An apparent weakness of the pseudophase ion-exchange (PIE) model for micellar catalysis by cationic surfactants with nonreactive counterions

Article abstract of DOI:10.1039/b101840j

Pseudo-first-order rate constants (k_obs) for alkaline hydrolysis of phenyl benzoate (PB) show maxima in k_obs-[D_n] profiles at constant [NaOH] in the presence of cetyltrimethylammonium chloride (CTACl) micelles (D_n). These observed data have been explained in terms of the pseudophase ion-exchange (PIE) model. But the quality of the data fit to the kinetic equation derived from the PIE model approach remains essentially unchanged with a large change in ion-exchange constant (K_Cl^OH) and in fractional micellar neutralization (β). The use of the PIE model for k_obs, obtained under varying concentrations of added NaCl salt at a constant [NaOH] and [CTACl]_T, gives a K_s value significantly different from the K_s value obtained from k_obs-[D_n]. These observed data (k_obs-[MX], where X = Cl and Br) were also treated in terms of the pseudophase micellar (PM) model coupled with an empirical relationship:K_OH = K_OH⁰/(1 + Ψ_X-OH[MX]) where K_OH is the CTACl micellar binding constant of HO^- in the presence of MX and Ψ_X-OH is an empirical parameter. This data treatment, which does not require the constancy of K_X^OH and β as needed in the PIE model, gives data fitting as good as the PIE model. The values of Ψ_X-OH for Cl^- and Br^- are explained with conceivable chemical reasoning.

Full text of DOI:10.1039/b101840j

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An apparent weakness of the pseudophase ion-exchange (PIE)
model for micellar catalysis by cationic surfactants with
nonreactive counterions†
2
Mohammad Niyaz Khan* and Emran Ismail
Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur,
Malaysia. E-mail: niyaz@kimia.um.edu.my
Received (in Cambridge, UK) 27th February 2001, Accepted 21st May 2001
First published as an Advance Article on the web 22nd June 2001
Pseudo-first-order rate constants (kobs) for alkaline hydrolysis of phenyl benzoate (PB) show maxima in kobs–[Dn]
profiles at constant [NaOH] in the presence of cetyltrimethylammonium chloride (CTACl) micelles (D ). These
observed data have been explained in terms of the pseudophase ion-exchange (PIE) model. But the quality of the
n
data fit to the kinetic equation derived from the PIE model approach remains essentially unchanged with a large
OH
change in ion-exchange constant (KCl ) and in fractional micellar neutralization (β). The use of the PIE model for
kobs, obtained under varying concentrations of added NaCl salt at a constant [NaOH] and [CTACl] , gives a K value
T
S
significantly different from the K value obtained from k –[D ]. These observed data (kobs–[MX], where X = Cl and
S
obs
n
Br) were also treated in terms of the pseudophase micellar (PM) model coupled with an empirical relationship:
0
Ϫ
KOH = KOH /(1 ϩ ΨX–OH[MX]) where KOH is the CTACl micellar binding constant of HO in the presence of MX and
OH
ΨX–OH is an empirical parameter. This data treatment, which does not require the constancy of K
in the PIE model, gives data fitting as good as the PIE model. The values of ΨX–OH for Cl and Br are explained with
X
Ϫ
and β as needed
Ϫ
conceivable chemical reasoning.
Introduction
ment between values of selectivity coefficients for exchange of
highly hydrophilic counterions determined by different methods
1
The pseudophase ion-exchange (PIE) model was developed
12
is also attributed to the breakdown of the PIE model.
due to failure of the kinetic model of micellar-mediated reac-
13
Bunton, in an excellent review, called this whole exercise a
2
tion to explain the kobs–[D ] profiles involving maxima
n
somewhat jaundiced view of quantitative models of micellar
obtained for ionic micellar-mediated bimolecular reactions
where one of the reactants was ionic with charge similar to the
charge of counterions of micelles (D ). The success of the PIE
14
rate effects. Germani et al. have pointed out, without giving
the details of analytical data, that a major problem in using ion-
n
exchange treatments is that rate data can be fitted by using
model is generally emphasized within the domain of consider-
ably lower residual errors between observed and calculated rate
constants at varying concentrations of micelles. The increase in
the number of assumptions decreases the diverse applicability
of a model. There are a greater number of assumptions in the
PIE model than in the kinetic model of micellar-mediated reac-
OH
a variety of values for parameters such as β, KBr , or K Ј.
