Investigation of surfactant conformation and order at the liquid-liquid interface by total internal reflection sum-frequency vibrational spectroscopy

Article abstract of DOI:10.1021/jp953616x

The conformational order of sodium dodecyl sulfate (SDS) adsorbed at the D₂O-CCl₄ interface has been examined by total internal reflection sum-frequency vibrational spectroscopy. A change in conformation of the alkyl chain with increased surface coverage at the liquid-liquid interface is observed. A series of aqueous surfactant concentrations have been examined in order to determine the effect of surface coverage on the conformation of the alkyl chains at the interface. Polarization studies indicate that, for the concentration range examined, the symmetry axis of the terminal methyl group on the alkyl chain is oriented primarily along the surface normal. Identification of spectral features in the C-H region of the infrared region is facilitated by examination of the sum-frequency spectrum from an analogous deuterated compound.

Full text of DOI:10.1021/jp953616x

J. Phys. Chem. 1996, 100, 7617-7622
7617
Investigation of Surfactant Conformation and Order at the Liquid-Liquid Interface by
Total Internal Reflection Sum-Frequency Vibrational Spectroscopy
John C. Conboy, Marie C. Messmer,† and Geraldine L. Richmond*
Department of Chemistry, UniVersity of Oregon, Eugene, Oregon 97403
ReceiVed: December 4, 1995X
The conformational order of sodium dodecyl sulfate (SDS) adsorbed at the D2O-CCl4 interface has been
examined by total internal reflection sum-frequency vibrational spectroscopy. A change in conformation of
the alkyl chain with increased surface coverage at the liquid-liquid interface is observed. A series of aqueous
surfactant concentrations have been examined in order to determine the effect of surface coverage on the
conformation of the alkyl chains at the interface. Polarization studies indicate that, for the concentration
range examined, the symmetry axis of the terminal methyl group on the alkyl chain is oriented primarily
along the surface normal. Identification of spectral features in the C-H region of the infrared region is
facilitated by examination of the sum-frequency spectrum from an analogous deuterated compound.
Introduction
(SFG). This technique, pioneered by Shen and co-workers,16
has been exploited previously, resulting in significant progress
Intermolecular forces play a large role in reactivity, orienta-
tion, and structural conformation in molecular systems. At
surfaces and interfaces, molecules possess unique properties,
undergoing reactions and ordering caused by the asymmetric
nature of the interfacial region.1,2 In recent years there has been
increased interest in the adsorption, molecular structure, and
electrochemical properties of molecules at liquid interfaces. The
interface between two immiscible liquids has proven to be a
particularly difficult surface to characterize due to the inability
to access this interface with conventional techniques which
provide molecular level information. What is known has
primarily come from techniques that probe only macroscopic
properties of the interface such as interfacial tension and
electrochemical and transport properties. Much more remains
to be learned about this interface concerning molecular con-
formation and adsorption in the interfacial region.
Molecular adsorption at liquid-liquid interfaces plays an
important role in many biological and chemical systems such
as biological membranes, extraction processes, and electron
transfer.1,2 While studies have been done to investigate the
orientation and adsorption of surface active dyes at the oil-
water interface using fluorescence,3,4 resonance Raman
scattering,5-7 and second harmonic generation (SHG),8-10 little
is known about the conformation of simple alkyl surfactants at
the liquid-liquid interface. Studies of the liquid-liquid
interface using surfactant dye molecules alone and in the
presence of common ionic surfactants have shown that the dye
molecules assume different conformations at varying surface
pressures.4,11 In addition, fluorescence depolarization studies
have shown the importance of surface roughness on the diffusion
of molecules on the interface.12 Recent molecular dynamics
calculations have been done that predict molecular ordering at
the alkane-water interface.13 These calculations coupled with
recent SHG studies has led to an understanding of molecular
ordering at the interface between a hydrocarbon and an aqueous
phase.14,15
in the understanding of many interfacial systems such as solid-
adsorbate-air, solid-liquid, and liquid-air interfaces.17-31 The
inherent advantage of SFG over conventional spectroscopies
arises from its dependence on the induced second-order
nonlinear polarization, which is generated only in noncen-
trosymmetric media. This feature makes SFG an ideal technique
for studying interfacial systems of centrosymmetric materials,
where the symmetry is broken only at the interfacial region.
