Dynamics in the interior of AOT lamellae investigated with two-dimensional infrared spectroscopy

Article abstract of DOI:10.1021/ja312676e

The dynamics inside the organic regions of aerosol-OT (AOT)/water mixtures in the lamellar mesophase, bicontinuous cubic (BC) phase, and in an analogous molecule without the charged sulfonate headgroup are investigated by observing spectral diffusion, orientational relaxation and population relaxation using ultrafast two-dimensional infrared (2D IR) vibrational echo spectroscopy and IR pump-probe experiments on the asymmetric CO stretch of a vibrational probe, tungsten hexacarbonyl (W(CO)₆). The water layer thickness between the bilayer planes in the lamellar phase was varied. For comparison, the dynamics of W(CO)₆ in the normal liquid bis(2-ethylhexyl) succinate (EHS), which is analogous to AOT but has no charged sulfonate headgroup, were also studied. The 2D IR experiments measure spectral diffusion, which results from the structural evolution of the system. Spectral diffusion is quantified by the frequency-frequency correlation function (FFCF). In addition to a homogeneous component, the FFCFs are biexponential decays with fast and slow time components of ～12.5 and ～150 ps in the lamellar phase. Both components of the FFCF are independent of the number of water molecules per headgroup for the lamellae, but they slow somewhat in the BC phase. The dynamics in the ordered phases are in sharp contrast to the dynamics in EHS, which displays fast and slow components of the FFCF of 5 and 80 ps, respectively. As the hydration level of AOT increases, vibrational lifetime decreases, suggesting some change in the local environment of W(CO)₆ with water content.

Full text of DOI:10.1021/ja312676e

Article
pubs.acs.org/JACS
Dynamics in the Interior of AOT Lamellae Investigated with Two-
Dimensional Infrared Spectroscopy
S. K. Karthick Kumar, A. Tamimi, and Michael D. Fayer*
Department of Chemistry, Stanford University, Stanford, California 94305, United States
ABSTRACT: The dynamics inside the organic regions of
aerosol-OT (AOT)/water mixtures in the lamellar mesophase,
bicontinuous cubic (BC) phase, and in an analogous molecule
without the charged sulfonate headgroup are investigated by
observing spectral diffusion, orientational relaxation and
population relaxation using ultrafast two-dimensional infrared
(
2D IR) vibrational echo spectroscopy and IR pump−probe
experiments on the asymmetric CO stretch of a vibrational
probe, tungsten hexacarbonyl (W(CO) ). The water layer
6
thickness between the bilayer planes in the lamellar phase was
varied. For comparison, the dynamics of W(CO) in the normal liquid bis(2-ethylhexyl) succinate (EHS), which is analogous to
6
AOT but has no charged sulfonate headgroup, were also studied. The 2D IR experiments measure spectral diffusion, which
results from the structural evolution of the system. Spectral diffusion is quantified by the frequency−frequency correlation
function (FFCF). In addition to a homogeneous component, the FFCFs are biexponential decays with fast and slow time
components of ∼12.5 and ∼150 ps in the lamellar phase. Both components of the FFCF are independent of the number of water
molecules per headgroup for the lamellae, but they slow somewhat in the BC phase. The dynamics in the ordered phases are in
sharp contrast to the dynamics in EHS, which displays fast and slow components of the FFCF of 5 and 80 ps, respectively. As the
hydration level of AOT increases, vibrational lifetime decreases, suggesting some change in the local environment of W(CO)6
with water content.
1
1
. INTRODUCTION
AOT. The lamellar repeat distances have been characterized
by X-ray diffraction and scale linearly with the reciprocal of the
AOT volume fraction.
Mesomorphic phases appear in many surfactant systems and
with increasing surfactant concentration. For example, the
surfactant AOT (aerosol-OT, sodium bis(2-ethylhexyl) sulfo-
succinate) in mixtures with water with increasing AOT
concentration at room temperature goes through the phases:
isotropic, lamellar, bicontinuous cubic (BC), and inverse
hexagonal. The lamellar structure occurring in surfactant
systems has been shown by a variety of techniques as being
composed of macroscopically extended bilayers of amphiphilic
molecules alternating with water layers. Lyotropic lamellar
phases are important as models of logical membranes and in
various scientific and technological applications. Biological
membranes have a basic bilayer structure. In addition to a
permeability barrier, the plasma membrane serves as the
3
,16,17
The structure of AOT is shown in
Figure 1A. It contains a sulfonate headgroup and a sodium
counterion that preferentially partition into the water phase.
There are ester moieties near the headgroup that may also
interact with the water as well as the sodium counterion
depending on the packing and configuration of the AOT
1
18−21
molecules.
The alkyl chains are branched and bulky and
their steric interactions with one another play an important role
in determining the surface area per AOT molecule in lamellar
phases and in reverse micelles.
carried out to investigate the hydration of lipid mem-
2
,3
4
−6
3,18,22
Various studies have been
7,8
5
,23−25
branes,
and experiments and simulations have shown
that water molecules penetrate the interfacial region of bilayers
to some extent, but have a low probability of being found in the
hydrophobic core.
system in the lamellar phase have been somewhat sparse. Some
of these studies have focused on water diffusion
others have explored the effect of water/surface interactions
and confinement geometry on the dynamics of the water.
