A low temperature phase transition in Langmuir-Blodgett films

Article abstract of DOI:10.1021/jp806226e

The influences of temperature on the SFG spectra of Langmuir-Blodgett films of cadmium steì?arate, ferric stearate, stearic acid and octadecanamide are reported. Upon cooling, all films display reversible discontinuous shifts of a??8 cm^-1 in the r⁺, r^- and r _fermi modes of the terminal methyl groups at a??150 K. Reversible changes in the relative intensities of these methyl group peaks, most pronounced in the PPP spectra, are also observed and attributed to a change in the environment of the methyl group that accompanies a discontinuous transition in the ordering of their alkyl chains. The onset of new spectral features at higher frequency is attributed to the observation of ordered water molecules contained within the films. The correlation between the onset of the water features and the onset of the reversible, discontinuous, spectroscopic changes of the amphiphiles argues for a causal connection between the two. In addition to the discontinuous behavior upon cooling, monolayer films of stearic acid and octadecanamide display activity of methylene modes upon exposure to vacuum. Films displaying SFG-active methylene groups at room temperature had them gradually become completely SFG-inactive by 100 K. Heating the films to room temperature revealed that the methylene group activity was reversible. Monolayer films of cadmium ste;arate and ferric stearate do not display this methylene activity upon exposure to vacuum, suggesting that this behavior may be linked to solvation of the amphiphile's headgroup. These observations suggest that water plays a key role in the stability and structure of LB supported monolayers, and have important implications to those interested in low temperature (cryogenic) effects of biological systems. ? 2008 American Chemical Society.

Full text of DOI:10.1021/jp806226e

J. Phys. Chem. B 2008, 112, 13823–13833
13823
A Low Temperature Phase Transition in Langmuir-Blodgett Films
Thomas P. Johansson and Gary W. Leach*
Laboratory for AdVanced Spectroscopy and Imaging Research, 4D LABS and Department of Chemistry,
Simon Fraser UniVersity, Burnaby, B.C., V5A 1S6 Canada
ReceiVed: July 14, 2008
The influences of temperature on the SFG spectra of Langmuir-Blodgett films of cadmium stearate, ferric
stearate, stearic acid and octadecanamide are reported. Upon cooling, all films display reversible discontinuous
-
1
+
-
shifts of ∼8 cm in the r , r and rfermi modes of the terminal methyl groups at ∼150 K. Reversible changes
in the relative intensities of these methyl group peaks, most pronounced in the PPP spectra, are also observed
and attributed to a change in the environment of the methyl group that accompanies a discontinuous transition
in the ordering of their alkyl chains. The onset of new spectral features at higher frequency is attributed to
the observation of ordered water molecules contained within the films. The correlation between the onset of
the water features and the onset of the reversible, discontinuous, spectroscopic changes of the amphiphiles
argues for a causal connection between the two. In addition to the discontinuous behavior upon cooling,
monolayer films of stearic acid and octadecanamide display activity of methylene modes upon exposure to
vacuum. Films displaying SFG-active methylene groups at room temperature had them gradually become
completely SFG-inactive by 100 K. Heating the films to room temperature revealed that the methylene group
activity was reversible. Monolayer films of cadmium stearate and ferric stearate do not display this methylene
activity upon exposure to vacuum, suggesting that this behavior may be linked to solvation of the amphiphile’s
headgroup. These observations suggest that water plays a key role in the stability and structure of LB supported
monolayers, and have important implications to those interested in low temperature (cryogenic) effects of
biological systems.
Introduction
the monolayers formed by self-assembly of n-alkanethiols on
gold substrates. The structure and physical properties of these
Surface films are of fundamental importance because of their
many interesting and useful physical and chemical character-
films have been extensively studied using a variety of techniques
1
4
15
including electron and He diffraction methods, transmission
1
-5
istics.
In addition to their broad current utility and potential
16
17
electron microscopy, scanning tunneling microscopy as well
as many spectroscopic methods.
for many future applications, surface films represent environ-
ments characterized by rich and diverse physical phenomena.
As such, they represent model systems of reduced dimensional-
ity and offer the potential for experiment and theory to intersect.
Indeed, this potential has sustained an ongoing interest in the
18,19
In their studies of docosaneth-
1
2
iol self-assembled monolayers, Nuzzo et al. used infrared
spectroscopy to show that these ordered monolayers were
characterized by a significant degree of gauche conformations
concentrated in the vicinity of the chain termini and that the
spectroscopic features observed as a function of temperature
were akin to those observed for bulk phase n-alkane crystals.
They observed slow continuous spectral changes over a broad
temperature range, characteristic of complex phases and het-
erogeneity in the organic monolayers. Rowntree and co-workers
have since undertaken an IR study of SAMs of hexadecanethiol
6
phase transitions of two-dimensional surface films. The devel-
opment and application of new and powerful surface charac-
terization techniques, together with new theoretical develop-
ments have led to the broader understanding of surface phase
transitions and the distinction between melting phenomena in
7
-12
two and three dimensions.
It is now generally accepted that
two-dimensional melting processes are not necessarily char-
acterized by a single discontinuous transition, and that the
transition between ordered and disordered phases can proceed
via intermediate phases described by quasi-long-range orienta-
tional ordering. The detailed structure-property characteristics
and dynamics of these two-dimensional phase transitions are
dictated by the nature of the surface, the size and shape of the
adsorbed species in question as well as the type of interaction
1
3
and its deuterated counterpart formed on Au substrates. They
also observed continuous changes in peak position (on the order
-
1
of 2 cm spectral shift) and increases in intensity (up to 75%)
for both the methyl and methylene modes upon cooling from
room temperature to 25 K and attributed the effects to intra-
and intermolecular interactions of the alkyl chains. The continu-
ous nature of the spectral changes observed in both of these
studies suggest that, although there may well exist complex
phase transitions of the order-to-order and order-to-disorder type,
no phase transitions of a discontinuous nature occur within these
n-alkyl SAMs at low temperature.
1
0
between the surface and adsorbate. To date, the behavior of
films composed of molecules of many shapes and sizes have
been investigated, including the limiting class of single molecule
1
1-13
thickness films formed through self-assembly chemistry.
Though the structure of monolayer films deposited via the
Langmuir-Blodgett technique is qualitatively similar to those
formed by self-assembly, the nature of the adsorbate-substrate
interactions are markedly different between the two and may
be expected to result in different structural arrangements, degrees
Previous efforts to examine the low temperature behavior in
this type of ordered ultrathin film have focused primarily on
*
Author to whom correspondence should be addressed. E-mail: gleach@
sfu.ca.
