Interaction between Ionic Liquids and Gemini Surfactant: A Detailed Investigation into the Role of Ionic Liquids in Modifying Properties of Aqueous Gemini Surfactant

Article abstract of DOI:10.1007/s11743-015-1747-x

Tuning physicochemical properties of aqueous surfactant solutions comprised of normal or reverse micelles by external additives is of utmost importance due to the enormous application potential of surfactant-based systems. Unusual and interesting properties of environmentally benign ionic liquids (IL) make them suitable candidates for this purpose. To understand and establish the role of IL in modifying properties of aqueous gemini surfactants, we studied the effect of the IL, 1-hexyl-3-methylimidazolium bromide ([Hmim][Br]) and 1-octyl-3-methylimidazolium bromide ([Omim][Br]) on the properties of the aqueous cationic gemini surfactant 1,6-hexanediyl-α,ω-bis(dimethyltetradecyl)ammonium bromide (14-6-14,2Br^-). The behavioral changes were investigated by measuring the critical micelle concentration (CMC) using electrical conductance, surface tension, dye solubilization and fluorescence probe measurements at 298.15 K. It was observed that the CMC of 14-6-14,2Br^- gemini surfactant decreases with addition of IL, thus favoring the micellization process. An increase in micellar size was observed at lower IL concentration using dynamic light scattering, with a decrease in aggregation number (N _agg) determined from fluorescence probe quenching measurements. It is noteworthy that the extent of modulation of the micellar properties is different for both the IL due to their structural differences. IL behave like electrolytes at lower concentrations and cosurfactants at higher concentrations and form mixed micelles with the cationic gemini surfactant showing an increase in N _agg.

Full text of DOI:10.1007/s11743-015-1747-x

J Surfact Deterg
DOI 10.1007/s11743-015-1747-x
ORIGINAL ARTICLE
Interaction Between Ionic Liquids and Gemini Surfactant:
A Detailed Investigation into the Role of Ionic Liquids
in Modifying Properties of Aqueous Gemini Surfactant
Utkarsh More1 Pallavi Kumari1 Zuber Vaid1 Kamalakanta Behera2
Naved I. Malek1
•
•
•
•
Received: 23 January 2014 / Accepted: 30 September 2015
Ó AOCS 2015
Abstract Tuning physicochemical properties of aqueous
surfactant solutions comprised of normal or reverse
micelles by external additives is of utmost importance due
to the enormous application potential of surfactant-based
systems. Unusual and interesting properties of environ-
mentally benign ionic liquids (IL) make them suit-
able candidates for this purpose. To understand and
establish the role of IL in modifying properties of aqueous
gemini surfactants, we studied the effect of the IL, 1-hexyl-
3-methylimidazolium bromide ([Hmim][Br]) and 1-octyl-
3-methylimidazolium bromide ([Omim][Br]) on the
properties of the aqueous cationic gemini surfactant
1,6-hexanediyl-a,x-bis(dimethyltetradecyl)ammonium bro-
mide (14-6-14,2Br-). The behavioral changes were inves-
tigated by measuring the critical micelle concentration
(CMC) using electrical conductance, surface tension, dye
solubilization and fluorescence probe measurements at
298.15 K. It was observed that the CMC of 14-6-14,2Br-
gemini surfactant decreases with addition of IL, thus
favoring the micellization process. An increase in micellar
size was observed at lower IL concentration using dynamic
light scattering, with a decrease in aggregation number
(Nagg) determined from fluorescence probe quenching
measurements. It is noteworthy that the extent of modu-
lation of the micellar properties is different for both the IL
due to their structural differences. IL behave like elec-
trolytes at lower concentrations and cosurfactants at higher
concentrations and form mixed micelles with the cationic
gemini surfactant showing an increase in Nagg
.
Keywords Ionic liquids Á Gemini surfactants Á
Fluorescence quenching Á Dynamic light scattering Á
Aggregation number
Abbreviations
ANS
8-Anilino-1-naphthalenesulfonic acid
Critical micelle concentration
Cetylpyridinium chloride
CMC
CPC
DLS
Dynamic light scattering measurements
Ethylene glycol
EG
[Hmim][Br]
IL
1-Hexyl 3-methyl imidazolium bromide
Ionic liquids
Nagg
Aggregation numbers
[Omim][Br]
1-Octyl 3-methyl imidazolium bromide
SANS
Small angle neutron scattering
14-6-14,2Br- 1,6-Hexanediyl-a,x-
bis(dimethyltetradecyl)ammonium
Electronic supplementary material The online version of this
article (doi:10.1007/s11743-015-1747-x) contains supplementary
material, which is available to authorized users.
bromide
PIL
Protic ionic liquids
& Naved I. Malek
navedmalek@yahoo.co.in; navedmalek@ashd.svnit.ac.in
List of symbols
b
DG0m
c
Counter ion binding
1
Applied Chemistry Department, S.V. National Institute of
Technology, Surat, Gujarat 395 007, India
Standard free energy of micelle formation
Surface tension
2
Division of Chemistry and Biological Chemistry, School of
Amin
Area per molecule at the air–water interface
Maximum surface excess concentration
Physical and Mathematical Sciences, Nanyang Technological
University (NTU), 21 Nanyang Link, Singapore 637171,
Singapore
Cmax
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J Surfact Deterg
R
Universal gas constant
Avogadro’s number
monomeric surfactants. The aggregation behavior in mono-
meric conventional surfactants is mainly controlled by inter-
molecular interactions, whereas in gemini surfactants it is
controlled by the co-operativity of both inter-molecular and
intra-molecular interactions. The properties of gemini sur-
factants in aqueous solution significantly depend upon con-
centration, temperature, nature of hydrophobic tails, spacer
properties, hydrophobic tails asymmetry and salt concentra-
tion [13–16].
NA
PCMC Surface pressure at the CMC
Introduction
Due to their immense application potential, surfactant-based
systems are a topic ofmajor research interestin both academia
and industry. Surface-active molecules self-assemble in
aqueous media to form nano-sized aggregates called micelles.
Recently, dimeric or gemini surfactants having two identical
or different hydrophobic chains and two ionic/polar head
groups covalently connected by a spacer group (Scheme 1)
have been investigated [1–6]. The superiority of gemini sur-
factants over conventional monomeric surfactants includes,
remarkably low critical micelle concentration (CMC), high
surface activity, low Krafft temperature, unusual rheological
properties, antimicrobial activity, better wetting ability and
multifarious aggregate structures [7–9], to name a few. Such
unique properties encouraged their applications in various
fieldofstudy,suchasgenetransfection, drugformulationsand
drug delivery, food industry, cosmetics, catalytic reaction, oil
recovery and polymerization, etc. [10–12]. In aqueous solu-
tion, gemini surfactants behave differently than conventional
If conditions are unchanged, aqueous gemini surfactant
solutions have more or less fixed physicochemical properties
that are difficult to modify. One possible way to modify the
solution properties is to use some external means such as
temperature/pressure and/or addition of a variety of modi-
fiers such as organic salts, organic solvents, hydrotropic salt,
cosurfactants, electrolytes, polymers, nonpolar organics,
inorganic salts, biomolecules, conventional surfactants or
polyelectrolytes [17–24] to name a few.
