The Journal of Physical Chemistry B
Article
high accuracy, which depict an apparent molecular arrange-
ment in the L film. The IR and Raman techniques provide rich
information on molecular conformation and orientation in the
film regardless of the degree of the crystallinity. Although these
spectroscopic techniques are quite useful, they reveal the struc-
tural information only of the film molecules, and no information
is given for the ions interacted with the film molecules.
on a zinc bromide aqueous solution at a concentration of
1.0 × 10− mol L at pH of ca. 6, which was a good condition
to make all of the stearic acid molecules reacted with zinc
cations to form zinc stearate as shown later. The trough was
placed in an external-reflection optics designated for the FTIR
spectrometer equipped with a PM-IRRAS accessory. The sub-
phase water was obtained by a Millipore (Molsheim, France)
Elix UV-3 pure-water generator and a Yamato (Tokyo, Japan)
Autopure WT100U water purifier, which is a compatible model
with Milli-Q. The water exhibited a high electric resistivity higher
3
−1
For the purpose of analyzing chemical atmosphere about an
ion, X-ray absorption fine structure (XAFS) plays an important
9
−12
role.
XAFS spectra are conveniently discussed by separat-
−1
ing into two regions called X-ray absorption near edge struc-
ture (XANES) and extended X-ray absorption fine structure
than 18.2 MΩ cm and the surface tension was 72.8 mN m at
the temperature, which guaranteed that the water was organic
contaminants free.
(EXAFS) regions. The XANES region that appears near the
absorption edge measures an X-ray absorption due to transi-
tions from an electronic core level to unoccupied levels, which
reveals the local electronic structure about the ion. The prom-
inent absorption band at the edge is called the white line and it
accompanies some informative peaks about the coordination
structure. However, the EXAFS region in a higher energy range
than XANES measures energy-dependent variation of the exci-
tation probability (absorptivity) due to the interference be-
tween the electromagnetic wave going out from the atom generated
by the core−electron excitation and the scattered electrons by
Two Types of a Langmuir Film Used for the Present
Study. Two types of an L film were used for the present study:
uncompressed and compressed L films. The uncompressed L
film was prepared by spreading the stearic acid solu-
tion on a fixed surface area of the zinc bromide aqueous
solution using no compression bar. The volume of the spread
solution was designed, so that the average molecular cross-
2
section area of the L film would be 0.2 nm that corresponds to
the full molecular packing with a perpendicular molecular stance
17
to the water surface. The surface pressure of the uncompressed
L film 10 minutes after the preparation was less than 15
2
+
surrounding atoms. In the case of Zn analysis, the white line
of XANES is known to respond to the coordination number
−1
mN m . Unfortunately, however, the precise monitoring of the
surface pressure for a long time period during the IR
measurements is difficult because the water-level change due
to the evaporation makes the monitoring disturbed. Therefore,
it should be noted that the molecular density is kept unchanged
through the experiments.
1
3−15
about the ion,
which is a fortunate condition for highly
sensitive measurements. In the present study, therefore,
XANES spectra measured at the air/water interface have
been analyzed to reveal the structure on a cation coordinated
with water and stearic acid molecules.
16
In a previous study, Watanabe et al. were interested in the
molecular aggregation (surface condensation) effect induced by
zinc cation on an L film of stearic acid. Because alkali metal ions
are known to exhibit no assist for the molecular aggregation,
the aggregation caused by zinc cation suggests that it is not
driven only by electric charge. They focused on the coordina-
tion structure about zinc cation at the interface of the L film
and water. Through the analysis of the XANES spectra, they
found that the white line developed with time, which suggested
that the coordination number increased. In the discussion of
the study, the time-dependent variation was attributed to metal
complex formation as a bottleneck. The discussion is, however,
based on the chemical information about the zinc cation only,
and the correlation with the structure of the fatty acid is still
unclear.
However, the compressed L film was prepared in a usual
manner of the Langmuir technique using a compression bar.
−1
After the L film was once compressed to attain 15 mN m , the
surface area was fixed.
PM-IRRAS Measurements. The IR spectra of the L films
were measured by using polarization-modulation infrared reflec-
tion absorption spectrometry (PM-IRRAS), which is powerful to
effectively reduce the absorption bands due to ambient water
vapor. The main bench of the PM-IRRAS measurements was a
Thermo Fischer Scientific (Madison, WI) Nicolet 6700 FT-IR
spectrometer. The FT-modulated IR ray was led out of the
spectrometer, and it was passed though the photoelastic modu-
lator (PEM) with an intrinsic resonance frequency of 50 kHz
generating alternately s- and p-polarization rays operated by a
HINDS Instruments (Hillsboro, OR) PEM-90 PEM controller.
The double-modulated (interferometer and PEM) IR ray was
directly introduced onto the L film, and the reflected ray was
led to an MCT detector liquid-nitrogen cooled through an
IR transparent KBr lens. No mirror was used in the optics to
prevent disturbance of polarizations. The angle of incidence
was 76° in accordance with the optimal conditions analyzed in
In the present study, the IR and X-ray techniques were co-
operatively employed to understand the correlation between
stearic acid and zinc cation. Both spectra exhibited time-
dependent variations, through which the molecular dynamic
rearrangement after the film preparation has first been revealed
as a function of the preparation technique.
1
8,19
former studies.
The half-wave-retardation frequencies were
EXPERIMENTAL SECTION
−1
■
2900 and 1404 cm for the measurements of the C−H and
fingerprint regions, respectively.
19−21
Chemicals. Octadecanoic acid (stearic acid) with a purity
higher than 99% was purchased from Sigma-Aldrich (St. Louis,
MO), and it was used as received without further purification.
For preparation of Langmuir (L) films, a chloroform solution of
The modulation fre-
quency of the interferometer was 20 kHz, and the band resolu-
−1
tion was 8 cm . The accumulation number of the interfero-
gram collection was 3000 to improve signal-to-noise (SN)
−1
22
stearic acid at a concentration of ca. 1.0 mg mL was used. The
solvent, chloroform, was a spectra grade reagent purchased
from Wako Pure Chemical Industries Ltd. (Osaka, Japan).
L films for the PM-IRRAS measurements were prepared on a
USI (Fukuoka, Japan) Langmuir−Blodgett film trough with a
ratio, which took about 2000 s.
Bandpass filters were used to remove an excess IR energy
2
0,21
that can make the L film move around on water.
were Spectrogon (Tab
The filters
̈
y, Sweden) BBP-2800−3800 nm and
LP-5100 nm IR filters, and they were set on the automatic filter
exchanger equipped in the spectrometer.
3
size of 97 × 300 × 5 mm by spreading the chloroform solution
3
149
dx.doi.org/10.1021/jp212028d | J. Phys. Chem. B 2012, 116, 3148−3154