Langmuir 2000, 16, 6597-6600

6597

Role of the Liquid/Liquid Interface in a Phase-Transfer
Catalytic Reaction As Investigated by in Situ
Measurements Using the Quasi-Elastic Laser Scattering
MethodYoshiko Uchiyama, 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

Isao Tsuyumoto
Department of Environmental Systems Engineering, Kanazawa Institute of Technology,
7-1 Ohgigaoka, Nonoichi, Ishikawa 921-8501, Japan
Received October 8, 1999. In Final Form: May 15, 2000
The relationship between the concentration of a phase-transfer catalyst, tetrabutylammonium bromide(TBAB), and the rate of its cyclic reaction was investigated using the quasi-elastic laser scattering (QELS)
method. TBAB forms an ion pair (TBA+C6H5O-) with sodium phenoxide (C6H5ONa) at the interface. Thision pair moves from the interface to the organic phase where it reacts with diphenylphosphoryl chloride
and produces triphenyl phosphate. During this reaction, tetrabutylammonium chloride, which is generated
simultaneously with the product, returns to the water phase. An increase in the concentration of thephase-transfer catalyst (TBAB) dose not promote the cyclic reaction. The QELS study indicates that thisis because some of the TBAB catalyst adsorbs on the interface and disturbs mass transfer of the otherchemical species (TBA+C6H5O-). Interfacial coverage of the catalyst should be taken into account to findthe optimum concentration of the catalyst. This paper shows for the first time that the interfacial catalyst
promotes the reaction and at the same time disturbs mass transfer of the other chemical species.

Introduction
The liquid/liquid interface plays important roles invarious processes such as phase-transfer catalysis, solvent
extraction and ion-selective electrode reactions. Amongthem, phase-transfer catalysis has attracted much attention because the catalyst activates the reaction bytransferring across the liquid/liquid interface and brings
about high product yield and high selectivity by therecurred cycle.1-5 Tetrabutylammonium bromide (TBAB)
is a phase-transfer catalyst, which behaves as a surfactant
and activates organic synthesis reactions.6,7 For example,
a 1 mM TBAB solution lowers the interfacial tension ofwater and nitrobenzene by 1 mN/m. The concentration of
TBAB usually used in the phase-transfer catalysis isaround 1 mM, which has been determined empirically.
High concentrations of phase-transfer catalysts do notalways bring about high product yield; i.e., they have an
optimum concentration. The reason of this has not beenclarified so far. To clarify the mechanism that determines
the optimum concentration of the catalyst, in situ measurement of the interfacial molecular number density at
various concentrations is highly desirable.
Many studies have been reported on the behavior ofphase-transfer catalysts by conventional batch methods* To whom correspondence should be addressed. E-mail:ksawada@mail.ecc.u-tokyo.ac.jp.
(1) Starks, C. M. J. Am. Chem. Soc. 1971, 93, 195.
(2) Weber, W. P.; Gokel, G. W. Phase Transfer Catalysis in Organic
Synthesis; Springer-Verlag: Berlin, 1977.
(3) Starks, C. M.; Liotta, C. Phase Transfer Catalysis; Academic
Press: New York, 1978.
(4) Dehmlow, E. V.; Dehmlow, S. S. Phase Transfer Catalysis; Verlag
Chemie: Weinheim, 1983.
(5) Kimura, Y.; Regen, S. L. J. Org. Chem. 1982, 47, 2493.
(6) Mason, D.; Magdassi, S.; Sasson, Y. J. Org. Chem. 1990, 55, 2714.
(7) Uchiyama, Y.; Tsuyumoto, I.; Kitamori, T.; Sawada. T. J. Phys.
Chem. B 1999, 103, 4663.

