Study of the bromide ion reaction with methyl naphthalene-2-sulfonate in water-DMSO TTAB micellar solutions

Article abstract of DOI:10.1002/poc.1115

The reaction of bromide ions with methyl naphthalene-2-sulfonate (MeNS) has been investigated in water-dimethyl sulfoxide, DMSO, tetradecyltrimethylammonium bromide, TTAB, micellar solutions, with the weight percentage of DMSO up to 50%. In order to quantitatively rationalize the micellar kinetic effects observed, conductivity, surface tension, and steady-state fluorescence measurements were used to get information about the micellar reaction media. Results showed that changes caused by the addition of different amounts of DMSO to TTAB aqueous micellar solutions are made evident from the kinetic micellar effects, these being a helpful tool to obtain information on the micellar reaction media in the presence of the added organic solvent. Copyright

Full text of DOI:10.1002/poc.1115

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2006; 19: 676–682
Published online 29 November 2006 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/poc.1115
Study of the bromide ion reaction with methyl
naphthalene-2-sulfonate in water–DMSO
TTAB micellar solutions
*
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Marı´a Luisa Moya, Amalia Rodrıguez, Marıa Munoz, Marıa del Mar Graciani and Gaspar Fernandez
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Departamento de Quımica Fısica, Universidad de Sevilla, C/Profesor Garcıa Gonzalez 2, 41012 Sevilla, Spain
Received 13 March 2006; revised 5 July 2006; accepted 7 July 2006
ABSTRACT: The reaction of bromide ions with methyl naphthalene-2-sulfonate (MeNS) has been investigated in
water–dimethyl sulfoxide, DMSO, tetradecyltrimethylammonium bromide, TTAB, micellar solutions, with the weight
percentage of DMSO up to 50%. In order to quantitatively rationalize the micellar kinetic effects observed,
conductivity, surface tension, and steady-state fluorescence measurements were used to get information about the
micellar reaction media. Results showed that changes caused by the addition of different amounts of DMSO to TTAB
aqueous micellar solutions are made evident from the kinetic micellar effects, these being a helpful tool to obtain
information on the micellar reaction media in the presence of the added organic solvent. Copyright # 2006 John Wiley
& Sons, Ltd.
KEYWORDS: TTAB; micelles; DMSO; kinetics; thermodynamics; structure
INTRODUCTION
DMSO and, more so in water–DMSO mixtures. On this
basis, DMSO seems to be an appropriate organic solvent
in order to investigate how the addition of an organic
solvent to an aqueous micellar solution affects the rate of
a micelle-modified reaction. The chemical process chosen
was the well known SN2 substitution reaction methyl
naphthalene-2-sulfonate (MeNS) þ Brꢀ. In order to
quantitatively rationalize kinetic micellar effects, it is
necessary to obtain information about the micellar
reaction media used. Conductivity, surface tension, and
steady-state measurements were used to this purpose.
All measurements were done at 298.2 K.
Dimethyl sulfoxide (DMSO) is a common organic solvent
that has been used widely in biological studies and as a
drug carrier across cell membranes.1–3 Due to two lone
pairs on its oxygen atom, DMSO can interact with water
forming strong hydrogen bonds. A single DMSO
molecule is thought to form hydrogen bonds with as
many as four water molecules simultaneously.4 In
addition, the two methyl groups provide a non-polar
attribute to the molecule causing effects of hydrophobic
hydration and hydrophobic association of DMSO
molecules. This combination of polar and non-polar
characteristics makes DMSO and its aqueous solutions an
important solvent in organic chemistry and fine chemical
technology.2,5 The ability of a solvent to bring about the
self-association of conventional amphiphiles can be
EXPERIMENTAL
Materials
characterized by its Gordon parameter,6G ¼ go=V1=3
,
Tetradecyltrimethylammonium bromide (TTAB) was
from Aldrich as were NaBr and DMSO. The surfactant
was used without further purification and its cmc in
water was in agreement with literature data (see Table 2).7
Hexadecylpyridinium chloride was from Fluka. Pyrene
was from Aldrich and was purified before use. MeNS was
synthesized following the method in the literature.8
where go is the solvent surface tension and V its molar
volume. For pure DMSO, the Gordon parameter is
1.02 J mꢀ3. Since G values equal or higher than 1.0–
1.2 J mꢀ3 seem to be required for self-association of the
micellar type, micellization is expected to occur in pure
´
*Correspondence to: M. L. Moya, Departamento de Quımica Fısica,
Universidad de Sevilla, C/Professor Garcıa Gonzalez 2, 41012 Sevilla,
´
´
´
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Spain.
