ANALYTICAL SCIENCES NOVEMBER 2000, VOL. 16
2000 © The Japan Society for Analytical Chemistry1199

Effect of Organic Phase on Dynamic and Collective Behavior ofSurfactants at Liquid/Liquid Interfaces by a Time-Resolved
Quasi-Elastic Laser-Scattering Method
Zhenhui ZHANG,* Takehiko KITAMORI,* Tsuguo SAWADA,*† and Isao TSUYUMOTO***Department of Applied Chemistry, Graduate School of Engineering, The University of Tokyo,
7-3-1 Hongo, Bunkyo, Tokyo 113–8656, Japan**Department of Environmental Systems Engineering, Kanazawa Institute of Technology,
7-1 Ohgigaoka, Nonoichi, Ishikawa 921–8501, Japan

A time-resolved quasi-elastic laser-scattering method was applied to monitor the dynamic and collective behavior ofmolecules of an anionic surfactant, sodium dodecyl sulfate (SDS), around the critical micelle concentration (cmc) atseveral water/chlorinated hydrocarbon interfaces. The time courses of the capillary wave frequencies after injection of
the SDS solutions into the water phase were investigated. We found that when the injected concentration is above thecmc, the relative number density of SDS molecules decreases with an increase in the bulk aqueous concentration at all theexamined interfaces. We also found that the relative number density of SDS molecules adsorbed onto the interface
increases with a decrease in the permittivity of the organic phase. Our results suggested that the adsorption of SDS
molecules was affected by the permittivity of the organic phase.
(Received February 17, 2000; Accepted August 16, 2000)

Introduction
The study of liquid/liquid (L/L) interfaces is receiving growing
attention, because the transport or reactions of molecules at L/Linterfaces are fundamentally important for understanding many
chemical processes in the fields of electrochemistry, biology,
separation science, and environmental studies.
Thecharacterization of these chemical processes is often achievedby spectroscopically monitoring the adsorption, orientation,organization, and reactivity of submonolayer quantities of
surfactant molecules adsorbed to the interface.1–22 Second
harmonic generation (SHG) and sum frequency generation(SFG) techniques have been used to probe L/L interfaces andhave the potential to provide interface dynamic properties.5,11,16
Neutron specular reflectivity measurements have been used to
study the effect of surfactants on the roughness of L/Linterfaces.6 Many researchers have investigated liquid surfaces
by measuring capillary waves.23–31 Since capillary waves reflect
the fundamental properties of the surface or interfacial region,
much effort has been made to observe them and to analyze theirspectra. The theoretical understanding of these waves is wellestimated in hydrodynamics, and light scattering from thewaves is also widely used to monitor the L/L interfaces.2 In ourprevious papers,13,21,22 we reported that a developed timeresolved quasi-elastic laser-scattering (QELS) method was apowerful tool for studying molecular dynamics at L/Linterfaces. The QELS method monitors the change of thefrequencies of the capillary waves, which are spontaneously
generated by thermal fluctuation at L/L interfaces. The QELS
† To whom correspondence should be addressed.
E-mail: ksawada@mail.ecc.u-tokyo.ac.jp

method has good interface selectivity for overcoming the
problem of interference by bulk phases, because the capillary
wave is a characteristic phenomenon at the interface.
Chemical processes, such as formation of monolayers ortransformation of chemical species at the interface, causechanges of Gibbs free energy there.32,33 In turn, these Gibbs freeenergy changes cause changes of interfacial tension. So it has
been assumed that these chemical processes can be monitoredby observing changes of interfacial tension. As we described
previously,13,21 the capillary wave frequency is a function ofinterfacial tension and the change in the interfacial tension
depends on the change in the number density of surfactant
molecules at the interface; then the QELS method allows
observation of the dynamic change of L/L interfaces such as theformation of a lipid monolayer,13 or the adsorption of surfactant
molecules.21,22 Compared with conventional interfacial tension
measurements, the QELS method has the obvious advantage ofbeing able to probe interfaces between two bulk phases without
any mechanical perturbation.
It is suitable for in situ
monitoring of interfaces. Furthermore, owing to its improved
time resolution, each power spectrum can be obtained in 1 ms to
1 s, so the method can be used to monitor the dynamic change
in number density of surfactants at L/L interfaces in a realenvironment. The simplicity of the measurement is also anadvantage.
Using the QELS method, we have successfully monitored the
dynamic and collective behavior of three kinds of surfactant
molecules with different charges at the water/nitrobenzene(W/NB) interface, and found that the behavior of thesemolecules differs from each other.21,22 Particularly for anionicsodium dodecyl sulfate (SDS) molecules, when the injectedconcentration is above the critical micelle concentration (cmc),
the relative number density decreases with an increase in the

