Insight into the formation mechanism of durable hexadecylphosphonic acid bilayers on titanium alloy through interfacial analysis

Titanium alloy (Ti6Al4V) surfaces were successfully modified by self-assembled hexadecylphosphonic acid (HDPA) bilayers (thickness of about 4 nm) using a reproducible method. Conformation of the HDPA bilayers was put forward and then verified by X-ray photoelectron spectroscopy (XPS), Atomic force microscopy (AFM) and Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) measurements. The first molecular layer contains two kinds of HDPA molecules which interpenetrate each other. Formation of the second molecular layer was associated with reverse vesicles adhesion and then their rupture on the first molecular layer. Mechanism of HDPA bilayers formation on Ti6Al4V was deduced via an observation on the interfacial adsorption of HDPA molecules. High oxide/hydroxyl content on Ti6Al4V surfaces greatly promotes the formation of HDPA bilayers. Annealing after self-assembly enhances the stability of HDPA bilayers due to formation of pyrophosphates and P-O-Ti which can withstand ultrasonic cleaning for a period of 1 h at least. The HDPA-modified Ti6Al4V will have some potential good tribological properties and protein adsorption properties.