Effects of pure non-ionic and mixed non-ionic-cationic surfactants on the rates of hydrolysis of phenyl salicylate and phenyl benzoate in alkaline medium

Article abstract of DOI:10.1002/poc.414

The pseudo-first-order rate constants (k_obs) for the alkaline hydrolysis of phenyl benzoate (PB) decrease monotonically from 72.0 × 10^-4 to 7.4 × 10^-4 s^-1 with increase in the concentration of polyoxyethylene 23 lauryl ether ([C₁₂E₂₃]_T) from 0.0 to 0.01 M at 0.01 M NaOH and 35 °C. The results fit satisfactorily to a pseudophase model of micelles. The values of k_obs for the hydrolysis of ionized phenyl salicylate (PS^-) are almost independent of [C₁₂E₂₃]_T within the range 0.0-0.02 M at 0.01 and 0.03 M NaOH and 35°C. The rate of alkaline hydrolysis of PB appears to be negligible in the micellar pseudophase than in the aqueous pseudophase, which is ascribed to the extremely low concentration of HO^- in the micellar region of micellized PB molecules. The values of kobs for hydrolysis of PB and PS^- at 0.01 M NaOH become almost zero at ≥0.02 and ≥0.03 M C₁₂E₂₃, respectively. The effects of mixed C₁₂E₂₃-CTABr on k_obs for the alkaline hydrolysis of PB reveal that mixed micelles behave like pure CTABr micelles within the [C₁₂E₂₃]_T range 0.0-5 × 10^-4 M whereas an increase in [C₁₂E₂₃]_T from ≥7 × 10^-4 to 0.02 M decreases k_obs at 0.006, 0.01, 0.02 and 0.03 M CTABr. However, the values of kobs are significantly higher at [CTABr]_T = 0.006, 0.01,0.02 and 0.03 M than at [CTABr]_T = 0 in the presence of [C₁₂E₂₃]_T in the range >5 × 10^-4 - 0.02 M. An increase in [C₁₂E₂₃]_T from 0.0 to 0.015 M causes nearly a twofold increase in k_obs at 0.01, 0.02 and 0.03 M CTABr and 0.01 M NaOH. Copyright

Full text of DOI:10.1002/poc.414

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2001; 14: 669–676
DOI:10.1002/poc.414
Effects of pure non-ionic and mixed non-ionic±cationic
surfactants on the rates of hydrolysis of phenyl salicylate
and phenyl benzoate in alkaline medium
M. Niyaz Khan,* Emran Ismail and M. Rapandy Yusoff
Department of Chemistry, Faculty of Science, Universiti Malaya, 50603 Kuala Lumpur, Malaysia
Received 7 February 2001; revised 23 April 2001; accepted 2 May 2001
ABSTRACT: The pseudo-first-order rate constants (kobs) for the alkaline hydrolysis of phenyl benzoate (PB) decrease
À4
À4 À1
monotonically from 72.0 Â 10 to 7.4 Â 10
s
with increase in the concentration of polyoxyethylene 23 lauryl
ether ([C E ] ) from 0.0 to 0.01 M at 0.01 M NaOH and 35°C. The results fit satisfactorily to a pseudophase model
1
2 23 T
À
of micelles. The values of k for the hydrolysis of ionized phenyl salicylate (PS ) are almost independent of
obs
[
C E ] within the range 0.0–0.02 M at 0.01 and 0.03 M NaOH and 35°C. The rate of alkaline hydrolysis of PB
1
2 23 T
appears to be negligible in the micellar pseudophase than in the aqueous pseudophase, which is ascribed to the
extremely low concentration of HO in the micellar region of micellized PB molecules. The values of kobs for
À
À
hydrolysis of PB and PS at 0.01 M NaOH become almost zero at ꢀ0.02 and ꢀ0.03 M C E , respectively. The
1
2 23
effects of mixed C E –CTABr on k for the alkaline hydrolysis of PB reveal that mixed micelles behave like pure
1
2
23
obs
À4
À4
CTABr micelles within the [C E ] range 0.0–5 Â 10 M whereas an increase in [C E ] from ꢀ7 Â 10 to
1
2
23 T
12 23 T
0
[
0
.02 M decreases k at 0.006, 0.01, 0.02 and 0.03 M CTABr. However, the values of k are significantly higher at
obs obs
À4
CTABr] = 0.006, 0.01, 0.02 and 0.03 M than at [CTABr] = 0 in the presence of [C E ] in the range >5 Â 10
–
T
T
12 23 T
.02 M. An increase in [C E ] from 0.0 to 0.015 M causes nearly a twofold increase in k at 0.01, 0.02 and 0.03 M
12 23 T obs
CTABr and 0.01 M NaOH. Copyright  2001 John Wiley & Sons, Ltd.
KEYWORDS: phenyl salicylate; phenyl benzoate; hydrolysis; kinetics; non-ionic micelles; mixed non-ionic–cationic
surfactants
INTRODUCTION
nature when X ꢁ0.3 and anionic in nature when X
4, where X = [SDS]/[CTABr] (SDS = sodium dode-
ꢀ
Studies on the effects of mixed surfactants or mixed
micelles on reaction rates have been started only
cyl sulfate and CTABr = cetyltrimethylammonium bro-
5
mide). Most studies on the effects of mixed non-ionic–
1–3
recently.
