Effects of non-ionic and mixed non-ionic-cationic micelles on the rate of aqueous cleavages of phenyl benzoate and phenyl salicylate in alkaline medium

Article abstract of DOI:10.1002/poc.739

Pseudo-first-order rate constants (k_obs) for the hydrolysis of phenyl salicylate (PSH) and phenyl benzoate (PB) in the alkaline medium show a monotonic decrease with the increase in [C₁₆E₂₀] _T (total concentration of Brij 58) at constant [CH₃CN] and [NaOH], This micellar effect is explained in terms of the pseudophase model of micelles. These results show a characteristic difference between the effects of [C₁₆E₂₀]_T and previously published effects of [C₁₂E₂₃]_T (total concentration of Brij 35) on the rates of aqueous cleavage of PSH and PB at [C_nE_m] _T/[NaOH] ≥ ～3. The values of k_obs, obtained at different [C₁₆E₂₀]_T in the presence of a constant amount of CTABr, follow the empirical relationship k_obs = (k₀ + θK.[C₁₆E₂₀]_T)/ (1+K[C₁₆E₂₀]_T), where θ and K are empirical parameters. The values of θ are not affected whereas the values of K decrease with increase in [CTABr]_T in a mixed C ₁₆E₂₀-CTABr micellar system. The values of θ at different [CTABr]_T show that θ > k₀ for hydrolysis of PSH and θ > k₀ for hydrolysis of PB in the presence of 0.01 M NaOH. The effects of [CTABr]_T on K and θ are explained in terms of the pseudophase model of micelles coupled with the empirical relationship K_S = K_S0/(1 + K _C16E20/S[C₁₆E₂₀]_T). Copyright

Full text of DOI:10.1002/poc.739

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2004; 17: 376–386
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.739
Effects of non-ionic and mixed non-ionic–cationic
micelles on the rate of aqueous cleavages of phenyl
benzoate and phenyl salicylate in alkaline medium
M. Niyaz Khan* and Emran Ismail
Department of Chemistry, Faculty of Science, Universiti Malaya, 50603 Kuala Lumpur, Malaysia
Received 15 March 2003; revised 11 December 2003; accepted 18 December 2003
ABSTRACT: Pseudo-first-order rate constants (kobs) for the hydrolysis of phenyl salicylate (PSH) and phenyl
benzoate (PB) in the alkaline medium show a monotonic decrease with the increase in [C16E20]T (total concentration
of Brij 58) at constant [CH3CN] and [NaOH]. This micellar effect is explained in terms of the pseudophase model of
micelles. These results show a characteristic difference between the effects of [C16E20]T and previously published
effects of [C12E23]T (total concentration of Brij 35) on the rates of aqueous cleavage of PSH and PB at [CnEm]T/
[NaOH] ꢀ ꢁ3. The values of kobs, obtained at different [C16E20]T in the presence of a constant amount of CTABr,
follow the empirical relationship kobs ¼ (k0 þ ꢀK[C16E20]T)/(1 þ K[C16E20]T), where ꢀ and K are empirical para-
meters. The values of ꢀ are not affected whereas the values of K decrease with increase in [CTABr]T in a mixed
C16E20–CTABr micellar system. The values of ꢀ at different [CTABr]T show that ꢀ > k0 for hydrolysis of PSH and
ꢀ < k0 for hydrolysis of PB in the presence of 0.01 M NaOH. The effects of [CTABr]T on K and ꢀ are explained in terms
of the pseudophase model of micelles coupled with the empirical relationship KS ¼ KS0/(1 þ KC16E20/S[C16E20]T).
Copyright # 2004 John Wiley & Sons, Ltd.
KEYWORDS: phenyl salicylate; phenyl benzoate; base; hydrolysis; kinetics; micelles; mixed micelles; cationic surfactant;
non-ionic surfactants
INTRODUCTION
micelles on reaction rates have been started only re-
cently.10 Most of the kinetic data on mixed micellar-
mediated reactions have been attempted to explain in
terms of micellar models used for the pure micellar
system.11 However, it is almost certain that the structural
characteristics of mixed micelles, formed from two
different surfactants S1 and S2, change from S1 mi-
celle-type to S2 micelle-type as the X values increase
from very low to very high where X ¼ [S2]/[S1].12
Davies and Foggo13 explained the observed data on the
effects of mixed anionic–non-ionic micelles on the rate of
reaction of m-chloroperbenzoic acid and iodide in terms
of the MMPP model of the micelles. The effects of mixed
cationic–non-ionic micelles on the rate of SN2 reactions
of Brꢂ with fully bound sulfonate ester have been
explained in terms of the PP micellar model coupled
with a factor R defined as mole fraction of cationic
surfactant.14 This empirical approach appears to be
incapable of explaining the observed data on the effects
of mixed C12E23–CTABr [where CTABr ¼ cetyltri-
methylammonium bromide and C12E23 ¼ C12H25-
(OCH2CH2)23OH] micelles on aqueous cleavages of
phenyl benzoate, phenyl salicylate,15 4-nitrophthali-
mide16 and phthalimide17 in alkaline medium.
The study of the effects of pure micelles, formed from
cationic, anionic and non-ionic surfactants, on reaction
rates has been the subject of great interest for the last 4–5
decades. Most of the kinetic data obtained in these studies
have been explained in terms of one of the various
proposed micellar models [such as enzyme kinetic
type,1 pseudophase (PP),2 pseudo-phase ion-exchange
(PIE),3 mass-action,4 Coulombic5 and multiple micellar
pseudophase (MMPP),6 micellar kinetic models]. All
these micellar models appear to be very approximate
and some of them have apparent weaknesses.5a,7 One of
the main reasons for the lack of a perfect micellar model
to use in quantitative analysis of kinetic data on micellar-
mediated reactions is the incomplete understanding of the
fine details of micellar structure.8 Structural aspects of
mixed micelles have been studied theoretically in the last
few years,9 but the fine details of the structure of a mixed
micellar system are far less clear than those of a pure
micellar system. Kinetic studies on the effects of mixed
*Correspondence to: M. N. Khan, Department of Chemistry, Faculty of
Science, Universiti Malaya, 50603 Kuala Lumpur, Malaysia.
E-mail: niyaz@um.edu.my
Contract/grant sponsor: University of Malaya; Contract/grant number:
Vot F, F0718/2002B.
We have previously reported the effects of pure C12E23
and mixed C12E23–CTABr micelles on the rates of
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 376–386

EFFECTS OF MICELLES ON CLEAVAGE OF PHENYL BENZOATE AND SALICYLATE
377
alkaline hydrolysis of PSꢂ and PB. However, the kinetic
data could not be fully explained by any existing kinetic
model, because the rate of reaction dropped to almost
zero in a narrow range of [C12E23]T. This behaviour of
C12E23 we called as unusual behaviour of C12E23 mi-
celles, and it restricted the attainment of a sufficiently
high range of [C12E23]T. However, we could explain the
kinetic data obtained in the limited concentration range of
pure C12E23 micelles in terms of the PP model of
micelles. However, the kinetic data obtained in mixed
C12E23–CTABr micelles did not appear to fit to PP model
and, consequently, the observed results were explained
only qualitatively.
micelles within a wide concentration range of micelles at
0.02 M NaOH.
It is well known that micellar structure, behaviour and
characteristics are sensitive to the structural features
and nature of additives. Although both phthalimide and
phenyl salicylate exist in the anionic form at > 0.005 M
NaOH, the hydrophobic and structural difference caused
PSꢂ and PTꢂ to have very different CTABr and C16E20
micellar binding constants. Kinetic observations also
showed a deeper micellar penetration of PSꢂ than PTꢂ.