Under ambient condition, judicious selection of envi-
ronmentally benign substances, such as ionic liquids (IL)
can play a major role in modifying the properties of gemini
surfactants. IL, composed entirely of cations and anions are
known to be effective green solvents due to their negligible
vapor pressure and ability to recycle. The other fascinating
properties of IL, which draws the attention of academic as
well as industrial researchers includes, wide liquid ranges,
good thermal stabilities, electrolytic conductivity, wide
range of viscosities, adjustable miscibility, nonflammabil-
ity, etc. Long alkyl chain IL can also show surfactant like
properties [25–30]. Comprehensive studies on the interac-
tions between mixtures of gemini and conventional single
chain surfactants of different charge classes to form mixed
micellar aggregates have been reported [31–34]. In recent
decades, studies on the interactions between single chain
surfactants and IL have been reported and the results
obtained are discussed at length [35–46]. Pandey et al. [35–
38] reported the dual behavior of IL in aqueous solutions of
single chain surfactants. However interactions between
gemini surfactants and IL are hardly studied.
Br
CH2
CH2
CH3
N
12
CH2
6
CH2
CH2
CH3
N
12
Br
Hexamethylene 1, 6-bis (N-tetradecyl-N,
N-dimethylammonium bromide) (14-6-14 2 Br-)
CH3
Recently, Chai et al. [47] studied the composition and
structural parameters of W/O microemulsions containing
the gemini imidazolium surfactant and its monomeric form
of IL using the dilution method. Li et al. [48] has studied
the aggregation behavior of a cationic gemini surfactant
with methylene spacers in two different protic IL (PIL),
propylammonium nitrate (PAN) and butylammonium
nitrate (BAN), by small and wide-angle X-ray scattering,
polarized optical microscopy and rheology measurements.
The study reveals that changing from PAN to BAN, phase
transition of the aggregate was observed from the reversed
to normal hexagonal phase due to the attractive interaction
between the IL and cationic gemini surfactants. Wang et al.
[49] studied the aggregation behavior of dissymmetric
H3C
N
N+
Br-
1-Hexyl 3-methylimidazolium bromide ([Hmim][Br])
CH3
N+
H3C
N
Br-
1-Octyl 3-methylimidazolium bromide ([Omim][Br])
Scheme 1 Structures of the cationic gemini surfactant (14-6-
14,2Br-), Ionic liquids [Hmim][Br] and [Omim][Br]
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J Surfact Deterg
cationic gemini surfactants in PIL and observed that
structure dissymmetry plays an important role in aggrega-
tion process. Shang et al. [50] have investigated the mixed
micellization of binary mixtures containing cationic gemini
surfactant, trimethylene-1,3-bis(dodecyl ammonium bro-
mide) (12-3-12) with the conventional cationic surfactant
dodecyltrimethyl-ammonium bromide (DTAB) and with
IL, [Cnmim]Br (n = 4, 6, 8). The concentration-dependent
dual role of the IL is well documented [6, 37].
purchased from Spectrochem PVT Ltd (Mumbai, India).
N,N-dimethyltetradecylamine was purchased from Aldrich
Ltd and used as received. Ethyl acetate (Merck, 99 %), n-
hexane (Finar, 95 %), acetone (Merck, C99 %), ethyl
alcohol (J.H.I.T. Co. Ltd. 99.9 %), acetonitrile (R.F.C. Ltd.
99 %). Water, double distilled over KMnO4 and deionized,
with
a
conductivity of 6.1–6.4 lS cm-1 was used
throughout the experiments. 8-Anilino-1-naphthalenesul-
fonic acid (ANS) (used as a chromophore) was obtained
from Aldrich and used as received. Stock solutions of IL
and chromophore (1 mM) were prepared using distilled
water. All of the IL were dried under vacuum at 343 K for
a few days to remove any trace amounts of water. Karl–
Fisher analysis of the samples indicated that the water
content was reduced to \100 ppm in all the IL.
Jn a lower concentration range, IL behave like common
electrolytes, however, in a higher concentration range they
behave like polar co-solvents [6, 37]. Shang et al. studied
the effect of [Cnmim][Br] (where n = 4, 6 and 8) IL and
NaBr on the aggregation behavior of cationic gemini sur-
factants. IL plays the role of inorganic electrolyte at lower
concentration whereas at higher concentration it acts like
cosurfactant and cosolvent [50]. The influence of the long
chain IL (1-tetradecyl-3-methylimidazolium bromide) on
the micellar properties of single and double chain cationic
surfactants was investigated [51]. The results showed an
attractive interaction between the IL and the gemini sur-
factant with a short spacer and promoted micellization due
to the more hydrophilic character [51].
Synthesis of 14-6-14,2Br2
14-6-14,2Br- was synthesized [52, 53] by the reaction of
1,6-dibromohexane with N,N-dimethyltetradecylamine in
dry ethanol. A 1:2.1 reaction mixture (slight excess of 1,6-
dibromohexane ensured complete bis-quaternization) was
refluxed (at 80 °C) for 48 h. After completion (the reaction
progress was monitored by TLC), solvent was removed
under vacuum from the reaction mixture and the solid was
recrystallized five times from a mixture of hexane and ethyl
acetate to obtain the compound in pure form. The overall
yield of the surfactant was 80 %. Purity of the surfactant
was checked on the basis of C, H, N analysis, which was
The concentration dependent dual role of IL as inorganic
electrolytes (at lower concentration) or as cosurfactants (at
higher concentrations) encouraged us to study the effect of
IL on the aggregation behavior of cationic gemini surfac-
tants. The IL used in the current report were chosen based
on their relatively high interfacial tensions. Mixtures of IL
with gemini surfactants have better surface properties, can
show remarkable physicochemical properties and are more
cost effective than single chain surfactants [31–33].
Additionally, studying the complex molecular interactions
in these systems can help us to better understand biological
systems and functions [10–12, 34].
1
further characterized by H-NMR [53] spectroscopy (M.P.
221 °C).
1
The H-NMR of 14-6-14,2Br- is as follow: (200 MHz,
CDCl3, d ppm): 0.88 (t, 6H, 29 –CH3), 1.26–1.36 (m, 44H,
29 –(CH2)11–CH3), 1.79 (s, 2H, –CH2–(CH2)11–CH3), 2.41
(s, 2H, –N–CH2–CH2–(CH2)11–CH3), 3.39 (s, 12H, 29 –N–
(CH3)2, 3.51 (m, 8H, 29 –N–CH2 from spacer and 29 –N–
CH2 from alkyl chain, 3.86 (s, 8H, 29 –N–CH2–CH2–
CH2– from spacer).
