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C30 Self-Assemblied Monolayers on Silica, Titania, and Zirconia: HPLC Performance, Atomic Force Microscopy, Ellipsometry, and NMR Studies of Molecular Dynamcis and Uniformity of Coverage.


pdf icon C30 Self-Assemblied Monolayers on Silica, Titania, and Zirconia: HPLC Performance, Atomic Force Microscopy, Ellipsometry, and NMR Studies of Molecular Dynamcis and Uniformity of Coverage. (2811 K)
Pursch, M.; Vanderhart, D. L.; Sander, L. C.; Gu, X.; Nguyen, T.; Wise, S. A.; Gajewski, D. A.

Journal of the American Chemical Society, Vol. 122, No. 29, 6997-7011, 2000.

Keywords:

self-assembled monolayers; predictive models; silica; titania; zirconia; HPLC

Abstract:

C30 self-assembled monolayers (SAMs) have been prepared on zirconia, titania and two different silica gels by reacting C30 trichlorosilane with the humidified surfaces. 13 deg C solid-state NMR spectroscopy indicated higher alkyl chain order on titania and zirconia materials than on the silica C30 phases. Order is inferred from the relative intensity of the main methylene carbon resonance assigned to an all-trans conformation. Carbon longitudinal relaxation time (T1C) data reveal that these ordered alkyl chains still have large-amplitude motions on submicrosecond timescales at ambient temperature. Since fast diffusional rotation about the chain axis is compatible with an all-trans conformation, T1C, carbon chemical shift, and proton line width data for the alkane rotator phase (C19) and the C30 phases were compared. Proton spin diffusion experiments were also conducted using an initial polarization gradient based on mobility differences. These experiments indicated both a higher mobility for the free end of the immobilized chains as well as heterogeneity in the density of coverage on at least the 20-nm distance scale. The methyl carbon lineshape was also discussed in detail since its chemical shift conveys information about both mobility and interactions with an air interface in a dry sample. Atomic force microscopy (AFM) and contact angle studies indicated a greater surface roughness for C30 SAMs compared to C18 SAMs prepared on silicon. Ellipsometry revealed film thicknesses of 2.82 nm for the C18 SAM and 4.05 nm for the C30 SAM. High shape selectivity was found in correspondent liquid chromatographic (LC) separations of polycyclic aromatic hydrocarbons (PAHs), carotenoids and tocopherols. The LC data confirm the highly organized alkyl chain arrangement on zirconia and titania, which provide an alternative to the silica-based reversed phases.