The conformational aspects of adsorption are investigated here
by examining the C-H stretching region of the sum-frequency
vibrational spectrum with a variety of polarization- and con-
centration-dependent experiments. A total internal reflection
geometry is used which significantly enhances the sensitivity
of this technique for the surfactant studied, allowing measure-
ments to be made over a wide range of surface concentrations.
Results show that an ordering process takes place for the
surfactant at the oil-water interface. In addition, results from
deuterated analogues have facilitated assignment of spectral
features.
Background
Sum-Frequency Generation. In SFG, two light beams (ω1
and ω2) impinge on a surface, generating a light beam of the
sum frequency (ω3 ) ω1 + ω2), the intensity of which is
governed by surface polarization.32 Sum-frequency intensities
are determined by the induced nonlinear polarization in a media
given by
P(2)(ωSF) ) ø(2):eˆω eˆω Iω Iω
(1)
x
x
1
2
1
2
where eˆi ) ꢀˆifi; ꢀˆ is the unit polarization vector, fi is the
geometric Fresnel factor, I is the laser intensity for each of the
input beams ω1 and ω2, and ø(2) is the second-order polarizability
tensor. The resulting sum-frequency intensity is proportional
to the square of the induced polarization by
In this paper, the ordering of the alkyl chain of sodium
dodecyl sulfate (SDS) at the water-carbon tetrachloride inter-
face is examined in detail, by surface sum-frequency generation
2
I(ωSF) ) |˜fSFP(2)(ωSF)|
(2)
† Present address: Department of Chemistry Lehigh University, 6 East
Packer Avenue, Bethlehem, PA 18015-3172.
where ˜fSF is the nonlinear Fresnel factor for the generated sum-
frequency field.
X Abstract published in AdVance ACS Abstracts, April 1, 1996.
S0022-3654(95)03616-1 CCC: $12.00 © 1996 American Chemical Society

7618 J. Phys. Chem., Vol. 100, No. 18, 1996
Conboy et al.
SFG has been widely used as a spectroscopic probe of
interfaces as the hyperpolarizability of adsorbates on the surface
strongly affects sum-frequency intensities. The surface polar-
izability can result from both nonresonant and resonant com-
ponents. If resonant and nonresonant contributions from the
bare interface are negligible, the molecular resonances from the
adsorbate will determine the total polarization induced at a
surface. In the case of an IR vibrational resonance, the resonant
component of the polarizability is given by
Figure 1. Diagram of the experimental setup and cell design. The
infrared (ωIR) and the visible (ωvis) 532 nm beams are incident through
the high-index CCl4 phase.
NAn Mlm ∆p
ν
ν
(ø(R2)(ωIR))lmn
)
(3)
∑
ων - ωIR - iΓν
ν
product was recrystallized from ethanol and characterized by
FTIR and proton NMR. Sodium hexadecyl-d3 sulfate was
prepared by LiAlH4 reduction of palmitic-d3 acid in tetrahy-
drofuran. The resulting alcohol was then sulfonated as above.
Purity was determined by FTIR and NMR.
where N is the adsorbate surface density, An is the IR transition
ν
moment, Mlm is the Raman transition strength, ∆p is the
ν
population difference, ων is the transition frequency with a
damping constant of Γν for a specific transition, ν, and ωIR is
the frequency of the incident infrared beam. In the case of sum-
frequency vibrational spectroscopy, a tunable IR source is used
in combination with a fixed-frequency visible light source. As
the IR light is tuned through vibrational resonances, for example
the C-H stretch region of the spectrum, the intensity of the
emitted sum-frequency signal is dependent on the vibrational
modes of the molecules at the interface, resulting in a sum-
frequency vibrational spectrum. For a vibrational transition to
be sum frequency active it must satisfy the constraint of eq 3
which requires that the vibrational mode be both infrared and
Raman active.
