Quantum Chemical and Electrochemical Evaluation of Alkyl Phosphine Oxide in Corrosion Inhibition of Carbon Steel in Formation Water

Abstract

Abstract In this study, quantum chemical calculations and molecular dynamics simulations studies of a series of five nonionic surfactants namely: dimethyl alkyl phosphine oxide surfactants (C10-18PO) containing decyl, dodecyl-, tetradecyl-, hexadecyl-, and octadecyl alkyl chains were implemented using the density functional theory (DFT) method to interpret the correlation between the inhibition efficiency and the molecular structure of the different inhibitors in formation water. The global quantities including: highest occupied molecular orbital energy (HOMO), lowest unoccupied molecular orbital energy (LUMO), energy gap (ΔE), dipole moment (μ), total energy (TE), ionization potential (I), electron affinity (A), electronegativity (χ), chemical potential (π), global hardness (η), global softness (σ), global electrophilicity (ω), polarizabilities <α> and fraction of electrons transferred (ΔN) were calculated for the different inhibitors in formation water. The dependence of surface activities of the nonionic surfactants on the alkyl chain length was studied in distilled water. The surface activities and surface parameters of the studied surfactants were described using: surface tension and interfacial tension measurements. The determined surface parameters of the studied surfactants were surface tension, critical micelle concentration, effectiveness, efficiency, maximum surface excess and minimum surface area at 25 °C. The thermodynamic evaluation of the surfactants was performed by calculating the standard free energies of micellization and adsorption. The structure-corrosion inhibition performance was estimated using potentiodynamic polarization, electrochemical impedance measurements and quantum chemical studies at 25 °C in true sample formation water.