Orientation in Methyl- and Hydroxyl-Terminated Self-Assembled Alkanephosphate Monolayers on Titanium Oxide Surfaces Investigated with Soft X-ray Absorption

Titanium oxide surfaces were coated with methyl- and hydroxyl-terminated dodecyl phosphate monolayers by immersion of the substrates into aqueous solutions of the corresponding ammonium salts (DDPO4-(NH4)(2) and OH-DDPO4(NH4)(2)). The samples were examined by contact-angle measurements and near-edge X-ray absorption fine structure spectroscopy. DDPO4 was found to form self-assembled films of comparable quality to those of the longer octadecyl phosphate prepared from an organic solvent and to those of a similar thiol/gold system. The hydroxyl-terminated adsorbates, however, established films with a much lower degree of orientation under the same preparation conditions and showed ''high'' water contact angle values (50-55degrees) for a hydrophilic surface. Mixed films of both species establish a degree of orientation between those of both pure films, which is also reflected by their wettability.