Environmental Engineering Reference
In-Depth Information
the preparation and characterization of SA coatings using bolaamphiphiles on
metal or metal oxide powders, particularly of magnetic iron oxides which have
potential applications in drug-delivery, magnetic carrier technology, raw mate-
rial recovery, biological cell separation, magnetic fluids, magnetic ink, and
magnetic memory media [65, 66, 67, 68, 69, 70].
Inour laboratory [71], 16-mercaptohexadecanoic acid (MHA) (HS(CH
2
)
15
COOH)
was assembled onto nanosized magnetic particles (g-Fe
2
O
3
) from chloroform.
To elucidate the reactivity of polar groups with g-Fe
2
O
3
, stearic acid
[CH
3
(CH
2
)
16
COOH], are also self-assembled into monolayer coatings on
magnetic particles and characterized by X-ray photon spectroscopy (XPS),
diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS), and
film flotation. It is expected that the carboxylic head group (
COOH) of
MHA preferentially anchors to the surface of the magnetic particles so that
the thiol (
SH) on the other end remains available for different reactions.
The most significant spectral change in the XPS spectra of g-Fe
2
O
3
with and
without self-assembled layers is the appearance of two C
1s
bands at 288.3
(COO
) and 284.6 eV (C-C) [72], when the g-Fe
2
O
3
powders were treated by
stearic acid and MHA. The ratio of area under the C
1s
band of higher binding
energy to that of lower binding energy was calculated to be ca. 1:17 and 1:15 for
g-Fe
2
O
3
self-assembled with stearic acid and MHA, respectively. These values
are in excellent agreement with those derived from the molecular structure. For
the MHA treated sample, a sulfur band of S
2p
at 163.3 eV was observed. This
band is characteristic of -SH or -S-S- groups without oxidization to sulfate.
These spectral changes indicate the presence of self-assembled stearic acid and
MHA layer on g-Fe
2
O
3
.
To determine the orientation of surfactant molecules in the SA monolayers,
a surface-sensitive, DRIFTS was used, and the spectra are shown in Fig. 6.3.
Over the high wavenumber region, the CH
2
stretching vibration bands at 2,924
and 2,851 cm
1
are observed, suggesting the presence of a hydrocarbon chain as
anticipated. The CH
2
bands appeared at the same band positions for bulk
MHA and for MHA coated on g-Fe
2
O
3
. However, the bands are sharper for
the MHA on g-Fe
2
O
3
than for bulk MHA, indicating a more ordered poly-
methylene chain and confirming the assembly of a densely packed surfactant
monolayer. However, over the low wavenumber region, the MHA on g-Fe
2
O
3
resulted in a different spectral feature than bulk MHA. The absence of the
carbonyl band at 1,703 cm
1
and the presence of a carboxylate band at
1,433 cm
1
for the MHA on g-Fe
2
O
3
suggest not only the anchoring of carbo-
nyl groups on g-Fe
2
O
3
, transferring a carbonyl to the carboxylate functionality,
but also the absence of the second MHA layer, i.e., only a monolayer coverage.
Should the second layer be present, a mixed functionality of carboxylate from
the first layer (band at 1,433 cm
1
) and carbonyl from the second layer (band at
1,703 cm
1
) would be observed.
From film flotation test, the differences of critical surface tensions of MHA-
coated g-Fe
2
O
3
, stearic acid-coated particles, and DTDPA (3,3
0
-dithiodipropionic
acid)-coated particles further confirmed that the terminal group of the MHA
