Effects of organic salts on the rate of intramolecular general base-catalyzed piperidinolysis of ionized phenyl salicylate in the presence of cationic micelles

Article abstract of DOI:10.1039/b102567h

Pseudo-first-order rate constants (k_obs), obtained for the cleavage of ionized phenyl salicylate (PS^-) at constant [NaOH], [MeCN], [CTABr]_T (total concentration of cetyltrimethylammonium bromide), [Pip]_T (total

Full text of DOI:10.1039/b102567h

View Article Online / Journal Homepage / Table of Contents for this issue
Effects of organic salts on the rate of intramolecular general base-
catalyzed piperidinolysis of ionized phenyl salicylate in the
presence of cationic micelles
2
Mohammad Niyaz Khan* and Soo Y. Kun
Department of Chemistry, Faculty of Science, Universiti Malaya, 50603 Kuala Lumpur,
Malaysia. E-mail: niyaz@kimia.um.edu.my
Received (in Cambridge, UK) 20th March 2001, Accepted 15th May 2001
First published as an Advance Article on the web 5th July 2001
Pseudo-first-order rate constants (kobs), obtained for the cleavage of ionized phenyl salicylate (PSϪ) at
constant [NaOH], [MeCN], [CTABr]T (total concentration of cetyltrimethylammonium bromide), [Pip]T
(total concentration of piperidine) and varying concentrations of sodium o-, m- and p-toluate ([MX]), follow
the relationship: kobs = (k0 ϩ θK[MX])/(1 ϩ K[MX]), where θ and K are empirical parameters. The values of θ are
almost independent of [CTABr]T, while the K values decrease with increasing [CTABr]T within its range 0.005–0.020
mol dmϪ3. The values of θ and K are explained in terms of a pseudophase model of the micelle coupled with an
0
empirical relationship: KS = KS /(1 ϩ kX/S[MX]), where KS is the CTABr micellar binding constant of PSϪ in the
presence of MX. The value of KX/S, for the o-toluate ion is nearly 2.5-fold smaller than those for the m- and
p-toluate ions. The values of θ/(knW[Pip]T) {where knW[Pip]T = kobs at [CTABr]T = [MX] = 0} vary from nearly 0.5
to 0.6 within the [CTABr]T range 0.005–0.020 mol dmϪ3
.
of an ion, XϪ (such as BrϪ) to expel another counterion, SϪ
(such as PSϪ) from a micellar pseudophase to the aqueous
pseudophase. The results of the effects of [C6H5COONa] on the
rate of reactions of PSϪ with piperidine and n-butylamine sup-
ported the validity of eqn. (2).8 Spectrophotometric techniques
have been also used to determine KS (CTABr micellar binding
constant of PSϪ) at different [NaBr] and [C6H5COONa] in the
absence of any amine and these data also support the validity
of eqn. (2).9 In the continuation of our work on testing the
validity of eqn. (2), the effects of a few organic salts on the rate
of piperidinolysis of PSϪ in the presence of cationic micelles
have been studied. The observed results and their probable
explanation(s) are described in this paper.
Introduction
The ion exchange between HOϪ and BrϪ was first detected
kinetically in the reactions of HOϪ with neutral organic
substrates in the presence of cationic surfactants such as cetyl-
trimethylammonium bromide (CTABr). A pseudophase ion-
exchange (PIE) model,1 which is the extension of the older
pseudophase micellar (PM) model,2 has been developed to
explain quantitatively such ion exchange. However, some
serious weaknesses in the PIE model have recently been
realized.3 The use of the PIE model in micellar-mediated reac-
tion systems containing two or more ion-exchange processes is
rare. To best of our knowledge, there are only a few reports
where the PIE model has been used in reaction systems involv-
ing two ion-exchange processes with relatively more restrictive
experimental conditions (because of the conceptual and
mathematical complexity involved in formulating a practically
workable kinetic equation for data analysis) compared to those
involving only one ion-exchange process.4 The effects of an
inert inorganic salt (here, KBr) on the pseudo-first-order rate
constants (kobs) for alkaline hydrolysis of moderately hydro-
phobic anionic esters5 and imides,6 in the presence of cationic
micelles, have been explained in terms of the PIE model
Experimental
Materials
Reagent grade chemicals such as phenyl salicylate, cetyl-
trimethylammonium bromide (CTABr), piperidine and toluic
acids were supplied by Fluka, BDH or Aldrich and were of the
highest commercially available purity. All other chemicals used
were also of reagent grade. Stock solutions (0.01 mol dmϪ3) of
phenyl salicylate were prepared in acetonitrile. Stock solutions
(1 mol dmϪ3) of piperidine were freshly prepared in distilled
water. Stock solutions (y mol dmϪ3) of toluic acids were freshly
prepared in (y ϩ 0.05) mol dmϪ3 NaOH.