X
Recently, it has been reported that the quality of the fitting of
observed data on alkaline hydrolysis of securinine in terms of
the PIE model remained almost unchanged with the change in
OH
4
15
15
K
Br from 5 to 10 at β = 0.8. In this study, the kobs–[CTABr]
profile revealed an insignificant maximum. Very strong maxima
in kobs–[CTABr] profiles were obtained in the alkaline
hydrolysis of phenyl benzoate in the presence of CTABr
2
tion, and hence the applicability of the PIE model should be
expected to be more limited than the kinetic model of micellar-
mediated reaction. The applicability of the PIE model has
been subjected to experimental verification using various non-
kinetic techniques. Although the PIE model explained quanti-
2
16
micelles. In order to test the apparent weakness of the PIE
model described in ref. 15, we considered the present reacting
3
system which is expected to show strong maxima in the kobs
–
tatively or semi-quantitatively a large amount of kinetic data on
bimolecular semi-ionic or ionic reactions in the presence of
[CTACl] profiles. The observed rate data on alkaline hydrolysis
of phenyl benzoate and insensitivity of the fitting of these rate
1
b,4
5
6
ionic micelles,
vesicles, reversed micelles and microemul-
Ϫ
data (kobs versus [D ] at a constant [HO ]) in terms of the PIE
7
n
sions, it failed to explain the rates of reactions of very hydro-
OH
model to a large change (i) in KCl at a constant β and (ii) in β at
Ϫ
Ϫ 1b,8
philic ions such as HO and F .
explained in terms of the mass action (MA) model, the
Poisson–Boltzmann equation (PBE) in spherical symmetry
and allowing different β values (estimated conductimetrically)
at different concentrations of CTAOH and CTAF (where CTA
These data have been
OH
a constant KCl , are described in this manuscript.
9
10
Experimental
Materials
11
represents the cetyltrimethylammonium group‡). Poor agree-
All the chemicals used were supplied by Fluka, Sigma or
Aldrich and were of the highest commercially available purity.
The stock solutions of phenyl benzoate (PB) were prepared in
acetonitrile.
†
Electronic supplementary information (ESI) available: Tables I–IV
listing kobs at different [CTACl] and at 0.005, 0.010, 0.015 and 0.020 M
NaOH; Tables V–VII listing kobs at different [NaCl] and at 0.01 M
NaOH and at 0.007 and 0.01 M CTACl; Table VIII listing kobs at differ-
ent [NaBr] and at 0.007 and 0.01 M CTACl in the presence of 0.01 M
NaOH. See http://www.rsc.org/suppdata/p2/b1/b101840j/
Kinetic measurements
The rate of hydrolysis of PB was studied spectrophoto-
metrically by monitoring the appearance of the product, the
‡
The IUPAC name for cetyl is hexadecanyl.
1
346
J. Chem. Soc., Perkin Trans. 2, 2001, 1346–1350
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b101840j

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phenolate ion, as a function of time at 290 nm and 35 ЊC. The
The reaction scheme, in terms of the PM model, can lead to
eqn. (6) where k2,W is the second-order rate constant for the
details of the kinetic procedure, and data analysis are described
17
elsewhere.
(
6)
Results and discussion
Several kinetic runs were carried out within the total cetyltri-
methylammonium chloride concentration ([CTACl] ) range of
T
Ϫ
Ϫ3
Ϫ3
reaction of HO with phenyl benzoate (PB) in the aqueous
0
.0 to 0.1 mol dm at 0.005 mol dm NaOH at 35 ЊC in an
pseudophase, K is the CTACl micellar binding constant of PB
S
aqueous solvent containing 2% v/v CH CN. Similar observ-
3
Ϫ3
and the rate of bimolecular reaction in the micellar pseudo-
phase is defined as rate = kЈ2,M mOH [PBM].
The unknown parameters, kЈ2 and K , as well as the least-
squares (Σd , where d = k
from eqn. (6) using observed data, kobs versus [D ], and calcu-
lated values of mOH. The values of mOH at different [D ] were
calculated by solving quadratic eqn. (5) at constant given values
of β and KCl with known values of [HO ] , [Cl ] , and critical
ations were obtained at 0.01, 0.015 and 0.02 mol dm NaOH.
Pseudo-first-order rate constants (kobs) for these runs are sum-
marized in Tables I–IV in the electronic supplementary infor-
mation (ESI). These results show the presence of maxima in the
,M
S
2
Ϫ kcalcd i) value were calculated
i
i
obs
i
n
kobs–[CTACl] profiles at a constant [NaOH] within the range
T
Ϫ3
n
0
.005–0.020 mol dm .
A few kinetic runs were also carried out within the [NaX]
OH
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ4
Ϫ3
T T
(
where X = Cl and Br ) range of 5 × 10 to ≤ 0.3 mol dm
Ϫ3 Ϫ3
micellization concentration (c.m.c). The values of c.m.c. at dif-
19
at 0.007 mol dm CTACl, 0.01 mol dm NaOH, 35 ЊC and 2%
ferent [NaOH] were obtained by a graphical technique. The
v/v CH CN in aqueous solvent. Similar observations were car-
3
2
Ϫ3
calculated values of kЈ2 , K and Σd at different combinations
,M
S
i
ried out at 0.01 mol dm CTACl. Pseudo-first-order rate con-
OH
of KCl and β and at different [NaOH] are summarized in Table
1. The quality of the fit of observed data to eqn. (6) in terms of
the PIE model is evident from the calculated values of rate
constants (kcalcd) and the least-squares values (Tables I–IV in the
ESI and Table 1).