Sum-Frequency Experiments. Tunable IR light was gener-
ated using a LiNBO3 optical parametric oscillator (OPO). The
OPO was pumped with the fundamental output of a Q-switched
Nd:YAG laser generating 1064 nm pulses at 10 Hz and a pulse
duration of 12 ns. Tunability through the desired wavelength
region was achieved by angle tuning the crystal. IR radiation
in the region 2700-3100 cm-1 with a pulse width of 6 cm-1 at
1-2 mJ was obtained over the entire spectral region. Calibra-
tion of the OPO was performed using a polystyrene sample.
The remainder of the 1064 nm YAG fundamental was frequency-
doubled in a KDP crystal to generate the visible 532 nm.
Total Internal Reflection. In addition to a strong resonant
contribution, the intensity is also dependent upon the Fresnel
factors for the input fields, fi, and the outgoing SF, ˜fsf, as shown
in eqs 1 and 2. The detected SF signal is dependent upon the
optical properties of the two bulk media through their refractive
indices. A comparison of the SF intensity in an internal and
external reflection geometry shows that an enhancement of
several orders of magnitude is expected for an internal reflection
geometry.20,22,33-36 For the case of internal reflection, the linear
and nonlinear Fresnel factors take on imaginary values at
incident angles above the critical angle. The local field intensity
exceeds those in an external reflection geometry due to the
formation of an evanescent wave at the interface. The maximum
in the SF intensity occurs at the critical angles for both incoming
fundamental beams. As reported in our initial communication,31
this technique is exploited here for the first time to measure
the sum-frequency vibrational spectrum of surfactants adsorbed
at the liquid-liquid interface. This has allowed the investigation
of this interface which has previously been inaccessible by
conventional SFG.
The sum-frequency experiments were performed using a
cylindrical quartz sample cell shown in Figure 1. In order to
achieve the desired optical geometry for total internal reflection,
both the visible and IR beams were directed onto the interface
through the CCl4, high-index phase, in a copropagating manner.
The IR was focused on the interface at an angle of 70°. The
visible 532 nm was collimated to a diameter of 1-2 mm and
incident on the interface at an angle of 66°. Laser powers used
were typically 1-2 mJ/pulse between 2800 and 3100 cm-1 and
5 mJ/pulse at 532 nm.
The sum-frequency signal was collected in reflection at an
angle of 66°. The resulting sum-frequency light was polariza-
tion selected by a broad-band Glann-Taylor polarizer. The
residual 532 nm light was removed by a combination of
absorptive, interference, and holographic filters. Detection was
accomplished using a PMT and gated electronics. Variation
of the input IR polarization was accomplished with a Soleil-
Babinet compensator and an IR polarizer. Polarization of the
visible light was selected with a half-wave plate. Data points
were collected every 2 cm-1 by averaging 200 pulses. The SF
spectra were corrected for Fresnel contributions and the intensity
variation of the infrared beam throughout the spectral region.
Experimental Section
The sample cell used in the SF experiments was cleaned with
MicroCleaner and Nochromix solution and rinsed with
NANOpure water, which had a resistivity of 17.9 Mohm cm.
The cell was dried in an oven at 100 °C to remove residual
water. The solvents CCl4 and D2O were introduced into the
sample cell and allowed to equilibrate. No detectable SF signal
was detected from the bare D2O/CCl4 interface. This measure-
ment also served as a means of determining the presence of
any surface-active contaminants. SDS was added to the cell
by removing an aliquot of the D2O and dissolving the desired
amount of SDS. The solution was then returned to the cell and
mechanically stirred. SF spectra were collected after allowing
the solutions to equilibrate for 20 min. Spectra were collected
for concentrations of SDS ranging from 0.1 to 10 mM.
