Concentration and medium micellar kinetic effects caused by morphological transitions

Article abstract of DOI:10.1021/la102857d

The reaction methyl naphthalene-2-sulfonate + Br^- was investigated in several alkanediyl-α-ω-bis(dodecyldimethylammonium) bromide, 12-s-12,2Br^- (with s = 2, 3, 4, 5, 6, 8, 10, 12), micellar solutions in the absence and in the presence of various additives. The additives were 1,2-propylene glycol, which remains in the bulk phase, N-decyl N-methylglucamide, MEGA10, which forms mixed micelles with the dimeric surfactants, and 1-butanol, which distributes between the aqueous and micellar phases. Information about the micellar reaction media was obtained by using conductivity and fluorescence measurements. In all cases, with the exception of water-1,2-prop 12-5-12,2Br^- micellar solutions, with 30% weight percentage of the organic solvent, a sphere-to-rod transition takes place upon increasing surfactant concentration. In order to quantitatively explain the experimental data within the whole surfactant concentration range, a kinetic equation based on the pseudophase kinetic model was considered, together with the decrease in the micellar ionization degree accompanying micellar growth. However, theoretical predictions did not agree with the experimental kinetic data for surfactant concentrations above the morphological transition. An empirical kinetic equation was proposed in order to explain the data. It contains a parameter b which is assumed to account for the medium micellar kinetic effects caused by the morphological transition. The use of this empirical equation permits the quantitative rationalization of the kinetic micellar effects in the whole surfactant concentration range.

Full text of DOI:10.1021/la102857d

pubs.acs.org/Langmuir
©
2010 American Chemical Society
Concentration and Medium Micellar Kinetic Effects Caused by
Morphological Transitions
Marı
´
a del Mar Graciani, Amalia Rodrı
´
guez, Victoria Isabel Martı
a Luisa Moy ꢀa *
´
n, Gaspar Fern ꢀa ndez, and
Marı
´
Departamento de Quı ´m ica Fı´sica, Universidad de Sevilla, C/Profesor Garcı´a Gonz aꢀ lez 1, 41012 Sevilla, Spain
Received July 19, 2010. Revised Manuscript Received October 24, 2010
-
The reaction methyl naphthalene-2-sulfonate þ Br was investigated in several alkanediyl-R-ω-bis(dodecyldimethyl-
-
ammonium) bromide, 12-s-12,2Br (with s=2, 3, 4, 5, 6, 8, 10, 12), micellar solutions in the absence and in the presence
of various additives. The additives were 1,2-propylene glycol, which remains in the bulk phase, N-decyl N-methylglucamide,
MEGA10, which forms mixed micelles with the dimeric surfactants, and 1-butanol, which distributes between the
aqueous and micellar phases. Information about the micellar reaction media was obtained by using conductivity and
-
fluorescence measurements. In all cases, with the exception of water-1,2-prop 12-5-12,2Br micellar solutions, with 30%
weight percentage of the organic solvent, a sphere-to-rod transition takes place upon increasing surfactant concentration.
In order to quantitatively explain the experimental data within the whole surfactant concentration range, a kinetic
equation based on the pseudophase kinetic model was considered, together with the decrease in the micellar ionization
degree accompanying micellar growth. However, theoretical predictions did not agree with the experimental kinetic data
for surfactant concentrations above the morphological transition. An empirical kinetic equation was proposed in order
to explain the data. It contains a parameter b which is assumed to account for the medium micellar kinetic effects caused
by the morphological transition. The use of this empirical equation permits the quantitative rationalization of the kinetic
micellar effects in the whole surfactant concentration range.
Introduction
by the additives can be made evident through the study of ade-
quately chosen chemical reactions in the micellar solutions. Since
the process between methyl naphthalene-2-sulfonate, MNS, and
Dimeric surfactants represent a new class of surfactants. They
are formed by two amphiphilic moieties connected at the level of
5
1
,2
bromide ions meets the requirements, this process was investigated
the head groups by a spacer. The possibilities of using dimeric
surfactants to create new types of surfactants have recently been
in several alkanediyl-R-ω-bis(dodecyldimethylammonium) bromide,
-
3
12-s-12,2Br (with s=2, 3, 4, 5, 6, 8, 10, 12), micellar solutions in
attracting increasing attention. The interest in such surfactants
the absence and in the presence of various additives. It is worth
noting that these dimeric surfactants are particularly interesting
because they undergo morphological transitions when surfactant
arises from their physicochemical properties that are better than
those of conventional surfactants, such as much lower critical micelle
concentrations (cmc), better wetting, greater surface tension low-
ering, and unusual morphologies. These properties could make
them potentially useful in many fields of application, for example,
in soil remediation, enhanced oil recovery, drug entrapment and
1,2
concentration increases, with the dimeric micelles changing shape
from spherical aggregates into elongated ones. The surfactant con-
centration at which this morphological transition occurs is often
referred to as “second cmc” (C*). The sphere-to-rod transition is
followed by variations in the characteristics of the micellar aggre-
4
release, and so forth.
At concentrations above the cmc, dimeric surfactants tend to
self-associate in water to form micelles whose characteristics depend
-
gates which will affect the rate of the MNS þ Br process.
1
,2
The additives used were the polar organic solvent 1,2-propyl-
on surfactant nature as well as on temperature. Additives affect
the self-aggregation process and the features of the formed aggregates.
This influence operates through variations in the chemical poten-
tial of the surfactant molecules in the bulk phase, μꢀsurfactant(bulk),
as well as in the chemical potential of the surfactant molecules
in the micelles, μꢀsurfactant(m). The importance of one or another
effect depends on the nature of the additive. The changes caused
6
englycol, which remains in the bulk phase, 1-butanol, that distri-
7
butes between the bulk and micellar pseudophases, and the sur-
factant N-decyl N-methylglucamide, MEGA10, which forms mixed
8
micelleswith thegemini surfactants. Most of thekinetic measure-
-
ments in the presence of additives were carried out in 12-5-12,2Br
micellar solutions. Nonetheless, kinetic data obtained in other
-
1
2-s-12,2Br micellar solutions showed that the kinetic effects
provoked by the additives were similar for all the surfactants
investigated.
In this work, the use of an empirical equation was proposed
for the quantitative rationalization of the kinetic micellar effects
*
To whom correspondence should be addressed. E-mail: moya@us.es.
(
(
(
1) Menger, F. M.; Keiper, J. N. Angew. Chem., Int. Ed. 2000, 39, 1906.
2) Zana, R. Adv. Colloid Interface Sci. 2002, 97, 205.
3) See, for example, (a) Viscardi, G.; Quagliotto, P.; Barolo, C.; Savaino, P.;
Barni, E.; Fisicaro, E. J. Org. Chem. 2000, 65, 8917. (b) Faustino, C. M. C.; Calado,
A. R. T.; García-Río, L. J. Phys. Chem. B 2009, 113, 977. (c) Stjerndahl, M.; Lundberg,
D.; Zhao, H.; Menger, F. M. J. Phys. Chem. B 2007, 111, 2008. (d) Mishra, B. K.;
Mukerhee, P.; Dash, S.; Patel, S.; Pati, H. N. Synth. Commun. 2009, 39, 2529. (e) Brito,
R. O.; Marques, E. F.; Silva, S. G.; do Vale, M. L.; Gomes, P.; Ara ꢀu jo, M. J.;
Rodríguez-Burgos, J. E.; Infante, M. R.; García, M. T.; Ribosa, I.; Mitjans, M.
