Micellization studies of dicationic gemini surfactants (m-2-m Type) in the presence of various counter- and Co-ions

Article abstract of DOI:10.1007/s11743-013-1453-5

Salts have the ability to influence the water activity and self-association of ionic micelles. In the present case, gemini surfactants; ethanediyl-α,ω-bis(dimethyl alkyl ammonium bromide) (referred to as m-2-m, m = 10, 12, 14) are synthesized and their micellization study in aqueous medium in presence of monovalent inorganic (NaBr, NaNO₃, NaCl, KCl, LiCl) and organic salts (NaTos, NaBenz, NaSal) at 303 K is systematically investigated by conductometric and tensiometric methods. All the salts have the tendency to lower the critical micelle concentration of the surfactants. The effect of inorganic salts on the micellization properties has been found to obey the Hofmeister series. Organic salts reduce the CMC more effectively as compared to inorganic salts. The theoretical models of Rubingh and Rosen have been used to compare the results and obtain the interaction parameters, minimum area per molecule, surface excess, mixed micelle composition, activity coefficients and free energies of micellization/adsorption.

Full text of DOI:10.1007/s11743-013-1453-5

J Surfact Deterg (2013) 16:693–707
DOI 10.1007/s11743-013-1453-5
ORIGINAL ARTICLE
Micellization Studies of Dicationic Gemini Surfactants
(
m-2-m Type) in the Presence of Various Counter- and Co-Ions
Jeenat Aslam
Wajid Husain Ansari
•
Umme Salma Siddiqui
Kabir-ud-Din
•
•
Received: 30 July 2012 / Accepted: 6 February 2013 / Published online: 7 March 2013
Ó AOCS 2013
Abstract Salts have the ability to influence the water
activity and self-association of ionic micelles. In the
present case, gemini surfactants; ethanediyl-a,x-bis
Introduction
Over the last few decades, a new type of surfactant, gemini
or dimeric, has received keen attention worldwide. The
reason is that these surfactants possess superior physico-
chemical properties over conventional surfactants. They are
made up of two amphiphilic moieties connected at the level
of the headgroups by a spacer group. Owing to their unique
structure, they have much lower critical micelle concen-
trations (CMC), better wetting, foaming, solubilizing abil-
ities, and unusual aggregation morphologies [1–7]. Gemini
surfactants of the type m-s-m are generally used for most of
the research studies, where m is the carbon number in the
alkyl chain length and s is the carbon number in the spacer
chain length. The stereochemistry of the spacer and the
alkyl chain length of gemini surfactants play significant
roles in understanding the micellization phenomenon [3–7].
Gemini surfactants with short spacers have a shorter
distance between the two alkyl chains, a characteristic
which enhances the hydrophobic interaction, restricts
hydrophobic hydration and minimizes electrostatic repul-
sion between the two alkyl chains of gemini molecule
leading to low CMC values. As a result, they reflect higher
functionalities that could lead to a reduction in their con-
sumption. This is a point worth mentioning regarding their
environmental toxicity. These surfactants are capable of
forming worm-like micelles, causing micellar growth at
low concentrations [8, 9].
(
dimethyl alkyl ammonium bromide) (referred to as m-2-
m, m = 10, 12, 14) are synthesized and their micellization
study in aqueous medium in presence of monovalent
inorganic (NaBr, NaNO , NaCl, KCl, LiCl) and organic
3
salts (NaTos, NaBenz, NaSal) at 303 K is systematically
investigated by conductometric and tensiometric methods.
All the salts have the tendency to lower the critical micelle
concentration of the surfactants. The effect of inorganic
salts on the micellization properties has been found to obey
the Hofmeister series. Organic salts reduce the CMC more
effectively as compared to inorganic salts. The theoretical
models of Rubingh and Rosen have been used to compare
the results and obtain the interaction parameters, minimum
area per molecule, surface excess, mixed micelle compo-
sition, activity coefficients and free energies of micelliza-
tion/adsorption.
Keywords Cationic gemini surfactants ꢀ Critical micelle
concentration ꢀ Mixed micelles ꢀ Synergism ꢀ Counterion ꢀ
Coion
Electronic supplementary material The online version of this
article (doi:10.1007/s11743-013-1453-5) contains supplementary
material, which is available to authorized users.
The self-aggregation or micellization of surfactants in
solution is a well-known phenomenon, which depends on
the amphiphilic species and on the conditions of the system
in which they are dissolved. It is mainly controlled by two
opposing tendencies: the removal of the nonpolar hydro-
carbon chains from the aqueous environment and the
repulsions among polar head groups, which is reasonably
J. Aslam ꢀ U. S. Siddiqui (&) ꢀ W. H. Ansari (&) ꢀ
Kabir-ud-Din
Department of Chemistry, Aligarh Muslim University,
Aligarh 202002, India
e-mail: drsalsidd@gmail.com
W. H. Ansari
e-mail: wajidhusain.chem@gmail.com
1
23

6
94
J Surfact Deterg (2013) 16:693–707
pacified by the presence of counterions to the surface of the
micelle. The narrow concentration range over which sur-
factant solutions show an abrupt change in the physico-
chemical properties is called the CMC, where micelle
formation starts taking place [10, 11]. Thus, the charac-
teristics of these aggregates are easily controlled by the
changes in the surfactant molecular structure and the
solution conditions, such as pH, concentration, tempera-
ture, additives, etc.
H C
CH3
3
2 C
H
2m+1 Br +
N
m
N
CH2
2
CH3
H C
3
CH3
CH3
H2m+1Cm
N
CH2
N
C H
m
2Br
2
2m+1,
CH3
CH3
Depending on size and nature of counterions, micellar
morphology can be controlled by addition of salts [12, 13].
Salts have the tendency to lower the CMC of ionic sur-
factants. In the past, the effect of salts on the micellization
process has been investigated in the light of Hofmeister
series (HS), where ions have been classified in order of
their ability to salt-out or salt-in proteins [14].
(
m = 10, 12, 14)
Scheme 1 Protocol for the synthesis of m-2-m geminis
-
SO3 Na+
-
+
-
+
The properties of ionic surfactants possessing small
counterions, such as halides, generally follow HS, which is
related to the polarizability of ions. Hydrophobic/chao-
tropic counterions are bound more strongly to the micellar
surface than hydrophilic/kosmotropic counterions. As
compared to kosmotropic anions, chaotropic anions are
more effective in promoting the micellar growth of ionic
surfactants [15, 16]. Therefore, a decrease of hydropho-
bicity (polarizability) of the counterions generally reduces
its affinity towards ionic micellar surfaces and the tendency
to form ion pairs leads to higher ionization degrees. This
disfavors the micellization process and gives higher CMC
values.
COO Na
OH
COO Na
CH3
sodium salicylate (Sal¯ ) sodium benzoate (Benz¯ ) sodium tosylate (Tos¯)
Scheme 2 Molecular structures of the aromatic salts (hydrotropes)
used
hydrochloric acid medium. Due to the formation of a
protective layer on the electrode surface [21], they show
maximum inhibition efficiency near their CMC values.
The micellization behavior of gemini surfactants is
significantly affected by the presence of electrolytes and
the effect is generally attributed entirely to the interaction
of counterions with the gemini micelles, as reported in the
literature [19, 22–25]. There are, furthermore, some reports
[26, 27] which indicate that co-ions can also affect the
micellization phenomenon of ionic surfactants.
However, in the case of polyatomic ions, the steric
effects and inter/intramolecular interactions play important
roles in the aggregation behavior of amphiphiles. A liter-
ature review [17–20] has thrown light on the micellization
of surfactants in the presence of various anions such as
halides or alkylsulfonates, as well as benzoate derivatives.
It was observed that the CMC and ionization degree of
micelles do not depend on a single known parameter or the
nature of the counterion (i.e., ion size, polarizability, or ion
hydrophilicity, etc.).
Experimental
Materials
Thus, for a proper understanding of the fundamental
micellar solution properties, we report micellization studies
of more efficient gemini surfactants having a shorter spacer
The chemicals 1-bromodecane (Sigma-Aldrich, USA,
C98 %), 1-bromododecane (Sigma-Aldrich, USA, C97 %),
1-bromotetradecane (Sigma-Aldrich, USA, C98 %), N, N,
N, N-tetramethylethylenediamine (s. d. fine-Chem., Mum-
bai, C99 %), propanol (E. Merck, Mumbai, C99 %), NaNO3
(Merck, Mumbai, C98 %), NaBr (s. d. fine-Chem., Mumbai,
C98 %), NaCl (s. d. fine-Chem., Mumbai, C98 %), KCl
(Merck, Mumbai, 99.5 %), LiCl (s. d. fine-Chem., Mumbai,
C98 %), sodium salicylate NaSal (Fluka, Switzerland,
C99 %), sodium tosylate NaTos (Fluka, Switzerland,
70–80 %), and sodium benzoate NaBenz (Merck, Germany,
99.5 %) were used as received. Water was distilled twice
(
m-2-m) (Scheme 1) in the presence of inorganic and
organic salts by conductometric and tensiometric tech-
-
-
niques. The small inorganic counterions (Br , Cl , NO
-
,
3
which are principally from the Hofmeister series) and
-
-
-
aromatic counterions (Tos , Benz , Sal , Scheme 2) have
been taken into account. In order to evaluate the effect of
alkyl chain length variation (m = 10, 12, 14) on the mic-
ellization process, we have kept the spacer chain length
(
s = 2) and headgroups the same in all the geminis. The
gemini surfactants used in the study have proven them-
selves to be good inhibitors of iron corrosion in a
over alkaline KMnO in an all-glass still.
4
1
23

