The hydrolysis of C12 primary alkyl sulfates in concentrated aqueous solutions. Part 1. General features, kinetic form and mode of catalysis in sodium dodecyl sulfate hydrolysis

Article abstract of DOI:10.1039/b102957f

As part of an investigation into the observed rapid hydrolysis of sodium primary alkyl sulfates in commercial concentrated aqueous mixtures (typically 70% by weight of surfactant) at 80 °C, the rate of hydrolysis of sodium dodecyl sulfate (SDS) in water has been followed acidimetrically over a wide range of initial SDS concentrations in aqueous buffers, in the presence of added sulfuric acid (0.20 mol kg^-1) and also in unbuffered, initially neutral solution. First order rate coefficients derived from the initial rates of sulfuric acid-catalysed reactions showed unexpected, non-monotonic variations with increasing initial [SDS] at constant [H₂SO₄] and with [H₂SO₄] at 70% SDS. Reactions in initially neutral solutions were found to have an autocatalytic form arising from the existence of both an uncatalysed and an acid catalysed pathway from reactants to products. Both pathways are characterised by rate coefficients (derived by computer simulation of the initial phase of the hydrolysis) that vary with the initial [SDS]; those for the acid-catalysed pathway show similar but less dramatic variation than observed in reactions in the presence of sulfuric acid. Possible reasons for the difference in behaviour are discussed. The autocatalysis observed in initially neutral solutions is shown to arise from the production of hydrogen sulfate ions during the hydrolysis, but experiments in buffer solutions at both high and low SDS concentrations show the characteristics of specific hydrogen ion catalysis. Solvent kinetic deuterium isotope effects on both pathways are, however, small. Examination of the dodecanol produced by SDS hydrolysis at low and high initial concentrations in¹⁸O-enriched water showed no incorporation of the label, signifying exclusive S-O cleavage in the acid-catalysed pathway. It is argued that the results, taken in conjunction with literature data, are consistent with an S_N2 displacement of sulfate ion by water in the uncatalysed hydrolysis pathway. While, for the hydrogen ion catalysed pathway, a previously suggested unimolecular cleavage of SO₃ from dodecyl hydrogen sulfate, with concerted intramolecular proton transfer, appears more consistent with the observations, proton transfer concerted with direct transfer of SO₃ to a preassociated water molecule is a plausible alternative.

Full text of DOI:10.1039/b102957f

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The hydrolysis of C12 primary alkyl sulfates in concentrated
aqueous solutions. Part 1. General features, kinetic form and
mode of catalysis in sodium dodecyl sulfate hydrolysis†
2
Donald Bethell,*a Roger E. Fessey,a Ernest Namwindwaa and David W. Robertsb
a Department of Chemistry, University of Liverpool, Liverpool, UK L69 3BX
b Unilever Research, Port Sunlight Laboratory, Bebington, Wirral, UK CH63 3JW
Received (in Cambridge, UK) 2nd April 2001, Accepted 13th June 2001
First published as an Advance Article on the web 20th July 2001
As part of an investigation into the observed rapid hydrolysis of sodium primary alkyl sulfates in commercial
concentrated aqueous mixtures (typically 70% by weight of surfactant) at 80 ЊC, the rate of hydrolysis of sodium
dodecyl sulfate (SDS) in water has been followed acidimetrically over a wide range of initial SDS concentrations
in aqueous buffers, in the presence of added sulfuric acid (0.20 mol kgϪ1) and also in unbuffered, initially neutral
solution. First order rate coefficients derived from the initial rates of sulfuric acid-catalysed reactions showed
unexpected, non-monotonic variations with increasing initial [SDS] at constant [H2SO4] and with [H2SO4] at 70%
SDS. Reactions in initially neutral solutions were found to have an autocatalytic form arising from the existence
of both an uncatalysed and an acid catalysed pathway from reactants to products. Both pathways are characterised
by rate coefficients (derived by computer simulation of the initial phase of the hydrolysis) that vary with the initial
[SDS]; those for the acid-catalysed pathway show similar but less dramatic variation than observed in reactions
in the presence of sulfuric acid. Possible reasons for the difference in behaviour are discussed. The autocatalysis
observed in initially neutral solutions is shown to arise from the production of hydrogen sulfate ions during the
hydrolysis, but experiments in buffer solutions at both high and low SDS concentrations show the characteristics
of specific hydrogen ion catalysis. Solvent kinetic deuterium isotope effects on both pathways are, however, small.
