Field-Selective Adsorption of Saccharin on Nickel: Mechanistic DFT Insights into Solvation, Protonation, and Coating Morphology

The molecular mechanisms by which organic additives such as saccharin control microstructure in nickel electrodeposition remain inadequately understood, particularly the role of the intense interfacial electric field. This study employs density functional theory (DFT) calculations to elucidate the field dependent adsorption behavior of neutral saccharin and its deprotonated anion (saccharinate) on nickel. By employing the B3LYP functional and implicit solvent models, the field dependent adsorption energetics, frontier orbitals and electrostatic potentials are calculated on a nickel surface. Key findings reveal that while saccharinate dominates in bulk plating baths, its strong solvation shell impedes surface adsorption. In contrast, neutral saccharin exhibits energetically favorable adsorption via sulfonyl oxygen or aromatic $π$-face interactions, with specific orientations further stabilized by the interfacial field.This selective adsorption at growth sites rationalizes saccharin's role in inhibiting rapid crystallization, promoting grain refinement, and producing bright, level deposits.The results directly link field-modulated molecular stereochemistry to macroscopic coating properties, providing a mechanistic foundation for the rational design of electroplating additives beyond empirical approaches.