1200

ANALYTICAL SCIENCES NOVEMBER 2000, VOL. 16

bulk aqueous concentration, indicating that a possible local
change in aggregation state of SDS molecules occurs, such as
the formation of micelles or reversed micelles at the interface.22
In the previous work,21,22 we focused on the effect of the ionicnature of surfactants on the dynamic and collective behavior. In
the present work, we focus on the effect of the organic phase,such as its dielectric constant, which is an important
macroscopic property of a liquid at an interface. We can expectthat the behavior of surfactant molecules will reflect the uniqueproperties or structures of the interfaces. We look at how theinterfacial behavior of SDS molecules changes around the cmc
by monitoring the change in number density of SDS molecules
at several water/chlorinated hydrocarbon interfaces, and at how
the property of the organic phase affects the dynamic andcollective behavior of SDS molecules.

Experimental
The principle of the QELS measurement has been described indetail previously.13,21 The experimental setup of the QELS
method is shown in Fig. 1. The signal/noise ratio (S/N) was
improved by using a YAG laser (CrystaLaser, Model GCL025s, 532 nm) with 20 mW power instead of the He-Ne laser
we used previously. The noise that originates from thefluctuation of a gas laser was decreased. Furthermore, the YAG
laser has a single longitudinal mode which is better suited to ourmeasurement system; the multi longitudinal modes of the HeNe laser resulted in a small S/N ratio in the measurement region.
Owing to this improvement, we can apply the QELS method to
not only the W/NB interface but to various other interfaces aswell. The power of the incident laser beam was adjusted byusing a neutral density filter positioned in front of the laser.
There exist capillary waves with different wavelengths at L/Linterfaces, but one of them can be selected by a grating. In
detail, we used a transmission diffraction grating having a
spacing of 0.33 mm, the fifth order diffracted beam was selected
for the measurement. Thus, the wavelength of the observed
capillary wave in this work was 66 µm.21 As shown in Fig. 1,
the beam is incident on the grating, and passes through thebottom of the sample cell. The cell is made of quartz glass andhas an optically flat bottom. In this work, an optical heterodyne
technique was used to detect the capillary wave frequency. The
incident laser beam normal to the interface is quasi-elasticallyscattered by the capillary wave with a Doppler shift. The
scattered beam is optically mixed with the diffracted beam fromthe grating to generate an optical beat in the mixed light. Thebeat frequency obtained here is the same as the Doppler shift,i.e. the capillary wave frequency. The optical beat of the mixed
light is measured by the photodiode (Hamamatsu Photonics
S1290). The signals are Fourier transformed and saved by adigital spectrum analyzer (Takeda Riken TR9404).
An L/L interface was prepared by slowly adding 10 mL ofpurified water (from a Millipore Milli-Q system) to 10 mL oforganic solvent in the sample cell. To observe how the dynamic
and collective behavior of surfactant molecules at the interfacechanges around the cmc, an SDS (C12H25OSO3Na) solution (0.5mL; Kanto Chemical Co., Inc.; first grade) was injected into thewater phase with a microsyringe. The concentrations of theinjected SDS solution (C) were in the range of 1 – 20 mM; thebulk aqueous concentrations were in the range of 0.05 – 1 mM.
The cmc of an SDS solution above which surfactant molecules
form micelles, is 8.1 mM at 20˚C.14 To observe the effect of theorganic phase on the interfacial behavior of SDS molecules,
three kinds of organic solvents: dichloromethane (CH2Cl2)

Fig. 1 Schematic diagram of the experimental setup for the QELS
method: ND filter, neutral density filter; O, organic phase; W, waterphase; S, microsyringe; F and f are light frequencies. Inset: A typicalpower spectrum for the capillary wave at the neat W/CCl4 interface at
20˚C.

