Reactivity of typical solvolytic reactions in SDS and TTABr water-in-oil microemulsions

Article abstract of DOI:10.1021/jp970460n

The kinetics of the solvolysis reactions of diphenylmethyl chloride, 4-nitrophenyl chloroformate, benzoyl chloride, anisoyl chloride, and bis(4-nitrophenyl)carbonate were studied in isooctane/SDS/1-hexanol/water and isooctane/TTABr/1-hexanol/water microemulsions with a constant [1-hexanol]/[surfactant] ratio of 5. The results were interpreted by means of a pseudophase model for (isooctane+hexanol)/(surfactant+hexanol)/water systems, the distribution of hexanol between isooctane and surfactant being calculated from previously published data. The intrinsic rate constant for the reaction in the interfacial pseudophase varied with the water content of the microemulsion droplets in a way that depended on both the nature of the surfactant and the mechanism of the reaction.

Full text of DOI:10.1021/jp970460n

5514
J. Phys. Chem. B 1997, 101, 5514-5520
Reactivity of Typical Solvolytic Reactions in SDS and TTABr Water-in-Oil Microemulsions
L. Garc´ıa-R´ıo, J. R. Leis,* and C. Reigosa
Departamento de Qu´ımica F´ısica, Facultad de Qu´ımica, UniVersidad de Santiago,
15706 Santiago de Compostela, Spain
ReceiVed: February 5, 1997X
The kinetics of the solvolysis reactions of diphenylmethyl chloride, 4-nitrophenyl chloroformate, benzoyl
chloride, anisoyl chloride, and bis(4-nitrophenyl)carbonate were studied in isooctane/SDS/1-hexanol/water
and isooctane/TTABr/1-hexanol/water microemulsions with a constant [1-hexanol]/[surfactant] ratio of 5.
The results were interpreted by means of a pseudophase model for (isooctane + hexanol)/(surfactant + hexanol)/
water systems, the distribution of hexanol between isooctane and surfactant being calculated from previously
published data. The intrinsic rate constant for the reaction in the interfacial pseudophase varied with the
water content of the microemulsion droplets in a way that depended on both the nature of the surfactant and
the mechanism of the reaction.
Introduction
SCHEME 1
Three-component water-in-oil (w/o) microemulsions are
transparent, thermodynamically stable, isotropic systems con-
sisting of reverse micelles (water droplets surrounded by a
monolayer of surfactant) dispersed in a nonpolar bulk solvent.1,2
The most intensively studied three-component w/o microemul-
sions have been systems of the form water/AOT/oil, where AOT
is sodium bis(2-ethylhexyl)sulfosuccinate.3 An important prop-
erty of these systems is the highly structured nature of the water
at the interface when the [H2O]/[surfactant] ratio W is small.4
For W e 6, all water molecules are immobilized by strong
interactions with the ion pair formed by the polar head of AOT
and its counterion. With W ≈ 11 the ion pairs have dissociated,
but all water is still engaged in solvation of the individual anions
and cations. Only for W > 12 are there water molecules with
properties approximating those of the molecules of ordinary bulk
water. These alterations in the properties of water can have a
significant influence on the kinetics of reactions taking place
at the micellar interface.
If a surfactant has a small packing parameter,5 as is the case
of sodium dodecyl sulphate (SDS)6,7 and alkyl ammonium
surfactants,8 it may only form a stable microemulsion in the
presence of a fourth component, the “cosurfactant” (usually an
alcohol7 or a carboxylic acid9). Spectroscopic studies of the
cosurfactant action of alcohols in oil-in-water microemulsions
and direct micelles have shown that the alcohol molecules
reduce the surface charge density of the micelle by intercalating
among the interfacial surfactant molecules and increasing the
separation between their ionic headgroups.10 Alcohol cosur-
factants also influence the kinetics of reactions carried out in
o/w microemulsions and direct micelles both by changing the
polarity of the intramicellar medium and by diluting the reagents
therein.11,12 Four-component w/o microemulsions have been
investigated as regards the influence of alcoholic cosurfactants
on the fluidity of the interface and droplet-droplet interaction,13
but the effects of such cosurfactants on the properties of the
micellar water and on the kinetics of reactions carried out in
these media have not yet been clarified.
properties of the reaction medium has in the past been used to
study the effects of additives on the structure of water14,15 and
whose behavior in water/AOT/isooctane microemulsions we
have recently reported.16 Since all the substrates used are very
poorly soluble in water, it may be assumed that in microemul-
sions they are present only in the nonpolar and interfacial regions
and that the solvolysis reactions take place only in the latter.