J Surfact Deterg (2013) 16:693–707
695
Surface Tension Measurements
Pure 10-2-10
Pure 12-2-12
Pure 14-2-14
5
4
4
3
3
0
5
0
5
0
The surface tension measurements were done by a Kr u¨ ss 11
Tensiometer by the platinum ring detachment method.
Concentrated stock solution of surfactant prepared in dif-
ferent fixed concentrations of salt solution was added in
installments to a known quantity of distilled water (or salt
solution of fixed concentration as taken in the stock) in a
vessel and the readings were taken after thorough mixing
and temperature equilibration. The corrections to the
c values were made according to the procedure of Harkins
and Jordan in-built in the instrument software. The accuracy
γ
-
1
of the c value measurements was within ± 0.1 mNm
.
-
5.0
-4.5
-4.0
-3.5
-3.0
-2.5
-2.0
log [surfactant]
Results and Discussion
Fig. 1 Variation in surface tension versus concentration for the pure
gemini surfactants at 303 K
The surface tension of water decreases with an increase in
surfactant concentration. At low concentrations, the sur-
factant molecules have the tendency to adsorb at the liquid/
air interface until the surface of the solution is totally
occupied and then the excess molecules tend to self-asso-
ciate in the bulk-solution forming micelles. This results in
the invariant surface tension. The CMC for pure surfactants
and surfactant–additive systems were determined by dis-
tinct breaks in the surface tension/specific conductivity
versus concentration plots. The surface tension measure-
ment technique has been found to be more advanced over
conductometric technique, as small micellar aggregates can
be detected by the former method. Here, we have given
representative plots of surface tension versus log conc. of
pure m-2-m (m = 10, 12, 14) (Fig. 1). The CMC values for
pure gemini surfactants are in accordance with the reported
literature values [21]. The determination of CMC and the
other related parameters are dependent upon the method-
ology adopted and these values show variation by different
methods. Therefore, we have taken average CMC values
determined by tensiometry as well as conductometry for
general correlation. Variation of CMC with salt concen-
tration is presented in Fig. 2, which makes the role of
coion/counterion quite evident with regard to CMC values.
Synthesis and Characterization of Gemini Surfactants
A mixture of N, N, N, N-tetramethylethylenediamine with a
corresponding bromoalkane in dry propanol was refluxed
with continuous stirring for 48 h to synthesize ethanediyl-
a,x-bis(dimethylalkylammonium bromide) (10-2-10, 12-2-
1
2, 14-2-14) gemini surfactants [28, 29] (Scheme 1). After
evaporating the solvent, the residue was recrystallized in a
mixture of acetone and ethanol three times to give a white
product. The compound was dried in an oven for 3 days
until constant weight was attained. The yields were almost
quantitative, 90–97 %. The purities were checked via
1
H-NMR spectra recorded in CDCl solution with BRUKER
3
AVANCE 300 MHz spectrometer.
1
H NMR (300 MHz, CDCl ) (m-2-m): d = 0.9 [t, 6H,
3
2
(CH )], 1.26–1.38 [m, 36 H, (-CH -) alkyl chain], 1.81
3 2
?
?
[
m, 4H, CCH CN alkyl chain], 3.50 [s, 12H, (CH ) N ],
2 3 2
?
3
.69 [t, 4H, CCCH N alkyl chain], and 4.67 ppm [s, 4H,
2
?
N CH CH N ].
?
2
2
Conductometric Measurements
The conductance measurements were made with a Sys-
tronics conductivity meter 306, using a dip cell (cell con-
Role of Co- and Counterions on the Micellization
of Geminis
-
1
stant 0.1 cm ). The experiments were performed at 303 K
by circulating water through a jacketed cell holding the
solutions under study. Concentrated stock solutions of
surfactants were prepared in double-distilled water or in
salt of desired concentration. These stock solutions were
added to a known quantity of distilled water or salt solution
of concentration as in the stock. The conductivity at each
concentration was measured by successive addition of
concentrated surfactant stock solution to the thermostated
solution.
The CMC and counterion dissociation for the self-aggre-
gating gemini in the presence of different fixed concen-
?
?
trations of salts of Na , K and Li were determined at
?
?
303 K. The CMC forming efficiency order was K [
?
?
Na [ Li . The charge on the counterion plays significant
role in the micellization phenomenon. In Fig. 2a, the CMC
values obtained are plotted against the added salt concen-
trations. Due to the synergistic effect, on increasing salt
1
23