X-ray, NMR, and fluorescence experiments have also been used
to study the model membranes mainly to study the effect of
“
solvent” for membrane proteins and other biological
molecules. Therefore, the nature of the internal dynamics of
lamellar structures (bilayers) is important for understanding
processes that occur within them.
interesting thermodynamically stable structure consisting of
what is essentially a branching network of surfactant cylinders
in three dimensions with congruent networks of water
19,26−28
Dynamical studies of the AOT/water
9
−13
The BC phase has an
29−31
while
18,32
1
4,15
channels.
It has been suggested that the BC phase is an
intermediate between the inverted hexagonal phase (parallel
cylinders) and the lamellar phase.
33−37
cholesterol on the depth of water penetration.
AOT gives rise to lamellar structures when mixed with water
in a wide range of AOT/water ratios. At room temperature, the
AOT lamellar phase occurs in the range from ∼12 to ∼78 wt%
Received: December 28, 2012
Published: March 6, 2013
©
2013 American Chemical Society
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Article
molecule vibration within the inhomogeneously broadened
absorption line. Spectral diffusion is the result of the structural
evolution of the system that causes the local environments of
the probe molecules to change, which in turn changes the
probe vibrational frequency. Therefore, spectral diffusion
reports on the time dependence of the structural evolution of
the system. The 2D IR measurements of spectral diffusion yield
the frequency−frequency correlation function (FFCF), which
quantifies the dynamic time scales and amplitudes of
43−47
contributions to the structural changes in the system.
The polarization-selective IR pump−probe experiments meas-
ure the vibrational lifetime and orientational relaxation. The
vibrational lifetime is very sensitive to the local environment.
Changes in local environment of the vibrational probe can
result in changes in the lifetime.
The FFCFs obtained from the 2D IR experiments yield a
homogeneous (ultrafast structural fluctuations) component and
two spectral diffusion components. In the lamellar phase, the
spectral diffusion components are independent of the number
of water molecules per headgroup (λ) within experimental
error, and have fast and slow component values of ∼12.5 and
∼
150 ps, respectively. When the amount of water is reduced,
the phase changes to BC, and both components of the spectral
diffusion slow (∼15 and ∼180 ps), showing that the dynamics
are sensitive to the structural phase but not sensitive to the
amount of water so long as the system remains in the lamellar
phase. In contrast to these ordered phases, the two components
of the spectral diffusion in the EHS disordered liquid model of
AOT are ∼5 and ∼80 ps, which shows that the mesophase
structure has a substantial impact on the internal dynamics of
the lamellar and BC phases. Although the spectral diffusion
does not change with λ (number of water molecules between
bilayers) in the lamellar phase, the vibrational lifetime does
change to some extent, indicating a local change in the
environment of the vibrational probe.
Figure 1. Structures of (A) AOT (aerosol-OT, sodium bis(2-
ethylhexyl) sulfosuccinate) and (B) bis(2-ethylhexyl) succinate (EHS).
In this paper we investigate the internal dynamics of AOT
lamellar structures, the BC phase, and a related model
compound. Measurements of spectral diffusion, orientational
relaxation, and population relaxation were made using two-
dimensional infrared (2D IR) vibrational echo spectroscopy
and pump probe experiments. The experiments were
performed on a vibrational dynamics probe molecule, the
triply degenerate asymmetric CO stretching mode of tungsten
hexacarbonyl (W(CO) ) at room temperature. W(CO) is
6
6
completely insoluble in water and therefore resides in the
hydrophobic interior of the AOT lamellar and BC structures.
The number of water molecules per surfactant is given by the
2
. EXPERIMENTAL PROCEDURES
Succinic anhydride, 2-ethyl-1-hexanol, toluene, and p-toluenesulfonic
acid (Sigma-Aldrich) were used as received. Aerosol-OT (AOT,
sodium bis(2-ethylhexyl) sulfosuccinate) (Sigma-Aldrich) was purified
by dissolving the compound in methanol and stirring overnight with
activated charcoal. The charcoal was removed by vacuum filtration,
and methanol was removed by rotary evaporation.
parameter λ = [H O]/[AOT]. The lamellar phase was studied
2
over a wide range of hydration levels from λ = 98 to 16. The
BC phase only occurs in a very narrow range of hydration, and
measurements were made with λ = 6.
To investigate the effect of structural ordering in the
mesophases on their dynamics, the same experiments were
Bis(2-ethylhexyl) succinate (EHS), the model compound, was
synthesized and purified following a related procedure for the synthesis
performed on W(CO) in a disordered liquid that is chemically
6
4
8
equivalent to the interior of the AOT lamellar and BC
structures. The model system is the room temperature liquid
bis(2-ethylhexyl) succinate (EHS, C H O ), which has the
of a variant of AOT as follows: 10 g of succinic anhydride and 21 g
of 2-ethyl-1-hexanol were dissolved in 250 mL of toluene, and 2.2 g of
p-toluenesulfonic acid were added as a catalyst. The reaction mixture
was heated to 120 °C for 15 h in a round-bottom flask with an
attached Dean−Stark water-trap. The reaction mixture was sub-
2
0
38
4
same chemical structure as AOT but without the charged
sulfonate headgroup. The structure of EHS is shown in Figure
sequently washed a few times with aqueous NaHCO solution. The
3
1
B.
organic phase was dried over MgSO , and the solvent was removed
4
Recently, using ultrafast infrared spectroscopy, dynamics of
using a rotary evaporator. The mixture was then purified using flash
water between the bilayers in AOT lamellar structures and in
the water nanopools in AOT spherical reverse micelles were
column chromatography over silica with a 10−20% EtO /petroleum
2
ether solvent gradient.
