Self-assembled monolayers of alkylphosphonic acids on metal oxides

Dense, highly ordered monolayers can be prepared by the adsorption of octadecylphosphonic acid (ODPA) onto nonporous ZrO₂, TiO₂, and zirconated silica powders. ODPA reacts strongly with Al₂O₃ to form a bulk (aluminoalkyl)phosphonate. The infrared spectra of the ODPA monolayers indicate a degree of conformational order comparable to self-assembled monolayers on planar substrates. From the solid-state ³¹P and ¹³C NMR spectra, the strength of the surface interaction and the degree of conformational order decrease in the following order: zirconated silica > ZrO₂ > TiO₂. Although the inner methylenes are primarily in an all-trans conformation, the chain termina are highly disordered. In the case of zirconated silica, there is evidence for the formation of some bulk zirconium alkylphosphonate. Proton line width measurements from 2D WISE NMR experiments reveal that considerable chain mobility is present in the ODPA monolayers on ZrO₂ and TiO₂, in contrast to bulk crystalline phases.