In situ study of the impact of acidic and neutral deposition pH on alkane phosphate film formation and stability on TiO2

Abstract

TiO2 has been used as a model system for the surface of medical implant devices based upon titanium alloys. Self Assembled Monolayers (SAM) can be applied via a specific phosphate interaction to modify the surface properties of the TiO2 substrate. Here it is demonstrated via in situ Quartz Crystal Microbalance and Neutron Reflectometry experiments that the deposition of dodecyl phosphate onto TiO2 proceeds more rapidly at pH 4.5 than at pH 7.0. Conversely, the film stability was enhanced for films deposited at pH 7.0, demonstrating that while the initial association with the surface is driven by electrostatics it does not determine the SAM density. While the adsorbed amount appeared to be relatively constant after a few minutes incubation time it was found that washing with buffer removed about 50% of the adsorbed material after these short incubation times. With incubation time of the order of hours the proportion of the film washed off the surface decreased demonstrating that the specific phosphate–TiO2 interaction was a slow process. The slower initial surface interaction at pH 7.0 therefore allowed greater re-arrangement of the dodecyl phosphate resulting in more complete and robust monolayers than at pH 4.5. This was demonstrated by washing the film with buffer of increasing pH of up to 9.5. For SAM longevity on titanium alloys it is clear that slow deposition at pH 7 produces more robust films than rapid deposition at lower pH values.