It is logical that the generality of the empirical Eqn (1)
and the conceivable chemical meaning of empirical
constant, KCnEm=S or KX/S, should be studied under
variety of reaction conditions. This is the driving force
that led us to carry out the present study and the observed
results and their probable explanation(s) are described
here.
We proposed the empirical equation KS ¼ KS0/(1 þ
K
X/S[MX]) in 199722 to explain the effects of inert
inorganic salts on rates of methanolysis of PSꢂ. Since
then, this empirical equation has been under test through
rate studies of micellar-mediated reactions carried out
under a variety of reaction conditions. In 2001,16 this
empirical equation was used in a modified form, Eqn (1),
coupled with the PP model of micelles to explain the
effects of mixed C12E23–CTABr micelles on the rates of
alkaline hydrolysis of 4-nitrophthalimide.
EXPERIMENTAL
Materials
Reagent-grade chemicals such as phenyl salicylate
(PSH), phenyl benzoate (PB), cetyltrimethylammonium
bromide (CTABr) and polyoxyethylene (20) cetyl ether,
C16H33(OCH2CH2)20OH (Brij 58 or C16E20), were ob-
tained from Fluka and Aldrich and were of the highest
commercially available purity. All other chemicals used
were also of reagent grade. Stock solutions (0.01 M) of
PSH and PB were prepared in acetonitrile.
KS ¼ K0=ð1 þ K
½CnEmꢃÞ
ð1Þ
CnEm=S
S
where KS and KS0 are CTABr micellar binding constants
of organic substrate (S) in the presence and absence of
CnEm, respectively, and KC E =S is an empirical constant
n
m
whose magnitude is a measure of the ability of the CnEm
surfactant to change the micellar affinity of S from pure
cationic micelle to very close to pure CnEm micelle (i.e. to
ꢄ KS0 to KSC E , where KS
CTABr
CTABr
n
m
change KS from KS
and KSC E are the CTABr and CnEm micellar binding
constants of S, respectively).Thus, the value of KC E =S is
Kinetic measurements
n
m
n
m
The rate of hydrolysis of PSH, in an alkaline medium,
was studied by monitoring the disappearance of reactant
(PSH) spectrophotometrically at 350 nm. The rate of
alkaline hydrolysis of PB was studied spectrophotome-
trically by monitoring the appearance of product (pheno-
late ion) at 290 nm. The details of the kinetic procedure
have been described elsewhere.18 All the kinetic runs
were carried out under pseudo-first-order kinetic condi-
tions. Pseudo-first-order rate constants (kobs) were calcu-
lated from either Eqn (2) (if disappearance of reactant
was monitored periodically) or Eqn (3) (if appearance of
product was monitored periodically):
expected to depend on the nature of CTABr, CnEm and S.
Recently, we studied the effects of pure C12E23, C16E20
and mixed C12E23–CTABr and C16E20–CTABr micelles
on the rates of alkaline hydrolysis of phthalimide
(PTꢂ).17 The observed unusual behaviour of pure
C12E23 and mixed C12E23–CTABr micelles beyond a
certain [C12E23]/[NaOH] ratio prompted us to include
another non-ionic C16E20 surfactant and to compare the
behaviour of both C12E23 and C16E20 micelles. Unlike
C12E23 micelles, C16E20 micelles showed no unusual
behaviour even up to 0.18 M C16E20 at 0.02 M NaOH,
which is much higher than 0.015 M C12E23 at 0.02 M
NaOH, where a sudden change in the micellar effect on
reaction rate occurred. The effects of [C16E20]T on kobs
followed the PP model of micelles within the [C16E20]T
range 0.0–0.18 M. The effects of mixed C16E20–CTABr
micelles on kobs were explained in terms of Eqn (1)
coupled with the PP model of micelles. However, again,
the observed data on the effects of C12E23 and mixed
C12E23–CTABr micelles could not be explained in terms
of PP model and Eqn (1) coupled with the PP model of
Aobs ¼ Eapp½Xꢃ expðꢂkobstÞ þ A1
ð2Þ
ð3Þ
Aobs ¼ Eapp½Xꢃ0½1 ꢂ expðꢂkobstÞꢃ þ A0
0
by the use of the non-linear least-squares technique
considering Eapp (apparent molar absorptivity of the
reaction mixture) and A (the absorbance at reaction
1
time t ¼ 1) or A0 (the absorbance at reaction time t ¼ 0)
also as unknown parameters. In Eqns (2) and (3), Aobs is
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 376–386

378
M. N. KHAN AND E. ISMAIL
the absorbance value at any time t and [X]0 is the initial
concentration of PSH or PB. The reactions were carried
out for up to 3–8 half-lives and the absorbance data (Aobs
versus t) fitted well Eqn (2) or (3).
at both 0.01 and 0.05 M NaOH (Fig. 2). The values of Eapp
remained essentially constant within the [C16E20]T range
2 ꢆ 10ꢂ 4–0.18 M, which revealed an increase in the
initial absorbance (A0 ¼ Aobs at reaction time t ¼ 0) values
with increase in [C16E20]T at ꢀ 0.01 M C16E20 because
A0 ¼ Eapp[X]0 þ A . Such an increase in A0 at high
1
RESULTS AND DISCUSSION
values of [C16E20]T may be attributed to the development
of micro-turbidity under such conditions.17 This micro-
turbidity appeared to be independent of wavelength, as is
evident from the results observed on hydrolysis of phtha-
limide17 and phenyl benzoate, as described later.
The values of Eapp were found to be independent of
[C16E20]T within the range 2 ꢆ 10ꢂ 4 ꢂ 0.18 M at 0.01 M
NaOH. However, the value of Eapp became almost zero at
0.03 M C12E23 and 0.01 M NaOH.15 These observations
show the characteristic behaviour of C12E23 [C12E23 ¼
C12H25(OCH2CH2)23OH], which is different from that of
C16E20.