Systematic kinetic studies on the effects of
cationic surfactants on the rates of reactions involved
C H (OCH CH ) OCH CH OH (C E ) as the non-
mixed non-ionic –cationic micelles on the rates of
organic reactions have shown that the kinetic data can
be explained in terms of a pseudophase model of
1
0
21
2
2 3
–4
2
2
10 4
2
ionic surfactant.
The head groups of C E micelles
10 4
are not as voluminous as those of C E (Brij 35) and the
12 23
2–4
micelles.
The effects of mixed cationic–anionic
size of the head groups affects the characteristic proper-
ties of micellar and presumably mixed micellar systems.
In a recent study on the effects of pure CTABr, C E
surfactants on the rates of methanolysis of ionized phenyl
À
salicylate (PS ) and hydrolysis of phenyl benzoate (PB)
1
2 23
have revealed the mixed micelles behaving as cationic in
and mixed CTABr–C E surfactants on the acid –base
12 23
behavior of phenyl salicylate, an unusual result was
6
obtained. We became interested in finding out whether
there are some unusual effects of pure C E and mixed
12 23
*
Correspondence to: M. N. Khan, Department of Chemistry, Faculty
CTABr–C E surfactants on the rates of alkaline
12 23
of Science, Universiti Malaya, 50603 Kuala Lumpur, Malaysia.
E-mail: niyaz@kimia.um.edu.my
Contract/grant sponsor: National Scientific Research and Develop-
ment Council of Malaysia, IRPA Program; Contract/grant number:
hydrolysis of esters. The present study was aimed at
determining the effects of pure C E and mixed
12 23
CTABr–C E surfactants on the rates of hydrolysis of
12 23
0
9-02-03-0785.
phenyl salicylate and PB under mild alkaline conditions
and the results and their probable explanations are
described in this paper.
Contract/grant sponsor: National Scientific Research and Develop-
ment Council of Malaysia, IRPA Program; Contract/grant number:
0
9-02-03-0785.
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 669–676

6
70
M. N. KHAN, E. ISMAIL AND M. R. YUSOFF
À
Figure 1. Plots showing the relationship between Aobs and reaction time, t, for the hydrolysis of PB '*) and PS '~) under
4
À1
,
alkaline conditions. '~) The solid line is drawn through the calculated data points using Eqn. '1) with 10 k = 6.51 Æ 0.18 s
obs
À1
À1
À4
E
app
= 6284 Æ 61 M cm and A = 0.015 Æ 0.013, obtained under the reaction conditions [X] = 2 Â 10 M, [C E ] =
?
0
12 23 T
0
.01 M, [NaOH] = 0.03 M, 35°C and l = 350 nm. '*) The solid line is drawn through the calculated data points using Eqn.
3
À1
À1
À1
'
2) with 10
k
= 2.16 Æ 0.04 s , E = 2248 Æ 18 M cm
app
and A = 0.137 Æ 0.003, obtained under the reaction
obs
0
À4
conditions [X] = 2 Â 10 M, [C E ] = 0.01 M, [CTABr] = 0.03 M, [NaOH] = 0.01 M, 35°C and l = 290 nm
0
12 23 T
T
EXPERIMENTAL
period of 4–18 half-lives of the reactions. Pseudo-first-
order rate constants (kobs) were calculated from either
À
Materials. Phenyl salicylate (PSL), phenyl benzoate
PB), cetyltrimethylammonium bromide (CTABr) and
C H (OCH CH ) OCH CH OH (C E or Brij 35)
Eqn. (1) [if disappearance of reactant (PS ) was
(
monitored periodically] or Eqn. (2) [if the appearance
of product(s) was monitored periodically]
12
25
2
2 22
2
2
12 23
were obtained from Fluka, Sigma and Aldrich and were
of the highest commercially available purity. All other
chemicals used were of reagent grade. Stock solutions
Aobs ˆ Eapp‰XŠ expꢂÀkobst† ‡ A1
ꢂ1†
ꢂ2†
0
Aobs ˆ Eapp‰XŠ ‰1 À expꢂÀkobst†Š ‡ A
(
0.01 M) of PSL and PB were prepared in acetonitrile.
0
0
Kinetic measurements. The rate of hydrolysis of ionized
where Eapp is the apparent molar absorptivity of the
À
PSL (PS ) was studied spectrophotometrically by
reaction mixture, [X] is the initial concentration of ester
0
À
monitoring the disappearance of PS as a function of
(PSL or PB), A = A at t = ? and A = A at t = 0.
?
obs
0
obs
time at 350 nm. Similarly, the rate of alkaline hydrolysis
of PB was studied spectrophotometrically by monitoring
the appearance of the product (phenolate ion) as a
function of time at 290 nm. The absorbance values (Aobs),
at different reaction times (t) were measured using a
Shimadzu Model UV-1601 UV–visible spectrophot-
ometer equipped with a thermoelectrically temperature-
controlled cell positional Model CPS 240A. Two to four
kinetic runs were carried out simultaneously under
reaction conditions where the reaction rates were fairly
slow. The reaction rates were monitored for a reaction
The non-linear least-squares technique was used to
calculate kobs, Eapp and A? or A from Eqn. (1) or (2).
0
The fitting of the observed data to either Eqn. (1) or (2)
was good, as is evident from the typical plots of Aobs
versus t shown in Fig. 1, where solid lines are drawn
through the calculated data points using either Eqn. (1) or
(2) with the calculated values of unknown parameters
kobs, Eapp and A? or A and from the standard deviations
0
associated with the calculated parameters (Tables 1 and
2). The details of the kinetic procedure and data analysis
7
have been described elsewhere.