E-mail: moya@us.es
Conductivity measurements
´
Contract/grant sponsors: DGCYTBQU2002-00691; Consejerıa de
´
Conductivity was measured with a Crison microCM 2201
conductimeter connected to a water flow thermostat
´
Educacion y Ciencia de la Junta de Andalucıa; contract/grant
number: FQM-274.
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 676–682

BROMIDE ION REACTION WITH METHYL NAPHTHALENE SULFONATE IN WATER–DMSO TTAB MICELLAR SOLUTIONS 677
maintained at 298.2 ꢁ 0.1 K. The conductimeter was
calibrated with KCl solutions of the appropriate
concentration range.
was added in 10 ml of acetonitrile to 1 ml of the reaction
solution at 298.2 K, so that the organic substrate
concentration in the reaction medium was 10ꢀ4 M. The
kinetics were followed for more than five half-lives in all
the water–DMSO micellar media. The observed rate
constants were obtained from the slopes of the
ln(At ꢀ A ) against time plots, At and A being the
Surface tension measurements
/
/
The surface tensions were measured by a du Nou¨y ring
method using a KSV 703 digital tensiometer (Finland)
equipped with an automatic device to set the time
between two consecutive measurements and to select the
rising velocity of the platinum ring. A water-jacketed
sample beaker connected to a cryostat was used to control
the sample temperature. Prior to each measurement, the
ring was rinsed with ethanol and then heated briefly by
holding it above a Bunsen burner until glowing. The
vessel was cleaned by using chromic sulfuric acid, boiled
in distilled water, and then flamed with a Bunsen burner
before use. The precision in the measurements was
ꢁ1 mN mꢀ1. Care has to be taken in using the du Nou¨y
ring method to deduce surfactant properties, because the
surfactant adsorption kinetics can influence the results.9
In our experiments, the ring rising velocity was chosen
low enough to allow the surfactant adsorption to reach
equilibrium.
absorbances at time t and at the end of the reaction,
respectively. Each experiment was repeated at least twice,
and the observed rate constants were reproducible within
precision of better than 5%. The temperature was
maintained at 298.2 ꢁ 0.1 K using a water-jacketed cell
compartment connected to a water flow thermostat.
In order to check the reliability of our kinetic data, the
reaction was followed in hexadecyltrimethylammonium
bromide micellar solutions, at [surfactant] ¼ 0.04 M, in
the presence and in the absence of NaBr 0.02 M. The
observed rate constants obtained were 8.3 ꢂ 10ꢀ4 and
7.4 ꢂ 10ꢀ4 sꢀ1, respectively, in good agreement with
literature data.8
RESULTS
Figure 1 shows the dependence of the observed rate
constant for the SN2 reaction MeNS þ Brꢀ on TTAB
concentration in various water–DMSO TTAB micellar
solutions, in the presence of NaBr 0.1 M.
Fluorescence measurements
The cmc of the water–DMSO TTAB solutions, in the
presence of NaBr 0.1 M, were determined by using
surface tension measurements (see Fig. 2). These data are
summarized in Table 1. Surface tension measurements
also provide information about the adsorption of
surfactants at the air–liquid interfaces. The surface
excess concentration, Gmax, and the minimum area per
surfactant molecule, Amin, at the air–solvent interface
Fluorescence measurements were made by using a
Hitachi F-2500 fluorescence spectrophotometer. The
temperature was kept at 298.2 ꢁ 0.1 K by a water flow
thermostat connected to the cell compartment.