Effects of [C16E20]T on kobs for hydrolysis of PSꢂ
at 35 ꢅC
A few kinetic runs were carried out within the total
C16E20 [ ¼ C16H33(OCH2CH2)20OH] concentration ([C16
E20]T) range 2 ꢆ 10ꢂ 4–0.18 M at 0.05 M NaOH and 35 ꢅC
in mixed aqueous solvent containing 2% (v/v) CH3CN
and 2 ꢆ 10ꢂ 4 M phenyl salicylate. Similar observations
were also obtained at 0.01 M NaOH. The calculated
parameters, kobs and A , at both 0.01 and 0.05 M NaOH
1
are shown graphically in Figs 1 and 2, respectively. Non-
ionized (PSH) and ionized (PSꢂ) phenyl salicylate
showed non-detectable and strong absorption, respec-
tively, at 350 nm.18 The hydrolysis products, phenol,
salicylic acid, phenolate and salicylate ions, have essen-
The values of kobs (Fig. 1) show a monotonic decrease
with increase in [C16E20]T, which could be explained
quantitatively in terms of the PP model of micelles.1,2
It can easily be shown that the observed rate law and
the rate law derived based on the reaction mechanism
in terms of the PP model of micelles can lead to the
equation
tially no absorption at 350 nm.18 Hence, Eapp and A
1
must be equal to nearly EPSꢂ (molar absorptivity of PSꢂ)
and zero, respectively, at different [C16E20]T. However,
the values of A seem to be ꢁ0 only at < 0.01 M C16E20
1
and the A values show a modest but definite increase
kWh þ kMh KS ½Dnꢃ
1
kobs
¼
ð4Þ
with increase in [C16E20]T at ꢀ 0.01 M C16E20 followed by
1 þ KS ½Dnꢃ
constancy of A within the [C16E20]T range 0.06–0.18 M
1
Figure 1. Plots of kobs versus [C16E20]T for hydrolysis of PSꢂ at 0.01 M NaOH (*) and 0.05 M NaOH (~) and for hydrolysis of PB
at 0.01 M NaOH (!). Solid lines are drawn through the calculated data points as described in the text
Copyright # 2004 John Wiley & Sons, Ltd.
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EFFECTS OF MICELLES ON CLEAVAGE OF PHENYL BENZOATE AND SALICYLATE
379
Figure 2. Plots of Az versus [C16E ] where A ¼ A for hydrolysis of PSꢂ at 0.01 M NaOH (*) and 0.05 M NaOH (~) and
1
20 T
Az ¼ A0 for hydrolysis of PB at 0.01 M NaOH (!z)
where the subscripts W and M represent aqueous pseu-
dophase and micellar pseudophase, respectively, kWh and
kMh are pseudo-first-order rate constants for the hydrolysis
of PSꢂ in the aqueous pseudophase and micellar pseu-
dophase respective, KS is the apparent C16E20 micellar
binding constant of PSꢂ and [Dn] ¼ [C16E20]T ꢂ c.m.c.,
c.m.c. being the critical micelle concentration. The value
of c.m.c. was determined by an iterative technique.19 The
values of c.m.c. at 0.01 and 0.05 M NaOH turned out to be
ꢁ0 and 1 ꢆ 10ꢂ4 M, respectively. The change in [NaOH]
from 0.01 to 0.05 M is not expected to increase the c.m.c.
The main reason for such unexpected values of c.m.c is
the extremely low sensitivity of kobs to the c.m.c. value.
with increase in [C12E23]T from 0.0 to 0.01 M at 0.01 M
NaOH and from 0.0 to 0.03 M at 0.03 M NaOH.15 How-
ever, the values of kobs revealed a monotonic decrease
with increase in [C16E20]T from 0.0 to 0.18 M at both 0.01
and 0.05 M NaOH (Fig. 1). The values of kobs decreased
by nearly 25 and 43% with respective increases in
[C16E20]T from 0.0 to 0.01 M at 0.01 M NaOH and from
0.0 to 0.03 M at 0.05 M NaOH. The value of kobs dropped
to ꢁ0 and the concentration of PSꢂ became zero at
reaction time t ¼ 0 at 0.03 M C12E23 and 0.01 M NaOH.
However, such an abrupt drop in kobs was not found even
up to [C16E20]T ¼ 0.18 M and the concentration of PSH
remained zero until [C16E20]T ¼ 0.18 M at both 0.01 and
0.05 M NaOH (Fig. 1). Hence the present results show a
very different behaviour of C16E20 micelles from that of
C12E23 micelles15, which may be considered unusual.
The abrupt drop in kobs at a certain ratio of [C12E23]T/
[NaOH] has been attributed to the sudden drop in hydro-
xide ion concentration in the vicinity of micellized PSꢂ
ions.15 The real mechanism by which C12E23 micelles
cause the expulsion of hydroxide ions from the neigh-
bourhood of micellized PSꢂ ions or cause the irreversible
entrapment of phenyl salicylate molecules in the micellar
environment of nearly zero hydroxide ion concentration
at a certain value of [C12E23]T/NaOH (ꢀ 2) is unknown,
at least to these authors. However, the effects of [C16E20]T
on kobs are not unusual.
For example, a change in c.m.c. from 0 to 1.0 ꢆ 10ꢂ 4
M
changed kMh , KS and Ædi2 (where di ¼ kobs i ꢂ kcld i, with
kobs i and kcld i being the observed and calculated values,
respectivly of rate constants at the ith [C16E20]T) by 0.4,
1.6 and 2–10%, respectively. Such an insignificant
change in the kinetic parameters shows that an
appreciable error in c.m.c. (ꢁ0) is not a setback for
data analysis. The reported values of c.m.c. for C12E12,
C16E12, C12E10 and C12E23 are 140 ꢆ 10ꢂ 6 and
2 ꢆ 10ꢂ 6, 100 ꢆ 10ꢂ 6 and 60 ꢆ 10ꢂ 6 M, respectively,
obtained in the absence of any ionic or non-ionic solute.20
The least-squares calculated respective values of kMh aꢂnd1
KS are (2.79 ꢇ 0.06) ꢆ 10ꢂ 4 sꢂ 1 and 62.6 ꢇ 3.4 M
with 104 kWh ¼ 7:90 sꢂ1 and c.m.c. ¼ 0 M at 0.01ꢂM1
NaOH and (2.72ꢇ 0.04) ꢆ 10ꢂ 4 sꢂ1 and 52.7 ꢇ 1.9 M
with 104 kWh ¼ 8:25 sꢂ1 and c.m.c. ¼ 0 M at 0.05 M
NaOH.
The value of kMh ( ¼ 2.7 ꢆ 10ꢂ4 sꢂ1) is nearly three
times smaller than that of kWh ( ¼ 8.1 ꢆ 10ꢂ4 sꢂ1). Hence
the nearly threefold lower rate of hydrolysis of PSꢂ in the
micellar pseudophase than in the aqueous pseudophase
shows that PSMꢂ ions remained in the micellar environ-
ment of considerably low water concentration compared
with [H2OW].
In view of the observed results described pre-
viously,15,21,22 the cleavage of phenyl salicylate, under
the present experimental conditions, involves PSꢂ and
H2O as reactants. The values of kobs remained unchanged
Copyright # 2004 John Wiley & Sons, Ltd.
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380
M. N. KHAN AND E. ISMAIL
Effects of [C16E20]T on kobs for hydrolysis of PB
(60 ꢆ 10ꢂ6 M),20 C12E12 (140 ꢆ 10ꢂ6 M),20 and C16E12
(2 ꢆ 10ꢂ6 M),20 the c.m.c. value of C16E20 should prob-
ably be < 2 ꢆ 10ꢂ6 M.
at 0.01 M NaOH and 35 ꢅC
The effects of [C16E20]T on the rate of alkaline hydrolysis
of phenyl benzoate (PB) [a molecule which is similar to
phenyl salicylate (PSH), except that the PSH molecule
contains an o-OH group in its benzoyl moiety] were
studied by carrying out a series of kinetic runs at different
[C16E20]T ranging from 2 ꢆ 10ꢂ5 to 0.06 M in a mixed
aqueous solvent containing 2% (v/v) CH3CN. The kinetic
parameters, kobs and A0, calculated from Eqn (3) at
different [C16E20]T are shown graphically in Figs 1 and
2, respectively. The increase in [C16E20]T from 0.0 to
0.06 M caused essentially no change in Eapp and a mono-
tonic decrease in kobs. The values of A0 remain unchanged
with increase in [C16E20]T from 0 to 0.01 M and a modest
increase in A0 is evident at [C16E20]T > 0.01 M (Fig. 2). A
similar increase in A0 can be seen from the calculated
Although it appeared that the values of kobs obtained in
the presence of C12E23 micelles obeyed the PP model of
micelles only within the [C12E23]T range where [C12
E23]T/[NaOH] > 1.5–2.0, the least-squares-calculated va-
lue of KS ( ¼ 1 ꢆ 103 M
ꢂ1 15
) may be compared with KS
) obtained in the present study.
