Study on premicellar and micellar aggregates of gemini surfactants with hydroxyl substituted spacers in aqueous solution using a probe showing TICT fluorescence properties

Article abstract of DOI:10.1016/j.jphotochem.2011.07.009

The aggregation properties of two gemini surfactants, 12-3(OH)-12,2Br ^- and 12-4(OH)₂-12,2Br^- with hydroxyl substituted spacer group have been studied. The changes in photophysical properties of a single probe, trans-2-[4-(dimethylamino)styrylbenzothiazole (DMASBT) showing dipolar nature in its twisted intramolecular charge transfer (TICT) excited state have been exploited rather than using multiple probes to describe various properties of micellar aggregates. Formation of a number of premicellar aggregates has been demonstrated in addition to the description of the micropolarity and the microviscosity of environment using steady-state fluorescence spectroscopy and fluorescence anisotropy of DMASBT. Conductometric measurements have been carried out to determine degree of micellar ionization (α) and to verify critical micelle concentration (CMC) values estimated by fluorescence method. Hydroxyl substituted spacer group induces the formation of premicellar aggregates. The micropolarity of environment around probe molecules increases on going from premicellar to micellar aggregates. The growth of micellar aggregates has been demonstrated by a continuous increase in the microviscosity of environment. The micropolarity of micellar environment of 12-4(OH)₂-12 is found to be less than that of 12-3(OH)-12. The microviscosity of premicellar and micellar aggregates of 12-4(OH)₂-12 are higher than that of 12-3(OH)-12. CMC increases, whereas α decreases with increasing spacer chain length as well as number of hydroxyl substitution of a spacer group.

Full text of DOI:10.1016/j.jphotochem.2011.07.009

Journal of Photochemistry and Photobiology A: Chemistry 223 (2011) 6–13
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Study on premicellar and micellar aggregates of gemini surfactants with
hydroxyl substituted spacers in aqueous solution using a probe showing TICT
fluorescence properties
Amit K. Tiwari, Sonu, M. Sowmiya, Subit K. Saha∗
Department of Chemistry, Birla Institute of Technology and Science, Pilani 333031, Rajasthan, India
a r t i c l e i n f o
a b s t r a c t
The aggregation properties of two gemini surfactants, 12-3(OH)-12,2Br− and 12-4(OH)2-12,2Br− with
hydroxyl substituted spacer group have been studied. The changes in photophysical properties of a
single probe, trans-2-[4-(dimethylamino)styryl]benzothiazole (DMASBT) showing dipolar nature in its
twisted intramolecular charge transfer (TICT) excited state have been exploited rather than using mul-
tiple probes to describe various properties of micellar aggregates. Formation of a number of premicellar
aggregates has been demonstrated in addition to the description of the micropolarity and the micro-
viscosity of environment using steady-state fluorescence spectroscopy and fluorescence anisotropy of
DMASBT. Conductometric measurements have been carried out to determine degree of micellar ion-
ization (˛) and to verify critical micelle concentration (CMC) values estimated by fluorescence method.
Hydroxyl substituted spacer group induces the formation of premicellar aggregates. The micropolarity
of environment around probe molecules increases on going from premicellar to micellar aggregates. The
growth of micellar aggregates has been demonstrated by a continuous increase in the microviscosity of
environment. The micropolarity of micellar environment of 12-4(OH)2-12 is found to be less than that of
12-3(OH)-12. The microviscosity of premicellar and micellar aggregates of 12-4(OH)2-12 are higher than
that of 12-3(OH)-12. CMC increases, whereas ˛ decreases with increasing spacer chain length as well as
number of hydroxyl substitution of a spacer group.
Article history:
Received 15 April 2011
Received in revised form 21 June 2011
Accepted 15 July 2011
Available online 23 July 2011
Keywords:
DMASBT
Gemini surfactant
TICT fluorescence
Premicellar aggregation
Micropolarity
Microviscosity
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
3(OH)-16 with chloride ions as counter ions in 0.1 M NaCl at 50 ◦C
and of 16-4(OH)2-16 with bromide ions as counter ions in aqueous
A class of surfactants called ‘Gemini’ containing two hydropho-
bic tails and two hydrophilic headgroups connected by a spacer
group has attracted special research interests for their enhanced
surface properties [1–3]. There are reports on aggregation behavior
of gemini surfactants depending on the chemical nature of hydro-
carbon chains and spacer groups [4–7]. A recent article [4] describes
the effect of spacer group on the association behavior of gemini
surfactants in aqueous medium.