1
0.1021/jp806226e CCC: $40.75  2008 American Chemical Society
Published on Web 10/15/2008

1
3824 J. Phys. Chem. B, Vol. 112, No. 44, 2008
Johansson and Leach
of freedom, and phase behavior. Indeed, it is well-known that
in the vicinity of room temperature, there exists a wealth of
complex phase behavior, even for many simple LB films.
group has C3V symmetry and possesses three nonzero indepen-
dent elements in its hyperpolarizability tensor. Associated with
3
+
-
the r mode are Rzzz and Rxxz ) Ryyz, and associated with the r
Unfortunately, there is little or no detailed information available
regarding the structural characteristics of these films at lower
temperatures, despite their relevance to low temperature effects
in biological systems, such as the cryogenic freezing of cell
membrane structures. The purpose of this work is to investigate
the low temperature structural characteristics and phase behavior
in ordered amphiphiles deposited by the LB technique. We
employ the method of sum frequency generation (SFG) vibra-
tional spectroscopy because of its surface specificity and
modes (assumed degenerate) is Rzxx. It should be recognized
that one also generally expects a nonzero hyperpolarizability
component Rxxx, which, under conditions of free rotation or a
random distribution of orientations, averages to zero. Assuming
a delta function distribution of orientation angles, θ, an isotropic
sample under rotation about the surface normal, and averaging
2
3
over Euler angles yields
(
2)
(2)
1
2
ꢀ
+
) ꢀ
+
) NR [(R + 1)〈cos(θ)〉 +
YYZ,r
XXZ,r zzz
2
0
excellent sensitivity.
3
(
R - 1)〈cos (θ)〉] (6)
Theory of SFG
(
2)
3
ꢀ
+
) NR [R〈cos(θ)〉 + (1 - R)〈cos (θ)〉]
(7)
ZZZ,r
zzz
The theoretical description of SFG has been presented in
2
1
detail elsewhere. Here, we provide only a brief description.
Irradiation from two optical fields E1 and E2 with frequencies
ω1 and ω2 generates a second-order nonlinear polarization
(2)
YZY,r
(2)
(2)
ZXX,r
(2)
ZYY,r
1
2
ꢀ
+
) ꢀ
XZX,r
+
) ꢀ
+
) ꢀ
+
) NR (1 -
zzz
3
R)[〈cos(θ)〉 - 〈cos (θ)〉] (8)
(2)
(2)
eff
P (ω)ω +ω ) ) ꢀ : E (ω ) E (ω )
(1)
where R ) Rxxz/Rzzz, N is the number density, and θ is the angle
between the C-CH3 axis and the surface normal.
1
2
1
1
2
2
where
Similar expressions hold for each of the CH3 asymmetric
-
-
(
eff
2)
(2)
a b
stretch modes, r , r :
ꢀ
) [e(ω ) K(ω )]ꢀ : [e(ω ) L(ω )][e(ω ) L(ω )] (2)
3 3 1 1 2 2
-
2
1
3
e being the unit polarization vector, L the linear Fresnel factor
describing the incident fields at the interface and K, a combina-
tion of linear and nonlinear Fresnel factors describing the
outgoing SFG field. The geometry of total internal reflection
(2)
YYZ,r
(2)
XXZ,r
ꢀ
-
) ꢀ
-
)
NR [〈cos(θ)〉 - 〈cos (θ)〉] (9)
xzx
(2)
3
ꢀ
ZZZ,r
-
) NR [〈cos(θ)〉 - 〈cos (θ)〉]
(10)
xzx
(
TIR) was chosen to take advantage of the large Fresnel factors
(2)
YZY,r
(2)
XZX,r
(2)
ZXX,r
(2)
1
2
3
ꢀ
-
) ꢀ
-
) ꢀ
-
) ꢀ
-
) NR 〈cos (θ)〉]
2
2
ZYY,r xzx
associated with this configuration.
(2)
The macroscopic susceptibility, ꢀ , is related to the molecular
(
11)
hyperpolarizability by
In many SFG analyses, the CH3 group is considered to be of
(
2)
-
-
ꢀ
) N 〈FijkIJK 〉 Rijk
(3)
C3V symmetry and the methyl group ra and rb modes are
eff,IJK
∑
-
i,j,k
assumed degenerate and treated as a single r mode. However,
attachment of the methyl group to the alkyl chain results in a
where Rijk is the molecular hyperpolarizability in the molecular
(2)
methyl group of C symmetry and removes any degeneracy of
s
(
ijk) frame, ꢀeff,IJK is the macroscopic susceptibility in the
-
the r modes, because the in-plane and out-of-plane CH bonds
laboratory (IJK) frame, N is the number of surface active
molecules and 〈FijkIJK〉 is the function describing the average
over molecular orientation.
When the IR frequency (ω) is near a vibrational resonance,
Rijk can be written as
are no longer equivalent. Because most SFG analyses involve
data obtained with picosecond or femtosecond light sources,
their relatively large bandwidth usually precludes resolution of
-
-
the ra and rb modes and only in relatively few circumstances
24,25
have the two near degenerate modes been resolved.
In cases
Λijk,ν
of degeneracy, or where the two asymmetric modes cannot be
R ) R +
(4)
(2)
ijk
nr
∑
resolved, the corresponding susceptibility is taken to be ꢀIJK,r-
ω - ω0,ν - Γν
(2)
(2)
)
ꢀIJK,r - + ꢀIJK,r -.
a
b
where ν represents the νth vibrational mode, ω0,ν is the resonant
frequency of that mode, Γν is the damping constant of that mode,
Λν represents the transition strength of that mode and Rnr
represents the signal contribution from the nonresonant
background.
The relative values of R , R , and R are functions of the
xxz
zzz
xzx
single bond polarizability ratio. When this ratio has a value of
0.11, the value for R becomes 2.4 and the value of R /R
xzx zzz
becomes 2.0. These values have previously been used with some
success in analyzing films of this nature and we employ these
assumptions in our subsequent analysis. It should also be noted
that the presence of a Fermi resonance band perturbs the
Similarly, the macroscopic susceptibility can be written as
(
2)
A
+
(
2)
(2)
IJK,ν
intensity of the r mode. To evaluate the unperturbed intensity,
ꢀ
) ꢀnr
+
(5)
IJK
∑
ω - ω0,ν - Γν
in terms of Αν and ꢀnr , the macroscopic analogues of Λν and
the following expression has been used.
(2)
(2)
+
+ 2
2
Aunperturbed(r ) ) A(r ) + A(rfermi)
(12)
(
2)
√
Rnr, respectively. If the components ꢀ IJK can be obtained
experimentally from the SFG spectrum and the Rijk are known,
then the average orientation of the functional groups contributing
to that vibrational signal can be deduced.
Experimental Section
Films were prepared using standard LB protocols. Specifi-
cally, amphiphiles were spread from chloroform solutions of 1
mM concentration onto the subphase. The subphase was
composed of ultrapure water (18.2 MΩ cm) for films of
The orientation of the terminal CH3 group can be determined
by analyzing its symmetric stretch mode, as well as by its
asymmetric stretch mode. In the limit of free rotation, the CH3

Phase Transition in Langmuir-Blodgett Films
J. Phys. Chem. B, Vol. 112, No. 44, 2008 13825
octadecanamide, stearic acid and ferric stearate; however, the
cadmium stearate film was prepared by deposition of stearic
-
3
acid onto a subphase containing 10 M CdCl2, pH adjusted to
2
6
6
.9. A PTFE LB trough (Nima 600) was used for controlling
the surface pressure while dipping. Surface pressures were read
using a Wilhelmy plate balance. Following chloroform evapora-
2
tion, the barriers were compressed at a rate of 10 cm /min. Films
were deposited at surface pressures of 35 mN/m and dipped at
a rate of 1 mm/min onto a CaF2 prism. The orientation of the
CaF2 substrate was uncontrolled. In all cases, the hydrophilic
substrate was immersed in the subphase prior to spreading of
the film and deposition occurred on the upstroke. Film transfer
ratios were 1.0 ( 0.1.
