Molecular modeling of organic corrosion inhibitors: Calculations, pitfalls, and conceptualization of molecule–surface bonding

Highlights: • Various approaches to molecular modeling of corrosion inhibitors are described. • Pitfalls that appear more or less frequently in the literature are illuminated. • Conceptualization of the molecule–surface bonding is provided. • Molecular electronic parameters and their connection to the molecule–surface bonding are examined. • Various aspects of the explicit modeling of inhibitor adsorption are discussed. Molecular modeling of corrosion inhibitors has gained momentum in the last decade. This paper describes various approaches, starting with the so-called MEPTIC (Molecular Electronic Properties To Inhibition-efficiency Correlation) approach that is among the most commonly used in the literature, and illuminates some pitfalls that appear more or less frequently therein, such as the "$\Delta N<3.6$" rule, inferences based on correlations obtained for only a few inhibitors, attributing significance to total energies, reliance on too small differences, etc. The relevance of the often used molecular electronic parameters in the MEPTIC approach and their connection to molecule–surface bonding are examined, because the basic premise of many modeling studies is that corrosion inhibition is achieved by adsorption of inhibitor molecules. To this end, the conceptualization of molecule–surface bonding is provided and some aspects of the explicit modeling of inhibitor adsorption, either by quantum-mechanical DFT methods or empirical force-field molecular-dynamics methods, are discussed, such as the use of too simple surface models, non-validated force-fields and solid/vacuum instead of electrified solid/liquid interfaces, applicability of coarse-grained simulations, distinction between physisorption and chemisorption, and bond-breaking and bond-making during adsorption.