Effects of ionic surfactants and cyclodextrins on hydride-transfer reaction of l-Benzyl-l,4-dihydronicotinamide with methylene blue

Article abstract of DOI:10.1246/bcsj.80.1383

The kinetics of the hydride-transfer reaction between methylene blue (MB⁺) and 1 -benzyl-1,4-dihydronictinamide (BNAH) were studied in media containing cyclodextrins (β- and γ-CD) and surfactants (sodium dodecyl sulfate (SDS), dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, and hexadecyltrimethylammonium bromide). Cationic surfactants decreased the apparent first-order rate constant (k _obsd) above the cmc, while SDS increased k_obsd just above the cmc and then decreased k_obsd with increasing surfactant concentration. This behavior for cationic surfactants was typical of micellar effects due to a separation of the reactants by the micelles. BNAH associated with micelles, whereas MB⁺ ions were repelled from the cationic interface of the micelles. Binding of BNAH and MB to the same SDS micelle enhanced the reaction, but dilution of reagents within the micellar interface with the increase in [SDS] caused a decrease in K_obsd. In β-CD-cationic surfactant mixtures, the results were interpreted in terms of the model which takes into account the formation of CD-BNAH, CD-MB₊, and CD-surfactant complexes and the association of BNAH with micelles. The decrease in K_obsd with increasing surfactant concentration observed in γ-CD-cationic surfactant mixtures can be explained by the decrease in the concentration of free γ-CD by the formation of 1:1 and 2:1 complexes of surfactant monomer with β-CD.

Full text of DOI:10.1246/bcsj.80.1383

Ó 2007 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 80, No. 7, 1383–1390 (2007) 1383
Effects of Ionic Surfactants and Cyclodextrins on Hydride-Transfer
Reaction of 1-Benzyl-1,4-dihydronicotinamide with Methylene Blue
ꢀ
Takeshi Matsumoto, Yingjin Liu, Yoshimi Sueishi, and Shunzo Yamamoto
The Graduate School of Natural Science and Technology, Okayama University,
3-1-1 Tsushimanaka, Okayama 700-8530
Received December 13, 2006; E-mail: yamashun@cc.okayama-u.ac.jp
The kinetics of the hydride-transfer reaction between methylene blue (MBþ) and 1-benzyl-1,4-dihydronictinamide
(BNAH) were studied in media containing cyclodextrins (ꢀ- and ꢁ-CD) and surfactants (sodium dodecyl sulfate (SDS),
dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, and hexadecyltrimethylammonium bro-
mide). Cationic surfactants decreased the apparent first-order rate constant (kobsd) above the cmc, while SDS increased
kobsd just above the cmc and then decreased kobsd with increasing surfactant concentration. This behavior for cationic
surfactants was typical of micellar effects due to a separation of the reactants by the micelles. BNAH associated with
micelles, whereas MBþ ions were repelled from the cationic interface of the micelles. Binding of BNAH and MBþ
to the same SDS micelle enhanced the reaction, but dilution of reagents within the micellar interface with the increase
in [SDS] caused a decrease in kobsd. In ꢀ-CD–cationic surfactant mixtures, the results were interpreted in terms of the
model which takes into account the formation of CD–BNAH, CD–MBþ, and CD–surfactant complexes and the association
of BNAH with micelles. The decrease in kobsd with increasing surfactant concentration observed in ꢁ-CD–cationic sur-
factant mixtures can be explained by the decrease in the concentration of free ꢁ-CD by the formation of 1:1 and 2:1
complexes of surfactant monomer with ꢁ-CD.
Aqueous micellar solutions appear to be homogeneous since
these aggregates are of colloidal size, but in reality they are
highly anisotropic solvents whose properties change gradually
from those of pure water to those of hydrocarbon-like liquids
upon going from the bulk water phase to the interior of the mi-
cellar core.1–3 Micelles can cause acceleration or inhibition of
a given reaction relative to the equivalent reaction in aqueous
solutions. The influence of micellar systems on chemical reac-
tivity is often analyzed in terms of the pseudo-phase model.4–9
Extensive research has been carried out to investigate the ef-
fects of surfactants on the electron absorption spectra of many
dyes.10–17 Recently, we observed that the absorbance of meth-
ylene blue (MBþ) at 655 nm decreases rapidly at first with
increasing SDS concentration but increases gradually near
the cmc.18 On the other hand, the absorbance of MBþ is almost
independent of the hexadecyltrimethylammonium bromide
(HTAB) concentration. At low SDS concentrations, formation
of an MBþ–SDS aggregate occurs starting with the ion pair
(MBþ–SDSꢁ) and continues to a aggregate represented by
(MBþSDSꢁ)n. Near and just below the cmc (MBþSDSꢁ)n
aggregates reorganize into premicelles with a monomeric
MBþ content, resulting in an increase in the absorbance of
MBþ. With further increases in SDS concentration, all MBþ
molecules are accommodated into normal micelles as mono-
meric molecules.18
plexes.19 Changes in the reactivities of the guest molecules
results from such host–guest interactions. The effects of inclu-
sion complexes on reactivities vary widely and depend on the
guest, the CD, and the reaction.20,21
The inclusion of MBþ with ꢀ- and ꢁ-CD in aqueous solu-
tions was studied using absorption and fluorescence spectros-
copy. It was found that the position of the monomer/dimer
equilibrium of MBþ was suppressed by the addition of ꢀ-
CD and enhanced by addition of ꢁ-CD. These findings were
explained by the inclusion of a monomer by ꢀ-CD and a dimer
by ꢁ-CD.22–24
The properties of surfactant solutions are changed by the
addition of a CD because of the formation of inclusion com-
plexes of surfactant molecules.25–31 In general, complex for-
mation increases the cmc of surfactants. There have been sev-
eral studies of the effects of CD–surfactant systems on reaction
rate constants.21,32–36
Micellar effects on bimolecular reaction rates are due main-
ly to the increase or decrease of reactant concentrations in
the micellar pseudophase and the changes in the reaction rate
with surfactant concentration often can be explained in these
terms.37 Generally, it is easier to evaluate the partition of
hydrophobic reactants between the aqueous phase and micellar
pseudophase. In the case of ionic reactants, the Coulombic in-
teraction between the ions and the charge of the micellar sur-
face takes an important role.