0
coupled with an empirical equation [eqn. (1)], where KS and KS
KS = KS0 Ϫ L[KBr]
(1)
represent the micellar binding constant of a moderately
hydrophobic anionic organic substrate in the presence and
absence of KBr, respectively, and L is an empirical parameter,
the measure of the ability of an ion (such as BrϪ) to expel
another counterion (such as SϪ) from a micellar pseudophase to
the aqueous pseudophase.
Kinetic measurements
The rate of piperidinolysis of ionized phenyl salicylate (PSϪ)
was studied by monitoring the disappearance of PSϪ spectro-
photometrically at 350 nm in the presence of CTABr micelles.
Stock solutions (0.25 or 0.5 mol dmϪ3) of NaOH were used to
produce 0.02 mol dmϪ3 NaOH into the reaction mixture for
each kinetic run. The concentrations of NaOH produced by the
stock solutions of sodium toluates were in the range 0.001 to
0.050 mol dmϪ3 within the [sodium toluate] range 0.01 to 0.40
mol dmϪ3 or 0.60 mol dmϪ3. Thus, the total concentrations of
The effects of [KBr] on the rate of methanolysis of ionized
phenyl salicylate (PSϪ) revealed the occurrence of eqn. (2),7
0
KS = KS /(1 ϩ KX/S[KBr])
(2)
where KX/S is an empirical parameter, the measure of the ability
DOI: 10.1039/b102567h
J. Chem. Soc., Perkin Trans. 2, 2001, 1325–1330
This journal is © The Royal Society of Chemistry 2001
1325

View Article Online
Fig. 1 Plots showing the dependence of kobs upon [NaTA] where
NaTA represents sodium o-toluate at 0.005 (᭺), 0.007 (᭝), 0.010 (ᮀ),
0.015 (᭞) and 0.020 mol dmϪ3 (᭛) CTABr. The solid lines are drawn
through the least-squares calculated points.
Fig. 2 Plots showing the dependence of kobs upon [NaTA] where
NaTA represents sodium m-toluate at 0.005 (᭺), 0.007 (᭝), 0.010 (ᮀ),
0.015 (᭞) and 0.020 mol dmϪ3 (᭛) CTABr. The solid lines are drawn
through the least-squares calculated points.
added NaOH into the reaction mixtures were in the range 0.021
to 0.070 mol dmϪ3. The final concentration of PSϪ in each
kinetic run was kept constant at 2 × 10Ϫ4 mol dmϪ3. The details
of the kinetic procedure, data analysis and product character-
ization have been described elsewhere.10 The products of the
reaction of piperidine with phenyl salicylate are N-piperidinyl-
salicylamide and phenol.
Results and discussion
A series of kinetic runs was carried out within a [o-CH3-
C6H4COONa] range of 0.0–0.60 mol dmϪ3 at 0.1 mol dmϪ3
piperidine (Pip), ≥0.021 mol dmϪ3 NaOH, 0.005 mol dmϪ3
CTABr and 35 ЊC. The pseudo-first-order rate constants (kobs
)
obtained under these conditions are shown in Fig. 1. Similar
results were obtained at 0.007, 0.010, 0.015 and 0.020 mol dmϪ3
CTABr. These results are also shown as the plots of kobs versus
[o-CH3C6H4COONa] in Fig. 1. The effects of [m-CH3C6H4-
COONa] and [p-CH3C6H4COONa] on kobs were also studied at
different [CTABr]T (= total concentration of CTABr) and these
results are shown in Figs. 2 and 3, respectively.
The nonlinear increase in kobs with increasing concentration
of the sodium salt of toluic acid, [NaTA] (Figs. 1–3), cannot be
attributed to the salt effect or to the probable nucleophilic reac-
tion between PSϪ and toluate ion because carboxylate groups
are poorer nucleophiles than amines toward esters.11 Further-
more, the increase in [sodium benzoate] from 0.0 to 0.8 mol
dmϪ3 at ≥0.01 mol dmϪ3 NaOH and in the absence of micelles
changed kobs for the hydrolysis of PSϪ from 7.59 × 10Ϫ4 to
6.40 × 10Ϫ4 sϪ1 at 35 ЊC. Pseudo-first-order rate constants (kobs
)
Fig. 3 Plots showing the dependence of kobs upon [NaTA] where
NaTA represents sodium p-toluate at 0.005 (᭺), 0.007 (᭝), 0.010 (ᮀ),
0.015 (᭞) and 0.020 mol dmϪ3 (᭛) CTABr. The solid lines are drawn
through the least-squares calculated points.
for the reaction of piperidine with PSϪ showed monotonic
decrease with the increase in [CTABr]T from 0.0 to ≤0.004 mol
dmϪ3 and almost leveled off at [CTABr]T > 0.004 mol dmϪ3 12
.