6
96
J Surfact Deterg (2013) 16:693–707
(
a) 6.6
(b)
10-2-10+NaSal
10-2-10+NaBenz
10-2-10+NaTos
1
1
0-2-10+NaBr
2
.5
0-2-10+NaNO3
6
6
5
5
5
4
4
.3
.0
.7
.4
.1
.8
.5
1
1
1
0-2-10+KCl
0-2-10+ NaCl
0-2-10+LiCl
2.0
1
1
0
0
.5
.0
.5
.0
0
1
2
3
4
5
0
1
2
3
4
5
[
salt] (mM)
[
salt] (mM)
(a)
(b)
1
1
1
2-2-12+NaSal
2-2-12+NaBenz
2-2-12+NaTos
0
.12
0
0
0
0
0
0
.7
.6
.5
.4
.3
.2
0
0
0
0
.10
.08
.06
.04
1
2-2-12+NaBr
1
2-2-12+NaNO3
0.02
1
1
1
2-2-12+KCl
2-2-12+NaCl
2-2-12+LiCl
0
.00
0
1
2
3
4
5
0
1
2
3
4
5
[
salt] (mM)
[
salt] (mM)
(
a)
(b) 0.05
0
.10
.08
.06
.04
.02
14-2-14+NaBr
14-2-14+NaSal
14-2-14+NaBenz
14-2-14+NaTos
1
4-2-14+NaNO3
1
1
1
4-2-14+KCl
4-2-14+NaCl
4-2-14+LiCl
0
0
0
0
.04
.03
.02
.01
0
0
0
0
0
1
2
3
4
5
0
1
2
3
4
5
[
salt] (mM)
[
salt] (mM)
Fig. 2 Values of CMC of the gemini surfactants m-2-m (m = 10, 12, 14) at different concentrations of a inorganic salts, and b organic salts
concentration, a decrease in CMC values of gemini sur-
factants is observed. This can be understood by considering
the positive and negative contributing factors in the mic-
ellization process. The primary driving force in micelliza-
tion is the hydrophobic effect associated with the alkyl
chain association [10], which promotes the release of water
molecules and solvates the apolar chain. Due to the
hydrophobic effect or the assembly of the amphiphilic
monomers, a net entropy increase in the system takes place.
There is a balance between the electrostatic force among
the amphiphilic headgroups with their counterions and
water at the micellar surface. Neutral ion pairs are formed
1
23

J Surfact Deterg (2013) 16:693–707
697
is more favorable in the presence of NaBr. As a result, the
CMC decrease is larger in NaBr than NaCl. As regards
other physical properties of the ions, it has been found that
-
14-2-14
0
0
0
0
0
0
0
.026
.024
.022
.020
.018
.016
.014
Cl
the hydration number (n ) increases with the CMC,
H
whereas the partial molar volume (v ), the polarizability
s
(
p) and the lyotropic number (N) decrease with increasing
CMC [34–36]. In the present case too, a decrease in the
CMC is found with the increasing polarizability of coun-
terions (Fig. 3, where the polarizability (p) versus the CMC
of 14-2-14 in the presence of 5 mM of NaBr, NaNO and
3
-
NO
NaCl is plotted).
3
To extend our study further we have also chosen salts
having an aromatic benzene ring in their structure (NaBenz,
NaTos, NaSal, Scheme 2). These organic salts, also refer-
red to as ‘hydrotropes’, are surface active and highly water
soluble, which increase the solubility of solutes in water.
Like surfactants, they have hydrophilic and short/cyclic
hydrophobic groups. NaBenz contains a carboxylate
group, NaSal contains a carboxylate and a hydroxyl group,
whereas NaTos has a sulfonate group attached to the
benzene ring. The difference in behavior can be explained
by taking into account the structure, nature, and the relative
basicity of the groups attached to these salts which leads to
the following hydrophilic ranking [37].
-
Br
3
.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0
Polarizability (p)
Fig. 3 Correlation between the polarizability (p) and the CMC of
4-2-14? inorganic salts (NaBr, NaNO , and NaCl)
1
3
by the positively charged headgroups and negatively charged
counterions in solution that are less hydrated than free ions,
which ultimately leads to the release of water into the bulk
with the entropy increase. The more hydrophobic the coun-
terion, the more strongly it interacts with an amphiphilic
micellar interface (leading to stronger ion-pair formation),
hence favoring micelle formation by reducing the CMC.
The position of an ion in the Hofmeister series (HS) is
ꢁ
ꢁ
ꢁ
COO [ ꢁ SO
As NaTos has a –SO , which is less hydrophilic than
3
-
3
-
–COO of NaBenz, the CMC decrease is slower with the
former salt. It is well known that among different
hydrotropes, NaSal is the most effective towards cationic
surfactants [38]. Salicylate is the benzoate-derived
counterion, which has a delocalized negative charge
making it more hydrophobic (despite the presence of a
hydrophilic –OH group). Salicylate has planar geometry
which restricts the rotation of the carboxylate and further
stabilizes the hydrogen bond. Due to this intramolecular
hydrogen bond in the salicylate ion, it is strongly
considered to depend upon its hydrated radius (r ), or
h
polarizability (p) and charge. The hydrated radius of the
anion is inversely proportional to its polarizability. An
anion having large polarizability is expected to enhance the
binding of the counterion at the micellar surface and also
decreases the electrostatic repulsion between the head-
groups of the surfactant molecules, thus increasing the
tendency of micellization and lowering both the CMC and
p. Therefore, a typical HS for anions is as follows: Ace-
-
hydrophobic [23]. The orientation of –COO (with respect
to –OH) is responsible for the growth.
-
-
-
-
-
-
-
tate \ Cl \ NO -\ Br \- ClO \ I \ CNS (the
We can see that in the absence of salts, the CMC is not
as lowered as in the presence of salts (Fig. 2b) which
clearly indicates that at zero salt concentration, the sur-
factant molecules are not as tightly packed as when the
micelles are formed in the presence of salts. In the absence
of any salt counterion, the positively charged headgroups
of gemini molecules tend to keep surfactant molecules
away from each other due to electrostatic repulsion. As the
addition of a salt takes place, a screening of the effective
positive charges in the headgroup weakens the electrostatic
3
4
positions of the NO3 and Br ions are often switched in
the HS) [30–32]. All the physical properties of the ions
correlate well to the CMC as long as the ions have similar
electronic configuration and similar morphologies. As the
-
-
hydrated size of Cl is larger than that of Br [33], the
mobility of the former is smaller, and its ability to increase
micropolarity is weaker. The counterions are adsorbed at
the positively charged head groups of the micelle. Less
-
hydrated Br binds more strongly and, therefore, it can
-
more strongly screen the interaction between gemini
-
headgroups. Br , with a larger diameter, has a stronger
repulsion. In addition, with cationic amphiphiles, Sal /
-
-
Benz intercalates between the headgroups. The –COO
ability to suppress the Stern layer and reduces the curvature
of the aggregate. Thus, the formation of larger aggregates
group interacts with the positive charge of another micelle
reducing its surface charge. In this way the micelles come
1
23