ꢂ1
( ¼ 1.5 ꢆ 103 M
However, the value of kmMr KOH[HOꢂ]T for the C12E23
micellar-mediated alkaline hydrolysis of PB was not
different from zero at a similar value of [HOꢂ]T
15
whereas, although the value of kmMr KOH[HOꢂ]T
[ ¼ (9.3 ꢇ 3.5) ꢆ 10ꢂ5 sꢂ1] for C16E20 is associated with
a considerably large standard deviation, it is not definitely
zero. The rate of reaction of PBM with HOꢂM seems to be
much lower than that of PBW with HOꢂW, which may be
attributed to both the medium effect and different average
locations of HOꢂ and PB in the micellar pseudophase.24
The value of KS for S ¼ PB is nearly 25–30 times larger
than that for S ¼ PSꢂ which is conceivable because the
C12E23 micellar binding constants of pernonanoic acid,
pernonanoate ion, 3-chloroperbenzoic acid and 3-chlor-
operbenzoate ion are 1010, 282, 172 and ꢁ0 Mꢂ1, respec-
tively.6
values of A0 ( ¼ Eapp [X]0 þ A ) for PSꢂ obtained at
1
350 nm (Fig. 2). These results are attributed to the
presence of micro-turbidity.
The increase in [C16E20]T from 2 ꢆ 10ꢂ5 to 0.06 M at
0.01 M NaOH did not affect the values of Eapp. However,
the values of Eapp remained independent of [C12E23]T
only within the range 0–0.003 M and dropped from 2400
to ꢁ250 Mꢂ1 cmꢂ1 with increase in [C12E23]T from 0.003
to 0.020 M at 0.01 M NaOH.15
It may be of interest that an increase in [C16E20]T and
15
[C12E23]T from 0 to 0.01 M decreased kobs from
The rate of alkaline hydrolysis of PB has been shown to
involve PB and HOꢂ as reactants.23 Thus, under the
present experimental conditions, kobs ¼ kOH[HOꢂ]. The
values of kobs at different [C16E20]T were found to fit
reasonably well to the equation
66.3 ꢆ 10ꢂ4 to 5.42 ꢆ 10ꢂ4 sꢂ1 (Fig. 1) and from
69.7 ꢆ 10ꢂ4 to 7.42 ꢆ 10ꢂ4 sꢂ1, respectively. However,
the value of kobs became almost zero at 0.02 M C12E23 and
0.01 M NaOH15 whereas the value of kobs did not become
zero even at 0.06 M C16E20 and 0.01 M NaOH (Fig. 1). A
similar unexpected behaviour of C12E23 was observed in
the pH-independent hydrolysis of PSꢂ.15
kOH;W½HOꢂꢃT þ kmrKOHKS½HOꢂꢃT½Dnꢃ
1 þMKS½Dnꢃ
kobs
¼
ð5Þ
Effects of [C16E20]T on kobs for hydrolysis of PSꢂ in
the presence of a constant [CTABr]T at 0.01 M
NaOH and 35 ꢅC
A series of kinetic runs were carried out within the
[C16E20]T range 0–0.12 M at 0.006 M CTABr. Similar
observations were obtained at 0.01 and 0.02 M CTABr.
The values of Eapp were independent of [C16E20]T (range
0–0.12 M) and [CTABr]T (range 0.006–0.020 M). How-
ever, the value of Eapp dropped to almost zero at
[C12E23]T > 0.015 M and [CTABr]T in the range 0.01–
0.03 M in the presence of 0.01 M NaOH.15 The values of
which is derived based on the reaction mechanism
described in terms of the PP model of micelles, where
1 ꢈ KOH [Dn] and the rate of reaction (vM) in the micellar
pseudophase is defined as vM ¼ kmMr mOH [PBM] with
mOH ¼ [HOMꢂ]/[Dn], [HOꢂ]T is the total concentration of
hydroxide ion, kOH,W is the second-order rate constant for
the reaction between HOꢂW and PBW and KOH is the
micellar binding constant of HOꢂ.
The values of kmMr KOH[HOꢂ]T and KS were calculated
from Eqn (5) considering kOH,W[HOꢂ]T and c.m.c. as
known parameters. The value of kOH,W[HOꢂ]T was ob-
tained experimentally by carrying out kinetic run at
[C16E20]T ¼ 0 and the value of c.m.c. was determined
by an iterative technique.19 The best c.m.c. value turned
out to be nearly zero, which is similar to that obtained for
C16E20 micellar-mediated hydrolysis of PSꢂ. However, a
change in c.m.c. from 0 to 6 ꢆ 10ꢂ6 M changed the
A
1
were not affected by the presence of CTABr within
the concentration range 0.006–0.02 M. The value of kobs
shows an almost monotonic increase with increase in
[C16E20]T at constant [CTABr]T and [NaOH] (Table 1).
However, the rate of hydrolysis of PSꢂ at 0.01 M NaOH,
>0.015 M C12E23 and within the [CTABr]T range 0.01–
0.03 M became too slow to monitor easily. Mixed micelles
behave as if pure CTABr micelles at [CTABr]T/
[C16E20]T ꢀ ꢁ10 at both 0.01 and 0.02 M CTABr because
under such conditions the kobs values remain independent
of [C16E20]T (Table 1).