1
8,38−42
probed.
The experiments investigated the influence of
Tungsten hexacarbonyl was dissolved in methylene chloride and
added to the surfactant or EHS, such that the ratio of probe molecule
to surfactant or to EHS was 1:600. In the surfactant systems, water was
added after slow evaporation of methylene chloride overnight. AOT/
water mixtures were made with AOT weight percentages 20, 30, 40,
interactions with the AOT interface on the dynamics of water.
The experiments compared water dynamics in the planar
lamellar geometry and the spherical reverse micelle geometry as
a function of the number of water molecules per AOT
headgroup. In contrast to those experiments on water, the
current experiments examine the dynamics in the surfactant
interiors in the lamellar and BC mesophases.
5
5
0, 60, and 80, which correspond to water to surfactant ratios λ = 98,
7, 37, 24, 16, and 6. The water thickness of lamellar samples of
varying water to surfactant ratios has been determined by X-ray
3,16,17
diffraction.
Samples for FT-IR absorption experiments and 2D IR
The 2D IR experiments measure spectral diffusion, which is
the time evolution of the vibrational frequencies of the probe
and IR pump−probe experiments were housed in sample cells with
CaF windows separated by 12 μm Teflon spacers.
2
5
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A Ti:sapphire oscillator/regenerative amplifier with a repetition rate
of 1 kHz, which produced 100 fs pulses with pulse energies of ∼700 μJ
and a wavelength centered at 800 nm, was used to pump a home-built
mid-IR optical parametric amplifier to generate ∼170 fs pulses
centered at 1980 cm with ∼5 μJ pulse energy. The mid-IR pulses
were then co-aligned with a He−Ne beam and a 90:10 ZnSe beam-
splitter was used to split the incoming pulse into a weaker probe pulse
and a stronger pump pulse. The weak pulse is routed through a
spectral diffusion time constants and amplitude factors. The difference
between the T = 0 intercept of the CLS plot and 1 is related to the
w
homogeneous component of the FFCF. To obtain homogeneous
component, the CLS fitting results are used to simultaneously fit the
−1
46,47
FT-IR absorption spectrum and the CLS data.
component is included in the full FFCF.
The homogeneous
The polarization selective pump−probe technique tracks the decay
of the probe transmission with polarizations parallel (I ) and at the
∥
mechanical delay line which is used to set the waiting time T (see
magic angle (MA = 54.7°) relative to the pump pulse polarization. The
data contain information about both population and orientational
dynamics.
w
below). The strong pulse was sent to a mid-IR Fourier-domain pulse-
shaper. Details of the laser system and the pulse shaper have been
49
presented. The output beam from the pulse shaper, which generated
pulses 1 and 2 in the vibrational echo experiments, is focused into the
I = P(t)(1 + 0.8C (t))
2
(3)
(4)
sample using a 150 mm focal length CaF lens and the weak probe
2
IMA = P(t)
pulse (pulse 3) is focused in the sample using a 100 mm focal length
CaF2 lens to give spot size diameters of ∼250 and ∼180 μm,
respectively. The probe is collimated after the sample then sent to a
spectrometer equipped with a 32-element HgCdTe IR array detector.
In a 2D IR vibrational echo experiment, τ (the time between pulses
P(t) is the vibrational population relaxation and C (t) is the second
2
Legendre polynomial correlation function, which is the orientational
correlation function for the vibrational mode. Measurements of IMA
enable C (t) to be extracted from measurements of I . Here C (t) is a
2
||
2
1
and 2) is scanned for a fixed T (the time between pulse 2 and 3),
w
single-exponential decay characterized by the orientational relaxation
time, τr.
and a 2D spectrum is obtained by Fourier transforming along the τ
dimension. Then T is changed, and τ is scanned to obtain another 2D
w
spectrum. A series of such spectra is recorded. The desired information
is contained in the peak positions and the Tw dependence of the
amplitudes and shapes of the peaks in the 2D spectrum. In the pump−
probe experiments, two pulses, a strong pump and a weak probe,
impinge on the sample. The time between the two pulses is t. The
pump produces a transient change in the transmission of the probe.
The probe is frequency resolved by the spectrograph, and its
amplitude at each frequency is measured as a function of t. The
change in probe transmission is obtained with the pump polarization
parallel to the probe and at the magic angle of 54.7°.
3
. RESULTS AND DISCUSSION
.1. FT-IR Absorption Spectra. W(CO) is used as a
3
6
vibrational spectral probe of dynamics and structure. W(CO)6
is substantially smaller than AOT. It has a molecular volume of
3
∼95 Å , compared to the molecular volume of AOT, which is
3
∼455 Å . So the probe is about a fifth the size of AOT. In
addition, its concentration in the bilayers is small, with 1
W(CO) per 600 AOT. Therefore, the presence of W(CO)
6
6
should not have a significant influence on the global structure
of the surfactant bilayers.
The FFCF, which quantifies the spectral diffusion in terms of the
amplitudes and time scales of the dynamics, is the joint probability that
a vibration with an initial (t = 0) frequency in the inhomogeneous
spectral distribution will maintain its frequency at a later time t,
averaged over all initial frequencies. To extract the FFCF from the 2D
Figure 2 displays FT-IR absorption spectra of the asymmetric
CO stretching mode of W(CO) in AOT/water mixture with λ
6
4
6,47
spectra, center line slope (CLS) analysis was employed.