This both simplifies analysis of kinetic data and allows
interpretation of results directly in terms of the structure of the
interface and interfacial water. The microemulsion systems used
were water/SDS/1-hexanol/isooctane, which allowed evaluation
of the effects of 1-hexanol by comparison with the earlier
results16 for water/AOT/isooctane (both AOT and SDS being
anionic surfactants), and water/TTABr/1-hexanol/isooctane (TTA-
Br ) tetradecyltrimethylammonium bromide), which allowed
evaluation of the effects of surfactant charge17 by comparison
with the results for water/SDS/1-hexanol/isooctane.
In this work we studied the behavior, in four-component w/o
microemulsions, of a series of solvolytic reactions (the solvoly-
ses of diphenylmethyl chloride, 4-nitrophenyl chloroformate,
benzoyl chloride, p-anisoyl chloride, and bis(4-nitrophenyl)
carbonate; see Scheme 1) whose sensitivity to the physical
Experimental Section
Sodium dodecyl sulfate (SDS) and tetradecyltrimethylam-
monium bromide (TTABr) were supplied by Sigma and used
without further purification. 1-Hexanol and isooctane were
X Abstract published in AdVance ACS Abstracts, June 1, 1997.
S1089-5647(97)00460-4 CCC: $14.00 © 1997 American Chemical Society

Solvolytic Reactions
J. Phys. Chem. B, Vol. 101, No. 28, 1997 5515
Aldrich products of the maximum commercially available purity.
Deuterated water (99.7% D) was supplied by the Spanish
Nuclear Energy Board. Diphenylmethyl chloride (DPhMeCl),
4-nitrophenyl chloroformate (NPhCl), benzoyl chloride (BzCl)
and p-anisoyl chloride (AnCl) (all from Aldrich), and bis(4-
nitrophenyl) carbonate (BNPhC) (from Sigma) were of the
maximum commercially available purity. Microemulsions of
the desired compositions were prepared from stock water/
surfactant/1-hexanol/isooctane microemulsions by addition of
appropriate amounts of isooctane and/or water.
The solvolysis reactions were carried out in a Spectronic 3000
diode array or Kontron-Uvikon spectrophotometers fitted with
thermostated cell holders (all experiments were carried out at
25 °C) and were followed by monitoring the absorbance at the
maximum of the spectrum of the aromatic products (238 nm
for DPhMeCl, 340 nm for NPhCl, 288 nm for BzCl, 290 nm
for AnCl, and 315 nm for BNPhC). The integrated first-order
expression
At ) A0 + (A∞ - A0)exp(-kot)
(1)
(where A0, At, and A∞ are the absorbances at times 0, t and
infinity, respectively, and ko is the observed first-order constant)
was fitted to the absorbance-time data by nonlinear regression
to avoid the problems associated with the linearization of
exponential data.18
Figure 1. Dependence of [hexi]/[TTABr] on [TTABr] in isooctane/
TTABr/1-hexanol/water w/o microemulsions at 25 °C for [hex]/
[TTABr] ) 5: (O) W ) 3; (b) W ) 5; (0) W ) 10; (9) W ) 20; (4)
W ) 30; (2) W ) 40.
1H NMR spectra were recorded in a Brucker AMX300
apparatus operating at 300.1 MHz with a coaxial tube of DMSO-
d6 for locking.