Cyclodextrins (CDs) possess cavities with a truncated cone
shape with a nonpolar and hydrophobic interior and two hydro-
philic rims formed by primary and secondary alcohol groups.
The CD cavity is capable of encapsulating (in whole or in part)
a variety of guest molecules of the appropriate size, shape, and
polarity and forming noncovalent host–guest inclusion com-
There has been little research either on how cyclodextrins
affect the effects of the micelles on the reaction rate or how
the presence of surfactants affects the behavior of cyclodex-
trins. It seems generally to have been assumed that micelles
are only formed once all the cyclodextrin present has been
Published on the web July 11, 2007; doi:10.1246/bcsj.80.1383

1384 Bull. Chem. Soc. Jpn. Vol. 80, No. 7 (2007)
Effects of CD and Surfactants on Reaction Rate
rendered inactive by complexation with surfactant monomers.
The experimental investigation of this assumption could be
made using the reactions which are influenced by micelles as
well as cyclodextrins.
The kinetics of the hydride-transfer reaction between
Methylene Blue (MBþ) and 1-benzyl-1,4-dihydronicotinamide
(BNAH) were studied in 10% ethanol–90% water mixed sol-
vents containing ꢀ- and ꢁ-CD. The reaction was found to be
suppressed by the addition of ꢀ-CD but enhanced by the addi-
tion of ꢁ-CD.38 Since the reaction between BNAH and MBþ
involves neutral and cationic species, the different effects
of the cationic and anionic surfactants on the reaction rate
are expected.
(1)
1
0.5
0
(11)
(1)-(11)
300
400
500
600
700
800
Wavelength / nm
In this manner, MBþ shows various interactions with CDs
and surfactants. Furthermore, in the mixed system of CD–
surfactant, CD can change the properties of surfactant solu-
tions. It is interesting to know how surfactants alone or in com-
bination with CDs affect the kinetics of the reaction of MBþ.
In this paper, we study the effects anionic and cationic surfac-
tants on the reaction of MBþ with BNAH in the absence and
presence of CDs.
Fig. 1. Repetitive scan of absorption spectrum for the hy-
dride-transfer reaction of BNAH with MBþ in 0.2
mol dmꢁ3 NaOH–sodium tetraborate buffer (10 vol % etha-
nol–90 vol % water) at pH 9.4 and 25 ꢂC in the presence of
O2. ½BNAHꢄ0 ¼ 1:41 ꢃ 10ꢁ4 mol dmꢁ3, ½MBþꢄ0 ¼ 4:27 ꢃ
10ꢁ6 mol dmꢁ3. (1) Taken immediately after mixing: (2)–
(11) taken at subsequent 5 min intervals.
tension versus surfactant concentration.41 To ensure that the in-
struments properly worked and that the method of determination
of the cmc was appropriate, the cmcs of SDS and HTAB in aque-
ous solutions were determined. The results were 8:2 ꢃ 10ꢁ3 and
9:0 ꢃ 10ꢁ4 mol dmꢁ3 for SDS and HTAB, respectively, and were
in good agreement with literature values (8:26 ꢃ 10ꢁ3 and 9:02 ꢃ
10ꢁ4 mol dmꢁ3 for SDS and HTAB, respectively).41
Experimental
Materials. MBþ, ꢀ- and ꢁ-cyclodextrins (ꢀ- and ꢁ-CD), and
surfactants (sodium dodecyl sulfate (SDS), dodecyltrimethyl-
ammonium bromide (DTAB), tetradecyltrimethylammonium
bromide (TTAB), and hexadecyltrimethylammonium bromide
(HTAB)) were of the highest commercially available purity
(Wako Pure Chemical Industries) and were used without further
purification. BNAH was purchased from Tokyo Kasei Kogyo
Chemical Industry and was used as supplied. Because of the
low solubility of BNAH in water and in order to reduce the spon-
taneous decomposition of BNAH, 10 vol % ethanol aqueous basic
buffer solutions (0.2 mol dmꢁ3 NaOH–sodium tetraborate buffer;
pH 9.4) were used for kinetic measurements.38
Results and Discussion
Kinetics of the Oxidation of BNAH by MBþ in the Pres-
ence of O2. The reaction of BNAH with MBþ was studied
in 10 vol % ethanol aqueous buffer solutions in the presence
of O2.