respective values of kmMr KOH[HOꢂ]T and KS from
ꢂ1
(9.3 ꢇ 3.5) ꢆ 10ꢂ5 sꢂ1 and 1538 ꢇ 38 M
to (10.4 ꢇ
4.0) ꢆ 10ꢂ5 sꢂ1 and 1575 ꢇ 43 Mꢂ1. In view of the re-
ported c.m.c. values of C12E10 (100 ꢆ 10ꢂ6 M),20 C12E23
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 376–386

EFFECTS OF MICELLES ON CLEAVAGE OF PHENYL BENZOATE AND SALICYLATE
381
a
Table 1. Values of kobs and K for hydrolysis of PSꢂ at different [C16E20]T in the presence of a constant value of [CTABr]T
[CTABr]T (M)
0.006
0.010
104kccld
0.020
b
d
[C16E20]T
(M)
104kobs
(sꢂ1
104kcld
K
(M
104kobs
(sꢂ1
K
(M
104kobs
(sꢂ1
104kcld
K
ꢂ1
ꢂ1
ꢂ1
(M )
)
(sꢂ1
)
)
)
(sꢂ1
)
)
)
(sꢂ1
)
0.0
1.34 ꢇ 0.01e
1.28 ꢇ 0.012e
1.30 ꢇ 0.01
1.34 ꢇ 0.01
1.32 ꢇ 0.00
1.61 ꢇ 0.01
1.67 ꢇ 0.02
1.94 ꢇ 0.02
0.0002
0.0005
0.0010
0.0050
0.0100
0.0200
0.0240
0.0260
0.0280
0.0300
0.0400
0.0500
0.0600
0.1000
0.1200
Average
1.29
1.31
1.34
1.54
1.71
1.92
15f
19f
1.23 ꢇ 0.01e
1.26 ꢇ 0.01
1.13 ꢇ 0.02
1.27 ꢇ 0.02
1.42 ꢇ 0.02
1.68 ꢇ 0.02
1.22
1.23
1.24
1.37
1.48
1.63
15f
16f
6.5f
ꢂ13f
1.76 ꢇ 0.03
2.02 ꢇ 0.03
2.01 ꢇ 0.02
2.15 ꢇ 0.03
2.14 ꢇ 0.02
2.37 ꢇ 0.02
2.40 ꢇ 0.02
2.55 ꢇ 0.03
1.67
1.88
2.17
2.24
2.27
2.30
2.35
2.46
17
17
11
12
12
16
16
17
13
9.0
10
2.2f
4.6
6.8
2.06 ꢇ 0.03
2.09 ꢇ 0.04
2.18 ꢇ 0.04
2.25 ꢇ 0.04
2.41 ꢇ 0.05
2.06
2.13
2.19
2.23
2.40
10
9
10
10
10
1.78 ꢇ 0.03
1.85 ꢇ 0.03
1.91 ꢇ 0.04
1.83 ꢇ 0.03
2.06 ꢇ 0.04
2.10 ꢇ 0.05
1.74
1.81
1.87
1.91
2.10
2.07
6.6
6.6
6.5
5.1
5.6
6.4
2.65 ꢇ 0.04
2.81 ꢇ 0.04
2.88 ꢇ 0.05
2.60
2.80
2.87
18
19
26
16 ꢇ 4e
10 ꢇ 1
6.0 ꢇ 0.8
a
Conditions: [phenyl salicylate]0 ¼ 2 ꢆ 10ꢂ4 M, [NaOH] ¼ 0.01 M, 35 ꢅC, ꢂ ¼ 350 nm, aqueous reaction mixture for each kinetic run contained 2% (v/v)
obs
CH3CN and the values of K were calculated from Eqn (6) with F ¼ 1 and kC
obs
( ¼ kobs) values shown in Fig. 1.
16E20
C16E20
ꢂ1
b Calculated from Eqn (6) with 104k0 ¼ 1.34 sꢂ1, k
( ¼ kobs) values shown in Fig. 1, F ¼ 1.23 and K ¼ 10 M
.
C16E20
obs
C16E20
obs
ꢂ1
c Calculated from Eqn (6) with 104k0 ¼ 1.28 sꢂ1, k
( ¼ kobs) values shown in Fig. 1, F ¼ 0.98 and K ¼ 10 M
.
d Calculated from Eqn (6) with 104k0 ¼ 1.21 sꢂ1, k
( ¼ kobs) values shown in Fig. 1, F ¼ 1.01 and K ¼ 5.9 M
.
ꢂ1
e Error limits are standard deviations.
f These values were not included in the calculation of the average value of K.
The value of tꢂhe1 CTABr micellar binding constant of
PSꢂ is ꢁ7000 M and the pseudo-first-order rate con-
stant for hydrolysis of PSꢂ is nearly eight times larger in
the aqueous pseudophase than that in pure CTABr mi-
cellar pseudophase;25 probable causes of such an inhibi-
tory effect have been discussed in detail elsewhere.21
This shows that PSꢂ ions are almost fully micellized at
[CTABr]T ꢀ 0.006 M in the absence of any additive. The
increase in kobs with increase in [C16E20]T at a constant
[CTABr]T is therefore attributed to the transfer of PSꢂ
ions from the less to the more hydrated mixed micellar
environment. A limited number of studies on the effects
of mixed cationic–non-ionic micelles on the rates of
reactions showed that an increase in non-ionic surfactant
concentration at a constant cationic surfactant concentra-
tion increased both the volume of mixed micelles and the
degree of micellar ionization (ꢁ).11,14 Voluminous poly-
oxyethylene headgroups of CnEm micelles are known to
be extensively hydrated.26 The increase in ꢁ values due to
the increase in [CnEm]T at a constant [CTABr]T is most
likely due to a decrease in the electrostatic affinity of
counterions for the cationic headgroups because of the
apparent increase in both [H2O] and size of the cationic
headgroups. It is apparent that an increase in [CnEm]T at a
constant [cationic surfactant]T is bound to decrease the
cationic micellar affinity of both counterions and non-
ionic solubilizates. It should be noted that the cationic
headgroups of CTABr micelles surfactant are buried deep
inside the voluminous headgroup mantle of C16E20.27 It
has been reported that polar solubilizates reside in the
headgroup mantle rather than in the hydrophobic core of
C12E23.28
The kinetic data on the effects of mixed micelles on
reaction rates show that the characteristic features of
mixed micellar system (S1–S2 with S1 and S2 represent-
ing two different surfactants) change gradually from very
close to pure S1 micelle type to very close to pure S2
micelle type as the X values increase from very low to very
high, where X ¼ [S2]/[S1]. Hence it is obvious that the
increase in [C16E20]T at a constant [CTABr]T will change
CTABr micellar binding constant of PSꢂ from KS
( ¼ KS0) to very close to KS
CTA
C16E20
(pure C16E20 micellar
binding constant of PSꢂ). Based on related studies,16,17
such an effect of [C16E20] on KSCTA is expected to follow
Eqn (1) with CnEm replaced by C16E20.
Recently, the effects of [C12E23]T and [C16E20]T on the
CTABr micellar binding constants, KS, of anionic 4-
nitrophthalimide16 and phthalimide17 have been ex-
plained in terms of the pseudophase model of micelles
coupled with an empirical relationship shown by Eqn (1).