In the present work, two bis(quaternary ammonium bromide)
surfactants having n-alkyl tails of 12 carbon atoms in length and
a spacer group of 3 methylene units in length with one hydroxy
(12-3(OH)-12)- and a spacer group of 4 methylene units in length
with two hydroxy (12-4(OH)2-12)-substituted methylene groups
are chosen. Their structures are represented by Scheme 1 (denoted
as Gemini-1 and Gemini-2, respectively hereafter). Mathias et al.
[6] reported the premicellar aggregation of 14-3(OH)-14 and 16-
medium at 25 ◦C using steady-state and time-resolved fluorescence
quenching of pyrene. However, they did not notice the formation
of premicellar aggregates in cases of 12-3(OH)-12 and 12-4(OH)2-
12.
It has been observed in many cases of surfactants that the occur-
rence of the micellar phase is preceded by the formation of different
comparatively smaller aggregates known as premicellar aggregates
[6,8–11]. Hadgiivanova et al. [12,13] have developed a thermody-
namic model to study the mechanism of aggregation. There are
reports in the literature [5,6,14–18] for some gemini surfactants
without hydroxyl substituted spacer group in aqueous solution
that they start to self-aggregate at concentrations below the crit-
ical micelle concentration (CMC) only when the surfactant alkyl
chains are long enough. However, Pei et al. [19] in their recent work
have indicated the formation of premicellar aggregates of Gemini-
1. In view of these reports, present work is based on aggregation
behavior of both Gemini-1 and Gemini-2 in their submicellar con-
centration regions. The highly sensitive fluorescence properties of a
probe, trans-2-[4-(dimethylamino)styryl]benzothiazole (DMASBT)
(Scheme 2) to the polarity [20–22] and to the viscosity [23,24]
∗
Corresponding author. Tel.: +91 1596 515238; fax: +91 1596 244183.
E-mail addresses: subitksaha@gmail.com, sksaha@bits-pilani.ac.in (S.K. Saha).
1010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2011.07.009

A.K. Tiwari et al. / Journal of Photochemistry and Photobiology A: Chemistry 223 (2011) 6–13
7
micropolarity and microviscosity of environment around probes in
various aggregates.
2Br-
H3C
H3C
CH3
+
+
N
N
2. Experimental
CH3
C
12H25
OH
C12H25
2.1. Materials
1,3-bis(dodecyl-N,N-dimethylammonium bromide)-2-propanol (Gemini-1)
2Br-
The procedures for synthesis of surfactants used in this study
were reported in the literature [27]. Synthesis of Gemini-1 and
Gemini-2 were carried out by reacting required amounts of
1,3-dibromo-2-propanol and 1,4-dibromo-2,3-butanediol, respec-
tively with 2 molar equivalent (plus a 10% excess) of N,N-
dimethyldodecylamine. The mixture was refluxed in dry ethanol
for 72 h and then cooled at the end of the reaction. The solid material
obtained from the reaction was recovered by filtration and recrys-
tallized several times from a mixture of ethylacetate/methanol
(10:1 v/v). The structures of synthesized compounds were con-
firmed by FT-IR and 1H NMR data.
DMASBT was procured from Aldrich Chemical Company, WI,
USA. The methods of recrystallization and purity check of DMASBT
are mentioned elsewhere [22]. Triple distilled water was used for
the preparation of aqueous solutions. All other solvents used were
of spectroscopic grades and procured from Spectrochem Chemi-
cal Company, India. KH2PO4 and Na2HPO4 used for the adjustment
of pH of the solutions were procured from Qualigens, India. To
record UV–visible absorption and fluorescence spectra, the aque-
ous solutions of different concentrations of a gemini surfactant
were prepared with constant concentration of DMASBT. 0.05 mL
of a stock solution of DMASBT (1 mM) in methanol was added to
required amount of an aqueous solution of gemini with an extra
0.05 mL of methanol and the final volume of it was adjusted to
10 mL using water. The concentration of DMASBT in final solution
was 5 ␮M. Methanol was added due to low solubility of DMASBT
in water. However, only one percent of methanol was present in
each experimental solution. The fluorescence quantum yields were
determined with respect to that of quinine sulfate in 0.1 N H2SO4
(0.55). All aqueous solutions were adjusted to a pH value of 7.4 by
the use of a dilute solution of phosphate buffer.