The sample was placed in a helium cooled cryostat (APD
Cryogenics) equipped with a coldfinger fabricated from red
copper. The sample was compressed between two pieces of
indium foil to facilitate thermal conductivity between the sample
and the thermal finger. The pressure was reduced using a turbo-
molecular vacuum pump (Leybold-Heraeus). The cryostat
Figure 1. Room temperature SFG spectra of cadmium stearate: SSP
(
lower), PPP (middle) and SPS (upper). Plots are offset for clarity.
-6
system allowed for typical operating pressures ∼(0.5-5) × 10
Solid lines represent best fits to the data.
Torr. The cryostat temperature was controlled using a Lakeshore
Cryotronics Inc. temperature controller (DRC 80C) and allowed
for temperature stability of approximately (5 K. Temperature
control to 25 K was routinely achieved under these conditions.
vibrationally nonresonant response from a spin cast sample of
malachite green.
A detailed description of the SFG apparatus has been
2
7
previously reported. A brief description is given below. SFG
spectra were obtained using tunable, broadband mid-IR light
produced by difference frequency mixing of the signal and idler
beams produced from a two-stage, ꢁ-BBO-based, optical
parametric amplifier (OPA) in an AgGaS2 crystal. The OPA
was pumped by the output of a regenerative amplifier (Positive
Light) operating at 800 nm and a 1 kHz rep rate. A portion of
the residual 800 nm light unused in the parametric amplification
process was employed for sum frequency generation. Variable
resolution was achieved by passing the residual 800 nm beam
through a spectral manipulator. Spectral and temporal manipula-
tion of the 800 nm light was performed by reflecting the beam
from a diffraction grating and focusing it onto a high reflector.
The amplitude of the various frequency components of the 800
nm beam were filtered by placing a slit of variable width in the
focal plane. The resulting beam was then redirected along a
reverse path toward the diffraction grating where it was spatially
reconstructed. The resolution of our instrument, as dictated by
the bandwidth of the 800 nm light, was 7 cm-1 for the studies
presented here.
Results and Discussion
The SFG spectra of a monolayer of each amphiphile was
obtained prior to placing each sample into the cryostat. Figure
1
displays the SSP, PPP and SPS spectra of a freshly prepared
monolayer of cadmium stearate. The spectra in Figure 1 are
characteristic of all of the materials reported here, with at most
only minor intensity variations distinguishing the various
amphiphiles. The spectra show well-defined peaks at 2880, 2940
-1
and 2966 cm . These peaks are associated with the alkyl
chain’s terminal methyl groups and have been previously
+
-
28
identified as the r , rfermi and unresolved r bands, respectively.
The observation that the intensity of methylene modes associated
with the alkyl chain are virtually nonexistent in all polarization
combinations is taken as an indication that the alkyl chains of
the film possess an all-trans configuration.
The solid lines in Figure 1 represent nonlinear least-squares
best fits to the data, where the spectral features of all polarization
combinations are fit simultaneously. The figure shows that these
spectra can be well fitted, in this case, with contributions from
three Lorentzian peaks interfering with a small complex
nonresonant background signal contribution. This nonresonant
signal is generally small (∼3%) relative to the magnitude of
the resonant vibrational contributions and it is assumed that all
peaks are of the same phase with respect to each other or are
Sum frequency generation signals were produced by directing
the IR and 800 nm light beams collinearly toward the hypote-
neuse of a CaF2 right angle prism containing the monolayer of
interest. The SFG beam was spectrally filtered and then reflected
from a mirror before entering a dual port Oriel spectrograph
and detected using a CCD camera (Andor) or photomultipier
tube (Hammamatsu R928). Spectra were typically recorded as
the sum of two 1 min acquisitions of the CCD camera.
Polarization selection was achieved by using a tunable half-
wave plate for the IR (Alphalas), a zero-order, half-wave plate
for the visible beam (CVI) and a Glan Thompson polarizer
2
9
180° out-of-phase. Applying eqs 6-11 and following an
3
0
analysis method published previously, we have determined
the orientation of the terminal methyl group and hence the tilt
of the overall alkyl chain.
The ability to obtain the SFG spectra in many polarization
combinations makes possible the determination of the averaged
orientation by a number of different models and therefore
provides a check on the self-consistency of the analysis and
interpretation. By examining the susceptibility of the symmetric
stretch in the PPP and SSP spectra, we obtain a tilt angle of
38.9° for the methyl group. Alternatively, the ratio of the
susceptibilities of the asymmetric stretch in the SPS and PPP
spectra yields a tilt angle of 40.3°. A third possible approach
employs the ratio of the susceptibilities of the asymmetric stretch
to the symmetric stretch in the PPP spectrum, yielding a methyl
(
CVI) for the resulting SFG signal. Spectra were taken in the
SSP, PPP, SPS and PSS polarization combinations, where the
designations refer to the polarizations of the SFG, visible and
infrared beams respectively. Without exception, the SPS and
PSS spectra were indistinguishable to within the noise level
and a constant intensity factor. For simplicity, we report only
the SPS spectra. The bandwidth of our mid-IR pulse was taken
into account in our analysis by normalizing spectra to the

1
3826 J. Phys. Chem. B, Vol. 112, No. 44, 2008
Johansson and Leach
Although the observation of methylene activity in monolayer
films has typically been associated with the presence of gauche
defects usually concentrated near the chain termina, or other
mechanisms of disorder that characterize the alkyl chains, these
effects would be expected to be accompanied by a loss of
intensity in the methyl group modes, as disorder within the alkyl
chains would naturally effect methyl group orientations and
diminish the coherent superposition of contributions that define
their nonlinear response. The observation of increased methylene
intensity at no expense to the observed terminal methyl group
intensity suggests an alternative explanation. The observation
of increased methylene activity in the case of stearic acid and
octadecanaminde and no such activity in the case of the fatty
acid salts, ferric stearate and cadmium stearate, may suggest
that this behavior is linked to the amphiphile’s head groups and
differences in their solvation and orientation.
Methylene activity in SFG spectra can also arise due to
systematic kinks or bends in the alkyl chains or defects
concentrated in the vicinity of the headgroup. One potential
explanation for the vacuum-induced effects is that the increased
methylene intensity is due to the removal of water molecules
that solvate and stabilize the polar head groups of the am-
phiphiles. Changes in headgroup solvation may lead to potential
reorientation and relaxation of the head groups to optimize their
overall structural energetics. This, in turn, can lead to changes
in the orientation of the methylene units in the vicinity of the
head groups and lead to uncompensated methylene contributions
from the alkyl chains, while maintaining favorable packing of
the outer portions of the chains and similar orientations of the
terminal methyl groups. Though we have no direct evidence
for this mechanism, we have previously observed that for certain
amphiphiles, the SFG spectra can appear with substantial
methylene activity and minimal alkyl chain defect content near
the chain termini, as manifest in their molecular resolution AFM
images, which are characterized by crystalline packing over
Figure 2. SFG spectra (PPP polarization combination) of stearic acid
before (circles) and after (triangles) vacuum pumping for one day. The
solid lines represent best fits to the data.
group tilt angle of 34.0°. These angles are self-consistent and
indicate that the alkyl chains are oriented approximately
perpendicular (<5° tilt) to the surface, consistent with previous
3
1
observations.