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 669–676

EFFECTS OF SURFACTANTS ON HYDROLYSIS OF PHENYL SALICYLATE AND BENZOATE
671
a
Table 1. Effect of [C E ] on k for hydrolysis of PB at 0.01 M NaOH and 35°C
1
2
23 T
obs
4
4
4
b
1
0 [C E ]
10 k
E
10 k
1
M)
2
23 T
obs
)
app
calcd
)
À1
À1
À1
À1
c
d
(
(s
(M cm
)
A0
(s
Qobs
Qcalcd
e
e
e
0
0
1
2
3
5
7
0.0
0.0
0.0
0.0
.0
.5
.0
.0
.0
.0
.0
69.7 Æ 0.5
2420 Æ 6
2358 Æ 9
2363 Æ 6
2322 Æ 5
2371 Æ 6
2368 Æ 8
2382 Æ 7
2541 Æ 7
2544 Æ 8
2346 Æ 3
0.100 Æ 0.001
0.106 Æ 0.001
0.106 Æ 0.001
0.098 Æ 0.001
0.098 Æ 0.001
0.113 Æ 0.002
0.089 Æ 0.001
0.094 Æ 0.001
0.102 Æ 0.000
0.093 Æ 0.000
0.099 Æ 0.001
0.098 Æ 0.000
0.099 Æ 0.002
74.6 Æ 0.8
72.3 Æ 0.5
67.4 Æ 0.5
62.1 Æ 0.6
53.0 Æ 0.5
45.4 Æ 0.4
39.9 Æ 0.3
25.2 Æ 0.2
19.3 Æ 0.1
11.9 Æ 0.3
7.42 Æ 0.15
67.6
61.9
53.0
46.4
39.1
25.7
19.2
11.0
7.14
5.04
1.071
1.163
1.362
1.590
1.810
2.865
3.741
6.067
9.730
1.137
1.225
1.400
1.575
1.837
2.712
3.587
6.213
9.713
1
2
3
6
2061 Æ 22
1289 Æ 16
1
1
2
00.0
50.0
00.0
4.13 Æ 0.17
1077 Æ 14
f
f
a
b
c
d
e
f
À4
[
PB] = 2 Â 10 M, 0.01 M NaOH, 35°C, l = 290 nm, reaction mixture for each kinetic run contained 2% (v/v) CH CN.
0
3
Calculated from Eqn. (3) with the parameters mentioned in the text.
Qobs = k W/kobs, where 10 k = 72.2 s
Calculated from Eqn. (4) as described in the text.
Error limits are standard deviations.
The rate of reaction was too slow to measure kobs and Eapp accurately (the value of Aobs changed from 0.10 to 0.15 with change in
h
4
h
À1
.
W
reaction time (t) from ꢄ20 s to 3 h).
RESULTS AND DISCUSSION
scheme for alkaline hydrolysis of PB, in terms of the PP
model of micelles, may be expressed by Scheme 2.
Effect of [C E ] on k for hydrolysis of PB
obs
12
23 T
KS
SW ‡ D
n
„ S
M
A series of kinetic runs were carried out within the total
C E concentration ([C E ] ) range 0.0–0.015 M at
kOH; W
SW ‡ HOÀ
À! Products
12
23
12 23 T
W
kOH; M
0
E
.01 M NaOH and 35°C. First-order rate constants (kobs),
SM ‡ HOÀ À! Products
M
and A , obtained under these experimental condi-
app
0
tions, are shown in Table 1. An attempt to obtain kobs at
0.02 M C E failed because the absorbance at 290 nm
remained almost unchanged within the reaction period of
nearly 3 h.
Scheme 2
ꢀ
12 23
S stands for reactive substrate (PB), K
is the micellar
S
binding constant of S, D represents the micelle, kOH,W
n
The kobs values decreased monotonically with increase
in [C E ] until [C E ] ꢃ 0.01 M at 0.01 M NaOH.
and kOH,M are second-order rate constants for the
reactions occurring in the aqueous pseudophase and
micellar pseudophase, respectively, and the subscripts W
and M stand for the aqueous pseudophase and micellar
pseudophase, respectively. The observed rate law
1
2
23 T
12 23 T
The rate of alkaline hydrolysis of PB should involve
uncatalyzed (k
[PB]) and hydroxide ion-catalyzed
H O
2
À
(kOH [HO ] [PB]) kinetic steps, as shown in Scheme 1.
(
rate = kobs [S] , where [S] = [S ] ‡ [S ]) and the rate
T
T
W
M
1
: kH2
O
law for the reaction, based on the reaction steps in
Scheme 2, can lead to
phenyl benzoate ÀÀÀÀ ÀÀ ! phenolate ion ‡ benzoate ion
À
2
: kOH‰HO Š
À
ꢂPB†
ꢂPhO †
ꢂBZ†
kh ‡ k K ‰4oxD Š
h
W
M
S
n
Scheme 1
kobs ˆ
ꢂ3†
1
‡ K ‰D Š
S
n
However, the contribution of kH2O [PB] has been shown
to be negligible compared with k [HO ] [PB] in the
rate law at pH >9. Thus, k = kOH [HO ] under the
present experimental conditions.
