J. Phys. Chem. B 2000, 104, 4699-4702

4699

Observation of Dynamic Molecular Behavior in a Phase Transfer Catalytic Reaction at a
Liquid/Liquid Interface by Using the Time-Resolved Quasi-Elastic Laser Scattering Method
Yoshiko Uchiyama, Masanori Fujinami, 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: December 13, 1999; In Final Form: March 1, 2000

The dynamic behavior of the phase transfer catalyst, quaternary ammonium bromide R4N+Br- (R: n-C4H9,n-C3H7, C2H5), at a water/nitrobenzene (W/NB) interface was monitored by time-resolved quasi-elastic laser
scattering (QELS) measurements. The time courses of the QELS spectrum after injection of the (n-C4H9)4N+Br- phase transfer catalyst, which brings about high product yield, showed high interfacial molecular number
density in the stationary state of the reaction, whereas its adsorption rate in the initial stage was almost thesame as the rates of the other two. The product yield was considered to reflect the interfacial amount of thephase transfer catalyst, and the interface acted as the reaction site controlling the phase transfer catalytic
reaction.

Introduction
Liquid/liquid interfaces play important roles in various
processes such as phase transfer catalysis, solvent extraction,and ion-selective electrode reactions. Among them, phasetransfer catalysis has attracted much attention because thecatalyst activates the reaction by transferring across the liquid/liquid interface and brings about high product yield and highselectivity by the recurred cycle.1-5 Tetrabutylammonium bromide (TBAB, (n-C4H9)4N+Br-) is a commonly used phase transfer catalyst, which behaves as a surfactant and activates organic
synthesis reactions.6,7 It is empirically known that TBAB, whose
carbon chain number is 4, brings about higher product yieldthan other quaternary ammonium bromides. Longer carbon
chains of the quaternary ammonium bromide cause moredifficult separation of product, and thus shorter carbon chains
have an advantage as a phase transfer catalyst.8,9 However, thereason TBAB shows a higher product yield than tetrapropyllammonium bromide (TPRAB, (C3H7)4N+Br-) and tetraethylammonium bromide (TEAB, (C2H5)4N+Br-) has not beenclarified so far. Clarification by an analytical approach to theinterfacial reaction as well as a conventional study using thebulk phase concentration is highly desirable.
In recent years, various spectroscopic techniques have beendeveloped to probe liquid/liquid interfaces. Second harmonicgeneration (SHG) and sum frequency generation (SFG) methods
have provided great insight into molecular orientation at theliquid/liquid interfaces.10-19 In addition to these studies oninterfacial static structures, experimental approaches to theinterfacial molecular density are of great significance in revealing the interfacial molecular dynamics.
We have developed a time-resolved quasi-elastic laser scattering (QELS) method and have reported on the dynamic* To whom correspondence should be addressed. E-mail: tsawada@mail.ecc.u-tokyo.ac.jp.

changes of interfacial molecular density of surfactants at thewater/nitrobenzene (W/NB) interface.20-25 This method allows
nonperturbative in situ measurement of liquid/liquid interfaces
and provides insight into the interfacial mass transfer. We
focused on a phase transfer catalytic reaction system withquaternary ammonium bromide, R4N+Br- (R: n-C4H9, n-C3H7,n-C2H5), whose reaction scheme is well-known and whoseoperating conditions are mild. We previously reported that theliquid/liquid interface was the reaction site for formation of ionpairs (TBA+C6H5O-) based on observation of one process inthe phase transfer catalysis.7 The main aims of the present study
are to clarify the dynamic molecular behavior in the phasetransfer catalytic reaction and the mechanism determining theproduct yield. We first present time courses of the relative
molecular number densities at the W/NB interface in the phasetransfer catalytic reaction after injection of the phase transfer
catalyst and then discuss the importance of the reaction site in
the formation of ion pairs.
Experimental Section
Principle. The principle of the QELS method has alreadybeen described.20-27 The relationship between the capillary wave
frequency f and the interfacial tension γ is expressed by Lamb’s
equation:28f)()

1/21γk3/2
2π Fw + Fo

(1)

where Fw is the density of the water phase, Fo is the density ofthe organic phase, and k is the wavenumber of the capillarywave. Here, the interfacial tension γ is inversely proportional
to the interfacial number density G based on the Gibbs adsorption isotherm.23 Therefore, the relative change of the number
density G can be estimated from a change in the capillary wave
frequency. We have reported this previously. 21, 22

10.1021/jp994386f CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/26/2000

4700 J. Phys. Chem. B, Vol. 104, No. 19, 2000

Uchiyama et al.

