Alkyl Phosphate Monolayers, Self-Assembled from Aqueous Solution onto Metal Oxide Surfaces

Abstract

Metal oxide surfaces have been coated by self-assembled monolayers (SAMs) of dodecyl phosphate (DDPO 4 ) and 12-hydroxy dodecyl phosphate (OH-DDPO 4 ), by means of a novel surface-modification protocol based on the adsorption of the alkyl phosphate ammonium salts from aqueous solution. Formation of DDPO 4 SAMs has been successfully demonstrated on anodized aluminum (Al 2 O 3 ) as well as on smooth, thin films of tantalum oxide (Ta 2 O 5 ), niobium oxide (Nb 2 O 5 ), zirconium oxide (ZrO 2 ), and titanium oxide (TiO 2 ) deposited on glass substrates, resulting in highly hydrophobic surfaces with advancing water contact angles ≥110°. SAM formation does not occur on silica surfaces under the same conditions. The formation of SAMs based on hydroxy-terminated alkyl phosphates (OH-DDPO 4 ) onto Ta 2 O 5 and Nb 2 O 5 has also been observed. DDPO 4 and OH-DDPO 4 were codeposited onto Ta 2 O 5 from aqueous solutions at different concentration ratios. It is shown that the water contact angle can be precisely controlled within the range of 110−50° by adjusting the molar ratio of the two different molecules in the aqueous SAM forming solution. The SAMs were characterized by advancing water contact angle (wettability), microdroplet density measurements (condensation figures to judge homogeneity), and X-ray photoelectron spectroscopy (coverage and orientation of molecules).