Materials. D2O (99%) and HPLC grade CCl4 were pur-
chased from Aldrich. The CCl4 was further distilled in order
to remove any residual hydrocarbon compounds as determined
by transmission FTIR. D2O was shaken with purified CCl4 prior
to use and decanted. Sodium dodecyl sulfate (SDS) (99.8%)
was obtained from Aldrich and used as received. Sodium
hexadecyl sulfate (SHDS) was prepared by sulfonation of
1-hexadecanol (99%, Aldrich) with chlorosulfonic acid (99%,
Aldrich).37 The reaction was carried out by stepwise addition
of a solution of chlorosulfonic acid in 1,2-dichloroethane to a
solution of 1,2-dichloroethane containing hexadecanol. The
reaction was cooled in an ice bath to reduce unwanted side
reactions. Sodium bicarbonate was added, and the resulting
precipitate was filtered and rinsed with 1,2-dichloroethane. The

Surfactant Conformation and Order
J. Phys. Chem., Vol. 100, No. 18, 1996 7619
Figure 2. Sum-frequency vibrational spectrum of 5.0 mM SDS at the
D2O-CCl4 interface. The polarizations used were p for visible and p
for infrared. The sum-frequency output light was collected with no
polarization selection. The solid lines represent a fit to the spectra
using a combination of Gaussian and Lorentzian functions for each
peak.
Figure 4. Sum-frequency vibrational spectrum of (a) hexadecyl sulfate,
sodium salt, and (b) hexadecyl-d3 sulfate, sodium salt. The spectra
were obtained with p-polarized visible, p-polarized infrared, and no
polarization selection of the output sum-frequency beam. Resonances
at 2872 and 2960 cm-1 are assigned to the methyl symmetric and
asymmetric stretch modes, respectively. The solid lines represent a
fit to the spectra using a combination of Gaussian and Lorentzian
functions for each peak.
Figure 3. Molecular structure of the surfactant sodium dodecyl sulfate
(SDS).
TABLE 1: SFG Peak Frequencies (cm-1) of SDS Adsorbed
at the D2O-CCl4 Interface and the Measured Infrared and
Raman Band Frequencies (cm-1) for Alkyl Chains38-43
Spectral assignments for the SF spectrum of SDS shown in
Figure 2 are made as follows. The peaks at 2872 and 2960
cm-1 are assigned to the terminal methyl symmetric and
asymmetric stretch modes, respectively. These peaks cor-
SFG
infrared
Raman
d+(π)
2850
2871
d+(0)
r+
respond to the r+ at 2872 cm-1 and r- at 2963-2953 cm-1
,
2848 CH2 SS
2872 CH3 SS
2850
2873
r+
which are seen in both the IR and Raman spectra of
polymethylene.38-42 The peak at 2848 cm-1 is assigned to the
CH2 symmetric stretch (SS) resonance of the hydrocarbon chain.
This correlates well with the IR spectrum of an extended
polyethylene chain which shows a band at 2850 cm-1 for the
d+(π) mode.38-42 The corresponding Raman spectrum displays
a band at 2850 cm-1 for the symmetric stretch (d+(0)).38-42 The
assignment of the peaks in Figure 2 which are centered at 2896
and 2925 cm-1 are somewhat more complicated to make. As
shown in Table 1, there is a difference between the IR and
Raman spectra for these modes. The peak centered at 2896
cm-1 is assigned to the symmetric methylene Fermi resonance
(FR). This peaks appears in the IR (d+(π)FR) at 2898-2904
cm-1 and in the Raman (d+(0)FR) at 2890 cm-1 of polymeth-
ylene.39,40 It is also possible that there is some degree of a
Raman asymmetric methylene mode at 2890 cm-1 (d-(0)) that
is contributing intensity. The peak at 2925 cm-1 is assigned to
the methylene asymmetric stretch (AS), in agreement with the
value observed in the IR spectrum (2928 cm-1),38 although
previous Raman studies have assigned a peak at 2890 cm-1 to
the methylene asymmetric stretch.39 It is possible that intensity
from the methylene Fermi resonance (d+(0)FR) may contribute
to this peak. Previous SFG studies have assigned a peak at
2930-2940 cm-1 to a Fermi resonance resulting from interac-
tion of an overtone of the CH3 bending mode with the methyl
symmetric stretch.23,24,44-47 However, this peak is not observed
in the present SF spectrum of SDS.
