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(AFM) operating in noncontact mode at ambient conditions
was used to confirm the uniformity of the film on the surface.
The cantilever tip with a resonance frequency of 160-170 kHz
and a typical spring constant of 40 N/m were obtained from
Agilent-Molecular Imaging Corp.
Mass Spectrometry. A high-resolution MALDI-TOF MS
purchased from Agilent Technologies with pulsed dynamic focusing
was used to characterize the molecules on the substrate. MS
analyses of the ions were detected in positive mode. The matrix,
R-cyano-4-hydroxycinnamic acid (CHCA) (Sigma-Aldrich,>99.0%
purity), was used without further purification and added in THF
(10 mg/mL). Samples forMALDIMS analysis were prepared by
the “dried-drop method”. A 1 μL aliquot of the matrix solution
was placed on each of the modified substrates and then dried at
room temperature. The samples were then placed directly onto a
MALDI sample plate using double-sided tape and loaded into the
MALDI-TOF. Analyte ionization was achieved by focusing a
pulse of a laser light onto the sample/matrix preparation.
Results
Self-assembled monolayers were formed on the native oxide
surface of stainless steel 316L and Nitinol alloys. X-ray photo-
electron spectroscopy (XPS) analysis of the stainless steel 316L
substrate was performed previously using a Phoibos 150 hemi-
spherical energy analyzer and a monochromatized Al (1486.6 eV)
source. Compositional results for the SS316L reference substrate
were in reasonable agreement with the nominal SS316L bulk
elemental composition50
(surface composition: Fe 66.01%, Cr
19.19%, Ni 9.17%, Mn 3.22%, and Mo 2.42%). The Nitinol
substrate’s surface composition was 50% each of nickel and
titanium oxides.
45,51
Thus, iron, chromium, nickel, manganese,
molybdenum, and titanium were used as the constituent metals
for the alloys in this study. A long carbon chain of 18 was utilized
as the standard to optimize van der Waals interactions and to
prevent chain length variation from affecting the formation of
ordered self-assembled monolayers. The films formed were char-
acterized by DRIFT spectroscopy after deposition, rinse in THFfor 15min, and after sonication in THF for 15min. In the spectra
of the modified substrates, the C-H stretches of the methylene
group are used as the reference peaks for alkyl chain organization
in the SAM.
13,52,53
Aspectra which contained νCH2 asyme2918 cm-1
and νCH2 symm e 2848 cm-1
indicated that the alkyl chains in the
film were ordered with the molecules organized in all-trans
configuration on the substrate (Figure 1). The positions of the
peaks corresponding to νCH2 asym after rinsing with THF and after
sonication in THF are provided in Table 2 for the acid-metal
oxide and acid-alloy oxide systems. If the organicmoleculeswere
removed by rinse or sonication, the data are represented by dotted
lines.
From Table 2, it is clear that phosphonic acid forms the most
stable interaction with all of the substrates as the films remain
intact after sonication on the metals and alloys. Sulfonic acid
forms films on all of the surfaces and is stable to rinsing in solvent
but only remains on SS316L after sonication. Carboxylic acid
only formed films on SS316L and its major component Fe oxide,
and the films were stable on both surfaces.
Hydroxamic acid formed ordered films on more substrates
than carboxylic acid. Previous reports on hydroxamic acid films
indicated that they were more stable than carboxylic or phos-
phonic acid onmetal oxides such as copper, aluminum, and iron16
and had good corrosion inhibition efficiency due to their well-
ordered alkyl chains and film formation.