In order to find out the effects of a large variation in KCl
values at a constant β and in β values at a constant KCl on the
quality of the fit of the observed data to eqn. (6), the values of
kcalcd at different [D ] were obtained by the nonlinear least-
squares calculated values of kЈ2 and K at constant values of
KCl and β (Table 1). The values of KCl were arbitrarily varied
from 2 to 500 at β = 0.7 (the reported value of β, not strictly
obtained under the kinetic conditions of the present study, is
.7 ). Similarly, the values of β were varied from 0.1 to 0.9 at
Cl = 5 (the value of KCl = 5 was considered for the reason that
the experimentally determined values of KBr , KCl and KBr are
–31, 3–11 and 2.5, respectively). Among the various pos-
stants (kobs) for these runs are summarized in Tables V–VIII in
Ϫ3
the ESI. The increase in [NaX] from 0.0 to ≤0.3 mol dm
decreased kobs monotonically by ∼16- to 27-fold at 0.007 and
Ϫ3
0
.01 mol dm CTACl.
Pseudo-first-order rate constants (kobs), obtained at
OH
[
CTACl] = 0, show a linear increase with the increase in [NaOH]
OH
Ϫ3
from 0.005 to 0.020 mol dm with the intercept essentially at
zero and slope = 0.68 ± 0.02 dm mol s . Similar observa-
tions have been reported previously. The appearance of an
3
Ϫ1 Ϫ1
18
n
,M
S
intercept at zero was concluded to be due to the insignificance
OH
OH
Ϫ
of kH2O[H O][PB] compared to k [HO ][PB] in the rate law
2
OH
18
for alkaline hydrolysis of PB. Thus, under the present experiϪ-
mental conditions, the rate of hydrolysis of PB involves HO
and PB as the reactants.
1
1
0
K
OH
OH
The maxima in the kobs–[CTACl] profiles (Tables I–IV in the
T
OH OH Cl
ESI) are typical for ionic micellar-mediated bimolecular reac-
tions involving neutral and ionic reactants with the charge on
the ionic reactant similar to the charge of the counterion of the
1b
1b
3d
7
sible inferences that could be drawn from the values of kЈ2,M,
K , Σd (Table 1), percentage residual error (PRE) and k at
calcd
different values of KCl and β (Tables I–IV in the ESI), the most
obvious one may be described as follows.
2
S
i
ionic micelle, D . Such observed rate data are generally
n
OH
1
explained in terms of the PIE model. This model contains
essentially all the assumptions involved in the kinetic model of
2
2
Although the least-squares Σd values show a change with
i
micellar-mediated reaction and some additional assumptions
OH
1
,4
the change in KCl from 2 to 500 at β = 0.7 and in β from 0.1 to
which are well documented in several excellent reviews.
OH
Ϫ
0.9 at KCl = 5, the calculated values of rate constants (kcalcd)
and residual error do not differ appreciably under such condi-
tions (Tables I–IV in the ESI). The lowest values of Σd for the
observed data at 0.005, 0.01, 0.015 and 0.02 mol dm NaOH
are at KCl = 2, β = 0.7; KCl = 5, β = 0.1; KCl = 5, β = 0.1 and
KCl = 5, β = 0.9, respectively (Table 1). The values of both kЈ2,M
and K vary by many-fold in an irregular order under such
In view of the PIE model, the concentrations of HO (ionic
reactant) and Cl (micellar counterion) in the micellar pseudo-
Ϫ
2
i
phase are governed by an ion-exchange equilibrium, eqn. (1)
Ϫ3
OH
OH
OH
OH
Ϫ
Ϫ
Ϫ
Ϫ
W
OH
K
Cl = [HO W][Cl M]/([HO M][Cl ]) =
Ϫ
Ϫ
[
HO ]m /(mOH[Cl W]) (1)
W
Cl
S
specific conditions (Table 1). Thus, the criterion of minimum
2
where subscripts W and M represent aqueous pseudophase
value of Σdi for the best fit is meaningless in this case.
Ϫ
Ϫ3
2
and micellar pseudophase, respectively, m = [Cl ]/[D ] and
Furthermore, at 0.01 mol dm NaOH, the value of Σdi
Cl
M
n
Ϫ
Ϫ6
OH
mOH = [HO M]/[D ]. The fraction of counterions bound to the
micelle, β, total concentrations of Cl , [Cl ] , and HO ,
( = 22.36 × 10 ) at K = 2, β = 0.7 is not appreciably different
2 Ϫ6
OH
n
Cl
Ϫ
Ϫ
Ϫ
T
from Σd ( = 21.63 × 10 ) at K = 5, β = 0.1. But the values of
i
Cl
Ϫ
OH
Cl
[
HO ] , may be given by eqns. (2)–(4).
kЈ2,M and K at K = 2, β = 0.7 are very much different from the
S
OH
T
corresponding values at KCl = 5, β = 0.1 (Table 1). It may also
OH
be noted that at any set of values of KCl and β, the values of KS
mCl ϩ mOH = β
(2)
(3)
(4)
change by more that 50% with the change in [NaOH] from
Ϫ3
Ϫ
Ϫ
0.005 to 0.02 mol dm (Table 1) which cannot be explained
[
Cl ] = [Cl ] ϩ m [D ]
T
W
Cl
n
easily. Thus, it is difficult to ascertain with confidence the exact
OH
values of KCl and β in terms of the PIE model. It is apparent
Ϫ
Ϫ
[
HO ] = [HO W] ϩ mOH[Dn]
T
that kcalcd values change only slightly with a large change in
OH
OH
either KCl at constant β or β at constant KCl (Tables I–IV in the
Eqns. (1)–(4) yield eqn. (5).
