Stability of Phosphonic Acid Self-Assembled Monolayers on Amorphous and Single-Crystalline Aluminum Oxide Surfaces in Aqueous Solution

Abstract

The formation of octadecylphosphonic acid (ODPA) self-assembled monolayers (SAMs) and their stability in water has been studied on four distinctly different aluminum oxide surfaces. The aim was to improve the understanding of the state of binding between the phosphonic acid to the oxide surface and how this interaction depends on the structure and termination of the oxide surface. Single crystalline Al(2)O(3)(0001) and Al(2)O(3)(1102) surfaces were compared to amorphous oxide passive films on aluminum and physical vapor deposited (PVD) amorphous aluminum oxide films on gold. The monolayers were adsorbed from ethanol solution, characterized by means of high-resolution in situ atomic force microscopy (AFM), contact angle measurements, polarization modulated infrared reflection absorption spectroscopy (PM-IRRAS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and proved to be self-assembled. On Al(2)O(3)(1102) surfaces and amorphous Al(2)O(3) surfaces, the ODPA self-assembled monolayers showed high stability in aqueous environments. However, the adsorbed ODPA monolayers were substituted by the adsorption of interfacial water on the Al(2)O(3)(0001) surface via the intermediate formation of micelles. The different stability of the monolayers in aqueous environments is explained by the variation of interfacial binding states ranging from ionic interactions between phosphonate groups and the positively charged hydrolytated oxide surface to directed coordination bonds between the phosphonate group and Al ions.