Results and Discussion
Incorporation of Alcohol in the Interface. Microemulsions
and other types of surfactant aggregates such as monolayers,
micelles, and vesicles all have an interfacial region that separates
the oil and water regions and is composed of surfactant
headgroups, associated counterions and co-ions, and any added
polar additives. Determining the compositions of aggregate
interfaces is an active area of research because interfacial
compositions, not stoichiometric concentrations of components,
reflect the balance of forces controlling aggregate structure and
stability.19
To simplify analysis of the kinetic results, it was assumed
that the concentration of the very poorly water-soluble 1-hexanol
in the aqueous pseudophase was negligible, and the microemul-
sion was accordingly treated as an (isooctane + hexanol)/
(surfactant + hexanol)/water pseudophase system, the distri-
bution of hexanol between isooctane and surfactant being
calculated using the partition coefficient
Figure 2. Dependence of [hexi]/[SDS] on [SDS] in isooctane/SDS/
1-hexanol/water w/o microemulsions at 25 °C for [hex]/[SDS] ) 5:
(b) W ) 7; (O) W ) 10; (9) W ) 15; (0) W ) 20; (2) W ) 25.
hexanol) was taken from published data for direct micelles21 as
K1 ) 2459 for SDS systems and K1 ) 550 for TTABr systems;
these values of Kw and K1 afford K2 values of K2 ) 145 for
o
SDS systems and K2 ) 32.4 for TTABr systems. Figures 1
and 2 show how the mole ratio [hexi]/[surfactant] varied with
[surfactant] in TTABr and SDS systems, respectively. The fact
that the interfacial hexanol content was greater in SDS systems
than in TTABr systems is in keeping with a previous report
that SDS is more reluctant to form inverse micelles than
TTABr.6d
K2 ) ([hexi][isooctane])/([hexo][surfactant])
(2)
where [hexi] and [hexo] are the bulk concentrations of interfacial
and oil-phase hexanol, respectively. K2 was obtained from the
expression
Kwo ) K2/K1
(3)
Analysis of Kinetic Data. For each substrate, all kinetic
experiments were performed using the same concentration of
substrate, which was low enough not to perturb micelle structure.
For each of various values of W, a series of experiments were
carried out in which the surfactant concentration was varied
while the [hex]/[surfactant] ratio was kept equal to 5.
Because of their poor solubility in water, it was hypothesized
that all the substrates, such as 1-hexanol, would be distributed
exclusively between the isooctane pseudophase and the interface
and that the reaction with water would accordingly take place
only in the latter region (Scheme 2).22,23
where Kw is the partition coefficient for the distribution of
o
1-hexanol in a two-phase water/isooctane system (estimated as
Kw ) 0.059 on the grounds that it must be close to the
o
coefficients for octane/water, decane/water, and dodecane/
water,20 and
K1 ) ([hexi][H2O])/([hexw][surfactant])
(4)
(where [hexw] is the bulk concentrations of aqueous-phase

5516 J. Phys. Chem. B, Vol. 101, No. 28, 1997
Garc´ıa-R´ıo et al.
Figure 4. Data of Figure 3 linearized as plots of 1/ko against Z* in
accordance with eqs 6 and 7: (4) W ) 2.5; (9) W ) 20; (0) W ) 15;
(b) W ) 10; (O) W ) 7.
Figure 3. Influence of [SDS] on the first-order pseudoconstant ko for
the solvolysis of benzoyl chloride in isooctane/SDS/1-hexanol/water
w/o microemulsions with [hex]/[SDS] ) 5 and various values of W )
[H2O]/[SDS] at T ) 25 °C: (b) W ) 7; (O) W ) 10; (9) W ) 15; (0)
W ) 20; (2) W ) 25.
SCHEME 2
i
Figure 5. Influence of Z* ) ([isooctane] + [hexo])/[TTABr] on the
reciprocal of the first-order pseudoconstant ko for the solvolysis of
benzoyl chloride in isooctane/TTABr/1-hexanol/water w/o microemul-
sions with [hex]/[TTABr] ) 5 and various values of W ) [H2O]/
[TTABr] at T ) 25 °: (O) W ) 3; (0) W ) 5; (9) W ) 15; (b) W )
20.