Colloids Surf., B 2009, 72, 80.
(5) Moy ꢀa , M. L.; Graciani, M. M.; Rodrı
Catal. 2008, 7, 43–53.
(6) Rodrı
guez, A.; Graciani, M. M.; Fern ꢀa ndez, G.; Moy ꢀa , M. L. J. Colloid
´
guez, A.; Fern ꢀa ndez, G. Curr. Top.
´
Interface Sci. 2009, 338, 207.
(7) Zana, R. Adv. Colloid Interface Sci. 1995, 57, 1.
(4) See, for instance, Gemini Surfactants: Synthesis, Interfacial and Solution-Phase
(8) Martın, V. I.; Rodrı
´ ´
guez, A.; Graciani, M. M.; Robina, I.; Moy ꢀa , M. L.
Behavior and Applications; Zana, R., Xia, J., Eds.; Marcel Dekker Inc.: New York, 2004.
J. Phys. Chem. B 2010, 114, 7817.
Langmuir 2010, 26(24), 18659–18668
Published on Web 11/23/2010
DOI: 10.1021/la102857d 18659

Article
Graciani et al.
Scheme 1
Scheme 2
Second Critical Micelle Concentration, C*, Determination.
-6
The 1ꢀ10 M SPQ surfactant solutions were prepared in double
distilled water. The fluorescence intensities were measured at 443 nm
by the excitation at 346 nm, as indicated in ref 19. Excitation
and emission slits were 5 and 10 nm, respectively, and a scan speed
of 60 nm was used. Surfactant concentrations were well above
the cmc.
Study of the Polarity of the Micellar Interfacial Region. The
mol dm P3C micellar solutions were prepared as in ref 20.
-5
-3
10
Pyrene-3-carboxaldehyde was excited at 356 nm, and fluorescence
spectra were recorded from 380 to 600 nm. A scan speed of 60 nm/min
was used, and the excitation and emission slits were changed depend-
ing on the micellar media. The fluorescence maximum shown in
observed in the whole surfactant concentration range, below and
above the morphological transition. This equation was also shown
to be useful in the presence of the additives which alter not only
the aggregation process but also the surfactant concentration
range in which the spherical-to-rod transition takes place. It is
interesting to point out that few kinetic studies have been carried
out in dimeric micellar solutions undergoing morphological
2
1
the spectra is strongly dependent on the polarity of the medium.
In order to check the reliability of the results, the fluorescence
emission spectrum of P3C in a 0.1 M aqueous DTAB micellar
solution was recorded at 303 K. The fluorescence maximum was
21
446 nm, in good agreement with literature data.
-
Kinetic Measurements. The reaction MNS þ Br (Scheme 2)
9-16
transitions,
and, to the authors’ knowledge, this is the first
was recorded at 326 by using Hitachi 3100 and Shimadzu 1800
UV-visible spectrophotometers. MNS was added in 10 μL of aceto-
nitrile to 1 mL of the reaction solution, so that the organic sub-
attempt at quantitatively explaining the kinetic micellar effects
caused by the changes in the micelle characteristics accompanying
morphological transitions.
-4
strate concentration in the reaction medium was 10 M. The
bromide ions involved in the process come from the surfactant
molecules, and no additional Br was added. Kinetics were followed
-
Experimental Section
for more than five half-lives in all the micellar media. The observed
rate constant was obtained from the slopes of the ln(A - A )
Materials. Pyrene-3-carboxaldehyde, P3C, was from Fluka.
-Methoxy-N-(3-sulfopropyl)quinolinium, SPQ, was purchased
t
¥
6
against time plots, with A
t and at the end of the reaction, respectively. The A
t
and A
¥
being the absorbance at time
value was
from Molecular Probes, Inc. and used as received. N-Decanoyl
N-methylglucamide, MEGA10 (see Scheme 1), and 1-butanol were
from Fluka and used without further purification. 1,2-Propylene
glycol, 1,2-prop, was from Aldrich.
¥
experimentally obtained by letting the reaction finish. Each experi-
ment was repeated at least twice, and the observed rate constants
were reproducible within a precision better than 5%. The tem-
perature was maintained at 303.0 ( 0.1 K using a water-jacketed
cell compartment connected to a water-flow cryostat.
Dodecyltrimethylammonium bromide (DTAB) was from Aldrich.
The synthesis of the dimeric surfactants (see Scheme 1) was done
as described in ref 17. The surfactants were characterized by H
1
-
-
Kinetics in 12-2-12,2Br and 12-3-12,2Br could not be done
for surfactant concentrations higher than 0.04 and 0.07 M,
respectively, because of solubility problems.
1
3
NMR, C NMR, and elemental analysis (CITIUS, University of
Seville), with the results being in agreement with those previously
reported. Methyl naphthalene-2-sulfonate, MeNS, was synthesized
The reactions of the organic substrates with water can make a
although this
1
8
-
22,23
following the method in the literature.
Conductivity Measurements. Conductivity was measured
with a Crison GLP31 conductimeter as described previously.
contribution to the reaction MNS with Br ,
contribution is not significant except at low surfactant concentra-
tions. Kinetic data have been corrected, when necessary, from the
spontaneous hydrolysis contribution as in ref 24.
8
Steady-State Fluorescence Measurements. Fluorescence
measurements were done by using a Hitachi F-2500 fluorescence
spectrophotometer. The temperature was kept at 303 K via a water
flow cryostat connected to the cell compartment.
Results and Discussion
Figure 1 shows the dependence of the observed rate constant,
-
kobs, for the reaction MNS þ Br on surfactant concentration for
(
9) Bunton, C. A.; Robinson, L.; Schaak, J.; Stam, M. F. J. Org. Chem. 1971, 36,
-
the different pure 12-s-12,2Br micellar solutions investigated. The
2346.
(
10) Bunton, C. A.; Minch, M. J.; Hidalgo, J.; Sep uꢀ lveda, L. J. Am. Chem. Soc.
973, 95, 3262–3272.
kinetic data in the presence of additives and those corresponding
to DTAB are displayed in Figure 2. The latter were obtained for
the sake of comparison (DTAB can be considered as the mono-
meric counterpart of the dimeric surfactants). The kinetic data in
1
(
11) (a) Brinchi, L.; Germani, R.; Goracci, L.; Savelli, G.; Bunton, C. A.
Langmuir 2002, 18, 7821. (b) Bhattacharya, S.; Kumar, V. P. J. Org. Chem. 2004,
9, 559.
12) Liveri, M. L. T.; Lombardo, R.; Sbriziolo, C.; Viscardi, G.; Quagliotto, P.
New J. Chem. 2004, 28, 793.
6
(
(
(
13) Qiu, L.-G.; Xie, A.-J.; Shen, Y.-H. Colloid Polym. Sci. 2005, 283, 1343.
(19) Kuwamoto, K.; Asakawa, T.; Ohta, A.; Miyagishi, S. Langmuir 2005, 21,
7691.
14) Rodrı
´
guez, A.; Graciani, M. M.; Bitterman, K.; Carmona, A. T.; Moy ꢀa ,
M. L. J. Colloid Interface Sci. 2007, 313, 542.
15) Graciani, M. M.; Rodrı
guez, A.; Moy ꢀa , M. L. J. Colloid Interface Sci. 2008,
28, 324.