In the present investigation, the IL, 1-hexyl 3-methyl
imidazolium bromide ([Hmim][Br]) and 1-octyl 3-methyl
imidazolium bromide ([Omim][Br]) were added in fixed
concentrations to an aqueous solution of the cationic gemini
surfactant 1,6-hexanediyl-a,x-bis(dimethyltetradecyl)am-
monium bromide (14-6-14,2Br-) with varied concentra-
tions. The physicochemical properties of aqueous 14-6-
14,2Br- with and without addition of IL were characterized
by conductance, surface tension, UV–visible and steady state
fluorescence and dynamic light scattering (DLS) techniques.
Synthesis of IL
IL reported here were prepared according to the procedure
reported in the literature [54–56]. Briefly, the reaction of
1-methylimidazole and excess molar amount of appropriate
alkyl bromide was performed in 1,1,1-trichloroethane
under reflux at ca. 343 K for 48 h. The 1,1,1-tri-
chloroethane was then removed using a rotary evaporator
under reduced pressure. The product was purified by
recrystallization from ethyl acetate/acetonitrile (3:2 by
volume) at least four times, to remove any unreacted
reagent. Any residual solvents were removed by heating at
343 K under vacuum. The water content was measured
using Karl–Fisher titration (Metrohm, 890 Titrando) and
Experimental Section
Materials
1,6-Dibromohexane (98 %), 1-methylimidazole (99 %),
1-bromooctane (98 %) and 1-bromohexane (98 %) were
123

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was lower than 100 9 10-6 mass%. The DSC thermo-
grams (Mettler Toledo, DSC-1) for both the IL samples
showed a sharp single endothermic peak supporting the
high purity of both IL samples. The IL was also analyzed
used for the calibration purpose. Surfactant concentration
was varied by adding concentrated surfactant solution in
small installments, and the readings were noted after
thorough mixing and temperature equilibration. The mea-
sured surface tension values were corrected according to
the procedure of Harkins and Jordan using the instrument
software. The accuracy of c measurements was within
0.1 mNm-1. The CMC values were determined by noting
inflections in the c versus the logarithm of surfactant
concentration isotherms.
1
by H NMR and 13C NMR (Advance DPX 200 Bruker,
CSMCRI) to confirm the absence of any major impurities.
1
The H NMR of [Hmim][Br] is as follows: (200 MHz,
DMSO-d6, d ppm):0.86 (3H,t, N–(CH2)5–CH3), 1.27 (br. S,
6H, N–CH2– CH2– (CH2)3–CH3), 1.78 (2H, quintet, N–
CH2–CH2–(CH2)3–CH3), 3.85 (3H, singlet, N–CH3), 4.15
(t, 2H, N–CH2–CH2–(CH2)3–CH3), 7.70 (s, 1H), 7.77 (s,
1H), 9.10 (s, 1H). 13C NMR (200 MHz, 16.14,24.18, 27.43,
31.69, 38.10, 41.85, 51.02, 124.56, 125.86, 136.82).
UV–Vis Absorbance Measurements
1
The H NMR of [Omim][Br] is as follows: (200 MHz,
UV–Vis absorption spectra were measured using a Carry
50 spectrophotometer (Varian, Switzerland) equipped with
a thermostated cell compartment. The absorption spectrum
of the aqueous solution of gemini surfactant ? ANS ? IL
mixture was collected against the reference solution con-
taining the ANS ? IL at 298.15 0. 1 K temperatures,
using a quartz cuvette with a path length of 1 cm. ANS was
used as the probe with a concentration of 1.6 9 10-6
mol dm-3 in all experiments to avoid any interference in
the micelle formation.
DMSO-d6, d ppm): 0.86 (3H, t, N–(CH2)5–CH3), 1.25 (br.
S, 10H, N–CH2– CH2– (CH2)5–CH3), 1.78 (2H, quintet,
N–CH2–CH2–(CH2)5–CH3), 3.85 (3H, singlet, N–CH3),
4.15 (t, 2H, N–CH2–CH2–(CH2)5–CH3), 7.70 (s, 1H), 7.77
(s, 1H), 9.10 (s, 1H). 13C NMR (200 MHz, 16.13, 24.14,
27.83, 30.59, 33.27, 38.37, 51.57, 79.13, 121.21, 123.82,
125.65, 138.13).
Electrical Conductivity Measurements
Aqueous solutions of IL were prepared by weight using an
analytical balance with a precision of 0.0001 g (B 204-S,
Mettler Toledo, Switzerland, operated in a dry box) in
Millipore grade deionized double distilled water. The cal-
culated amount of 14-6-14,2Br- solution was added from
the stock solution to the IL solutions. Electrical conduc-
tivities were measured at 298.15 0.1 K by a EUTECH
PC 6000 digital conductivity meter (Eutech Instruments,
Singapore) having a sensitivity of 0.1 lS cm-1 with an
accuracy of 0.5 %. The temperature was maintained within
0.1 K using a constant temperature bath. The dip type
conductivity probe (EC-CONSEN 21B) provided with the
instrument has an inbuilt temperature probe. Prior to
measurements, the probe was calibrated with aqueous KCl
Steady-State Fluorescence Measurements
Steady-state fluorescence measurements were performed
using a Fluorolog spectrofluorometer (Horiba Jobin Yvon)
using a 1-cm path length quartz cuvette. Pyrene was used
as the polarity probe with a concentration of 1.0 9 10-6
mol dm-3 in all experiments to avoid any interference in
micelle formation. The emission spectra of pyrene were
recorded in the wavelength range 350–500 nm at an exci-
tation wavelength of 334 nm using 1 nm excitation and
emission slit widths. The first (I1) and third (I3) vibronic
peaks of pyrene appeared at 373 and 384 nm, respectively.
CMC values were determined by the I1/I3 of pyrene spec-
trum as a function of surfactant concentration. The fluo-
rescence spectra were corrected for the instrumental
response. To determine surfactant aggregation numbers,
steady-state fluorescence quenching measurements were
performed using pyrene and cetylpyridinium chloride
(CPC) as the probe and static quencher, respectively. All
the measurements were performed at 298.15 0.1 K
using a constant temperature control bath.
solutions in the concentration range of 0.01–1.0 mol kg-1
.
At least three repetitive measurements were made for each
concentration and only the mean values were taken into
consideration. The standard uncertainty of the measure-
ments was \0.3 %.