CH 3
+
H3C
N
+
N
CH 3
H3C
OH
C12H25
C12H25
OH
1,4-bis(dodecyl-N,N-dimethylammonium bromide)-2,3-butanediol (Gemini-2)
Scheme 1. Chemical structure of gemini surfactants.
of environment have been explored earlier to study the aggrega-
tion behavior of conventional surfactants [20,21]. In the present
work, we have used steady-state fluorescence and fluorescence
anisotropic properties of DMASBT showing twisted intramolecular
charge transfer (TICT) fluorescence [22] to characterize the self-
aggregation behavior of gemini surfactants. Studies have shown
that the dipolar nature of DMASBT in its TICT excited state, makes
it useful as a surface probe for phenomena such as premicellar and
micellar aggregation of surfactants.
DMASBT gives normal fluorescence from the locally excited (LE)
state in a nonpolar medium, whereas a highly Stokes-shifted fluo-
rescence from the TICT state is observed in a polar medium [22].
The TICT state (S3 state) is originated as a result of transfer of
electron from the twisted donor group, –N(CH3)2 (torsion angle,
ꢀ = 90◦, Scheme 2) to the acceptor group, styrylbenzothiazole [22].
The nonradiative processes become faster in a highly polar medium
because of closer proximity of the TICT state towards triplet as well
as SToICT states as a result of greater stabilization of the TICT state.
With decreasing the polarity of the medium, the TICT fluorescence
quantum yield increases with the blue shift of fluorescence due to
the concomitant increase in the energy gap between ST3ICT and SoTICT
states as well as triplet state.
2.2. Methods
In addition to fluorescence, conductometric measurements have
also been performed to support the CMC values obtained from the
fluorescence method and also to determine the degree of micel-
lar ionization (˛). Gemini-1 and Gemini-2 are chemically different
only in terms of length of spacer groups and number of hydroxyl
substitution in it. This work aims to examine the combined effect
of increase of spacer chain length and the number of hydroxyl
substitution in it on the properties of aggregates viz. premicel-
lar aggregation concentration, CMC, degree of micellar ionization,
micropolarity and microviscosity of environment around probe
molecule. Generally, pyrene is used to determine the micropolar-
ity [25,26], whereas diphenylhexatriene (DPH) is used to determine
the microviscosity [26]. However, in the present study, the changes
in fluorescence properties of a single probe molecule enabled us to
do multiple things like demonstration of an important event in the
colloid science, i.e. premicellar aggregation and determination of
The absorption spectra were recorded using a Hitachi U-
2900 UV–visible spectrophotometer. Fluorescence measurements
were performed using a Horiba Jobin Yvon Fluoromax-4 scan-
ning spectrofluorimeter. The steady state fluorescence anisotropy
measurements were performed with the same steady state spec-
trofluorimeter fitted with a polarizer attachment. The excitation
and emission bandwidths used for the anisotropy measurements
were 5 nm each. The details of estimation of steady-state anisotropy
(r) values are given elsewhere [23,28]. All spectroscopic measure-
ments were done at room temperature, 25 1 ◦C. The conductivity
and pH measurements were performed using direct reading Eutech
Instruments combined pH and conductometer, model PC 510.
The conductivity dip cell (cell constant = 1.0 cm−1) was calibrated
with a standard KCl solution of specific conductivity 1413 ␮S cm−1
procured from Merck. Concentrated stock solutions of gem-
ini surfactants were prepared by dissolving required amounts
of surfactants in triple distilled water of specific conductivity
2–4 ␮S cm−1. Stock solution was then added progressively using
a micropipette to a container containing 20 mL of water kept
in a thermostat with a temperature accuracy of 0.01 ◦C. Before
the measurement of specific conductance, ꢁ, proper mixing and
equilibration of solutions were ensured. Molar conductivity value
was estimated as (ꢁ − ꢁo)/C (where ꢁo is specific conductance of
water) from the experimental value of ꢁ [5,19]. All pH and con-
S
φ
N
N
Scheme 2. Molecular structure of DMASBT.

8
A.K. Tiwari et al. / Journal of Photochemistry and Photobiology A: Chemistry 223 (2011) 6–13
160
120
80
40
0
[Gemini-1]
(mM)
0
Gemini-1
Gemini-2
a
0.01
0.07
0.10
0.15
0.20
0.40
0.72
0.90
1.00
2.00
cmc = 0.78 mM
cmc = 0.67 mM
κ
0.0000
0.0008
0.0016
0.0024
0.0032
450
500
550
600
650
[Gemini] (M)
Wavelength (nm)
Fig. 2. Plots of specific conductivity (ꢁ) versus concentration of aqueous solutions
of gemini surfactants at 25 ◦C.