The non-linear decrease in kobs with increase in
C E ] (Table 1) may be explained by the use of the
pseudophase (PP) model of micelles. The assumptions
involved in this model and its usefulness and weaknesses
were critically discussed by Bunton. The reaction
where [D ] = [C E ] À cmc, with cmc representing
n
12 23 T
À
the critical micelle concentration of the micelle-forming
OH
8
À
h
W
À
h
M
surfactant C E , k = k
mOH/VM (where mOH = [HO ]/[D ] and VM represents
the micellar molar volume ). The values of cmc,
obtained by an iterative technique and Broxton’s
[HO W] and k = kOH,M
obs
12 23
OH,W
À
M
n
1
0
11
[
1
2 23 T
9
12
À4
À4
graphical technique, are 1.5 Â 10 and 1.3 Â 10 M,
respectively.
10
The non-linear least-squares technique was used to
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 669–676

6
72
M. N. KHAN, E. ISMAIL AND M. R. YUSOFF
À
a
Table 2. Effect of [C E ] on k and Eapp for hydrolysis of PS at 35°C
1
2
23 T
obs
0
.01 M NaOH
0.03 M NaOH
4
4
4
1
[
0
10 k
E
10 k
E
obs
)
app
obs
)
app
À1
À1
À1
À1
À1
À1
C E ] (M)
1
(s
(M cm
)
A?
(s
(M cm
)
A?
2 23 T
b
b
b
b
b
b
0
5
5
1
1
2
3
5
6
6
1
1
2
3
.0
.0
.0
7.48 Æ 0.12
6339 Æ 40
6021 Æ 46
5813 Æ 44
5993 Æ 12
5780 Æ 39
6173 Æ 64
6091 Æ 59
5944 Æ 45
5866 Æ 13
5792 Æ 28
5838 Æ 14
5482 Æ 16
À0.007 Æ 0.008
7.51 Æ 0.14
6290 Æ 44
6100 Æ 42
À0.020 Æ 0.009
7.52 Æ 0.13
7.11 Æ 0.14
7.89 Æ 0.04
7.13 Æ 0.13
7.11 Æ 0.21
7.11 Æ 0.19
7.18 Æ 0.15
7.37 Æ 0.04
6.95 Æ 0.09
7.54 Æ 0.05
7.27 Æ 0.06
À0.053 Æ 0.010
À0.006 Æ 0.010
0.001 Æ 0.003
À0.011 Æ 0.008
À0.012 Æ 0.013
À0.006 Æ 0.012
0.007 Æ 0.009
0.037 Æ 0.003
0.086 Æ 0.006
0.065 Æ 0.003
0.032 Æ 0.003
0.067 Æ 0.004
7.85 Æ 0.12
À0.019 Æ 0.009
0.0
0.0
7.83 Æ 0.09
8.26 Æ 0.33
7.50 Æ 0.22
6.94 Æ 0.19
8.07 Æ 0.04
7.30 Æ 0.06
6.51 Æ 0.18
7.43 Æ 0.05
6.40 Æ 0.10
6.41 Æ 0.07
6138 Æ 28
6205 Æ 99
6199 Æ 73
6330 Æ 62
5822 Æ 11
6249 Æ 17
6284 Æ 61
6154 Æ 14
6211 Æ 35
6078 Æ 23
0.005 Æ 0.005
À0.005 Æ 0.018
0.002 Æ 0.014
0.001 Æ 0.013
0.028 Æ 0.002
0.052 Æ 0.003
0.015 Æ 0.013
0.023 Æ 0.003
0.051 Æ 0.008
0.074 Æ 0.005
0.0
0.0
0.0
0.0
0.0
00.0
00.0
00.0
00.0
5.89 Æ 0.17
1804 Æ 18
c
c
a
b
c
À4
[
Phenyl salicylate] = 2 Â 10 M, 35°C, l = 350 nm, reaction mixture for each kinetic run contained 2% (v/v) CH CN.
0
3
Error limits are standard deviations.
The absorbance value (Aobs = 0.012) remained unchanged with change in the reaction time (t) from ꢄ20 s to 3 h.
h
h
calculate k and K from Eqn. (3) considering k as a
occur at the micellar surface (i.e. palisade or Stern layer)
where the microscopic hydrophilicity (i.e. microscopic
dielectric constant) of the medium is considerably lower
compared with the macroscopic hydrophilicity (i.e.
dielectric constant) of the aqueous pseudophase. An
M
S
W
known parameter. The least-squares calculated values of
h
À4 À1
k
and K are (0.47 Æ 0.53) Â 10
s
and 987 Æ
M
S
À1
h
2
7 M , respectively. The calculated value of k
is
M
associated with a relative standard deviation of >100%
and hence it is not statistically different from zero, and
increase in acetonitrile content from 2 to 70% (v/v)
À3
therefore K was also calculated from Eqn. (4), which is
decreased k for hydrolysis of PB from 14.9 Â 10 to
S
obs
À3 À1
h
M
8
the rearranged form of Eqn. (3) with k = 0 and
1.41 Â 10
s
at 0.02 M NaOH and 35°C. However,
h
M
ꢀ
= 1 À cmc KS:
the value of k ꢃ 0 cannot be explained in terms of a
medium effect only. The possible reason for the slower
rate of hydrolysis in the micellar pseudophase is the
khW=kobs ˆ ꢀ ‡ K ‰C E Š
ꢂ4†
S
12 23 T
À
presence of a kinetically insignificant amount of HO in
the vicinity of micellized PB molecules. A similar
proposal has been made to explain the inhibitory effects
The linear least-squares calculated values of f and K are
S
À1
À
0
.962 Æ 0.046 and 875 Æ 11 M , respectively. The value
of non-ionic micelles on the rate of reaction of HO with
À1
14
of K is only 11% lower than that (987 M ) obtained
p-nitrophenyl diphenylphosphate. The evidence for the
S
from Eqn. (3). It may be worth mentioning that the non-
presence of different locations or the non-uniformity in
linear least-squares technique used in the calculation of
the concentrations of two different reactants in the
h
14–17
k
and K from Eqn. (3) involves a simulation where it
micellar pseudophase is no longer rare.