Examination of the dodecanol produced by SDS hydrolysis at low and high initial concentrations in 18O-enriched
water showed no incorporation of the label, signifying exclusive S–O cleavage in the acid-catalysed pathway. It is
argued that the results, taken in conjunction with literature data, are consistent with an SN2 displacement of sulfate
ion by water in the uncatalysed hydrolysis pathway. While, for the hydrogen ion catalysed pathway, a previously
suggested unimolecular cleavage of SO3 from dodecyl hydrogen sulfate, with concerted intramolecular proton
transfer, appears more consistent with the observations, proton transfer concerted with direct transfer of SO3
to a preassociated water molecule is a plausible alternative.
examine the possibility that the high reactivity might be associ-
Introduction
ated with the state of aggregation of the surfactant at high
concentration. Our starting point was the hydrolysis of sodium
dodecyl sulfate (SDS), a material that features commonly in
academic studies of surfactant behaviour and which is found
to the extent of 40 to 70% by weight in commercial PAS
preparations.
Long chain primary alkyl sulfuric acids, usually as their sodium
salts, are important commercial surfactants. They are normally
produced commercially by treatment of mixtures of naturally
produced fatty alcohols with sulfur trioxide, followed by
neutralisation of the sulfated product with sodium hydroxide.
From this process is obtained a neutral mixture of sodium
primary alkyl sulfates (PAS), containing around 70% by weight
of PAS with chain lengths in the range C12 to C14, in the form of
a viscous paste at ambient temperatures. This paste becomes a
free-flowing, and therefore more easily handled, liquid at higher
temperatures, typically around 80 ЊC. Prolonged storage of
the material at such elevated temperature, however, leads to
complete hydrolysis according to the formal reaction (1).
Although there is a substantial literature on the hydrolysis of
aryl sulfates as a consequence of their biological significance,
there are relatively few reports in the open literature on the
hydrolysis of primary alkyl sulfates. Much of what has
appeared has been concerned with the differences in the kinetics
of hydrolysis of SDS when conducted below and just above the
critical micelle concentration (c.m.c = 8.6 mM at 40 ЊC1). Thus
Kurz,2 Motsavage and Kostenbauder3 and Nogami et al.4
demonstrated from initial rate measurements the occurrence of
an uncatalysed and a proton-catalysed hydrolysis pathway.
They showed that micellisation of SDS, for example, led to a
30- to 40-fold increase in rate constants for the acid-catalysed
hydrolysis, but that there was a negligible effect on the
uncatalysed reaction. Mixed micelles of SDS and non-ionic
additives, particularly dodecanol, showed substantial rate
enhancements,3 further complicating the kinetic form of SDS
hydrolysis. Although in these early investigations the term
specific acid catalysis was used in connection with the
hydrolysis, the precise form of the catalysis was not explicitly
studied using buffer solutions of varying concentration,
RCH2OSO3ϪNaϩ ϩ H2O → RCH2OH ϩ HSO4ϪNaϩ (1)
Notwithstanding variations in the alkyl-chain composition of
different batches of PAS, complete hydrolysis usually occurs in
about 12 hours at 80 ЊC. The kinetic and mechanistic investi-
gation described in this and an accompanying paper was under-
taken to throw more light on this situation, particularly to
† Dedicated to the memory of Lennart Eberson in gratitude for his
friendship and in recognition of his many contributions to Physical
Organic Chemistry.