The pseudophase model of micelles i.e. Eqns (4) and (1),
with replacement of CnEm by C16E20, and with kWh ¼ kobs
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 376–386

382
M. N. KHAN AND E. ISMAIL
at [CTABr]T ¼ [C16E20]T ¼ 0 and kWh ¼ koCb1s6E20 at
[CTABr]T ¼ 0 but [C16E20]T ¼ 0, can lead to the equation
Table 2. Values of F and K, calculated from Eqn (6), for
hydrolyses of PSꢂ and PB at different values of [CTABr]T
a
Ester
(S)
[CTABr]T
(M)
F
K
(M
KX/S
(M
ꢂ1
ꢂ1
16E20
k0 þ F kC
K½C16E20ꢃT
)
)
1 þoKbs½C16E20ꢃ
kobs
¼
ð6Þ
T
PSꢂ
0.006
0.010
0.020
0.006
0.010
0.020
0.006
0.010
0.015
0.020
1.23 ꢇ 0.10b
1.0
10 ꢇ 2b
(16 ꢇ 4)c
10 ꢇ 1
688
710
0.98 ꢇ 0.04
where
1.0
1.01 ꢇ 0.12
1.0
(10 ꢇ 1)
5.9 ꢇ 1.4
(6.0 ꢇ 0.8)
651 ꢇ 126
(505 ꢇ 84)
495 ꢇ 74
(269 ꢇ 55)
121 ꢇ 13
(142 ꢇ 25)
16 ꢇ 1
kWh þ kMh KS0 ½Dnꢃ
848
k0 ¼
ð7Þ
ð8Þ
1 þ kS0½Dnꢃ
PB
1.73 ꢇ 0.48
1.0
3.35 ꢇ 0.32
1.0
2020
1614
1562
390
=ð1 þ K0 ½DnꢃÞ
0.70 ꢇ 0.53
K ¼ KC
16E20=S
S
1.0
1.08 ꢇ 0.04
1.0
PTꢂd
and F ¼ ꢀ=koCb1s6E20 with F representing the fraction of pure
CTABr micellized PSꢂ transferred to pure C16E20 mi-
celles by the limiting concentration of C16E20 (the limit-
ing concentration of C16E20 is the optimum value of
[C16E20]T at which kobs values become independent of
[C16E20]T). Hence, conceptually, the value of F should be
ꢉ 1.0. It may be noted that a slightly different form of
Eqn (6) was used in the previous work.16,17
19 ꢇ 3
1.26 ꢇ 0.05
5.7 ꢇ 0.3
8.0 ꢇ 0.9
1.7 ꢇ 0.7
5.2 ꢇ 1.5
0.4 ꢇ 0.3
3.6 ꢇ 1.0
1.0
2.44 ꢇ 0.79
1.0
268
259
6.50 ꢇ 4.10
1.0
238
a
0
ꢂ1
KX/S ¼ K(1 þ KS0 [CTABr]T) with X ¼ C16E20
,
KS ¼ 7000 M
for
S ¼ PSꢂ, KS0 ¼ 500 M for S ¼ PB and KS0 ¼ 3250 M for S ¼ PTꢂ.
ꢂ1
ꢂ1
b
Error limits are standard deviations.
The values of the unknown parameters F and K were
calculated from Eqn (6) by the use of the non-linear least-
c Values in parentheses were obtained from Tables 1 and 3.
d The values of K were calculated from Eqn (6) by the use of data obtained
from Ref. 17.
16E20
squares technique. The values of k0 and koCbs
were
obtained experimentally by carrying out experiments
under similar experimental conditions with [C16E20]T ¼
0, [CTABr]T ¼ 0 and [C16E20]T ¼ 0, [CTABr]T ¼ 0. The
least-squares calculated values of F and K are summar-
ized in Table 2. The fitting of observed data to Eqn (6)
appears to be satisfactory, as is evident from the values of
the calculated rate constants (kcld) summarized in Table 1.
The larger value of F ( ¼ 1.23 at 0.006 M CTABr) com-
pared with its expected maximum value of 1.0 is con-
sidered to be fortuitous and therefore K values were also
calculated from the equation
from Eqn (6) that the maximum value of kobs should be
ꢁꢉ kMh (i.e. first-order rate constant for the hydrolysis of
PSꢂ in pure C16E20 micelles).
Effects of [C16E20]T on kobs for hydrolysis of PB in
the presence of a constant [CTABr]T at 0.01 M
NaOH and 35 ꢅC
Several kinetic runs were carried out with [C16E20]T in
the range 0.0–0.06 M at 0.006 M CTABr. Similar observa-
tions were obtained at 0.01 and 0.02 M CTABr. These
results, as kobs versus [C16E20]T, are shown in Table 3.
The values of Eapp were independent of [C16E20]T (range
0–0.06 M) and [CTABr]T (range 0.006–0.020 M). How-
ever, the values of Eapp remained almost unchanged
within the [C12E23]T range 0–0.015 M and dropped from
ꢁ2100 to 300–500 Mꢂ1 cmꢂ1 with increase in [C12E23]T
from 0.015 to 0.025 M at 0.01 M NaOH and [CTABr]T in
the range 0.006–0.030 M.15 It is evident from these results
that mixed micelles behave as pure CTABr micelles at
[CTABr]T/[C16E20]T ꢀ ꢁ10–20. The values of both Eapp
and A0 were not affected by the presence of 0.006–
0.020 M CTABr. The values of kobs decrease with increase
in [C16E20]T at [CTABr]T/[C16E20]T of < ꢁ10–20. How-
ever, the value of kobs even at 0.06 M C16E20 with
[CTABr]T/[C16E20]T ¼ 0.1 is about double the value ob-
tained at 0.06 M C16E20 with [CTABr]T ¼ 0. Similar
kobs ꢂ k0
K ¼
ð9Þ
ðkoCb1s6E20 ꢂ kobsÞ½C16E20ꢃT
which is the rearranged form of Eqn (6) with F ¼ 1. These
calculated values of K are shown in Table 1. It should be
noted that the calculation of K from Eqn (9) with F ¼ 1
has the disadvantage that the calculated values of K may
not be reliable at both very low and very high values of
[C16E20]T because, under such conditions, kobs ! k0 and
16E20
obs
kobs ! kC
, respectively, and consequently (kobs–
k0) ! 0 and ðkC
ꢂ kobsÞꢂ1 ! 1. These mathematical
16E20
obs
limits show that under such conditions, the values of
(kobs–k0) and ðkC
ꢂ kobsÞꢂ1 should be very sensitive
16E20
obs
to errors.
