10284

J. Phys. Chem. B 1998, 102, 10284-10287

Observation of the Dynamic and Collective Behavior of Surfactant Molecules at a Water/
Nitrobenzene Interface by a Time-Resolved Quasi-Elastic Laser-Scattering MethodZhenhui H. Zhang, Isao Tsuyumoto, Takehiko Kitamori, and Tsuguo Sawada*
Department of Applied Chemistry, School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan
ReceiVed: July 13, 1998; In Final Form: October 7, 1998

A spectroscopic technique of time-resolved quasi-elastic laser-scattering measurement was applied to monitor
the dynamic and collective behavior of anionic surfactant sodium dodecyl sulfate (SDS) and neutral surfactant
Triton X-100 molecules around their critical micelle concentrations (cmc’s) at a water/nitrobenzene liquid/
liquid interface. The time courses of the capillary-wave frequencies after the injection of the surfactant solutions
into the water phase were investigated. We found that the dynamic and collective behavior of these twokinds of surfactant molecules at the interface differ from that of the cationic surfactant cetyltrimethylammoniumbromide we reported previously. Our results suggested that for SDS the monomolecules at the interface coexist
with some molecular aggregates such as micelles at the interface; for Triton X-100, the monolayer at theinterface was disrupted and some molecular aggregates, such as micelles, were formed at the interface.

Introduction
Amphiphilic surfactant molecules which contain both ahydrophilic and a hydrophobic group can be used as a uniquekind of probe molecule at liquid/liquid interfaces and thus aremeaningful for the study of interface properties. Moreover,interest in the behavior of surfactants is growing because theyplay an increasingly important role in various chemical andbiological areas.1-4
Although linear and nonlinear spectroscopic and spectrochemical studies of surfactants at liquid/liquid interfaces havebeen performed,5-25 little has been reported on the dynamic andcollective behavior of surfactants at the interface. Without adoubt, the observation of the dynamics of collective behavior
is one of the most important experiments which can be undertaken to understand interfaces and surfactants.
We have developed a time-resolved quasi-elastic laserscattering (QELS) method and have reported that it is a powerfultool for studying the dynamics at liquid/liquid interfaces.26,27
The QELS method monitors the frequencies of capillary waves,which are spontaneously generated by thermal fluctuation atliquid/liquid interfaces. The interfacial tension acts as a restoring
force on the fluctuation, and it excites an interfacial wave, whichis called a capillary wave or ripplon. The relationship between
the capillary-wave frequency and the interfacial tension follows
Lamb’s equation as we have described it in our previous paper.27
Because the capillary-wave frequency is a function of interfacial
tension and the change in the interfacial tension depends onthe change in the number density of surfactant molecules at
the interface, we can obtain various kinds of information on
the dynamic change of liquid/liquid interfaces such as theformation of a lipid monolayer,26 the change in number densityof surfactant molecules,27 and the evaluation of interfacial
density profiles.28 Compared with conventional interfacial
tension measurements, the QELS method has the advantage ofprobing interfaces between two bulk phases without any
mechanical disturbance. It is suitable for in situ monitoring of* To whom all correspondence should be addressed.

interfaces. In our work, the in situ, noncontact monitoring of
the interface is of great importance, because the observation ofcollective and dynamic behavior of surfactant molecules would
be affected by mechanical disturbance. 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 liquid/liquid interfaces in areal environment. Furthermore, this method has good interface
selectivity for overcoming the problem of interference by bulk
phases, because the capillary wave is a characteristic phenomenon at interfaces and the frequency of this wave can be detectedby an optical heterodyne technique. The purpose of this work
is to monitor the change in number densities of two kinds ofsurfactant molecules at a water/nitrobenzene (W/NB) interface
and to study how the interfacial molecular behavior of thesesurfactants changes around the critical micelle concentration(cmc) using the QELS method we have developed.
Experimental Section
The principle and experimental setup of the QELS measurements has been described previously.26,27 In this work, a liquid/
liquid interface was prepared by slowly adding 10 mL ofpurified water (from a Millipore Milli-Q system) to 10 mL ofnitrobenzene (Kanto Chemical Co., Inc.; special grade; 99.5%
purity) in a sample cell. The cell is made of quartz glass andhas an optical flat bottom, which is indispensable to maintaining
good reproducibility of the experimental results. To observe thedynamic and collective behavior of surfactant molecules withdifferent charges, an anionic surfactant sodium dodecyl sulfate(SDS) solution (0.5 mL; Kanto Chemical Co., Inc.; first grade)or a neutral surfactant Triton X-100 solution (0.5 mL; Kanto
Chemical Co., Inc.; first grade) was injected into the water phasewith a microsyringe. The injected concentrations (C) of the SDS
and Triton X-100 solutions were in the ranges of 1-20 and
0.075-1.5 mM, respectively; the bulk aqueous concentrations(Ceq) were in the ranges of 0.05-1 and 0.00375-0.075 mM.
The cmc’s of an SDS solution and a Triton X-100 solution above
which surfactant molecules form micelles are 9.8 and 0.2 mM