A study of the fluorescence quenching of pyrene by N-
hexadecylpyridinium chloride, CePyCl, was carried out.
The introduction of pyrene in the water–DMSO micellar
solutions was done as in Ref. [10]. The pair pyrene/
CePyCl ensures that the residence time of the quencher in
the micelles is much longer than the fluorescence lifetime
of the probe.11 The probe concentration was kept low
enough (2 ꢂ 10ꢀ6 mol dmꢀ3) to avoid excimer formation,
and the quencher concentration was varied from 5 ꢂ 10ꢀ5
to 25 ꢂ 10ꢀ5 mol dmꢀ3. These values give [pyrene]/
[micelles] and [quencher]/[micelles] ratios low enough
to ensure a Poisson distribution.12 Some comments about
the aggregation numbers obtained will be made below.
Kinetics
The reaction between MeNS þ Brꢀ was recorded at
326 nm in a Unicam Helios-g spectrophotometer in the
presence of NaBr 0.1 M. Sodium bromide was added to
the reaction media because in its presence, the
reproducibility of the kinetics in water–DMSO TTAB
micellar solutions was better than in its absence. MeNS
Figure 1. Influence of the TTAB, on the obꢀserved rate
constant, kobs(sꢀ1), for the reaction MeNS þ Br in various
water–DMSO TTAB micellar solutions (T ¼ 298.2 K)
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 676–682
DOI: 10.1002/poc

´
M. L. MOYA ET AL.
678
molecule corresponding to pure water obtained in this
work is in agreement with previous results,15 although it
is smaller than that obtained through neutron reflection
measurements.16
Table 2 shows the values of the critical micelle
concentration and of the micellar ionization degree, a, of
the water–DMSO TTAB solutions investigated in the
absence of NaBr 0.1 M. The authors determined these
cmc and a values from inflections in plots of conduc-
tivity, k, against the surfactant concentration (Williams
method17), as described in Ref. [18] (see Fig. 3).
Nonetheless, since this method has been criticized,19
the authors also used the Phillips method20 in order to
estimate the cmc values. This method was applied
through an integration by the Runge-Kutta method and a
least-square Levenberg–Maquardt fitting, as described in
Ref.[21]. The cmc values obtained by the two methods
were in good agreement, thus giving reliability to the cmc
and a values listed in Table 2. It is worth noting that the
cmc and a values corresponding to pure aqueous
solutions are in agreement with literature values.8
The aggregation number, Nagg, of the micelles present
in the micellar solutions studied are also listed in Table 2.
These values were obtained from the quenching of pyrene
fluorescence by hexadecylpyridinium chloride (see
Fig. 4). The Nagg value in TTAB aqueous micellar
solution is small as compared with the literature value.22
This could be due to the quenching processes not being
particularly effective in the large TTAB micelles.23 There
is another point that could also be responsible for the
small aggregation numbers obtained: bromide ions can
quench the pyrene excited state.23 With this in mind,
attention will be paid to how the presence of the organic
solvent influences the aggregation numbers of TTAB
micelles and it seems reasonable to conclude that Nagg
decreases as the percentage by weight of DMSO in the
mixture increases.
Figure 2. Dependence of the surface tension, g (mN mꢀ1),
of various water–DMSO TTAB solutions on ln([Surfactant])
(T ¼ 298.2 K)
were obtained by using the following equations:
ꢀ
ꢁ
1
@g
Gmax ¼ ꢀ
(1)
(2)
qRT @ ln C
T;P
1
Amin
¼
GmaxNA
Here R is the gas constant, NA is Avogadro’s number, g
is the surface tension, and C is the concentration of
surfactant in solution. The quantity q is the number of
solute species whose concentration at the interface
changes with change in the value of C. q is 1 for non-
ionic and zwitterionic surfactants and 2 for ionic
surfactants. In the water–DMSO surfactant solutions
studied, NaBr 0.1 M was present. Therefore, the ionic
strength of the solutions can be considered constant for
the surfactant solutions in the premicellar region and, as a
consequence, the value of the coefficient q is unity.13 The
values of Gmax and Amin are listed in Table 1. These values
correspond to water–DMSO TTAB solutions in the
presence of NaBr 0.1 M. Nonetheless, the presence of this
moderate concentration of salt is not expected to affect the
surface tension data strongly14 and the values of the Gmax
and Amin corresponding to the TTAB solutions in the
absence of salt are expected to be similar to those
summarized in Table 1. The minimum area per surfactant
Table 4 shows solvent surface tension, go, the molar
volume, V, and the Gordon parameter, G ¼ go=V1=3, for
the water–DMSO mixtures used as bulk phase in the
cationic micellar solutions investigated. The molar
volume of the mixtures were estimated from
V ¼ VDMSOXDMSO þ Vwaterð1 ꢀ XDMSOÞ.