Surface Tension Measurements
Surface tension measurements were made using a K9
tensiometer (Kru¨ss Instruments, Germany) using a plat-
inum ring by the ring detachment method at
298.15 0.1 K. The temperature was maintained constant
within 0.1 K using a constant temperature bath. The sur-
face tension of deionized double distilled water having
surface tension of 72.2 mNm-1 at 298.15 0.1 K was
Dynamic Light Scattering Measurements
Dynamic light scattering measurements were performed at
298.15 0.1 K on a Spectro Size 300 (Malvern Instru-
ments, UK) with a He–Ne laser (633 nm, 4 Mw). An
appropriate amount of IL was added by weight to the 14-6-
14,2Br- solution (2 mL) taken in a cylindrical quartz
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cuvette. The aqueous solution of gemini 14-6-14,2Br-
were prepared above the CMC. The temperature of the
measurements was controlled within an accuracy of
0.1 K.
micellization behavior of two cationic gemini surfactants
and the results suggest that the CMC decreases with
increasing concentration of the electrolyte. Tiwari and
Subit [62] investigated the aggregation behavior of cationic
gemini surfactants with hydroxyl groups in the spacer with
IL in water and water–organic solvent media and reported
that the CMC increases with increasing cosolvent con-
centration. Further, with decreasing the polarity of the
solvent, the surfactant tails have more affinity toward the
bulk phase solvent which results in decreased hydropho-
bicity of the surfactant molecules and results in higher
CMC.
All the solutions were prepared in deionized double
distilled water on a weight basis. The concentrations of
individual surfactants were kept higher than CMC values
and then mixed with appropriate volumes of IL to obtain
different bulk concentrations of the binary systems. All the
measurements were taken in triplicate and averaged. All
the data were analyzed using Microsoft Excel and Orig-
inPro 8.0 softwares.
Among the most important observations from the con-
ductance data is the degree of counter ion binding (b). In the
current investigation, as the concentration of IL increases,
surfactant molecules bind more closely with the IL, which
results in increased b values as shown in Table 2. This result
is quite logical as strongly bound counter ions weaken head
group repulsion, which would lead to micelle formation at a
lower concentration. The observed increase in b is probably
due to the decrease in the charge density at the micellar
surface caused by the decrease in the aggregation number of
the micelle. This observation is well supported by the fluo-
rescence quenching measurement, which clearly indicates
that as the IL concentration increases, the aggregation num-
ber initially decreases up to 0.4 wt% addition of the IL and
then increases, which may be due to the cosurfactant like
behavior at higher concentration. The lower CMC is either
due to the increasing counterion binding or to formation of
mixed micelles (cations of the surfactant ? cation of the IL).
One important observation from the CMC measurement
by electrical conductance is, at a fixed IL concentration of
0.6 wt%, the CMC decreases more due to [Omim][Br] than
for [Hmim][Br]. At higher concentration no appreciable
change in CMC was noticed [50, 60, 61]. Increasing the
salt [22, 63] concentration reduces the electrostatic repul-
sion between the charged groups and favors micelle
aggregation. In our case, at lower concentration the same
phenomena were observed; where the IL forms charged
ions, which reduces the electrostatic repulsion between the
charged head groups of the gemini surfactant and lowers
the CMC. It is reported that increasing the chain length of
alcohols and alkyl amines as additives, the CMC of the
aqueous cationic gemini surfactants decreased [64].
As observed in the supplementary material (Figs. S1–
S2), conductance increased as the wt% of the IL increased,
which is due to the dissociation of the IL in the solution. At
a fixed concentration, [Hmim][Br] containing surfactant
solutions have significantly higher conductance as com-
pared to those containing [Omim][Br] as shown in Fig. 2.
This may be due to the higher limiting ionic conductivity
of [Hmim?] as compared to that of [Omim?] at ambient
conditions.
Results and Discussion
Interaction Studies by Electrical Conductance
Measurement
Measurement of electrical conductivity is widely used to
provide information regarding the critical micelle concen-
tration (CMC) and degree of counter ion binding (b) of
aqueous surfactant solutions [57, 58]. We investigated the
properties of aqueous 14-6-14,2Br- in the presence and
absence of IL, [Hmim][Br] and [Omim][Br]. One expects
two linear regimes for conductance in the pre- and post-
micellar region of an aqueous ionic surfactant solution
where the slope becomes smaller, once the micelles have
been formed as per the Onsager theory of electrolyte
conductivity [57]. In the current investigation, we noticed
similar linear fragments (Fig. 1a). The concentration at
which the two linear fragments intersect is assigned to the
CMC. The slope change at the CMC is due to an effective
loss of ionic charges because a fraction of the counterions
are confined to the micellar surface. The aggregation of the
molecules, or micelles, start to form at the concentration
corresponding to this break point. The CMC of aqueous
14-6-14,2Br- gemini surfactant is in excellent agreement
with the reported value [20, 29, 31, 59] 1.58 9 10-4
mol dm-3. The CMC of the aqueous 14-6-14,2Br-
decreases as the IL concentration increases as shown in
Figures S1–S2 and reported in Table 1.
Farah et al. [60] studied the effect of linear primary
alcohols and amines on the CMC values of 12-4-12 gemini
surfactants and observed that CMC decrease with
increasing the concentration as well as the alkyl chain
length of the additives. Further, alcohols are more effective
than amines in decreasing the CMC due to the less specific
interaction between the surfactant head groups and amines
than alcohols. Increasing the concentration of organic salts
as well as the hydrophobicity of the salt anion also
decreases the CMC [17]. Yu et al. [61] investigated the
effect of organic and inorganic electrolytes on the
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Fig. 1 Representative plots of variation of different physical quan-
tities with concentration for determining the critical micelle concen-
tration (CMC) of 14-6-14,2Br- in water at 298.15 K. a Specific-
conductance versus 14-6-14,2Br- concentrations in water, b surface
tension versus log concentration, c UV–Vis absorbance versus
wavelength (inset shows the absorbance versus concentration of
gemini surfactant), d pyrene fluorescence intensity versus concentra-
tion of cationic gemini surfactant in water (inset shows the
normalized fluorescence versus wavelength). (Filled square) I1/I3,
(open circles) d2(I1/I3)/dc2
The CMC of a surfactant is regarded as a measure of
the stability of its micellar form relative to its monomeric
form, the lower the CMC, the greater the stability.
According to the mass action model and approximation
suggested by Zana [65], DG0m value depends on both the
CMC and b and can be written as (in solution, gemini
surfactants give 2 alkyl chains and two polar head
groups):
combination of the electrolyte effect and hydrophobic
effect. The dominance of these two factors depends on the
various physical parameters such as concentration and the
alkyl chain length of the added IL.
Interaction Studies by Surface Tension
Measurement
One of the significant aspects of surfactants is the ability
to lower the interfacial tension between an aqueous
solution and some other phase. In the current investiga-
tion, we examined the surface tension (c) of aqueous
14-6-14,2Br- in the presence and absence of IL. The
threshold surfactant concentration required to saturate the
air/solution interface is the CMC which results in a break
point in the c versus log [14-6-14,2Br-] plot as shown in
Fig. 1b. Eastoe et al. [5] reported on possible interfer-
ences of inorganic as well as surface active impurities on
the surface tension of dimeric anionic surfactants. The
presence of residual inorganic and surface active
CCMC
DG0m ¼ 2RTð1:5 À aÞ ln XCMC ¼ 2RTð1:5 À aÞ ln
55:4
ð1Þ
where XCMC is the CMC in mole fraction unit, CCMC is that
in mol dm-3, a the degree of counter ion binding and 55.4
comes from that 1 dm3 of water corresponds to 55.4 mol of
water at 298.15 K. The value of DG0m becomes more
negative as the IL concentration increases, which is due to
stronger hydrophobic–hydrophobic interactions.