The effects of [NaTA] on kobs were studied under [CTABr]T
(≥0.005 mol dmϪ3) where the PSϪ ions are fully micellar bound
in the absence of NaTA. The value of kobs is nearly 13-fold
larger at [CTABr]T = 0 than at [CTABr]T ≥ 0.004 mol dmϪ3 (see
ref. 12 and Table 1). Thus, the most obvious cause for the
increase in kobs with the increase in [NaTA] at ≥0.005 mol dmϪ3
CTABr may be attributed to the transfer of micellized ionized
1326
J. Chem. Soc., Perkin Trans. 2, 2001, 1325–1330

View Article Online
Table 1 Values of the empirical parameters, θ and K, calculated from eqn. (3)a
[CTABr]T/
c
KX/S
/
mol dmϪ3
103 k0 b/sϪ1
103 θ/sϪ1
K/dm3 molϪ1
dm3 molϪ1
Fd
o-Toluate
0.005
0.007
0.010
0.015
0.020
2.42 0.08e
1.95 0.11
2.30 0.07
2.21 0.09
2.07 0.07
19.1 1.4e
15.2 0.9
15.2 2.5
15.8 2.3
11.3 1.4
6.00 1.48e
4.48 0.72
2.95 1.22
1.67 0.47
1.82 0.49
216
224
209
177
257
0.59
0.47
0.47
0.49
0.35
m-Toluate
0.005
0.007
0.010
0.015
0.020
2.62 0.06
2.52 0.04
2.11 0.06
2.36 0.05
2.12 0.06
21.0 0.9
20.5 2.0
17.0 1.3
16.6 1.3
17.1 1.9
14.6 1.9
10.9 3.1
7.80 1.60
5.90 1.20
3.60 0.80
526
545
554
625
508
0.65
0.63
0.53
0.51
0.53
p-Toluate
0.005
0.007
0.010
0.015
0.020
2.37 0.07
2.26 0.08
2.10 0.09
2.00 0.07
1.99 0.07
19.4 0.8
18.3 0.9
17.4 1.3
15.8 1.3
16.8 2.1
17.3 2.2
11.5 1.5
7.12 1.31
5.30 1.00
3.30 0.80
623
575
506
562
465
0.60
0.56
0.54
0.49
0.52
a Conditions: [phenyl salicylate]0 = 2 × 10Ϫ4 mol dmϪ3; [NaOH] > 0.02 mol dmϪ3; [Pip] = 0.1 mol dmϪ3; 35 ЊC; λ = 350 nm; the aqueous solvent for
0
each kinetic run contained 2% v/v MeCN. b k0 = kobs at [MX] = 0. c KX/S = K(1 ϩ KS [CTABr]T) where KS0 = 7000 dm3 molϪ1
.
d F = θ/(knW[Pip]T) where
knW = 0.324 dm3 molϪ1 sϪ1 and [Pip]T = 0.1 mol dmϪ3 e Error limits are standard deviations.
.
phenyl salicylate (PSϪM) to the aqueous pseudophase by the
of kn[Pip]T at 0.1 mol dmϪ3 Pip.12 Thus, under the present
experimental conditions, kobs ≈ kn[Pip]T.
added toluate ions (TAϪ) through the ion exchange TAϪW/PSϪ
M
(subscripts W and M represent the aqueous pseudophase and
micellar pseudophase, respectively). The concentrations of
nonionic piperidine in aqueous and micellar pseudophases
should not be affected by the occurrence of ion exchange within
the domain of the assumptions of the pseudophase model of
the micelle.1–3
The brief reaction scheme for the cleavage of PSϪ may be
shown in Scheme 1, where kH O(H2O) [= k0 in eqn. (4)] is neg-
2
The values of kobs obtained at 0.1 mol dmϪ3 Pip, ≥0.021 mol
dmϪ3 NaOH, a constant concentration of CTABr and different
values of [NaTA] were found to fit to the empirical equation,
eqn. (3), where k0Ј = kobs at [NaTA] = 0, and θ and K are empir-
ical parameters. Values of k0 for different reaction conditions
were obtained by carrying out experiments in the absence of
NaTA. The values θ and K were calculated from eqn. (3) using
k0Ј ϩ θK[NaTA]
kobs
=
(3)
1 ϩ K[NaTA]
the nonlinear least-squares technique. These calculated values
of θ and K at different [CTABr]T for o-, m- and p-CH3C6-
H4COONa are summarized in Table 1. The fitting of the
observed data to eqn. (3) is evident from the plots of Figs. 1–3,
where solid lines are drawn through the calculated data points.