DISCUSSION
Table 1. Critical micelle concentration, cmc, surface excess
concentration, Gmax, and minimum area per surfactant mol-
ecule, Amin, for TTAB in water–DMSO mixtures in the pre-
sence of NaBr 0.1 M (T ¼ 298.2 K)
The dependence of the observed rate constant on
surfactant concentration for the reaction of bromide with
MeNS þ Brꢀ (Scheme 1) in various water–DMSO TTAB
micellar solutions are shown in Fig. 1. This Figure shows
that kobs increases upon increasing surfactant concen-
tration in all the water–DMSO mixtures investigated,
which can be explained by considering that an increase in
[TTAB] causes a further incorporation of MeNS into the
cationic micelles, where the interfacial bromide ion
concentration is large. On the other hand, for a given
Amin
(ꢂ1020m2)
Gmax
(ꢂ106 mol mꢀ2
DMSO
(wt%)
cmc
(ꢂ104 mol dmꢀ3)
)
0
20
30
40
50
3.4 ꢁ 0.2
8.6 ꢁ 0.4
17.4 ꢁ 0.8
41 ꢁ 2
2.6 ꢁ 0.1
2.4 ꢁ 0.1
2.3 ꢁ 0.1
2.1 ꢁ 0.1
2.0 ꢁ 0.2
64 ꢁ 2
69 ꢁ 3
72 ꢁ 4
79 ꢁ 4
85 ꢁ 4
61 ꢁ 3
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 676–682
DOI: 10.1002/poc

BROMIDE ION REACTION WITH METHYL NAPHTHALENE SULFONATE IN WATER–DMSO TTAB MICELLAR SOLUTIONS 679
Table 2. Cmc, micellar ionization degree, a, aggregation number, Nagg, and standard Gibbs energy of micelle formation, DGoM,
values for water–DMSO TTAB micellar solutions (T ¼ 298.2 K)
a
DMSO(wt%)
cmc (ꢂ104 mol dmꢀ3)
a
Nagg
ꢀDGoMðkJmolꢀ1
Þ
0
20
30
40
50
3.6 ꢁ 0.1
7.3 ꢁ 0.2
10 ꢁ 0.2
0.23 ꢁ 0.01
0.28 ꢁ 0.01
0.31 ꢁ 0.01
0.36 ꢁ 0.02
0.39 ꢁ 0.02
58 ꢁ 3
48 ꢁ 5
35 ꢁ 4
23 ꢁ 4
19 ꢁ 3
24.6
20.9
19.2
17.0
15.1
15.1 ꢁ 0.5
22.4 ꢁ 0.7
a These aggregation numbers are approximated (see the text).
surfactant concentration, the observed rate constant is
smaller when the weight percentage of the organic solvent
is higher. If pseudophase kinetic models are considered,
kobs can be written as:24
is not significant except at low surfactant concentrations.
Kinetic data have been corrected, when necessary, from
the spontaneous hydrolysis contribution as in Ref. 26].