6
98
J Surfact Deterg (2013) 16:693–707
closer to each other, making the hydrophobic interaction
relatively stronger. Both these factors are responsible for
lowering the CMC.
of hydrotropes are observed [42]. All these changes are
responsible for enhancing the hydrophobic environment in
the gemini-hydrotrope system (Scheme 3).
Ion-specificity is considered to be an important factor
for micellar transition. A carboxylate headgroup is con-
sidered ‘hard’ and a sulfonate headgroup is considered as
Thus, the reduction in CMC on addition of hydrotropes
to gemini surfactant solutions is due to two important
factors. The partitioning of hydrotropes and their adsorp-
tion on the headgroups of geminis (i) reduces the electro-
static repulsions, and (ii) enhances the hydrophobic
character, thereby reducing the concentration where
aggregation begins. Besides, the possibility of hydrogen
bonding cannot be ignored.
‘
soft’, therefore, their interactions with soft ammonium
headgroups are different and thus hydrotropes can be
compared to inorganic ions. This can be attributed to lower
hydrophilicity of the sulfonate compared to the carboxylate
moiety [39] or by a stronger interaction of the cationic
surfactant headgroups with sulfonate groups compared to
carboxylates as per their relative positions in the Hofmei-
ster series [40]. There are two types of ions: ‘hard’ ions
have high charge density and high polarizing power and
Role of Spacer and Alkyl Chain Length in the Gemini
Micellization
‘
soft’ ions have low charge density and low polarizing
The micellization and adsorption properties of cationic
gemini surfactants are strongly affected by the alkyl chain
length and the nature of the spacer. For s = 2, as the distance
between two alkyl chains is short, the hydrophobic interac-
tion is promoted. As a result, the hydrophobic hydration is
restricted and electrostatic repulsion is minimized between
the two alkyl chains of the gemini molecules, producing
lower CMC values. It can be inferred from the Table 1 data
that, when we move from m = 10 to m = 14 (alkyl chain
length), the hydrophobic interactions are considered to be a
major driving force. During micelle formation, the water
molecules in hydration shell around the hydrophobic parts of
monomeric amphiphiles are released, also resulting in a
greater entropy increase and giving rise to micellization at a
lower concentration (lower CMC). With s remaining the
same in the three gemini surfactants used herein, the decrease
in CMC follows a behavior akin to the alkyl chain length
effect of conventional surfactants.
power. According to the ‘matching water affinity’ concept
of Collins [41], soft ions come into contact with soft ions
and hard ions come into contact with hard ions. However,
if hard ones come into contact with soft ions, they do not
come into proximity and their hydration spheres remain
intact, hence interacting weakly with each other. Therefore,
such mixtures, where quaternary ammonium headgroups
behave like soft ions, are responsible for enhancing the
potential performance of gemini surfactants.
The reduction in CMC values in the case of the anionic
hydrotropes and gemini surfactant systems indicates the
existence of synergism between the two. An attractive
interaction is operating between the two components. In
addition to charge neutralization, intercalation of the
hydrophobic part of the hydrotropes into gemini micelles
also occurs. As can be seen (Scheme 2), all the hydrotropes
used herein contain a hydrophobic benzene ring. Due to
the interaction of the positively charged headgroup of the
gemini with the p-electron cloud of the benzene ring, the
hydrophobic interaction increases. Whereas, at high con-
centration of hydrotropes, due to higher interaction, the
additional p-p aromatic interactions between the headgroups
Surface Properties
Gemini molecules are found to be more tightly packed at
the interface and the cCMC decreases considerably with
Scheme 3 Schematic
representation of mixed micelle
formation of cationic gemini-
organic counterion (hydrotrope)
systems
1
23

J Surfact Deterg (2013) 16:693–707
699
0
m
0
ads
Table 1 Values of CMC, PCMC, Cmax, Amin, DG and DG for gemini surfactant 14-2-14? salt systems at 303 K
-
1
)
7
-2
)
2
0
m
-1
0
ads
-1
˚
Additives (mM)
CMC (mM)
PCMC (mN m
U
max 10 (mol m
A
min (A )
DG (kJ mol
)
DG (kJ mol )
LiCl ? 14-2-14
0
0.118
0.098
0.087
0.058
0.053
0.044
35.41
37.56
37.66
38.14
39.58
43.83
7.66
8.33
8.41
9.54
9.97
10.50
217
199
197
174
167
158
-32.90
-33.36
-33.66
-34.69
-34.90
-35.39
-37.52
-37.87
-38.14
-38.69
-38.87
-39.56
0
1
2
3
5
.5
NaCl ? 14-2-14
0
1
2
3
5
.5
0.074
0.057
0.044
0.030
0.026
38.12
39.50
39.53
39.75
40.67
8.99
9.81
185
169
146
132
130
-34.06
-34.71
-35.39
-36.31
-36.71
-38.30
-38.74
-38.86
-39.47
-39.90
11.38
12.59
12.76
KCl ? 14-2-14
0
1
2
3
5
.5
0.068
0.054
0.038
0.028
0.024
38.95
40.26
40.98
39.62
40.92
9.19
10.87
11.50
13.31
13.55
181
153
144
125
123
-34.27
-34.87
-35.72
-36.53
-36.92
-38.51
-38.58
-39.29
-39.50
-39.94
3
NaNO ? 14-2-14
0
1
2
3
5
.5
0.052
0.036
0.028
0.021
0.018
41.52
42.92
43.31
45.19
45.11
10.34
11.50
11.76
13.36
14.33
161
144
141
124
116
-34.97
-35.86
-36.48
-37.25
-37.64
-38.98
-39.59
-40.16
-40.63
-40.79
NaBr ? 14-2-14
0
1
2
3
5
.5
0.050
0.035
0.024
0.018
0.015
41.20
42.02
43.12
41.04
43.96
12.27
13.25
14.36
14.45
15.36
135
125
116
115
109
-35.04
-38.40
-39.10
-39.92
-40.41
-40.96
-35.928
-36.915
-37.570
-38.099
NaTos ? 14-2-14
0
1
2
3
5
.5
0.046
0.040
0.021
0.014
0.011
38.67
41.98
42.76
43.06
43.38
12.46
13.03
13.58
14.87
15.37
133
127
122
112
108
-35.25
-35.63
-37.25
-38.18
-38.77
-38.35
-38.85
-40.40
-41.08
-41.59
NaBenz ? 14-2-14
0
1
2
3
5
.5
0.043
0.037
0.012
0.009
0.007
40.01
40.63
41.28
41.33
41.57
12.71
13.62
14.35
15.52
15.81
131
122
116
107
105
-35.45
-35.79
-38.56
-39.39
-40.02
-38.60
-38.77
-41.44
-42.05
-42.65
NaSal ? 14-2-14
0
1
2
3
5
.5
0.029
0.024
0.009
0.007
0.006
35.52
35.57
36.70
36.88
36.71
13.08
14.63
14.67
16.28
16.66
127
114
113
102
100
-36.40
-36.86
-39.25
-39.85
-40.41
-39.11
-39.29
-41.75
-42.11
-42.61
1
23