DOI: 10.1039/b102957f
J. Chem. Soc., Perkin Trans. 2, 2001, 1489–1495
This journal is © The Royal Society of Chemistry 2001
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Table 1 Relationship of instantaneous rate of SDS hydrolysis and
sodium hydrogen sulfate concentration at 80 ЊC
Exp.
initial
SDS
concentration Conversion Instantaneous [NaHSO4]/ rate/
(weight%)
(%)
rate/M sϪ1
M
M sϪ1
5
4
8
22
40
4
2.7
4.0
8.1
11
8.0
14
20
37
4.5
7.0
14
0.00694a
0.0139a
0.0381a
0.0694a
0.0142b
0.0282b
0.0741b
0.1428b
0.10c
2.5, 2.9
4.3, 4.7
8.0, 9.8
11, 12
8.1, 9.5
12, 16
18, 24
34, 40
4.0, 5.0
6.2, 7.8
12, 16
10
70
8
21
41
4
8
21
41
0.20c
0.50c
17
1.0c
15, 19
a Constant ionic strength (NaCl) 0.24 M. b Ionic strength (NaCl)
0.49 M. c Ionic strength (NaCl) 3.43 M.
Fig. 1 pH-Dependence of the initial first order rate coefficients for
hydrolysis of SDS (0.1 M = 2.8 weight%) in aqueous sulfate buffers at
100 ЊC. Ionic strength 0.4 M.
reliance being placed on the approximate proportionality of
observed rate constants and [H3Oϩ].3 Subsequently it was
reported that the proportionality was not exact4a and that a
‘saturation’ effect was evident; this was interpreted5 in terms of
the pseudophase ion-exchange model of micellar behaviour.6
The effect on rate constants for hydrolysis of increasing the
surfactant concentration substantially beyond the c.m.c. has
received relatively little attention. Garnett et al.5 reported a
ca. 30% linear decrease in rate constants for perchloric acid
catalysed hydrolysis of SDS on increasing the concentration
from 0.02 to 0.6 mol dmϪ3 (i.e. up to ca. 18% by weight), an
effect attributed to dilution of the hydrogen-ion concentration
in the Stern layer of the micelles, offset by ionic strength effects.
The present investigation addresses the following aspects of
SDS hydrolysis: the effects on the rate of hydrolysis of increas-
ing the reactant/surfactant concentration from a little above the
c.m.c. to a value comparable with the PAS concentration in
commercial pastes, the nature of catalysis involved in the
hydrolysis, and possible changes in the reaction mechanisms
over the concentration range studied. In a subsequent paper we
shall discuss certain properties of concentrated SDS solutions
that throw light on the origin of the kinetic behaviour described
here, and also compare the behaviour of SDS with other C12
PAS.
Fig. 2 Variation of SDS concentration with time in the hydrolysis
of SDS at 100 ЊC in initially neutral aqueous solution: (ᮀ) curve A, 1%
SDS; (ꢀ) curve B, 10% SDS.
solutions, initially at pH 7, one containing 1% by weight of SDS
(0.035 mol kgϪ1) and the other 10% SDS (0.347 mol kgϪ1) with
no other additives. In both instances the course of the reaction
is sigmoidal, characterized by an initial slow phase followed by
a rapid acceleration and a final slowing down as the hydrolysis
approaches completion. It should be noted that the time to
complete hydrolysis is very much shorter in the more concen-
trated SDS solution. These results are entirely consistent with
earlier views of SDS hydrolysis as involving two pathways, one
a slow, uncatalysed hydrolysis and the other an acid-catalysed
process. The hydrolysis shows the characteristic autocatalytic
form, the catalysis presumably arising from the production of
hydrogen sulfate ions according to reaction (1).