The PB
M
S
h
may be possible that large errors in both k and K may
molecules, being highly hydrophobic, are expected to be
dragged deeper inside the micellar pseudophase whereas
hydroxide ions, being highly hydrophilic, are expected to
remain in the considerably polar and hydrophilic region
of the micellar pseudophase.
M
S
compensate each other, producing a seemingly good fit of
the observed data to Eqn. (3). However, the linear least-
squares treatment gives an exact solution of Eqn. (4) and
hence there is no possible compensatory errors in the
calculated values of f and K . The fitting of the observed
The values of kobs at ꢀ0.015 M C E show a sharp
S
12 23
decrease with increase in [C E ] (Table 1), which is
h
data to Eqn. (4) is evident from the (k W/k
)
values
obs calcd
12 23 T
largely due to depletion of HO ions in the micellar
À
listed in Table 1. The pseudo-first-order rate constants
kobs) for the alkaline hydrolysis of securinine, in the
presence of C E micelles, were found to fit a kinetic
(
environment of micellized PB molecules, and these kobs
values fail to obey the PP model, i.e. Eqn. (3) or (4). This
is a rather unusual observation in view of the theoretical
micellar models, such as the PP model, used to explain
such data quantitatively.
1
2 10
13
equation similar to Eqn. (4).
h
M
The insignificant value of k shows that the rate of
hydrolysis of PB is much slower in the micellar
pseudophase than in the aqueous pseudophase. The
micellar-mediated reactions are generally believed to
The molar absorptivities of non-ionized and ionized
phenol at 290 nm and 1 M ionic strength are 0 and
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 669–676

EFFECTS OF SURFACTANTS ON HYDROLYSIS OF PHENYL SALICYLATE AND BENZOATE
18
673
À1
À1
2
350 M cm , respectively. In Eqn. (2), Eapp = EP
can be used to calculate the values of A at different
0
ÀE , where E and E represent the molar absorp-
[C E ] with the values of E and A? summarized in
PB
P
PB
12 23 T
app
À4
tivities of products, phenolate and benzoate ions, and
Table 2 where [X] = 2 Â 10 M}. If the change in the
0
PB, respectively, and A = E [X] . The value of E
reaction conditions, such as an increase in [C E ] ,
0
PB
0
app
12 23 T
À1
À1
(
= 2420 M cm , Table 1) at [C E ] = 0 and the
increases [PSH] (= [PSH ] ‡ [PSH ]), then the values
12
23 T
T
M
W
relationship Eapp = E À E reveal that E ꢃ E (where
of A should have been dependent upon [C E ] . The
0 12 23 T
P
PB
P
BZ
BZ represents benzoate ion) at 290 nm. The values of A0
and Eapp are almost independent of [C E ] within the
unchanged values of A with the change in [C E ]
0 12 23 T
À
show the presence of 100% ionized form (PS ) of phenyl
salicylate in the reaction mixtures under such experi-
12
23 T
range 0.0–0.02 and 0.0–0.003 M, respectively. An
increase in [C E ] beyond 0.003 M decreased Eapp
mental conditions. However, the value of A (ꢄ0.012) at
12
23 T
0
and the value of Eapp at 0.015 M C12E23 became almost
half of that at [C E ] = 0. Thus, the value of E at
0.01 M NaOH and 0.03 M C E and also at 0.03 M
12
23
NaOH and >0.03 M C12E23 remained unchanged within
12
23 T
app
À
0
.015 M C E indicates that the hydroxide ion con-
the reaction period of ꢄ3 h, which shows that [PS ] = 0
12
23
T
centration in the micellar environment of micellized
product phenol molecules was insufficient to cause their
complete ionization. It should be noted that phenol is
apparently more hydrophilic than PB and hence the
location in the micellar pseudophase for phenol should be
and the entire PSH molecules are fully bound by
micelles, i.e. [PSH ] = 0 under such conditions because
W
7
the aqueous pK of PSH is 9.25. Similar results were
a
W
obtained in the study on the effect of [C E ] on the pK
of PSH.
12
23 T
a
6
18
more polar than that for PB. Thus, the concentration of
The first-order rate constants (kobs) for the hydrolysis
À
HO in the vicinity of micellized PB molecules must be
of phenyl salicylate were found to be independent of
M
À
even lower than that of micellized phenol.
[HO ] within the range 0.005–0.060 M in the absence of
18
The possibility that the decrease in Eapp with the
increase in [C E ] at 0.006 M is due to the effect of the
micelles.
