The formation and stability of self-assembled monolayers of octadecylphosphonic acid on titania

Octadecylphosphonic acid (OPA) forms monolayers on amorphous smooth titania surfaces. The OPA was deposited by immersion in tetrahydrofuran solutions. The process takes place rapidly at 2 × 10−4 M OPA, evidence of which is an advancing water contact angle of approx. 100° after 10 s of immersion. The advancing water contact angle plateaus at 110 ± 2° after several hours. At lower concentrations (2 × 10−6 and 2 × 10−8 M OPA), only partial coverage of OPA on titania films is achieved.OPA self-assembled monolayers (SAMs) are resistant to exposure to common solvents. Water wettability is a function of the molecular volume of the solvents. Whereas low molecular volume liquids penetrate OPA SAMs, no penetration is observed for high molecular volume liquids. The cross-sectional area of pores in the OPA monolayers is estimated at 27 Å2.UV light (λ = 250 nm), heat treatment and air plasma all significantly improve the wettability of OPA SAMs, resulting in a water contact angle of 0°. The presence of phosphorus shows that the bond of the phosphate headgroup to titania through oxygen atom(s) is strong and can withstand these conditions.