using NMR, UV-visible spectrophotometry, and gas
chromatography, etc., where part of the solution is sampledfrom the bulk phase.8-15 These analyses of behavior havebeen mainly based on information from bulk phase.
However, to clarify the behavior, direct measurements of
the interface are essential. There are a number of studies
using interfacial tension measurements that have provided useful information on the interface; in them, theinterfacial tension was measured by the Wilhelmy method,
the pendant drop method, etc., which perturb the interface,causing a certain amount of change in the interfacial
tension.16-19 These methods are thus still indirect. However, in situ measurements of liquid/liquid interfaces canprovide direct information on interfacial molecular number
density during the whole reaction.
We have developed a quasi-elastic laser scattering(QELS) method and have reported on the dynamics ofmass transfer of surfactants at the water/nitrobenzene(8) Krishnakumar, V. K.; Sharma, M. M. Ind. Eng. Chem. Process
Des. Dev. 1985, 24, 1297.
(9) Motoi, M.; Shimamura, K.; Shimamura, C.; Muramoto, S.; Kanoh,S.; Suda, H.. Bull. Chem. Soc. Jpn. 1989, 62, 2553.
(10) Asai, S.; Nakamura, H.; Tanabe, M.; Sakamoto, K. Ind. Eng.
Chem. Res. 1993, 32, 1438.
(11) Asai, S.; Nakamura, H.; Tanabe, M.; Sakamoto, K. Ind. Eng.
Chem. Res. 1994, 33, 1687.
(12) Iizawa, A.; Yamada, Y.; Ogura, Y.; Sato, Y. Polym. Phase Transfer
Catal. 1994, 2057.
(13) Wang, M. L.; Ou, C. C. Ind. Eng. Chem. Res. 1994, 33, 2034.
(14) Wu, H. S.; Meng, S. S. J. Chem. Eng. Jpn. 1996, 29, 1007.
(15) Ma, G.; Freiser, H.; Muralidharan, S. Anal. Chem. 1997, 69,2827.
(16) Chida, Y.; Watarai, H. Bull. Chem. Soc. Jpn. 1996, 69, 341.
(17) Sawarkar, C. S.; Juvekar, V. A. Ind. Eng. Chem. Res. 1996, 35,2581.
(18) Nahringbauer, I. Langmuir. 1997, 13, 2242.
(19) Ferrari, M.; Liggieri, L.; Ravera, F.; Amodio, C.: Miller, R. J.
Colloid Interface Sci. 1997, 186, 40.

10.1021/la9913295 CCC: $19.00 © 2000 American Chemical Society
Published on Web 07/14/2000

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Uchiyama et al.

(W/NB) interface.20-24 This method features a nonperturbative in situ measurement of liquid/liquid interfaces
and an interfacial tension measurement. Besides, thismethod allows real time measurements with time resolution of 1 s, though this advantage was not taken in thepresent study. We focused on a phase-transfer catalytic
reaction system with TBAB, whose reaction scheme iswell-known and operating conditions are mild. In ourprevious study, we presented that the liquid/liquidinterface was the important reaction place to form the ion
pairs (TBA+C6H5O-) by observing one process in the phasetransfer catalysis.7 The aim of the present study is to clarify
the mechanism determining the optimum concentrationof the phase-transfer catalyst (TBAB). We present therelative molecular number densities at the W/NB interface
in the phase-transfer catalytic reaction and discuss therelationship between the concentration of phase-transfer
catalyst, TBAB, and the rate of its cyclic reaction.

Figure 1. Phase-transfer catalytic reaction scheme between
C6H5ONa and DPC in the W/NB system.

Experimental Section
Principle. The principle of the QELS method has alreadybeen described.20-26 In brief, an incident laser beam normal tothe interface is scattered quasi-elastically with a Doppler shiftby the capillary wave. The scattered beam is optically mixed
with a local beam produced by a diffraction grating to generate
an optical beat in the mixed light. The obtained beat frequency
is the same as the Doppler shift, i.e., , the capillary wave
frequency.

f)

(

)

1/2
1
γ
k3/2
2π Fw + Fo

(1)