The values of kobs in Fig. 1 and Table 1 reveal that an
increase in [C16E20]T decreases kobs compared with kWh
and increases kobs compared with kobs at
[CTABr]T ꢀ 0.006 M and [C16E20]T ¼ 0. It is evident
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 376–386

EFFECTS OF MICELLES ON CLEAVAGE OF PHENYL BENZOATE AND SALICYLATE
383
observations were obtained in the alkaline hydrolysis of
PB in the presence of mixed CTABr–C12E23 micelles.15
The value of kobs( ¼ kWh ) for hydrolysis of PB is
6.77 ꢆ 10ꢂ3 sꢂ1 at 35 ꢅC, 0.01 M NaOH and
[CTABr]T ¼ [C16E20]T ¼ 0.23 The values of kobs at differ-
ent [CTABr]T with [C16E20]T ¼ 0 are significantly higher
than kWh (Table 3), which is due to HOꢂ–Brꢂ ion
exchange at the cationic micellar surface.23 The increase
in [C16E20]T at a constant [CTABr]T is expected to alter
gradually the characteristic features of the mixed micellar
system from cationic micelle type to non-ionic micelle
type and consequently it should change the micellar
affinity of the reactants, HOꢂ and PB, from cationic to
non-ionic micelles. Such a change in [C16E20]T at a
constant [CTABr]T is expected to change the CTABr
micellar binding constants of both reactants, HOꢂ and
PB. If we assume that the changes in KS and KOH due to
increase in [C16E20]T at a constant [CTABr]T follow
empirical equations similar to Eqn (1), then
KS ¼ KS0=ð1 þ KX=S½C16E20ꢃTÞ
ð10Þ
and
KOH ¼ KO0H=ð1 þ KX=HO ½C16E20ꢃTÞ
ð11Þ
where the subscript X represents C16E20 and KX/S and
KX/HO are empirical constants. Equation (5) with the
condition that 1 is not negligible compared with
KOH[Dn], and Eqns (10) and (11) can lead to
2
k00 þ ðꢀKPB þ ꢃKHOÞ½C16E20ꢃT þ ꢀꢃKPBKHO½C16E20ꢃT
kobs
¼
ð1 þ KPB½C16E20ꢃTÞð1 þ KHO½C16E20ꢃTÞ
ð12Þ
a
Table 3. Values of kobs and K for hydrolysis of PB at different [C16E20]T in the presence of a constant value of [CTABr]T
[CTABr]T (M)
0.006
0.010
0.020
b
c
d
[C16E20]T
(M)
103kobs
(sꢂ1
103kcld
K
(M
103kobs
(sꢂ1
103kcld
K
(M
103kobs
(sꢂ1
103kcld
K
(M
ꢂ1
)
ꢂ1
ꢂ1
)
)
(sꢂ1
)
)
)
(sꢂ1
)
)
(sꢂ1
)
0.0
0.02
13.5 ꢇ 0.1e
13.7 ꢇ 0.1
13.6 ꢇ 0.2
10.7 ꢇ 0.02e
7.81 ꢇ 0.11e
0.00005
0.00010
0.00015
0.00020
0.00025
0.0003
0.0005
0.0007
0.0010
0.0012
0.0014
0.0015
0.0016
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0.010
0.020
10.8 ꢇ 0.5
11.3 ꢇ 0.1
7.44 ꢇ 0.18
7.67 ꢇ 0.08
13.6 ꢇ 0.1
12.8 ꢇ 0.2
13.0 ꢇ 0.1
12.8 ꢇ 0.1
11.5 ꢇ 0.2
10.9 ꢇ 0.1
10.1 ꢇ 0.1
9.66 ꢇ 0.13
9.34 ꢇ 0.07
7.88 ꢇ 0.12
11.1 ꢇ 0.3
7.43 ꢇ 0.10
11.7 ꢇ 0.5
11.3 ꢇ 0.2
7.25 ꢇ 0.09
7.20 ꢇ 0.07
11.6
10.9
9.86
9.28
8.74
8.50
468
452
430
417
394
639
10.2 ꢇ 0.4
6.83 ꢇ 0.09
6.88
152
9.78 ꢇ 0.30
9.46
114f
8.81 ꢇ 0.12
8.14 ꢇ 0.09
6.99 ꢇ 0.07
6.20 ꢇ 0.10
5.15 ꢇ 017
4.84 ꢇ 0.10
4.42 ꢇ 0.05
3.80 ꢇ 0.09
3.22 ꢇ 0.04
1.58 ꢇ 0.01
1.07 ꢇ 0.01
0.707 ꢇ 0.005
0.534 ꢇ 0.004
9.07
8.33
7.01
5.94
5.29
4.61
4.15
3.77
3.36
1.79
1.27
0.992
0.930
200
221
234
231
274
250
249
280
290
362
373
458f
554f
6.52 ꢇ 0.07
6.26 ꢇ 0.13
5.94 ꢇ 0.09
5.52 ꢇ 0.07
5.10 ꢇ 0.09
4.59 ꢇ 0.09
4.35 ꢇ 0.08
4.20 ꢇ 0.09
3.64 ꢇ 0.08
2.64 ꢇ 0.04
1.51 ꢇ 0.01
1.09 ꢇ 0.01
0.851 ꢇ 0.006
6.62
6.37
5.84
5.37
4.99
4.63
4.34
4.07
3.66
2.35
1.74
1.39
1.17
154
151
122
118
121
132
127
119
129
105
154
173
195
6.60 ꢇ 0.12
6.07 ꢇ 0.09
5.35 ꢇ 0.09
4.61 ꢇ 0.05
3.94 ꢇ 0.02
3.35 ꢇ 0.05
2.89 ꢇ 0.05
2.41 ꢇ 0.05
1.05 ꢇ 0.01
0.638 ꢇ 0.009
0.422 ꢇ 0.005
0.342 ꢇ 0.006
0.284 ꢇ 0.008
7.42
5.98
4.95
4.30
3.72
3.31
2.98
2.58
1.36
0.949
0.734
0.658
0.585
681
504
457
467
486
526
563
583
770f
928f
0.030
0.040
0.050
0.060
Average
1162f
1393f
1619f
505 ꢇ 84e
269 ꢇ 55e
142 ꢇ 25e
a Conditions: [phenyl benzoate]0 ¼ 2 ꢆ 10ꢂ4 M, [NaOH] ¼ 0.01 M, 35 ꢅC, ꢂ ¼ 290 nm, aqueous reaction mixture for each kinetic run contained 2% (v/v) CH3CN
and the values of K were calculated from Eqn (6) with F ¼ 1 and koCb1s6E20 ( ¼ kobs) values shown in Fig. 1.
C16E20
obs
C16E20
obs
C16E20
obs
ꢂ1
ꢂ1
b Calculated from Eqn (6) with 103k0 ¼ 13.3 sꢂ1, k
c Calculated from Eqn (6) with 103k0 ¼ 11.0 sꢂ1, k
d Calculated from Eqn (6) with 103k0 ¼ 7.64 sꢂ1, k
e Error limits are standard deviations.
( ¼ kobs) values shown in Fig. 1, F ¼ 1.73 and K ¼ 651 M
( ¼ kobs) values shown in Fig. 1, F ¼ 3.35 and K ¼ 495 M
( ¼ kobs) values shown in Fig. 1, F ¼ 0.70 and K ¼ 121 M
.
.
ꢂ1
.
f These values were not included in the calculation of the average value of K.
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 376–386

384
M. N. KHAN AND E. ISMAIL
where
to compensatory effects of the errors in F and K because
of the term FK in the numerator of Eqn (6) may be ruled
out for the reason that exactly the same values of F, K and
residual errors were obtained from Eqn (18), which is a
rearranged form of Eqn (6):
ðkOH;W þ kMmrKO0HKS0½DnꢃÞ½HOꢂꢃT
ð1 þ KS0½DnꢃÞð1 þ KO0H½DnꢃÞ
k00 ¼
ð13Þ
ꢀ ¼ kOXH;W½HOꢂꢃT=ð1 þ KO0H ½DnꢃÞ
ꢃ ¼ kOXH;W½HOꢂꢃT=ð1 þ KS0 ½DnꢃÞ
KPB ¼ KX=S=ð1 þ KS0 ½DnꢃÞ
ð14Þ
ð15Þ
ð16Þ
k00 K0 þ FKoCb1s6E20 ½C16E20ꢃT
kobs
¼
ð18Þ
K0 þ ½C16E20ꢃT
where K0 ¼ 1/K. Perhaps the more reliable values of K
were obtained from Eqn (6) with F ¼ 1. These calculated
values of K are shown in Table 3. The values of K seem to
be constant within the [C16E20]T/[CTABr]T range ꢁ0.1–
1.5 (Table 3). However, the K values increase with
increase in [C16E20]T/[CTABr]T beyond a value of ꢁ1.5
(Table 3).
and
KHO ¼ KX=HO=ð1 þ KO0H ½DnꢃÞ
ð17Þ
In Eqns (14) and (15), kOXH;W ¼ kOH;W at [X]
( ¼ [C16E20]T) ¼ 0.