The FFCF is described with a multiexponential model,
2
i
C (t) = ⟨δω (t)δω (0)⟩ = ∑ Δ exp(−t/τ)
1
1,0
1,0
i
i
(1)
where the Δ are the frequency fluctuation amplitudes of each
i
component, and the τ are their associated time constants. A
i
component of the FFCF with Δτ < 1 is motionally narrowed, and it
is the dominant source of the homogeneous broadening of the
absorption line. When a component is motionally narrowed, Δ and τ
cannot be determined separately. The motionally narrowed homoge-
neous contribution to the absorption spectrum has a pure dephasing
2
line width given by Γ* = Δ τ = 1/πT*, where T* is the pure dephasing
2
2
time. The observed homogeneous dephasing time, T , also has
2
contributions from the vibrational lifetime and orientational relaxation:
Figure 2. Background subtracted infrared absorbance spectrum of the
1
T2
1
1
2T1
1
3Tor
CO stretching mode of tungsten hexacarbonyl (W(CO) ) in AOT/
6
=
+
+
T*
water binary mixture with various water to surfactant ratios. λ =
2
(2)
[
H O]/[AOT] = 98 to 16 are in the lamellar phase and λ = 6 is in the
2
where T*, T , and T are the pure dephasing time, vibrational lifetime,
bicontinuous cubic phase.
2
1
or
and orientational relaxation times, respectively. The total homoge-
neous line width is Γ = 1/πT .
2
CLS analysis of the T -dependent 2D IR line shapes permits the
(water molecules per AOT) ranging from 96 to 6. For λ = 96 to
16, the samples are lamellae, while for λ = 6, the sample is in
the BC phase. The spectra with λ = 16 and 37 are identical. The
spectra are not symmetrical. They have a steeper edge on the
blue (high-frequency) side of the line than on the red side of
the line. The λ = 57 and 98 samples appear to have increasingly
longer tails on the red side of the lines. However, at these high
water contents, there is a very large water background that must
be subtracted out to obtain the spectra. It is possible that
background subtraction errors at high water content give rise to
w
4
6,47
FFCF to be determined in the following manner.
The 2D
spectrum at a given T is sliced parallel to the ω axis (the vertical axis
w
m
in the 2D spectra) over a range of frequencies about the center of the
spectrum. Each slice is fit to a Gaussian line shape function to obtain
its peak position. The peak positions are plotted against the
corresponding ω frequencies of the slices. (ω is the horizontal axis
τ
τ
in the 2D spectra.) The set of peak positions (ω ,ω ) are referred to as
center line data, and the resulting line is fit with a linear regression to
obtain its slope. Slopes are obtained for each Tw and plotted. The
τ
m
slopes can range from 1 to 0. A fit to the CLS vs T curve gives the
w
5
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the larger red tails. The λ = 6 (BC phase) spectrum is clearly
narrower than the other spectra and its peak is shifted by ∼0.5
As can be seen in Figure 3A, W(CO) in dodecane has an
6
−1
exceedingly narrow line width of 4.2 cm compared to its
−1
−1
cm to the blue. The full widths at half-maximum (fwhm)
from low to high water content are 11.02, 11.92, 12.27, 12.62,
width in EHS (14.8 cm ). The line width in EHS is more
typical. For example, the CO antisymmetric stretches of
−
1
−1
and 12.62 cm .
W(CO)6 in CCl and in CHCl3 are 10 and 19 cm ,
4
50
W(CO) is completely insoluble in water. Therefore, it
respectively. Long linear hydrocarbons like dodecane tend
to have all-trans configurations. The all-trans configuration will
produce little variation in the local structure experienced by
6
resides in the hydrophobic interior of the lamellar and BC
structures. Spectra can shed some light on the location of the
vibrational probe within the lamellae and BC phase. Figure 3A
W(CO) , giving rise to the narrow line width. It is not
6
unreasonable to assume that the branched alkane inside the
lamellae is less ordered than in dodecane. Therefore, W(CO)6
in the alkane region of AOT will have a wider the line width
then it has in dodecane. The branching of the alkane portion of
AOT and differences in packing are likely responsible for the
somewhat increased width of Gaussian 1 (green curve) in the
fit to the AOT spectrum shown in Figure 3B compared to the
W(CO)6 spectrum in dodecane. In EHS, W(CO)6 can
experience a wider range of environments giving rise to a
broader line. In EHS, W(CO)6 can be in an all alkane
environment or it could be in contact with or near the ester
moieties (see Figure 1). Given the spectrum in dodecane,
presumably the blue side of the EHS line corresponds to more
alkane-like environments and the on the red side of the line
there are more interactions with the esters. It is important to
note that W(CO) does not have to come in direct contact with
6
an ester oxygen to have its frequency shifted. As has been
shown by comparison between experiments and simulations,
frequency shifts that give rise to inhomogeneous broadening of
vibrational absorption lines can arise from variations in the local
51−54
electric field.
