Self-Assembled Monolayers of Dodecyl and Hydroxy-dodecyl Phosphates on Both Smooth and Rough Titanium and Titanium Oxide Surfaces

Abstract

Dodecyl phosphate and hydroxy-terminated dodecyl phosphate are shown to spontaneously assemble on smooth titanium oxide and titanium metal coated glass and silicon substrates, as well as on rough titanium metal implant surfaces. The surfaces were dipped in aqueous solutions of the corresponding ammonium salts for 48 h. The molecules are shown by X-ray photoelectron spectroscopy (XPS) to form densely packed, self-assembled monolayers (SAMs) on all surfaces investigated. The phosphate headgroups are believed to attach to the titanium (oxide) surface with the terminal end group (either methyl or hydroxy) pointing toward the ambient environment (air, vacuum, or water). Mixed SAMs are shown to be formed from mixed aqueous solutions of the two amphiphiles, with the hydroxy-terminated dodecyl phosphate adsorbing more favorably than the methyl-terminated molecule. The advancing water contact angles can be easily tailored via the composition of the self-assembly solution in the range of 110° (pure methyl) to 55° (pure hydroxy) on flat, smooth titanium surfaces. Surface roughness strongly modifies the wetting properties, with advancing contact angles in the range of 150−100° being observed, as well as the degree of hysteresis (difference between advancing and receding angles). Model calculations based on XPS intensities have been successfully used to quantify the adlayer composition and molecular surface densities across the whole range of mixed adlayer chemistry. The organophosphate monolayers on titanium are believed to have a significant potential for precise control of the surface chemistry and interfacial tension on both smooth and rough titanium surfaces in application areas such as medical implants and other devices where independent control of surface chemistry and topography is essential to performance.