The overall effect of the synthesized IL on the CMC of
the aqueous 14-6-14,2Br- gemini surfactant is mainly a
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Table 1 CMC of aqueous
cationic gemini surfactant (14-
6-14,2Br-) obtained using
different methods in the
presence of [Hmim][Br] and
[Omim][Br]
IL in wt%
104 CMC (mol dm-3
)
Conductivity
Surface tension
UV–Vis
Fluorescence
[Hmim][Br]
0.0
0.2
1.60 (1.60) [29]
1.60
1.30
1.00
0.60
0.50
0.12
1.70
1.60
1.30
1.00
0.15
0.06
1.80
1.60
–
1.30
1.00
0.94
0.50
0.15
0.4
0.6
1.30
–
0.8
1.0
1.10
[Omim][Br]
0.2
1.00
0.60
0.20
0.10
0.05
1.00
1.00
0.20
0.10
0.05
1.40
1.10
0.80
0.08
0.05
1.10
–
0.4
0.6
0.80
–
0.8
1.0
0.60
Standard uncertainties u are: u(CMC) = 0.03 9 10-4 mol dm-3. All the experiments were carried out in
an inert atmosphere and at atmospheric pressure
Table 2 Counter ion binding (a), interfacial parameters: surface pressure at CMC (PCMC), surface excess (Cmax 9 106), minimum area per
molecule (Amin) and Gibbs free of micellization (ÀDG0mic) for aqueous 14-6-14,2Br- in absence and presence of different additives at 298.15 K
2 a
PCMC (mNm-1
)
Cmax 9 106 (mol m-2 a
)
Amin (A )
ÀDG0mic (kJ mol-1
)
˚
IL in wt%
a
[Hmim][Br]
0.0
0.2
0.35 (0.36) [29]
32.10
35.57
37.70
38.30
39.13
41.77
0.50 (0.64)
0.52 (0.77)
0.56 (0.84)
0.50 (0.75)
0.45 (0.68)
0.40 (0.60)
329 (259)
322 (215)
296 (197)
332 (222)
369 (245)
415 (275)
-79.80 (-71.71) [29]
0.14
0.23
0.66
0.86
–
-73.26
-80.80
-90.70
-143.68
–
0.4
0.6
0.8
1.0
[Omim][Br]
0.2
0.10
0.42
0.51
0.82
–
35.70
38.60
39.10
40.70
42.93
0.51 (0.74)
0.52 (0.76)
0.48 (0.71)
0.43 (0.67)
0.38 (0.57)
326 (224)
319 (218)
346 (233)
386 (248)
437 (289)
-72.20
-80.70
-94.66
-143.30
–
0.4
0.6
0.8
1.0
2
u(Amin) = 0.60 A ,
Standard uncertainties
u
are: u(b) = 0.04, u(PCMC) = 0.70 mN m-1
,
u(Cmax) = 0.05 9 10-6 mol m-2
,
˚
u(DG0mic) = 5.00 kJ mol-1. All the experiments were carried out in an inert atmosphere and at atmospheric pressure
a
Values in the parentheses for Cmax and Amin are calculated by taking the pre-factor value n = 2
impurities can greatly affect the surface tension of aque-
ous anionic gemini surfactant solutions and it was well
documented by recording the neutron reflection and sur-
face tension for the synthesized anionic surfactants
adsorption isotherm at the air–water interface. In the
current investigation, we have taken proper care in the
synthesis of the cationic gemini surfactants so that the
level of the impurities will be as low as possible.
of the IL increases surface tension of the aqueous 14-6-
14,2Br- decreases. The surface tension decreases more
for the [Omim][Br], which may be due to the longer
alkyl chain, which increases the hydrophobicity in the
system. The addition of IL causes the compression of
the diffuse electric double layer, which reduces the
repulsions between the surfactant head groups and
reduces the CMC. Similar synergistic effects for mix-
tures of cationic gemini surfactants with different aro-
matic hydrotropes and salt anions have also been
reported [17, 63].
A careful examination of the c versus log [14-6-
14,2Br-] plots after addition of the IL (Figures S3–S4)
reveals that as the concentration and alkyl chain length
123

J Surfact Deterg
counter ions. Comparison of small angle neutron scattering
(SANS) and surface tension studies of cationic gemini
surfactants resulted in a value for n close to 2, except for
the compound with the xylyl spacer, for which n was about
3 [63]. Further, Eastoe et al. [5] had reported the pre-factor
value as 2 for the dimeric anionic surfactants. As reported
earlier, cationic surfactants form surfactant-counter ion
complexes at the interface [68, 69]. Looking at all these
aspects of the cationic gemini surfactants we have used
both values of the pre-factor; 3 and 2 (values in the
parentheses) and reported in Table 2. For the comparison,
we used the value of n = 3 throughout the manuscript.
Nevertheless, the value of n used to calculate Cmax, and so
A
min, does not affect the role of IL, which is an important
Fig. 2 Specific-conductance versus 14-6-14,2Br- concentration with
the addition of 0.2 wt% of (filled squares) [Hmim][Br] and (open
circles) [Omim][Br]
point. The effective area of surfactant molecule at the air/
water interface, Amin, can be calculated by:
Amin ¼ 1=ðNACmax
Þ
ð3Þ
It is important to mention that the CMC of our aqueous
14-6-14,2Br- obtained by surface tension measurements is
in good agreement with that reported in the literature [20,
29, 31, 59]. The CMC of the aqueous 14-6-14,2Br-
decreases from 1.60 9 10-4 to 0.10 9 10-4 mol dm-3
with the addition of only 0.8 wt% of [Omim][Br]. For the
[Hmim][Br], the same trend was observed but the CMC
decreases up to 0.12 9 10-4 mol dm-3. Generally, the
addition of inorganic salt decreases the CMC of ionic
surfactants [22, 66, 67], the depression being mainly due to
the decrease in the thickness of the ionic atmosphere sur-
rounding the ionic head groups and consequently decreased
electrostatic repulsion between ionic surfactants in the
micelle.