Exposure of stearic acid and octadecanamide films to vacuum
led to pronounced changes in the appearance of their spectra.
The spectral changes induced by vacuum exposure for a
monolayer film of stearic acid are displayed in Figure 2 for the
PPP polarization combination. The primary spectral change
induced by exposure to vacuum was a substantial increase in
+
the intensity of modes associated with methylene activity (d ,
-
1
-
-1
2
860 cm ; d , 2910-2930 cm ). The activity of the meth-
ylene peaks grew with exposure time, reaching a maximum
beyond which, no further spectral changes were observed. This
maximum intensity appeared to be different for different
amphiphiles and could, in some cases, take several days to attain.
Films of ferric stearate and cadmium stearate did not exhibit
these effects and remain unperturbed by vacuum exposure.
Interestingly, in the case of stearic acid and octadecanamide,
the growth of methylene mode intensity was accompanied by
no appreciable loss in intensity from the methyl group modes.
This observation suggests that at least for the amphiphiles and
substrate employed here, the monolayers remain on the substrate,
despite previous reports regarding the questionable vacuum
3
0,33
relatively large length scales.
The SFG spectra of monolayer films of stearic acid and
octadecanamide were investigated as a function of temperature.
These films were cooled from room temperature to 25 K in
-
1
+
∼
30 K increments. The methylene bands at 2860 cm (d )
-
1
-
and 2910-2930 cm (d ) induced by exposure to vacuum,
drop steadily in intensity until they are no longer measurable
at 100 K. We infer from these observations that the vacuum
induced defects are effectively quenched as the relative intensity
+
-
of the d and d modes drops away with temperature. Upon
heating the films from 25 K to room temperature, we observe
that the activity of the methylene groups return to approximately
the same levels as those before cooling. The effect of the low
temperature ordering on the alkyl chains thus appears to be
reversible. The consistency of this observation was checked over
several cycles of cooling and heating for several different LB
films. In all cases, the methylene activity observed upon
exposure to vacuum showed the same temperature dependent
behavior.
3
2
stability of certain LB deposited films. Indeed, Brandl et al.
observed that monolayer and bilayer LB films composed of fatty
acid salts deposited on metallic substrates were much more
vacuum stable compared to films of just the fatty acid and that
a film of arachidic acid deposited on glass had a characteristic
decay time constant on the order of one minute when exposed
to vacuum. Though we did not observe the signal losses that
would be expected from material removal from the surface, a
large increase in methylene activity was observed to occur for
similar films of stearic acid. We interpret the stability of the
SFG signals originating from the terminal methyl groups upon
exposure to vacuum as an indication of their vacuum stability.
It is unlikely that the observed methylene activity is due to
contaminants in the vacuum chamber adhering to the surface,
because the magnitude of the effect appears to be sample
dependent. Also, we have placed fresh samples into the cryostat,
but without the application of vacuum and these effects were
not observed. This suggests that the presence of the vacuum is
required to observe the increase in methylene activity.
In addition to these gradual changes in the appearance of the
SFG spectra with temperature, the spectra also display other
distinct and abrupt changes in intensity and spectral position
when cooled. To investigate these changes in more detail, films
of the fatty acid salts, cadmium stearate and ferric stearate were
examined. Because these films showed no evidence of vacuum
induced methylene activity, the interpretation of any spectral
features and changes therein, are greatly simplified.
Figures 3, 4 and 5 display the spectra and corresponding
temperature contours of the PPP, SSP and SPS polarization

Phase Transition in Langmuir-Blodgett Films
J. Phys. Chem. B, Vol. 112, No. 44, 2008 13827
Figure 4. (a) The SFG spectrum of cadmium stearate at 25 K. The
solid line represents a best fit to the data. (b) Temperature dependent
SFG spectra (SSP polarization combination) displayed as a contour
plot. Data collected every 20 K and have been normalized to the r+
mode in the SSP spectrum.
Figure 3. (a) SFG spectrum of cadmium stearate at 25 K. The solid
line represents a best fit to the data. (b) Temperature dependent SFG
spectra (PPP polarization combination) displayed as a contour plot.
Data collected every 20 K and have been normalized to the r mode
in the SSP spectrum.
+
combinations, respectively, from a monolayer film of cadmium
stearate. The spectra displayed in Figures 3a-5a were obtained
at 25 K, the lowest temperature examined in this study. The
contour plots (Figures 3b-5b) illustrate the temperature de-
pendent changes to the spectra upon cooling. Intensity variations
and spectral shifts observed upon cooling can be identified by
following the contours from room temperature to 25 K (bottom
to top of the contour plot). The low temperature spectra (Figures
but these are of a more subtle nature. Examination of Figures
3-5 illustrate that the spectra undergo clear changes with
cooling and that these changes occur in the vicinity of 150 K.
Figure 6 displays the SFG spectra for the three polarization
combinations obtained at temperatures of 230 and 100 K. These
spectra are generally representative of the spectra obtained at
high (>150 K) and low (<150 K) temperature and illustrate
the principal changes to the spectra upon cooling. Figure 6a
shows the SFG spectra at high and low temperatures obtained
with the SSP polarization combination. Aside from the distinct
frequency shift, the SSP spectra at 230 and 100 K appear very
similar, with a minor (∼20%) decrease in the intensity of the
Fermi band. There is, in addition, a small but appreciable
increase in the intensity of the baseline at the high energy side
of the spectrum. Figure 6b displays the high and low tempera-
ture spectra for the SPS polarization combination. The spectrum
is characterized by one prominent peak that drops in intensity
by roughly 30% and broadens slightly upon cooling. There is
3
a-5a) are aligned and registered with the contour plots to
facilitate interpretation of the data. Note that spectra were
collected with every 20 K of temperature change and that the
contours interpolate between these data points. It should be noted
that, due to small variations in the overall signal during the time
scale of this data collection (many hours), all spectra have been
+
normalized to the r peak in the SSP spectrum at each
temperature. Upon cooling, no appreciable changes in any of
the spectra were observed from room temperature to ∼150 K.