Measurements. In a previous study, the reactions of MBþ with
BNAH were followed by recording the decrease in absorbance
of MBþ at 665 nm under deaerated conditions where the initial
concentrations of BNAH were 10-fold or greater in excess over
MBþ. In this study, however, the reactions were followed by
recording the decrease in absorbance of BNAH at 360 nm in the
presence of O2 using a Hitachi spectrophotometer, model 2001,
at 25.0 ꢂC, because the absorption spectra of MBþ significantly
depended on the concentrations of MBþ, CDs, and surfactants.
In the presence of O2 (oxygen-saturated condition), the absorb-
ance of BNAH at 360 nm decreased smoothly, while the absorb-
ance of MBþ at 665 nm decreased a little at first, then became al-
most constant, even under conditions of excess concentration of
BNAH over that of MBþ (Fig. 1). Figure 1 shows that the leuco
methylene blue (MBH) produced by reduction with BNAH was
re-oxidized by dissolved oxygen and the steady-state was attained
for MBþ. This suggests that the reaction obeys pseudo-first-order
kinetics after the steady-state was reached and the absorbance
at 360 nm could be used to obtain the apparent first-order rate con-
stants.39 All runs gave good first-order plots over three or 4 half-
lives. The steady-state concentration of MBþ was assumed to be
equal to the initial concentration.
H
H
CONH2
N
N
S
CH2
+
(H3C)2N
N+(CH3)2
BNAH
MB+
ð1Þ
CONH2
+
N
k0
H
N
CH2
+
N(CH3)2
(H3C)2N
S
BNA+
MBH
Figure 1 shows the time dependence of the absorption spec-
trum for the reaction of BNAH with MBþ. The absorption of
BNAH at 360 nm gradually decreased after mixing a BNAH
solution with an MBþ solution (½BNAHꢄ0 ¼ 1:41 ꢃ 10ꢁ4
mol dmꢁ3, ½MBþꢄ0 ¼ 4:27 ꢃ 10ꢁ6 mol dmꢁ3), while that of
MBþ at 665 nm showed only slight decrease. Under the condi-
tions that the initial concentrations of MBþ were less than that
Surface tensions of the surfactant solutions in 10% ethanol
aqueous buffer solutions in the absence and presence of ꢀ- and
ꢁ-CD were measured by the drop-weight method.40,41 The cmcs
of the surfactant solutions were determined by plots of surface

T. Matsumoto et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 7 (2007) 1385
0
30
20
10
-1
-2
-3
0
1000
2000
3000
0
2
β
4
6
t / s
103[
-CD] / mol dm-3
Fig. 2. Pseudo-first-order plots of the reaction of BNAH
with MBþ at pH 9.4 in the presence of O2. ½BNAHꢄ0 ¼
1:40 ꢃ 10ꢁ5 mol dmꢁ3; ½MBꢄ0 ¼ 0:57 ( ), 1.18 ( ), 2.36
Fig. 3. Effect of ꢀ-CD on the observed first-order rate con-
stant (kobsd) for the hydride-transfer reaction of BNAH
with MBþ in 0.2mol dmꢁ3 NaOH–sodium tetraborate buff-
er (10 vol % ethanol–90 vol % water) at pH 9.4 and 25 ꢂC.
(
), 3.53 ( ), 4.27 ( ), 5.70 ( ), 11.8 ( ), 23.6 ( ),
34:2 ꢃ 10ꢁ6 mol dmꢁ3
( ).
of BNAH, the absorbance of MBþ was kept nearly constant
while the absorbance of BNAH decreased. As mentioned
above, the leuco methylene blue (MBH) produced by the reac-
tion with BNAH was rapidly re-oxidized by dissolved O2 and
the steady-state for MBþ was attained. All the runs gave good
first-order plots (Fig. 2). The apparent first-order rate constants
(kobsd) were obtained at some initial concentrations of MBþ. A
saturation curve was obtained by plotting the kobsd-values
against [MBþ]0. The saturation curve shows the existence
of a 1:1 complex between reactants in this reaction. As men-
tioned in a previous study, the reaction of BNAH with MBþ
resulted in a 1:1 complex between BNAH and MBþ, and
this complex is the real intermediate. However, the plots be-
tween kobsd and [MBþ]0 at concentrations below 5:72 ꢃ 10ꢁ6
mol dmꢁ3 approximately gave a straight line with a small inter-
cept. This intercept showed the rate constant for the spontane-
ous decomposition of BNAH. The equation for kobsd in these
regions of MBþ concentration is expressed as
100
80
60
40
20
0
1
2
103[
γ
-CD] / mol dm-3
Fig. 4. Effect of ꢁ-CD on the observed first-order rate con-
stant (kobsd) for the hydride-transfer reaction of BNAH
with MBþ in 0.2mol dmꢁ3 NaOH–sodium tetraborate buff-
er (10 vol % ethanol–90 vol % water) at pH 9.4 and 25 ꢂC.
kobsd ¼ k0½MBþꢄ0 þ kSD
;
ð2Þ
k0
BNAH þ MBþ ! BNAþ þ MBH;
ð3Þ
ð4Þ
ð5Þ
where k0 is the rate constant for the reaction (3) shown later
and kSD is the first-order rate constant for the spontaneous
K1
BNAH þ ꢀ-CD ꢀ BNAH{CD;
decomposition of BNAH (k0 ¼ 54:7 dm3 molꢁ1 sꢁ1 and kSD
¼
2:1 ꢃ 10ꢁ6 sꢁ1). This indicates that at the low concentrations
of MBþ and BNAH, the concentration of the complex could
be neglected. All measurements of the effects of CD and
surfactants on the reaction rate were done in these concentra-
tion regions.