The results of the fits in Figure 3B suggest that the W(CO)6
experiences both alkane and ester environments but with the
alkane environments dominating. Based on the molecular
3
volume of EHS (392 Å ) and the volume of the ester portion
3
only (124 Å ), EHS, (see Figure 1B) is very approximately
composed of a one-third ester region and a two-thirds alkane
Figure 3. (A) Spectra of the CO stretching mode of W(CO) in λ =
6
3
7 lamellae (red curve), in dodecane (green curve), and in EHS, the
model liquid that is AOT without the sulfonate headgroup (see Figure
). (B) The λ = 37 spectrum (red curve) and a two-Gaussian fit (black
region. The spectrum of W(CO) in the pure alkane shows that
6
the absorption is on the blue side of the peak of the spectrum in
the lamellae. To fit the spectrum in the lamellae requires the
EHS spectrum plus additional absorbance on the blue side of
1
dashed curve). The two Gaussians that result from the fit are the blue
and green curves. The blue Gaussian is virtually identical to the EHS
spectrum shown in A. The green curve is slightly shifted and
somewhat broader than the dodecane spectrum shown in A.
the line, indicating that the W(CO) is exposed more to the
6
alkane portion of the AOT than to the ester portion. Given the
areas of the Gaussians in the fits in Figure 3B (ratio blue/green
= 2.1) and the very approximate partitioning of the EHS into
33%/66% ester/alkane, a very crude estimate suggests that the
shows the FT-IR absorption spectra of the CO asymmetric
stretch of W(CO) in the λ = 37 lamellae (red), dodecane
6
(
green), and EHS (the AOT model compound) (blue). The
W(CO)
6
is exposed to 77%/23% alkane/ester in the interior of
spectrum in the lamellae is clearly asymmetrical with steep rise
on the blue side of the line and a broader red side. The peak of
the spectrum in the lamellae falls between the other two peaks.
Figure 3B shows the spectrum in the lamellae (red) again
with a fit (black dashed curve) to two Gaussians. The fit is
basically perfect. The two Gaussians that compose the fit are
also shown as the green and blue curves. The blue Gaussian in
Figure 3B has parameters that are almost identical to a fit to the
EHS spectrum in Figure 3A. The fit to the EHS data in Figure
the lamellae. This is one extreme. However, in EHS, W(CO)
6
will frequently be exposed to ester and alkane portions of the
surrounding solvent simultaneously. If we take all of the EHS
portion of the fit in Figure 3B (blue curve) to represent some
interactions of W(CO) with ester moieties and the green curve
6
to represent interactions only with the alkane portions of AOT,
then the ratio is 32%/68% alkane/ester. Here ester means some
influence on W(CO) because it is either in contact or near an
6
ester. This is a wide range, but the results indicate that the
−1
3
A yields a center position of 1976.9 cm and a fwhm of 14.8
ensemble of W(CO) molecules is not located strictly in the
6
−1
cm . The blue Gaussian in Figure 3B has center position of
1
data in Figure 3A yields a center position of 1982.0 cm and a
fwhm of 4.2 cm . The green Gaussian in Figure 3B has center
position of 1980.7 cm and a fwhm of 7.6 cm . The fit
Gaussian is shifted somewhat to the red and is significantly
broader than spectrum in dodecane.
alkane portion of the bilayers. It may be possible to obtain more
976.4 cm− and a fwhm of 14.9 cm . The fit to the dodecane
1
−1
detailed information on the W(CO) environments through
6
−1
simulations.
−
1
W(CO)6 is not polar. However, it is polarizable. Its
polarizability will produce some attraction to the polar ester
region of AOT. Returning to the spectra in Figure 2, the
increasing tails to the red with increasing water content may
−1
−1
5
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47
indicate that the additional water, which penetrates to some
extent into the ester regions of the AOT headgroup, causes
method. The CLS data are presented in Figure 5. The data
have important qualitative features. All of the AOT lamellar
18
W(CO) to partition to a greater extent into the ester region.
6
Conversely, the fact the spectrum in the BC phase (λ = 6) has
less spectral density to the red may indicate that in this phase,
tungsten hexacarbonyl partitions less into the ester region of
the AOT head groups.
3.2. Ultrafast IR Spectroscopy. To illustrate the nature of
the data, 2D IR vibrational echo spectra of the asymmetric CO
stretch of W(CO) inside a lamellar structure (λ = 24) are
6
shown in Figure 4 at two waiting times (T ). The positive
w
Figure 5. CLS curves for the CO stretching mode of W(CO) in
6
AOT/water binary mixtures in the lamellar phase, λ = 98 to 16, in the
BC phase, λ = 6, and in the model liquid bis(2-ethylhexyl) succinate
(EHS).
samples (λ = 98, 75, 37, 24, and 16) have almost identical decay
curves. However, the AOT sample in the BC phase (λ = 6, blue
curve) displays a significantly slower decay. In contrast, the data
for W(CO) in the EHS model liquid (red curve, AOT like
6
structure but without the sulfonate, see Figure 2B) decay much
faster than any of the other data. These results show that the
dynamics interior to the lamellar structures have little
dependence on the number of water molecules between the
AOT bilayers. However, when the phase changes from lamellar
to BC there is a change in dynamics. In addition, the decay
curves show that the dynamics interior to the ordered phases
are quite different from the dynamics in EHS liquid, which is a
model of AOT without the sulfonate headgroup that causes
AOT to form ordered phases in the presence of water.
The qualitative ideas are borne out by the quantitative
analysis of the CLS curves that yield the FFCFs for each
Figure 4. 2D IR vibrational echo spectra of the CO stretch of W(CO)6
46,47
sample.