The spectral evidence, as described elsewhere,12 revealed the
presence of 100% ionized form of phenyl salicylate (PSϪ) under
the experimental conditions of the entire kinetic runs of the
present study. The rate constants kobs for the cleavage of PSϪ
under the present experimental conditions have been found to
follow eqn. (4),12 where [Pip] ≈ [Pip]T (= [Pip] ϩ [PipHϩ]), k0 is
Scheme 1
ligible compared with kn(Pip) under the present reaction condi-
tions. The detailed reaction mechanisms for the kn and k0 steps
in the absence of micelles have been described elsewhere.13,14
The rate of reaction of Pip with PSϪ in the presence of
CTABr micelles may be explained quantitatively in terms of a
pseudophase model (PM) of the micelle, i.e. Scheme 2.2 The
assumptions involved in this model and its usefulness, as well as
weaknesses, are critically discussed by Bunton.3c,15
kobs = k0 ϩ kn[Pip]
(4)
In Scheme 2, KS and KN are the CTABr micellar binding
constants of PSϪ and Pip, respectively, kn and kn represent
W
M
the pseudo-first-order rate constant for the intramolecular
general base (IGB)-catalyzed hydrolysis of PSϪ and kn is the
second-order rate constant for the IGB-catalyzed nucleophilic
reaction of Pip with PSϪ. The source for IGB catalysis is the
ionized o-OH group in PSϪ, which acts as an intramolecular
general base catalyst for the nucleophilic reactions of the
nucleophiles (with a hydrogen atom attached to the nucleophilic
atom such as H2O, ROH, RNH2 and R2NH) with PSϪ.10,13,14
But the contribution of k0 is negligible compared to that
the nucleophilic second-order rate constants for the reactions
of Pip with PSϪ in the aqueous and micellar pseudophases,
respectively, and Dn stands for the CTABr micelle. Scheme 2
and the observed rate law (rate = kobs[PSϪ]T) can lead to
eqn. (5), where knsM = knM/VM (VM is the micellar molar
(knW ϩ knsMKNKS[Dn])[Pip]T
kobs
=
(5)
(1 ϩ KS[Dn])(1 ϩ KN[Dn])
J. Chem. Soc., Perkin Trans. 2, 2001, 1325–1330
1327

View Article Online
Eqns. (2) (with replacement of KBr by NaTA) and (6) lead to
eqn. (3) with k0Ј, θ and K defined by eqns. (7)–(9).
k0Ј = (knW[Pip]T ϩ knsMKNKS [Dn][Pip]T)/(1 ϩ KS [Dn]) (7)
0
0
θ = knW[Pip]T
(8)
(9)
0
K = KX/S/(1 ϩ KS [Dn])
Scheme 2
volume15) and [Dn] = [CTABr]T Ϫ cmc (cmc is the critical
micelle concentration). The reported value of the cmc is
<1 × 10Ϫ4 mol dmϪ3 in the presence of 2 × 10Ϫ4 mol dmϪ3 PSϪ
and the absence of any inert salt.12,16 The presence of an inert
salt is expected to decrease the cmc and, thus, under the present
experimental conditions, the cmc value must be much lower
than 1 × 10Ϫ4 mol dmϪ3. Since the lowest value of [CTABr]T
is 50 × 10Ϫ4 mol dmϪ3, it is therefore apparent that [Dn] ≈
[CTABr]T.
The reported value of kn is 0.324 dm3 molϪ1 sϪ1 at 35 ЊC.12
W
Thus, the maximum value of θ should be 0.0324 sϪ1 at 0.1 mol
dmϪ3 Pip, which could be obtained only if the limiting concen-
tration of TAϪ can cause 100% transfer of micellized PSϪ ions
(PSϪM) from the micellar pseudophase to the aqueous pseudo-
phase, provided the PSϪ ions are completely micellar bound in
the absence of MX. The limiting concentration of a salt such as
NaTA is defined as the concentration of the salt at which the
rate of a reaction becomes independent of the salt concen-
tration [i.e., at the limiting concentration of NaTA,
K[NaTA] ӷ 1 and θK[NaTA] ӷ k0Ј in eqn. (3)]. Thus, the ratio
F {= θ/(knW[Pip]T)} may be considered to be the measure of the
fraction of the fully micellized PSϪ ions transferred from the
micellar pseudophase to the aqueous pseudophase by the limit-
ing concentration of an inert salt such as NaTA.
The reported value of KN for a tetradecyltrimethylam-
monium bromide (TTABr) micelle is 0.3 dm3 molϪ1 17,18
The
.
value of KN for a CTABr micelle should not be significantly
different from the KN for TTABr.19 The value of KN would not
be expected to change appreciably in the presence of sodium
toluate of varying concentrations because piperidine is a neu-
tral and highly hydrophilic molecule. The value of the cetyltri-
methylammonium chloride (CTACl) micellar binding constant
of neutral benzimidazole, a relatively much more hydrophobic
molecule than piperidine, increased from 43 to 68 dm3 molϪ1
with the increase in [NaCl] from 0.0 to 1.0 mol dmϪ1 at 0.01
mol dmϪ3 CTACl.4b Thus, it is apparent that 1 ӷ KN[Dn] at
[CTABr]T ≤ 0.02 mol dmϪ3 and, under such conditions, eqn. (5)
can be reduced to eqn. (6).