Similarly, kobs for the hydrolysis and
methanolysis of phenyl salicylate turned out to be
12 23 T
À
medium on E and E may be ruled out because E and
independent of [HO ] within the respective [NaOH]
P
PB
P
EPB remained essentially unchanged with the change in
range 0.01–0.04 and 0.005–0.050 M in the presence of
19
20
the acetonitrile content from 2 to 70% (v/v) in mixed
0.0015 M and 0.01 M CTABr. It has been shown
unequivocally that pH-independent hydrolysis of phenyl
8
aqueous solvents. The most obvious reason for the
À
21
decrease in Eapp with the increase in [C E ] at a
salicylate involves PS and H O as the reactants. The
12
23 T
2
constant [NaOH] is the depletion of hydroxide ions in the
region of micellized PB molecules in the palisade layer
through a mechanism (unknown to us at the moment) by
which C E micelles affect the pK or pK of phenyl
brief reaction scheme for the cleavage of phenyl
salicylate, under the present experimental conditions,
may be as shown in Scheme 3.
1
2
23
a
b
6
k
H
O
2
‰H OŠ
2
salicylate.
À
À
À
À
o- OC H COOC H ÀÀÀÀÀ! o- OC H CO ‡ C H O
6
4
6
5
6
4
2
6
5
À
NaOH
À
ꢂPS †
ꢂsalicylate ion†
ꢂPhO †
À
Effect of [C E ] on k for the hydrolysis of PS
12
23 T
obs
Scheme 3
In order to find out the effect of [C E ] on the rate of
The observed data, summarized in Table 2, show that
12 23 T
À
hydrolysis of PS , a few kinetic runs were carried out at
different [C E ] ranging from 0.0 to 0.02 M at 0.01 M
kobs remained almost independent of [C E ] within the
12
23 T
range 0.0–0.01 M at 0.01 M NaOH and 0.0–0.03 M at
12 23 T
NaOH, and 35°C. Similar results were obtained at 0.03 M
NaOH. First-order rate constants (kobs), Eapp and A?
obtained under these experimental conditions are sum-
marized in Table 2. The values of kobs could not be
obtained at [C E ] ꢀ0.03 M within the [NaOH] range
0.03 M NaOH. These results may be explained in terms of
À
either K = 0 (i.e. PS ions do not have a detectable
S
binding affinity with C E micelles) or K ≠ 0, but the
1
2
23
S
À
location of micellized PS in the micellar pseudophase is
very similar to that in the aqueous pseudophase in terms
of medium polarity and water concentration. The
1
2 23 T
0
.01–0.03 M because, under such conditions, the absor-
bance (Aobs) at 350 nm became ꢄ0.02 at reaction time
t ꢃ 15 s and remained unchanged within the reaction
period of nearly 3 h.
possibility that K = 0 can be ruled out for the following
S
À
À
reason. If K = 0 {where K = [PS M]/([PS ][D ])},
S
S
W
À1 †
.
n
app
À
W
then KS = [PSH ]/([PS ][D ]) ꢃ 100 M
Under
M
n
À
The values of the molar absorptivities for PS and PSH
†
app
ÀM
ÀW
(
5
non-ionized form of phenyl salicylate) at 350 nm are
(By definition, K
S
= {[PS ] ‡ [PSHÀ
M
]}/{([PS ] ‡ [PSH
] = 0 because K = 0 and
M and, therefore,
W
])
À
À1
À1
18
[
[
D
n
] = [PSH
M
]/([PS
W
][D
n
])}. Where [PS
of PSH is 5.67 Â 10
M
S
700–6000 and ꢄ0 M cm , respectively. The values
À10
7
PSH
W
] = 0 because the K
a
of the initial absorbance (A = A at t = 0) of the reaction
0
obs
W app
at 0.01 M NaOH, [PSH ] = 0. The values of E (= molar absorptivity
of PS at 350 nm because the molar absorptivities of PSH, non-ionized
and ionized products, salicylic acid and phenol are nearly zero at
À
mixtures at 0.01 and 0.03 M NaOH were found to be
unchanged with change in [C E ] from 0.0 to 0.01 M
12
23 T
18
À1
À1
3
50 nm ) at 0.0 and 0.02 M C12
E23 are 6300 and 1800 M cm
and from 0.0 to 0.03 M, respectively {it is evident from
(
Table 2), respectively, in the presence of 0.01 M NaOH. Thus,
app
S
À1
Eqn. (1) that at t = 0, Aobs = A = E [X] ‡ A , which
K
= (6300 À 1800)/(1800 Â 0.02) = 125 M
.
0
app
0
?
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 669–676

6
74
M. N. KHAN, E. ISMAIL AND M. R. YUSOFF
such conditions, the PP model can lead to
h
app
kobs ˆ k W=ꢂ1 ‡ KS ‰D Š†
ꢂ5†
n
h
which shows that kobs = k /3 at 0.02 M C E . However,
W
12 23
À4 À1
the observed value of kobs (= 5.89 Â 10 s ) at 0.02 M
h
W
À4
C E is only 22% smaller than k (= k = 7.48 Â 10
1
2
23
obs
À1
s ) at [C E ] = 0. The possibility that K = 0 may be
12
23 T
S
also ruled out based on the observed effect of C E and
12 23
6
mixed C E –CTABr micelles on pK or pK of PSH.
1
2
23
a
b
À
The kinetic data reveal that PS ions exist in the
micellar interface or micellar pseudophase of CTABr
and sodium dodecyl sulfate micelles of very low [H O]
17
18
2
compared with [H O] of the aqueous pseudophase.