Below 150 K, all spectral features appear to have shifted
-
1
-1
abruptly to lower frequency by ∼6-8 cm and the relative
intensities of certain peaks appear to have changed. Cooling to
lower temperatures leads to additional changes in the spectra,
also a slight increase in the intensity around 2930 cm , the
region associated with the rfermi mode. Figure 6c shows the PPP
spectra obtained at high and low temperature. This polarization

1
3828 J. Phys. Chem. B, Vol. 112, No. 44, 2008
Johansson and Leach
Figure 5. (a) The SFG spectrum of cadmium stearate at 25 K. The
solid line represents a best fit to the data. (b) Temperature dependent
SFG spectra (SPS polarization combination) displayed as a contour
+
plot. Data collected every 20 K and have been normalized to the r
mode in the SSP spectrum.
combination is characterized by the greatest spectral changes
upon cooling. In addition to the frequency shift, the spectrum
+
shows substantial changes to the relative intensities of the r ,
-
-
rfermi and r peaks. The intensity of the r band drops by
approximately 50% and the intensity of the rfermi band ap-
+
proximately doubles. The intensity of the r mode remains
approximately the same. The peak shape of the r band also
-
appears to change from a well-defined sharp peak to a smaller,
broader peak. Interestingly, this change in peak shape for the
-
r mode was more pronounced in the PPP spectrum than in the
SPS spectrum. The increase in the intensity of the baseline at
the high energy side of the spectrum observed in the SSP
polarization is also observed in the PPP spectrum. However,
the smaller signal-to-noise in the SPS polarization combination
precludes its definitive observation. Upon further cooling, the
Figure 6. (a) SFG spectra (SSP polarization combination) of cadmium
stearate at 230 K (circles) and 100 K (triangles). (b) SFG spectra (SPS
polarization combination) of cadmium stearate at 230 K (circles) and
100 K (triangles). (c) The SFG spectra (PPP polarization combination)
of cadmium stearate at 230 K (circles) and 100 K (triangles). Solid
lines represent best fits to the data.
+
-
r and r bands remain unchanged, whereas the rfermi band loses
intensity slowly with decreasing temperature.
Each spectrum has been fitted as described above and the fit
parameters have been examined as a function of temperature.
Figure 7a demonstrates that upon cooling, the spectral features
shift abruptly to lower frequency at ∼140 K. This shift of peak
frequency coincides with the change in the relative peak
intensities in the spectra. Upon heating, at ∼165 K the peaks
shift back to their original frequencies and the intensity changes
seen in the PPP spectrum reverse. The coinciding changes in

Phase Transition in Langmuir-Blodgett Films
J. Phys. Chem. B, Vol. 112, No. 44, 2008 13829
-
1
a value of ∼7 to ∼14 cm . Below 100 K the line widths
extracted from the fits remain approximately independent of
temperature.
We have observed similar changes in the SFG spectra for
the terminal methyl group as a function of temperature for all
of the LB films examined, although as mentioned previously,
the effects are less apparent in films which display the vacuum
induced activity of the methylene modes. The spectra were
analyzed to determine whether or not the observed spectral
changes reflect changes in orientation of the amphiphiles or
general film structure. Using the SFG spectra of a monolayer
of cadmium stearate at 25 K and at room temperature as an
example, if we make use of the relative susceptibilities of the
+
-
r
and r modes in the PPP spectrum as a measure of the
orientation of the methyl group, we find that the average tilt
angle changed from 34° at high temperature to 42° at low
temperature. In contrast, employing the ratio of the susceptibili-
+
ties of the r mode in the PPP spectra to the SSP spectra, would
result in a tilt angle change from 39° at high temperature to
2
4° at lower temperature. Taking the ratio of the susceptibilities
-
of the r mode in the SPS spectra to the PPP spectra shows a
change from 40° at high temperature to 30° at low temperature.
These analyses no longer yield a self-consistent measure of the
methyl group tilt angle as obtained at room temperature and as
one would expect if the observed spectral changes reflected a
change in molecular orientation. This underscores both the
importance of obtaining the SFG spectra in multiple polarization
combinations to obtain a self-consistent measure of averaged
molecular orientation, and points to the need for a more
sophisticated analysis of the temperature dependent spectral data
at hand.
It has been previously established from high resolution
vibrational spectroscopy studies of bulk alkanes that at tem-
-
-
peratures of ∼10 K, a splitting of the ra and rb bands as large
-
1
12
as 12 cm can been observed. With increase in temperature,
motional narrowing effects dominate in these systems, and by
temperatures of >150 K, such splittings are reduced to a few
Figure 7. (a) Band positions (ω
0
) for the r+ (squares), rfermi (circles),
and r (triangles) modes extracted from the spectral fits, for cooling
open) and heating (filled) cycles. Solid lines are fits to the data
according to the functional form R ) (R + R )/2 - (R - R )/2 tanh[(T
)/∆T], which has been used previously to model the glass transition
-
-
1
cm , and typically not resolved in most SFG spectroscopy. It
is a common assumption in most SFG analyses that the methyl
group undergoes free rotation, which allows for a simplified
(
1
2
1
2
-
T
g
-
34-36
in polymer systems (ref 48). According to this model, the transition
occurs at 143 K on the cooling cycle and 172 K upon heating. The
temperature range over which the transition occurs (∆T) is 4.8 K. (b)
Line widths (Γ) for the r+ (circles), rfermi (squares) and r- (triangles)
modes extracted from the spectral fits.
analysis in terms of a single r mode.
However, as others
have pointed out, the barrier to internal rotation in these systems
is far too large for free rotation, and torsional motion is
characterized by rms torsion angles of roughly 10° at room
2
5,37
temperature.
frequency and intensity suggest that the driving force behind
each are related. The temperature at which the changes occur
is different for heating than for cooling suggesting hysteretic
behavior for this process. However, the hysteresis may also
result from a lag in the temperature between the monolayer film
and the cryostat temperature set point and reflect our inability
to control the sample temperature with as great a precision as
desired. Lacking appropriate data to eliminate this possibility,
we can only state that the spectral changes appear to be
reversible and that they occur at 155 ( 10 K.
To our knowledge, there are only a few reported cases where
2
4,25
these bands have been spectrally resolved using SFG,
2
4
and this splitting was not always observable. Others have
-
inferred the splitting of the degenerate r mode and the relative
-
-
phases of the ra and rb modes based on the inability to
-
38-40
adequately fit their data to a single r mode.
In similar
-
fashion, we have attempted to fit our spectra with a single r
mode and observe this vibrational contribution to appear to
double in line width and drop to half-its initial intensity in the
PPP polarization combination, as the temperature is decreased.
Note that we have also observed that the increase in line width
Figure 7b shows the changes in the Lorentzian line widths
+
-
-
(
Γ) with temperature of the r , r and rfermi modes upon cooling.
for the r mode appearing in the SPS spectrum is very modest.
+
As the temperature drops, it is apparent that the r and rfermi
modes remain relatively constant in width. There is a slight
increase in width in the vicinity of where the spectral changes
occur (150 K), but any such changes are very modest. In
Figure 5a displays the fit of the SPS spectrum at 25 K. The
-
quality of the fit in the vicinity of the r mode in the SPS
spectrum is slightly inadequate. The fitting procedure entails
that all spectra are fit simultaneously and, due to the larger line
width of the peak observed in the PPP spectrum, it becomes
impossible to fit both the PPP and SPS spectra without
compromise. Although the observation of a temperature inde-
-
contrast, the Lorentzian line width associated with the r mode
appears to double in width upon cooling. Between 150 and 100
K there appears to be a steady increase in the peak width from

1
3830 J. Phys. Chem. B, Vol. 112, No. 44, 2008
Johansson and Leach
relative to higher temperatures. Attachment of the methyl group
to the alkyl chain and the concomitant change of methyl group
-
symmetry from C3V to Cs results in the doubly degenerate r
mode of E symmetry splitting into two distinct modes, one with
-
-
A′ symmetry (ra ) and the other mode with A′′ symmetry (rb ).
