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

摘要

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_2O_3) as well as on smooth, thin films of tantalum oxide (Ta_2O_5), niobium oxide (Nb_2O_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 deg. 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_2O_5 and Nb_2O_5 has also been observed. DDPO_4 and OH-DDPO_4 were codeposited onto Ta_2O_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 deg by adjusting the molar ratio of the two different molecules in the aqueous SAM forming solution. The SAIvIs were characterized by advancing water contact angle (wettability), microdroPlet density measuremepts (condensation figures to judge homogeneity), and X-ray photoelectron spectroscopy (coverage and orientation of molecules).