Adsorption and desorption of stearic acid self-assembled monolayers on aluminum oxide

Development of coatings to minimize unwanted surface adsorption is extremely important for their use in applications, such as sensors and medical implants. Self-assembled monolayers (SAMs) are an excellent choice for coatings that minimize nonspecific adsorption because they can be uniform and have a very high surface coverage. Another equally important characteristic of such coatings is their stability. In the present study, both the bonding mechanism and the stability of stearic acid SAMs on two aluminum oxides (single-crystal C-plane aluminum oxide (sapphire) and amorphous aluminum oxide (alumina)) are investigated. The adsorption mechanism is investigated by ex situ X-ray photoelectron spectroscopy and infrared (IR) spectroscopy. The results revealed that stearic acid binds to sapphire surfaces via a bidentate interaction of carboxylate with two oxygen atoms while it binds to alumina surfaces via both bidentate and monodentate interactions. Desorption kinetics of stearic acid self-organized on both aluminum oxide surfaces into water is explored by ex situ tapping mode atomic force microscopy, IR spectroscopy, and contact angle measurements. The results exhibit that the SAMs of stearic acid formed on sapphire are not stable in water and are continuously lost through desorption. Water contact angle measurements of SAMs that are immersed in water further indicate that the desorption rate of adsorbates from atomically smooth terrace sites is substantially faster than that of adsorbates from the sites of surface defects due to weaker molecular interaction with the smooth surface. A time-dependent desorption profile of SAMs grown on amorphous alumina reveals that contact angles decrease monotonically without any regional distinction, providing further evidence for the presence of adsorption sites with different types of affinity on the amorphous alumina surface.