-
The symmetric ra mode has the same symmetry as the rfermi
and r modes. The proximity of an additional band to rfermi with
+
the same symmetry as rfermi, could provide an additional avenue
of intramolecular coupling. This coupling could take the form
of an anharmonic interaction between the Fermi mode and the
-
+
ra mode (as well as the r mode), or equivalently, the coupling
could be considered a more complex case of Fermi interaction
whereby the overtone of the bending mode that gains its intensity
through Fermi resonance with the symmetric stretch mode, now
-
also interacts and gains intensity through the ra mode. In this
description, the observed vibrational modes are admixtures of
what we would normally refer to as pure symmetric stretch,
antisymmetric stretch, and bending overtone modes. In such a
scenario, one might expect the line strength ratios to change
abruptly to reflect an alternative coupling mechanism. Specif-
ically, the additional coupling pathway allows the rfermi peak to
+
Figure 8. Ratio of line strengths, Aeff(fermi)/Aeff(r ), extracted from
the spectral fits for the SSP (filled diamonds) and PPP (open squares)
polarization combinations, as a function of temperature.
-
increase in intensity at the expense of the ra band, as it borrows
intensity.
pendent line width in the SPS spectrum may appear to be
inconsistent with the observations in the PPP polarization
combination and the argument for splitting, it must be recog-
nized that the SPS spectrum probes different susceptibility
components than does the PPP spectrum. These seemingly
disparate observations may suggest the presence of two peaks,
close in frequency, but appearing with different intensities in
different polarization combinations. Thus, we attribute these
observations to an increase in the splitting of the ra and rb modes
at temperatures below 150 K and to differences in their behavior
depending upon which polarization combination is used to probe
them.
This notion is consistent with our observations. In the PPP
polarization combination, at temperatures of ∼150 K, the Fermi
+
band increases its relative intensity to the r band (Figure 8)
and there is a concomitant inversion in the relative intensities
of the rfermi and r bands (Figure 6c). A similarly consistent
-
observation is seen in the SPS spectrum (Figure 6b), where the
-
r mode intensity observed above 150 K decreases in response
-
-
-
to low temperature. Though the loss of intensity in the r
mode(s) and gain of intensity in the Fermi mode are not exactly
equivalent in the SPS and PPP spectra, they are very similar.
To understand the SSP spectra, it must be recognized that in
this polarization combination, the r mode generally appears
as a small contribution with a relative phase opposite to the r
and rfermi bands and is sometimes observable as a small shoulder
on the Fermi band. The mixing of the ra with the Fermi mode
at temperatures below 150 K leads to new band positions and
intensities for these now mixed modes. The new arrangement
of eigenstates leads to a greater destructive interference between
the rfermi and ra bands in the SSP spectrum, in the same way
that this arrangement of eigenstates leads to an increase in
intensity in the PPP polarization combination, where the relative
phase of the ra mode is the same as that for the r and rfermi
modes and they constructively interfere.
-
To understand the observed spectral changes with temperature
in all polarization combinations, it proves useful to consider
the nature of the intensity distribution appearing in the CH
region of these amphiphiles. It is generally accepted that the
Fermi band is an overtone of a bending mode that gains its
intensity through Fermi resonance with the symmetric stretch
+
-
4
1
mode. These interactions lead to two molecular eigenstates
-
+
(
r and rfermi) of mixed character. Within this coupling scenario,
the Fermi intensity should therefore be a constant fraction of
+
the intensity of the r mode, independent of temperature. Figure
-
+
8
displays the ratio of line strengths extracted from fits of the
+
SSP and PPP spectra for the rfermi and the r modes (Afermi/Ar+
)
as a function of temperature for a monolayer of cadmium
stearate. This ratio reflects the fractional intensity of the Fermi
band relative to that of the r band, and on the basis of this
The temperature dependent behavior of the line strength ratio
at lower temperatures appears to be more difficult to understand.
The ratio of line strengths at temperatures below 150 K shows
a steady drop with temperature for both the SSP and PPP
polarization combinations. It should be noted that these decreas-
ing ratios reflect a decreasing intensity of the Fermi band with
+
argument, one might expect such a ratio to be independent of
temperature. However, as shown in Figure 8, the ratios of line
strengths display large temperature dependent variations. Specif-
ically, the line strengths observed in both the SSP and PPP
polarization combinations appear to be roughly equal and
independent of temperature (as expected) from room temperature
to ∼150 K, where they both appear to show an abrupt departure
from this behavior. In the PPP spectra at temperatures near 150
K, the Fermi band appears to gain substantial intensity relative
+
temperature, as the intensity of the r band appears to be roughly
-
independent of temperature. Although the splitting of the ra
-
and rb bands is expected to be temperature dependent, the
magnitude of this temperature dependence is generally small,
and spectral simulation indicates that an explanation of the
steady decrease in line strength ratio due only to a temperature
dependent splitting is not feasible. It is possible that the
temperature dependent splitting leads to temperature dependent
+
to r , whereas in the SSP spectra, the Fermi band appears to
+
lose a small amount of intensity relative to r . As the
-
+
temperature is lowered, the ratio of line strengths in both SSP
and PPP polarization combinations decrease steadily.
changes in the coupling between the ra , rfermi and r modes
sufficient to explain this observation. Alternatively, in their
investigation of the temperature dependent spectral changes of
alkanethiols self-assembled on gold, Rowntree and co-workers
-
-
The observation of both the ra and rb modes at temperatures
below 150 K reflects a significant increase in their splitting

Phase Transition in Langmuir-Blodgett Films
J. Phys. Chem. B, Vol. 112, No. 44, 2008 13831
Figure 9. SFG spectra of cadmium stearate at 25 K: SSP (lower),
Figure 10. Low temperature SFG spectrum (SSP polarization com-
bination) of cadmium stearate. The spectrum is a convolution of two
separate spectra, normalized for IR bandwidth, by scaling them
appropriately to common spectral features. The solid line represents a
best fit to the data.
PPP (middle) and SPS (upper). Plots are offset for clarity. Solid lines
-
represent best fits to the data, assuming two nondegenerate r modes.
have observed temperature dependent intermolecular as well
1
3
as intramolecular couplings. It is perhaps not surprising that
the intramolecular coupling scheme suggested above should be
insufficient to explain all of the temperature dependent spectral
changes observed, as the observed spectra may be influenced
by temperature dependent intermolecular couplings as well.
temperature, following one cooling and heating cycle, there
-
1
remains a small residual signal at ∼3100 cm . Otherwise, this
behavior appears to be reversible, with large increases in the
intensity of this spectral feature occurring at ∼150 K upon
cooling and dramatic decreases in intensity at temperatures
greater than ∼165 K upon heating.