Figure 3. Power spectra for capillary waves excited at the W/NBinterface (283 K).

Figure 1. Schematic diagram of the experimental setup: T, glass tube;
W, water phase; NB, nitrobenzene; PD, photodiode; AMP, preamplifier;
FFT, FFT analyzer.

Figure 4. Capillary wave frequency dependence on the concentrationsof C6H5ONa (283 K).

Figure 2. The phase transfer catalytic reaction scheme between
C6H5ONa and DPPC in the W/NB system.

Apparatus. A schematic diagram of the experimental setup
is shown in Figure 1. 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 ofquartz glass and has an optically flat bottom. After passing
through the sample, one of the diffracted beams, which is mixed
with the scattered light, is selected by the aperture in front of
the photodiode (Hamamatsu Photonics S1290). Signals fromthe 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
is 6.6 × 10-2 mm. The quartz cell is placed inside a copper
cell that has a thermostat (Tokyo Rikakikai Co., UA-110) tokeep the measurement temperature at 283 K because the W/NB
interface is most stable at that value.
Sample Preparation. The reaction scheme between sodium
phenoxide (C6H5ONa) and diphenylphosphoryl chloride (DPPC)in the W/NB system with quaternary ammonium bromide,
R4N+Br- (R: n-C4H9, n-C3H7, C2H5), is shown in Figure 2. Atthe beginning of the reaction, R4N+ Br- reacts with C6H5ONa
to form R4N+C6H5O- at the interface.7 This ion pair moves
into the organic phase, where it reacts with DPPC to produce
triphenyl phosphate ((C6H5O)3PO). During this reaction, R4N+Clis also formed and is transported into the water phase to reactagain with C6H5ONa. R4N+Br- activates the production oftriphenyl phosphate by circulating between the two phases.
In this study, we focused on the liquid/liquid interface in thephase transfer catalytic reaction and investigated the dynamicbehavior of the ion pair (R4N+Cl-, R4N+C6H5O-) formed inthe presence of R4N+Br- (1 mM), DPPC (1 mM), and C6H5ONa (70 mM). We used three kinds of quaternary ammonium

bromides, R4N+Br-: tetrabutylammonium bromide (TBAB),(C4H9)4N+Br-; tetrapropylammonium bromide (TPRAB),(C3H7)4N+Br-; and tetraethylammonium bromide (TEAB),(C2H5)4N+Br-. A liquid/liquid interface was prepared by adding
9.8 mL of an aqueous solution of C6H5ONa (Aldrich) and NaOH
(Kanto Chemical Co.) to 10 mL of nitrobenzene containing
DPPC (Kanto Chemical Co.) in the quartz cell. After 0.2 mLof R4N+Br- (Kanto Chemical Co.) was injected in the waterphase with a microsyringe, the power spectrum was obtained
every 1 s. Ultrapure water (from Millipore Milli-Q system) wasused for all sample preparations. NaOH was added to adjust
the ionic strength to 0.2 and to prevent the production of phenolor benzoic acid. All chemicals were reagent grade and wereused without further purification.
Results and Discussion
First, a typical power spectrum of capillary waves excited atthe W/NB interface is shown in Figure 3. The signal-to-noise
ratio of the present system was improved by increasing the dataacquisition frequency from 4 Hz to 20 Hz and integrating thedata every 1 s. The errors on the values of the capillary wave
frequency were 0.1 kHz obtained as the standard deviation of
10 repeated measurements.
Capillary wave frequency dependence on C6H5ONa is shownin Figure 4. The frequency decreased significantly with increasing C6H5ONa concentration. This indicated that interfacial tension was decreased by the interfacial adsorption of C6H5ONa.
To examine the dynamic molecular behavior in the phasetransfer catalytic reaction, we investigated the time courses ofthe capillary wave frequencies after the injection of the TEAB,TPRAB, and TBAB solutions into the water phase. These curves
were obtained with good reproducibility. The time just prior tothe injection was regarded as 0 s, and the power spectrum wasobtained every 1 s. Capillary wave frequencies after the injectionof the TEAB, TPRAB, and TBAB solutions are plotted as a

Dynamic Molecular Behavior in a Catalytic Reaction

Figure 5. Time courses of capillary wave frequency after an injectionof TEAB solution (1mM) at 283 K.