The bromide ion concentrations [Brꢀbulk] and [Brmꢀ] can
be estimated by considering that the incorporation of the
bromide anions into the TTAB micelles can be described
by Eqn (4):27
k2bulk½Brꢀbulkꢃ þ ðk2m=VmÞ½BrꢀmꢃKm
kobs
¼
(3)
1 þ Km½TTABmꢃ
½Brꢀmꢃ
Here [Brꢀbulk] and [Brꢀm] are the bromide ion concen-
trations in the bulk and micellar pseudophases referred to
the total solution volume. Vm is the molar volume of
the reactive region at the micellar surface and Km is the
equilibrium binding constant which describes the
distribution of the organic substrate between the bulk
and micellar pseudophases. [TTABm] is the micellized
surfactant concentration, equal to the total surfactant
concentration minus the cmc. (km2 /Vm) ¼ k2m (sꢀ1) is the
second order rate constant in the micellar pseudophase
written with concentrations expressed as molar ratios,
[Brꢀm]/[TTAB], and kb2ulk is the second order rate constant
of the reaction in the bulk phase. kb2ulk values were
KBrꢀ
¼
(4)
½Brꢀbulkꢃð½TTABmꢃ ꢀ ½BrꢀmꢃÞ
Considering Eqn (4) and the mass balance, one can
write:
2
KBrꢀ½Brꢀmꢃ ꢀ ðKBrꢀ½TTABmꢃ þ KBrꢀ½BrꢀT ꢃ þ 1Þ
ꢂ ½Brꢀmꢃ þ KBrꢀ½TTABmꢃ½BrꢀT ꢃ ¼ 0
ð5Þ
where bromide ions come from the surfactant and from
the NaBr present in the reaction media. The authors
considered a KBrꢀ value of 900 dm3 molꢀ1 in TTAB
aqueous micellar solutions.26 For the other micellar
reaction media, KBrꢀ was estimated from its dependence
on a (Eqn (6)). In this way, for any water–DMSO TTAB
obtained experimentally, they being equal to 7.7 ꢂ 10ꢀ5
,
8.3 ꢂ 10ꢀ5, 9.5 ꢂ 10ꢀ5, 10.7 ꢂ 10ꢀ5 molꢀ1 dm3 sꢀ1 for 0,
20, 30, and 50% weight percentage of DMSO,
respectively. At this point, it is worth noting that the
reaction of MeNS with water can make a contribution to
the reaction MeNS þ Brꢀ,9,25 although this contribution
1 ꢀ a
KBrꢀ
¼
(6)
a2½TTABmꢃ
micellar solution with an ionization degree equal to a,
KBrꢀ can be calculated by:
KBrꢀ ð1 ꢀ aÞ 0:232
¼
(7)
900
ð1 ꢀ 0:23Þ a2
where 0.23 is the micellar ionization degree of the
aqueous TTAB micelles and 900 their corresponding
KBrꢀ value. The kinetic measurements were carried out in
the presence of NaBr 0.1 M, whereas the micellar
Table 3. Fitting parameters for the reaction MeNS þ Brꢀ in
DMSO–water TTAB bromide micellar solutions at 298.2 K
DMSO (wt%) Km(molꢀ1 dm3) 104 ꢂ k2m ¼ (k2m/Vm) (sꢀ1
)
0
20
40
50
505 ꢁ 12
150 ꢁ 4
43 ꢁ 3
9.41 ꢁ 0.08
8.8 ꢁ 0.1
8.7 ꢁ 0.2
9.3 ꢁ 0.3
Figure 3. Dependence of the specific conductivity, k,
in mS cmꢀ1, on surfactant concentration for water–DMSO
TTAB solutions (T ¼ 298.2 K)
20 ꢁ 2
Copyright # 2006 John Wiley & Sons, Ltd.
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DOI: 10.1002/poc

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M. L. MOYA ET AL.
680
Table 4. Solvent surface tension, go, solvent molar volume,
V, and Gordon parameter, G, for various water–DMSO
mixtures. T ¼ 298.2 K
Vm) and Km values gives the best fitting for each of the
micellar solutions investigated. Nonetheless, given the
simplicity of the model used to rationalize the kinetic
data, only substantial changes in the adjustable
parameters are worth discussing.
