PERGAMONElectrochimica Acta 44 (1998) 165±169

Monitoring of molecular behavior of a chemical oscillation
system at a liquid/liquid interface using a time-resolved
quasi-elastic laser scattering method
Satoshi Takahashi, Isao Tsuyumoto, Takehiko Kitamori, Tsuguo Sawada *Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113, Japan
Received 30 September 1997; received in revised form 8 February 1998

Abstract
A time-resolved quasi-elastic laser scattering (QELS) technique was applied to monitor the molecular numberdensity at a water/nitrobenzene (W/NB) interface of a W±NB±W three phase chemical oscillation system. We
successfully monitored the oscillations of interfacial molecular number density, electric potential, and turbidity
simultaneously by a newly developed chemical oscillation Ð QELS combination system. We observed for the ®rst
time that monolayer of the surfactants covers the interface almost throughout the oscillation and that the oscillation
occurs simultaneously with a catastrophic disintegration of the monolayer after the increase of molecular numberdensity. We proposed a new model for the chemical oscillation based on the obtained molecular description. # 1998
Elsevier Science Ltd. All rights reserved.
Keywords: Laser scattering; Chemical oscillation; Liquid/liquid interface; CTAB

1. Introduction
Nonlinear phenomena have attracted much attention
in chemistry since they are fundamentally important inphysicochemical systems such as chemical sensors, biological membranes, and solvent extractions. Chemical
oscillation systems have been studied extensively as
part of the nonlinear phenomena from a variety ofviewpoints, including physicochemical interests in theirchaotic behavior and as chemical approaches to biological phenomena [1]. Yoshikawa and Matsubara [2]were the ®rst to report a chemical oscillation system
made up of three layers. It consisted of a nitrobenzene
layer (NB) separating two aqueous layers, one ofwhich contained a surfactant and an alcohol (W1), andthe other a saccharide (W2). With this system, it ispossible to observe a sustained and spontaneous oscillation of the electric potential dierence between the* Corresponding author.

two aqueous layers. The oscillation has spike-shaped
patterns, which resemble those observed in biomembranes such as neuron systems. It is desirable to clarify
the mechanism of the oscillation at a molecular level
because this would provide great insight into excitability in biological systems and facilitate the design of
new oscillation systems suitable for biomimetic sensors.
A variety of studies have been carried out to elucidate the mechanism of the chemical oscillation. As
mentioned above, the electrical potential oscillation
has been ®rst observed for the system: Pt electrode/
W1/NB/W2/(Ag/AgCl electrode). It is natural to thinkthat the phenomena at the electrode/liquid interfaces
are not essential for the electrical oscillation becausesuch oscillations have not been observed for electrode/liquid interfaces. Thus a number of studies have been
performed to clarify the dynamic behavior of theliquid/liquid interfaces. The oscillation patterns of electric potential and pH have been examined with various
chemical species in the liquid phases [3, 4], and the iontransfer potential has been determined using voltam-0013-4686/98/$19.00 # 1998 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 1 3 - 4 6 8 6 ( 9 8 ) 0 0 1 6 5 - 0

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metric techniques [5]. Two types of oscillation models
have been proposed so far. Yoshikawa andMatsubara [2] proposed that the dominant factor forthe electric potential oscillation was the behavior of
surfactant cations at one of the interfaces in the system, and that changes, like spike patterns, were attributable to the disappearance of an interfacialmonolayer. On the other hand, Maeda et al. [5]reported that the oscillation amplitude corresponded
to the dierence of the ion transfer potential,
suggesting the dominant factor was molecular transfer
across the interface. Although no explanation for thechemical oscillation systems has been established sofar, there is no doubt that molecular behavior at theliquid/liquid interface plays an important role in thechemical oscillations. Therefore, in order to elucidate
their mechanism, monitoring of the solute molecules at
the interface is highly desirable.
We have reported an application of a quasi-elastic
laser scattering (QELS) method to monitoring theinterfacial molecular number density of lipid and surfactant systems [6±9]. The interfacial molecular number
density can be monitored as a function of capillary
wave frequency by using the QELS, because this frequency is a function of interfacial tension, which
depends on molecular number density at the interface.
Since the QELS technique does not require any mechanical probe be inserted into the interface and it does
not give any chemical and mechanical disturbances, it
should be one of the most suitable ways to probe themolecular behavior at the interface of the chemical oscillation in situ.
The QELS has been mainly used to measure liquid/gas interfaces and has been scarcely applied to liquid/
liquid interfaces. Furthermore, most of the previous
studies using the QELS have been performed on staticsystems [10, 11]. In order to apply the QELS to themonitoring of the molecular behavior at chemicallyoscillating liquid/liquid interfaces, time required fordata acquisition has to be shortened, unlike when observing static systems. Owing to our developments inthe optical equipment and the electronic signal processing, we are able to obtain a QELS spectrum with a 1
s data acquisition time. In this paper, we present theresults of coincident measurements of the QELS, theelectric potential, and the turbidity in the water±nitrobenzene±water (W±NB±W) chemical oscillation system. Since the behavior of the surfactants has beenreported to play an important role in theoscillation [12], we focus on the molecular behavior at
the W1/NB interface and propose a model for molecular behavior at the interface based on the experimental
results.