(Kanto Chemical Co., Inc.; special grade; 99.0% purity),
chloroform (CHCl3) (Kanto Chemical Co., Inc.; special grade;
99.0% purity), and carbon tetrachloride (CCl4) (Kanto ChemicalCo., Inc.; special grade; 99.5% purity) were chosen. We
monitored the change of capillary wave frequencies at theW/CH2Cl2, the W/CHCl3, and the W/CCl4 interfaces after the
injection of SDS solution using the QELS method.

Results and Discussion
To examine the dynamic behavior of SDS molecules at theinterfaces, we investigated the time courses of the capillary
wave frequencies after the injection of different amounts of
SDS solution into the water phase. The time just prior to theinjection is regarded as 0 s, a power spectrum like that shown in
the inset of Fig. 1 was obtained every 3 s. The capillary wave
frequencies at different interfaces are plotted as a function oftime in Fig. 2. An anomalous temporary decrease in capillary
wave frequency was observed at all the interfaces examined.
The capillary wave frequency decreases on injection, and after
attaining its minimum value, it increases gradually. As we havediscussed previously,21 the initial decrease in the capillary wave
frequency means an increase in the molecular number density at
the interface that can be attributed to the adsorption of
surfactant molecules onto the interface from the water phase,whereas the later increase is due to the gradual decrease of themolecular number density by such phenomena as desorption
from the interface to the organic phase. At the minimum
frequency, we assume that a transient equilibrium between thewater phase and the interface is approximately established. On
the basis of this assumption, we estimated the relative molecularnumber density of the surfactants at the interface from theminimum values of the capillary wave frequencies.
Experiments with equilibrated phases were performed, but theinterfacial tension only decreased with the increase inconcentration. From the measurement of the time course ofcapillary wave frequencies, further dynamical or structural
information can be expected by the analysis of the minimum

ANALYTICAL SCIENCES NOVEMBER 2000, VOL. 16

1201

Fig. 3 Relationships between the minimum value of the capillary
wave frequency and the bulk aqueous concentration. Experimental
for W/CHCl3 andfordata are shown as
for W/CH2Cl2,
W/CCl4 interfaces.

Fig. 2 Time courses of the capillary wave frequencies after
injection of SDS solution into (a) W/CH2Cl2, (b) W/CHCl3 and (c)W/CCl4 interfaces. The concentrations of the injected solutions (C)are shown, along with the bulk aqueous concentrations (Ceq).

Fig. 4 Relationships between the relative number density of SDS
and the bulk aqueous concentration. Experimental data are shown as
for W/CCl4, for W/CHCl3 and for W/CH2Cl2 interfaces. The
solid lines are a guide to the eye.

value, such as surfactant transport across the interface or
possibly dynamic changes in state of molecular aggregation at
the interface.
Relationships between the minimum values of the capillary
wave frequencies and the bulk aqueous concentrations of SDS
are shown in Fig. 3. We found that, when the injectedconcentration C is below the cmc, the frequency decreases withan increase in concentration. When the C is above the cmc, thefrequency increases with an increase in concentration. Fromthese results, we calculated the interfacial tension by using theLamb’s equation34 (Eq. (1)), where f is the capillary wave
frequency, γ is the interfacial tension, ρU is the density of theupper phase, ρL is the density of the lower phase, and k is thewavenumber of the capillary wave.