All of the CLS curves shown in Figure 5 are fit very
in λ = 24 lamellae. The top panel is at short time (T = 4 ps). It is
w
well by a biexponential function. The difference between the Tw
0 value of the CLS and 1 is related to the homogeneous
component. Combining the CLS fit with the linear absorption
substantially elongated along the diagonal (black dashed line). The
=
bottom panel is at long time (T = 95 ps). The shape of the spectrum
w
has changed substantially, and it has little elongation along the
diagonal showing spectral diffusion is almost complete.
46,47
spectrum makes it possible to calculate the full FFCF.
The
FFCF parameters are given in Table 1. First, consider the
lamellar samples, λ = 98 to 16. The slowest component of the
decay of the FFCF is τ . All of the τ values for the five lamellar
going bands (red) arise from the 0-1 vibrational transition. The
negative going bands (blue), which are only partially shown
below the 0-1 bands, arise from vibrational echo emission at the
2
2
samples are basically the same within experimental error. The
average of the values is 147 ps. The faster components, τ , are
1
1
-2 vibrational frequency. These bands are shifted by the
also all the same within experimental error with an average for
the five lamellar samples of 12.6 ps. The corresponding
amplitude factors, Δ and Δ , are also all the same within
vibrational anharmonicity to lower frequency along the ωm
frequency axis. Changes are clearly visible in the 0-1 spectra. At
early time (T = 4 ps, top panel), the spectrum is substantially
1
2
experimental error. Note that the deltas in Table 1 are the
w
elongated along the diagonal (black dashed line). The width
perpendicular to the diagonal shows there is substantial
homogeneous broadening of the absorption line. The
elongation along the diagonal at short time arises because the
line is inhomogeneously broadened and structural evolution of
the system has not caused significant sampling at this short time
standard deviations. The total inhomogeneous line width is the
2
convolution of these two Gaussians with standard deviation (Δ
1
2
1/2
+ Δ ) . The fwhm of the inhomogeneous line width is the
2
total standard deviation multiplied by 2.35.
Table 1 also gives the homogeneous linewidths, Γ, and the
corresponding homogeneous dephasing times, T . As with the
2
of the range of inhomogeneous frequencies. At long time (T =
spectral diffusion dynamics, the homogeneous dephasing is the
same for the five lamellar samples (λ = 98 to 16) within
experimental error. As discussed above, the homogeneous line
width has contributions from pure dephasing, the vibrational
lifetime, and the orientational relaxation (see eq 2). The
vibrational lifetime (see Table 2) contributes only ∼0.07 cm−
w
9
5 ps, bottom panel), the 2D IR spectrum is close to
symmetrical, which shows that spectral diffusion is almost
complete.
For each sample, 2D IR spectra are taken at many T values,
w
1
and the change in shape with time is quantified with the CLS
5
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Table 1. FFCF Parameters for the CO Stretching Mode of Tungsten Hexacarbonyl in AOT Lamellar Phase (λ = 98 to 16), in the
Bicontinuous Cubic Phase (λ = 6), and in Bis(2-ethylhexyl) Succinate (EHS, AOT Model Liquid)
sample
T2 (ps)
Γ (cm−1)
Δ (cm−1)
τ1 (ps)
Δ (cm−1)
τ2 (ps)
1
2
λ = 98
λ = 57
λ = 37
λ = 24
λ = 16
λ = 6
2.5 ± 0.2
2.4 ± 0.2
2.2 ± 0.4
2.1 ± 0.3
2.2 ± 0.2
2.5 ± 0.2
1.3 ± 0.3
4.3 ± 0.4
4.4 ± 0.6
4.9 ± 0.2
5.0 ± 0.4
4.9 ± 0.4
4.1 ± 0.5
8.2 ± 0.4
3.0 ± 0.2
3.2 ± 0.4
2.9 ± 0.6
2.7 ± 0.5
2.9 ± 0.3
2.8 ± 0.2
4.4 ± 0.2
12.0 ± 0.5
13.4 ± 0.5
12.5 ± 0.3
13.4 ± 0.5
11.7 ± 0.3
14.7 ± 0.4
4.8 ± 0.3
2.2 ± 0.4
2.2 ± 0.6
2.2 ± 0.6
2.2 ± 0.4
2.1 ± 0.3
2.3 ± 0.5
2.4 ± 0.4
149 ± 9
154 ± 12
144 ± 5
139 ± 6
149 ± 5
178 ± 4
80 ± 9
EHS
Table 2. Vibrational Lifetimes (P(t)) and Orientational
While going from the lamellar phase to the BC phase causes
a modest slowing of the structural dynamics, the dynamics in
the EHS model liquid (see Figure 1) are substantially different.
The CO mode of W(CO) in EHS has τ = 80 ± 9 ps and τ =
4.8 ± 0.3 ps. These times are almost factors of two and three
times faster, respectively, than the corresponding times in the
ordered phases. In addition, the homogeneous line width in
EHS is almost a factor of 2 wider than it is in the ordered
phases. EHS is a normal liquid with no long-range ordering in
its structure. Given the disorder, W(CO)6 should be in
proximity to both the alkane and ester portions of the EHS
molecule. It was argued in connection with the FT-IR spectra in
Figures 2 and 3 that the absorption spectra of the asymmetric
Decay Times (τ ) for the CO Stretching Mode of Tungsten
r
Hexacarbonyl in AOT Lamellar Phase (λ = 98 to 16), in the
Bicontinuous Cubic Phase (λ = 6), and in Bis(2-ethylhexyl)
Succinate (EHS, AOT Model Liquid)
6
2
1
sample
P(t) (ps)
τr (ps)
λ = 98
λ = 57
λ = 37
λ = 24
λ = 16
λ = 6
69 ± 0.6
73 ± 0.3
74 ± 0.3
80 ± 0.5
85 ± 0.3
88 ± 0.3
86.0 ± 0.8
4.0 ± 0.3
4.7 ± 0.4
3.7 ± 0.3
4.0 ± 0.1
4.5 ± 0.1
3.4 ± 0.2
3.9 ± 0.1
EHS
CO stretch of W(CO) in the AOT/water ordered phases is
6
similar to that in EHS but with more contribution from the
alkane-like spectrum in the AOT/water samples.