7
00
J Surfact Deterg (2013) 16:693–707
increasing salt concentration. The variation of PCMC (the
surface pressure at the CMC), Cmax (the maximum surface
gemini surfactant ions at the micellar surface, the Amin value
shows a decrease with increasing concentration of salts
(Fig. 5). It also confirms that the gemini surfactant molecules
are almost perpendicularly located at the micellar interface
0
excess), Amin (minimum surface area per molecule), DG
m
0
(
the standard Gibbs energy of micellization) and DGads (the
[44].
standard Gibbs energy of adsorption) values, obtained at
different concentrations of the added salts in 14-2-14
solutions, are collected in Table 1. However, for the other
two geminis (10-2-10 and 12-2-12), these parameters are
given in supporting information (Tables T1a, T1b).
Detailed explanations for these parameters are given
below.
Effect of Salt Counterions on g-Values
Various factors play an important role in the addition of salt
counterions. The present counterions in the solution are
electrostatically attracted to the charged micelles and are
adsorbed into the inner electrical double layer, partially
neutralizing the surface charge. The extent of charge neu-
tralization is known as counterion binding. The degree of
dissociation (g) was obtained from the ratio of the post- and
pre-CMC linear courses (using the conductance isotherms)
by the relation, g = post-CMC slope/pre-CMC slope. It has
been explained that the degree of dissociation (g) depends on
the nature and concentration of added salts, the coions do not
affect the values of g much. As the degree of counterion
binding also depends upon the surface charge density of the
micelle, the greater the charge density, the greater will be the
counterion binding with a smaller surface area per head
group. It shows no regular variation of g values (Supporting
information, Fig. S1). The g values were found to increase
with the increase in the salt concentration and this could be
due to the reduction in the charge density on the micellar
surface and release of counterions, while the decrease at still
higher values of salt concentration may be due to higher
counterion binding on the micelle surface.
Effect of Salt Counterions on PCMC
The PCMC values were obtained by using equation:
PCMC ¼ c ꢁ c
ð1Þ
are the surface tension of the solvent
0
CMC
where c and c
0
CMC
and the surface tension of the mixture at the CMC,
respectively. With an increase in salt concentration, the
increase in PCMC values indicates increased efficiency.
The trend is as follows: 10-2-10 \ 12-2-12 \ 14-2-14
(
Tables 1, TS1a, TS1b).
Effect of Salt Counterions on Cmax
The Gibbs Eq. (2) [43] is used for calculating Cmax of the
gemini molecules at the air/water interface
Cmax ¼ ðꢁ1=2:303 nRTÞðdc=d log CÞT
ð2Þ
where
R
and
-
T
-1
K
are the universal gas constant
) and temperature, respectively. The
1
(
8.314 J mol
Effect of Salt Counterions on the Thermodynamics
of Micelle Formation
prefactor n is the number of species which are adsorbed at
the air/water interface. In the present case, we have taken
n = 3. The slope of the tangent at the given concentration
of the c versus log C plot was used to calculate Cmax. The
Cmax values increase with an increase in salt concentration
A thermodynamic description of the process of micelle for-
mation includes a description of both electrostatic and
hydrophobic contributions to the overall Gibbs energy of the
system. The hydrophobic Gibbs energy (transfer Gibbs
energy) is defined as Gibbs energy for the process of trans-
ferring the hydrocarbon solute from the hydrocarbon solvent
to water. Micelle formation in an aqueous medium is a ther-
modynamically favored and a spontaneous process accom-
panied by a significant decrease in free energy, and the driving
force behind it is the hydrophobic bonding accompanied by
desolvation. Thus, the phenomenon of micellization is an
energetically controlled process, where the formation of the
micelle is well under thermodynamic control.
(
Tables 1, TS1a, TS1b, Fig. 4). It can be inferred from the
results that, in the presence of salts, the gemini surfactant
molecules have a greater tendency to be adsorbed at the air/
water interface, compared to that in the absence of salts.
The presence of salts reduces the repulsion among head
groups and more gemini surfactant molecules can be
adsorbed at the interface. This can be directly related to the
minimum area per headgroup (Amin) values as given below.
Effect of Salt Counterions on Amin
Amin is calculated using the equation:
For all the geminis, the Gibbs free energy of micelli-
zation [45] was calculated using equation:
2
0
0
m
Amin ¼ 10 =N C
ð3Þ
where N is Avogadro’s number. Due to the effective charge
DG ¼ ð3 ꢁ 2gÞ RT: ln CMC12
ð4Þ
A
max
A
In this equation, the CMC12 is the CMC of the mixture of
the two components at a given mole fraction. For
shielding in the presence of salts and the tight packing of the
1
23

J Surfact Deterg (2013) 16:693–707
701
(
a)
(b)
1
0-2-10+NaSal
10-2-10+NaBenz
0-2-10+NaTos
1
0-2-10+NaBr
1
7
1
1
1
1
1
4
3
2
1
0
9
10-2-10+NaNO3
1
1
1
0-2-10+KCl
0-2-10+NaCl
0-2-10+LiCl
1
16
1
1
1
5
4
3
Γ
Γ
12
11
0
1
2
3
4
5
0
1
2
3
4
5
[
salt] (mM)
[
salt] (mM)
(
a)
(b) 16.5
14
1
1
1
1
1
1
6.0
5.5
5.0
4.5
4.0
3.5
1
1
1
1
3
2
1
0
9
Γ
Γ
13.0
12-2-12+NaBr
1
2-2-12+NaNO3
1
1
1
2.5
2.0
1.5
12-2-12+NaSal
2-2-12+NaBenz
12-2-12+NaTos
1
2-2-12+KCl
1
12-2-12+NaCl
2-2-12+LiCl
1
0
1
2
3
4
5
0
1
2
3
4
5
[
salt] (mM)
[
salt] (mM)
(
a)
(b) 17
1
1
1
1
1
1
5
4
3
2
1
0
9
8
1
1
6
5
Γ
14
Γ
1
1
4-2-14+NaBr
4-2-14+NaNO3
1
1
3
2
14-2-14+NaSal
14-2-14+NaBenz
4-2-14+NaTos
14-2-14+KCl
1
1
4-2-14+NaCl
4-2-14+LiCl
1
0
1
2
3
4
5
0
1
2
3
4
5
[
salt] (mM)
[
salt] (mM)
Fig. 4 Values of Cmax of the gemini surfactants m-2-m (m = 10, 12, 14) at different concentrations of a inorganic salts, and b organic salts
1
23

7
02
J Surfact Deterg (2013) 16:693–707
1
1
1
1
1
0-2-10+NaBr
0-2-10+NaNO3
0-2-10+KCl
0-2-10+NaCl
0-2-10+LiCl
(
a) 180
(b)
10-2-10+NaSal
1
0-2-10+NaBenz
140
130
120
110
100
10-2-10+NaTos
1
1
1
1
1
1
70
60
50
40
30
20
A
A
0
1
2
3
4
5
0
1
2
3
4
5
[
salt] (mM)
[salt] (mM)
(
a) 200
(
b)140
1
2-2-12+NaSal
2-2-12+NaBenz
12-2-12+NaTos
1
2-2-12+NaBr
12-2-12+NaNO3
2-2-12+KCl
12-2-12+NaCl
1
1
1
1
1
1
1
1
1
90
80
70
60
50
40
30
20
1
1
1
1
1
1
1
1
35
30
25
20
15
10
05
00
1
1
2-2-12+LiCl
A
A
0
1
2
3
4
5
0
1
2
3
4
5
[salt] (mM)
[salt] mM
(
a) 200
14-2-14+NaBr
(b)135
14-2-14+NaSal
1
4-2-14+NaNO3
14-2-14+NaBenz
1
1
1
1
1
1
1
1
1
90
80
70
60
50
40
30
20
10
130
14-2-14+NaTos
1
1
1
4-2-14+KCl
4-2-14+NaCl
4-2-14+LiCl
1
1
1
1
25
20
15
10
A
A
105
00
1
95
0
1
2
3
4
5
0
1
2
3
4
5
[salt] (mM)
[salt] (mM)
Fig. 5 Values of Amin of the gemini surfactants m-2-m (m = 10, 12, 14) at different concentrations of a inorganic salts, and b organic salts
1
23