[Gemini-2]
(mM)
0
b
0.010
0.012
0.100
0.125
0.400
0.450
0.790
2.000
3.000
∼450 nm and a highly Stokes-shifted TICT fluorescence in a polar
medium with a broad structureless fluorescence band of peak max-
imum at ∼520 nm (in aqueous medium). Therefore, the structured
fluorescence bands observed in the present study at a low concen-
tration range of both the surfactants are because of the presence of
DMASBT molecules in a microenvironment of low polarity giving LE
fluorescence to some extent. Evolution of structured fluorescence
bands at ∼506 nm at low concentration of solutions of both Gemini-
1 and Gemini-2 surfactants suggests that the probe molecules
are residing neither in a very hydrophobic region nor in a highly
polar region but in a region with an intermediate polarity favor-
ing some extent of LE fluorescence. The decrease in the polarity of
the environment around the probe molecules due to the penetra-
tion of more and more molecules inside the surfactant aggregates is
responsible for the blue shift and the accompanying increase in the
fluorescence intensity of the fluorophore. The initial blue shift of the
fluorescence band of DMASBT indicates its interaction with the sur-
factant aggregates and its possible location somewhere in the upper
part of the hydrophobic region [20,21] near the headgroups of the
premicellar aggregates (discussed later). It is important to note that
when the concentration of Gemini-1 reaches 0.72 mM, the fluores-
cence band is red-shifted by 3 nm (Fig. 1a) giving peak maximum at
509 nm. Similar change in a fluorescence band occurs at 0.79 mM
concentration of Gemini-2 (Fig. 1b) with a band of same peak maxi-
mum at 509 nm. In fact, at this concentration of Gemini-2, there is a
prominent change in the structured nature of the band. The shoul-
der of the band at lower wavelength side is completely diminished.
Based on the changes in the nature of fluorescence bands and inten-
sity (discussed in Section 3.2), the CMCs of Gemini-1 and Gemini-2
are found to be 0.72 mM and 0.79 mM, respectively. The CMC val-
ues are also determined by the conductivity measurements. Fig. 2
represents the plots of specific conductivities (ꢁ) of aqueous solu-
tions of Gemini-1 and Gemini-2 against their concentrations. The
CMC values at 25 ◦C have been measured from the inflection point
of the plot of ꢁ versus surfactant concentration [29]. Although,
the conductivity measurements give CMC of Gemini-1 as 0.67 mM,
the same is 0.78 mM for Gemini-2. The CMC values determined by
two methods are well corroborated. The structured characteristics
of a fluorescence band starts disappearing with a red-shift of the
band after the formation of micelles indicating the fact that the
microenvironment around the probe is more polar in micelles than
that in the premicellar aggregates [20,21]. It is pertinent to note
that the fluorescence bands become completely structureless and
broad at 1 mM of Gemini-1 and 2 mM of Gemini-2, and undergo a
440
480
520
560
600
640
Wavelength (nm)
Fig. 1. Fluorescence emission spectra of DMASBT in various concentrations of aque-
ous solutions of (a) Gemini-1 and (b) Gemini-2. ꢂex = 370 nm, [DMASBT] = 5 ␮M.
ductometric measurements were carried out at a temperature of
25 0.01 ◦C.
3. Results and discussion
3.1. Fluorescence spectral properties of DMASBT in aqueous
solutions of Gemini-1 and Gemini-2
The absorption spectra of DMASBT in aqueous solutions of
Gemini-1 and Gemini-2 of various concentrations did not show
much variation indicating the low sensitivity of absorbance by the
change in concentration of surfactant. The fluorescence spectra
of DMASBT in some solutions of Gemini-1 are shown by Fig. 1a.
A fluorescence band appears at ∼520 nm in water, which con-
tinued to be blue shifted up to ∼506 nm at a concentration of
∼0.07 mM of Gemini-1 with the development of a structured band.
It is noteworthy that the peak maxima almost remain constant
after this concentration, however, intensities of structured fluores-
cence bands increase with increasing concentration of surfactant
with some irregularity at a few concentrations (discussed later).