DMSO (wt%) go(mN mꢀ1) V(dm3 molꢀ1) G ¼ go/V1=3(J mꢀ3
)
The (km2 /Vm) and Km values obtained from the fittings
are listed in Table 3. This Table shows that k2m seems to
be independent of the weight percentage of DMSO
present in the micellar reaction media. On the other hand,
the equilibrium binding constant, Km, of the MeNS
molecules to the TTAB micelles decreases strongly when
the amount of DMSO increases. In regard to the latter, an
increase in the amount of DMSO results in a decrease in
the polarity of the bulk phase (the dielectric constant is
78.39 and 46.68 for pure water and pure DMSO at
298.2 K31). This means that the water–DMSO bulk phase
is a better solvent for the organic substrate at higher
weight percentages of DMSO. Consequently, the affinity
of MeNS for the micellar pseudophase decreases when
wt% DMSO increases, also decreasing Km.
In order to compare the reactivity in water and in
micelles, the second order rate constant in the micellar
pseudophase expressed in molꢀ1 dm3 sꢀ1, k2m, has to be
calculated (one cannot compare k2m, a second order rate
constant expressed in sꢀ1, to k2bulk, a second order rate
constant expressed in molꢀ1 dm3 sꢀ1). Since k2m ¼ k2m Vm,
from the k2m values listed in Table 3 and the Vm values, k2m
can be estimated. From the data in Ref.,[26] the molar
reaction volume in TTAB aqueous micellar solutions was
estimated to be 0.33 dm3 molꢀ1. Therefore, the km2 value
calculated for the reaction MeNS þ Brꢀ in TTAB
aqueous micelles was 3.1 ꢂ 10ꢀ4 dm3 molꢀ1 sꢀ1, to be
compared to kb2ulk ¼ 7.7 ꢂ 10ꢀ5 dm3 molꢀ1 sꢀ1. That is,
the presence of cationic TTAB micelles in pure water
increases the rate constant. The main factors involved in
this increase would be the electrophilic interaction of the
ammonium head groups and the forming naphthalene-2-
sulfonate ion and the disruption of the hydration shell of
the bromide ion.32 The km2 values in water–DMSO
micellar solutions could not be estimated because the
corresponding Vm values are not known. Taking into
account the decrease in the aggregation number found in
the micellar solutions investigated by increasing the %wt
of DMSO in the solutions, a reasonable assumption would
be that Vm decreases upon increasing the amount of
DMSO, or at least, a substantial increase in the molar
reaction volume is not expected by increasing wt% of
DMSO. Therefore, taking the k2m ¼ (km2 /Vm) and k2bulk
values into account, one can conclude that the rate
constants of the reaction MeNS þ Brꢀ are larger in the
micellar pseudophases than in the bulk phase not only in
pure water, but also in the water–DMSO mixtures studied.
Table 3 shows that k2m is not practically affected by
changes in the weight percentage of DMSO. Therefore,
the diminution observed in kobs when wt% of DMSO
increases is mainly due to two factors: (i) A large decrease
in Km, which results in an increase of the contribution of
0
20
71.8a
18.07a
28.7
34.0
2.74
2.2
69 ꢁ 1
64 ꢁ 1
57 ꢁ 1
53 ꢁ 1
42.8a
30
40
50
100
2.0
1.7
1.5
1.03
39.4
44.7
71.3a
V ¼ VDMSOXDMSO þ Vwaterð1 ꢀ XDMSO
Þ
a Taken from CRC Handbook of Chemistry and Physics, 8th ed., 2004–
2005.
ionization degrees listed in Table 2 correspond to the
micellar solutions in the absence of salt. However,
previous work has shown that TTAB micelles remain
spherical in water and in NaBr aqueous solutions at salt
concentrations up to 0.1 M28,29 and no substantial changes
in the micellar ionization degree were considered within
that salt concentration range. Besides, both theory and
experiment indicate that ionization of the micelles for
ions that are not extremely hydrophilic, such as Brꢀ, are
not particularly sensitive to the concentration of added
salt.30 On this basis, the KBrꢀ values corresponding to the
different micellar reaction media were calculated by
using the micellar ionization degrees listed in Table 2 and
Eqn (7). KBrꢀ values equal 900, 691, 478, and 420 dm3
molꢀ1 for water–DMSO TTAB micellar solutions with 0,
20, 40, and 50% weight percentage of DMSO in the
mixture, respectively.