J Surfact Deterg (2013) 16:693–707
703
surfactants with low CMC values (below 10 mM), the
0
In order to investigate interactions between two com-
pounds at an interface or in micelles, the so-called
b parameters (interaction parameters) are calculated by
using Rubingh’s and Rosen’s approach [48], which are
conveniently obtained from surface (or interfacial) tension
or from CMC data by using the well-known Eqs. (6–9). By
knowing the b parameters, the nature and strength of the
values of DG would only differ by constant &ln 55.5
when using one or the other unit [5] (for the micellization
m
in pure water, the number of moles of solvent is taken as
-
3
0
5
5.5 mol dm ). The negative values of the DGm indicate
that micellization process is spontaneous in an aqueous
medium (Table 1). The low CMC values of the m-2-m
surfactants arise mainly because more than one chain is
transferred simultaneously from background solvent to the
micelle.
interaction between the two components can be ascertained
m
(
b
is the interaction parameter for mixed micelle forma-
r
tion in an aqueous medium and b is the interaction
parameter for mixed monolayer formation at an aqueous
solution/air interface). The average CMC values of NaTos,
NaBenz, and NaSal used in the calculation are 199.10,
290.17, 577.84 mM, respectively.
Further, the standard Gibbs energy of adsorption [46],
0
ads
DG is calculated using
0
ads
0
m
DG ¼ DG ꢁ PCMC=Cmax
ð5Þ
For mixed micellar systems the Rubingh’s approach is
applied as:
The standard state for the adsorbed surfactant is a
hypothetical monolayer at its minimum surface area per
molecule, but at zero surface pressure. The last term in Eq.
h
i
ꢀ
ꢁ
ꢀ
ꢁ
2
m
1
m
1
X
ln CMC12 a1=CMC1X
(
5) expresses the work involved in transferring the
ꢀ
ꢁ
ꢂ
ꢀ
ꢁꢃ ¼ 1
2
m
m
1
ð6Þ
ð7Þ
surfactant molecule from a monolayer at a zero surface
pressure to the micelle. All the negative values obtained
imply that the adsorption of the surfactants at the air/water
1 ꢁ X1 ln CMC ð1 ꢁ a Þ=CMC 1 ꢁ X
12
1
2
ꢀ
ꢁ
m
1
interface takes place spontaneously in the order: 10-2-
ln CMC12a1=CMC1X
m
b ¼
ꢀ
ꢁ
0
ads
2
1
0 \ 12-2-12 \ 14-2-14. The average values for DG
m
1 ꢁ X1
for salts show the following trend: NaSal [ NaBenz [
(
CMC , CMC , and CMC denote the experimental CMC
1
2
12
NaTos [NaBr [NaNO [KCl [NaCl [LiCl (Table 1),
3
values of hydrotrope, surfactant, and their binary mixture,
m
respectively, and X1 is the micellar mole fraction of the
which is in accordance with our previous discussion.
hydrotrope in the mixed micelle).
Organic Salt Effect on Interaction Parameters
Analogously, for a mixed monolayer of micelles,
Rosen’s approach is applied as:
As reported earlier, organic salts have additional hydro-
phobic interactions besides an electrostatic one. They have
a tendency to penetrate the micellar surface leading to
micellar growth with lower loading of bulky organic
counterions. An induction of strong hydrophobic interac-
tion and reduction of electrostatic repulsion between the
headgroups lead to the formation of tightly packed reduced
curvature aggregates. Addition of hydrotropes (organic
salts), bearing an opposite charge and hydrophobicity,
reduces the electrostatic repulsion between the headgroups,
whereas the hydrophobic interaction increases to the extent
of being more with a surfactant having a larger alkyl chain
length. As a result, more stable mixed systems with higher
alkyl chain lengths of geminis are formed due to the syn-
ergistic interactions. Thus, a mixture of hydrotropes with
gemini surfactants leads to the formation of mixed aggre-
gates because of the different surface activity of the
components.
h
i
ꢀ
ꢁ
ꢀ
ꢁ
2
r
1
r
X
ln conc12a1=conc1 X
1
ꢀ
ꢁ2
ꢂ
ꢀ
ꢁꢃ ¼
1
ð8Þ
ð9Þ
r
r
1
1
ꢁ X1 ln conc ð1 ꢁ a Þ=conc 1 ꢁ X
12
1
2
ꢀ
ꢁ
r
1
ln conc a =conc X
12
1
1
r
b ¼
ꢀ
ꢁ
2
r
1
ꢁ X1
(where, conc , conc , and conc denote the concentrations
1
2
12
of hydrotrope, surfactant, and their binary mixture,
r
respectively, and X is the micellar mole fraction of the
1
hydrotrope in the mixed micelle).
A positive b value signifies repulsive interaction among
mixed species, whereas a negative value signifies an
attractive interaction. The more negative values evidence
m
strong interaction. In all the mixed systems, negative b
values are obtained (Table 2, TS2a, TS2b, Fig. 6), which
suggest that the interaction between the two components is
more attractive in mixed micelles as compared to the self-
interaction of the two components before mixing. With an
increasing mole fraction of hydrotropes, due to the inter-
calation of counterions in the gemini micelles, attractive as
well as hydrophobic interactions increase and hence more
Mixed micelles formed in the solutions of such nonho-
mogeneous surface active materials are expected to be
nonideal. The nonideal mixing is quantified using
Rubingh’s model [47]. This model is based on Regular
Solution Theory (RST) for nonideal mixed systems.
m
negative b values and low CMC values are obtained.
1
23

7
04
J Surfact Deterg (2013) 16:693–707
r
For the mixed monolayer (Rosen’s approach), the b
factors, such as chain length and structure, also affect the
interactions.
values also show a similar trend (Table 2, TS2a, TS2b,
Fig. 6), i.e., the salt-gemini mixtures have a stronger
attractive interaction at the solution/air interface. Further,
in most of the cases, with increase in alkyl chain length, the
attractive interactions of organic salt-gemini systems
increase.
Conclusions
A comprehensive study of the ion specific effect of the salt
counterions on the micellization of cationic gemini surfac-
tants (m-2-m, m = 10, 12, 14) were performed by conduc-
tometric and tensiometric measurements: The micellization
of gemini surfactants in the presence of salts occurs at lower
concentrations. The inorganic counterions have been found
to affect the micellization of geminis by obeying the Hof-
meister series. The organic salts have been found to decrease
the CMC more effectively than inorganic salts and the trend
observed is found to be NaSalicylate [ NaBenzoate [
The b values are related to the activity coefficients (f ) in
i
the mixed systems as per the following equations:
h
i
ꢀ
ꢁ
2
m
m
m
f1 ¼ exp b 1 ꢁ X1
ð10Þ
ð11Þ
ð12Þ
ð13Þ
h
i
ꢀ
ꢁ
2
m
m
m
f2 ¼ exp b X1
h
i
ꢀ
ꢁ
2
r
1
r
r
f ¼ exp b 1 ꢁ X
1
h
i
ꢀ
ꢁ
2
r
2
r
r
1
NaTosylate [NaBr [NaNO [KCl [NaCl [LiCl. With
f ¼ exp b X
3
an increase in salt concentration, the increase in PCMC
values indicates increased efficiency. The trend is as fol-
lows: 10-2-10 \ 12-2-12 \ 14-2-14. The Amin value shows
a decrease with an increasing concentration of salts. It also
confirms that the gemini surfactant molecule is almost
perpendicularly located at the micellar interface. The Cmax
values increase with an increase in salt concentrations,
which confirm that in the presence of salts, the gemini sur-
factant molecules have a greater tendency to become
adsorbed at the air/water interface, compared to that in the
The activity coefficients of gemini surfactants (f ) are
2
found to be higher than that of the hydrotropes (f1)
(
Table 2), which are less than unity, indicating nonideal
behavior and synergistic interaction between the two
components.
The greater the value of the interaction parameter, the
greater the extent of nonideality in the system and the
smaller the value of the activity coefficient. The low
hydrophobicity and the higher CMC values, besides other
m
r
m
r
m
m
r
r
Table 2 Micellar compositions (X
gemini surfactant 14-2-14 at different mole fractions of salts at 303 K
1 1 1 2 1 2
, X ), interaction parameters (b , b ), and activity coefficients (f , f , f , f ) of binary mixtures of cationic
m
1
m
b
4
10 . f
m
1
m
2
r
1
r
4
10 . f
r
1
r
a
salt
X
f
X
b
f
2
NaTos ? 14-2-14
0
0
0
0
0
.5
0.289
0.323
0.370
0.388
0.405
-15.457
-17.014
-21.129
-23.344
-25.310
4.04
4.11
2.28
1.60
1.28
0.2750
0.1694
0.0554
0.0298
0.0158
0.290
0.337
0.376
0.391
0.406
-15.704
-18.840
-22.570
-24.399
-25.988
3.6464
2.5312
1.5245
1.1745
1.0419
0.2669
0.1177
0.0411
0.0239
0.0138
.66
.8
.85
.9
NaBenz ? 14-2-14
0
0
0
0
0
.5
0.289
0.321
0.377
0.392
0.407
-16.357
-17.858
-23.961
-25.989
-27.979
2.56
0.2550
0.1588
0.0332
0.0184
0.0097
0.291
0.326
0.383
0.394
0.410
-16.144
-17.930
-24.605
-25.782
-28.117
2.9891
2.9004
0.8550
0.7727
0.5616
0.2549
0.1488
0.0270
0.0183
0.0089
.66
.8
2.66
0.914
0.673
0.533
.85
.9
NaSal ? 14-2-14
0
0
0
0
0
.5
0.301
0.331
0.375
0.387
0.401
-19.188
-20.954
-26.260
-27.850
-29.729
0.848
0.845
0.351
0.285
0.233
0.1758
0.1007
0.0249
0.0154
0.0084
0.282
0.310
0.366
0.380
0.394
-17.459
-18.648
-24.633
-26.326
-27.828
1.2336
1.3941
0.5009
0.4029
0.3646
0.2494
0.1667
0.0369
0.0223
0.0133
.66
.8
.85
.9
1
23

