C 30 Self-Assembled Monolayers on Silica, Titania, and Zirconia: HPLC Performance, Atomic Force Microscopy, Ellipsometry, and NMR Studies of Molecular Dynamics and Uniformity of Coverage Matthias Pursch,* ,² David L. Vanderhart, ‡ Lane C. Sander, ² Xiaohong Gu, § Tinh Nguyen, § Stephen A. Wise, ² and Donald A. Gajewski | Contribution from the Analytical Chemistry DiVision, Chemical Science and Technology Laboratory, Polymers DiVision, Materials Science and Engineering Laboratory, Building Materials DiVision, Building and Fire Research Laboratory, and Semiconductor Electronics DiVision, Electronics and Electrical Engineering Laboratory, National Institute of Standards and Technology (NIST), Gaithersburg, Maryland 20899 ReceiVed October 15, 1999. ReVised Manuscript ReceiVed May 2, 2000 Abstract: C 30 self-assembled monolayers (SAMs) have been prepared on zirconia, titania, and two different silica gels by reacting C 30 trichlorosilane with the humidified surfaces. 13 C solid-state NMR spectroscopy indicated higher alkyl chain order on titania and zirconia materials than on the silica C 30 phases. Order is inferred from the relative intensity of the main methylene carbon resonance assigned to an all-trans conformation. Carbon longitudinal relaxation time (T 1 C ) data reveal that these ordered alkyl chains still have large-amplitude motions on submicrosecond time scales at ambient temperature. Since fast diffusional rotation about the chain axis is compatible with an all-trans conformation, T 1 C , carbon chemical shift, and proton line width data for the alkane rotator phase (C 19 ) and the C 30 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 and heterogeneity in the density of coverage on at least the 20-nm distance scale. The methyl carbon line shape is 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 and contact angle studies indicated a greater surface roughness for C 30 SAMs compared to C 18 SAMs prepared on silicon. Ellipsometry revealed film thicknesses of 2.82 nm for the C 18 SAM and 4.05 nm for the C 30 SAM. High shape selectivity was found in correspondent liquid chromatographic (LC) separations of polycyclic aromatic hydrocarbons, 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. Introduction Self-assembled monolayers (SAMs) are receiving widespread and increasing interest in materials research. 1-5 The majority of monolayers are formed by reaction of sulfur-containing ligands with transition metal (gold, silver, copper) surfaces. A thoroughly described example is the SAM formation of al- kanethiols on gold surfaces. 2 SAMs are also easily prepared by silanization of silicon surfaces. 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