16) Rodrı
B 2009, 113, 7767.
17) Menger, F. M.; Keiper, J. S.; Mbadugha, B. N. A.; Caran, K. L.; Romsted,
L. S. Langmuir 2000, 16, 9095.
18) Bacaloglu, R.; Bunton, C. A.; Ortega, F. J. Phys. Chem. 1989, 93, 1497.
(20) Zana, R. J. Phys. Chem. B 1999, 103, 9117.
(
´
(21) Kalyanasundaram, K.; Thomas, J. K. J. Chem. Phys. 1977, 81, 2176.
(22) Bonan, C.; Germani, R.; Ponti, P. P.; Savelli, G.; Cerichelli, G.; Bacaloglu,
R.; Bunton, C. A. J. Phys. Chem. 1990, 94, 5331.
(23) Brinchi, L.; Di Profio, P.; Germani, R.; Savelli, R.; Bunton, C. A. Langmuir
1997, 13, 4583.
3
(
´
guez, A.; Graciani, M. M.; Cordob ꢀe s, F.; Moy ꢀa , M. L. J. Phys. Chem.
(
(24) Rodrı
´
guez, A.; Graciani, M. M.; Mu n~ oz, M.; Moy ꢀa , M. L. Langmuir 2003,
(
19, 8685.
1
8660 DOI: 10.1021/la102857d
Langmuir 2010, 26(24), 18659–18668

Graciani et al.
Article
-
1
-
Figure 1. Dependence of the observed rate constant, kobs/s , for the reaction MNS þ Br on surfactant concentration in various aqueous
-
-
-
-
-
-
dimeric micellarsolutions at303 K:(a) 12-2-12,2Br ; (b) 12-3-12,2Br ; (c) 12-4-12,2Br ; (d) 12-5-12,2Br ; (e) 12-6-12,2Br ; (f) 12-8-12,2Br ;
(g) 12-10-12,2Br ; (h) 12-12-12,2Br .
-
-
Figures 1 and 2 are presented in independent plots in order to make
the visualization of the experimental results easier. Figures 1 and 2
show that in all cases kobs increases upon augmenting the sur-
factant concentration. With the exception of 12-5-12,2Br in the
-
Langmuir 2010, 26(24), 18659–18668
DOI: 10.1021/la102857d 18661

Article
Graciani et al.
Table 1. Critical Micelle Concentration, cmc, Micellar Ionization
Degree, r, Second Critical Micellar Concentration, C*, and
Wavelength of the Fluorescence Maximum of Pyrene-3-carboxaldehyde
in the Micellar Phase, λmax(m), for Various Alkanediyl-r-ω-
-
bis(dodecyldimethylammonium bromide), 12-s-12,2Br ,
Surfactants at 303 K
a
surfactant
cmc (mM)
R
C* (M)
λmax(m)
-
1
1
1
1
1
1
1
1
2-2-12,2Br
2-3-12,2Br
2-4-12,2Br
2-5-12,2Br
2-6-12,2Br
2-8-12,2Br
0.95
0.99
1.1
0.17
0.22
0.25
0.28
0.31
0.40
0.45
0.45
0.016
0.036
0.025
0.029
0.028
0.031
0.028
0.023
438.8
439.2
439.4
439.6
440.1
441.2
441.6
442.1
-
-
-
-
-
1.1
0.99
0.88
0.59
0.36
-
-
2-10-12,2Br
2-12-12,2Br
a
[
m
Surfactant ]=0.01 M; data corresponding to s=2, 5, 6, 8, 10, and
1
2 were taken from ref 8.
concentration brings about a further incorporation of the organic
substrate molecules into the micelles. As a consequence, the contri-
bution of the reaction occurring in the micellar pseudophase
increases and, given that the bromide ion concentration in this
pseudophase is much higher than that in the bulk phase, the
observed rate constant augments. One would expect kobs to reach
a constant value when the surfactant concentration was high
enough for the MNS molecules to be fully bound to the micelles.
However, in most cases, no plateau is observed. This behavior can
-
be explained by taking into account that in 12-s-12,2Br micellar
solutions an increment in surfactant concentration causes a
sphere-to-rod transition. The change in size and shape of the
micelles is accompanied, among other effects, by a diminution in
1
9,25
the micellar ionization degree,
which leads to an increment in
the interfacial bromide ion concentration and thus in kobs. There
are other changes following the morphological transition which
can influence kobs. They will be considered below.
cmc and r Values in the Micellar Reaction Media. In order
to quantitatively rationalize the kinetic micellar effects observed,
it is necessary to estimate the bromide ion concentration in the
bulk and micellar pseudophases, where the process occurs. In order
to do this, the critical micelle concentration, cmc, and micellar ion-
ization degree, R, of the aggregates present in the micellar reaction
media have to be known. These magnitudes were determined by
means of conductivity measurements and applying Carpena’s
2
6
method to the experimental specific conductivity data (see
Figure 1 in the Supporting Information). The cmc and R values
are listed in Tables 1 and 2. The dependence of these magnitudes
8
on the spacer length has been previously discussed.
The influence of additives on the cmc and R values of 12-2-
-
-
1
2,2Br and 12-5-12,2Br surfactants can be rationalized by
considering the effects of the additive on different contributions
2
7
to the Gibbs energy of micellization, ΔGꢀ. 1,2-Propylene glycol
M
6
remains in the bulk phase, lowering its polarity and making it a
-
1
Figure 2. Dependence of the observed rate constant, kobs/s , for
better solvent for the surfactant molecules. As a consequence, the
Gibbs energy change accompanying the transfer of the surfactant
hydrophobic chain from the bulk phase (water þ 1,2-prop) into
the micellar interior diminishes (it is less negative), with this
-
the reaction MNS þ Br on surfactant concentration in various
aqueous dimeric micellar solutions in the presence of additives and
inDTAB micellarsolutions at303 K:(a) 12-2-12,2Br ; (b) 12-5-12,
-
-
2
Br ; (c) DTAB.
2
8,29
leading to an increment in the cmc.
The presence of the
presence of 1,2-prop 30 wt %, and of DTAB micellar solutions, the
observed rate constant does not reach a plateau at high surfactant
concentrations, but it increases steadily when [12-s-12,2Br ]
(
25) (a) Rodrı
´
guez, A.; Graciani, M. M.; Mu n~ oz, M.; Robina, I.; Moy ꢀa , M. L.
Langmuir 2006, 22, 9519. (b) Rodríguez, A.; Graciani, M. M.; Angulo, M.; Moy ꢀa , M. L.
Langmuir 2007, 23, 11496.
(
-
26) Carpena, P.; Aguiar, J.; Bernaola-Galv ꢀa n, P.; Carrnero Ruiz, C. Langmuir
increases. The first observation can be explained by considering
2
002, 18, 6054.
-
that both the MNS and Br reagents are distributed between the
(27) Camesano, T. A.; Nagarajan, R. Colloids Surf., A 2000, 167, 165.
(
(
28) Nagarajan, R.; Wang, Ch.-Ch. Langmuir 2000, 16, 5242.
bulk and micellar pseudophases. Therefore, the process is taking
place simultaneously in the two pseudophases and the reaction
rate is the sum of the two contributions. An increase in surfactant
29) (a) Moy ꢀa , M. L.; Rodrıguez, A.; Graciani, M. M.; Fern ꢀa ndez, G. J. Colloid
´
Interface Sci. 2007, 316, 787. (b) Rodríguez, A.; Graciani, M. M.; Moy ꢀa , M. L.