J Surfact Deterg (2013) 16:693–707
705
m
r
Fig. 6 Values of b , b of the
gemini surfactants m-2-m
1
1
0-2-10+NaSal
0-2-10+NaBenz
10-2-10+NaTos
-12
10-2-10+NaSal
10-2-10+NaBenz
10-2-10+NaTos
-
-
-
-
-
12
13
14
15
16
(
m = 10, 12, 14) at different
-
-
-
13
14
15
mole fractions (asalt) of organic
salts (NaSal, NaBenz, NaTos)
β
β
-17
-16
-
-
-
-
18
19
20
21
-
-
17
18
-19
0
.00
0.04
0.08
0.12
0.16
0.20
0
.00
0.04
0.08
0.12
0.16
0.20
α
α
salt
salt
-
-
18
20
-
-
-
-
-
-
-
-
18
20
22
24
26
28
30
32
-22
1
1
1
2-2-12+NaSal
2-2-12+NaBenz
2-2-12+NaTos
1
2-2-12+NaSal
-
-
-
-
24
26
28
30
β
β
12-2-12+NaBenz
12-2-12+NaTos
0.2
0.3
0.4
α
0.5
0.6
0.7
0.8
0.2
0.3
0.4
α
0.5
0.6
0.7
0.8
salt
salt
-
-
-
-
-
-
-
-
-
14
16
18
20
22
24
26
28
30
-14
1
1
1
4-2-14+NaSal
4-2-14+NaBenz
4-2-14+NaTos
1
4-2-14+NaSal
14-2-14+NaBenz
4-2-14+NaTos
-
-
-
-
-
-
-
-
16
18
20
22
24
26
28
30
1
β
β
0.5
0.6
0.7
0.8
0.9
0.5
0.6
0.7
0.8
0.9
α
salt
α
salt
0
absence of salts. The negative values of the DGm indicate
that micellization process is spontaneous in an aqueous
surfactants at the air/water interface takes place spontane-
0
0
ously. In both the cases of DG and DG , more negative
m ads
0
ads
medium. The average values for DG for salts show the
following trend: NaSalicylate [ NaBenzoate [ NaTosy-
late [ NaBr [ NaNO [ KCl [ NaCl [ LiCl. All the
negative values obtained imply that the adsorption of the
values are obtained with increasing alkylchain length of
gemini and shows following trend: 10-2-10 \ 12-2-12 \
14-2-14. Mixtures of hydrotropes (organic salts) with
gemini surfactants lead to the formation of mixed aggregates
3
1
23

7
06
J Surfact Deterg (2013) 16:693–707
because of the different surface activity of the components.
With an increasing mole fraction of hydrotropes, due to the
intercalation of salt counterions in the gemini micelles, the
attractive interaction as well as hydrophobic interaction
15. Romsted L, Yao J (1996) Arenediazonium salts: new probes of
the interfacial compositions of association colloids. 4. 1–3 Esti-
mation of the hydration numbers of aqueous hexaethylene glycol
monododecyl ether, C12E6. Micelles by chemical trapping.
Langmuir 12:2425–2432
16. Romsted L (2007) Do amphiphile aggregate morphologies and
interfacial compositions depend primarily on interfacial hydration
and ion-specific interactions? The Evidence from chemical trap-
ping. Langmuir 23:414–424
m
increase and hence more negative b values and low CMC
values are obtained. For the salt-gemini mixtures, attractive
interactions and X1 values are nearly equal in mixed
micelles and monolayers. Further, with an increase in alkyl
chain length of gemini, the attractive interactions of organic
salt-gemini systems increase. The activity coefficients of
17. Brady JE, Evans DF, Kachar B, Ninham BW (1984) Spontaneous
vesicles. J Am Chem Soc 106:4279–4280
1
8. Moroi Y, Murata Y, Fukuda Y, Kido Y, Seto W, Tanaka M
1992) Solubility and micelle formation of bolaform-type sur-
(
gemini surfactants (f ) are found to be higher than those of
2
factants: hydrophobic effect of counterion. J Phys Chem 96:
8610–8613
the hydrotropes (f ), which are less than unity, indicating
1
nonideal behavior and a synergistic interaction between the
two components. The nature and the structure of salts pri-
marily govern the morphology of the gemini. Thus, these
systems may be utilized for tuning the micellar morphology
or for reduction of CMC.
19. Bijma K, Engberts J (1997) Effect of counterions on properties of
micelles formed by alkylpyridinium surfactants. 1. Conductom-
1
etry and H-NMR chemical shifts. Langmuir 13:4843–4849
2
2
0. Debnath S, Dasgupta A, Mitra R, Das P (2006) Effect of coun-
terions on the activity of lipase in cationic water-in-oil micro-
emulsions. Langmuir 22:8732–8740
1. Achouri MEl, Bensouda Y, Gouttaya HM, Nciri B, Perez L,
Infante MR (2001) Gemini surfactants of the type 1,2-Etha-
nediylbis-(dimethylalkylammonium bromide). Tenside Surf
Deterg 38:208–215
Acknowledgments USS and JA acknowledge CST-UP, Lucknow
(CST-790) and UGC, respectively, for providing financial assistance.
KU is grateful for UGC-BSR Faculty Fellowship award.
2
2
2. Wattebled L, Laschewsky A (2007) Effects of organic salt
additives on the behavior of dimeric (‘‘Gemini’’) surfactants in
aqueous solution. Langmuir 23:10044–10052
3. Jiang L, Peng Y, Yan Y, Deng M, Wang Y (2004) Micellization
of cationic gemini surfactants with various counterions and their
interaction with DNA in aqueous solution. J Phys Chem B
108:15385–15391
References
1
2
3
4
. Rosen MJ (2004) Surfactants and interfacial phenomena. Wiley
Interscience, New York
. Rosen MJ, Tracy DJ (1998) Gemini surfactants. J Surf Deterg
24. Manet S, Karpichev Y, Bassani D, Ahmad RK, Oda R (2010)
Counteranion effect on micellization of cationic gemini surfac-
tants 14-2-14: hofmeister and other counterions. Langmuir
26:10645–10656
25. Khan F, Siddiqui US, Khan IA, Kabir-ud-Din (2012) Physico-
chemical study of cationic gemini surfactant butanediyl-1,4-bis
(dimethyldodecylammonium bromide) with various counterions
in aqueous solution. Colloid Surf A 394:46–56
26. Muller N, Birkhahn RH (1968) Investigation of micelle structure
by fluorine magnetic resonance. II. Effects of temperature chan-
ges, added electrolyte, and counterion size. J Phys Chem 72:583–
588
1
:547–554
. Zana R (1998) In: Holmberg K (ed) Novel surfactants. Marcel
Dekker, New York
. Zana R, Benrraou M, Rueff R (1991) Alkanediyl-a,x-bis(dim-
ethylalkylammonium bromide) surfactants. 1. Effect of the spacer
chain length on the critical micelle concentration and micelle
ionization degree. Langmuir 7:1072–1075
5
. Zana R (2002) Dimeric and oligomeric surfactants. Behavior at
interfaces and in aqueous solution: a review. Adv Colloid Inter-
face Sci 97:205–253
6
. You Y, Zhao J, Jiang R, Cao J (2009) Strong effect of NaBr on
self-assembly of quaternary ammonium gemini surfactants at air/
water interface and in aqueous solution studied by surface tension
and fluorescence techniques. Colloid Polym Sci 287:839–846
. De S, Aswal VK, Goyal PS, Bhattacharya S (1996) Role of spacer
chain length in dimeric micellar organization. Small angle neu-
tron scattering and fluorescence studies. J Phys Chem 100:11664–
27. Paul BC, Islam SS, Ismail K (1998) Effect of acetate and pro-
pionate co-ions on the micellization of sodium dodecyl sulfate in
water. J Phys Chem B 102:7807–7812
28. Zana R, Levy H (1997) Alkanediyl-a,x-bis(dimethylalkylam-
monium bromide) surfactants (dimeric surfactants) Part 6. CMC
of the ethanediyl-1,2-bis(dimethylalkylammonium bromide) ser-
ies. Colloid Surf A 127:229–232
7
1
1671
29. Sun Y, Feng Y, Dong H, Chen Z, Han L (2007) Synthesis and
aqueous solution properties of homologous gemini surfactants
with different headgroups. Central Eur J Chem 5:620–634
30. Kunz W, Nostro PLo, Ninham BW (2004) The present state of
affairs with Hofmeister effects. Curr Opin Colloid Interface Sci
9:1–18
31. Vlachy N, Cwiklik BJ, V a´ cha R, Touraud D, Jungwirth P, Kunz
W (2009) Hofmeister series and specific interactions of charged
headgroups with aqueous ions. Adv Colloid Interface Sci 146:
42–47
32. Vlachy N, Drechsler M, Touraud D, Kunz W (2009) Anion
specificity influencing morphology in catanionic surfactant mix-
tures with an excess of cationic surfactant. C R Chim 12:30–37
33. Nightangle ER Jr (1959) Phenomenological theory of ion solva-
tion. Effective radii of hydrated ions. J Phys Chem 63:1381–1387
8
9
. Han L, Chen H, Luo P (2004) Viscosity behaviour of cationic
gemini surfactants with long alkyl chains. Surf Sci 564:141–148
. Zana R, Talmon Y (1993) Dependence of aggregate morphology
on structure of dimeric surfactants. Nature 362:228–230
0. Tanford C (1980) The hydrophobic effect: formation of micelles
and biological membranes. Wiley, New York
1. Wennerstrom H, Lindman B (1980) Micelles. Amphiphile
aggregation in aqueous solution. Top Curr Chem 87:1–87
2. Zana R (1987) Surfactant solutions: new methods of investiga-
tion. Dekker, New York
3. Israelachvili JN (1992) Intermolecular and surface forces. Aca-
demic, London
1
1
1
1
1
4. Hofmeister F (1888) About the science of the effect of salts. Arch
Exp Pathol Pharmakol 24:247–260
1
23