2. ExperimentalThe newly developed QELS system for coincident
measurements is shown in Fig. 1. Since experimental
details of the QELS equipment were described in ourprevious reports [6±9], only a brief outline follows
here. A beam from a He±Ne laser passes through aliquid/liquid interface, at which the beam is quasi-elastically scattered by the interfacial capillary waves. The
scattered light is accompanied by a Doppler shift, andthe frequency shift is the same as the capillary wave
frequency. The frequency shift is measured using an
optical heterodyne technique. The incident beam isdivided in front of the cell using a diraction grating,and one of the diracted beams is mixed with the scattered light to generate an optical beat. The optical beatis detected by a photodiode and the beat frequency isrecorded by a digital spectrum analyzer to obtain apower spectrum. The peak position of the power spectrum is the same as the beat frequency, i.e. the capillary wave frequency.
In order to achieve coincident measurements of theQELS and electric potential, the QELS equipment wasoptimized regarding several points. The measurements
were performed in the transmission con®guration,Fig. 1. Schematic illustration of the setup for the chemical oscillation system including the optical path of the QELS experiment.

S. Takahashi et al. / Electrochimica Acta 44 (1998) 165±169

making the system more stable to low frequency vibrations. The scattering angle was ®xed at 9.5 mrad,
for which the power spectrum of the signal had sucient intensity and separation from the low frequency
noises due to the external vibrations. In order to suppress the external mechanical vibrations, the optics
and chemical oscillation cell were mounted on a suspended optical platform.
The chemical oscillation cell was a beaker-shapedcell with a glass tube inside, which was supported witha clamp on a stand (Fig. 1). First a nitrobenzene solution (NB) of picric acid was poured into the cell. The
bottom of the tube was set at the NB level. Next, anaqueous solution (W2) of glucose was poured into thecell, along the outside of the tube and an aqueous solution (W1) of cetyltrimethylammonium bromide(CTAB) and 1-butanol was poured into the glass tube.
In this way, the two aqueous phases were successfully
separated by the NB phase, forming the W1±NB±W2
three-phase system. After the formation of the phases,
Ag/AgCl electrode and Pt electrode were immersed in
W2 and W1, respectively. The inner tube was adjusted
to minimize the distance between the end of the tube
and the W1(W2)/NB interfaces.
The data storage system of the QELS was activatedaccording to demand by triggering with a personal
computer, which monitors the electric potential of thechemical oscillation system via an analogue/digital (A/D) converter board connected to the electrodes in thechemical oscillation cell.
Concentrations of the substances in the liquid phases
were 5 mM CTAB plus 0.5 M 1-butanol for W1, 0.1
M glucose for W2, and 5 mM picric acid for NB. All
the chemicals were purchased and reagent grade. Sincepurchased picric acid is sold wet for safety, it wasdried in a desiccator before dissolving into nitrobenzene. The other substances were used without furtherpuri®cation.
As it has been reported that molecular transfer fromW1 to NB plays an important role in the oscillation [2±4], the QELS and electric potential measurements werecarried out on the W1±NB side. Turbidity measurements were also performed simultaneously by monitoring the intensity of the transmitted He±Ne laser light
across the W1 and NB phases during the chemical oscillation.
3. Results and discussion
The time-courses of the capillary wave frequency
and the electric potential during the chemical oscillation processes are shown in Fig. 2. The capillary
wave frequency and the electric potential weremeasured simultaneously using trigger signals from thepersonal computer. The electric potential was moni-167

Fig. 2. Time-courses of the electric potential between the two
aqueous phases and capillary wave frequency.