Langmuir 2008, 24, 12785.
1
8662 DOI: 10.1021/la102857d
Langmuir 2010, 26(24), 18659–18668

Graciani et al.
Article
Table 2. Critical Micelle Concentration, cmc, Micellar Ionization
Degree, r, Second Critical Micellar Concentration, C*, and
Wavelength of the Fluorescence Maximum of Pyrene-3-carboxaldehyde
-
in the Micellar Phase, λmax(m), for Various 12-s-12,2Br (s=2,5)
Surfactants in the Presence of Additives at 303 K
a
surfactant
cmc (mM)
R
C* (M) λmax(m)
;
b
b
b
b
1
1
1
1
1
1
1
1
2-2-12,2Br -MEGA, Xdimeric =0.9
1.0
1.4
1.3
1.6
1.3
2.7
0.96
0.80
0.18
0.22
0.30
0.34
0.29
0.019 439.2
0.053 442.8
0.048 441.3
0.089 442.9
0.060 440.1
;
2-2-12,2Br -MEGA, Xdimeric =0.6
;
2-5-12,2Br -MEGA, Xdimeric =0.8
;
2-5-12,2Br -MEGA, Xdimeric =0.6
-
2-5-12,2Br , 1,2-prop 10 wt %
-
2-5-12,2Br , 1,2-prop 30 wt %
-
0.39 >0.1
445.9
2-5-12,2Br , [1-butanol]=0.1 M
-
0.33
0.40
0.060 440.6
0.025 439.9
2-5-12,2Br , [1-butanol]=0.2 M
a
b
[
propylene glycol.
Surfactant
m
] = 0.01 M. Data taken from ref 8; 1,2-prop = 1,2-
organic solvent also affects other energetic contributions to ΔGꢀ ,
M
6,25
1
but the diminution in ΔGꢀ
mainly controls the cmc change.
Figure 3. SPQ fluorescence quenching in aqueous 12-5-12,2Br-
micellar solutions at high surfactant concentrations in pure water
and in the presence of 1,2-prop 30% wt. T = 303 K.
transf
The cmc increase results in an increment in the ionic concentra-
tion (the monomer concentration augments), which leads to a
decrease in the electrostatic repulsions between the positively
charged head groups of the dimeric surfactants through a screen-
ing effect. Besides, the average aggregation number of the dimeric
aggregates is expected to decrease because of the presence of the
although there are other size and shape dependent Gibbs energy
2
7
contributions to ΔGꢀ . The extra-packing term accounts for the
M
packing constraints on the surfactant tails (as they are connected
by the spacer) so as to pack within the micellar core. This term
favors micellar growth, since it decreases as micellar size increases
and makes the transition from sphere to cylinder easier. The electro-
static term is large in magnitude and limits micellar growth.
With regard to the presence of additives, Table 2 shows that an
increase in the weight percentage of 1,2-prop causes an increment
in C*. In fact, in the presence of 30 wt % 1,2-prop, no morphological
transition is observed (for [surfactant]<0.1 M). The dependence
of C* on 1,2-prop weight percentage can be discussed by taking
into account that addition of 1,2-prop is expected to decrease the
6
,16,25,28
organic solvent.
The two effects are followed by a diminu-
tion in the electrostatic charge density at the micellar interfacial
region, with this bringing about an increment in the micellar
ionization degree.
MEGA 10 molecules formed mixed micelles with the dimeric
surfactants, as has been shown previously through circular
8
dichroism. The experimental cmcs were found to be lower than
those expected while considering an ideal behavior. This nonideal
behavior was explained by considering the diminution in the electro-
static repulsions due to the presence of the nonionic component
1
6,25,29
8
aggregation number.
This size diminution brings about
in the mixed aggregates. The spacer length was shown not to play
-
a decrease in the tail deformation Gibbs energy and in the extra-
packing Gibbs energy contributions, making the sphere to rod
transition less favorable. As a result, C* diminishes upon increas-
ing the weight percentage of 1,2-prop.
an important role in the behavior of the 12-s-12,2Br -MEGA 10
binary systems. The ΔGꢀ reduction can also explain the increase
elect
in R following the addition of the nonionic surfactant.
1
-Butanol distributes between the aqueous and the micellar
MEGA10 molecules incorporate into the dimeric micelles forming
mixed aggregates. In contrast to the dimeric micelles, pure MEGA10
micelles do not show a tendency to micellar growth upon in-
pseudophases. The changes in the cmc and R caused by 1-butanol
addition can be explained by considering that alcohol molecules
intercalate between the dimeric surfactant molecules within the
micelles, diminishing the electrostatic repulsions between the
3
0
creasing surfactant concentration. It has been previously shown
that addition of a spherical micelle-forming surfactant, such as
positively charged head groups. This decrease in ΔGꢀ promotes
elect
MEGA10, to a treadlike micelle-forming surfactant, such as 12-s-
micellization and reduces the electrostatic charge density at the inter-
facial region, thus causing the cmc to decrease and R to increase.
Micellar Growth in the Microheterogeneous Reaction
-
1
2,2Br surfactants, results in the progressive inhibition of the
capacity of the latter to form such micelles when the surfactants
3
1
-
comicellized. This is in agreement with cryogenic transmission
electron microscopy (cryoTEM) measurements carried out with
Media. 12-s-12,2Br surfactants show morphological transitions
1
,2,8,16,25
when surfactant concentration increases.
Micellar growth
3
2
DTABþ12-2-12 mixtures.
is significant from the kinetic point of view since changes in the
size and shape of micelles affect the rate of micelle-modified
The presence of 1-butanol molecules in the aqueous phase
influences C* similarly to 1,2-propylene glycol, making the mor-
phological transition less favorable. On the other hand, intercalation
of 1-butanol molecules in the interfacial region reduces the electro-
static repulsions between the positively charged head groups. As a
consequence, the transition from spherical to spherocylindrical
micelles would be more favorable and C* would diminish. For
1
4
reactions. Therefore, information about the second cmc, C*, is
necessary in order to rationalize the kinetic data. C* values were
1
9
determined by using the method developed by Kuwamoto et al.
Figure 3 shows the SPQ fluorescence quenching in aqueous 12-5-12,
-
2
Br micellar solutions in the absence and in the presence of
,2-propylene glycol 10 wt %, at high surfactant concentrations,
1
1-butanol 0.1 M, an increase in C* is observed as compared to
with [surfactant] well above the cmc (see Tables 1 and 2). The C*
values are listed in Tables 1 and 2. Each second cmc value was
repeated at least twice, and the precision is C* ( 0.002 M.
The dependence of C* on the spacer length was discussed in a
pure water, thus pointing out that the changes in the bulk phase
(30) Hierrezuelo, J. M.; Molina Bolivar, J. A.; Carnero Ruiz, C. J. Phys. Chem.
B 2009, 113, 7178.
8
previous work. The ionic interactions and the additional extra-
(
31) Rodrı
´
guez, A.; Graciani, M. M.; Moreno-Vargas, A. J.; Moy ꢀa , M. L.
packing term to the deformation of the surfactant tails were
J. Phys. Chem. B 2008, 112, 11942.
considered in order to account for the experimental C* trend,
(32) Zana, R.; Talmon, Y. Nature 1993, 362, 228.