K2
MBþ þ ꢀ-CD ꢀ MB{CD;
k0½MBꢄ0
kobsdðꢀ-CDÞ ¼
þ kSD
;
ð6Þ
ð1 þ K1½ꢀ-CDꢄÞð1 þ K2½ꢀ-CDꢄÞ
Effects of CD on the Reaction of BNAH with MBþ.
Figure 3 shows the changes in kobsd with ꢀ-CD concentration.
As shown in Fig. 2, kobsd decreased with increasing ꢀ-CD con-
centrations. As mentioned in a previous paper, the decrease in
kobsd with the addition of ꢀ-CD is due to separate inclusions
of BNAH and MBþ in the cavity of ꢀ-CD.25 Taking into
account the formation of the complexes of NMAH and MBþ
with ꢀ-CD and assuming that these inclusion complexes did
not react with each other, nor with free reactants, Eq. 6 was
obtained from the following reactions:
where kobsd(ꢀ-CD) was the observed rate constant in the pres-
ence of ꢀ-CD. The solid line in Fig. 3 shows the values calcu-
lated by Eq. 6 using K1 ¼ 220 dm3 molꢁ1 and K2 ¼ 130
dm3 molꢁ1, which are values obtained in a previous study.38
The observed and calculated values were in good agreement.
As shown in Fig. 4, the kobsd-value increased with increas-
ing ꢁ-CD concentration. In a previous study, this increase in
kobsd was explained by the inclusion of the complex between
BNAH and MBþ in the cavity of ꢁ-CD. To explain the effect
of ꢁ-CD, the following reactions were considered:

1386 Bull. Chem. Soc. Jpn. Vol. 80, No. 7 (2007)
Effects of CD and Surfactants on Reaction Rate
This behavior was typical of micellar effects due to a separa-
tion of the reactants by the micelles. BNAH associated with
micelles, whereas MBþ ions were repelled from the interface
of the micelles due to electrostatic effects.
Taking into account that the overall reaction rate was the
sum of the rate in solution and in the micellar phase (in the
present case, since MBþ could not approach BNAH in the
micelle due to the electrostatic interaction between MBþ and
the cationic surface of micelles, the latter rate was neglected),
the expression of kobsd was obtained from reactions (1) and
(12) and given in Eq. 13:
KC
MBþ þ BNAH ꢀ C;
ð7Þ
ð8Þ
K3
C þ ꢁ-CD ꢀ C{CD;
C ! MBH þ BNAþ;
C{CD ! MBH þ BNAþ þ ꢁ-CD:
In the low ꢁ-CD concentration region, the following approxi-
mate equation can be obtained:
k1
ð9Þ
k2
ð10Þ
ꢀ
ꢁ
k2
kobsdðꢁ-CDÞ ¼ k0½MBꢄ0 1 þ K3½ꢁ-CDꢄ0 þ kSD
;
ð11Þ
k1
KS
BNAH þ Dn ꢀ BNAH{Dn;
where Dn shows the micelle.
ð12Þ
where kobsd(ꢁ-CD) was the observed rate constant in the pres-
ence of ꢁ-CD. The value of ðk2=k1ÞK3 was determined from
the slope of the straight line shown in Fig. 4. Using the values
of k1 and k2 (0.13 and 0.11 sꢁ1) shown in a previous paper, the
value of 1100 dm3 molꢁ1 for K3 was obtained. This value is
about a half of the value obtained in a previous study. Since
the experimental conditions in both studies were similar (the
buffer solutions differed in both studies and the present and
previous studies were carried out in the presence and absence
of O2, respectively), the reason for the discrepancy in K3-value
was not clear.
The equilibrium constants for the dimerization of MBþ and
the formation of the inclusion complex of the dimer with ꢁ-CD
were obtained by the dependences of the absorption intensity
of MBþ on the concentrations of MBþ and ꢁ-CD in 10% etha-
nol–90% water mixed solvent (4300 and 910 dm3 molꢁ1 for
the dimerization and the inclusion). From these equilibrium
constants the value of ð½Dꢄ þ ½D{CDꢄÞ=½MBþꢄ0 ¼ 0:035 was
obtained for ½MBþꢄ0 ¼ 4:27 ꢃ 10ꢁ6 mol dmꢁ3 (the initial con-
centration of MBþ in Fig. 4) and ½ꢁ-CDꢄ0 ¼ 2:56 ꢃ 10ꢁ3
mol dmꢁ3 (the highest concentration of ꢁ-CD in Fig. 4). This
shows that the existence of the dimer species was negligible
under the conditions shown in Fig. 4.
k0½MBꢄ0
1 þ KS½Dnꢄ
kobsdðSUR) ¼
þ kSD
;
ð13Þ
where kobsd(SUR) is the apparent rate constant in the presence
of surfactant, KS is the associate constant of BNAH to micelles
and [Dn] is the concentration of micellized surfactants (½Dnꢄ ¼
½SURꢄtotal ꢁ cmc).
Herzfeld et al. measured the cmc of dodecylammonium
chloride in the presence of alcohols and salts and found that
the cmc depended on alcohol and salt concentrations.42 It is
also known that the cmc increased with the addition of ꢀ-
CD.30,35 Since in the present study we measured the reaction
rates in 10% ethanol–90% water solutions with buffer and
examined the effects of ꢀ- and ꢁ-CD, we obtained the cmc
of surfactants under several conditions mentioned above and
show the results in Table 1.