-
In response to the proposed splitting of the r band and the
-1
resulting coupling scheme, we have attempted to fit the low
temperature SFG spectra with a more realistic model. Figure 9
displays fits of the SFG spectra for a monolayer of cadmium
stearate at a temperature of 25 K. These spectra are those already
displayed in Figures 3a-5a. However, in Figure 9 the data have
been fitted with two antisymmetric stretching modes, ra and
rb . Although the fits to the data in Figures 3a-5a were
reasonably good, a marked improvement in the fits is evident.
The 3100 cm feature, and others observed at higher energy,
are consistent with the presence of ordered water molecules on
4
2,43
the surface.
We have conducted a variety of experiments to
elucidate the nature and origin of this signal, including tem-
perature cycling studies, studies on films of various thickness,
films of deuterated amphiphiles, mixed films of deuterarated
and nondeuterated amphiphiles, and films deposited from a
subphase of D2O. The details of these studies are beyond the
scope of this manuscript and will be presented in a forthcoming
publication. However, it is clear from these studies that the origin
-
-
-
Peak fitting would suggest that the two r modes have
frequencies of 2955 and 2965 cm , each with a width Γ ) 7
cm . The splitting of 10 cm obtained from the fits is close
-
1
-
1
-1
-1
of the spectral feature at 3100 cm is due to surface water,
37
to that observed by MacPhail et al. The improvement in fitting
obtained by inclusion of the splitting of the degenerate anti-
symmetric stretching modes supports our analysis.
We have, thus far, only addressed aspects of the CH region
of the spectrum. Upon cooling, the onset of a new feature also
appears in the spectrum. The feature appears at energies higher
than the CH bands of the amphiphiles (∼3100 cm ) and
appears as a small peak. It should be noted that the frequency
range of the spectra reported here are limited by the bandwidth
of the IR pulses employed, which had a typical fwhm of 200
cm centered about 2900 cm . This new spectral feature
appears at the extreme high energy edge of our bandwidth and
its observation would indicate substantial intensity. When
normalized for incident infrared intensity, the integrated intensity
of this new spectral feature is much larger than the signals
originating from the methyl groups of the monolayer. Figure
and that the presence of water in these films is linked with the
deposition method, and not other sources. Further, the abrupt
transition from disordered water to ordered water on the surface
appears to be reversible and coincides with the observed
reversible changes in the SFG spectra originating from the
amphiphiles’ methyl groups. Although we have no direct
evidence to support the causal nature, we propose that the
temperature dependent spectral changes observed in the CH
region of all amphiphiles reported in this manuscript, occur in
response to the appearance of ordered water molecules in the
film. That is, the observed temperature dependent spectral
changes reflect a reversible phase transition of surface water
molecules. One potential causal link is an abrupt change in the
packing of alkyl chains that might accompany the onset of
ordering of water molecules within the film. The room tem-
perature structure of fatty acid LB films is typically a pseudohex-
agonal rotator phase, lacking long-range orientational order
between the planes of alkyl chains. A phase transition to a long-
range ordered orthorhombic phase would result in a change of
packing of the alkyl chains, a change in methyl group interac-
tions, and the potential for the onset of a new vibrational
coupling mechanism as outlined above. Consistent with this
conjecture and our observation of changes in the spectral features
of the asymmetric modes, is the observation by Snyder and co-
-
1
-
1
-1
1
0 displays this new feature, together with the CH bands of the
monolayer.
Figure 10 was obtained by stitching together separate
normalized spectra by scaling them appropriately to common
spectral features. When the samples are reheated from 25 K,
the intensity of the SFG signal at ∼3100 cm increases
dramatically in the vicinity of 165 K, and then decreases
dramatically as the temperature is further increased. At room
-1

1
3832 J. Phys. Chem. B, Vol. 112, No. 44, 2008
Johansson and Leach
workers that orthorhombic phases show factor-group splitting
The onset of SFG signals originating from ordered water
molecules at the surface at the same temperatures have also
been reported and suggests a connection between the two. Given
the generality of the observation for the range of amphiphiles
studied here and similar unexplained observations in other
studies, we have proposed that the phase transition manifested
in the amphiphilic monolayer CH region is a response to the
formation of ordered water molecules contained within the
monolayer film. These experiments provide supporting evidence
for the idea that the structural stability of these model surfaces
rely on the incorporation of solvent transferred from the
subphase to the solid substrates upon deposition. Further studies
are required to understand the detailed nature of this solvation
process and how changes in the degree of headgroup solvation
affect the structure and stability of the supported organic
monolayer. In addition, the observation of a phase transition of
the incorporated water molecules raises interesting questions
regarding their character and thermodynamic properties. The
onset of order in these surface water molecules may reflect the
unique anisotropic solvation environment of the amphiphile or
may reflect a more general thermodynamic behavior of water
in ultrathin films.
-
-
(
Davydov splitting) of the ra and rb modes associated with two
2
8
molecules in the unit cell.
To the best of our knowledge, this report represents the first
detailed structural study of LB films at low temperature.
Previous low temperature studies of LB films have dealt with
the paraelectric to ferroelectric or paramagnetic to ferromagnetic
phase transitions of specialized amphiphiles; however, these
reports have primarily focused on the details of these different
ordering phenomena, in some cases over narrow temperature
ranges, and have not generally investigated structural aspects
of the amphiphiles themselves. Nevertheless, one might expect
that on the basis of our findings, others would have observed
similar discontinuous phenomena in their studies. Interestingly,
Saito et al. have performed ESR experiments on LB films of
icosanoic acid mixed with bis(ethylenedioxy)tetrathiofulvalene
and decyltetracyanoquinodimethane and observed “an anoma-
4
4
lous phase transition” at 140 K.
The results presented here raise some interesting questions
regarding the structure of LB films, the behavior of water
associated with these films and the behavior of water at surfaces
more generally. The low temperature phase behavior of water
is very complex and not entirely well understood. Water is
known to exist in many stable and metastable phases depending
on its deposition temperature, film thickness and the substrate
onto which it is deposited and undergoes a phase transition from
a supercooled viscous phase to form amorphous solid water at
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada and Simon Fraser
Unversity for financial support and Dr. James W. Hager of MDS
Sciex Corp. for the donation of vacuum equipment employed
in these studies.
4
5
temperatures in the vicinity of 150 K. Films of water and
mixed films containing water and other solvents have been
reported to undergo a glass transition from a viscous fluid like
phase to the amorphous solid phase at comparable tempera-
Note Added after ASAP Publication. This article posted
ASAP on October 15, 2008 with an error in the caption to
Figure 7. The final correct version posted October 30, 2008.
46
tures, although there has been some disagreement on the glass
47
transition temperature. Our results indicate that at comparable
temperatures, water deposited in the LB process is altered from
an isotropic distribution to a highly ordered structure within
these films. This structure may be the result of the anisotropic
environment of the amphiphile, or may represent more general
behavior of ultrathin water films deposited in this manner. We
are currently trying to address this question with further
experiments.
References and Notes
(1) Handbook of Thin Film Technology; Maissel, L. I., Glang, R., Eds.;
McGraw-Hill: New York, 1970.
(2) Introduction to Surface and Thin Film Processes; Venables, J., Ed.;
Cambridge University Press: New York, 2000.
(3) Petty, M. C. Langmuir Blodgett Films; Cambridge University Press:
New York, 1996.