Langmuir 2010, 26(24), 18659–18668
DOI: 10.1021/la102857d 18663

Article
Graciani et al.
caused by the presence of the alcohol molecules in this phase control
the secondcmc variation. When 1-butanol concentration is 0.2 M,
the reduction in ΔGꢀelect induced by the incorporation of the
alcohol molecules into the micellar pseudophase is the energetic
contribution mainly controlling the second cmc variations.
Kinetic Micellar Effects on the Reaction between Methyl
Naphthalene-2-Sulfonate and Bromide Ions. Figure 1 shows
the influence of the dimeric surfactant concentration on the process
range in order to help the reader. The adjustable parameters
-
-
obtained are listed in Table 3. Subsequently, [Br ] and [Br ]
w
m
values were calculated for the whole surfactant concentration
range by taking the variations in R following micellar growth into
account. Using these bromide ion concentrations and the K and
m
R
obs
k2m values listed in Table 3, k was estimated by using eq 1. The
medium dashed lines in Figure 4a and in Supporting Information
Figure 2a-g show the dependence of kR on surfactant concen-
obs
-
MNS þ Br . The observed rate constant for the reaction can be
tration. One can see that, in all cases, consideration of R changes
improves the agreement between the theoretical and the experi-
mental rate constant values. However, the agreement is not good,
which is not a surprise, since, as was mentioned above, micellar
growth is followed by other changes in the aggregates character-
istics not taken into account in eq 1.
3
3
written as
Â
Â
Ã
À
ÁÂ
Ã
w
2
-
m
2
-
k
Br þ k =V
m
Br
K
m
w
m
k
obs
¼
¼
Â
Ã
-
1
þ K
m
12-s-12, 2Br
m
Ã
Â
Ã
w
-
-
k2 Br þ k2m Br
K
m
w
m
It is interesting to point out that micelles and other association
colloids can influence the rates of micelle-modified reactions
through two different effects. The concentration or depletion of
reactants in the interfacial region has a major effect on the rates of
bimolecular processes, such as the one investigated in this work.
This is called the concentration effect. However, micelles also exert
a medium effect that influences reactivity. This effect depends on
the transfer of substrate from water to micelles, on the reaction
mechanism, and on the properties of the interfacial region, such as
local charge, polarity, and water content. Usually, the kinetic
micellar effects observed in bimolecular processes are mainly due
to the concentration of one or both reactants in the small volume
of the micelles. However, there are processes for which large
kinetic micellar effects, not connected to concentration effects, are
Â
Ã
ð1Þ
-
1
þ K
m
12-s-12, 2Br
m
-
-
Here [Br ] and [Br ] are the bromide ion concentrations in the
w
m
aqueous and micellar pseudophases, respectively, 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 of the MNS molecules to the dimeric micelles. [12-s-
-
w
m
1
2,2Br ] is the micellized surfactant concentration. k and k are
m 2 2
the second order rate constants in the aqueous and micellar
-
1
3
-1
m
pseudophases (in mol dm s ), respectively, and (k /V ) =
2
m
-1
k2m (in s ) is the second order rate constant in the micellar
pseudophase written with the concentrations expressed as molar
-
-
ratios, [Br ]/[12-s-12,2Br ].
m
m
w
In order to fit the kinetic data by using eq 1, the k2 value
35
w
-
4
operative. In the reaction studied, the difference between k
2
and
was experimentally determined, with its value being 1.6 ꢀ 10
m
-1
3
-1
k
2
would account for the micellar medium effects in the dimeric
mol dm s at 303 K. The micellized surfactant concentration
can becalculated by using the cmc values listed in Table 1. Finally,
the bromide ion concentrations in the aqueous and micellar
pseudophases can be estimated by using eqs 2 and 3
micellar solutions assuming that no morphological transition
occurs. When the changes in the micellar ionization degree caused
by the morphological transition were taken into account, the con-
centration kinetic micellar effects provoked by micellar growth
Â
Ã
Â
Ã
-
-
R
obs
Br
¼ ð1 - RÞ 12-s-12, 2Br
ð2Þ
were considered. Therefore, the disagreement between the k and
m
m
the experimental rate constants has to be due to medium kinetic
micellar effects provoked by micellar growth. The difficulty is that
discussion of micellar medium effects is always qualitative because
they are the result of many factors influencing reactivity simul-
taneously and it is not possible to obtain experimental informa-
tion about how each of them affects reactivity. Because of this, the
authors examined the suitability of various empirical equations
for explaining the data and found that if kobs is expressed by eq 4,
the kinetic data can be quantitatively fitted for all the micellar
reaction media investigated. It has to be indicated that eq 4 is an
empirical equation and was not deduced from eq 1.
Â
Ã
Â
Ã
-
-
-
Br
¼ ½12-s-12, 2Br ꢁ - Br
ð3Þ
w
total
m
where R is the micellar ionization degree. From the fittings, the
adjustable parameters, Km and k2m, would be obtained. These
fittings will only render reliable adjustable parameters for kinetic
data within the range cmc<[12-s-12,2Br ] <C*, where, for the
surfactants investigated, R remains constant. Nonetheless, [Br ]
and [Br ] can be calculated within the whole surfactant concen-
-
-
w
-
m
tration range from the steady-state fluorescence data by using
1
9,14,34
Kuwamoto’s method.
Micellar growth leads to a decrease
not only in the micellar ionization degree but also in the micro-
polarity and in the microviscosity of the interfacial region as well
Â
Ã
Â
Ã
w
2
-
-
k
Br þ k2m Br
K
m
Â
Ã
w
m
-
2
k
obs
¼
Â
à þ b 12-s-12, 2Br
ð4Þ
as in V and in the water content. This means that, for [12-s-
-
m
m
1 þ Km 12-s-12, 2Br
-
m
m
1
2,2Br ] > C*, Km and k2 can vary upon increasing surfactant
concentration. Since the goal of thisArticle is to discussthe kinetic
micellar effects within the whole surfactant concentration range,
the authors proposed the following procedure in order to do so.
The kinetic data corresponding to the surfactant concentration
In this equation, K and k2m are the adjustable parameters
m
summarized in Table 3. Solid lines in Figure 4a and in Supporting
Information Figure 2a-g show the results of the fittings using
eq 4. The values of the adjustable parameter b are listed in Table 3.
One can see that the agreement between the theoretical and the
experimental data are good for all the dimeric micellar solutions
used as reaction media. The question is: has b any meaning? The
authors proposed that b can be considered as an empirical
parameter that involves the medium kinetic micellar effects caused
by the changes in the aggregates characteristics following micellar
-
range cmc<[12-s-12,2Br ]<C* were fitted by using eq 1 and a
set of Km and k2m adjustable parameters was obtained for each
of the dimeric surfactants. The dotted lines in Figure 4a and in
SupportingInformation Figure2a-g correspond to these fittings.
The lines have been drawn for the whole surfactant concentration
(
33) Bunton, C. A.; Yao, J.; Romsted, L. S. Curr. Opin. Colloid Interface Sci.
996, 65, 125.
34) Asakawa, T.; Kitano, H.; Ohta, A.; Miyagishi, S. J. Colloid Interface Sci.
001, 242, 284.
1
2
(
(35) Rodrı
´
guez, A.; Graciani, M. M.; Mu n~ oz, M.; Fern ꢀa ndez, G.; Moy ꢀa , M. L.