According to Stilbs,24 the partition coefficient of substrate
was defined as
Ksub ) {[subi]([isooctane] + [hexo])/([subo][surfactant])
(5)
TABLE 1: Partition Coefficients KBzCl and Intrinsic Solvo-
lytic Rate Constants k for Benzoyl Chloride in Isooctane/
SDS/1-Hexanol/Water and Isooctane/TTABr/1-Hexanol/
Water w/o Microemulsions at 25 °C, Together with
Published Values for k in Isooctane/AOT/Water Systemsa
(An analogous definition has previously been used successfully
in a study of direct micellar systems12.) These hypotheses lead
to the following expression for the first-order pseudoconstant
ko:
AOTb
k/s-1
SDS/1-Hexanol
k/s-1
KBzCl
TTABr/1-Hexanol
ko ) kKsub/(Ksub + Z*)
(6)
W
k/s-1
KBzCl
9.92 × 10-4
8.29 × 10-4
9.03 × 10-4
1.94 × 10-3
1.96 × 10-3
1.39 × 10-3
1.17 × 10-3
1.65 × 10-3
1.56 × 10-3
1.85 × 10-3
2.36 × 10-3
8.2
6.0
8.4
11.4
6.5
8.3
where k is the intrinsic rate constant for the reaction in the
interfacial region and
3
4
5
7
10
8.27 × 10-4 3.10 × 10-4
9.64 × 10-4 3.73 × 10-4
4.9
6.9
4.7
6.5
6.8
Z* ) ([isooctane] + [hexo])/[surfactant]
(7)
15 =2.8 × 10-3
20 =3.3 × 10-3
25 =4.0 × 10-3
7.76 × 10-4
9.00 × 10-4
1.10 × 10-3
is calculable using eq 2. For each substrate, the adequacy of
the above simplifying hypotheses, and of the pseudophase model
employed, was tested by verifying whether, in accordance with
eq 6, 1/ko depended linearly on Z* and whether the values of
Ksub obtained from these plots were independent of W as
required by the pseudophase model.
Solvolysis of Benzoyl Chloride. In the experiments with
BzCl in SDS microemulsions, the SDS concentration was varied
between 0.30 and 0.53 M for each of a number of W values
ranging from 7 to 25 (it was not possible to use lower W values
because of the reluctance of SDS to form stable reverse micelles
without an appreciable quantity of water).6d Figure 3 shows
that the reaction rate increased significantly with SDS concen-
tration and with W, and Figure 4 shows that plots of 1/ko against
Z* were linear as required by eq 6. In the experiments in
7.7
6.6
a [1-hexanol]/[surfactant] ) 5. b J. Phys. Chem. 1995, 99, 12318.
TTABr microemulsions, the TTABr concentration was varied
between 0.17 and 0.50 M for each of a number of W values
ranging from 3 to 25; Figure 5 shows the linearity of plots of
1/ko against Z*. For both surfactants, Table 1 lists the values
of k and KBzCl obtained from the 1/ko vs Z* plots (in both cases,
the W-independence of KBzCl supports the validity of the
pseudophase model used), together with values of k obtained
previously16 under similar conditions in water/AOT/isooctane
microemulsions.
In water, the solvolysis of BzCl takes place via the formation
of an acylium ion intermediate (Scheme 3),25 but in mixtures

Solvolytic Reactions
J. Phys. Chem. B, Vol. 101, No. 28, 1997 5517
SCHEME 3
of water with less polar solvents there is evidence that the
reaction is a concerted process, albeit one in which cleavage of
the C-Cl bond has progressed further than formation of the
C-OH bond in the transition state.26 In view of this, the W
dependence of k in water/AOT/isooctane microemulsions has
been explained16 in terms of the effects of W on both the polarity
of the interfacial medium and the nucleophilicity of interfacial
water; the observed fall in k with W at W values greater than
10 is attributed to the decreasing availability of water for
solvation of the leaving group, while the W independence of k
at W values less than 10 is attributed to the continuing fall in
interfacial polarity being offset by an increase in the nucleo-
philicity of the remaining water molecules, which are increas-
ingly strongly bound to the anionic SDS headgroups.