tored continuously as shown by the solid line in the®gure. The circles on the solid line indicate the trigger
points of the QELS measurement. The obtained capillary wave frequencies are depicted as triangles, and thedotted line is for an eye-guide. Spike-shaped oscillations are typically observed in the electric potential.
There is a marked tendency that the peaks of the electric potential almost correspond to the minimum pointof the capillary wave frequency, indicating that theinterfacial tension changes simultaneously with theelectric potential. It should be noted that the electric
potential re¯ects the change of the whole interfacialarea, whereas the QELS method monitors only a partof the interface which is equal to the beam diameter(0.68 mm in this case). The slight discrepancy between
the changes in the potential and frequency is probably
due to the dierence of the observed areas of respective parameters.
It is obvious that the electric potential oscillation
corresponds with the oscillation of the interfacial molecular number of CTAB. Since the capillary wave frequency is proportional to the square root of theinterfacial tension which re¯ects the molecular number
density of surfactants, the minimum points of thecapillary wave frequency correspond to the maximum
points of molecular number density of CTAB. In orderto estimate the interfacial molecular number density,
we used our previous results on the relation between
capillary wave frequency and the molecular coveragewith dipalmitoylphosphatidylcholine (DPPC) at the W/
NB interface [6]. We have monitored phase transitionsof DPPC in four states: gaseous state, liquid expandedstate, liquid condensed state, and oversaturated (stacking state). In the liquid condensed and the oversaturated states, the observed capillary wave frequency wasless than ca. 10 kHz. Based on the assumption that
DPPC and CTAB occupy approximately the same area

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S. Takahashi et al. / Electrochimica Acta 44 (1998) 165±169

Fig. 3. Time-courses of the electric potential between the two
aqueous phases and the intensity of the transmitted laser
beam.

per mole, the interfacial molecular number density of
the CTAB in the present study is as large as that of
the DPPC in the liquid condensed and the oversaturated states. It is noteworthy that such a large numberof molecules, forming a monolayer, are always present
at the interface, even during the spike-shaped oscillation.
Moreover, we found a great change in the interfacial
turbidity during the oscillations. The time-courses oftransmitted laser intensity through the W1±NB phases
and the simultaneously measured electric potential arepresented in Fig. 3. Intensity of the transmitted light isa maximum at the beginning of the spike-shaped oscillation of electric potential, and it drops to a minimum
around the end of the oscillation of electric potential.
Since the CTAB micelles are the only light-scattering
substance in the W1±NB phases, the change in turbidity is due to the change in the aggregation states of
CTAB molecules. The maximum point in the transmitted light intensity is the point at which the concentration of the CTAB micelles becomes the lowest. Thisindicates that the micelles in the W1 phase collapse toform monolayer at the interfacial regions just before
the beginning of the pulse-shaped oscillation of theelectric potential.
The above results cannot be explained by a simple
conventional assumption that the electric potential oscillation is due to the change of the density of the electrical double layer formed by the CTAB monolayer atthe W1/NB interface [2]. Thus, we propose a newmodel based upon our experimental results. The dominant factor for the oscillation is mass transfer acrossthe W1/NB interface, and the change in the electric potential is due to the transfer of charged species.
Although presently we cannot conclude directly which
species transfer across the interface during the chemical
oscillation, CTA cations composing micelles arereasonable candidates, as deduced from the averaged
capillary wave frequency during the whole period of
the oscillation and the results of turbidity measurements. The oscillation of the electrical potential which
occurs simultaneously with the changes of both theturbidity and the capillary wave frequency is considered to be due to the transfer of CTA cation fromthe interfacial region to the W1 phase and the NB
phase as micelles. Therefore, the decrease in the turbidity which occurs before the electrical oscillation is dueto the adsorption of CTA cations onto the interface
from the micelles in the W1 phase. In a word, the oscillation can be explained by the formation and rupture of interfacial monolayer.
In Fig. 4 we schematically illustrate the oscillation
based on our model. It consists of three stages: (1) The
surfactant molecules form both micelles and a monolayer at the interfacial region. (2) A catastrophic collapse of micelles occurs around the interface and themolecules are adsorbed as a monolayer onto the interface. (3) A rupture of monolayer occurs when attaining
a critical value of molecular number density. Somemolecules at the interface are transferred to the NB
phase, forming molecular aggregates such as reversed
micelles. The interfacial molecular number density
returns to that in stage (1) and molecular transfer
across the interface pauses. At present we think ourresults suggest that the chemical oscillation is generated by the ¯uctuation between the transfer rate fromthe W1 phase to the interface and that from the interface to the NB phase.
In summary, we applied the QELS method to monitoring molecular number density at the W/NB interface of a chemical oscillation system. We observed aninterfacial molecular transfer which occurs simultaneously with the generation of spike-shaped peaks inthe electric potential and we proposed a model basedon the obtained molecular description of the chemicaloscillation. We believe that the time-resolved QELS

Fig. 4. Schematic illustration of the oscillation based upon
our present model.

S. Takahashi et al. / Electrochimica Acta 44 (1998) 165±169

method will be a powerful tool for monitoring thedynamics at liquid/liquid interfaces.
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