The solid lines in Fig. 5 show the values calculated by
Eq. 13 using KS ¼ 120, 80, and 40 dm3 molꢁ1 for HTAB,
TTAB, and DTAB, respectively (cmc for HTAB, TTAB, and
DTAB obtained in 10 vol % ethanol–90 vol % water mixed sol-
vent were 0.65, 1.41, and 11:1 ꢃ 10ꢁ3 mol dmꢁ3, respective-
ly). The observed and calculated values were in good agree-
ment. BNAH was more easily associated with the micelles
in the order: DTAB < TTAB < HTAB.
Effects of Surfactants on the Reaction of BNAH with
MBþ. Figure 5 shows the plots of kobsd against [SUR] for
HTAB, TTAB, and DTAB. As shown in Fig. 5, the kobsd
-
Figure 6 shows the kobsd–[SDS] profile. The kobsd value was
constant in the low [SDS] region (below the cmc), sharply in-
creased at higher [SDS] than the cmc, passed through a maxi-
mum value, then decreased at higher surfactant concentrations.
These findings were typical for micellar effects of anionic
micelles in reactions between a hydrophobic neutral substrate
and a cationic substrate.
As mentioned in a previous paper, the absorption spectrum
of MBþ depended on the SDS concentration.35 Figure 7 shows
the change in the absorbance of MBþ at 655 nm with increas-
values were almost constant below cmc and they decreased
with increasing the concentrations of surfactants above cmc.
25
20
15
10
Table 1. The cmc of HTAB, TTAB, and DTAB in 10 vol %
ethanol–90 vol % water Buffer Solutions (0.2 mol dmꢁ3
NaOH–Na4B4O8 Buffer; pH 9.4) in the Presence of ꢀ-CD
0
10
20
103 cmc/mol dmꢁ3
103[surfactants]0 / mol dm-3
103[ꢀ-CD]/mol dmꢁ3
HTAB
TTAB
DTAB
0
0.65
1.32
1.90
3.31
1.41
1.98
2.54
5.06
11.1
—
14.1
—
Fig. 5. Variations of kobsd in the hydride-transfer reaction
of BNAH with MBþ in micellar solutions of HTAB,
TTAB, and DTAB as functions of the surfactant concen-
tration. Solid lines show the calculated values (see text).
1.0
2.0
4.0

T. Matsumoto et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 7 (2007) 1387
150
6
4
2
0
100
50
0
0
0.01
0.02
0.03
-4
-3
log [SDS]0 / mol dm
-2
-3
-3
[SDS]0 / mol dm
Fig. 8. Effects of SDS concentration on kobsd=ko0bsd for the
reaction of BNAH with MBþ. ½BNAHꢄ0 ¼ 1:40 ꢃ 10ꢁ5
mol dmꢁ3; ½MBꢄ0 ¼ 4:27 ꢃ 10ꢁ6 mol dmꢁ3. The solid line
shows the calculated values (see text).
Fig. 6. Variations of kobsd in the hydride-transfer reaction
of BNAH with MBþ in micellar solutions of SDS as a
function of the SDS concentration.
1.1
1
above the cmc.
The tendencies of kobsd shown in Fig. 6 were consistent with
previous observations for other reactions between neutral and
cationic reactants. Binding of BNAH and MBþ to SDS mi-
celles began at the cmc, hence the concentration of both re-
agents in the small volume of the micelle explained the sharp
increase in kobsd, but the continuous dilution of reagents within
the micellar interface with the increase in [SDS] caused a de-
crease in kobsd (the fraction of BNAH accommodated into the
micelles which were not associated with MBþ at their surfaces
increased with increasing micelle concentration).
As mentioned above, the overall reaction rate was the sum
of the rate in solution and in the micellar phase (in the present
case, since MBþ was associated at the surface of SDS micelle
and BNAH was accommodated into the micelle, it was
assumed that the reaction occurred at the micelle which asso-
ciated both of MBþ and BNAH). The expression of kobsd was
obtained from reactions (1) and (14)–(16) and given in Eq. 17.
0.9
0.8
0
1
2
103[SDS]0 / mol dm-3
Fig. 7. Effect of SDS on the absorbance of MBþ at 664 nm
in 0.2 mol dmꢁ3 NaOH–sodium tetraborate buffer (10
vol % ethanol–90 vol % water) at pH 9.4.
MBþ þ Dn KꢀS1 MB{Dn;
ð14Þ
ð15Þ
ð16Þ
ing [SDS]. The change observed in this study was somewhat
different from that shown in a previous paper. This difference
was due to the differences in the cmc of SDS in water and
ethanol–water mixture (and also the presence of buffer re-
agents). As shown in Fig. 7, initially, the absorbance of MBþ
at 655 nm decreased with increasing SDS concentration, but
increased gradually above the cmc. At low SDS concentra-
tions, formation of MBþ–SDS aggregates started with the
ion-pair (MBþSDSꢁ). Above the cmc, aggregates reorganized
at the surface of micelles with a monomeric MBþ form, result-
ing in an increase in the absorbance of MBþ. With further in-
creases in [SDS], the absorbance of MBþ reached its limiting
value and all MBþ molecules were accommodated at the sur-
face of micelles as monomeric molecules.