Figure 6. Influence of Z* ) ([isooctane] + [hexo])/[TTABr] on the
reciprocal of the first-order pseudoconstant ko for the solvolysis of
anisoyl chloride in isooctane/TTABr/1-hexanol/water w/o microemul-
sions with [hex]/[TTABr] ) 5 and various values of W ) [H2O]/
[TTABr] at T ) 25 °C: (b) W ) 3; (O) W ) 5; (9) W ) 7; (0) W )
12; (2) W ) 25.
In the SDS system with W ) 25 used in this work, k was
about 80 times smaller than the rate constant observed in water,
TABLE 2: Partition Coefficients KAnCl and Intrinsic Solvo-
lytic Rate Constants k for Anisoyl Chloride in Isooctane/
SDS/1-Hexanol/Water and Isooctane/TTABr/1-Hexanol/
Water w/o Microemulsions at 25 °C, Together with
Published Values for k in Isooctane/AOT/Water Systemsa
8.6 × 10-2 s-1 27
, and it decreased further with W. As in AOT
systems, this decrease can be attributed to an increasing
proportion of droplet water content being committed to solvation
of the surfactant headgroups and hence being unavailable for
solvation of the chloride ion. The fact that the k-W curve did
not flatten for SDS systems as it did in AOT systems seems
likely to be due simply to the impossibility of using SDS systems
with low enough W values. The fact that the values of k are
smaller in SDS than in AOT seems unlikely to be due to any
difference between SDS and AOT headgroups as to the degree
to which they stabilize the transition state. Rather, it is
attributable to the hexanol in the interface of the SDS systems
being committed to solvation of headgroups and sodium ions,
with the result that interfacial polarity is less in SDS than in
AOT systems.
AOTb
k/s-1
SDS/1-Hexanol
k/s-1
KAnCl
TTABr/1-Hexanol
W
k/s-1
KAnCl k+/k-
3
4
5
7
10
7.18 × 10-4
1.31 × 10-3
2.64 × 10-3
1.71 × 10-3
9.9
3.83 × 10-3
8.5
7.35 × 10-3 2.28 × 10-3 22.4 5.66 × 10-3 14.8 2.48
1.53 × 10-2 5.74 × 10-3 19.6 1.07 × 10-2 12.0 1.86
12 =2.0 × 10-2
1.63 × 10-2 12.4
15 =3.0 × 10-2 1.65 × 10-2 12.2 2.16 × 10-2 13.2 1.31
17 =3.7 × 10-2
2.39 × 10-2 12.3
20 =4.5 × 10-2 2.23 × 10-2 24.9 3.32 × 10-2 10.8 1.51
25 =7.0 × 10-2 3.53 × 10-2 12.4 4.24 × 10-2 12.3 1.20
In TTABr systems, k remained virtually constant as W
decreased. This may be explained as due to the k-reducing
effect of decreasing interfacial polarity being offset by the
catalytic effect of the alkylammonium salt,25 which may be
attributed to the structure of water being enhanced by the alkyl
groups.15,28 Since decreasing W means increasing TTABr
concentration with respect to the aqueous phase, the catalytic
effect of TTABr may be expected to increase with falling W.
The value of k in all these systems was about 40 times less
than in water and about the same as in AOT systems with W )
15.
a [1-hexanol]/[surfactant] ) 5. b J. Phys. Chem. 1995, 99, 12318.
Although the presence of 0.9 M Me4NBr halves the rate of
solvolysis of AnCl in 90:10 trifluoroethanol/water mixtures,25
the reaction was faster in TTABr systems than in SDS systems.