Solid lines in Fig. 1 are the results of fitting the
experimental kinetic data by using Eqn (3). One can see
that the agreement between the experimental and the
theoretical data was good. The use of cmc, a, and k2bulk
experimental data has reduced as much as possible the
number of adjustable parameters and only one set of (km2 /
Figure 4. Influence of the quencher (N-hexadecylpyridi-
nium chloride) concentration on the intensity of the pyrene
fluorescence in water–DMSO TTAB micellar solutions with
20 wt% DMSO (T ¼ 298.2 K)
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 676–682
DOI: 10.1002/poc

BROMIDE ION REACTION WITH METHYL NAPHTHALENE SULFONATE IN WATER–DMSO TTAB MICELLAR SOLUTIONS 681
Scheme 1
the reaction taking place in the bulk phase. Since the
reaction is slower in the bulk phase than in the micellar
pseudophase, the observed rate constant diminishes. (ii)
The increase in the micellar ionisation degree upon
increasing the amount of DMSO present in the mixture.
This increase will produce a diminution of the bromide
ion concentration at the micellar surface, the reaction site,
retarding the reaction.
From the data listed in Tables 1 and 2, additional
information about the micellar solutions used as reaction
media can be obtained. The Gibbs energy of micelliza-
tion, DGoM, can be calculated by:33
responsible for the increase in the micellar ionization
degree observed when the weight percentage of DMSO
increases. An increase in the ionic strength due to the
increase in the monomer surfactant concentration
(accompanying the substantial increase in the cmc)
results in a diminution of the ionic interactions at the
micellar surface (screening effects) and, consequently, the
ionization degree increases upon increasing the weight
percentage of DMSO. With respect to the decrease in
the micellar aggregation numbers upon increasing the
amount of DMSO present in the mixture, it can be
explained by taking into account the decrease in
the interfacial energy contribution to DGoM due to a
decrease in the solution–hydrocarbon interfacial tension
when the amount of DMSO in the mixture increases (see
the dependence of go on wt% DMSO in Table 4).34
Table 1 shows that G decreases, and A increases, when
the amount of DMSO present in the mixture increases.
This result can be related to changes in the water structure
due to the presence of DMSO, interactions between
DMSO and surfactant molecules, and the presence of
DMSO at the air–solution interface. In regard to the
solvent–surfactant interactions, an increase in the wt%
DMSO causes a decrease in the dielectric constant, in the
Reichardt parameter, ET, in the Gutman donor number,
DN, or in the pꢄ polarity index31 of the mixture, this
meaning that the surfactant molecules will be solubilized
more easily and their tendency to be adsorbed at the air–
solution interface will decrease. As a consequence, the
surface excess concentration decreases and A increases
when wt% EG increases.
Summarizing, the thermodynamic and structural
changes caused by the presence of different amounts of
DMSO in aqueous TTAB micellar solutions control the
micellar effects observed on the reaction MeNS þ Brꢀ
taking place in these micellar solutions. The decrease in
kobs originated by the addition of DMSO is mainly the
result of two factors: (i) the decrease in the bromide ion
concentration at the micellar surface, where the reaction
takes place, due to an increase in the micellar ionization
degree, and (ii) the decrease in the equilibrium binding
constant due to the water–DMSO mixtures being a better
solvent for the organic substrate molecules than pure
water. The second-order rate constant in the micellar
pseudophase in any of the water–DMSO micellar
solutions studied is faster than in the bulk phase, although
it shows no dependence on changes in the wt% of DMSO.