As shown in Figs. 6 and 7, the change in kobsd with increas-
ing SDS concentration was not consistent with that in the ab-
sorbance of MBþ, except for the part of the increase in kobsd
above the cmc. The formation of the ion pair (MBþSDSꢁ)
and its aggregates ((MBþSDSꢁ)n) at low SDS concentrations
did not affect the kobsd value. The formation of the monomeric
MBþ molecules at the surface of the micelles increased kobsd
BNAH þ Dn KꢀS2 BNAH{Dn;
km
(MB{BNAH)m{Dn ! MBH þ BNAþ þ Dn;
where Dn shows the SDS micelle and (MB–BNAH)m denotes
the micelle which have MBþ and BNAH.
ðk0 þ kmKS1KS2½DnꢄÞ½MBþꢄ0
kobsd
¼
þ kSD
:
ð17Þ
ð1 þ KS1½DnꢄÞð1 þ KS2½DnꢄÞ
The following equation could be obtained by neglecting kSD
kobsd
ko0bsd
ð1 þ ðkm=k0ÞKS1KS2½DnꢄÞ
¼
;
ð18Þ
ð1 þ KS1½DnꢄÞð1 þ KS2½DnꢄÞ
where ko0bsd shows the kobsd-value in the absence of SDS
micelle.
Figure 8 shows the plots of kobsd=ko0bsd against [Dn]. The
solid line shown in Fig. 8 was obtained from Eq. 18 using
km=k0 ¼ 0:023 mol dmꢁ3, KS1 ¼ 6000 dmꢁ3 mol and KS2
¼
400 dmꢁ3 mol estimated by trial and error method (KS1 and
KS2 are symmetrical in Eq. 18 and they are not distinguishable.

1388 Bull. Chem. Soc. Jpn. Vol. 80, No. 7 (2007)
Effects of CD and Surfactants on Reaction Rate
kobsd
¼
25
20
15
10
(a)
k0½MBþꢄ
þ kSD
:
ð20Þ
ð1 þ K1½CDfꢄ þ KSUR½SURfꢄÞð1 þ K2½CDfꢄÞ
In the surfactant concentration region below the cmc, a com-
plex of surfactant monomers with ꢀ-CD was formed, and the
decrease in the concentration of free ꢀ-CD induced the dis-
placement of BNAH and MBþ from the cavity of ꢀ-CD to
the aqueous medium. The increase in the concentration of free
BNAH and MBþ increased kobsd until the effect of ꢀ-CD on
kobsd became negligible.
0
5
10
15
-3
103[HTAB]0 / mol dm
In this region of surfactant concentration, the total ꢀ-CD
and surfactant concentrations were expressed as
25
20
15
10
(b)
½CDtotalꢄ ¼ ½CDfꢄ þ ½BANH{CDꢄ
þ ½MB{CDꢄ þ ½SUR{CDꢄ
¼ ½CDfꢄ þ K1½BNAHꢄ½CDfꢄ þ K2½MBþꢄ½CDꢄ
þ KSUR½SURfꢄ½CDfꢄ;
½SURtotalꢄ ¼ ½SURfꢄ þ ½SUR{CDꢄ
¼ ½SURfꢄ þ KSUR½SURfꢄ½CDfꢄ:
ð21Þ
0
5
10
15
ð22Þ
103[TTAB]0 / mol dm-3
As mentioned above, KBNAH ¼ 220 dm3 molꢁ1, KMB ¼ 130
dm3 molꢁ1, and KSUR reported were about two orders larger
than these values in aqueous solution.31–33 Since the concentra-
tion of surfactants were higher than those of BNAH and MBþ
in this study, the concentrations of BANH–CD and MB–CD in
determining the concentration of free CD was negligible. From
the above equations, the following equation was obtained by
neglecting the concentrations of BANH–CD and MB–CD:
25
20
15
(c)
2
KSUR½CDfꢄ þ ð1 þ KSUR½SURtotal
ꢄ
0
10
20
30
ꢁ ½CDtotalꢄÞ½CDfꢄ ꢁ ½CDtotalꢄ ¼ 0:
ð23Þ
103[DTAB]0 / mol dm-3
By assuming the KSUR-values for HTAB, TTAB, and DTAB,
and using ½CDtotalꢄ ¼ 2:0 ꢃ 10ꢁ3 mol dmꢁ3, [CDf] was calcu-
lated as a function of [SURtotal].
Fig. 9. Effects of (a) [HTAB], (b) [TTAB], and (c) [DTAB]
on kobsd in the hydride-transfer reaction of BNAH with
MBþ in the presence of ꢀ-CD (½ꢀ-CDꢄ ¼ 2:0 ꢃ 10ꢁ3
mol dmꢁ3. Solid lines show the calculated values (see
text).
Using the values of [CDf] obtained (the values of 10000,
30000, and 100000 dm3 molꢁ1 for KSUR of DTAB, TTAB,
and HTAB were used), kobsd was calculated from Eq. 20.
The solid lines in Fig. 9 show the calculated values of kobsd. As
shown in Fig. 9, the calculated and observed values for kobsd
were in good agreement at low concentrations of surfactants.
As shown in Fig. 9, the roles of surfactants in the effect of
ꢀ-CD on kobsd were almost saturated near or below the cmc.
Therefore, above the cmc, the concentration of free ꢀ-CD
was almost zero, and the change in kobsd was exclusively
caused by the increasing association of BNAH with micelles.
