326
H.A. Al-Lohedan et al. / Journal of Molecular Liquids 206 (2015) 321–327
pseudophases. The increase in concentration of Brij-35 incorporates
more and more amount of triflusal molecules into the Brij-35 micelles
and, thereby, decreasing the amount to triflusal in aqueous
pseudophases. At the same time, the negatively charged OH− ions are
repelled by the electronegative polyoxyethylenated alcohol group of
the Brij-35 micelles and its major proportions exist in the aqueous
phase. As a result, triflusal and OH− ions are predominantly distributed
in the retroactive micellar and aqueous pseudophases. The chance of
interaction between triflusal and OH− ions is decreased and thus,
causing the inhibition in the rate of hydrolysis with the increase in
Brij-35 micelles. Additionally, the other factors like lower water activity
in the Stern layer, the stabilization of substrate by the micelles, destabi-
lization of the transition state, and unfavorable orientation of the
substrate may also contribute in lowering the rate of reaction. The
rearrangement of Eq. (7), gives Eq. (23):
18
16
14
12
10
8
6
4
2
0
1
1
1
¼
þ
:
ð23Þ
kw0 − kψ k0w− km0 ðk0w− km0 Þ Ks ½Dnꢀ
103 [Salt] (mol dm-3)
Fig. 8. Plot of kψ vs. [salt] for the alkaline hydrolysis of triflusal in the presence of different
salt (●; NaBr, ○; (CH3)4NBr, ▲; (C2H5)4NBr) in CTABr. Reaction conditions: [triflusal] =
8.0 × 10−5 mol dm−3, [NaOH] = 2.0 × 10−2 mol dm−3, Temperature = 25.0 0.1 °C.
The kψ–[Brij-35] profile shows that the rate of hydrolysis of
triflusal decreases and becomes extremely slow at [Brij-35] N 8.0 ×
10−3 mol dm−3. From the analysis of these data, it can be inferred
that with the increase in concentration of Brij-35 the observed rate
constant decreases progressively, because more and more triflusal
molecules are incorporated inside the micelles. The observed rate con-
stant in the presence of micelles is due to the reaction of aqueous
triflusal molecules with OH− ions and so, the value of micellar rate con-
stant can be neglected in the above equation (Bunton and Cerichelli
[29]). Eq. (23) can be rewritten in the form of Eq. (24) after eliminating
the term ‘km’ in the above equation:
the aggregation number of micelles or modify the micelle structure by
extending the hydrocarbon chain. The size of micelles increases,
resulting into the entrance of more and more organic substrate into
the interior of micelles.
4. Conclusion
The increase in the concentration of cationic surfactants (CTABr,
CTACl, and DTABr) increased the rate of hydrolysis in the lower concen-
tration ranges. The further increase in [surfactant] decreased the values
of rate constant, thus, giving maxima like curves for kψ–[surfactant]
profile. The kψ versus [CTAOH] gave plateau like curve in which the
rate of reaction increased with the increase in surfactant and then
reached to a constant value. The anionic SDS did not influence the rate
of reaction while the non-ionic Brij-35 inhibited the rates of alkaline
hydrolysis of triflusal. The presence of micelles of CTABr, CTACl and
DTABr increased the local molalities of organic substrate and OH−
ions in the interfacial region, thereby, increasing the reaction rate.
The amounts of reactants in the micellar region were quantitatively
estimated using the pseudophase ion exchange model. The inhibitive
1
1
1
¼
þ
:
ð24Þ
k0w−kψ k0w k0w Ks ½Dnꢀ
1
1
½Dn
Thus, according to Eq. (24), a plot of k0
versus
ꢀ should give a
w−kψ
1
1
straight line with intercept at
and slope =
. The value of Ks
kw0
k0w Ks
was calculated from this plot and the values are given in Table 4.
3.5. Hydrolysis of triflusal in the presence of salt
The rate of hydrolysis of triflusal decreased with the increasing
concentrations of NaBr, (CH3)4NBr and (C2H5)4NBr in the presence of
6.0 × 10−3 mol dm−3 CTABr (Fig. 8). Similarly, the salts of NaBr, NaCl,
and CH3COONa decreased the rate of hydrolysis in the presence of
CTAOH micelles (Fig. 9). The inhibitive effect by CTABr and CTAOH
micelles may be attributed to the substitution of the reactive OH− ions
by the nonreactive ions from the Stern layer. The addition of salts alter
the polarity of the micellar surface potential and thereby, altering the
rate of reactivity. After, certain concentrations of salt the hydroxide
ions are almost completely excluded from the micellar surface by the
nonreactive anions and results into the decrease in the reaction rate.
The kψ–[CTABr] and kψ–[CTAOH] profiles for the hydrolysis of triflusal
were treated in terms of pseudophase model and the kinetic parameters
were determined using Eq. (20). Besides the partitioning of the
reactants in the different pseudophases, the added salt may increase
50
40
30
20
10
Table 4
Values of kw′ and Ks obtained from the intercept and
slope using Eq. (24) for the hydrolysis of triflusal in
Brij-35.
0
102kw′ (s−1
Ks
)
9.05
916.43
103 [Salt] (mol dm-3)
Reaction conditions: [Triflusal] = 8.0 × 10−5 mol dm−3
,
Fig. 9. Plot of kψ vs. [salt] for the alkaline hydrolysis of triflusal in the presence of different
salt (●; NaBr, ○; NaCl, ▲; CH3COONa) in CTAOH. Reaction conditions: [triflusal] =
8.0 × 10−5 mol dm−3, [NaOH] = 2.0 × 10−2 mol dm−3, Temperature = 25.0 0.1 °C.
[NaOH] = 2.0 × 10−2 mol dm−3, Temperature =
25.0 0.1 °C.