ESI). It is therefore necessary to determine the exact values of β
Y
X
and the ion-exchange constant (K
) under the typical experi-
mental conditions of kinetic runs by using methods other than
a kinetic one in order to get reliable values of the unknown
(
5)
parameters kЈ2,M and K by using the PIE model. Alternatively,
S
J. Chem. Soc., Perkin Trans. 2, 2001, 1346–1350
1347

View Article Online
2
Table 1 Values of kinetic parameters (kЈ2,M and K ) and least-squares (Σd ) at different [NaOH] (in the absence of added inert salt) calculated from
S
i
a
eqn. (6)
4
1
0 [CTACl] range/
T
Ϫ3
4
Ϫ3
OH
Cl
3
Ϫ1
3
Ϫ1
6
2
Ϫ3
[NaOH]/mol dm
10 c.m.c./mol dm
K
β
10 kЈ2,M/s
K /dm mol
10 Σd
mol dm
S
i
b
b
0.005
0.010
0.015
0.020
3.4
2.6
3.2
2.7
2
0
00
5
5
5
2
0
00
5
5
5
2
0
00
5
5
5
2
0.7
0.7
0.7
0.1
0.7
0.9
0.7
0.7
0.7
0.1
0.7
0.9
0.7
0.7
0.7
0.1
0.7
0.9
0.7
0.7
0.7
0.1
0.7
0.9
77.3 ± 3.6
220 ± 15
8910 ± 855
1846 ± 144
133 ± 8
247 ± 24
150 ± 18
81 ± 12
4.724
8.111
11.15
8.237
6.740
8.415
22.36
30.84
62.51
21.63
25.90
34.65
20.58
18.36
42.01
10.77
18.14
33.79
19.32
22.58
35.06
41.65
19.33
17.91
4–1000
1
5
5
5
5
96 ± 12
186 ± 21
259 ± 32
332 ± 38
164 ± 20
57 ± 10
116 ± 12
221 ± 25
291 ± 39
430 ± 39
191 ± 14
61 ± 7
78.3 ± 4.6
88.8 ± 4.0
237 ± 15
9537 ± 1224
1803 ± 107
147 ± 8
3–1000
4–1000
3–1000
1
93.3 ± 5.4
96.7 ± 2.9
243 ± 8
1
8888 ± 661
1749 ± 50
155 ± 5
141 ± 8
267 ± 20
347 ± 37
568 ± 46
219 ± 16
53 ± 5
172 ± 17
324 ± 23
412 ± 29
102 ± 4
89.1 ± 2.1
211 ± 7
1
0
00
5
5
5
7481 ± 449
1447 ± 66
138 ± 4
94.1 ± 2.2
a
Ϫ4
Ϫ3
b
[
PB] = 2 × 10 mol dm , λ = 290 nm, 35 ЊC and the aqueous reaction mixture in each kinetic run contains 2% v/v CH CN. Error limits are
0
3
standard deviations.
Y
X
Y
X
β and K or K
and K or β, K
and K should not be considered
ESI). But such characteristic residual errors are not noticeable
S
S
S
as disposable parameters in order to get reliable values of kЈ2
in the treatment of the observed data kobs–[D ] shown in Tables
,M
n
Y
and K
X
or kЈ2,M and β or kЈ2,M. Although, in principle, all the
I–IV in the ESI.
The minimum value of Σd turned out to be at KCl = 3 and
2
OH
parameters except kЈ2,M can be measured independently, prac-
tically it is not so simple to measure these parameters under
typical reaction kinetic conditions. Furthermore, it has been
i
Ϫ3
β = 0.7 at 0.007 and 0.01 mol dm CTACl for kobs obtained at
Ϫ3
2
0.01 mol dm NaOH (Table 2). But the minimum value of Σdi
20
OH
shown that the value of the ion-exchange constant of an ion
was found to be at KCl ≤ 2 and β = 0.7 for kobs obtained at 0.01
Ϫ3
and most likely the micellar binding constant of a molecule are
technique-dependent.
mol dm NaOH, [NaCl] = 0 and a different value of [CTACl]T
(Table 1). It may be noted that statistical significance of the
OH
OH
If the ion-exchange constant KCl is defined as KCl = K /K
calculated value of K seems to be reliable and unreliable in the
Cl
OH
S
where KCl and KOH represent CTACl micellar binding or associ-
absence of NaCl (Table 1) and presence of NaCl (Table 2),
Ϫ
Ϫ
OH
ation constants for Cl and HO , respectively, then it may be
respectively, at KCl ≤ 2 and β = 0.7. The values of K , obtained
S
OH
OH
2
shown that KCl cannot remain constant for the kinetic runs
at KCl = 3 (i.e. the minimum values of Σd ), are nearly 3-fold
i
carried out under varying concentrations of added inert salt
smaller in the absence than in the presence of NaCl at 0.01 mol
dm NaOH (Tables 1 and 2) which cannot be explained easily.