New J. Chem. 2001, 25, 1084 and references therein. .
1
8664 DOI: 10.1021/la102857d
Langmuir 2010, 26(24), 18659–18668

Graciani et al.
Article
-
Figure 4. Results of the fittings of the kinetic data corresponding to the reaction MNS þ Br in various dimeric micellar solutions in the
absence and in the presence of additives at 303 K (see the text).
-
-
Table 3. Fitting Parameters for the Reaction MNS þ Br in DTAB and 12-s-12,2Br Micellar Solutions in the Absence
and in the Presence of Additives at 303 K
3 -1
(dm mol )
3
m
2
-1
) = k2m (s )
2
-1 -2
[
surfactant]
K
m
10 ꢀ (k
/V
m
10 ꢀ b/s M
-
-
-
-
-
-
1
1
1
1
1
1
1
1
2-2-12,2Br
2-3-12,2Br
2-4-12,2Br
2-5-12,2Br
2-6-12,2Br
2-8-12,2Br
2-10-12,2Br
2-12-12,2Br
368 ( 35
350 ( 28
363 ( 59
354 ( 22
355 ( 35
207 ( 34
197 ( 21
122 ( 19
182 ( 14
276 ( 34
195 ( 31
380 ( 52
270 ( 28
211 ( 22
53 ( 2
1.82 ( 0.06
2.27 ( 0.05
2.1 ( 0.2
2.29 ( 0.03
7.2 ( 0.2
3.1 ( 0.2
2.1 ( 0.2
3.0 ( 0.06
1.27 ( 0.06
1.8 ( 0.1
1.61 ( 0.08
1.96 ( 0.07
1.68 ( 0.04
2.30 ( 0.07
2.27 ( 0.02
2.30 ( 0.07
3.1 ( 0.1
19 ( 1
9.6 ( 0.5
6.5 ( 0.3
2.1 ( 0.2
1.7 ( 0.8
1.3 ( 0.3
1.1 ( 0.2
1.3 ( 0.2
-
-
DTAB
-
1
1
1
1
1
1
1
1
2-2-12,2Br -MEGA, Xdimeric = 0.9
2.3 ( 0.6
1.3 ( 0.0.3
1.8 ( 0.0.4
0.82 ( 0.0.02
1.6 ( 0.3
-
2-2-12,2Br -MEGA, Xdimeric = 0.6
-
2-5-12,2Br -MEGA, Xdimeric = 0.8
-
2-5-12,2Br -MEGA, Xdimeric = 0.6
-
a
a
2-5-12,2Br , 1,2-prop 10 wt %
2-5-12,2Br , 1,2-prop 30 wt %
-
-
2-5-12,2Br , [1-butanol] = 0.1 M
2-5-12,2Br , [1-butanol] = 0.2 M
152 ( 22
154 ( 23
1.3 ( 0.2
3.07 ( 0.02
-
a
1
,2-prop = 1,2-propylene glycol.
growth. Before going into the discussion of the b values listed in
Table 3, some comments about Km and k2m values will be made.
The equilibrium association binding constants of the MNS mole-
K seems to decrease, particularly for s=12. This could be related
m
to an increase in the polarity of the interfacial region, where the
organic substrate molecules are localized. In this regard, the wave-
length of the fluorescence maximum of pyrene-3-carboxaldehyde
cules to the dimeric micelles, K , are similar for s=2, 3, 4, 5, and 6,
m
-
within the experimental errors. For the spacers with s=8, 10, and 12,
in the micellar pseudophase (at [12-s-12,2Br ]=0.01 M), λmax(m),
Langmuir 2010, 26(24), 18659–18668
DOI: 10.1021/la102857d 18665

Article
Graciani et al.
listed in Table 1 shows that an increase in the spacer length brings
about an increase in the polarity of the interfacial region. On this
basis, a decrease in Km would be expected upon augmenting s.
Nonetheless, the localization of the P3C probe can change from
one to another micellar medium, as can the MNS molecule site in
the interfacial region vary. A reasonable conclusion is that, when
comparing K (s=2) to K (s=12), a decrease in the equilibrium
However, the strong tendency to micellar growth shown by this
dimeric surfactant can be examined by using rheology measure-
16,40
-
ments.
In fact, the viscoelastic behavior found for 12-2-12,2Br
micellar solutions was attributed to the entanglement of long and
flexible aggregates. The fact that for s>2 no viscoelastic behavior
is found in pure water points out that for s = 2 the tendency to
micellar growth is stronger than for s > 2. This conclusion is
m
m
binding constant is observed due to an increment in the polarity of
the interfacial region. The aggregation number of 12-2-12,2Br
also in agreement with cryoTEM measurements carried out in
-
-
1
2-s-12,2Br micellar solutions by increasing surfactant concen-
3
6,41
spherical micelles is substantially higher than that of 12-12-
tration.
2-12-12,2Br micelles by augmenting surfactant concentration is
These measurements also show that the growth of
-
8,36
-
1
2,2Br .
As a result, the intercalation of solvent molecules
1
into the interfacial region is easier for s=12 than for s=2, and this
increment in the interfacial water content leads to an increase in
polarity.
slow. If the b values summarized in Table 3 are examined, one can
see that b increases when the tendency of the surfactant to micellar
growth also increases. This is a reasonable result, since a strong
tendency to micellar growth will be accompanied by large changes
in the characteristics of the interfacial region (the reaction site).
Therefore, substantial medium kinetic micellar effects would be
expected and, thus, high b values would be found. This result has
to be consistent with that in the presence of additives. This point
will be examined in the next section.
Kinetic Micellar Effects on the Reaction between Methyl
Naphthalene-2-sulfonate and Bromide Ions in the Presence
of Additives. The expression for the observed rate constant depends
on the nature of the additive. When an organic polar solvent, such
as 1,2-propylene glycol, is added to the aqueous micellar solution
one can write:
k2m does not seem to depend on the spacer length (see Table 3),
-
with the exception of the value found in 12-6-12,2Br micellar
solutions, which is higher than those estimated for the rest of the
dimeric micellar media. No plausible explanation has been found
for this high k2m value. In order to get some information about the
capacity of the dimeric micelles as catalysts for the reaction
-
m
MNS þ Br with respect to water, k =k V has to be estimated
2
2m m
for the different micellar reaction media. V values for s=2, 3, 4,
m
5
0
, 6, 8, 10, and 12 were 0.56, 0.58, 0.59, 0.60, 0.63, 0.66, 0.70, and
3
37
m
.73 dm mol, respectively. The k values calculated are within
2
-3
-1
3
-1
m
2
-3
-1
3 -1
the range 1ꢀ10 mol dm s <k <2ꢀ10 mol dm s
,
-
4
-1
3 -1
to be compared to 1.6ꢀ10 mol dm s . That is, the reaction
is faster in dimeric micelles than in water. The electrophilic inter-
actions of the ammonium head groups and the forming nitro-
benzenesulfonate ion and the disruption of the hydration shell of
the bromide ion in the cationic dimeric micelles can be responsible
Â
Ã
Â
Ã
bulk
2
-
-
k
Br
þ k2m Br
K
m
Â
Ã
bulk
m
-
2
k
obs
¼
Â
Ã
þ b 12-s-12, 2Br
ð5Þ
-
m
1
þ K
m
12-s-12, 2Br
m
3
8
for the acceleration of the process. The lower polarity of the
interfacial region as compared to water can also contribute to this
acceleration, since charge is dispersed in the transition state and,
as a consequence, a diminution in the polarity of the reaction site
will favor the reaction.