This may be attributed to the exit of the leaving group, Cl-,
being more favored by the presence of cationic TTABr
headgroups than by that of anionic SDS headgroups. By
contrast, Bunton et al.17 found that in direct micelles the ratio
k+/k- between the reaction rates in systems with cationic and
anionic surfactants was only 0.1. This difference between the
behaviors observed in w/o microemulsions and direct micelles
may be attributed to a difference in the relative ease of
stabilization of the acylium ion and the leaving group. In direct
micelles, in which abundant water is available, solvation of the
leaving group is fast and easy and the reaction is faster with
anionic than with cationic surfactants because the former
contribute to stabilization of the acylium ion. In w/o micro-
emulsions, on the other hand, the contribution of the surfactant
headgroup to “solvation” of the leaving group outweighs its
contribution to stabilization of the acylium ion. This interpreta-
tion is in keeping with the finding that, in our experiments, the
ratio k+/k- increased with decreasing W (Table 2).
Solvolysis of Anisoyl Chloride. As expected, the solvolysis
of AnCl in both SDS and TTABr systems exhibited a linear
dependence of 1/ko on Z*. Figure 6 illustrates this for TTABr
systems, and Table 2 lists the corresponding values of KAnCl
and k, together with values of k for AOT systems.16
The results for AnCl differ from those for BzCl chiefly in
that, for AnCl, k fell with W in both SDS and TTABr systems,
whereas for BzCl k was practically independent of W in TTABr
systems. This difference may be attributed to the greater SN1
character of the AnCl reaction,29,30 which makes it more
sensitive to reduction in the polarity of interfacial water (when
W decreases31) and less sensitive to the enhancement of water
structure by TTABr (in fact, in water, alkylammonium bromides
inhibit rather than catalyze the solvolysis of AnCl25). The
greater sensitivity of the AnCl reaction to reduced polarity must
also be the reason why in SDS systems the AnCl reaction rate
fell by a factor of about 15 between W ) 20 and W ) 7, as
against a factor of only about 3 for BzCl.
Solvolysis of Diphenylmethyl Chloride. It is considered
that the mechanism of the solvolysis of DPhMeCl is close to
the SN1 limit,32 but in certain solvents non-first-order kinetics
are observed because the carbocation is sufficiently dissociated
to discriminate between the attacking solvent and other nucleo-
philes that may be present (including the chloride ion).27,33 To

5518 J. Phys. Chem. B, Vol. 101, No. 28, 1997
Garc´ıa-R´ıo et al.
TABLE 3: Partition Coefficients KDPhMeCl and Intrinsic
Solvolytic Rate Constants k for Diphenylmethyl Chloride in
Isooctane/SDS/1-Hexanol/Water w/o Microemulsions at 25
°C, Together with Published Values for k in Isooctane/AOT/
Water Systemsa
TABLE 4: Partition Coefficients KNPhCl and Intrinsic Solvo-
lytic Rate Constants k for 4-Nitrophenyl Chloroformate in
Isooctane/SDS/1-Hexanol/Water and Isooctane/TTABr/1-
Hexanol/Water w/o Microemulsions at 25 °C, Together with
Published Values for k in Isooctane/AOT/Water Systemsa
AOTb
k/s-1
SDS/1-hexanol
AOTb
k/s-1
SDS/1-Hexanol
k/s-1
KNPhCl
TTABr/1-Hexanol
W
k/s-1
KDPhMeCl
W
k/s-1
KNPhCl
7
10
15
20
25
9.36 × 10-4
1.35 × 10-3
=2.8 × 10-3
4.66 × 10-3
6.42 × 10-3
6.14 × 10-5
2.53 × 10-4
6.34 × 10-4
8.45 × 10-4
1.68 × 10-3
6.9
5.7
7.1
11.5
5.2
3
4
5
7
10
4.76 × 10-2
3.24 × 10-2
2.93 × 10-2
6.16 × 10-2
6.17 × 10-2
6.55 × 10-2
6.82 × 10-2
7.80 × 10-2
6.85 × 10-2
6.84 × 10-2
7.08 × 10-2
11.7
14.1
13.2
12.7
11.1
14.5
13.5
12.2
2.32 × 10-2 5.36 × 10-2
1.96 × 10-2 4.79 × 10-2
3.1
4.1
3.4
3.6
4.3
15 =1.7 × 10-2
20 =1.5 × 10-2
25 =1.5 × 10-2
5.53 × 10-2
5.36 × 10-2
4.86 × 10-2
a [1-Hexanol]/[surfactant] ) 5. b J. Phys. Chem. 1995, 99, 12318.