The fluorescence bands are structured up to 0.9 mM concentra-
tion of Gemini-1. However, a structureless broad single band starts
appearing above this concentration. In case of Gemini-2 (Fig. 1b),
similar broad band appears at a little higher concentration of it
as compared to that of Gemini-1. Our previous work [22] has
shown that DMASBT exhibits LE fluorescence in a nonpolar medium
giving a structured fluorescence band with a peak maximum at

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9
significant red shift (Fig. 1a and b). This suggests that the microenvi-
ronment around DMASBT molecules in the aggregates of Gemini-1
and Gemini-2 favors only TICT fluorescence at this concentration
range. It can be mentioned here that the DMASBT in its TICT form
has a positive charge on the nitrogen atom of –N(CH3)2 group and a
dispersed negative charge on the styrylbenzothiazole moiety [22].
Thus after the formation of micelles, DMASBT molecules are some-
what pushed out to the surface from the hydrophobic region due
to the interaction between its dipole and the surface charges of
micelles. Hence, DMASBT molecules experience a more polar envi-
ronment, which results in the less structured fluorescence band at
micellar region followed by completely structureless bands at post-
micellar concentrations. The probe molecules are located at a more
polar environment of aggregates in postmicellar solutions giving
completely structureless, broad TICT fluorescence bands. Location
of probe molecules at a more polar environment of micellar aggre-
gates compared to that of premicellar aggregates has also been
observed in our previous work with the micelles of conventional
surfactants [20,21].
The peak maxima of broad, structureless TICT fluorescence
bands of DMASBT in solutions of high concentrations (1.5–2.5 times
of CMC) of Gemini-1 and Gemini-2 appear at ∼537 nm and at
∼525 nm, respectively. The intensity of such fluorescence further
increases with increasing surfactant concentration (Fig. 1a and b)
without any change in peak position. It has been discussed later
that in this concentration range, at some concentrations of Gemini-
1 and Gemini-2, some rearrangements of surfactant molecules or
changes in sizes/shapes of aggregates occur before micellization
process approaches saturation. It is noteworthy that the fluores-
cence bands are red shifted with respect to the TICT band in pure
water at this concentration range of surfactant solutions. Moreover,
the fluorescence intensities of respective fluorescence bands are
also much higher than that in water. Greater stabilization of TICT
state (red shift of the band) in these micellar aggregates compared
to that in water is in accordance with the stronger interactions
between the dipole of a DMASBT molecule and the surface charges
of the micelles. The higher fluorescence intensity in this phase as
compared to that in water and also the increase in intensity with
increasing concentration of surfactant can be explained by the fact
that the probe molecules are staying more and more in the polar
region with increasing size of aggregates and at the same time the
negative end of dipole of a DMASBT molecule is possibly getting
increasingly attached to the positive surface charges of the aggre-
gates electrostatically [20,21]. As a result, the molecular motions
are getting restricted thus increasing the fluorescence intensity as
has been observed at a concentration of surfactant much higher
than CMC. This increase is supported by the increase in fluores-
cence anisotropy (discussed later), which reflects the rigidity of the
surrounding environment of DMASBT [20,21]. Unlike other TICT
probe, such as dimethylaminobenzonitrile (DMABN) which shows
a continuous increase in intensity with hypsochromic shift with the
increase in the surfactant concentration [30,31], DMASBT does not
penetrate to the hydrophobic core of the micelles. This is proba-
bly because of its cylindrical nature, with some degree of flexibility
in the middle. That is why DMASBT acts as a surface probe and
becomes efficient and sensitive to the changing environment in
every phase of the premicellar aggregation.
in the polarity of microenvironment of micellar aggregates with
increasing number of hydroxyl substitution and carbon atoms of
the spacer group of surfactant. Two conclusions can be drawn from
this observation: (i) highly Stokes-shifted charge-transfer bands
suggest that the probe molecules reside near to the wet Stern
layer [22,32], as a result of electrostatic interactions between the
dipole of a DMASBT molecule and the surface charges, and (ii) the
environment surrounding probe molecules in Gemini-2 micelles
is comparatively less polar than that in Gemini-1. The second fac-
tor could be due to the protection of the probe molecules to be in
contact with the water molecules as a result of greater extent of
hydrogen bonding interactions between the hydroxyl groups on a
spacer group in Gemini-2 and the water molecules [33]. It attributes
that the screening of water molecules through hydrogen bonding
interactions inhibits probe molecules to be in contact with water
molecules which results in giving less polar microenvironment of
probe in Gemini-2. Thus, probe molecules in case of Gemini-2 are
located in such a region which is comparatively less wet than that of
Gemini-1. Of course, it is becoming possible because of short chain
spacer group being located at the interface of aggregate and water
compared to longer spacer group being folded into the core of an
aggregate [4,16,33,34].