2896 CH2 FRa 2890
d+(π)FR 2898-2904 d+(0)FR
2890
d-(0)
2925 CH2 ASa 2928
2960 CH3 AS
a See text.
d-(π)
2920-2930 d+(0)FR
2952-2964 r-
2953-2962 r-
Interfacial Tension Measurements. Interfacial tension
measurements were performed using the Wilhelmy plate
method. The equipment consisted of an electronic balance
equipped with a platinum plate with a resolution of 4 µN/m.
Measurements were performed at the interface of an aqueous
solution of SDS and CCl4 which was allowed to equilibrate for
1 h prior to making a measurement. The neat H2O/CCl4
interface was used as a reference in determining the interfacial
pressure.
Results and Discussion
Spectral Analysis. Deriving information about molecular
conformation and order from the spectral information requires
accurate spectral assignments. Such spectral assignments of the
SF spectra are aided by comparison with the features of the
corresponding infrared and Raman spectra for similar com-
pounds. As was shown in eq 3, a transition must be both IR
and Raman active to be SF allowed. Figure 2 displays the SF
vibrational spectrum obtained from SDS at the D2O/CCl4
interface. The molecular structure of SDS is shown in Figure
3. The spectrum was obtained with a bulk concentration of
5.0 mM with the IR and visible light p polarized. No
polarization selection was made for the SF output. The peak
assignments for SDS are summarized in Table 1. Also shown
for comparison are the corresponding infrared and Raman
transition frequencies for polymethylene and long-chain
alkanes.38-43
These assignments have been further verified by deuteration
studies. Figure 4a,b displays the SF spectra obtained from
sodium hexadecyl sulfate (SHDS) and hexadecyl-d3 sulfate,
sodium salt, for which the terminal methyl group has been
deuterated. The most striking change between parts a and b of
Figure 4 is the disappearance of the methyl peaks at 2872 and
2960 cm-1 for the symmetric and asymmetric modes, respec-

7620 J. Phys. Chem., Vol. 100, No. 18, 1996
Conboy et al.
Figure 6. A plot of the interfacial pressure of sodium dodecyl sulfate
at the water-carbon tetrachloride interface versus the aqueous con-
centration of sodium dodecyl sulfate. Measurements were obtained
using the Wilhelmy plate method. Interfacial pressures were calculated
by subtracting the value of the interfacial tension of the neat D2O-
CCl4 interface. The solid line is shown as a guide to the eye.
polarization only the normal component of the vibrational
transition moments will contribute to the observed intensity,
and similarly for sps polarization, only the components parallel
to the surface will be active. For the terminal methyl group,
which possesses C3V symmetry, the transition dipoles of the
symmetric and asymmetric stretches are orthogonal to each
other. For the case of ssp polarization, in which the C3V axis
of the terminal methyl is assumed to lie along the surface
normal, only the symmetric stretch peak will be apparent with
no appreciable intensity due to the asymmetric stretch. How-
ever, if the symmetry axis of the terminal methyl lies parallel
to the surface, the CH3 symmetric stretch will have no intensity,
and the asymmetric stretch will dominate.
