Self-Assembly of Carboxyalkylphosphonic Acids on Metal Oxide Powders

The structures formed by adsorption of carboxyalkylphosphonic acids on metal oxide powders were characterized by solid-state NMR and FTIR-PAS (Fourier transform infrared photoacoustic spectroscopy). A series of diacids, HO 2 C(CH 2 ) n PO 3 H 2 ( n = 2, 3, 11, and 15), were deposited on nonporous TiO 2 and ZrO 2 powders and nanocrystalline ZrO 2 with average particle diameters of 21, 30, and 5 nm, respectively. Solid-state 31 P NMR, combined with FTIR-PAS, indicates that the phosphonic acid group binds selectively to the surface, producing a monolayer of carboxylic acid terminated chains. The average chain conformation depends on the substrate in addition to the chain length. Ordered samples display thermal order/disorder transitions similar to other self-assembled monolayer systems. The more restricted chain mobility relative to analogous methyl-terminated chains is attributed to hydrogen-bonding among the pendant carboxylic acid groups. These results demonstrate that phosphonic acids are useful for selectively introducing pendant polar functional groups on metal oxide surfaces.