It is well known that the structure of a cationic micelle
changes from spherical to disk to rod with increasing concen-
tration of the micelle-forming surfactant.21 This micellar struc-
tural transition is enhanced in the presence of inert inorganic
or organic salts.22,23 Such structural changes might affect KX/S
,
and hence, the value of KX/S should not be expected to remain
constant for a wide range of [NaTA] at constant [CTABr]T.
Such a salt effect on KX/S could also indirectly affect θ. It
might also cause the observed data to deviate from fitting
to eqn. (3). This could possibly be the reason for the negative
deviations of the observed data points from the theoretical lines
at very high values of [NaTA] and low values of [CTABr]T
(Figs. 1–3).
(knW ϩ knsMKNKS[Dn])[Pip]T
kobs
=
(6)
(1 ϩ KS[Dn])
The occurrence of the ion-exchange phenomenon in ionic
micellar-mediated bimolecular reactions involving both ionic,
or one ionic and one neutral, reactants appears to be a ubiqui-
tous feature of such reacting systems. The possible ion-
exchange processes in the present reacting system are PSϪ/TAϪ,
PSϪ/BrϪ, PSϪ/HOϪ, TAϪ/BrϪ, TAϪ/HOϪ and HOϪ/BrϪ. Here
the ion-exchange processes TAϪ/BrϪ, TAϪ/HOϪ and HOϪ/BrϪ
may be ignored, as the rate of the piperidinolysis of PSϪ would
remain unaffected by their occurrence. The rate constant for
the reaction of Pip with PSϪ is more than 95-fold larger in the
aqueous pseudophase than in the micellar pseudophase.12 The
rate of hydrolysis of PSϪ remained independent of [HOϪ] with-
in its range 0.005–0.060 mol dmϪ3 at [CTABr]T = 010,20 and
0.01–0.04 mol dmϪ3 at 0.0015 mol dmϪ3 CTABr.16 The
effectiveness of an ion-exchange process decreases as the differ-
ence in the hydrophilicity or hydrophobicity of the exchanging
ions increases. The relative effectiveness of two or more ion-
exchange processes is also determined by the relative concen-
trations of the exchanging ions. The hydrophobicity of HOϪ,
BrϪ, TAϪ and PSϪ is expected to vary in the order PSϪ > TA-
Ϫ > BrϪ > HOϪ and their maximum concentrations, attained in
Eqn. (8) shows that the value of θ (= knW[Pip]T) should be
independent of [CTABr]T. But the calculated values of θ indi-
cate a slight decrease with the increase in [CTABr]T for all o-,
m- and p-toluate ions (Table 1). This might be attributed to the
micellar structural changes discussed above.
0
0
In view of eqn. (9), K = KX/S/(1 ϩ KS [Dn]) ≈ KX/S/(1 ϩ KS -
[CTABr]T) under the experimental conditions imposed in
the present study. The values of KX/S, obtained at different
[CTABr]T with KS0 = 7000 dm3 molϪ1 24
are summarized in
,
Table 1. They turn out to be almost independent of [CTABr]T
within its range from 0.005 to 0.020 mol dmϪ3. These results
show that the KX/S values are insensitive to any micellar struc-
tural changes there might be. The value of KX/S for an anion
such as toluate ion can be rationalized in terms of the hydrogen
bonding, electrostatic, hydrophobic and steric (i.e., packing
constraints) interactions between the anion and the ionic
micellar pseudophase. The hydrogen bonding and electrostatic
interactions between the CTABr micellar pseudophase or
surface and the o-, m- and p-toluate ions are expected to be the
same, but the hydrophobic and possibly steric interactions are
not. The hydrophobicity of m- and p-nitrobenzoate anions is
nearly the same, while the o-nitrobenzoate ion is significantly
less hydrophobic.25 The relative values of hydrophobicity of the
o-, m- and p-toluate ions are also similar to the corresponding
values of hydrophobicity of o-, m- and p-nitrobenzoate ions.26
Thus, the hydrophobicity argument may be used to explain (i)
the lower (~2.5-fold) KX/S value for the o-toluate ion (217 19
the present study, are ≤0.07, 0.02, ≤0.6 and 2 × 10Ϫ4 mol dmϪ3
,
respectively. Thus, the ion-exchanges PSϪ/BrϪ and PSϪ/HOϪ
may be ignored compared to PSϪ/TAϪ under the experimental
conditions of the present study. The ion-exchange process
which should most affect the rate of piperidinolysis of PSϪ is
PSϪ/TAϪ. The TAϪ ions, being moderately hydrophobic and
present in large amounts, can expel PSϪ ions from the micellar
pseudophase to the aqueous pseudophase, which, in turn,
causes the increase in kobs with the increase in [TAϪ]. Such an
effect of [TAϪ] on the distribution of PSϪ between the aqueous
and micellar pseudophases is represented by the empirical
eqn. (2).
dm3 molϪ1) compared to those for the m- (552 45 dm3 molϪ1
)
and p-toluate ions (546 62 dm3 molϪ1) and (ii) the almost
similar KX/S values for the m- and p-toluate ions (Table 1).
