Structure–Function Relationships in Dithiocarbamate Cu(II) Systems: Insights from Interfacial Electrochemistry, Adsorption, and Surface Morphology

Dithiocarbamates are versatile ligands whose coordination behaviour can significantly modulate interfacial electronic processes. 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 environments, using mild steel in 1 M HCl as a chemically tractable 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, modeled with an Rs (Rct‖CPE) circuit, showed concentration-dependent increases in charge-transfer resistance with slightly elevated double-layer capacitance, revealing a thick, polarizable interfacial layer and a predominantly electronic/kinetic inhibition mechanism 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 SEM and AFM 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.