3.2. Determination of premicellar and critical micellar
concentrations by correlating steady-state fluorescence with
fluorescence anisotropy
Fluorescence intensity and fluorescence anisotropy of DMASBT
at various concentrations of Gemini-1 and Gemini-2 are plotted in
Fig. 3a and b, respectively with the intention of characterization
of their aggregates [20,21]. Depending on the polarity of aggre-
gates, DMASBT molecules get attached to different parts of them.
The rearrangements of surfactant molecules during the process
of formations of different premicellar and micellar aggregates are
demonstrated and indicated by thin arrows in Fig. 3a and b for
Gemini-1 and Gemini-2, respectively. The concentrations of sur-
factant solutions at which all these aggregates formed are also
mentioned in the same figures. The change in the fluorescence
intensity reflecting one such process has been correlated with the
fluorescence anisotropy [35]. As mentioned above, in the premi-
cellar aggregates and the micellar aggregates at CMC, DMASBT
presumably gives LE fluorescence. However, fluorescence emis-
sions of DMASBT in the postmicellar aggregates mostly occur from
its TICT state. Therefore, during the course of micellization through
the formation of smaller premicellar aggregates, there is a progres-
sive change in the nature of the probe molecule and its location
in the aggregates reflecting the changes in its photophysical prop-
erties [20,21]. In general, whenever there is a new rearrangement
of the surfactant molecules especially during the formation of a
new premicellar aggregate, DMASBT molecules experience more
polar environment as a result of greater exposure to the aque-
ous environment showing decrease in the fluorescence intensity
(Fig. 3) [22]. This is also supported by the reduction in fluores-
cence anisotropy because of greater extent of molecular motion
(Fig. 3) [26,35]. Increase in intensity happens right after this event,
mostly resembling the restricted rotational motion of DMASBT
molecules, which is supported by the increase in fluorescence
anisotropy (Fig. 3). The values of premicellar concentrations, CMC
and the postmicellar concentrations (CMCꢀ and/or CMCꢀꢀ) corre-
sponding to the rearrangements or changes in sizes/shapes of
micelles are determined by monitoring simultaneous increase in
the fluorescence intensity and the fluorescence anisotropy and are
given in Table 1 along with the CMC values reported in the lit-
erature [33,36]. Observed CMCs are well corroborated with the
literature CMC values. Four to six distinct kinds of aggregations
including rearrangements or changes in sizes/shapes of micelles
The same peak maxima of structured fluorescence bands noted
at the low concentration range of solutions of both Gemini-1 and
Gemini-2 depict that the microenvironment around the probe in
the premicellar aggregates and that in the micellar aggregates
at their CMCs are same for both the cases. However, the envi-
ronment around the probe molecules in Gemini-2 postmicellar
aggregates is less polar than that in Gemini-1. The peak position
of a TICT band in Gemini-2 (∼525 nm) is blue shifted by ∼12 nm as
compared to that in Gemini-1 (∼537 nm) indicating the decrease

10
A.K. Tiwari et al. / Journal of Photochemistry and Photobiology A: Chemistry 223 (2011) 6–13
a
120
100
80
a
0.32
0.30
0.28
0.26
0.24
0.22
0.20
d
c
a: 0.002 mM
b: 0.100 mM
c: 0.720 mM
d: 2.000 mM
60
b
a
40
0.00
0.01
0.02
0.03
C0.5(M0.5
0.04
0.05
0.06
-6
-5
-4
-3
-2
log([Gemini-1]/M)
)
b
140
120
100
80
b
0.3
0.2
0.1
0.0
f
e
d
a: 0.05 mM
b: 0.07 mM
c: 0.20 mM
d: 0.79 mM
e: 0.98 mM
f: 2.00 mM
60
40
c
20
b
a
-6
-5
-4
-3
-2
0
0.00
0.01 0.02
0.03 0.04
0.05 0.06
log([Gemini-2]/M)
C0.5(M0.5
)
Fig. 3. Variation of fluorescence intensity and fluorescence anisotropy (r) of
DMASBT as a function of (a) Gemini-1 (anisotropy at ꢂem = 535 nm) and (b) Gemini-2
(anisotropy at ꢂem = 520 nm) concentrations. The thin arrows indicate the premicel-
lar, critical micellar and concentrations of surfactants where micellar aggregates
change their arrangements or sizes/shapes and the thick grey arrows indicate the
corresponding y-axes.