Ϫ3
(
NaCl) at constant [NaOH] and [CTACl] . The mathematical
T
complexity in the use of the PIE model would be increased
compared to one exhibited by eqns. (1)–(6) if the added inert
salt is NaBr or MX instead of NaCl in the presence of CTACl
micelles.
The CTABr micellar binding constants (K ) of neutral benz-
S
imidazole, obtained at different [NaCl], increased from 43 to 68
3
Ϫ1
Ϫ3
dm mol with the increase in [NaCl] from 0.0 to 1.0 mol dm
Ϫ3
at 0.01 mol dm CTACl and this salt effect on K was most
probably attributable to the salting out effect. But the increase
in [NaBr] from 0.0 to 0.3 mol dm did not reveal an appre-
S
21
The rate constants (kobs), obtained at a constant [CTACl]T,
Ϫ3
Ϫ3
0
.01 mol dm NaOH and under varying concentrations of
NaCl, were treated in terms of the PIE model, i.e. through eqns.
ciable effect on K for the sodium dodecyl sulfate micellar bind-
ing constant of PB. Thus, it seems unlikely that <3-fold larger
S
2
18
(
1)–(6). The values of kЈ2,M, K , Σd and PRE, k at β = 0.7
S
i
calcd
OH
and different values of KCl are summarized in Table 2 and
values of K in the presence (Table 2) than in the absence (Table
S
Tables V and VI in ESI, respectively. The value of the c.m.c. of
1) of varying concentrations of NaCl are due merely to the salt
Ϫ4
Ϫ3
Ϫ3
CTACl micelles is 2.6 × 10 mol dm at 0.01 mol dm NaOH
and [NaCl] = 0. The increase in [NaCl] is expected to decrease
effect. It may be noted from the values of calculated parameters
2
(kЈ2,M and K ) in Table 2 that although the values of Σdi
S
Ϫ4
the c.m.c. Thus, the value of c.m.c. should be <2.6 × 10 mol
change, the values of kЈ2,M and K remain essentially unchanged
S
Ϫ3
Ϫ3
dm in the presence of NaCl. But the lowest value of [CTACl]
with the change in [CTACl] from 0.007 to 0.01 mol dm at a
constant KCl (within the KCl range 3–500).
T
T
Ϫ3
OH
OH
is 0.007 mol dm
which shows that [D ]/c.m.c. ≈ 26 at
n
[
NaCl] = 0. It is therefore evident that [D ] ≈ [CTACl] at 0.007
As has been mentioned earlier in the text the use of the PIE
model for the observed data listed in Tables V and VI in the ESI
is rather suspect and for the observed data listed in Table VIII in
the ESI, the use of the PIE model becomes even more difficult
and complex because of the increased mathematical complexity
in arriving at a practically workable kinetic equation. An alter-
native kinetic model to explain kinetic data similar to those
listed in Tables V, VI and VIII in the ESI may be described as
n
T
Ϫ3
Ϫ3
mol dm and 0.01 mol dm CTACl. The calculated values of
kЈ2,M, K and Σd at KCl = 2, 3 and 5 do not show any appre-
ciable change with the change in c.m.c. from 0 to 2 × 10 mol
dm (Table 2) which supports the assumption that [D ] ≈
2
i
OH
S
Ϫ4
Ϫ3
n
[CTACl] under the experimental conditions imposed. It may
T
be noted that the absolute values of residual errors increase
appreciably with the increase or decrease in KCl from its value
at 3, especially at ≥ 0.01 mol dm NaCl (Tables V and VI in the
OH
Ϫ3
follows. The CTABr micellar binding constant (K ) of ionized
S
1
348
J. Chem. Soc., Perkin Trans. 2, 2001, 1346–1350

View Article Online
2
a
Table 2 Values of kinetic parameters (kЈ2,M and K ) and least-squares (Σd ) at different [CTACl] (in the presence of NaCl) calculated from eqn. (6)
S
i
T
Ϫ3
4
Ϫ3
OH
Cl
3
Ϫ1
3
Ϫ1
7
2
3
Ϫ3
[
CTACl] /mol dm
10 c.m.c./mol dm
K
10 kЈ2,M/s
K /dm mol
10 Σd
i
10 [NaCl] range/mol dm
T
S
b
b
0
.007
0.0
2
2
3
3
5
5
7
67.8 ± 1.7
67.3 ± 1.6
89.1 ± 1.4