The addition of DMSO, up to a percentage by weight of
DGoM ¼ ð2 ꢀ aÞ RTlncmc
(8)
where a is the micellar ionization degree. This equation
applies when the aggregation number is large. Therefore,
taking into account the Nagg values listed in Table 2, DGMo
values obtained by using Eqn (8) for the higher weight
percentages of DMSO have to be considered as
approximated. DGoM values in Table 2 shows that the
TTAB aggregation process is less spontaneous when
the wt% DMSO is larger. In this regard, the dependence of
the ability of water–DMSO mixtures to bring about the
self-association of conventional amphiphiles on the
weight percentage of DMSO can be related to its
cohesive energy density,6 which can be characterized by
the Gordon parameter, G ¼ go =V1=3, where go is the
solvent surface tension and V its molar volume. Table 4
shows the Gordon parameter values for the different
mixtures used as bulk phases in the micellar solutions
studied. The G parameter points out that an increase in the
weight percentage of DMSO results in a decrease in the
solvent cohesiveness, thereby improving the solvation
of the hydrocarbon tails in the bulk phase and decreasing
the solvophobic effect. In relation to this, the increase
in the cmc by increasing wt%, EG can be explained by
taking into account that the transfer of the surfactant tail
from the water þ DMSO bulk phase into the micellar core
is less spontaneous when the amount of DMSO in the
mixture increases.33–35 This is due to the water-polar
organic solvent mixtures being better solvents for the
surfactant molecules than pure water.31 There is also a
dependence of the cmc on the interfacial energy
contribution to DGoM,34 but it is always much weaker
when compared to the dependence on the solvophobic
Gibbs energy contribution. As a consequence, cmc
increases upon increasing wt% DMSO. This increase is
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 676–682
DOI: 10.1002/poc

´
M. L. MOYA ET AL.
682
50, to TTAB aqueous micellar solutions results in an
increase in the critical micelle concentration and in the
micellar ionization degree, whereas the micellar aggrega-
tion number decreases.
13. Rosen MJ. Surfactants, Interfacial Phenomena (3rd edn). Wiley:
New York, 2004.
´
´
14. Graciani MM, Rodrıguez A, Mun˜oz M, Moya ML. Langmuir
2005; 21: 7161.
15. Venable RL, Nauman RV. J. Phys. Chem. 1964; 68: 3498.
16. Simister EA, Thomas RK, Penfold J, Aveyard J, Binks BP, Cooper
P, Fletcher PDI, Lu JR, Sokolowski A. J. Phys. Chem. 1992; 96:
1383.
17. Williams RJ, Phillips JN, Mysels KJ. Trans. Faraday Soc. 1955;
51: 728.
Acknowledgements
´
´
18. Rodrıguez A, Graciani MM, Mun˜oz M, Moya ML. Langmuir
This work was financed by DGCYT (grant BQU2006-
2003; 19: 7206.
19. Sarmiento F, del Rıo JM, Prieto G, Atwood D, Jones MN,
´
00597) and Consejeria de Innovacıon, Ciencia y Empresa
de la Junta de Andalucıa. The authors thank Professor
´
´
Mosquera V. J. Phys. Chem. 1995; 99: 17628.
´
Victor Mosquera, from the University of Santiago de
Compostela, Spain, for helping us in the application of
the Phillips method for obtaining the cmc values. The
authors also thank Professor Ana Troncoso, from the
University of Seville, Spain, for helping us in the fluor-
escence measurements.
20. Phillips JN. Trans. Faraday Soc. 1955; 51: 561.
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21. Mosquera V, Garcıa M, Varela LM. In Handbook of Surfaces and
Interfaces of Materials, vol. 3, Nalwa HS (ed). Academic Press:
New York, 2001; 401.
22. Berr S, Jones RRM, Johnson JS. J. Phys. Chem. 1992; 96:
5611.
23. Van Stam J, Depaemelaere S, De Schryver FC. J. Chem. Edu. 1998;
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24. (a) Bunton CA, Nome F, Quina F, Romsted LS. Acc. Chem. Res.
1991; 24: 357; (b) Savelli G, Germani R, Brinchi L. In Reactions
and Synthesis in Surfactants Systems, Texter J (ed.). Surfactant
Science Series/100, Marcel Dekker: New York, 2001, Ch. 8, p. 175
and references therein.
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Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 676–682
DOI: 10.1002/poc