This interpretation was confirmed by measuring initial rates
of SDS hydrolysis at 80 ЊC in 5, 10 and 70% SDS solutions to
which had been added concentrations of hydrogen sulfate ion
corresponding to ca. 5, 10, 20 and 40% conversion according to
reaction (1). The experimental results are compared in Table 1
with instantaneous rates of hydrolysis of SDS in analogous
reactions when no sodium hydrogen sulfate had been added
initially and determined from the conversion/time curves at the
appropriate time by differentiation with respect to time of the
second order binomial expression that fitted the experimental
[SDS] vs. time curve. The agreement is well within the limits of
Results and discussion
Preliminary kinetic experiments and hydrolysis in the presence of
added sulfuric acid
In all cases, initial rates of hydrolysis at 80–100 ЊC were fol-
lowed by titration at 25 ЊC of aliquots of reaction mixtures to
pH 7 using an autotitrator, yielding the total acid (H3Oϩ and
HSO4Ϫ) produced and hence the residual SDS concentration.
Preliminary experiments were carried out on SDS (0.025 M)
in aqueous sulfate buffer solutions with measured initial
pH-values, determined at 25 ЊC, in the range from 3.8 to 1.0 and
at a constant ionic strength adjusted to 0.4 M by the addition of
sodium chloride. From the results for hydrolysis at 100 ЊC,
initial first order rate coefficients were evaluated from plots of
ln [SDS] vs. time and these are shown in Fig. 1 as a function of
pH. Clearly, at the higher pH values, down to ca. 2.7, the initial
rate coefficient is invariant with changing pH, but in more
acidic solutions values increase; a line of unit slope has been
drawn in Fig. 1 and fits the results satisfactorily. Fig. 2 shows the
time-variation of the concentration of SDS in reactions in two
1490
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Table 2 Initial first order rate coefficientsa for SDS hydrolysis at 100 ЊC in the presence of added H2SO4 (0.2 mol kgϪ1
)
SDS conc.
(weight%)
∆H/kJ molϪ1
SDS conc.
(weight%)
∆H/kJ molϪ1
b
b
k0/10Ϫ4 sϪ1
(∆S/J KϪ1 molϪ1
)
k0/10Ϫ4 sϪ1
(∆S/J KϪ1 molϪ1
)
1
2
5
10
12
15
20
4.0, 3.8 (6.5)c
2.1, 1.7 (5.5)
4.4, 4.7 (5.1)
4.5, 4.6 (5.7)
4.5, 4.7
112 (Ϫ15)
114 (Ϫ16)
108 (Ϫ10)
142 (ϩ60)
147 (ϩ70)
151 (ϩ77)
129 (ϩ40)
25
30
40
50
60
65
70
2.7, 2.7
119 (ϩ5)
115 (Ϫ5)
129 (ϩ38)
120 (ϩ20)
112 (Ϫ20)
116 (Ϫ10)
120 (Ϫ5)
2.9, 3.2 (4.5)
3.0, 3.1 (4.1)
4.0, 4.3 (4.5)
4.3, 4.2 (5.3)
4.2, 4.4
4.0, 4.3 (3.5)
3.0, 3.0 (3.5)
4.2, 4.2 (5.3)
a Initial 7% reaction. b Average estimated uncertainties: in ∆H 5 kJ molϪ1; in ∆S 12 J KϪ1 molϪ1 c Average values of k1 × [H2SO4] at the same SDS
.
concentration are in parentheses.
manner that is also not monotonic. Up to around 2% H2SO4,
rate coefficients increase linearly with the concentration of
added acid, but a rate maximum is reached at around 2.5%
sulfuric acid, k0 values decreasing to a minimum at ca. 4
weight% H2SO4 whereafter the values increase again. This
result, unforeseen on the basis of previously described acidity
dependences in dilute SDS solutions, is presumably a con-
sequence of electrolyte effects on SDS aggregation,8 but it
prompted an investigation of the mode of acid catalysis of SDS
hydrolysis in aqueous solutions from 5 to 70% SDS.