88.5 ± 1.4
129 ± 5
10366 ± 27218
7446 ± 13165
1142 ± 224
1107 ± 210
507 ± 119
502 ± 121
360 ± 104
270 ± 92
177 ± 76
101 ± 60
83 ± 56
7278 ± 11480
5661 ± 6564
944 ± 178
919 ± 174
438 ± 107
434 ± 108
320 ± 90
28.90
26.43
0–300
2
0
2
0
2
0
.0
.0
.0
.0
.0
.0
9.595
9.902
46.02
50.50
102.7
185.2
384.6
799.6
970.2
9.608
8.826
6.339
6.890
35.81
38.24
72.43
122.2
235.2
447.7
526.6
128 ± 5
168 ± 10
225 ± 20
411 ± 63
1922 ± 560
9589 ± 3467
66.2 ± 1.0
65.8 ± 0.9
86.4 ± 1.1
85.8 ± 1.1
124 ± 4
1
2
0
0
1
5
00
00
2
2
3
3
5
5
0
.010
0.0
0–300
2
0
2
0
2
0
.0
.0
.0
.0
.0
.0
122 ± 4
159 ± 8
7
0
0
1
2
210 ± 14
374 ± 38
1648 ± 267
7974 ± 1484
248 ± 78
173 ± 64
113 ± 52
100 ± 49
1
5
00
00
a
Ϫ4
Ϫ3
Ϫ3
[
PB] = 2 × 10 mol dm , [NaOH] = 0.01 mol dm , β = 0.7, λ = 290 nm, 35 ЊC and the aqueous reaction mixture in each kinetic run contains 2% v/
0
b
v CH CN. Error limits are standard deviations.
3
Ϫ
phenyl salicylate (PS ), obtained at different [KBr] in meth-
seems to be meaningless. The reasonably good fit of observed
22
23
anolysis and [NaBr] in aminolysis, followed an empirical
relationship [eqn. (7)] where MX = KBr or NaBr and ΨBr–PS
data in Table VIII in the ESI to eqn. (8) indicates that the
Ϫ
Ϫ
ion-exchange Br to Cl is insignificant compared with ion-
Ϫ Ϫ
exchange Br to HO which is conceivable for the reason that
Ϫ
0
S
Ϫ
K = K /(1 ϩ ΨBr–PS[MX])
(7)
KBr–OH ӷ KBr–Cl. The effect of ion-exchange from Cl to HO
S
Ϫ Ϫ
may be also ignored compared with Br to HO for the
Ϫ
reasons that (i) ψBr–OH/ψCl–OH ≈ 2 and (ii) the value of [Cl ]
T
is an empirical parameter whose magnitude is the measure of
the ability of a counterion (such as Br ) to expel another coun-
terion (such as PS ) from the micellar pseudophase to the
aqueous pseudophase. The validity of eqn. (7) has been tested
in a few more related studies. If we assume that eqn. (7) is
applicable for the expulsion of HO from the CTACl micellar
pseudophase to the aqueous pseudophase by externally added
NaCl or NaBr at a constant [NaOH] and [CTACl] , then the
Ϫ
is constant and rather low and its effect is a maximum at
[NaBr] = 0.
The calculated values of k are associated with considerably
high standard deviations and consequently these values of k are
statistically unreliable. The values of k and K (Table 3) show
Ϫ
24
Ϫ
that the contribution of the kK[MX] term compared with k in
0
Ϫ3
eqn. (8) is <20% at [NaCl] ≤ 0.1 mol dm . The low contribu-
T
2
tion of kK[MX] compared with k in eqn. (8) is the cause of the
0
kinetic model of the micellar-mediated reaction and eqn. (7)
can lead to eqn. (8) where k = {k ϩ k K K [D ]}/{(1 ϩ
0
unreliability in the values of k summarized in Table 3. The
values of K were also calculated from eqn. (8) with k = 0. These
K values (Table 3) are not very much different from the corres-
ponding K values obtained with k ≠ 0. This shows the negligible
0
W
M
OH
S
n
(
8)
contribution of kK[MX] compared with k in eqn. (8) under the
0
present experimental conditions.
0
K [D ])(1 ϩ K [D )}, k = k /(1 ϩ K [D ]) and K = ΨX–OH/
(
In view of the empirical definition of ψX–Y (= ψBr–PS in eqn.
(7)) where the magnitude of ψX–Y is the measure of the ability
of counterion X to expel another counterion Y from ionic
S
n
OH
n
W
S
n
0
0
1 ϩ KOH [D ]) with k , k and KOH representing the pseudo-
n W M
first-order rate constants for alkaline hydrolysis of PB in aque-
ous pseudophase and micellar pseudophase, and the CTACl
micellar binding constant of HO at [MX] = 0, respectively.