the width could not be used for the analysis. In the present case,only the peak frequency is used for the analysis. Since thesquare of the capillary wave frequency is proportional to theinterfacial tension, which is approximately inversely
proportional to the number density of surfactant molecules at
the interface,13,21 details can be added to the above outline byestimating the molecular number density of surfactants at theinterface.
The relationships between the relative molecular number
densities and the bulk aqueous concentrations are shown in Fig.
4. Here, the relative number density is a ratio of the number
density of SDS at the examined interfaces and the number
density of SDS at the W/CCl4 interface. It was found that, when
the injected concentration C is below the cmc (8.1 mM at 20˚C),
the relative number densities of SDS increase with an increasein concentration at all the interfaces examined. This is due tothe decrease in interfacial tension caused by the adsorption of
SDS molecules onto the interface. When the C is above thecmc, it is interesting to note that the relative number densities of
SDS decrease with an increase in concentration. Using theresonant TIR-SFG, Richmond and coworkers observed a
reduction in ordering of the chain of the SDS at the D2O/CCl4interface at higher concentrations.14 Here, we thought thedecrease in the number density, i.e. the increase in theinterfacial tension in our systems could be due to the local
change in aggregation state or to the reorientation of SDS
molecules at the interface caused by the electrostatic

1/2
γ  k3/2
1  ———f= —
2π  ρU + ρL 

(1)

Although Eq. (1) neglects the effect of surface viscosity, it
gives a good fit to the observed relationship between thefrequency and wavenumber of capillary waves at a liquidsurface.29,35–37 In this work, we did not discuss the shape, or thewidth of the spectrum, because they are determined by the
instrumental function as well as by the interfacial
viscosity.24,38,39 In our study, however, the instrumental function
was undetermined because of the transmitting opticalconfiguration employed for the observation of the interface, and1202

Fig. 5 Relationships between the relative number density of SDS
and the dielectric constant of the organic phase. The dielectric
constants are 2.2 for CCl4, 4.8 for CHCl3, and 7.8 for CH2Cl2 at 20˚C.

interactions of the head groups or the chain conformation.
To clarify the effect of the organic phase on the behavior of
SDS molecules at the interface, we investigated the relationshipbetween the relative number density of the SDS molecules at
the interface and the dielectric constant of the organic phase byusing the data when the C is 4 mM. As shown in Fig. 5, therelative number density increases with a decrease in the
dielectric constant of the organic phase. This means that theadsorption of SDS molecules at the interface depends on someproperties, such as the permittivity of the organic phase, thatwould affect the interaction between the hydrocarbon tails of
surfactant molecules and the organic solvent molecules, and thetail-tail interaction of surfactant molecules as well. Fromcomparison of our results with those reported by Grubb et al.,3
we can consider that the increase in relative number densitywith a decrease in the dielectric constant of the organic phasereflects the increased orientation of the surfactant molecules that
depends strongly on the property of the organic phase.

Conclusions
We applied the time-resolved QELS method to study the
dynamic and collective behavior of surfactant molecules atseveral water/chlorinated hydrocarbon interfaces. We foundthat the number density of SDS molecules adsorbed onto theinterface increased with a decrease in the dielectric constant of
the organic phase. For all the examined interfaces, when theinjected concentration was above the cmc, the relative number
density of SDS molecules decreased with an increase in theconcentration. Our results suggested that SDS monomolecules
coexisted with some molecular aggregates at the interface. This
may reflect the electrostatic interaction of the headgroups of the
SDS molecules.

References
1. T. Takenaka and T. Nakanaga, J. Phys. Chem., 1976, 80,475.
2. B. B. Sauer, Y. L. Chen, G. Zografi, and H. Yu, Langmuir,
1988, 4, 111.
3. S. G. Grubb, M. W. Kim, Th. Rasing, and Y. R. Shen,
Langmuir, 1988, 4, 452.
4. Y. Tian, J. Umemura, and T. Takenaka, Langmuir, 1988, 4,1064.