By setting ½CDfꢄ ¼ 0, Eq. 20 was rewritten as
We assigned the value shown above for KS1 and KS2, because
KS1 reflected the spectral change of MBþ in the presence of
SDS (Fig. 7), and KS2 must be the similar values to the asso-
ciation constant s for BNAH–cationic surfactants systems).
As shown in Fig. 8, the observed and calculated values were
in good agreement.
Effects of Cyclodextrins and Cationic Surfactants on the
Reaction of BNAH with MBþ. Figure 9 shows the effects
of HTAB, TTAB, and DTAB on kobsd in the presence of ꢀ-
CD. In each series, kobsd increased with surfactant concentra-
tion up to a maximum and then decreased in the absence of
ꢀ-CD. The surfactant concentration required to begin decreas-
ing kobsd increased with a decrease in the chain length of the
surfactant (with increases in the cmc). The experimental
behavior shown in Fig. 9 was explained qualitatively by con-
sidering the overlap of the effects of ꢀ-CD and surfactants.
k0½MBþꢄ
1 þ KS½Dnꢄ
kobsd
¼
þ kSD
ð12Þ
As mentioned above, the cmc increased with increasing ꢀ-CD
concentration. At the concentration of ꢀ-CD used in this study,
the cmcs of HTAB, TTAB, and DTAB were 1.90, 2.54, and
14:1 ꢃ 10ꢁ3 mol dmꢁ3, respectively (see Table 1).
The solid lines above the cmc in Fig. 9 were obtained from
Eq. 12 by using the KS-value given above for the effects of
surfactants on kobsd in the absence of ꢀ-CD and by using the
cmc shown in Table 1. As shown in Fig. 9, the calculated
and observed values were in good agreement. Since the cmc
SUR þ ꢀ-CD KꢀSUR SUR{CD:
ð19Þ
According to reactions (2)–(4) and (19), the following equa-
tion for kobsd was obtained:

T. Matsumoto et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 7 (2007) 1389
25
80
60
40
20
20
15
10
0
5
10
15
0
5
10
15
103[HTAB]0 / mol dm-3
103[TTAB]0 / mol dm-3
Fig. 10. Effects of [HTAB] on kobsd in the hydride-transfer
Fig. 11. Effects of [HTAB] on kobsd in the hydride-transfer
reaction of BNAH with MBþ in the presence of ꢁ-CD
(½ꢁ-CDꢄ ¼ 2:56 ꢃ 10ꢁ3 mol dmꢁ3). Solid and dashed lines
show the calculated values (see text).
reaction of BNAH with MBþ in the presence of ꢀ-CD (
:
½ꢀ-CDꢄ ¼ 2:0 ꢃ 10ꢁ3 mol dmꢁ3 and : ½ꢀ-CDꢄ ¼ 4:0 ꢃ
10ꢁ3 mol dmꢁ3). Solid and dashed lines show the calculat-
ed values (see text).
T{CD þ TTAB Kꢀ2T T2{CD;
where T–CD and T2–CD are the 1:1 and 2:1 complexes be-
ð25Þ
of DTAB in the presence of ꢀ-CD is large, the influence of
DTAB in the effect of ꢀ-CD on kobsd was saturated consider-
ably below the cmc. Therefore, as shown in Fig. 9, kobsd is con-
stant from the saturation point to the cmc for DTAB.
Figure 10 shows the effect of HTAB concentration on kobsd
in the presence of ½ꢀ-CDꢄ ¼ 2:0 and 4:0 ꢃ 10ꢁ3 mol dmꢁ3. An
increase in ꢀ-CD concentration decreased the peak value of
kobsd and increased the HTAB concentration at which the peak
appears. In the case of ½ꢀ-CDꢄ ¼ 4:0 ꢃ 10ꢁ3 mol dmꢁ3, the
peak value of kobsd was smaller than that in the absence of
ꢀ-CD and HTAB. This indicated that some amount of free
ꢀ-CD existed at the cmc at which the peak of kobsd appeared.
Above the cmc, however, the effect of the association of
BNAH with micelles seemed to be predominant. For both ꢀ-
CD concentrations, the observed and calculated values were
in good agreement (the dashed lines were obtained by the simi-
lar method mentioned above). This indicated that the decrease
in kobsd above the cmc was explained by the effect of the asso-
ciation of BNAH with micelles. An increase in the surfactant
concentration at which the maximum value of kobsd appeared
with increasing ꢀ-CD concentrations was also observed for
TTAB. For TTAB, the peak value of kobsd was similar to that
in the absence of ꢀ-CD and TTAB. The increase of TTAB
concentration at the peak was explained again by the increase
in the cmc with increasing ꢀ-CD concentration.
Figure 11 shows the effects of TTAB concentration on kobsd
in the presence of ꢁ-CD (½ꢁ-CDꢄ ¼ 2:56 ꢃ 10ꢁ3 mol dmꢁ3). As
mentioned above, the addition of ꢁ-CD increased kobsd (shown
by Eq. 11). As shown in Fig. 11, kobsd decreased with increas-
ing TTAB concentration. The effects of TTAB on kobsd were
thought to be due to two factors (the formation of an inclusion
complex of TTAB monomer and the association of BNAH
with micelles) in the presence of ꢁ-CD, as in the presence
of ꢀ-CD. These factors decreased kobsd for the case of ꢁ-CD.
