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