ANALYTICAL SCIENCES NOVEMBER 2000, VOL. 16
5. Y. R. Shen, Nature, 1989, 337, 519.
6. L. T. Lee, D. Langevin, and B. Farnoux, Phys. Rev. Lett.,
1991, 67, 2678.
7. A. J. Bell, J. G. Frey, and T. J. Vandernoot, J. Chem. Soc.,
Faraday Trans., 1992, 88, 2027.
8. L. T. Lee, D. Langevin, E. K. Mann, and B. Farnoux,
Physica B, 1994, 198, 83.
9. D. C. Clark, A. R. Mackie, P. J. Wilde, and D. R. Wilson,
Faraday Discuss., 1994, 98, 253.
10. R. N. Ward, D. C. Duffy, and P. B. Davies, J. Phys. Chem.,
1994, 98, 8536.
11. R. M. Corn and D. A. Higgins, Chem. Rev., 1994, 94, 107.
12. C. D. Bain, P. B. Davies, and R. N. Ward, Langmuir, 1994,
10, 2060.
13. S. Takahashi, A. Harata, T. Kitamori, and T. Sawada, Anal.
Sci., 1994, 10, 305.
14. M. C. Messmer, J. C. Conboy, and G. L. Richmond, J. Am.
Chem. Soc., 1995, 117, 8039.
15. H. Watarai and Y. Saitoh, Chem. Lett., 1995, 283.
16. K. B. Eisenthal, Chem. Rev., 1996, 96, 1343.
17. I. Benjamin, Chem. Rev., 1996, 96, 1449.
18. J. M. Kovaleski and M. J. Wirth, J. Phys. Chem., 1996,
100, 10304.
19. H. J. Paul and R. M. Corn, J. Phys. Chem. B, 1997, 101,4494.
20. J. C. Conboy, M. C. Messmer, and G. L. Richmond, J.
Phys. Chem. B, 1997, 101, 6724.
21. Z. H. Zhang, I. Tsuyumoto, S. Takahashi, T. Kitamori, and
T. Sawada, J. Phys. Chem. A, 1997, 101, 4163.
22. Z. H. Zhang, I. Tsuyumoto, T. Kitamori, and T. Sawada, J.
Phys. Chem. B, 1998, 102, 10284.
23. M. A. Bouchiat and J. Meunier, J. Physique, 1971, 32, 561.
24. D. Langevin, J. Chem. Soc., Faraday Trans. 1, 1974, 70, 95.
25. D. Byrne and J. C. Earnshaw, “Lasers in Chemistry”, 1977,
Elsevier, New York, 29.
26. S. Hard and H. Lofgren, J. Colloid Interface Sci., 1977, 60,
529.
27. D. Byrne and J. C. Earnshaw, J. Colloid Interface Sci.,
1980, 74, 467.
28. D. Langevin, D. Chatenary, and J. Meunier, “Surfactants in
Solution”, ed. K. L. Mittal and B. Lindman, 1984, Plenum,
New York.
29. J. C. Earnshaw and R. C. McGivern, J. Phys. D, 1987, 20,82.
30. V. Thominet, C. Stenvot, and D. Langevin, J. Colloid
Interface Sci., 1988, 126, 54.
31. K. Sakai and K. Takagi, Langmuir, 1994, 10, 257.
32. J. Mingins, J. A. G. Taylor, B. Pethica, C. M. Jackson, andT. Y. T. Yue, J. Chem. Soc., Faraday Trans., 1982, 78,
323.
33. G. J. Hanna and R. D. Noble, Chem. Rev., 1985, 85, 583.
34. H. Lamb, “Hydrodynamics”, 6th ed., 1945, Dover, NewYork.
35. V. G. Levich, “Physicochemical Hydrodynamics”, 1962,
Chap. 11, Prentice Hall, New York.
36. M. Sano, M. Kawaguchi, Y. L. Chen, R. J. Skarlupka, T.
Chang, G. Zografi, and H. Yu, Rev. Sci. Instrum., 1986, 57,1158.
37. I. Tsuyumoto, N. Noguchi, T. Kitamori, and T. Sawada, J.
Phys. Chem. B, 1998, 102, 2684.
38. D. McQueen and I. Lundstrom, J. Chem. Soc., Faraday
Trans. 1, 1973, 69, 694.
39. J. C. Earnshaw, R. C. McGivern, A. C. McLaughlin, and P.
J. Winch, Langmuir, 1990, 6, 649.