Similar hydrophobicity arguments have been used to explain
1
the values of ion-exchange constants, obtained by H NMR
1328
J. Chem. Soc., Perkin Trans. 2, 2001, 1325–1330

View Article Online
Table 2 Values of the empirical constants, KX/S and KY/S, and ion-exchange constants, KXY, obtained from empirical constants as well as from
physical techniquesa
Y b
Y c
X
Y
KX/S/dm3 molϪ1
KY/S/dm3 molϪ1
KX
KX
Ϫ
C6H5CO2
BrϪ
BrϪ
BrϪ
BrϪ
BrϪ
BrϪ
BrϪ
BrϪ
BrϪ
BrϪ
ClϪ
ClϪ
(124)d,e
217 (102)
552 (315)
546 (295)
(25)e, f
(25)
(25)
(5)g
9 (4)
22 (13)
22 (12)
Ϫ
o-CH3C6H4CO2
m-CH3C6H4CO2
Ϫ
Ϫ
p-CH3C6H4CO2
(25)
Ϫ
C6H5SO3
11h
19
Ϫ
p-CH3C6H4SO3
o-OϪC6H4CO2
20i
Ϫ
Ϫ
o-NO2C6H4CO2
m-NO2C6H4CO2
p-NO2C6H4CO2
2,6-Cl2C6H4CO2
BrϪ
3.8
11
Ϫ
Ϫ
3.3
Ϫ
13–22j
(100)
75 (44)k
1.3 (2.3)
2.65,l 5h
a Unless otherwise noted, the surfactant used is cetyltrimethylammonium halide, S is ionized phenyl salicylate and the values of KXY were determined
Y
by using physical techniques at a single concentration of the surfactant. b KXY = KX/S/KY/S
.
c The values of KX were obtained by using physical
techniques. d Ref. 8. e Parenthesized values are normalized empirical constants, KX/Sn, (= FXKX/S). f Ref. 34. g Parenthesized values are derived from
n
normalized values of empirical constants, KX/S
.
h Ref. 36. i Ref. 25. j Ref. 37, where KXY values were determined within [CTAϩ]T range 0.01–0.03 mol
dmϪ3 by the use of 1H NMR spectroscopic techniques. k Ref. 35, where S represents ionized phthalimide. l Ref. 38.
techniques, for the competition between BrϪ and o-, m- and p-
nitrobenzoate ions for a tetradecyltrimethylammonium brom-
ide (TTABr) micellar surface, where the ion-exchange constant
for o-nitrobenzoate was found to be 2.9-fold smaller than that
for the m-nitrobenzoate ion.25 Menger et al.27 reported that the
m-phthalate dianion binds more strongly to the decyltrimethyl-
ammonium micellar interface compared to the o-phthalate
dianion.
micelles and this model leads to a kinetic equation similar to the
one obtained from the PM model with modified definitions
of the kinetic parameters. The rate constant for a micellar-
mediated reaction and the micellar binding constants of the
reactants are composite and phase-average parameters in view
of the MMPP model.
