Structure–function relationships in dithiocarbamate–Cu(II) systems: insights from interfacial electrochemistry, adsorption and surface morphology

Dithiocarbamate ligands are widely recognized for their versatile coordination chemistry; however, the extent to which metal coordination modulates their interfacial electrochemical behaviour and corrosion-inhibition performance in acidic environments remains insufficiently understood. In this work, sodium N-methyl-N-phenyldithiocarbamate (L) and its Cu(II) complex (CC) were investigated to elucidate how metal coordination influences ligand–surface interactions in acidic media, using mild steel in 1.0 mol L−1 HCl as a model system. Potentiodynamic polarization measurements show that Cu(II) coordination leads to a substantial reduction in corrosion current density, from 378.6 µA cm−2 in the uninhibited system to 48.1 µA cm−2 at 125 mg L−1. Electrochemical impedance spectroscopy, modelled with an Rs–(Rct‖CPE) circuit, revealed concentration-dependent increases in charge-transfer resistance with slightly elevated double-layer capacitance, indicating the formation of a thick, polarizable interfacial layer governed by electronic/kinetic inhibition rather than simple geometric blocking. Adsorption analysis indicates spontaneous adsorption for both species, with standard free energies of −33.3 and −34.5 kJ mol−1 for L and CC, respectively, consistent with mixed physisorption–chemisorption behaviour. Surface morphological studies by scanning electron microscopy and atomic force microscopy confirm that coordination promotes the formation of a coherent and chemically stable interfacial layer. These findings establish a clear structure–function relationship between metal–ligand coordination and interfacial electrochemical behaviour, highlighting the role of dithiocarbamate coordination chemistry in controlling surface processes under chemically demanding conditions.