2
Therefore, it seems surprising that the C E micellar
1
2 23
À
location of PS M does not differ from the aqueous
pseudophase in terms of water concentration. However, it
has been reported that in aqueous non-ionic micelles,
such as C E , the ethylene oxide palisade layer is
extensively hydrated and hence the water concentra-
tion in the palisade layer of C E micelles must be
1
2 23
22,23
12 23
larger than in the interior of the Stern layer of ionic
Figure 2. Plots showing the relationship between kobs and
C E ] for the hydrolysis of PB at '◊) 0.0, '*) 0.006, '~)
À
[
micelles. As mentioned earlier, the hydrolysis of PS
12 23 T
À
0.01, '&) 0.02 and '!) 0.03 M CTABr
involves PS and monomeric H O (i.e. H Ofree) as the
2
2
reactants. Ethers and monofunctional alcohols such as the
head groups of C E are known to cause disruption of
12 23
À4
the water structure at a high mole fraction of organic co-
An increase in [C E ] from ꢀ7 Â 10 M caused a
12 23 T
24
solvent in mixed aqueous solvents. Such an interaction
is bound to produce more monomeric water molecules
decrease in k
and this decrease became nearly
obs
À4
independent of [CTABr]T at ꢀ25 Â 10 M C E .
12
23
(
H O ) and consequently increase the rate of hydrolysis
The values of kobs are significantly higher at
2
free
À
of PS . If this argument is correct, then it is merely a
coincidence that the effect of the decrease in [H O ]
(
[CTABr] = 0.006, 0.01, 0.02 and 0.03 M than at
T
[CTABr] = 0 in the presence of [C E ] in the range
2
M
T
12 23 T
À
compared with [H O ]) in the vicinity of PS ions is
M
À4
À4
>5 Â 10 –200 Â 10 M. These observations reveal
2
W
nearly counterbalanced by the effect of the increase in
H O ] (compared with [H O W]), owing to
that mixed CTABr–C E micelles behave like pure
12 23
À4
[
CTABr micelles at [C E ] ꢁ5 Â 10 M and the
12 23 T
2
free,
M
2
free,
disruption of the water structure by headgroups, on kobs
.
decrease in kobs with increase in [CTABr] from 0.006
T
to 0.03 M at a constant [C E ] is due to dilution effect
1
2 23 T
À
on [HO M] as generally ascribed in the interpretation of
Effect of C E ±CTABr mixed micelles on k
the hydrolysis of PB
for
the k
model.
versus [CTABr]T profile by use of the PIE
It is clear from a number of reports that
12
23
obs
obs
8,10,25
organic solutes and non-ionic surfactants increase both
A series of kinetic runs were carried out within the
C E ] range 0.0–0.025 M at 0.01 M NaOH and 35°C
in the presence of 0.006 M CTABr. Similar results were
obtained at 0.01 M, 0.02 and 0.03 M CTABr. These results
are shown graphically in Fig. 2 as plots of kobs versus
the fractional micellar ionization (a) and the volume of
4
[
the micellar pseudophase. These two effects on the
1
2 23 T
À
concentration of HO M may be attributed to the decrease
in kobs with increase in [C E ] at a constant [CTABr]
12
23 T
T
(Fig. 2). A quantitative treatment of these observations is
difficult to provide because of the lack of a perfect kinetic
model and imperfect understanding of the structural
features of mixed micelles at varying mole ratios of its
surfactant components.
[
0
[
C E ] . The value of k for the hydrolysis of PB at
12 23 T
obs
À3 À1
.01 M NaOH is 6.77 Â 10
s
at 0.01 M NaOH and
CTABr] = [C E ] = 0. The effects of [CTABr] on
T 12 23 T T
kobs for alkaline hydrolysis of PB in the absence of C12E23
micelles have been explained in terms of the pseudophase
ion-exchange (PIE) model and this data treatment
It is perhaps noteworthy that the values of kobs became
almost independent of [CTABr] within the range 0.006–
T
resulted in a CTABr micellar binding constant of PB of
0.03 M at a constant [C E ] within the range
12 23 T
À1 8
À4
À4
3
00 M
.
25 Â 10 –250 Â 10 M (Fig. 2). However, even under
It is evident from the plots of Fig. 2 that the values of
kobs are almost independent of [C E ] within the range
such conditions, the presence of CTABr increased the
À
À
concentration of HO M compared with that of HO M at
12
23 T
À4
0
.0–5 Â 10 M at 0.006, 0.01, 0.02 and 0.03 M CTABr.
[CTABr] = 0. This is evident from the larger values of
T
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 669–676

EFFECTS OF SURFACTANTS ON HYDROLYSIS OF PHENYL SALICYLATE AND BENZOATE
675
micelles, such as C E micelles, containing bulky
12 23
polyoxyethylene head groups, the voluminous outer
mantle of the micelle can also serve as a locus of
26
solubilization of some polar solubilizate. The distribu-
tion ratios of the amounts in the mantle and the amounts
associated with the hydrocarbon core, which includes the
molecules in the interfacial region, increased in favor of
the core location as the molecules became more
26
hydrophobic.