(4) Functional Organic and Polymeric Materials: Molecular Func-
tionality-Macroscopic Reality; Richardson, T. H., Ed.; Wiley,, New York,
2
000.
(5) Films on Solid Surfaces: The Physics and Chemistry of Physical
Conclusion
Adsorption; Dash, J. G., Ed.; Academic Press: New York, 1975.
(6) Phase Transitions in Surface Films; Dash, J. G., Ruvalds, J., Eds.;
Plenum: New York, 1980.
We have investigated the low temperature phase behavior of
Langmuir-Blodgett films formed from stearic acid, octade-
canamide, ferric stearate and cadmium stearate using sum
frequency generation vibrational spectroscopy. Monolayer films
of stearic acid and octadecamide display activity of methylene
modes upon exposure to vacuum. Their temperature dependence
shows slow and gradual change and a complete quenching of
activity by 100 K. Subsequent heating leads to the recurrence
of the methylene activity, indicating that the behavior is
reversible. Monolayer films of the fatty acid salts, ferric stearate
and cadmium stearate, however, show no such methylene
activity upon vacuum exposure. We have attributed the disparate
behavior to differences in headgroup solvation. Removal of
weakly bound water molecules by vacuum, induces changes in
solvation of the headgroup and the introduction of uncompen-
sated vibrational activity in nearby methylene groups.
(7) Kosterlitz, J. M.; Thouless, D. J. J. Phys. C 1973, 6, 1181.
(8) Nelson, D. R.; Halperin, B. I. Phys. ReV. B 1979, 19, 2457.
(9) Young, A. P. Phys. ReV. B 1979, 19, 1855.
(10) Hostetler, M. J.; Manner, W. L.; Nuzzo, R. G.; Girolami, G. S. J.
Phys. Chem. 1995, 99, 15269.
(
(
11) Bhatia, R.; Garrison, B. J. Langmuir 1997, 13, 765.
12) Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H. J. Chem. Phys. 1990,
93, 767.
(
13) Garand, E.; Picard, J.; Rowntree, P. J. Phys. Chem. B 2004, 108,
8
182.
(14) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546.
15) Chidsey, C. E. D.; Liu, G. Y.; Rowntree, D.; Scoles, G. J. Chem.
(
Phys. 1989, 91, 3321.
(
(
(
16) Huie, J. C. Smart Mater. Struct. 2003, 12, 264.
17) Volcke, C.; Thiry, P. A. J. Phys.: Conf. Ser. 2007, 61, 1236.
18) Orendorff, C. J.; Gole, A.; Sau, T. K.; Murphy, C. J. Anal. Chem.
2005, 77, 3261.
19) Kondo, T.; Kohei, U. J. Photochem. Photobiol. C Photochem. ReV.
007, 8, 1.
20) Akamatsu, N.; Domen, K.; Hirose, C.; Onishi, T. Chem. Phys. Lett.
(
2
All amphiphiles display reversible discontinuous spectro-
scopic changes in the vicinity of 150 K. We have attributed
these changes to the onset of a change in the ordering of the
alkyl chains within the unit cell and the onset of a new
vibrational coupling mechanism that alters the spectral positions
and intensities of the methyl group modes at low temperature.
(
1991, 181, 175.
(21) Rao, Y; Comstock, M.; Eisenthal, K. J. Phys. Chem B 2006, 110,
1
727.
(
(
22) L o¨ bau, J.; Wolfrum, K. J. Opt. Soc. Am. B 1997, 14, 2505.
23) Garand, E.; Picard, J.; Rowntree, P. J. Phys. Chem. B 2004, 108,
8182.

Phase Transition in Langmuir-Blodgett Films
J. Phys. Chem. B, Vol. 112, No. 44, 2008 13833
(
24) Chow, B. C.; Ehler, T. T.; Furtak, T. E. Appl. Phys. B 2002, 74,
95.
25) Beattie, D. A.; Fraenkel, R.; Winget, S. A.; Petersen, A.; Bain,
C. D. J. Phys. Chem. B 2006, 110, 2278.
26) Takamoto, D. Y.; Aydil, E.; Zasadzinski, J. A.; Ivanova, A. T.;
Schwartz, D. K.; Yang, T.; Cremer, P. Science 2001, 293, 1292.
(37) MacPhail, R.; Snyder, R.; Strauss, H. J. Chem. Phys. 1982, 77,
3
1118.
(
(38) Bain, C. D.; Davies, P.; Ong, T. H.; Ward, R. Langmuir 1991, 7,
1563.
(
39) Goates, S. R.; Schofield, D. A.; Bain, C. D. Langmuir 1999, 15,
(
1
400.
(
40) Himmelhaus, M.; Eisert, F.; Grunze, M. J. Phys. Chem. B 2000,
(
(
27) Star, D.; Kikteva, T.; Leach, G. W. J. Chem. Phys. 1999, 111, 14.
28) Macphail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J.
1
04, 576.
41) Macphail, R. A.; Synder, R. G.; Strauss, H. L.; Elliger, C. A. J.
(
Phys. Chem. 1984, 88, 334.
(
13.
(
Phys. Chem. 1984, 88, 334.
29) Tanaka, Y.; Lin, S.; Aono, M.; Suzuki, T. Appl. Phys. B 1999, 68,
(
(
42) Shen, Y. R. Solid State Commun. 1998, 108, 399.
7
43) Wei, X.; Miranda, P. B.; Shen, Y. R. Phys. ReV. Lett. 2001, 86,
30) Johansson, T.; Leach, G. J. Phys. Chem. B 2006, 110, 16567.
31) Peng, J. B.; Barnes, G. T.; Gentle, I. R. AdV. Colloid Interface Sci.
1
554.
(
(
44) Ikegami, K.; Kuroda, S.; Akutagawa, T.; Konuma, T.; Nakamura,
T.; Matsumoto, M.; Horiuchi, S.; Yamochi, H.; Saito, G. Thin Solid Films
998, 327, 391.
2
001, 91, 163.
32) Brandl, D.; Schoppmann, Ch.; Tomaschko, Ch.; Schurr, M.; Voit,
H. Thin Solid Films 1995, 256, 220.
(
1
(
(
(
(
45) Sieger, M. T.; Orlando, T. M. Surf. Sci. 2000, 451, 97.
(
33) Johansson, T.; Leach. G. W. Manuscript in preparation.
46) Souda, R. Phys. ReV. Lett. 2004, 93, 235002.
(
34) Johal, M.; Usaid, E.; Davis, P. J. Chem. Soc., Faraday Trans. 1996,
47) Velikov, V.; Borick, S.; Angell, C. A. Science 2001, 294, 2335.
48) Zhang, C.; Hong, S.-C.; Ji, N.; Wang, Y.-P.; Wei, K.-H.; Shen,
9
1
2, 573.
(
(
35) Noguchi, H.; Ito, M.; Uosaki, K. Chem. Lett. 2005, 34, 950.
36) Nishi, N.; Hobara, D.; Yamamoto, M.; Kakiuchi, T. Anal. Sci. 2003,
Y. R. Macromolecules 2003, 36, 3303.
9, 887.
JP806226E