It was reported that ꢁ-CD forms 1:1 and 2:1 TTAB/CD
complexes.26 The formation of these complexes decreased
the concentration of free ꢁ-CD and decreased kobsd by sup-
pressing the enhancing effect of ꢁ-CD.
tween TTAB and ꢁ-CD.
Below the cmc, only the inclusion complex formation af-
fects kobsd. From reactions (24) and (25), the concentrations
of ꢁ-CD and TTAB were expressed as follows:
2
½CDtotalꢄ ¼ ½CDfꢄð1 þ KT½Tfꢄ þ KTK2T½Tfꢄ Þ;
ð26Þ
½Ttotalꢄ ¼ ½Tfꢄð1 þ KT½CDfꢄ þ 2KTK2T½Tfꢄ½CDfꢄÞ; ð27Þ
where [CDtotal] and [Ttotal] are total concentrations of ꢁ-CD and
TTAB, and [CDf] and [Tf] are concentrations of free ꢁ-CD and
TTAB. By assuming the values of KT and K2T, [CDf] were cal-
culated for various total concentrations of TTAB at ½CDtotalꢄ ¼
2:56 ꢃ 10ꢁ3 mol dmꢁ3 by using successive approximations.
The values of kobsd were determined by putting the [CDf] ob-
tained into Eq. 11. The solid line shown in Fig. 9 at the low
concentration region of TTAB were obtained using KT ¼
600 and K2T ¼ 6700 dm3 molꢁ1. These values are similar to
those reported for TTAB–ꢁ-CD system in aqueous solution
by Tominaga et al.26
As shown in Fig. 11, the dashed line fit the kobsd below
½TTABꢄ0 ¼ 2:5 ꢃ 10ꢁ3 mol dmꢁ3. Though the cmc of TTAB
in the presence of ꢁ-CD could not be determined in the present
study, the cmc of TTAB under the present experimental condi-
tions was thought to be about 2:5 ꢃ 10ꢁ3 mol dmꢁ3 from the
result shown in Fig. 11 (somewhat higher than that in the ab-
sence of ꢁ-CD). The effect of the complex formation must
have been constant above the cmc, because the concentration
of free TTAB became constant above the cmc.
As mentioned above, the change in kobsd above the cmc was
caused by increasing association of BNAH with micelles. In
this case, kobsd was expressed by the following equation:
cmc
k
obsd
kobsd
¼
þ kBO
;
ð28Þ
1 þ KS½Dnꢄ
cmc
obsd
where k
is kobsd at the cmc and was assumed to be 52:6 ꢃ
10ꢁ5 sꢁ1. The solid line above the cmc in Fig. 11 was obtained
from Eq. 28 by using KS ¼ 80 dm3 molꢁ1 obtained above for
KT
TTAB þ ꢁ-CD ꢀ T{CD;
ð24Þ

1390 Bull. Chem. Soc. Jpn. Vol. 80, No. 7 (2007)
Effects of CD and Surfactants on Reaction Rate
16 R. Sabate, J. Kstelrich, J. Phys. Chem. B 2003, 107, 4137.
17 J. Yang, J. Colloid Interface Sci. 2004, 63, 181.
18 S. Yamamoto, S. Kobashi, K. Tsutsui, Y. Sueishi,
Spectrochim. Acta, Part A in press.
the effect of TTAB on kobsd in the absence of CD. As shown in
Fig. 11, the calculated and observed values were in good
agreement.
Conclusion
19 A. Connors, Chem. Rew. 1997, 97, 1325.
´
20 E. Iglesias, A. Fernandez, J. Chem. Soc., Perkin Trans. 2
The rate of hydride-transfer reaction of BNAH with MBþ
was suppressed by ꢀ-CD and enhanced by ꢁ-CD. These ef-
fects of CD were explained by the formation of inclusion com-
plexes of BNAH and MBþ with ꢀ-CD and by the formation of
a 1:1:1 BNAH–MBþ–ꢁ-CD complex. Cationic surfactants
(DTAB, TTAB, and HTAB) also suppressed the rate above
the cmc, while anionic surfactant (SDS) showed a complex
kobsd–[SDS] profile. These effects of surfactants were due to
the association of BNAH with micelles for cationic surfactants
and due to the association of BNAH and the attachment of
MBþ on the surface of SDS micelle for SDS. The change in
kobsd observed in CD and cationic surfactant mixtures was ex-
plained by the model which involves the formation of inclu-
sion complexes of BNAH, MBþ, and surfactant with CD
and the association of BNAH with micelles.
1998, 1691.
21 L. Garcia-Rio, J. R. Leis, J. C. Mejuto, J. Perez-Juste,
J. Phys. Chem. 1997, 101, 7383.
22 C. Lee, Y. W. Sung, J. W. Park, J. Phys. Chem. B 1999,
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23 S. Hamai, H. Satou, Bull. Chem. Soc. Jpn. 2000, 73, 2207.
24 X. Guo, H. Xu, R. Guo, Colloid Polym. Sci. 2003, 281,
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25 T. Okubo, Y. Maeda, H. Kitano, J. Phys. Chem. 1989, 93,
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26 T. Tominaga, D. Hachisu, M. Kamada, Langumuir 1994,
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27 I. Topchieva, K. Karezin, J. Colloid Interface Sci. 1999,
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28 B. Dorrego, L. Garcia-Rio, P. Herves, J. R. Leis, J. C.
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