Two Steady-State Adsorption Modes of Phosphonic Acids on Aluminum Surfaces

The phosphonic acid (PA) surface treatment on various metal substrates is of high industrial relevance, and the PA molecular structure significantly affects its quality. In this work, systematic variation of the PA molecular steric and electron environment helps discern two steady-state adsorption modes on an aluminum surface. The PA molecular structure was varied systematically, which included inorganic phosphorus acid, alkyl phosphonic acids, and phenyl phosphonic acids. To explore their in situ dynamics of adsorption/desorption on the electrochemically unstable aluminum, techniques such as electrochemical impedance spectroscopy and inductively coupled plasma optical emission spectrometry were employed. A range of different types of interfacial layers are formed on the aluminum surface, namely, from the dissolution-limiting physisorbed layer to a quasi-inhibiting chemisorbed layer on the aluminum surface in acidic (pH ≈ 2.2) solution. Presented findings establish the dynamic steady-state nature of this type of interface. They reveal fundamental relationships among adsorbent steric or electronic effects, the steady-state interface morphology, and the steady-state aluminum dissolution rate. The study brings also a more differentiated molecular structure-related description of the aluminum dissolution inhibition of PAs and relates it to molecular density functional theory calculations.