The capillary wave frequency f is approximately expressed by
Lamb’s equation,27 where Fw is the density of the water phase,
Fo is the density of the organic phase, and k is the wavenumberof the capillary wave. The capillary wave frequency f is in
proportion to the square root of the interfacial tension γ, while
the interfacial number density Γ is inversely proportional to theinterfacial tension γ by Gibb’s equation. The relative change ofthe number density Γ can be estimated from the change in thecapillary wave frequency. The error of interfacial tension andmolecular number density is within 10%. This was estimatedfrom the deviation between the conventional direct measurement
and that from the QELS method. It is well-known that theinterfacial tension depends on methodologies of the measurement.28,29 The accidental error of the QELS method is within 1%
and low enough to estimate relative change. We have reported
previously about this. 21, 22
Apparatus. A schematic diagram of the experimental setup
is shown in Figure 2. The beam from a YAG laser (CrystaLaser,model GCL-025S, 532 nm, 20 mW) passes through a transmitting
diffraction grating in front of the cell. The cell is made of quartz
glass and has an optically flat bottom. After passing through thesample, one of the diffracted beams, which is mixed with thescattered light, is selected by the aperture in front of thephotodiode (Hamamatsu Photonics S1290). Signals from the(20) Takahashi, S.; Harata, A.; Kitamori, T.; Sawada, T. Anal. Sci.
1991, 7, 645.
(21) Takahashi, S.; Harata, A.; Kitamori, T.; Sawada, T. Bunseki
Kagaku 1991, 40, 761.
(22) Takahashi, S.; Harata, A.; Kitamori, T.; Sawada, T. Anal. Sci.
1994, 10, 305.
(23) Zhang, Z.; Tsuyumoto, I.; Takahashi, S.; Kitamori, T.; Sawada.
T. J. Phys. Chem. A 1997, 101, 4163.
(24) Zhang, Z.; Tsuyumoto, I.; Takahashi, S.; Kitamori, T.; Sawada.
T. J. Phys. Chem. B 1998, 102, 10284.
(25) Lofgren, H.; Neuman, R. D.; Scriven, E. L.; Davis, T. H. J. Colloid
Interface Sci. 1984, 98, 175.
(26) Hard, S.; Neuman, R. D. J. Colloid Interface Sci. 1987, 115, 73.
(27) Lamb, H. Hydrodynamics, 6th ed.; Cambridge University
Press: London, 1932.
(28) Mann, J. A.; Baret, J. F.; Dechow, F. J.; Hansen, R. S. J. Colloid
Interface Sci. 1971, 37, 14.
(29) Sano, M.; Kawaguchi, M.; Chen, Y. L.; Skarlupka, R. J.; Chang,T.; Zografi, G.; Tu, H. Rev. Sci. Instrum. 1986, 57, 1158.

Figure 2. Relationship between the amount of transferred
ion pairs and the initial concentration of C6H5ONa estimatedby using absorption spectrophotometry (µ ) 0.2).
photodiode are Fourier-transformed and saved by a digitalspectrum analyzer (Sony Tektronix Co., model 3056). In thepresent study, the wavelength of the observed capillary wave
was 6.6 × 10-3 cm.
The number of transferred ion pairs across the W/NB interface
was estimated using absorption spectorophotometry (Shimadzu
UV-1600PC). Samples were withdrawn from the water phase in
the equilibrated cyclic reaction.
Sample Preparation. The reaction scheme between sodium
phenoxide (C6H5ONa) and diphenylphosphoryl chloride (DPC)in the W/NB system with TBAB is shown in Figure 1. At the
beginning of the reaction, TBAB reacts with C6H5ONa to form
TBA+C6H5O- at the interface below 50 mM. This had been
deduced from the results that the ratio of the equilibrium
concentrations of the two reactants was not unity.7 Thus, we set
the TBAB concentration below 50 mM. This ion pair moves into
the organic phase where it reacts with DPC to produce triphenyl
phosphate ((C6H5O)3PO). During this reaction, TBA+Cl- is also
formed and transported into the water phase to react again withC6H5ONa. TBAB activates the production of triphenyl phosphateby circulating between the two phases.
In this study, we focused on the role of the liquid/liquid interface
in the phase-transfer catalytic reaction and investigated adsorption behavior of the ion pair at various concentrations of TBAB
(1, 25 mM), DPC (0-1 mM), C6H5ONa (0-100 mM), and tetrabutylammonium chloride (TBAC: 0-100 mM). A liquid/liquidinterface was prepared by adding 10 mL of mixed aqueous solution
of TBAB (Kanto Chemical Co.), C6H5ONa (Aldrich), NaOH, and
TBAC (Kanto Chemical Co.) to 10 mL of nitrobenzene containing
DPC (Kanto Chemical Co) in a quartz cell. Ultrapure water (fromMillipore Milli-Q system) was used for all sample preparations.
NaOH was added to adjust the ionic strength to 0.2 and to preventthe production of phenol or benzoic acid. All chemicals werereagent grade and used without further purification.