to the homogeneous line width. Orientational relaxation
−1
The spectral diffusion dynamics as well as the pure dephasing
dynamics are very different in the ordered lamellar and BC
phases than they are in the EHS model liquid (see Table 1)
even though the chemical nature of the environments in the
ordered and disordered systems are very similar. These results
demonstrate that the organization of the interiors of lamellae
and BC structures have a substantial influence on the structural
dynamics. Going from one ordered phase to another, lamellar
to BC, has some effect on dynamics. While in the lamellar
phase, the structures are planar, in the BC phase, the dominant
structure is more akin to cylinders. This change will affect the
packing of the AOT hydrocarbon tails in the interior of the
ordered structures, which evidently has an impact on the
structural dynamics. However, the change from one ordered
structure to another has a small influence on dynamics
compared to the effect of order vs disorder.
contributes ∼0.9 cm . Therefore, the homogeneous line
width is dominated by pure dephasing, which results from
ultrafast motions that produce a motionally narrowed
component of the total absorption line width.
When the phase changes, the dynamics change. In the BC
phase (λ = 6), the spectral diffusion is slower. The slow
component τ = 178 ± 4 ps, in contrast to the average of 147 ps
2
for the lamellar phase samples. The fast component τ = 14.7 ±
1
0
.4 ps, in contrast to the average of 12.6 ps for the lamellar
phase samples. Therefore, both components of the spectral
diffusion are slower in the BC phase.
The water at the surface of AOT lamellae and at the AOT
surface of large reverse micelles undergoes orientational
1
8,38,39
relaxation in 18 ps.
The water in reverse micelles
undergoes slower orientational relaxation as the number of
39
water molecules per AOT decreases. For 10 waters per
39
In both the ordered phases and the disordered liquid, there
are two time scales for the spectral diffusion that are more than
an order of magnitude different. The FFCF is described in
terms of a multiexponential model. The time constants, τ and
τ2 given in Table 1 should be thought of as time scales for
headgroup it is 26 ps, and for 5 waters, it is 30 ps. Given these
results, it is likely that the water orientational relaxation in the
BC phase with λ = 6 is somewhat slower than the 18 ps
orientational relaxation of water at the surfaces of the lamellae.
These water orientational relaxation times are much faster than
the slow components of the spectral diffusion in either the
lamellar or BC phases, although they are comparable to the fast
components. It is unlikely that the water dynamics play a
significant role in the spectral diffusion measured inside of the
surfactant structures, although it cannot be ruled out for the fast
component. Therefore, it is reasonable to assume that the
slowing of structural dynamics in the BC phase results from the
difference in geometry of the BC phase compared to the
lamellar phase rather than from the reduction in the number of
water molecule per AOT per se. The organization of the AOT
in the BC phase is different than it is in the lamellar phase. The
difference in the physical arrangement of the surfactant
molecules in the two phases results in a difference in the
rates of structural dynamics.
1
processes rather than time constants for specific processes as
there may be a collection of motions, which occur with about
the same rates, that give rise to the time constants obtained
from the fits. The time scales may reflect motions of the side
chains (fast) and motions of the total AOT or EHS molecules
(slow), with the disordered EHS liquid having overall faster
motions of both types. Regardless of the exact nature of the
motions, it is clear that the order imposed by lamellar and BC
phases substantially slows the structural dynamics.
Although the amount of water between the lamellae layers
does not influence the structural dynamics as reported by the
spectral diffusion within experimental error, there is evidence
that there may be minor changes in the local environment of
W(CO) with water content. Figure 6 shows the results of
6
5
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In all of the systems, the anisotropy decay is faster than either
of the spectral diffusion time constants. It is important to
recognize that the anisotropy relaxation correlation function,
the second Legendre polynomial correlation function, obtained
from the pump−probe experiments is a different correlation
function than the FFCF. Therefore, the resulting time
constants cannot be directly compared. As an example, for
Gaussian fluctuation anisotropy relaxation that is single
exponential, dielectric relaxation, which measures the first
Legendre polynomial correlation function, gives a time constant
that is three times as long as that which would be measured by
5
9,60
a pump−probe experiment on the same system.
4
. CONCLUDING REMARKS
Figure 6. Magic angle pump−probe data (lifetimes) for the CO
stretch of W(CO) in the AOT/water lamellar phase (λ = 98, 57, 37,
6
Surfactant bilayers occur very commonly in systems such as
vesicles and cell membranes. Here we have examined the
dynamics inside surfactant bilayers, which are important for
processes that involve bilayers. Molecules can pass through the
walls of vesicles and cell membranes. Such transmission of
molecules through a bilayer involves the structural fluctuations
of the bilayer interior. Cell membranes contain a wide array of
transmembrane proteins. For transmembrane proteins, the
bilayer is the “solvent” in which biological processes occur.
