenyl Phosphonium Bromide - Based Deep Eutectic Solvents as Passivation Enhancers – Density Functional Theory & Molecular Simulation Analysis

We carried out an investigation into the use of two deep eutectic solvent (DES) components—methyltrioctylammonium chloride (MTOAC) and methyltriphenylphosphonium bromide (MTPB), as green passivation enhancers for S275JR steel in acidic chloride environments. Atomistic modelling comprising DFT (ωB97XD/6-311++G(2d,2p)) optimizations, Mulliken and Fukui analyses, non-covalent interaction (NCI) mapping, and molecular dynamics (MD) simulations on an Fe(110) model to elucidate adsorption modes and electronic drivers of inhibition was adopted. DFT descriptors showed striking electronic differences between the two inhibitors: MTOAC is electronically softer and more polarizable (EHOMO = −4.268 eV, ELUMO = −1.696 eV, ΔE = 2.572 eV, ΔN ≈ 1.56), whereas MTPB is comparatively hard and less reactive (EHOMO = −6.706 eV, ELUMO = −0.241 eV, ΔE = 6.465 eV, ΔN ≈ 0.55). MD simulations on Fe(110) produce interaction and binding energies that corroborate these trends: Fe (110)+MTOAC shows significantly stronger adsorption (Einteraction ≈ −220.34 kcal•mol⁻¹, Ebinding ≈ 220.34 kcal•mol⁻¹) than Fe (110)+MTPB (Einteraction ≈ −150.57 kcal•mol⁻¹, Ebinding ≈ 150.57 kcal•mol⁻¹), indicating a more stable, denser protective film for MTOAC.