Results and Discussion
The amount of transferred ion pair TBA+C6H5O- wasestimated from the decrease in the C6H5ONa concentrationin the water phase; i.e., the difference between the initial
C6H5ONa concentration and that equilibrated in the waterPhase-Transfer Catalytic Reaction of TBAB

Figure 3. Capillary wave frequency dependence on theconcentration of TBAB and TBAC.

phase was regarded as the amount of the ion pair. The
C6H5ONa concentration in the water phase was determined using absorption spectrophotometry. As shown in
Figure 2, the amount of TBA+C6H5O- transferred acrossthe W/NB interface increased with increasing C6H5ONa
concentration. On the other hand, the amount of TBA+C6H5O- transferred was greater when the TBAB concentration was 1 mM than when it was 25 mM. This result
indicates that high concentrations of the phase-transfer
catalysts do not always bring about high product yield,
reflecting the specificity of the liquid /liquid interface asa reaction place. To investigate details about the role ofthe liquid/liquid interface and the mechanism determining
the optimum concentration of the catalyst, the molecularnumber density was investigated using the QELS method.
First of all, the effect of each chemical species in thereaction system on the QELS spectrum should be clarified.
Capillary wave frequencies were independent of the DPC
concentration. This indicates that DPC molecules are notadsorbed alone onto the interface. Capillary wave frequencies were also independent of both DPC and C6H5ONa concentrations. This indicates that the reaction
between DPC and C6H5ONa does not proceed without thephase-transfer catalyst TBAB. These results agree withearlier works on the phase-transfer catalyst.30,31 Figure
3 shows capillary wave frequencies at various concentrations of TBAB and TBAC. The frequency decreased
significantly on increasing the concentration of eachspecies and there is no difference in the frequencies
between TBAB and TBAC at each concentration. This
indicates that TBAB and TBAC molecules are adsorbed
onto the interface and the counterion (Br- or Cl-) does notaffect the frequencies. Figure 4 shows calculated molecularnumber density based on the Lamb’s equation and Gibb’s
equation. On the basis of these results and our previous
report,7 we conclude that the QELS is able to monitor
adsorption dynamics of TBAB, TBAC and TBA+C6H5O-.
Since the frequencies decrease in the presence of TBAB
and C6H5ONa, the effect of TBA+C6H5O- on the QELS
spectrum is larger than that of TBAB and TBAC.
Analysis of the phase-transfer catalytic reaction was
performed by monitoring the interfacial adsorption of
TBAB, TBAC, and TBA+C6H5O- in solutions with different
concentrations of TBAB, C6H5ONa, and DPC. Figure 5
shows the capillary wave frequencies for these solutions.
There was a great difference in the frequencies between
the solutions with 1 and 25 mM TBAB. For example, when(30) Krishnakumar, V. K.; Sharma, M. M. Ind. Eng. Chem. Process
Des. Dev. 1985, 24, 1293.
(31) Asai, S.; Nakamura, H.; Tanabe, M.; Sakamoto, K. Ind. Eng.
Chem. Res. 1994, 33, 1687.

Langmuir, Vol. 16, No. 16, 2000 6599

Figure 4. Molecular number density dependence on theconcentration of TBAB and TBAC.

Figure 5. Capillary wave frequency dependence on theconcentrations of DPC and C6H5ONa with TBAB (1 and 25
mM).