Here the different terms have the same meaning as in eq 4. In this
case, the authors used the term bulk phase instead of aqueous
phase in order to consider that now the bulk phase is a water-1,2-
prop binary mixture, with a determined weight percentage of 1,2-
prop. The procedure used to fit the experimental kinetic data
was similar to that in the absence of additives. The experimental
3
8
DTAB micelles do not undergo morphological transitions by
increasing surfactant concentration, and eq 1 can quantitatively
fit the kinetic data (see Figure 1h in the Supporting Information).
bulk
-4
-4
k2 values in 10 and 30 wt % 1,2-prop are 1.9ꢀ10 and 2.3ꢀ10
-1
3 -1
mol dm s , respectively, at 303 K. The K , k , and b values
3
-1 39
m
-4
m
2m
By considering V =0.30 dm mol
,
one obtains k =3.8ꢀ10
m
2
obtained from the fittings are listed in Table 3. The solid line in
Figure 4b shows that for 10 wt % 1,2-prop the agreement between
the theoretical and the experimental data is good. In the case
of 30 wt % 1,2-prop, no morphological transition is observed for
-1
3
-1
mol dm s . This result indicates that dimeric micelles are
better catalysts for the reaction investigated than the monomeric
DTAB micelles.
b values show the following trend b(12-2-12,2Br ) > b(12-3-
2,2Br )>b(12-4-12,2Br )>b(12-5-12,2Br )>b(12-6-12,2Br )>
b(12-8-12,2Br ) ∼ b(12-10-12,2Br ) ∼ b(12-12-12,2Br ). All the
dimeric micellar solutions investigated undergo a morphological
transition upon increasing surfactant concentration. However,
the tendency to micellar growth depends on the spacer length.
-
-
-
-
-
-
[12-5-12,2Br ]<0.1 M. In this case, eq 1 is expected to be able to
1
-
-
-
fit quantitatively the kinetic data, as is observed in Figure 3a in the
Supporting Information.
For dimeric-MEGA10 mixed micellar solutions, the observed
rate constant can be written:
3
6
Â
Ã
Â
Ã
Danino et al. have investigated the dependence of the aggrega-
w
2
-
-
k
Br þ k2m Br
K
m
-
w
m
2
tion number of 12-s-12,2Br , with s=3, 4, 5, 6, 8, and 10, micelles
k
obs
¼
þ b½surfactant
m
ꢁ
ð6Þ
1
þ K
m
½surfactant
m
ꢁ
on surfactant concentration by time-resolved fluorescence quench-
ing. They found that the tendency to micellar growth follows
where the terms have the same meaning as in eq 4 and m refers to
the micellar pseudophase of the mixed micelles. Following the
-
-
-
the trend 12-3-12,2Br > 12-4-12,2Br > 12-5-12,2Br > 12-6-
1
-
-
-
2,2Br ∼ 12-8-12,2Br ∼ 12-10-12,2Br . The dimeric surfactant
method explained above, the K , k2m, and b values were obtained
m
with s = 2 could not be studied due to experimental problems.
and are listed in Table 3. Figure 4c shows the results of the fittings
-
in 12-2-12,2Br -MEGA10 mixed micellar solutions with Xdimeric=
0
.9. The fittings corresponding to the rest of the mixed dimeric-
(
36) Danino, D.; Talmon, Y.; Levy, H.; Beinert, G.; Zana, R. Science 1995, 269,
1
420.
(
(
nonionic binary systems are shown inFigure 3a-c in the Support-
ing Information. In all cases, agreement between the theoretical
and the experimental kinetic data was good.
37) Wetting, S. S.; Verral, R. E. J. Colloid Interface Sci. 2001, 235, 310.
38) (a) Bohme, K. D.; Young, L. B. J. Am. Chem. Soc. 1970, 92, 7354.
(
b) Bohme, K. D.; Mackay, G. I.; Pay, J. D. J. Am. Chem. Soc. 1974, 96, 4027. (c) Tanaka,
K.; Mackay, G. I.; Payzant, J. D.; Bohme, D. K. Can. J. Chem. 1976, 74, 1643.
d) Olmsted, W. E.; Braumen, J. I. J. Am. Chem. Soc. 1977, 99, 4219. (e) Henchman,
(
M.; Paulson, J. F.; Hiel, P. M. J. Am. Chem. Soc. 1983, 105, 5509. (f) Dewar, M. J.; Storch,
D. M. J. Chem. Soc., Chem. Commun. 1985, 94.
(40) Oda, R.; Panizza, P.; Schmitz, M.; Lequeux, F. Langmuir 1997, 13, 6407.
(41) Berheim-Groswasser, A.; Zana, R.; Talmon, Y. J. Phys. Chem. B 2000, 104,
4005.
(
39) Imae, T.; Ikeda, S. J. Phys. Chem. 1986, 90, 5216.
1
8666 DOI: 10.1021/la102857d
Langmuir 2010, 26(24), 18659–18668

Graciani et al.
Article
In water-1-butanol dimeric micellar solutions, the observed
rate constant can be expressed as
concentration equal to 0.01 M. It was found that the emission
spectra were not simple, but the result of the sum of those
corresponding to the P3C molecules localized in the bulk phase
and in the micellar interfacial region. In order to resolve the two
contributions to the total fluorescence emission intensity, a
deconvolution program was used (see Figure 4 in the Supporting
Information). Fluorescence emission spectra of P3C in water
and in the water-1,2-prop and water-1-butanol mixtures, in the
absence of surfactant, were also recorded in order to get an
approximate value of λmax(bulk phase) to be used in the curve-
fitting deconvolution. The maximum wavelength at the micellar
phase values, λmax(m), are listed in Table 3.
Â
Ã
Â
Ã
bulk
-
-
-
k2
Br
bulk
þ k2m½12-s-12, 2Br ꢁ Br
m
m
K
m
k
obs ¼ À
Â
ÃÁÀÂ
Ã
Á
-
-
1
þ K
m
12-s-12, 2Br
12-s-12, 2Br þ ½alcohol
m
ꢁ
m
m
Â
Ã
-
2
4
5
þ b 12-s-12, 2Br
ð7Þ
m
The terms have the same meaning as in eq 4. Given that 1-butanol
molecules distribute between the micellar and aqueous phases, the
authors prefer the term bulk phase in order to point out that the
continuous phase is a water-1-butanol mixture. The term [12-s-
-
1
2,2Br ] þ [alcohol ] takes into account that incorporation of
m
m
The addition of 1,2-prop results in a substantial decrease in the
alcohol molecules into the micelles will increase the concentration
of the micellar pseudophase, which is equivalent to diluting micellar
bound reactants.