Figure 6. Time courses of capillary wave frequency after an injectionof TPRAB solution (1mM) at 283 K.

function of time in Figures 5, 6, and 7, respectively. In thesefigures, no changes in capillary wave frequencies were observed.
This indicated that R4N+Br- molecules were not adsorbed alone
onto the interface. Our measurements showed that C6H5ONa
molecules were present at the interface, whereas R4N+Brmolecules were in the bulk phase at 283 K.
Capillary wave frequencies after the injection of the TEAB,TPRAB, and TBAB solutions into the water phase containing
C6H5ONa are also plotted in Figures 5, 6, and 7, respectively.
No changes in the frequencies after the injection were observed.
This suggests the two following possibilities. One is thatinterfacial C6H5ONa does not react with R4N+Br- under suchconditions. The other is that interfacial C6H5ONa reacts with
R4N+Br- and forms ion pairs R4N+C6H5O- at the interface,
but this formation of ion pairs does not affect the capillary wavefrequency.
As shown at 0 s in Figures 5, 6, and 7, the capillary wave
frequencies at the interface between the W phase containing
C6H5ONa and the NB phase containing DPPC were the sameas those at the interface between the W phase containing
C6H5ONa and the NB phase without DPPC. This indicates thatthe DPPC molecules were not adsorbed at the interface and thereaction between C6H5ONa and DPPC did not proceed without
the phase transfer catalyst. On the other hand, when the phasetransfer catalyst (R4N+Br-) was injected into the interface
between the W containing C6H5ONa and the NB containing
DPPC, the capillary wave frequencies decreased gradually. Afterattaining their minimum, they increased gradually and thenbecame constant after 8-15 s. As shown within the dotted line
rectangle of Figure 8, the decrease in frequency corresponded
to the adsorption of R4N+Cl- onto the interface and the increase
corresponded to the desorption from the interface, because thechange in frequency was observed only when DPPC was added.
The times when the frequencies attained their minimum andbecame constant are summarized in Table 1. The time of theJ. Phys. Chem. B, Vol. 104, No. 19, 2000 4701

Figure 7. Time courses of capillary wave frequency after injection ofTBAB solution (1mM) at 283 K.

Figure 8. Reaction scheme that corresponds to the decrease andincrease in capillary frequency.

TABLE 1: Time of the Minimum and Stationary StateFrequencies and the Change of Frequency of R4N+BrEstimated in This Study
time of minimum frequency
time of stationary state frequency∆f (adsorption) (kHz)∆f (desorption) (kHz)ratio of adsorption onto the interface ofstationary state (%) (∆f(adsorption) ∆f(desorption))/ ∆f(adsorption)

TEAB

TPRAB

TBAB

6s
13 s
0.43
0.43

7s
14 s
0.78
0.47

6s
9s
0.43
0.16

040

64

minimum frequency was almost the same regardless of the phasetransfer catalyst, whereas the frequency of TBAB became
constant much faster than the other two. This meant that theadsorption rates of R4N+Cl- were almost the same for the threephase transfer catalysts, whereas the desorption behavior of theTBAB ion pair differed from that of the other two. Thissuggested that the desorption rate of the TBAB ion pair wasvery low and the stationary state could be established quickly
at high interfacial molecular density. Furthermore, the ratio ofinterfacial adsorption of the stationary state is defined andsummarized in Table 1. This parameter reflects the amount ofinterfacial adsorbate of the stationary state. This result indicatedthat the interface accommodated the most adsorbate in the cyclic
reaction of TBAB. In short, the TBAB ion pairs were difficult
to desorb from the interface to the W phase, and thus many
interfacial adsorbate molecules were present for the stationarystate. This behavior of TBAB was considered to be related tothe high product yield as a phase transfer catalyst.
Based on the experimental fact that the high interfacial
molecular density of TBAB ion pair was related to the highproduct yield, we considered that the interface was the reaction
site controlling the phase transfer catalytic reaction. Thus, weproposed a model for the phase transfer catalytic reaction in
the stationary state. It is shown within the dotted line rectangleof Figure 9. This model is a further explanation for our previous

4702 J. Phys. Chem. B, Vol. 104, No. 19, 2000

Figure 9. Model of the phase transfer catalytic reaction in the stationary
state. Reaction proceeds between the adsorbed chemical species at theinterface after adsorption from the water phase.

study, which demonstrated the interfacial reaction of TBAB
below 50 mM.7
Our results reflected an interfacial specificity for a chemical
reaction and mass transfer. We successfully observed dynamic
molecular behavior reflecting the interfacial specificity byfocusing on the phase transfer catalyst system using the timeresolved QELS method. We expect the QELS method willcontribute significantly to the development of an interfacial
reaction scheme by molecular dynamics at liquid/liquid interfaces.
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