The empirical definition of KX/S implies that the value of KX/S
is proportional to the value of KX (micellar binding constant of
X) and inversely proportional to the value of KS (micellar bind-
ing constant of S). Thus, KX/S = δSKX/KS, where δS is a pro-
portionality constant with dimensions dm3 molϪ1. The value of
δS depends only upon the nature and micellar affinity of S (the
ion expelled by another counterion from the micellar pseudo-
phase to the aqueous pseudophase); it is independent of the
nature and micellar affinity of X (the ion which expels another
counterion S from the micellar pseudophase to the aqueous
pseudophase). Similarly, for another ion Y, KY/S = δSKY/KS. It is
thus apparent that the ratio KX/S/KY/S should be equal to the
As mentioned earlier, the magnitude of F {= θ/(knW[Pip]T)} is
a measure of the fraction of the fully micellized PSϪ ions
expelled from the micellar pseudophase to the aqueous pseudo-
phase by the limiting concentration of sodium toluate. The
value of F varies from 0.5 to 0.6 within the [CTABr]T range
0.005–0.020 mol dmϪ3 for all o-, m- and p-toluate ions, which
indicates that the optimum concentrations of toluate ions can
expel only nearly 50–60% of the total [PSϪM] (= [PSϪ]T) to the
aqueous pseudophase. These observations show that o-, m- and
p-toluate ions cannot penetrate the CTABr micellar pseudo-
phase to the same extent as PSϪM ions. The phenyl salicylate ion
is apparently more hydrophobic than o-, m- and p-toluate ions
and it is perhaps because of this that the depth of penetration
into the micellar pseudophase is different for PSϪ and toluate
ions. The results described in this manuscript indirectly show
that the micellar pseudophase is not strictly a two-state micro-
environment (i.e., Stern layer and core) in terms of polarity,
water concentration and the distribution of ionic solubilizates
of different hydrophibicity. This conclusion is also supported
by other kinetic data, which indirectly show that the micellar
surface is not uniform in terms of water concentration,28–31
polarity,10,22 ionic strength22,32 and distribution of solubilizates
of different hydrophobicity.10,18 The fact that fluorescence
spectroscopy shows that very hydrophilic anions, e.g. HOϪ and
FϪ, are singularly ineffective in displacing BrϪ from the micellar
Y
usual ion-exchange constant KX (where KXY = KX/KY =
{[XϪM][YϪW]}/{[XϪW][YϪM]}). It should be noted that the
Y
relationship: KX/S/KY/S = KX is correct only if KX/S and KY/S
were obtained experimentally using eqn. (2). If the values of
KX/S and KY/S were obtained from eqn. (3), then these values
should be normalized, i.e. KX/Sn = FXKX/S and KY/Sn = FYKY/S
,
/
n
where F = θ/(knW[Pip]T) and, under such conditions, KX/S
KY/Sn = KXY. The reported values of KX/PS (X = C6H5CO2Ϫ)8 and
KY/PS (Y = BrϪ)34 as well as KX/PT and KY/PT (where PT repre-
sents ionized phthalimide)35 for various X and Y (Table 2) and
listed values of KX/PS in Table 1 were used to calculate the usual
Y
ion-exchange constants, KX (= KX/PS/KY/PS or KX/PT/KY/PT).
These results are summarized in Table 2. The values of KBr/PS
and KBz/PS (Bz = C6H5CO2Ϫ) were obtained from the kinetic
relationships, where the values of F (= θ/knW[Pip]T) were inher-
ently maintained as 1. But the values of KX/PS (Table 1) were
obtained from the kinetic relationship where the F values are
0.47, 0.57 and 0.54 for X = o-, m- and p-toluate ions, respect-
pseudophase to the aqueous pseudophase, while kinetic data
OH
are fitted with values of KBr
{= ([BrϪM][HOϪW])/([BrϪ
]
W
[HOϪM])} generally in the range of 12–30,3c indirectly shows the
different degrees of penetration of BrϪ, HOϪ or FϪ to the
cationic micellar surface.† Recently Davies and co-workers33
proposed a multiple micellar pseudophase (MMPP) model of
n
ively. The normalized values of the empirical constants, KX/PS
(i.e., KX/PS values at F = 1) would therefore be equal to FKX/PS
These normalized values of KX/PS where X = o-, m- and
.
n
p-toluate ions, respectively, are shown in parenthesis in Table 2.
n
n
These values of KX/PS and KBr/PS (= 25 dm3 molϪ1) were also
used to calculate KXBr, the values for which are shown in Table
† One of the referees has correctly pointed out that the BrϪ/HOϪ/FϪ
competition is not governed by location but by the balance between
Coulombic and non-Coulombic ion–micelle interactions. But, in a
multisite or multistate micellar model, each site or state differs from
others in terms of the balance between the Coulombic and non-
Coulombic ion–micelle interaction energies, and a specific ion should
reside in a specific site or state (in view of the energy barrier) in a
multistate micellar model.
Br
2. These values of KX for benzoate, o-, m- and p-toluate ions
Y
may be compared with KX obtained directly by the use of
spectrophotometric,36 1H NMR spectroscopic25,37 and trapping
of free counterions measurement38 techniques. Magid and
co-workers37 have also concluded that the value of such an
ion-exchange constant is technique-dependent.
J. Chem. Soc., Perkin Trans. 2, 2001, 1325–1330
1329

View Article Online
19 C. Bravo, P. Herves, J. R. Leis and M. E. Pena, J. Colloid Interface
Acknowledgements
Sci., 1992, 153, 529.
The authors thank the National Scientific Research and
Development Council of Malaysia under IRPA program for
financial support (Grant No. 09-02-03-0785).
20 M. N. Khan, I. L. Fatope, K. I. Isaac and M. O. Zubair, J. Chem.
Soc., Perkin Trans. 2, 1986, 655.
21 A. Heindl, J. Strand and H.-H. Kohler, J. Phys. Chem., 1993, 97, 742.
22 P. Mukerjee, in Solution Chemistry of Surfactants, ed. K. L. Mittal,
Plenum Press, New York, 1979, vol. 1, p. 153.
23 B. K. Mishra, S. D. Samant, P. Pradhan, S. B. Mishra and C.
Manohar, Langmuir, 1993, 9, 894.