Figure 5. Sum-frequency vibrational spectra of sodium dodecyl sulfate
with (a) s-polarized output, s-polarized visible, and p-polarized infrared
and (b) s-polarized output, p-polarized visible, and s-polarized infrared.
Spectra were obtained with a solution of 5.0 mM SDS in D2O. The
solid lines represent a fit to the spectra using a combination of Gaussian
and Lorentzian functions for each peak.
tively. Comparison of the spectra in Figure 4a,b also shows
that no detectable peak arising from the methyl Fermi resonance
is observed. There has been some discrepancy in the past as to
the exact assignment of the CH2 asymmetric stretch and the
Fermi resonance of the methyl stretch which both occur in the
region of 2920-2940 cm-1 31,46
The assignments obtained from
.
these deuteration studies establish that this peak is dominated
by the methylene asymmetric mode with the possibility that
intensity from the CH3 Fermi resonance may also contribute
slightly. This assignment is in contrast to SFG spectra from
self-assembled on metal substrates44 and Langmuir-Blodgett
films of cadmium arachidate on fused silica.23,48 where this
Fermi resonance is pronounced. The assignments for SDS at
the D2O-CCl4 interface are consistent with SF spectra obtained
from other liquid surfaces such as alcohols and alkanes at the
water-air interface.49
The local symmetry of the CH2 hydrocarbon backbone plays
an important role in the determination of peak intensities of
the CH2 resonances. Under the dipole approximation for SFG,
little contribution from methylene resonances should be observed
for a system of well-ordered (all-trans) hydrocarbon chains.18
It is well-known that although there is an odd number of
methylene groups, essentially no signal arises from the meth-
ylenes in a well-ordered system.18,47 The presence of a large
methylene intensity therefore suggests a significant number of
gauche defects in the hydrocarbon chain which relax the local
symmetry constraint. The terminal methyl, which possesses
both IR- and Raman-active modes, is by nature in a noncen-
trosymmetric environment regardless of chain conformation.
Conformational information can be obtained from the relative
intensity of the methyl peaks which reflect the orientational
average of the ensemble with respect to the surface normal. For
example, polarization dependence of the symmetric methyl
stretch has been used in the past as a means of determining the
average tilt angle of the hydrocarbon chain in these systems.18
Polarization Studies. The spectra of SDS shown in Figure
5a,b are obtained with the polarization combinations of ssp (s-
polarized SF, s-polarized visible, p-polarized IR) and sps (s-
polarized SF, p-polarized visible, s-polarized IR), respectively.
The spectra in Figure 5a,b have been corrected for Fresnel
contributions over the spectral region displayed. With ssp
The spectra in Figure 5a of SDS at a concentration of 5.0
mM for ssp polarization (p-polarized IR) shows a pronounced
CH3 symmetric stretch at 2872 cm-1 with negligible intensity
of the CH3 asymmetric resonance at 2960 cm-1. This suggests
that the terminal methyl group is pointing primarily along the
surface normal which is also substantiated by Figure 5b. For
the sps polarization (s-polarized IR) displayed in Figure 5b, the
symmetric stretch of the terminal methyl is completely absent,
and the most prominent feature in this spectra is the asymmetric
CH3 mode. These results, which are found for the entire
concentration range of SDS examined (0.1-10 mM), unam-
biguously support the conclusion that the terminal methyl group
is oriented normal to the surface. Any conclusions regarding
the absolute orientation or orientational average of the alkyl
chain, however, are complicated by the fact that there appears
to be a large number of gauche defects in the alkyl chains as
seen by the strong symmetric methylene peak of Figures 2 and
5a.
Concentration Studies. To investigate surface coverage
effects on surfactant conformation, SF spectra were collected
for a series of aqueous surfactant concentrations. Spectra were
taken from 0.1 to 10.0 mM bulk SDS concentration in D2O.
Surface coverages were estimated using interfacial pressure
measurements with the results shown in Figure 6. The minimum
tension achieved at 2.0 mM bulk concentration is taken to reflect
a surface coverage of one monolayer. Above this concentration,
the surface pressure does not change.