Eqn. (8) was applied to the rate constants (kobs), obtained at
constant [CTACl] , [NaOH] (= 0.01 mol dm ) and different
concentrations of NaCl and NaBr. The nonlinear least-squares
calculated values of k and K at 0.007 mol dm and 0.01 mol
dm CTACl for NaCl and NaBr are shown in Table 3. The
values of k used in the calculation of k and K from eqn. (8)
were obtained experimentally by carrying out kinetic runs at
MX] = 0. The quality of the fit of kobs to eqn. (8) is evident
from the standard deviations associated with the values of cal-
culated parameters, k and K (Table 3), as well as from the
values of calculated rate constants (kcalcd) as shown in Tables
VII and VIII in the ESI. Incidentally, the values of Σd (Table
) for kobs obtained at 0.007 and 0.01 mol dm CTACl
micellar pseudophase to the aqueous pseudophase, the value of
Ϫ
Y
ψ
X–Y must be proportional to K
X
(ion-exchange constant in PIE
and K representing ionic
Y
formalism where K
X
= K /K
X
with K
Y
X
Y
Ϫ3
micellar binding constants of counterions X and Y, respect-
T
OH
ively). Thus, ψCl–OH and ψBr–OH must be proportional to KCl and
Ϫ3
OH
KBr respectively and consequently, ψBr–OH/ψCl–OH should be
Ϫ3
Cl OH OH
equal to KBr (= KBr /KCl ). The average value of KBr–OH/KCl–OH
(= ψBr–OH/ψCl–OH ≈ 2) may be compared with the reported
0
Cl
1b,3d
values of KBr = 2–3.
Similar relationships between KX–S/KY–S
Y
[
(= ψX–S/ψY–S) and ion-exchange constant K
X
have been obtained
16,24b
for various ions X and Y with S = anionic phthalimide
and
These results strengthen the
24a,c–e
anionic phenyl salicylate.
validity of eqn. (7).
2
i
Ϫ3
3
Acknowledgements
through eqn. (8) are almost similar to the corresponding min-
imum values of Σd obtained through PIE treatment (Table 2).
But, as concluded earlier, the criterion of minimum least-
squares value for the best fit in the use of the PIE model
2
i
The authors thank the National Scientific Research and Devel-
opment Council of Malaysia under the IRPA program for
financial support (Grant No. 09–02–03–0003).
J. Chem. Soc., Perkin Trans. 2, 2001, 1346–1350
1349

View Article Online
2
a
Table 3 Values of kinetic parameters (k and K) and least-squares (Σd ) calculated from eqn. (8)
i
Ϫ3
3
Ϫ1
3
Ϫ1
7
2
3
Ϫ3
Salt
[CTACl] /mol dm
10 k/s
K/dm mol
10 Σd
10 [salt] range/mol dm
T
i
b
b
NaCl
0.007
0.51 ± 0.22
0
0.38 ± 0.15
0
Ϫ0.01 ± 0.29
0
Ϫ0.13 ± 0.21
0
104 ± 4
12.86
0–300
0–200
88 ± 17
86 ± 3
79 ± 14
192 ± 11
177 ± 33
167 ± 7
169 ± 21
0
.010
5.263
23.43
12.08
NaBr
0.007
0
.010
a
Ϫ4
Ϫ3
Ϫ3
[
PB] = 2 × 10 mol dm , [NaOH] = 0.01 mol dm , λ = 290 nm, 35 ЊC and the aqueous reaction mixture in each kinetic run contains 2% v/v
3
0
b
CH CN. Error limits are standard deviations.
1
0 (a) C. A. Bunton and J. R. Moffatt, Langmuir, 1992, 8, 2130;
b) A. Blasko, C. A. Bunton, C. Armstrong, W. Gotham, Z.-M. He,
References
(
1
(a) L. S. Romsted, in Micellization, Solubilization and Micro-
emulsions, ed. K. L. Mittal, Plenum Press, New York, 1977, vol. 2,
p. 509; (b) L. S. Romsted, in Surfactants in Solutions, eds. K. L.
Mittal and B. Lindman, Plenum Press, New York, 1984, vol. 2,
p. 1015.
J. Nikles and L. S. Romsted, J. Phys. Chem., 1991, 95, 6747;
(c) A. Blasko and C. A. Bunton, J. Phys. Chem., 1993, 97, 5435;
(d ) F. Ortega and E. Rodenas, J. Phys. Chem., 1987, 91, 837.
11 M. de F. S. Neves, D. Zanette, F. Quina, M. T. Moretti and F. Nome,
J. Phys. Chem., 1989, 93, 1502.
2
3
F. M. Menger and C. E. Portnoy, J. Am. Chem. Soc., 1967, 89,
12 (a) T. J. Broxton, Aust. J. Chem., 1981, 34, 2313; (b) T. J. Broxton and
D. B. Sango, Aust. J. Chem., 1983, 36, 711; (c) E. B. Abuin, E. Lissi,
P. S. Aravio, R. M. V. Aleixo, H. Chaimovich, N. Bianchi, L. Miola
and F. H. Quina, J. Colloid Interface Sci., 1983, 96, 293; (d ) M. G.
Nascimento, S. A. F. Miranda and F. Nome, J. Phys. Chem., 1986,
90, 3366.
13 C. A. Bunton, in Surfactants in Solution, eds. K. L. Mittal and D. O.
Shah, Plenum Press, New York, 1991, vol. 11, p. 17.