where NA is Avogadro’s number. Cmax and Amin are listed
in Table 2. As stated in Eq. 3, the higher the adsorption,
the smaller is the effective area of the surfactant molecule
at the surface. The calculated value of Amin in water for
2
14-6-14,2Br- (329 A ) is in good agreement with the
˚
2
recent literature (326 A ) [70]. The values of Amin increase
˚
with increasing spacer length for m-s-m type gemini sur-
factants for m = 12 and s \ 10, and decreases when
s [ 10 [71]. The variation in the molecular area with the
number of methylene groups in the spacer might be due to
the arrangement of the spacer as a flat surface at the air/
water interface when s \ 10, which at a higher spacer unit
(s [ 10, due to higher hydrophobicity of the spacer)
becomes wicket-like [71]. Similar results were found for
the m = 14, 16 and 18 and s = 3, 4, 5, 6 and 8 containing
gemini surfactants [70, 72]. Comparing our results with the
previously published results for the m = 14 and s = 2, 4, 5
The area per molecule at the air/water interface, effec-
tiveness and efficiency of the surfactants are also calculated
from the c - log[14-6-14,2Br-] plots. As reported earlier
[5], trace level impurities can also affect the area per
and 6, we found similar correlation, i.e. Amin for the 14-6-
14,2Br- is higher than the 14-2-14,2Br- (90 A [51]),
˚
2
2
˚
molecule at the interface from 72 3 A for untreated
2
samples to 56 3 A for the purified surfactant. Care was
2
-
2
14-3-14,2Br- (123 A [73]), 14-4-14,2Br (291 A [70])
˚
˚
˚
and 14-5-14,2Br- (295 A [70]).
2
˚
taken at the time of synthesis of the gemini surfactants to
remove possible impurities to avoid discrepancies in the
results. The maximum surface excess, Cmax, at the air/
water interface was calculated by application of Gibbs
adsorption isotherm to the surface tension (c) versus con-
centration data. The Gibbs adsorption isotherm may be
expressed as [66]:
The effect of alkyl chain length on the Amin for the m-s-
m type gemini surfactant were also correlated. The reported
-
min are: 8-3-8,2Br (156 A , [74]), 10-3-10,2Br (124 A
2
-
2
˚ ˚
A
2
2
[74]), 12-3-12,2Br- (105 A [71], 107 A [50], 128 [73]),
˚
˚
2
-
2
14-3-14,2Br- (123 A [73]), 16-3-16,2Br (121 A [74])
˚
˚
and 18-3-18,2Br- (128 A , [72]), which indicates quasi
2
˚
ꢀ
ꢁ
parabolic dependence of the Amin when the length of the
alkyl substituent increases in the range of m = 8–18 and
s = 3. For s = 6, the reported Amin are, 143, 329, 326, 200,
1
dc
Cmax ¼ À
ð2Þ
2:303nRT d log C
2
˚
205 A for m = 12 [71], 14 [Exp], 14 [70], 16 [75] and 18
where Cmax is the surface excess concentration, R the
universal gas constant, C is the concentration of the surface
active compound and n is taken as 3 corresponding to
complete dissociation of the m-s-m type gemini surfactant
to give one divalent surfactant ion and two monovalent
[72], respectively.
The data in Table 2 indicate that Cmax and Amin varies
with the kind and the concentration of IL added [76, 77]. It
reveals that the variation of the effective area of surfactant
123

J Surfact Deterg
interfacial area occupied by the surfactant molecule at the
air/solution interface. The PCMC values depend on the
structure of the surfactants and their orientation at the
interface. In addition, the greater the PCMC values, the
higher the effectiveness of the surfactants. As shown in
Table 2, the PCMC values increase with increasing IL
concentration. PCMC is larger in the case of [Omim][Br] as
compared to [Hmim][Br], which confirms the better effi-
ciency [51].
molecules at the surface decrease with the addition of IL.
As we know, the addition of inorganic electrolyte to ionic
surfactant solution can obviously influence the surface
adsorption. The presence of counter ions near the polar
heads of surfactant molecules decreases the electrostatic
repulsion force between the head groups and makes it
possible for the surfactant molecules to approach each
other more closely. Thus the maximum adsorption
increases and the area occupied per surfactant molecule
decreases. However, the effective area of surfactant
molecules at the surface decreases at the lower concen-
tration of the IL and then increases with increasing the
concentration of the IL from 0.4 wt%. The increase in the
Cmax at lower concentration of IL indicates the greater
preference of the gemini surfactant solutions to be adsor-
bed at the air/water interface than in pure water. The
presence of the IL at lower concentration decreases the
repulsion among head groups and more gemini surfactant
molecules can adsorb at the interface. This is confirmed by
low values of Amin [63]. With the addition of NaBr, a
continuous increase in Cmax and decrease in Amin was
observed, which is expected for inorganic electrolytes [50,
70]. It may be due to the reduction of repulsive forces
between the head groups of the surfactant in the presence
of inorganic electrolyte.
The surface parameters obtained by the surface tension
measurement suggest possible interaction between the
cationic gemini surfactant and IL ([Cnmim][Br] with n = 6
and 8) molecules, which can modify the interfacial and
aggregation behavior of the aqueous gemini surfactant
solution.
Interaction Studies by the UV–Vis Technique
The interaction between ionic surfactants and oppositely
charged dye can change the aggregation behavior to a large
extent. Different behavior of the dye in the pre and post-
micellization will be treated as the best characteristic of the
dye molecule in the spectroscopic determination of the
metal ions [77, 79]. Beyaz et al. [41] had studied the
aggregation behavior of the anionic single chain surfactant
in IL by using 8-ANS dye. In the current investigation, we
have used 8-ANS dye to study the interaction between the
14-6-14 and IL.
A 50-lL aliquot of ANS from a 1.6 9 10-6 mol dm-3
stock solution was added to a quartz cuvette containing
3 mL of the respective IL. Incremental amounts (10 or
100 lL) of gemini surfactant were added using a micro
syringe and a series of spectra were recorded. The com-
ponents were thoroughly mixed after each incremental
addition of 14-6-14,2Br-. The absorption spectra in the
range of 200–500 nm were recorded after allowing the
solution to reach steady state (about 4 min). Absorbance
maximum were observed at 270 and 380 nm. The absor-
bance at both wavelengths shows an inflection with
increasing surfactant concentration as shown in Fig. 1c.
The inflection point of the absorbance versus [14-6-
14,2Br-] plot was taken as the CMC [80–82]. About 40
spectra were accumulated for each CMC determination.
The CMC values obtained by this method are given in
Table 1 and they are somewhat higher than the CMC
obtained by conductivity, surface tension as well as with
literature values [20, 29, 31, 59], which may be due to the
interaction of the anionic dye with the cationic surfactant
[77, 79].
Rodrıguez et al. [67] obtained similar results in their
investigation on the surface area of the gemini surfactants
12-s-12 (s = 3–5) in pure water and water–EG mixtures.