10.1021/jp982993h CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/13/1998

Behavior of Surfactant Molecules

J. Phys. Chem. B, Vol. 102, No. 50, 1998 10285

Figure 1. Typical power spectra for capillary waves at the W/NBinterface (a) without SDS molecules and (b) 10 s after injection of aSDS solution (0.5 mL, 10 mM) into the water phase.

at 25 °C. All experiments were performed at room temperature.
We intended to study how the dynamic and collective behaviorof the surfactant molecules changes around the cmc’s.
Results and Discussion
Typical power spectra for capillary waves at the W/NB
interface are shown in Figure 1a without SDS molecules andin Figure 1b 10 s after the injection of an SDS solution (0.5mL, 10 mM) into the water phase. As we discussed previously,26,27 the peaks appearing around 5-13 kHz are the beatfrequencies, i.e., the capillary-wave frequencies. In Figure 1,
the peak of the capillary-wave frequency shifts from 13 to 5.5
kHz on the injection of an SDS solution. This is due to thedecrease in interfacial tension caused by the increased numberdensity of surfactant molecules at the interface.
To examine the dynamic change in the number densities ofsurfactant molecules at the interface in detail, we investigated
the time courses of the capillary-wave frequencies after theinjection of the SDS and Triton X-100 solutions into the waterphase. The time just prior to the injection was regarded as 0 s,
and a power spectrum was obtained every 3 s. The capillarywave frequencies after the injection of the SDS and Triton
X-100 solutions are plotted as a function of time in Figure 2.
From Figures 1 and 2, it is apparent that the time resolution (3s) of the present QELS method is sufficiently high to detect
the change in the number density of surfactant molecules at
the interface. In Figure 2, an anomalous temporary decrease incapillary-wave frequency is observed at almost all of theconcentrations of SDS and Triton X-100 we used. The capillarywave frequency decreases rapidly on injection, and after
attaining its minimum value, it increases gradually. The initialsudden decrease in the frequency means an abrupt increase in
the molecular number density at the interface that can be
attributed to the rapid adsorption of surfactant molecules onto
the W/NB interface from the W phase whereas the later gradualincrease is due to the gradual decrease of the molecular number
density by such phenomena as desorption from the W/NB
interface to the NB phase. It is clear from the figure that theadsorption rate onto the W/NB interface from the W phase ismuch higher than the desorption rate from the W/NB interface
to the NB phase. Thus, at the minimum frequency, it isreasonable to assume that the equilibrium between the W phaseand the W/NB interface is approximately established and thatthe mass transfer from the W/NB interface to the NB phase isnegligible. On the basis of this assumption, we estimated therelative molecular number densities of SDS and Triton X-100
at the interface from the minimum value of the capillary-wave
frequencies in Figure 2.
The relationship between the minimum values of the capillary-wave frequencies and the bulk aqueous concentrations ofSDS and Triton X-100 is shown in Figure 3. Because the square

Figure 2. Time courses of the capillary-wave frequencies after injectionof (a) SDS and (b) Triton X-100 solutions into the interface. The
concentrations of the injected solutions (C) are shown along with thebulk aqueous concentrations (Ceq).

Figure 3. Relationship between the minimum value of the capillarywave frequency and the bulk aqueous concentration for (a) SDS and(b) Triton X-100.

of the capillary-wave frequency is proportional to the interfacialtension, which is approximately inversely proportional to thenumber density of surfactant molecules at the interface,26,27
details can be added to the above outline by estimating themolecular number density of surfactants at the interface.
The relationship between the relative number density of SDS
molecules and the bulk aqueous concentration is shown in Figure
4a. We found that when the C is below the cmc (9.8 mM at 25°C), the number density increases with an increase in concentration. The experimental data are in agreement with the Langmuir

10286 J. Phys. Chem. B, Vol. 102, No. 50, 1998

Figure 4. Relative interface coverage of (a) SDS and (b) Triton X-100
at the W/NB interface as a function of bulk aqueous concentration.
Experimental data are shown as squares, the solid lines are the calculated
Langmuir adsorption isotherms, and the dotted lines are a guide to theeye.