It is evident from the empirical definition of KX/S or
KX/HO that the magnitude of say KX/S should be directly
and inversely proportional to KSX and KSCTA ðꢄ KS0Þ, re-
spectively, where KSX and KSCTA are the respective pure
C16E20 and CTABr micellar binding constants of S
( ¼ PB). The values of kinetically determined CTABr
micellar binding constants of anionic acetyl salicylic
The larger values of K than expected at [C16E20]T/
[CTABr]T > ꢁ1.5 may be due to the following reasons:
(i) an unexpected faster decrease in kobs with increase in
[C16E20]T/[CTABr]T beyond ꢁ1.5; (ii) the value of F is
no longer 1 at [C16E20]T/[CTABr]T > ꢁ1.5 and drops
sharply with increase in [C16E20]T/[CTABr]T beyond
ꢁ1.5; (iii) the value of the empirical constant KX/S does
not remain constant at [C16E20]T/[CTABr]T > ꢁ1.5;
(iv) the inequality 1 ꢈ KHO[C16E20]T is probably no
longer true at [C16E20]T/[CTABr]T > ꢁ1.5. All these
possibilities may be the consequence of the fact that the
nature and characteristics of the mixed micellar environ-
ment of a mixed micellar-mediated reaction cannot be
expected to remain the same within the entire range of
[C16E20]T at a constant [CTABr]T. Sommer et al.32
recently reported the formation of giant worm-like non-
ionic micelles in the presence of small amounts of ionic
surfactants. However, such a probable characteristic
mixed micellar structural change due to changes in the
mole fraction of mixed micellar components at a specific
value appeared to be kinetically insensitive to the rate of
non-catalysed hydrolysis of PSꢂ, which involves PSꢂ and
H2O as the reactants, and sensitive to the rate of hydro-
xide ion-catalysed hydrolysis of PB, which involves PB
and HOꢂ as the reactants.
and 4-acetoxybenzoic acids are in the range ꢁ200–
ꢂ1 29
The values of the ion-exchange constants,
300 M
.
K
( ¼ KB0z /KB0r, where Bz represents benzoate ion)
Br
Bz
and KBrOH ( ¼ KB0r/KO0H), for the respective ion-exchange
processes Bzꢂ–Brꢂ and Brꢂ–HOꢂ at the CTABr micellar
surface are ꢁ622,30 and 2–20,3,29,31 respectively. These
ꢂ1
ꢂ1
.
results show that KB0rꢊ33–50 M and KO0Hꢊ2–25 M
The value of KOXH(i.e. the C16E20 micellar binding con-
stant of HOꢂ) should be much smaller than KO0H
CTA
OH
( ¼ K
). The values of KSX and KSCTA ( ¼ KS0) are
ꢂ1 23
respectively. In
ꢁ1550 (Table 2) and 300–700 M
,
view of the empirical definition of KX/S and KX/HO and
the estimated values of KSX, KS0, KOXH and KO0H, it seems
that the value of KX/S should be much higher than that of
KX/HO and consequently ꢃKHO may be neglected
compared with ꢀKPB in Eqn (12). It can also be shown
that 1ꢈ ꢃKHO [C16E20]T under the experimental condi-
tions of the present study. These inequalities reduced
Eqn (12) to Eqn (6) provided 1ꢈ KHO[C16E20]T with
KPB ¼ K and kWh ¼ kOH,W [HOꢂ]T ¼ kobs at [CTABr]T ¼
[C16E20]T ¼ 0 and kWh ¼ koCb1s6E20 at [CTABr]T ¼ 0 but
[C16E20]T ¼ 0.
Equation (8) or (16) predicts that the value of K should
decrease with increase in [CTABr]T provided that KS[Dn]
is not negligible compared with 1 within the [CTABr]T
range considered in this study. The values of K for
S ¼ PSꢂ, PB and PTꢂ17 (PTꢂ represents ionized phtha-
limide) decrease with increase in [CTABr]T within the
range 0.006–0.020 M (Table 2). The CTABr micellar
binding constants (KS0) for S ¼ PSꢂ, PTꢂ and PB are
The unknown parameters F and K ( ¼ KPB) were
calculated from Eqn (6) by the use 0of the non-linear
least-squares technique considering k0 and kobsC16E20 as
known parameters. Although the residual errors ( ¼ kobs i
–
kcldi) appear to be reasonably low at [C16E20]T/
ꢂ1 23
7000,25 325024e and 500 M
,
respectively. The values
[CTABr]T < ꢁ1.5 (Table 3), the calculated values of F
are unreliable. This is not an unusual problem encoun-
tered in solving an equation similar to Eqn (6) by the use
of the non-linear least-squares technique. This is a
mathematical paradox which requires careful attention
in such data analysis. The likely possibility that the
low residual errors and fortuitous values of F were due
of the c.m.c. of CTABr vary in the range 1 ꢆ 10ꢂ4
–
3 ꢆ 10ꢂ4 M in the presence of 2 ꢆ 10ꢂ4 M S and therefore
[Dn] ¼ [CTABr]T ꢂ c.m.c.ꢊ[CTABr]T at [CTABr]T ꢀ
0.006 M. The known values of KS0 were used to calculate
KX/S from Eqn (8) or (16) with [Dn] ¼ [CTABr]T and the
results obtained are shown in Table 2.
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 376–386

EFFECTS OF MICELLES ON CLEAVAGE OF PHENYL BENZOATE AND SALICYLATE
385
The values of KX/S are almost independent of
[CTABr]T within the range 0.006–0.020 M. The average
values of KX/S for S ¼ PSꢂ (KX/S ¼ 750 ꢇ 90 Mꢂ1),
of KC
mide) at different [CTABr]T ranging from 0.006 to
for S ¼ PSꢂ, PB and PTꢂ (anionic phthali-
16E20=S
ꢂ1
0.020 M
definition of KC
are conceivable in view of the empirical
.
S ¼ PB (KX/S ¼ 1730 ꢇ 250 Mꢂ1) and S ¼ PTꢂ (KX/S
¼
16E20=S
290 ꢇ 70 M
)
ꢂ1 17 are conceivable in view of the empirical
definition of KX/S, which suggests that the magnitude of
X/S should be proportional to the magnitude of KSX (X,
K
Acknowledgement
C16E20, micellar binding constant of S) and inversely
proportional to the magnitude KSCTA (CTA, CTABr,
micellar binding constant of S). Thus, KX/S ¼ ꢄS KSX/
KSCTA, where ꢄS is a proportionality constant with dimen-
sions Mꢂ1. The value of ꢄS depends only on the nature and
micellar affinity of S. The value of KX/S for S ¼ PSꢂ
(KX/S ¼ 750 Mꢂ1) is nearly three times larger than KX/S
The authors thank the University of Malaya for financial
support (Vot F, F0718/2002B).
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( ¼ 290 Mꢂ1) for S ¼ PTꢂ because KSX ( ¼ 62 Mꢂ1) for
ꢂ1 17
S ¼ PSꢂ is about 10 times larger than KX/S ( ¼ 6 M
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The effect [C16E20]T on kobs for the hydrolysis of PSꢂ
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micellar pseudophase compared with that in the aqueous
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0.01 M NaOH and 35 ꢅC. Although the rate of alkaline
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The effects of mixed micelles, C16E20–CTABr, on the
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explained in terms of the pseudophase model of micelles
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Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 376–386