Blackberry leaf extract as a green inhibitor for copper corrosion: Insights from coordination complexes, thermodynamics, RSM optimization, and DFT calculations

• Physical adsorption of inhibitor occurs according to the Langmuir adsorption isotherm. • A large inhibitor concentration leads to change in the corrosion inhibition mechanism. • Better adsorption on the metal surface takes place via organometallic complexes. • The 3:1 caffeic acid complex with Cu(II) ions dominates as an inhibitory specie. • Simultaneous effect of parameters was optimized for better inhibitor efficiency. This study provides a comprehensive evaluation of blackberry leaf extract (BLE) as a green inhibitor of copper corrosion in 0.5 M NaCl. The experimental method of gravimetric analysis was applied, the results of which were used for data analysis methods: adsorption and thermodynamic studies and statistical modeling. The density functional theory (DFT) method was applied to determine the influence on the process of corrosion inhibition by comparing BLE molecules and possible organometallic complexes formed between Cu ions in solution and BLE molecules. BLE, containing caffeic acid, quercetin-3-O-glucoside and kaempferol-3-O-glucoside, significantly reduced copper dissolution across 298–328 K, reaching inhibition efficiencie up to 99% at concentrations ≥10 g/L. Adsorption obeyed the Langmuir isotherm at all temperatures, and negative Gibbs free energies (–12.5 to –14.6 kJ/mol) confirmed spontaneous physical adsorption. The increase in activation energy and positive adsorption entropy values indicated an endothermic, physisorption-controlled process accompanied by water displacement from the copper surface. Response surface methodology showed excellent predictive power (R² > 0.97), identifying inhibitor concentration as the most influential factor, followed by immersion time and temperature. Optimization predicted conditions yielding 99% inhibition, validated experimentally. DFT calculations established the stability and preferred geometries of Cu(II) complexes formed with the main BLE constituents, supporting their participation in adsorption, which represents a significant novelty in relation to previous research on plant corrosion inhibitors. Overall, the combined experimental-computational approach confirms BLE as a highly efficient, eco-friendly solution for copper protection in chloride environments.