adsorption isotherm, suggesting that the SDS molecules areadsorbed onto the interface as a monolayer. The adsorptionenergy of the SDS molecules onto the interface is estimated atca. -21.9 kJ/mol from the experimental data. The free-energychange, ∆G, when 1 mol of methylene groups (-CH2-) moves
from an aqueous solution to a water/oil interface is estimatedat -3.4 kJ/mol using Troube’s rule.29 Thus, the free-energychange of SDS molecules when they move from the W phaseto the interface should be higher than -37.6 kJ/mol. However,
the experimental value, -21.9 kJ/mol, is lower than this. This
discrepancy suggests that about 60% of the methylene groups
in SDS molecules is transported to the NB phase. This may bea reflection of molecular orientation, because the SDS does notform a well-ordered monolayer at the W/NB interface.
When the C is above the cmc, it is interesting to note thatthe number density of SDS molecules at the interface decreases
with an increase in the concentration. This may be due to the
local change in aggregation state at the interface from monolayer
to molecular aggregates. The molecular aggregates observed
here are assumed to be micelles or reversed micelles, but further
investigation is necessary to verify this. Furthermore, thisgradual decrease above the cmc suggests that the equilibrium
between interfacial molecules and the molecular aggregates isestablished and the change in the concentration causes the shiftof the equilibrium. It could be thought that the interfacialconcentration of SDS is higher than the bulk aqueous concentration, so it is possible for SDS molecules to form molecularaggregates at the interface while the bulk aqueous concentration
is below the cmc. We also find that when the C is above thecmc, the collective behavior of SDS at the W/NB interface isvery different from that of cationic surfactant cetyltrimethylammonium bromide (CTAB) which we reported previously.27
CTAB forms an oriented monolayer whereas SDS does not form
well-ordered layers (C > cmc). Using the resonant TIR-SFG,
Richmond and co-workers observed a reduction in ordering of
the chain of the SDS at the D2O/CCl4 interface at higher
concentrations,13 and they also reported that the alkyl chains of
the cationic surfactants DTAC and DAC possess the fewest

Zhang et al.
gauche defects whereas the anionic surfactants SDS and DDS
display more disorder in the hydrocarbon chains at similar
surface concentrations.23 Similar results were reported byothers30,31 for SDS adsorption onto hydrophobic substrates fromwater. On the basis of our experimental results and those of theothers, we thought that when the C of SDS is above the cmc,
the increase in capillary-wave frequency, i.e., the decrease in
number density, may be due to the change in aggregation stateof SDS molecules caused by the chain conformation or theelectrostatic interactions of the headgroups. It is generally said
that the formation of reversed micelles in the bulk NB phase isquite unlikely because of the high polarity of NB molecules.
However, it is reasonable to consider that the behavior of SDS
molecules at the interface differs from that in the bulk NB phasebecause W and NB molecules can mix at the interfacial region
and because the properties of the interface differ from those of
the bulk liquid.
By estimating the relative molecular number density of Triton
X-100 at the interface, we find that when the C is below thecmc (0.2 mM at 25 °C), the experimental data in Figure 4b arein good agreement with the calculated Langmuir adsorption
isotherm. This indicates that the adsorbed Triton X-100
molecules are confined to a monolayer. When the C is above
the cmc, a slight decrease in the number density was observed.
It is reasonable to consider that the slight decrease is due to thechange in aggregation state at the interface such as the formationof micelles whereas the latter slow increase is due to the increaseof molecular aggregates at the interface. This result suggeststhat the phase transition from a monolayer to molecularaggregates occurs at a certain concentration unlike the case withSDS.
The molecular behavior at the interface may be affected by
the ionic nature, ion pair formation, solvent polarity, or the chain
conformation of surfactants. Through comparison of the measurements of the behavior of different kinds of surfactants at
the liquid/liquid interface, the interfacial structure is also
expected to be deduced.
Conclusions
In summary, we observed the dynamic and collective behaviorof SDS and Triton X-100 at a W/NB interface by monitoring
the time courses of the capillary-wave frequency using the QELSmethod. These results suggested that below the cmc SDS
molecules were adsorbed onto the interface, forming a monolayer; above the cmc, the monolayer was disrupted and somemolecular aggregates such as micelles or reversed micelles wereformed. It could also be thought that below the cmc Triton
X-100 molecules were adsorbed onto the interface, forming amonolayer; above the cmc, the monolayer was disrupted andsome molecular aggregates such as micelles were formed, andwith further increases in concentration, the number of micelles
increased. The behavior of surfactant molecules described above
may be affected by the ionic nature, ion pair formation, solvent
polarity, or the chain conformation of surfactants. Throughcomparison of the measurements of the behavior of different
kinds of surfactants at the liquid/liquid interface, the interfacial
structure is also expected to be deduced.
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