As was discussed previously, the methyl and methylene
region of the infrared spectrum is especially indicative of the
conformational ordering of the surfactant alkyl chain. The
methyl and methylene region of the vibrational spectra of SDS
for 0.1 and 10.0 mM using ssp polarization is shown in Figure
7. In this figure, strong C-H resonances are observed for both
the methyl and methylene symmetric stretch. Even at a
relatively low surface coverage, a high signal-to-noise ratio is

Surfactant Conformation and Order
J. Phys. Chem., Vol. 100, No. 18, 1996 7621
Figure 8. A plot of the ratio of methyl symmetric stretch peak intensity
to the methylene symmetric stretch peak intensity versus bulk aqueous
concentration.
Figure 7. Sum-frequency vibrational spectrum of the symmetric stretch
region for sodium dodecyl sulfate at the D2O-CCl4 interface. Spectra
are shown for 0.1 (b) and 10 mM (9) SDS aqueous concentration.
The spectra were taken using s-polarized output, s-polarized visible,
and p-polarized infrared. The solid lines represent a fit to the spectra
using a combination of Gaussian and Lorentzian functions for each
peak.
The ratio of the methyl to methylene peak intensity, for the
range of concentrations examined, is plotted in Figure 8. At
low concentrations, a relatively low ratio is observed which
increases significantly with increased adsorption at the interface.
This ratio reaches a maximum at one monolayer coverage,
corresponding to 2.0 mM aqueous concentration. These results
suggest that the chains become more ordered (fewer gauche
defects) as more surfactant adsorbs to the interface. An ordering
of the alkyl chains may be caused by a closer packing of
molecules on the surface. The favorable energetics of the
anionic head groups occupying interfacial sites will further force
the hydrophobic alkyl chains to be driven away from the
interface as surface sites become limited. This process will
necessitate a more ordered configuration with more trans bonds
within the alkyl chain. Such ordering processes have been
observed at the air-liquid and solid-air interface.18,50 At the
air-liquid interface, molecules forming insoluble monolayers
can be physically restricted to a small surface area by mechanical
compression.51 Previous spectroscopic investigations by SFG
of Langmuir monolayers of pentadecanoic acid on water in fact
show a close correlation between molecular area and alkyl chain
disorder.18 Other studies by FTIR on solid substrates50 have
shown similar results for assembled molecules on solid supports.
observed, establishing the very high sensitivity of this technique.
Two features in this data warrant further discussion. First, the
presence of a strong methylene peak necessarily leads to the
conclusion that a number of gauche defects exist in the chain
throughout the concentration region examined, corresponding
to a small fraction of a monolayer to one full monolayer. This
result shows that the surfactant molecules at the D2O-CCl4
interface do not attain a well-ordered all-trans configuration at
even full monolayer coverage. This result is on contrast to many
other monolayer systems in which full monolayer coverage
corresponds to an all-trans close-packed configuration for the
alkyl chains.18,24,50 However, at the liquid-liquid interface,
solvent interaction between the carbon tetrachloride and the alkyl
portion of the surfactant molecules likely weakens the chain-
chain interactions which are responsible for the all-trans ordering
of the alkyl backbone during packing induced ordering. Other
processes responsible for ordering at an interface such as
physisorption or chemisorption of the adsorbate onto a solid
substrate are not possible at a liquid-liquid interface. In
addition, mechanical compression of an insoluble monolayer
is not possible because the adsorbate is in dynamic equilibrium
with the bulk solution.