avoid the extra risk of such complications that would have been
occasioned by the use of TTABr, the solvolysis of DPhMeCl
was studied only in SDS systems. Table 3 lists the values of k
and KDPhMeCl estimated in the same way as for the other
substrates, together with reaction rate data for AOT systems.
It has been suggested that in solvents of low polarity the
solvolysis of diarylmethyl halides is slowed by internal return
within a tight ion pair and that the rate-limiting reaction step
may be the attack of the nucleophilic solvent on the ion pair,34
but to distinguish experimentally between this mechanism and
a direct nucleophile-substrate interaction in which the transition
state exhibits extensive C-Cl bond breaking would require a
means of trapping the ion pair. In any case, the tight ion pair
mechanism is certainly not active in the solvolysis of diarylalkyl
halides in solvents with low water content, since these latter
reactions exhibit common ion effects proving the formation of
the free carbocation, which is therefore presumably also formed
in dry solvents, albeit with a shorter lifetime.35 Furthermore,
although it might be supposed that the carbocation should be
sufficiently stabilized by delocalization of charge for stabiliza-
tion by external agents to have little effect on the kinetics of
the reaction (especially since, in SN1 reactions, solvation of the
leaving group is generally more important than solvation of a
bulky, relatively stable carbocation),36 the fact is that arylalkyl
substrates with quite different extents of charge delocalization
are similarly affected by the nature of the reaction medium.37
In this work, the solvolysis of DPhMeCl was slower in SDS
than in AOT systems, which was expected and was attributed,
as in the cases of BzCl and AnCl, to the lower polarity caused
by the presence of hexanol in the former. Again, since the
polarity of interfacial water falls with W, the factor of 27.4 by
which the reaction rate in SDS systems fell when the water
content was reduced from W ) 20 to W ) 7 is in keeping with
the reaction of DPhMeCl being more purely SN1 than those of
AnCl and BzCl, for which the corresponding factors were 15.5
and 3.5.
Solvolysis of 4-Nitrophenyl Chloroformate. In water, the
mechanism of the solvolysis of NPhCl is similar to that of
addition to the carbonyl group of carbonate esters or acid
anhydrides in which the rate-limiting process is a nucleophilic
addition.27 In w/o AOT microemulsions, the reaction rate
increases as W falls because of the increasing nucleophilicity
of the interfacial water,38 which is caused by increasingly strong
hydrogen-bonding interaction with the AOT headgroups.16
Table 4 lists the values of k and KNPhCl determined in SDS
and TTABr systems in this work, together with the earlier k
results for AOT systems.
The absence, in SDS systems, of the increase in k with
decreasing W that is exhibited by the reaction in AOT systems
is attributed simply to the impossibility of preparing stable SDS
microemulsions with low enough W for the nucleophilicity of
the interfacial water to be markedly increased by interaction
a [1-hexanol]/[surfactant] ) 5. b J. Phys. Chem. 1995, 99, 12318.
Figure 7. Influence of W ) [H2O]/[surfactant] on the 1H NMR
chemical shift of the protons of water in isooctane/SDS/1-hexanol/water,
isooctane/TTABr/1-hexanol/water, and isooctane/AOT/water w/o
microemulsions: (b) AOT; (O) SDS; (0) TTABr.