24 M. N. Khan, Z. Arifin, I. A. Wahab, S. F. M. Ali and E. Ismail,
Colloids Surf., A, 2000, 163, 271.
25 S. J. Bachofer and U. Simonis, Langmuir, 1996, 12, 1744.
26 A. L. Underwood and E. W. Anacker, J. Phys. Chem., 1984, 88,
2390.
27 F. M. Menger, D. Y. Williams, A. L. Underwood and E. W.
Anacker, J. Colloid Interface Sci., 1982, 90, 546.
28 H. L. Casal, J. Am. Chem. Soc., 1988, 110, 5203.
29 F. M. Menger and C. E. Mounier, J. Am. Chem. Soc., 1993, 115,
12222.
References
1 L. S. Romsted, in Surfactants in Solution, ed. K. L. Mittal and B.
Lindman, Plenum Press, New York, 1984, vol. 2, p. 1015.
2 F. M. Menger and C. E. Portnoy, J. Am. Chem. Soc., 1967, 89, 4698.
3 (a) C. A. Bunton, F. Nome, F. Quina and L. S. Romsted, Acc. Chem.
Res., 1991, 24, 357; (b) R. R. Germani, G. Savelli, T. Romeo,
N. Spreti, G. Cerichelli and C. A. Bunton, Langmuir, 1993, 9, 55;
(c) C. A. Bunton, in Surfactants in Solution, ed. K. L. Mittal and
D. O. Shah, Plenum Press, New York, 1991, vol. 11, p. 17; (d) N. H.
Lajis and M. N. Khan, J. Phys. Org. Chem., 1998, 11, 209.
4 (a) L. S. Romsted, J. Phys. Chem., 1985, 89, 5107; (b) L. S. Romsted,
J. Phys. Chem., 1985, 89, 5113; (c) A. G. Oliveira, I. M. Cuccovia
and H. Chaimovich, J. Pharm. Sci., 1990, 79, 37.
30 A. Belmajdoub, N. Mahieu, P. Tekely and D. Canet, J. Phys. Chem.,
1992, 96, 1011.
5 S. Vera and E. Rodenas, J. Phys. Chem., 1986, 90, 3414.
6 M. N. Khan, Colloids Surf., A, 1997, 127, 211.
7 M. N. Khan, J. Org. Chem., 1997, 62, 3190.
31 M. N. Khan, J. Naaliya and M. Dahiru, J. Chem. Res. (S), 1988,
116; M. N. Khan, J. Naaliya and M. Dahiru, J. Chem. Res. (M),
1988, 1168.
8 M. N. Khan, Z. Arifin, E. Ismail and S. F. M. Ali, Colloids Surf., A,
2000, 161, 381.
9 M. N. Khan and E. Ismail, J. Chem. Res. (S), in the press.
10 M. N. Khan, J. Chem. Soc., Perkin Trans. 2, 1990, 445.
11 W. P. Jencks and J. Carriuolo, J. Am. Chem. Soc., 1960, 82, 1778.
12 M. N. Khan, Z. Arifin, M. N. Lasidek, M. A. M. Hanifiah and
G. Alex, Langmuir, 1997, 13, 3959.
13 M. N. Khan, J. Chem. Soc., Perkin Trans. 2, 1989, 199.
14 M. N. Khan and S. K. Gambo, Int. J. Chem. Kinet., 1985, 17, 419.
15 C. A. Bunton, Catal. Rev. Sci. Eng., 1979, 20, 1.
32 E. H. Cordes, Pure Appl. Chem., 1978, 50, 617.
33 (a) D. M. Davies, N. D. Gillitt and P. M. Paradis, J. Chem. Soc.,
Perkin Trans. 2, 1996, 659; (b) D. M. Davies, S. J. Foggo and
P. M. Paradis, J. Chem. Soc., Perkin Trans. 2, 1998, 1597.
34 M. N. Khan, Z. Arifin, E. Ismail and S. F. M. Ali, J. Org. Chem.,
2000, 65, 1331.
35 M. N. Khan and Z. Arifin, J. Chem. Soc., Perkin Trans. 2, 2000,
2503.
36 D. Bartet, C. Gamboa and L. Sepulveda, J. Phys. Chem., 1980, 84,
272.
16 M. N. Khan and Z. Arifin, J. Colloid Interface Sci., 1996, 180, 9.
17 E. Iglesias, J. R. Leis and M. E. Pefia, Langmuir, 1994, 10, 662.
18 A. Fernandez, E. Iglesias, L. Garcia-Rio and J. R. Leis, Langmuir,
1995, 11, 1917.
37 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.
38 I. M. Cuccovia, I. N. da Silva, H. Chaimovich and L. S. Romsted,
Langmuir, 1997, 13, 647.
1330
J. Chem. Soc., Perkin Trans. 2, 2001, 1325–1330