Effect of Mixed C E ±CTABr micelles on k for
obs
12
23
À
the hydrolysis of PS
A few kinetic runs were carried out within the [C E ]
2 23 T
1
range 0.005–0.015 M at 0.01 M CTABr and 0.01 M
NaOH. The first-order rate constants (kobs) showed a
linear increase with increase in [C E ] , as is evident
12 23 T
from the plot in Fig. 4. Similar results were obtained at
.02 and 0.03 M CTABr (Fig. 4). It is apparent that mixed
C E –CTABr micelles behave as pure CTABr micelles
0
12 23
Figure 3. Effect of [C E ] on E for hydrolysis of PB at '◊)
1
2
23 T
app
at [C E ] <0.005 M and [CTABr]T ꢀ0.01 M. As
12 23 T
0
.0, '*) 0.006, '~) 0.01, '&) 0.02 and '!) 0.03 M CTABr
mentioned earlier, the increase in [C E ] at a constant
12 23 T
[
CTABr] is expected to increase both a and the volume
T
4
kobs (Fig. 2) and Eapp (Fig. 3) in mixed CTABr–C12E23
of micellar pseudophase. These two effects are expected
to increase the water concentration in the vicinity of
micellized PS ions, which could be the cause of the
micelles compared with those in pure C E micelles.
12 23
À
These observations reflect the fact that in the interface of
a cationic micelle the hydroxide ion concentration will be
higher than that in the interface of a non-ionic micelle
and, therefore, the reaction will be faster in mixed
cationic–non-ionic than in non-ionic micelles. However,
increase in kobs with increase in [C E ] . The linear
12
23 T
increase in kobs with increase in [C E ] at constant
12
23 T
[CTABr] cannot be attributed to the probable reaction
T
À
the dilution effect on [HO M] due to the increase in
[
CTABr] at a constant [C E ] and the decreasing
T 12 23 T
À
effects of the increase in [C E ] on [HO M] and a at a
12 23 T
constant [CTABr] are no longer kinetically significant.
T
It is interesting that the pK of phenyl salicylate (PSL)
a
was unaffected by an increase in [CTABr] from 0.0 to
T
6
0
.03 M at [C E ] = 0.02 M. This shows that the
presence of CTABr did not cause an increase in [HO M]
compared with [HO ] at [CTABr] = 0) sufficient
12 23 T
À
À
(
M
T
enough to show an effect on the pK of PSL under such
a
conditions.
The value of Eapp at 0.006 M CTABr is nearly 30%
smaller than those at 0.01, 0.02 and 0.03 M CTABr in the
presence of 0.02 M C E (Fig. 3), whereas the values of
12 23
kobs are independent of [CTABr] under such conditions
T
(
Fig. 2). These observations may be explained with the
assumptions that PBM molecules are located deeper
inside the micellar pseudophase compared with the
relatively more hydrophilic product phenolate ions
À
(
PhO M) and the rate of hydrolysis of PB is insignificant
in the micellar pseudophase compared with that in the
aqueous pseudophase. However, the product, phenolate
ion, formed in the aqueous pseudophase is trapped by the
cationic interface of mixed C E –CTABr micelles
12 23
À
À
where [HO ] increases with increase in [CTABr] at
M
T
Figure 4. Effect of [C E ] on k for hydrolysis of PS at
1
2
23 T
obs
a constant [C E ] . Mukerjee pointed out that for
'*) 0.01, '~) 0.02 and '!) 0.03 M CTABr
12
23 T
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 669–676

6
76
M. N. KHAN, E. ISMAIL AND M. R. YUSOFF
À
between PS and ionized or non-ionized hydroxyl groups
of C E because the product (ionized alkyl salicylate)
Acknowledgements
12 23
of such a reaction could not be detected spectrophotome-
The authors thank the National Scientific Research and
Development Council of Malaysia under the IRPA
Program for financial support (Grant No. 09-02-03-
0785).
7
trically in the presence of either pure C E micelles or
12 23
mixed C E –CTABr micelles. Similarly, the reaction
12 23
of p-nitrophenyl diphenylphosphate with terminal alk-
oxide groups of non-ionic micelles was found to be
1
4,27
unimportant,
but the reaction of 2,4-dinitrochloro-
À
benzene (DNCB) with HO in C E and C12E23 gave
12 10
considerable amounts of the ether formed by the reaction
14
of alkoxide ion with DNCB. The apparent linear
variation of kobs with [C E ] at a constant [CTABr]
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23 T
obs
5
6
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À
9
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4
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12 23 T
some unusual effect of [C E ] on the distribution of
1
2 23 T
À
HO between the aqueous and micellar pseudophases
under such conditions. The effect of [C E ] on k for
1
2
23 T
obs
À
À
the hydrolysis of PS shows the presence of PS ions in
the micellar environment of a high concentration of
M
1
À
H O and hence PS
ions most likely remain in the
2
M
M
voluminous polyoxyethylene head groups of the mi-
celles. The presence of mixed micelles of CTABr and
2
À
C E causes an increase in [HO M] at the interface of
12 23
2
the cationic surfactant which makes kobs larger than that
at the corresponding [C E ] with [CTABr] = 0 for
PB. The addition of CTABr micellar solution to C E
micellar solution causes the presence of PS M at the
CTABr micellar interface where [H O ] is significantly
12
23 T
T
12 23
À
2
2
M
lower than [H O ]. However, an increase in [C E ]
2
W
12 23 T
2
from 0.005 to 0.015 M at a constant [CTABr] increases
kobs, which is due to the increase in [H O ] caused by the
T
(
ed). New York: Plenum Press, 1979; 153–174.
2
M
27. Gellman SH, Petter R, Breslow R. J. Am. Chem. Soc. 1986; 108:
swelling of the mixed micelles.
2388–2394.
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 669–676