Hydrolysis in unbuffered, initially neutral solution
The unusual variations in k0 values with [SDS] observed in
sulfuric acid-catalysed reactions discouraged attempts to
analyse curves such as those in Fig. 2 over the whole course of
the reaction. Instead recourse was made to kinetics simulation
based on the program Kinetics (Chemistry Courseware
Consortium, University of Liverpool) applied to data obtained
over the first 10% of hydrolysis. The program requires the input
of the initial concentrations of all species in the reaction
scheme and the simplified version in Scheme 1 was adopted
Fig. 3 Variation with SDS molality of observed first order rate
coefficients at 100 ЊC for the hydrolysis of SDS in aqueous solutions
containing 2% by weight of sulfuric acid.
accuracy of the two data sets, confirming that hydrogen sulfate
ions are responsible for the catalysis. The presence of the prod-
uct ions leads to an increased acidity in the reaction mixture,
and we show later that the autocatalysis arises solely from the
increasing hydrogen ion concentration and does not involve the
hydrogen sulfate ions per se, that is, as general acids.
Scheme 1
The sigmoidal form of [SDS] versus time curves was absent
when the hydrolysis was carried out in the presence of
added sulfuric acid (2% by weight = 0.204 mol kgϪ1); only the
acid-catalysed pathway was observable. Initial first order rate
coefficients (k0) were determined in duplicate runs from plots of
ln [SDS] against time over the early stages of reaction (≤7%
hydrolysis) which were linear (r ≥ 0.99); values were determined
at temperatures 80, 85, 90 and 100 ЊC over the range of SDS
concentrations from 1 to 70 weight% (0.035 to 2.43 mol kgϪ1).
Reproducibility of rate coefficients was usually 5%. Acti-
vation parameters, derived using the Eyring equation from
excellent linear plots of ln k0 vs. T Ϫ1, and rate coefficients at
100 ЊC are in Table 2. The values of k0 vary in an unexpected
way with [SDS] (see Fig. 3); the initial decrease is similar to that
previously reported in perchloric acid-catalysed hydrolyses,
but the most remarkable features are minima at 2 and ca. 25%
SDS separated by a prominent maximum at 12% SDS. This
behaviour would seem to be a consequence of the com-
plex nature of the surfactant solutions and the nature of the
aggregates within which hydrolysis takes place.
throughout. Note that the concentration of water was not
explicitly included, so that the derived rate constants contain
where appropriate the (constant) concentration of water (see
below). For the purposes of the simulation, a value of pKa for
hydrogen sulfate ion of 1.78, experimentally determined at
25 ЊC,7 was assumed to apply under all reaction conditions. We
justify this on the grounds that we could not directly measure
pKa under the range of conditions of the SDS hydrolysis, but
we recognise that any variation in the pKavalue for hydrogen
sulfate ion as the [SDS] changes will be reflected in the derived
rate coefficient for the acid-catalysed reaction. We shall return
to this matter later. Since the simulation program requires the
pKa-value to be introduced in the form of rate constants for the
forward and reverse reactions, kϪ1 was arbitrarily fixed at
1 × 1010 sϪ1 with k1 then 1.66 × 108 mol kgϪ1 Ϫ1. Changing
s
these values by an order of magnitude while maintaining the
ratio constant did not influence the outcome of the simulation.
Experimental data were simulated by adjustment of the two
remaining parameters in Scheme 1, the rate coefficients for the
uncatalysed (k2) and catalysed (k3) processes. All hydrolyses
were run in duplicate and separately simulated; values of the
derived rate coefficients generally agreed within 5–10% of
the mean for k3 for which changes of this magnitude could
readily be detected by the simulation procedure. Rather larger
In a further set of experiments, the effect of changing the
concentration of sulfuric acid initially present in reaction mix-
tures containing initially 70% by weight of SDS was investi-
gated using the same procedures. Values of k0 so obtained are
in Table 3, from which it can be seen that they increase in a
J. Chem. Soc., Perkin Trans. 2, 2001, 1489–1495
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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