Figure 6. Interfacial tension dependence on the concentrationsof DPC and C6H5ONa with TBAB (1 and 25 mM).

the concentration of C6H5ONa was 0 mM, the frequency
for 1 mM TBAB was around 12.8 kHz while that for 25
mM TBAB was around 11 kHz. This showed a big
difference in the interfacial molecular number density of
TBAB, which was simply due to the difference in concentration of TBAB. Figures 6 and 7 show the interfacial
tension and the molecular number density by using Lambvs and Gibb’s equation, respectively. For 1 mM TBAB,
the interfacial molecular number density of TBAB was
0.59 × 10-10 mol/cm2, and for 25 mM TBAB, it was 15 ×
10-10 mol/cm2. On increasing the concentration of C6H5ONa, the frequencies for both cases (1 and 25 mM TBAB)
decreased monotonically and reached equilibrium at about
50 mM C6H5ONa. This decrease was due to the formationof the ion pair TBA+C6H5O-, as we have reported
previously.7 For 1 mM TBAB, the frequency decreased
from 12.8 kHz to about 12 kHz, and for 25 mM TBAB, it

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Uchiyama et al.
Table 1. Relative Amounts of Each Chemical Species
Estimated in This Study

Figure 7. Molecular number density dependence on theconcentrations of DPC and C6H5ONa with TBAB (1 and 25
mM).

dropped from 11 to 10 kHz. These drops were approximately proportional to the amount of the formed ion pair
TBA+C6H5O- at the interface. The interfacial molecularnumber density of TBA+C6H5O- for 1 mM TBAB wasaround 8.2 × 10-10 mol/cm2 while that for 25 mM TBAB
was around 11 × 10-10 mol/cm2 from Figures 6 and 7.
Thus despite the great difference in the amount of the
TBAB adsorbed onto the interface between 1 and 25 mM
TBAB, the amount of the formed ion pair TBA+C6H5Owas estimated to be almost the same. In short, the amountof the TBAB adsorbed onto the interface had hardly any
affect on the amount of the ion pair TBA+C6H5O-, which
is significant in phase-transfer catalytic reaction. Combined with the results in Figure 2 that showed highconcentrations of the catalyst did not always bring abouta high product yield, we can discuss this observation
further.
The qualitative amounts of each chemical species
observed in this study are summarized in Table 1. The
large interfacial concentration of TBAB does not result inincreases in interfacial TBA+C6H5O- nor in NB phase
TBA+C6H5O-. This is because the liquid/liquid interface
plays a double role in this reaction: it acts as the place
for formation of the ion pair and as the place for selective[TBAB] initial

1 mM[TBAB] interfacial[TBA+C6H5O-] interfacial[TBA+C6H5O-]NB

small
same
large

25 mM
<
)
>

large
same
small

mass transfer. The interfacial TBAB promotes ion pairformation and simultaneously disturbs mass transfer
across the interface. Since reaction sites are limited at
the liquid/liquid interface in the stationary state, theoptimum concentration of the catalyst should be determined by taking into account the interfacial coverage,
adjusting the balance between the rate for ion pairformation and the rate for mass transfer. This suggests
the need for proposing a new reaction scheme for the liquid/
liquid interface. It should be noted that an experimental
approach to the interfacial reaction can be made only bydirect measurements of the liquid/liquid interface, andthe QELS method would be a powerful tool for such a
study. Furthermore, real time monitoring of the masstransfer of each molecule used in the present study will
give hard evidence of this conclusion and will be shownin the future.
In summary, the relationship between the concentrationof phase-transfer catalyst, tetrabutylammonium bromide(TBAB), and the rate of its cyclic reaction was investigatedusing the quasi-elastic laser scattering (QELS) method.
The increase in the concentration of the phase-transfer
catalyst (TBAB) did not promote the cyclic reaction. The
QELS study indicated that this was because TBAB
adsorbed on the interface disturbed the mass transfer ofthe other chemical species (TBA+C6H5O-). Interfacialcoverage of the catalyst should be taken into account tofind the optimum concentration of the catalyst. It is
expected that the QELS method will contribute significantly to the development of an interfacial reaction
scheme.
LA9913295