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41,54,55
In this study, the
hydroxamic acid films formed were stable to rinse but not to
sonication, which was not performed in the previous report. To
understand this result, AFM images of Nitinol before and after
the deposition and rinsing of the octadecylhydroxamic acid
(ODHA) film are shown in Figure 2. The rms roughness values
of the control and modified substrates were compared.Modified
surfaces with a similar rms roughness to the control surface are
considered to be uniform films that follow the contour of the
surface, while modified surfaces that have a much larger rms
roughness than the control aremultilayer or nonuniformfilms.
36,56
The rms roughness of the control and modified substrate was 14
and 75 A, respectively (Figure 2). These values and the images
Figure 1. DRIFT spectra of the C-H stretching region of octa-
deylsulfonic acid film on Mn after rinse (bottom) and sonication
(top). The filmis removed by sonication. The νCH2 asym=2914 cm-1
and νCH2 symm =2848 cm-1
.clearly indicate that the surface is nonuniformly covered by
multilayers and islands of agglomerates. These images are corre-
lated to the contact angle values obtained of ODHA on Nitinol
which are lower than those expected for a hydrophobic surface as
shown in Table 3. Moreover, the film was completely removed
after sonication, and so it is concluded that the hydroxamic acid
films are weakly bound, perhaps due in part to micelle formation
seen in the images.
The films stable to rinse on the alloys and the major compo-
nents of the alloy were further characterized by contact angle
goniometry (Table 3). Contact angles can be a useful indicator of
film quality and wettability of the surface when used in conjunc-
tion with other data. Contact angle values for the unmodified
substrates were in the range of 50-60 (specific values are in the
Supporting Information). Here, the contact angles for phospho-
nic acid SAMs were the most hydrophobic. Sulfonic acid films
produced lower contact angle values, suggesting heterogeneous or
incomplete surface films as can be seen in the AFM images of
these substrates (AFM in Supporting Information). For hydro-
xamic acid films on SS316L the standard deviations were high,
indicating that the filmswere not uniformon this surface.Contact
angle values for films on Nitinol, nickel, and titanium were
consistent with hydrophobic uniform films. Finally, carboxylicacid formedmonolayers only on SS316Land iron and presented a
hydrophobic interface on both of them, indicating that themethyl
tail group is presented at the interface.
The interaction between the organic moiety and the surface
oxide can be characterized by DRIFT spectroscopy. This inter-
action can be either mono-, bi-, or tridentate, and cases of each
have been reported in the literature (Figure 3).
17,36,37,57
The bonding mode of the acid with the metals and alloys after
sonication has been tabulated in Table 4 based on IR spectroscopy.
Figure 3. Monodentate, bidentate, and tridentate bonding of
phosphonic acid on stainless steel 316L.For phosphonic acid SAMs the IR stretch values in the P-O
region, which corresponds to 1300-950 cm-1
, provides valuable
information about the type of bonding of the headgroup with the
substrate. IR bands at 1227 and 950 cm-1
are assigned to νPdO
and νP-OH, respectively, and the peaks at 1074 and 1010 cm-1
are
assigned to the symmetric and asymmetric stretch of νPO3
.
19,20,40
If
the peak assigned to P-OandPdO stretches are observed,
coupled with the disappearance of νP-O-H =920cm-1
,a
bidentate coordination mode of the molecule to the surface is