46
polarity of the bulk phase and in an increment in the polarity of
the interfacial region. That is, the bulk phase is a better solvent for
the MNS molecules and the affinity of the organic substrate for
the interfacial region decreases. Therefore, a reduction in Km is
expected, as observed. The same reasoning can be used to explain
In order to estimate [alcohol ], the distribution of the alcohol
m
molecules between the micellar and bulk (water-alcohol) phases
4
2
was described by using eq 8:
the effect of 1-butanol on K . The polarity of water-1-butanol
m
7
mixtures decreases as 1-butanol content increases, although this
½
alcohol
T
ꢁ - ½alcohol
w
ꢁ
K ¼
ð8Þ
decreaseis not as large as that for 1,2-prop. In regard to MEGA10
addition, λmax(m) values show an increase in the polarity of the
interfacial region by increasing the molar fraction of nonionic
½
alcohol
w
ꢁð½surfactant
m
ꢁ þ ½alcohol
T
ꢁ - ½alcohol
w
ꢁÞ
where K is the distribution coefficient and subscripts T, m, and w
refer to the total, micellar, and aqueous alcohol concentrations,
respectively, referred to the total solution volume. The authors
surfactant in the mixed solutions. Therefore, a decrease in Km
would also be expected, in agreement with the results.
The presence of the additives does not seem to affect k2m
-3
considered a value of K=1 mol dm for 1-butanol in the dimeric
4
3
substantially. With regardto the b values, they follow the opposite
trend shown by the second cmc. That is, when the presence of
an additive causes an increment (reduction) in C*, b decreases
micellar solutions at 303 K, with the dimeric surfactant con-
4
4
centration in eq 8 expressed per surfactant tail chain. Since Zana
has pointed out that the alcohol distribution coefficients in micellar
7
(
increases). For instance, in the presence of 1-butanol 0.2 M, the
solutions show low temperature dependence, it was assumed that
second cmc is lower than that in pure water and b is higher than
that in water. An increase in C*, for a given dimeric sur-
factant, can be taken as indicative that the tendency to micellar
growth upon increasing surfactant concentration decreases. This
was shown through cryoTEM measurements for the 12-2-
K at 298 K was similar to K at 303 K. It is worth noting that
application of eq 8 in the whole surfactant concentration range
means that variations in the shape and size of micelles do not affect
their solubilization capacity. Therefore, the alcohol concentra-
tions estimated are approximate.
-
32
bulk
1
2,2Br -DTAB mixed micellar solutions. It was also shown
k2 was experimentally determined in water-1-butanol homo-
-
-4
-4
through rheological measurements in 12-2-12,2Br micellar solu-
tions upon addition of ethylene glycol (an organic polar solvent
which remains in the bulk phase). On the other hand, addition
of alcohols, which provoke a decrease in C*, has been shown to
result in a stronger tendency to micellar growth by rheology
geneous mixtures, with its value being 1.9ꢀ10 , 2.0ꢀ10 , and
bulk
-
4
-1
3
-1
2
.2 ꢀ 10 mol dm s for 1-butanol 0.1, 0.2, and 0.3 M,
1
6
respectively, at 303 K. In relation to the k
values considered in
2
the fitting of the kinetic data, it is necessary to take into account
-
that when [12-5-12,2Br ] varies [1-butanol ] also changes and, as
w
4
7
bulk
bulk
measurements. On the basis of the meaning proposed by the
authors for b, the variations observed in this parameter in the
presence of the different additives investigated is reasonable. It is
also consistent with the results obtained for the aqueous dimeric
micellar solutions.
a result, so does k2 . With this in mind, the dependence of k2
on [1-butanol ] was fitted to a polynomial equation. This equa-
T
bulk
tion allows the authors to estimate the values of k2 correspond-
ing to the [1-butanol ] calculated for each of the dimeric
w
surfactant concentrations present in the micellar reaction media.
From the fittings of the kinetic data using eq 7, K , k2m, and
m
Concluding Remarks
b values for the two water-1-butanol micellar solutions investigated
were obtained. They are listed in Table 3. Figure 4d, together
with Figure 3e in the Supporting Information, shows the results of
the fittings.
-
The reaction methyl naphthalene-2-sulfonate þ Br was investi-
gated in several alkanediyl-R-ω-bis(dodecyldimethylammonium)
-
bromide, 12-s-12,2Br (with s = 2, 3, 4, 5, 6, 8, 10, 12), micellar
Km values listed in Table 3 show that addition of 1,2-prop,
MEGA10, and 1-butanol results in a decrease in the equilibrium
binding constant. With the scope of explaining the results, the
micropolarity of the micellar interfacial region in the presence of
additives was investigated by using the fluorescence probe pyrene-
solutions in the absence and in the presence of various additives.
The additives were 1,2-propylene glycol, which remains in the
bulk phase, N-decyl N-methylglucamide, MEGA10, which forms
mixed micelles with the dimeric surfactants, and 1-butanol, which
distributes between the aqueous and micellar phases. In all cases,
2
0
-
3
-carboxaldehyde, P3C. The fluorescence emission spectra of
with the exception of water-1,2-prop 12-5-12,2Br micellar solutions,
P3C were recorded in the presence of a micellized surfactant
with 30% weight percentage of the organic solvent, a sphere-to-rod
(
42) Gettings, J.; Hall, D.; Jobling, P. L.; Rassing, J. E.; Wyn-Jones, E. J. Chem.
Soc., Faraday Trans. II 1978, 74, 1957.
43) Graciani, M. M.; Rodrıguez, A.; Martı
Interface Sci. 2010, 342, 382.
44) (a) Benrraou, M.; Zana, R. Tenside, Surfactants, Deterg. 2005, 42. (b) Zana,
(45) Sevilla, J. M.; Domı
Chem. 1989, 13, 197.
(46) Uosaki, Y.; Kitaura, S.; Moriyoshi, T. J. Chem. Eng. Data 2005, 50, 2008.
(47) Parathakatt, S.; George, J.; Saajev, M. S.; Sreejith, L. J. Surfactants Deterg.
2009, 12, 219.
nguez, M.; Garcı
´ ´
a-Blanco, M.; Bl ꢀa zquez, M. Comput.
(
´
´
n, V. I.; Moy ꢀa , M. M. J. Colloid
(
R.; Yiv, S.; Strazielle, C.; Lianos, P. J. Colloid Interface Sci. 1981, 80, 208.
Langmuir 2010, 26(24), 18659–18668
DOI: 10.1021/la102857d 18667

Article
Graciani et al.
transition takes place upon increasing surfactant concentration.
In order to quantitatively explain the experimental data within the
whole surfactant concentration range, a kinetic equation based on
the pseudophase kinetic model was considered, together with the
decrease in the micellar ionization degree accompanying micellar
growth. However, theoretical predictions did not agree with the
experimental kinetic data for surfactant concentrations above
the morphological transition. The discussion of micellar medium
effects is always qualitative because they are the result of many
factors influencing reactivity simultaneously and it is not possible
to obtain experimental information about how each of them
affects reactivity. Taking this into account, an empirical kinetic
equation was proposed in order to explain the data. It contains a
parameter b which is assumed to account for the medium micellar
kinetic effects caused by the morphological transition. The use
of this empirical equation permits the quantitative rationalization
of the kinetic micellar effects in the whole surfactant concentra-
tion range.
It is the first time, to the authors’ knowledge, that kinetic micellar
effects on a micelle-modified reaction have been quantitatively
explained in a micellar reaction media where a morphological
transition occurs.
Acknowledgment. This work was financed by the DGCYT
(
Grant BQU2009-07478) and Consejerı
Ciencia y Empresa de la Junta de Andalucı
P07-FQM-03056).
´
a de Innovaci oꢀ n,
´
a (FQM-274 and
Supporting Information Available: Additional figures. This
material is available free of charge via the Internet at http://
pubs.acs.org.
1
8668 DOI: 10.1021/la102857d
Langmuir 2010, 26(24), 18659–18668