Nature of the Interaction of N,N′‑Diphenyl-1,4-phenylenediamine with Iron Oxide Surfaces

Redox-active polymers and small molecules are of great interest in coatings, such as corrosion inhibitors for steel and other metals. In this work the interaction of the redox-active phenyl-capped aniline dimer (N,N′-diphenyl-1,4-phenylenediamine, DPPD) with iron oxide surfaces was investigated with the aim to understand the corrosion inhibition and self-healing properties of polyaniline and aniline oligomers on iron oxide surfaces. Raman, mid-IR, and visible spectroscopies all show that reduced DPPD transforms into the semiquinone form by interacting with α-Fe2O3. Thermal gravimetric analysis (TGA) was used to quantify the strength of these interactions, clearly within the chemisorption range. TGA analysis, mid-IR spectroscopy, and atomic force microscopy showed the DPPD molecules to be standing on their edge on the surface and changing their orientation to standing on end upon initiation of multilayer formation. DPPDand hence other reduced oligoanilines or polyanilineare therefore shown to strongly interact with iron oxide surfaces through hydrogen bonding and charge transfer to the surface. A full understanding of coatings will ultimately require the study of all oxidation states and their surface interactions. Here we provide the most detailed understanding to date of the reduced state as a first step.