The second interesting aspect of the data in Figure 7 is the
change in relative intensity of the symmetric methyl and
methylene peaks with bulk concentration. A decrease in the
methyl to methylene intensity ratio is observed with increased
concentration, suggesting a change in the conformation of the
alkyl chain. A variation in this ratio indicates a change in
orientation of the average component of the methyl and
methylene transition dipole along the surface normal. The
polarization data presented previously have shown that the C3V
axis of the methyl group lies primarily along the z axis, and
this holds true for all concentrations examined. This observation
suggests that no net change in methyl orientation occurs, but
rather a change in methylene chain conformation is causing the
observed variation in the CH3/CH2 ratio. With the infrared light
beam polarized along the z axis, an increase in the ratio most
likely reflects a decrease in the CH2 symmetric stretch intensity,
indicating that the alkyl chains are assuming a conformation
that contains fewer gauche defects, not simply a change in tilt
angle or orientation along the surface normal. This allows one
to use the methyl/methylene symmetric stretch intensity ratio
as a indicator of the relative order within the chains for various
concentrations. A low methyl to methylene ratio reflects a
relatively small amount of order in the alkyl chain consisting
of a relatively large amount of gauche defects in the alkyl chain.
Another important result of these concentration studies
pertains to information on the extent of the spatial region that
is probed by this technique. The evanescent wave generated
at the interface by the incoming light field decays into the low-
index medium exponentially with distance, generating a large
electric field in the low-index material up to microns from the
interfacial region. It is often unclear what the actual depth of
the sum-frequency active region at an interface between two
centrosymmetric materials is. Recent SFG experiments49 have
confirmed that the majority of SF signal arises from interfacial
region and that no measurable amount of signal arises from the
bulk material. This work confirms these conclusions for this
system. Changes in the SF spectral features with concentration
closely follow changes in interfacial pressures, suggesting that
SF is probing only the interfacial region. This is consistent
with the highly surface specific description of SFG.
Disorder in alkyl chains induced by laser heating has been
observed in a study of Langmuir-Blodgett films on CaF2 using
SFG.52 To rule out the possibility that this heating may
contribute to observed ordering effects, spectra were taken with
and without probing the methylene stretch excitation which is
thought to contribute to chain disorder. Identical spectra were
observed, showing that laser heating is not causing disorder in
this particular system. A plausible explanation for this is that
the fluid nature of the monolayer at the liquid-liquid interface
allows the monolayer to restructure and dissipate heat more
effectively than that of a solid-air interface.

7622 J. Phys. Chem., Vol. 100, No. 18, 1996
Conboy et al.
(5) Takenaka, T. T.; Nakanaga, T. J. Phys. Chem. 1976, 80, 475.
(6) Tian, Y.; Umemura, J.; Takenaka, T. Langmuir 1988, 4, 1064.
(7) Takenaka, T. Chem. Phys. Lett. 1978, 55, 515.
Conclusions
With the use of TIR SFG the first vibrational spectrum of
the simple alkyl surfactant sodium dodecyl sulfate at the D2O-
CCl4 interface has been obtained. By performing these experi-
ments in a TIR geometry, the sensitivity of the surface selective
technique of SFG is increased considerably. Infrared and
Raman spectra for similar compounds were used to assign the
C-H vibrational resonances of the SF spectra of SDS. Spectral
assignments were further verified by deuteration studies. The
SF spectra of sodium hexadecyl sulfate was compared with
sodium hexadecyl-d3 sulfate in order to determine the contribu-
tion from the terminal methyl group to the SF spectra of SDS.
Deuteration results reveal that, unlike monolayers adsorbed to
solid substrates, minimal contribution from the methyl Fermi
resonance is observed.
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Acknowledgment. The authors gratefully acknowledge the
skilled assistance of Jennifer Gage and the facilities provided
by Professor Bruce Branchaud for the synthesis of hexadecyl-
d3 sulfate, sodium salt. In addition, the authors express
appreciation to Ted Hinke for the construction of the cell. The
starting material palmitic-d3 acid was graciously provided by
Professor Frederick Dahlquist. Funding is gratefully acknowl-
edged from NSF (CHE 9416856) and the Office of Naval
Research.
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