A solid-state NMR study of self-assembled monolayers

Hydrocarbon and perfluorinated fatty acids deposited on high surface area ZrO2 and TiO2 substrates have been studied by solid-state nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy for characterization as potential surface modification agents and for fundamental studies of monolayer conformation and dynamics. Alkanoic acids of sufficient chain length (CH3(CH3)nCO2H, n > 15) were found to form conformationally ordered monolayers on ZrO2 powder via a chelating bidentate zirconium carboxylate surface bond. Variable temperature and 2D solid-state NMR experiments demonstrate that alkanoate monolayers display chain dynamics similar to other self-assembled monolayers (SAMs). o-Hydroxy-alkanoic acids (HO(CH3)15CO 2H) produce hydrophilic monolayers, with no evidence of the hydroxyl group looping to the metal oxide substrate.; Unlike the hydrocarbon analogues, perfluoroalkanoic acids chemisorbed on both ZrO2 and TiO2 powders via a carboxylate bond forming stable and thermally robust SAMs. Spin-lattice relaxation measurements showed that the adsorbed SAMs are more mobile than in the bulk state. The motions associated with this enhanced mobility are proposed to involve reorientations about the chain axis. No evidence was found for chain melting in the fluorocarbon SAMs at temperatures well above the melting point of the bulk acids.; Since both long chain carboxylic and phosphonic acids form SAMs, a series of diacids, (HO2C(CH2)nPO3H 2, n = 2, 3, 11, and 15), were deposited on TiO2 and ZrO 2 powders and ZrO2 nanopowder to examine the structures formed by bifunctional surfactants where both groups strongly interact with the substrate. Solid-state 31P NMR data in combination with IR showed 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 and chain length. Ordered samples display thermal order/disorder transitions similar to other hydrocarbon SAM systems demonstrating that phosphonic acids are useful for selectively introducing pendant polar functional groups on metal oxide surfaces. The chain mobility relative to analogous methyl terminated chains is more restricted and is attributed to hydrogen bonding among the pendant carboxylic acid groups. 1H fast MAS NMR experiments revealed the nature of the surface hydrogen bonding in these systems. Whereas dipolar coupled P-OH protons were observed on TiO2, only isolated residual P-OH groups were detected on ZrO2, reflective of the stronger interaction of phosphonic acids with the latter substrate. Two dimensional 1H double-quantum fast MAS experiments revealed that only hydrogen bonded homodimers occur in the bulk diacids while hydrogen bonding between the carboxylic and phosphonic acid groups exists in multilayers of the diacids on the ZrO2 nanopowder.