14 R. Germani, G. Savelli, T. Romeo, N. Spreti, G. Cerichelli and C. A.
Bunton, Langmuir, 1993, 9, 55.
4
698.
(a) E. Lissi, E. Abuin, I. M. Cuccovia and H. Chaimovich, J. Colloid
Interface Sci., 1986, 112, 513; (b) F. M. Menger, D. Y. Williams,
A. L. Underwood and E. W. Anacker, J. Colloid Interface Sci., 1982,
9
4
0, 546; (c) J. A. Loughlin and L. S. Romsted, Colloids Surf., 1990,
8, 123; (d ) I. M. Cuccovia, I. N. da Silva, H. Chaimovich and L. S.
Romsted, Langmuir, 1997, 13, 647; (e) S. J. Bachofer and U. Simonis,
Langmuir, 1996, 12, 1744.
(a) J. H. Fendler, Membrane Mimetic Chemistry, Wiley Interscience,
New York, 1982; (b) C. A. Bunton, Catal. Rev. Sci. Eng., 1979, 20, 1;
4
15 N. H. Lajis and M. N. Khan, J. Phys. Org. Chem., 1998, 11, 209.
16 M. N. Khan and Z. Arifin, J. Chem. Soc., Perkin Trans. 2, 2000,
2503.
(
2
c) C. A. Bunton and G. Savelli, Adv. Phys. Org. Chem., 1986, 22,
13; (d ) C. A. Bunton, F. Nome, F. H. Quina and L. S. Romsted,
Acc. Chem. Res., 1991, 24, 357.
17 M. N. Khan, J. Chem. Soc., Perkin Trans. 2, 1989, 199.
18 M. N. Khan, Colloids Surf., A, 1998, 139, 63.
19 (a) T. J. Broxton, J. R. Christie and R. P.-T. Chung, J. Org. Chem.,
1990, 53, 3081; (b) M. N. Khan and Z Arifin, Langmuir, 1997, 13,
6626; (c) A. Senz and H. E. Gsponer, J. Colloid Interface Sci., 1994,
165, 60; (d ) I. Rico, K. Halvorsen, C. Dubrule and A. Lattes, J. Org.
Chem., 1994, 59, 415.
20 L. J. Magid, Z. Han, G. G. Warr, M. A. Cassidy, P. D. Butler and
W. A. Hamilton, J. Phys. Chem. B, 1997, 101, 7919.
21 L. S. Romsted, J. Phys. Chem., 1985, 89, 5113.
22 M. N. Khan, J. Org. Chem., 1997, 62, 3190.
23 M. N. Khan, Z. Arifin, E. Ismail and S. F. M. Ali, J. Org. Chem.,
2000, 65, 1331.
24 (a) M. N. Khan, Z. Arifin, E. Ismail and S. F. M. Ali, Colloids Surf.
A, 2000, 161, 381; (b) M. N. Khan and A. B. H. Faujan, Colloids
Surf. A, 2001, 181, 11; (c) M. N. Khan and M. R. Yusoff, J. Phys.
Org. Chem., 2001, 14, 74; (d ) M. N. Khan and E. Emaran, J. Chem.
Res., in the press; (e) M. N. Khan and E. Emaran, Int. J. Chem.
Kinet., 2001, 33, 288.
5
6
(a) P. Herves, J. R. Leis, J. C. Mejuto and J. Perez-Juste, Langmuir,
1
997, 13, 6633; (b) J. H. Fendler and W. L. Hinze, J. Am. Chem.
Soc., 1981, 103, 5439; (c) I. M. Cuccovia, F. H. Quina and H.
Chaimovich, Tetrahedron, 1982, 38, 917.
(a) O. A. El Seoud and A. M. Chinelatto, J. Colloid Interface Sci.,
1
983, 95, 163; (b) O. A. El Seoud, Adv. Colloid Interface Sci., 1989,
3
0, 1; (c) C. J. O’Connor, T. D. Lomax and R. E. Ramage, Adv.
Colloid Interface Sci., 1984, 20, 21.
7
8
R. A. Mackay, J. Phys. Chem., 1982, 86, 4756.
(a) F. Nome, A. F. Rubira, C. Franco and L. G. Ionescu, J. Phys.
Chem., 1982, 86, 1881; (b) M. Gonsalves, S. Probst, M. C. Rezende,
F. Nome, C. Zucco and D. Zanette, J. Phys. Chem., 1985, 89,
1
127.
9
(a) C. A. Bunton, L. H. Gan, J. R. Moffatt, L. S. Romsted and G.
Savelli, J. Phys. Chem., 1981, 85, 1148; (b) S. Vera and E. Rodenas,
Tetrahedron, 1986, 12, 143; (c) A. Cuenca, Int. J. Chem. Kinet., 1998,
3
0, 777; (d ) C. R. A. Bertoncini, M. de F. S. Neves, F. Nome and
C. A. Bunton, Langmuir, 1993, 9, 1274.
1
350
J. Chem. Soc., Perkin Trans. 2, 2001, 1346–1350