The chain conformation and dynamics of hydrocarbon and perfluorocarbon fatty acids adsorbed on 4 nm ZrO2 particles were characterized by solid-state 13C chemical shift and 19F NMR relaxation measurements, respectively, and compared to those from previous studies on lower surface area fumed metal oxide powders. The interdigitation of chains between neighboring particles, which increases with chain length, can be detected from the splitting of the 13C NMR and 19F NMR signals of the CH3 and CF3 groups, respectively. Similar to the case of alkanethiol self-assembled monolayers (SAMs) on gold nanoparticles, this interdigitation allows for efficient chain packing despite the high surface curvature. The hydrocarbon chains on the ZrO2 nanoparticles are more ordered, and the reversible chain length dependent order-disorder transition temperatures are elevated relative to those of the same fatty acids adsorbed on fumed ZrO2 powder. Likewise, the 19F spin lattice relaxation times of the fluorocarbon chains approach those of the bulk acids with increasing chain length and interdigitation, indicating densely packed chains.

Deuterium NMR spectroscopy has been used to probe the dynamics of two samples, 2,2-d 2-hexadecanoate (HDCA) and perdeuterated HDCA monolayers on nonporous ZrO2 powder (surface area ∼ 40 m2/g) over the temperature range of 180 K to room temperature. The data indicate the presence of a considerable motional gradient in the tethered chain. Line shape simulations were performed on the spectra of 2,2-d 2-HDCA to extract detailed information on the dynamic processes involved. These quantitative line shape analyses clearly show that, over the whole temperature range studied, the C2D bonds have substantial motional freedom with respect to the characteristic 2H NMR time scale. In addition, at each temperature, a weighted superposition of several simulated line shapes with different rates and site populations is required to account for the observed spectral features, indicating the presence of considerable motional heterogeneity within the hexadecanoate monolayers. The results will be compared with dynamic behaviors of other self-assembled monolayers (SAMs), particularly those of alkylthiolate SAMs on gold nanoparticles and phosphonate SAMs on ZrO2 powder. Attempts will be made to delineate the effects of various factors on the alkyl chain dynamics.