Chemistry depends to a great extent on the nature of the
solvent, which provides dynamical intermolecular interactions
that are intimately involved in chemical processes. For cytosolic
proteins and enzymes, the water medium matters. MD
simulations use sophisticated water models that do a reasonable
job of reproducing the mechanical degrees of freedom of water.
Even when implicit water models are used, these are tweaked to
mimic atomistic water models. The bath degrees of freedom
can be included in MD simulations of cytosolic proteins and
enzymes because we have an extensive understanding of water
dynamics, and water simulations have been tested against
experiments, principally ultrafast 2D IR vibrational echo
2
4, 16), in the bicontinuous cubic phase (λ = 6), and in the model
liquid bis(2-ethylhexyl) succinate (EHS).
pump−probe data taken at the magic angle for all of the
samples. These decays are the vibrational lifetimes, P(t) (see
eqs 3 and 4). The vibrational lifetime is very sensitive to the
very local environment and intermolecular interactions. This is
in contrast to spectral diffusion that is caused by global
structural reorganization of a system. In a number of systems,
5
1−53
55,56
57
such as proteins,
water,
and water/salt solutions, the
influence of structural fluctuations on a vibrational frequency
has been modeled in terms of fluctuating electric fields. Global
motions of the environment cause the electric field along the
vibrational transition dipole direction to fluctuate, which via the
Stark effect, gives rise to time dependent frequency
5
1
fluctuations.
All of the decays in Figure 6 fit extremely well to single-
exponential functions. The results of the fits are given in Table
2
9
. As the number of water molecules per AOT decreases from
8 to 6, the vibrational lifetime increases from 69 to 88 ps.
61−64
Because the decays are single exponential and can be measured
over several lifetimes, the error bars are small (see Table 2).
The progression of the lifetimes indicates local structural
change with water content. Either these changes do no
influence the structural dynamics of the systems or the
influence is too small to be picked up in the measurements
of spectral diffusion. Note that while going from the lamellar
phase to the BC phase causes a jump in both the fast and slow
spectral diffusion time constants, the changes in the lifetimes
seems to progress smoothly without a noticeable discontinuity
when the phase changes.
experiments.
The equivalent detailed simulations based
on 2D IR experimental information that is available for
6
1,62
water
are not available for the interiors of surfactant
bilayers. The results presented here are an initial step in
increasing our understanding of the structural dynamics of
surfactant bilayers.
The results presented above provide the first detailed
examination of the fast structural dynamics of the interior of
surfactant bilayers in both the lamellar phase and the
bicontinuous cubic phase. For the lamellae, multibilayers with
16−98 water molecules per AOT surfactant molecule were
studied and it was determined that the thickness of the water
layer that separates bilayers does not influence the internal
structural dynamics of the bilayers. Sixteen water molecules are
sufficient to solvate the AOT headgroup and its sodium
Combining the pump−probe magic angle and parallel
polarization experiments, the W(CO) transition dipole
orientational anisotropy decays were measured (see eqs 3 and
6
4
). The results are given in Table 2. All of the anisotropy decays
18
are virtually the same. The anisotropy decay of the asymmetric
counterion. As the number of water molecules per AOT is
increased, the water layer away from the bilayer/water interface
becomes increasingly bulk-like. By 98 water molecules per
AOT, there is a substantial bulk-like slab of water between the
bilayers. The results show that an increased separation of the
bilayers by bulk-like water does not change the internal
structural dynamics.
CO stretch of W(CO) is not caused by normal molecular
6
reorientation. The CO stretch is triply degenerate. The
excitation pulse projects out a superposition of the three
modes that results in the initially excited transition dipole being
along the pump pulse electric field. Then fluctuations that
cause this initial superposition to change result in a change in
the direction of the transition dipole without physical
reorientation of the molecule. Therefore, the anisotropy
decays in these systems do not provide the type of information
that normal orientational relaxation measurements do and
cannot be interpreted in terms of shape factors, hydrodynamic
boundary conditions, or friction coefficients.
58
Changing the phase does change the dynamics to a small but
measurable extent. A reduction in the number of water
molecules per AOT to six causes a phase change to BC. The
internal structural dynamics in the BC phase are ∼20% slower
than in the lamellar phase. However, the dynamics in both the
lamellar and BC phases are much slower than those of the
58
5
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Journal of the American Chemical Society
Article
model liquid, bis(2-ethylhexyl) succinate (EHS), which is
identical to AOT except that it lacks the charged sulfonate
headgroup (see Figure 1). Without the sulfonate head groups
interacting with water, EHS does not form ordered structural
phases. It is essentially identical chemically to the interior of the
AOT bilayers but without the ordered structure. In EHS the
fast and slow time scale dynamics are approximately factors of 3
and 2 faster, respectively, than the corresponding dynamics in
the AOT ordered phases. Thus the order imposed by the
interaction of the charged head groups with water has a very
substantial impact on the internal structural dynamics of the
lamellar and BC phases.
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AUTHOR INFORMATION
Corresponding Author
fayer@stanford.edu
■
1183.
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(
6
(
■
This work was funded by the Division of Chemical Sciences,
Geosciences, and Biosciences, Office of Basic Energy Sciences
of the U.S. Department of Energy through Grant No. DE-
FG03-84ER13251. We thank Aanindeeta Banerjee for help with
the synthesis of EHS. A.T. thanks the Stanford Graduate
Fellowship program for a research fellowship.
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