The addition of IL changes the effective area of the sur-
factant molecule at the air/water interface parabolically
[50]. At higher concentrations of the IL ([0.4 wt%), Amin
increases, which suggests that IL can easily enter into the
micellar structure by replacing the monomers of the gemini
surfactants [59, 78]. Noori et al. [76] have studied similar
phenomena for their synthesized counter ion-coupled
gemini (COCOGEM) surfactants. At lower concentrations
of the gemini surfactant, Cmax decreases which on the
formation of a monolayer increases when the gemini sur-
factant concentration increases from a 0.025 mol fraction
of the total concentration. This phenomenon occurs due to
the hydrophobicity of the gemini surfactant, due to which it
also tries to adsorb at the interface and helps in the for-
mation of the monolayer. Amin is considerably more for the
[Omim][Br] as compared to [Hmim][Br] which is the result
of the more hydrophobic nature of the [Omim][Br]. The
increasing Amin at higher concentration of the IL suggest its
co-surfactant like behavior.
The surface pressure at the CMC (PCMC) was obtained
by using the following equation:
Careful examination of the absorption data suggests that
as the concentration of 14-6-14,2Br- increases, the
absorption maxima is red shifted, which might be due to
increased hydrophobicity of the solution. The red shift for
PCMC ¼ c0 À cCMC
where c0, and cCMC are the surface tension of the solvent
and the mixture at the CMC. PCMC is the measure of the
ð4Þ
123

J Surfact Deterg
[Omim][Br] is greater than [Hmim][Br] (Figs. S5–S6)
which supports this hypothesis. Using crystal violet dye,
Javadian et al. [44] reported a similar shift for single chain
anionic surfactants.
partition of pyrene between micelles and the bulk phase.
Similar behavior was also reported by Zana et al. [87]. It is
noteworthy to mention that the CMC decreases with
increasing concentration of IL in 14-6-14,2Br- aqueous
solutions. CMC for the addition of [Omim][Br] is greater
than that obtained with [Hmim][Br]. Our CMC data for the
aqueous [14-6-14,2Br-] by this technique is in good
agreement with that reported in the literature [20, 29, 31,
59].
Interaction Studies by Pyrene Fluorescence
We have used pyrene as a fluorescence probe to obtain the
CMC of 14-6-14,2Br- in the presence and absence of IL.
Pyrene fluorescence has been used to measure various
important micellar parameters such as the CMC, aggrega-
tion numbers (Nagg), dipolarity, microfluidity, etc. [57, 83–
86]. The pyrene polarity scale, defined by the ratio ‘‘I1/I3’’
of the monomer fluorescence intensities at bands I (ca.
373 nm) and III (ca. 384 nm), is a function of solvent
dielectric (e) and the refractive index (n) via the dielectric
cross term [f(e,n2)]. Pyrene I1/I3 changes dramatically with
change in the dipolarity of the cybotactic region of the
probe. A more polar microenvironment results in a higher
I1/I3 and vice versa. Pyrene fluorescence spectra were
collected from solutions of varying [14-6-14,2Br-] in the
presence of different concentrations of IL, [Hmim][Br] and
[Omim][Br]. Figure 1d shows the variation in I1/I3 versus
[14-6-14,2Br-] for the neat gemini surfactant in an aque-
ous medium. The plot of I1/I3 versus [14-6-14,2Br-] show
a rapid decrease as the surfactant concentration is
increased, associated with the formation of aggregates. The
minimum for the second derivative in the graph shows the
CMC.
Micellar Aggregation Number from Fluorescence
Quenching
The aggregation number (Nagg) of 14-6-14,2Br- micelles
in the presence and absence of [Hmim][Br] and
[Omim][Br], was obtained by fluorescence quenching of
pyrene by CPC according to the following equation [61,
88–91]:
ꢀ
ꢁ
I0
Qmicelle
½CPCŠ
micelle
ln
¼
¼
IQ
½micelleŠ ½micelleŠ
14À6À14;2BrÀ
ꢂ
ꢃ
Nagg
½14 À 6 À 14; 2BrÀŠ À CMC14À6À14;2Br
¼ ½CPCŠ
micelle
À
ð5Þ
where I0 and IQ are the fluorescence intensities of pyrene in
the absence and presence of quencher CPC, respectively.
[CPC]micelle and [14-6-14,2Br-] are
½micelleŠ
14À6À14;2BrÀ
the concentrations of 14-6-14,2Br- micelles, quencher
CPC within the micellar pseudo phase and 14-6-14,2Br-
gemini surfactant, respectively. The graph of ln(I0/IQ)
versus [CPC]micelle in 100 mM 14-6-14,2Br- solution in
the presence of [Hmim][Br] and [Omim][Br] are shown in
Fig. 3. The data fit a linear regression at each concentration
of [Hmim][Br] and [Omim][Br] from zero to 1.0 wt%. The
The pyrene fluorescence provides interesting and useful
information on IL-added aqueous [14-6-14,2Br-] solutions
(Fig. S7–S8). The CMC values determined by this method
are slightly larger than those determined by conductance as
well as surface tension (Table 1), which may be due to the
Fig. 3 Pyrene fluorescence quenching by CPC in 100 mM aqueous
14-6-14,2Br- solutions in the presence of different wt%
a [Hmim][Br] and b [Omim][Br]. (Filled squares) Neat 14-6-14,
(filled circles) 0.2 wt%, (filled triangles) 0.4 wt%, (open squares)
0.6 wt%, (open circles) 0.8 wt% and (open triangles) 1.0 wt%,
respectively, under ambient conditions
123

J Surfact Deterg
slopes decrease upon addition of IL up to 0.4 wt% and then
increase. The decrease in slope is greater for [Hmim][Br]
than for [Omim][Br]. We calculate that as the concentra-
tion of IL increases, Nagg decreased up to 0.4 wt% and then
increases; the decrease being significantly more drastic for
[Hmim][Br] compared to that of [Omim][Br]. The decrease
in slope is due to the decreased efficiency of quenching,
which increases after 0.4 wt% addition of IL. Nagg of
aqueous 14-6-14,2Br- is in good agreement with that
reported in the literature [59].
presence of [Hmim][Br] than [Omim] [Br]. The decrease in
Nagg is significantly greater in the presence of [Hmim][Br]
than [Omim][Br], which may be due to the lesser
hydrophobicity of [Hmim][Br] as compared to
[Omim][Br].
Dynamic Light Scattering for Average Aggregate
Size
Dynamic light scatteringwas used to measure the average size
ofmicellar aggregatesin100 mM14-6-14,2Br- solutionwith
and without the addition of IL. Figure 4 shows the scattering
intensity versus diameter (D) measured at 298.15 0.1 K
upon addition of [Hmim][Br] and [Omim][Br]. Monomodal
distribution is observed at all concentration of additives. From
the DLS data (Table 3) it is clear that the peak diameter of
micellar aggregates is similar for both IL. The diameter
increases sharply up to 0.4 wt% after which there is no change
in diameter with increasing IL concentration. Several factors
can influence the micellar structure of the cationic gemini
surfactant upon the addition of IL. These factors include
electrostatic interactions, changes in the structure of water,
and the hydrocarbon–hydrocarbon repulsion. The Br- ions
from the IL can diminish the repulsive force between the head
groups of the gemini surfactant, which makes it possible for
the surfactant molecules to pack tighter and form the larger
aggregates, which is reflected in the increase in the diameter.
