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Table 3 Influence of the concentration of added H2SO4 on the initial rate coefficients for SDS hydrolysis in 70% solution at 100 ЊC
[H2SO4] (weight%)
0
1.2
2.1
1.8
3.6
1.9
3.9
2.0
4.2
2.4
4.6
2.6
3.4
3.2
2.7
4.6
3.1
6.4
7.9
104 k0/sϪ1
Small
Table 4 Rate coefficients for SDS hydrolysis in initially neutral solution from computer simulations according to Scheme 1 of experimental [SDS]
versus time curvesa
k2/10Ϫ7 sϪ1
80 ЊC
k3/10Ϫ4 MϪ1 sϪ1
b
b
(weight%)
100 ЊC
(∆S/J KϪ1 molϪ1
)
80 ЊC
100 ЊC
(∆S/J KϪ1 molϪ1
)
1
2
5
2.3, 3.0
1.9, 2.2
21, 29 (26)c
16, 24
120 (Ϫ30)
130 (0)
150 (10)
130 (Ϫ50)
110 (Ϫ100)
140 (Ϫ3)
90 (Ϫ130)
140 (10)
3.6, 3.9
2.6, 2.9
2.9, 3.2
2.4, 3.2
2.3, 2.7
2.2, 2.6
1.5, 1.9
1.8, 2.2
1.8, 2.2
2.9, 3.2
5.7, 6.1
30, 34 (1.7)c
25, 29
120 (20)
120 (50)
110 (10)
120 (40)
100 (Ϫ20)
100 (Ϫ20)
140 (80)
120 (40)
130 (60)
110 (10)
80 (Ϫ80)
0.74, 0.80
0.055, 0.085
0.05, 0.09
0.11, 0.17
0.21, 0.29
0.30, 0.40
0.43, 0.47
0.80, 1.0
1.1, 1.2
10, 12 (10)d
0.7, 0.7 (0.8;c 0.7d)
0.6, 0.4
23, 27 (28)d
26, 30 (14;c 31d)
15, 19
10
15
20
30
40
50
60
70
1.4, 2.2 (1.8;c 2.0d)
1.3, 1.6
4.5, 4.9
15, 19 (9;c 19d)
20, 24
18, 22
5.7, 6.1
140 (10)
100 (Ϫ100)
90 (Ϫ130)
20, 24
5.5, 5.9
24, 28
5.3, 5.7 (5.5;c 5.5d)
24, 28 (12;c 28d)
a Initial 10% reaction. b Estimated uncertainties: in ∆H 20 kJ molϪ1; in ∆S 50 J KϪ1 molϪ1 c Reactions using LDS; average values from two
.
experiments. d Reactions carried out in D2O; average values from two experiments.
differences were obtained in repeat determinations of k2,
especially when the values were very low.
0.021 M perchloric acid in water at 70 ЊC for which observed
second order rate constants exceeded those for SDS by around
10%.5 However, the effect of added NaCl and LiCl on the
perchloric acid catalysed hydrolysis of SDS at higher concen-
trations (0.35 mol dmϪ3) was a rate enhancement that was
greater for Na than Li and this reversal of the effect observed at
lower [SDS] has been ascribed to an activity coefficient effect
superimposed on the ion-exchange equilibrium in which
hydrogen ions are displaced by alkali-metal cations in the
micellar Stern layer. As we saw, SDS hydrolysis catalysed by
added mineral acid shows somewhat different characteristics
from the autocatalysed process, and this can be ascribed in
part to electrolyte effects on surfactant aggregation. For the
present, suffice it to say that the change in counter-ion affects
only the acid-catalysed pathway, presumably by altering the
acidity experienced by sulfate head-groups in the SDS aggre-
gates which are expected to be quite different at 1 and 70%
SDS.
Results obtained at 80 and 100 ЊC for SDS concentrations in
the range 1 to 70 weight% are compiled in Table 4 from which it
can be seen that there are variations in values of both k2 and k3.
It may be noted that k3 values turn out to be of similar magni-
tude to values of k0/[H2SO4] taken from Table 2. The results in
Table 4 show that, for the uncatalysed process, rate coefficients
pass through a clear minimum in the concentration range 10–15
weight% SDS at both 80 and 100 ЊC. Variations in the values of
k3 are smaller and less clearly defined, but a minimum is dis-
cernible around 30 weight% SDS at 80 ЊC, and in the range 15–
20% at 100 ЊC. Values of the apparent activation enthalpies and
entropies were evaluated from the kinetic results at 80 and
100 ЊC for both the catalysed and uncatalysed reaction; the pre-
cision is necessarily not high, but, taken at face value, the results
in both series show interesting but different fluctuations as the
initial SDS concentration varies. Detailed analysis is not
warranted, but such complexity is to be expected from a
superposition of temperature effects on rate processes and
temperature effects on surfactant aggregation.
In a separate series of experiments, the influence of the reac-
tion product dodecanol on the rate coefficients derived from
simulations of the initial 10% of hydrolysis was examined.
Reaction mixtures contained an amount of dodecanol corre-
sponding to that which would have been formed after 50%
hydrolysis. It was established that dodecanol had no effect
within the experimental error on values of k2 over the whole
range of initial SDS concentrations. For k3, however, values
increased uniformly by ca. 50%.
Acid catalysis of SDS at low and high concentrations in aqueous
solution
The mode of acid catalysis by hydrogen sulfate ions in aqueous
solution was investigated at two SDS concentrations, 1 and 70
weight%, by the use of sulfate buffer solutions of varying con-
centration. In the experiments at the lower SDS concentration,
the ionic strength was maintained constant by the addition of
NaCl. In the far from ideal concentrated SDS solutions no
additional salts were added; formal ionic strengths were calcu-
lated assuming that all the salts were fully dissociated. Initial
rates of hydrolysis were measured as before; the results are in
Table 5.
It can be seen from the results that at both low and high SDS
concentrations the measured initial rates of hydrolysis increase
as the ratio [HSO4Ϫ]/[SO42Ϫ] increases and the pH decreases, but
remains essentially unchanged when [HSO4Ϫ] increases at con-
stant buffer ratio. This is the characteristic of reactions showing
specific hydrogen-ion catalysis and indicates that the hydrolysis
of dodecyl sulfate ions at both low and high concentrations in
water has a rate-limiting step involving a rapidly formed adduct
of the anion and a proton.9 Two different types of basic site for
proton attachment exist in alkyl sulfate anions, namely the
oxygen atoms bearing the formal negative charge (protonation
of which would yield the alkyl hydrogen sulfate) and the oxygen
atom directly attached to the alkyl group (protonation of which
The effect on values of k2 and k3 of replacing water in the
reaction mixtures by deuterium oxide was also investigated.
Over the whole range of SDS concentrations, the solvent
isotope effects were small, falling well within the experi-
mental uncertainty of the rate constants derived by the
D
simulation method; average values for k2H/k2 in solutions
containing 5, 10, 20 and 70% SDS at 100 ЊC were 1.0, and for
k3H/k3D 0.9.
Comparison of values of k2 and k3 with values obtained
using lithium dodecyl sulfate (LDS) at 1, 10, 20 and 70% by
weight showed that the change of counter-cation had little
effect on k2, but that k3 for SDS was roughly twice that for the
lithium salt at all concentrations (neglecting the small difference
in mol kgϪ1). These observations contrast with earlier results on
the hydrolysis of LDS (up to ca. 5% by weight) catalysed by
1492
J. Chem. Soc., Perkin Trans. 2, 2001, 1489–1495