J Surfact Deterg (2013) 16:693–707
707
3
3
4. Mukerjee P, Karematsu K, Obawauchi M, Sugihara G (1985)
Effect of temperature on the electrical conductivity and the
thermodynamics of micelle formation of sodium perfluorooct-
anoate. J Phys Chem 89:5308–5312
5. Rodriguez JR, Perez AG, Castillo JLD, Czapkiewicz J (2002)
Thermodynamics of micellization of alkyldimethylbenzylammo-
nium chlorides in aqueous solutions. J Colloid Interface Sci
46. Rosen MJ, Cohen AW, Dahanayake M, Hua X (1982) Rela-
tionship of structure to properties in surfactants. 10. Surface and
thermodynamic properties of 2-dodecyloxypoly(ethenoxyetha-
2 4 x
nol)s, C12H25(OC H ) OH, in aqueous solution. J Phys Chem
86:541–545
47. Rubingh DN (1979) Mixed micelle solutions. In: Mittal KL (ed)
Solution Chemistry of Surfactants, vol 1. Plenum, New York
48. Rosen MJ (1998) Molecular interaction and the quantitative
prediction of synergism in the mixtures of surfactants. Prog
Colloid Polym Sci 109:35–41
2
50:438–443
3
3
6. Rosen MJ (2004) Surfactants and interfacial phenomena. Wiley,
New York, p 215
7. Naqvi AZ, Rub MA, Kabir-ud-Din (2011) Effects of pharma-
ceutical excipients on cloud points of amphiphilic drugs. J Col-
loid Interface Sci 361:42–48
8. Rao URK, Manohar C, Valaulikar BS, Iyer RM (1987) Micellar
chain model for the origin of the viscoelasticity in dilute sur-
factant solutions. J Phys Chem 91:3286–3291
9. Laughlin RG (1981) HLB from a thermodynamic perspective.
J Soc Cosmet Chem 32:371–392
0. Cacace MG, Landau EM, Ramsden JJ (1997) The Hofmeister
series: salt and solvent effects on interfacial phenomena. Q Rev
Biophys 30:241–277
1. Collins KD (2004) Ions from the Hofmeister series and osmo-
lytes: effects on proteins in solution and in the crystallization
process. Methods 34:300–311
2. Haldar J, Aswal VK, Goyal PS, Bhattacharya S (2004) Aggre-
gation properties of novel cationic surfactants with multiple py-
ridinium headgroups. Small-angle neutron scattering and
conductivity studies. J Phys Chem B 108:11406–11411
3. Chattoraj DK, Birdi KS (1984) Adsorption and the Gibbs surface
excess. Plenum, New York
Author Biographies
3
Jeenat Aslam is currently a Ph.D. student in the Department of
Chemistry, Aligarh Muslim University, Aligarh, India. She received
her M.Sc. degree from the same university.
3
4
Umme Salma Siddiqui is currently working as a Young Scientist in
the Department of Chemistry, Aligarh Muslim University, Aligarh,
India in a CST-UP, Lucknow, funded project (CST-790). She
received her M.Sc. and Ph.D. degrees from the same university.
4
4
Wajid Husain Ansari received his M.Sc., M. Phil. and Ph.D. degrees
from Aligarh Muslim University.
Kabir-ud-Din is currently a UGC-BSR Faculty Fellow at Aligarh
Muslim University. He received his M.Sc. and Ph.D. degrees from the
same university. He held postdoctoral positions at the universities in
Prague (Czech Republic), Keele (UK) and Austin (USA). His present
lines of research are micellar kinetics, electrochemistry, physico-
chemical behavior of micellar solutions, clouding phenomenon in
amphiphilic systems, and so on.
4
4
4. Ananda K, Yadav OP, Singh PP (1991) Studies on the surface
and thermodynamic properties of some surfactants in aqueous
and water ?1, 4-dioxane solutions. Colloids Surf 55:345–348
5. Evans DF, Wennestorm H (1994) The colloidal domain: where
physics, chemistry and biology meet. VCH, New York
4
1
23