Additionofthe inorganic electrolyte (KBr) has a similareffect
on cationic gemini surfactants [15]. According to Yu et al.
[61], addition of organic electrolyte induces a morphology
change from micelles to vesicles whereas the inorganic elec-
trolytes only change the size of the aggregates. At higher IL
concentrations, the peak diameter remains constant but the
The Nagg behavior can be explained on the basis of
electrostatic and hydrophobic interactions between the
cations of the IL and the head group of the gemini sur-
factant. The results suggest that electrostatic repulsion
dominates over the hydrophobic interactions. This leads to
the formation of a loosely packed micellar structure. As the
concentration of the IL increases beyond 0.4 wt%, more IL
molecules enter into the micellar structure. The decrease of
Nagg along with the increased average size of aggregates
(Table 3) may be due to water penetration into at the
aggregates [37]. Water penetration is more apparent in the
Table 3 Aggregation number (Nagg) and micellar size (nm) of
aqueous gemini surfactant (14-6-14,2Br-) in presence of [Hmim][Br]
and [Omim][Br] at 298.15 K
IL in wt% [Hmim][Br]
[Omim][Br]
Nagg
Diameter (nm) Nagg Diameter (nm)
0.0
0.2
0.4
0.6
0.8
1.0
30 (32) [59] 2.96
30
28
24
32
29
43
2.96
3.46
4.07
4.09
4.09
4.09
29
22
29
29
31
3.46
4.08
4.10
4.10
4.10
Nagg increases slightly and its value is higher for [Omim][Br]
than for [Hmim][Br].
Standard uncertainties u are: u(Nagg) = 2, u(D) = 0.40 nm
Fig. 4 Dynamic light scattering (DLS) results of 100 mM aqueous
14-6-14,2Br- in the presence of increasing concentration of
a [Hmim][Br], b [Omim][Br]. (Filled squares) Neat 14-6-14, (filled
circles) 0.2 wt%, (filled triangles) 0.4 wt%, (open squares) 0.6 wt%,
(open circles) 0.8 wt% and (open triangles) 1.0 wt%, respectively,
under ambient conditions
123

J Surfact Deterg
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surfactant interactions: 5: equilibrium studies on the binding of
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Conclusion
The behavioral changes of cationic gemini surfactants in
the presence of cationic IL were studied using a number of
different techniques. The CMC of cationic gemini surfac-
tant,
1,6-hexanediyl-a,x-bis(dimethyltetradecyl)ammo-
nium bromide (14-6-14,2Br-) studied herewith decreased
with increasing IL concentration and the behavior was
compared with common electrolytes. Increasing the alkyl
chain length of the IL, decreases the CMC due to an
increased hydrophobicity of the medium and the spacing
between the adjacent molecules increases. The somewhat
higher value of the CMC measured by dye solubilization
may be due to the interaction of the anionic dye with the
cationic gemini surfactant. The increased hydrophobicity
of the medium was confirmed by the red shift of the kmax
.
Decreases in Nagg with increasing average size of aggre-
gates may be due to water penetration into the aggregates
[37] which is more apparent in the presence of the less
hydrophobic IL. Overall, IL behave like common elec-
trolytes at lower concentrations and more like cosurfactants
at higher concentrations.
13. Bernheim-Groswasser A, Zana R, Talmon Y (2000) Sphere-to-
cylinder transition in aqueous micellar solution of a dimeric
(gemini) surfactant. J Phys Chem B 104:4005–4009
14. Cl Oelschlaeger, Waton G, Candau SJ, Cates ME (2002) Struc-
tural, kinetics, and rheological properties of low ionic strength
dilute solutions of a dimeric (gemini) surfactant. Langmuir
18:7265–7271
15. Kabir-ud-Din, Siddiqui US, Ghosh G (2009) Growth of gemini
surfactant micelles under the influence of additives: DLS studies.
J Dispers Sci Technol 30:1310–1319
16. Alargova RG, Kochijashky II, Sierra ML, Kwetkat KJ, Zana R
(2001) Mixed micellization of dimeric (gemini) surfactants and
conventional surfactants: II. CMC and micelle aggregation
Acknowledgments UM is thankful for financial assistance by DST,
Government of India (Project No. SR/FT/CS-014/2010) and NM is
thankful for financial assistance by CSIR, Government of India
(Project No. EMR-II/2545/2011). ZV acknowledges the financial
assistance in the form of a Maulana Azad National Fellowship (F1-
17.1/2012-13/MANF-2012-13-MUS-GUJ-10818). NM acknowledges
Dr. Arvind Kumar, Scientist, CSMCRI for the DLS and Fluorescence
measurements.
numbers for various mixtures.
235:119–129
J Colloid Interface Sci
17. Wattebled L, Laschewsky A (2007) Effects of organic salt
additives on the behavior of dimeric (‘‘gemini’’) surfactants in
aqueous solution. Langmuir 23:10044–10052
18. Rodriguez A, Graciani MDM, Angulo M, Moya ML (2007)
Effects of organic solvent addition on the aggregation and
micellar growth of cationic dimeric surfactant 12-3-12,2Br-.
Langmuir 23:11496–11505
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Utkarsh U. More earned his M.Sc. from Shivaji University, India
and has worked as a project fellow under the Department of Science
and Technology, Government of India project for the last 4 years,
with a specialization in the field of surfactants and their interfacial
phenomena.
Pallavi Kumari earned her M.Sc. from the Sardar Vallabhbhai
National Institute of Technology, Surat in the field of Industrial
Chemistry. She did her master’s thesis work on ionic liquids as
modifiers of gemini surfactants.
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73:3385–3388
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electrolytes on surface tension and surface adsorption of 1-hexyl-
3-methylimidazolium chloride ionic liquid in aqueous solution.
J Chem Thermodyn 42:962–966
Zuber S. Vaid earned his M.Sc. from Veer Narmad South Gujarat
University, India and is working as a project fellow under the
Maulana Azad National Fellowship funded by the Government of
India for the last 3 years, specializing in the field of synthesis and
characterization of ionic liquids and their interfacial phenomena as
the surface active agents.
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Kamalakanta Behera obtained his Ph.D. from the Indian Institute of
Technology, New Delhi on the research topic of ionic liquids as
property modifiers of conventional cationic surfactants. He is doing
his postdoctoral work at the Division of Chemistry and Biological
Chemistry, School of Physical and Mathematical Sciences, Nanyang
Technological University (NTU), Singapore.
Naved I. Malek obtained his Ph.D. from Veer Narmad South Gujarat
University, India worked as the Post Doc Fellow at Universidade de
Sao Paulo, Brazil with Professor Omar El Seould and now working as
an Assistant Professor at the Sardar Vallabhbhai National Institute of
Technology. He has been involved in research dealing with surfac-
tants and ionic liquids for the past 10 years.
123