with the surfactant headgroups. The possibility that there might
be a difference in kind between water-surfactant interactions
in AOT and SDS systems was ruled out by investigating the W
1
dependence of the H NMR signals of the water in isooctane/
SDS/1-hexanol/water and isooctane/TTABr/1-hexanol/water mi-
croemulsions and by comparing them with published data for
AOT/(isooctane or cyclohexane)/water systems.39 The paral-
lelism between the δ-W curves for SDS and AOT systems
(Figure 7) suggests that there is no radical dissimilarity between
the water-headgroup interactions in these microemulsions. In
both cases, the marked shift to higher field as W decreases
indicates that the predominant effect of the sodium counterion,
the concentration of which increases with decreasing W, is to
shield the H2O protons by breaking hydrogen bonds rather than
to deshield them by polarizing the water molecule.40 That the
upfield shift is much smaller in TTABr systems than in AOT
and SDS systems seems likely to be a result of the “structure-
enhancing” property of tetraalkylammonium salts.28
In TTABr systems, the rate constant k was slightly greater
than in water, which is attributed to the enhancement of the
nucleophilicity of interfacial water by the “structure-enhancing”
surfactant, and hardly varied with W. The latter behavior is
attributed, as in the case of BzCl, to the enhancement of
nucleophilicity increasing with effective surfactant concentration
as W fell, and so offsetting the k-reducing effect of decreasing
interfacial polarity.
Solvolysis of Bis(4-nitrophenyl) Carbonate. The solvolysis
of BNPhC was only studied in TTABr systems by using BNPhC
concentrations of about 4 × 10-5 M and varying the TTABr
concentration over the range 0.17-0.67 M for each value of

Solvolytic Reactions
J. Phys. Chem. B, Vol. 101, No. 28, 1997 5519
TABLE 5: Partition Coefficients KBNPhC and Intrinsic
Solvolytic Rate Constants (kH and kD) for Bis(4-Nitrophenyl)
Carbonate in Isooctane/TTABr/1-Hexanol/Water (H2O and
D2O, Respectively) w/o Microemulsions at 25 °Ca
nucleophilic water of AOT systems will have less need of
catalytic assistance.
Conclusions
H2O
D2O
We have used a method for estimating the distribution of
hexanol between isooctane and surfactant. To simplify, it was
assumed that the concentration of the very poorly water-soluble
1-hexanol in the aqueous pseudophase was negligible, and the
microemulsion was accordingly treated as an (isooctane +
hexanol)/(surfactant + hexanol)/water system.
The alcohol required as cosurfactant to stabilize w/o micro-
emulsions based on SDS and TTABr reduces interfacial polarity
and thereby also tends to reduce the reaction rates of solvolysis
reactions taking place at the interface. As W falls, the
nucleophilicity of interfacial water is probably increased in SDS
systems by increasing the interaction with headgroups, and in
TTABr systems by the structure-enhancing properties of the
alkylammonium salt, whose effective concentration increases
with falling W. The W dependence of the intrinsic rate constant
for solvolysis at the interface depends on the solvolytic
mechanism.
W
kH/s-1
KBNPhC
kD/s-1
KBNPhC
kH/kD
3
4
5
6.84 × 10-5
7.52 × 10-5
9.28 × 10-5
1.23 × 10-4
1.38 × 10-4
1.66 × 10-4
1.72 × 10-4
1.74 × 10-4
1.95 × 10-4
1.99 × 10-4
34.1
86.5
60.4
48.6
50.4
48.7
43.2
42.5
33.9
37.1
4.38 × 10-5
4.80 × 10-5
5.77 × 10-5
6.82 × 10-5
6.94 × 10-5
7.38 × 10-5
7.63 × 10-5
7.95 × 10-5
7.91 × 10-5
7.56 × 10-5
36.4
47.9
45.4
43.9
41.2
39.9
40.2
36.6
36.8
39.4
1.56
1.57
1.61
1.80
1.99
2.25
2.25
2.19
2.46
2.63
7
10
15
20
25
30
40
a [1-hexanol]/[surfactant] ) 5.
Acknowledgment. Financial support from Xunta de Galicia
(Project XUGA 20906B93) and from the Direccio´n General de
Investigacio´n Cient´ıfica y Te´cnica of Spain (Project PB93-0524)
is gratefully acknowledged.
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