Fig. 4. Plots of variation of molar conductivity (ꢃ) with C0.5 (where C = concentration
of surfactant) of (a) Gemini-1 and (b) Gemini-2 at 25 ◦C. The arrow in each plot
indicates the CMC as obtained from the corresponding ꢁ versus C plot in Fig. 2.
have been observed for the concentration range studied in the
present work (Table 1). A rearrangement process occurring even
after the start of micellization at CMC is probably representing
a phase where there is a readjustment of the shapes or sizes
of micelles before approaching complete micellization [20]. The
irregular behavior of fluorescence intensity with concentration of
surfactant (Fig. 3) is because of surface-probing nature of DMASBT
with its dual emission properties (LE and TICT fluorescence) con-
trolled by the changing microenvironment of the probe in the
aggregates during micellization through the formation of premi-
cellar aggregates.
The occurrence of premicellar aggregation was noticed by many
groups [5,6,14–18]. Zana [5] reported self-aggregation at submicel-
lar concentration of solutions of gemini surfactants, m-s-m (where
m = number of carbon atoms in alkyl chain and s = number of carbon
atoms of an alkanediyl spacer group) with only m ≥ 14. However,
in the present study, the formations of premicellar aggregates
have been observed even if m = 12 with short chain hydroxyl sub-
stituted spacer groups. Therefore, it seems that hydroxyl groups
present in the spacer groups are inducing the formation of pre-
micellar aggregates may be through intermolecular hydrogen
bonding interactions [19]. More number of premicellar aggregates
for Gemini-2 surfactant molecules as compared to that for Gemini-
1 is in accordance with the greater extent of hydrogen bonding
interactions as the number of substituted hydroxyl groups in the
spacer group of the former is higher than that of the later. Recently,
premicellar aggregation of Gemini-1 has been indicated by Pei
et al. [19]. However, in the present work, formations of multiple
premicellar aggregates have been demonstrated. The occurrences
of premicellar aggregation have been confirmed by conductivity
measurements [4,5,37]. The variation of molar conductivity (ꢃ)
with C0.5 (where C = concentration of surfactant) of Gemini-1 and
Gemini-2 are represented by Fig. 4a and b, respectively. The ꢃ
versus C0.5 plot for both the geminis show a maximum. This max-
imum is the signature of premicellar aggregation in a solution of
surfactant. It arises because the equivalent conductance of a small
Table 1
Premicellar and micellar concentrations of Gemini-1 and Gemini-2 at 25 ◦C.
Gemini-1
Phasea
Gemini-2
Phase
[Gemini-1] (mM)
[Gemini-2] (mM)
Ag1
0.002
0.100
0.720
2.000
Ag1
Ag2
Ag3
0.05
0.07
0.20
0.79
0.98
2.00
Ag2
CMCb
CMCꢀ
CMCc
CMCꢀ
CMCꢀꢀ
a
Ag stands for a premicellar aggregation.
b
c
Literature value: 0.78 mM for Gemini-1 with Cl− ion as counter ion, Ref. [36].
Literature value: 0.87 mM, Ref. [33].

A.K. Tiwari et al. / Journal of Photochemistry and Photobiology A: Chemistry 223 (2011) 6–13
11
aggregate formed by surfactant ions is larger than the sum of the
equivalent conductances of the ions constituting it [4,5,37].
It can be seen from Table 1 that the CMC values of Gemini-
1 and Gemini-2 are 0.72 mM and 0.79 mM, respectively at 25 ◦C.
There is a very small difference between the CMC values. These
values obtained from the conductivity measurements at 25 ◦C are
0.67 and 0.78 mM, respectively (Fig. 2). Fluorescence as well as
conductivity measurements show higher CMC value of Gemini-2
than that of Gemini-1. Wettig et al. [33] have reported the decrease
in CMC of gemini surfactants with increasing number of substi-
tuted hydroxyl groups on a spacer group containing 4 methylene
units (s = 4) in length. They have explained the decrease in CMC on
hydroxyl substitution on the basis of the fact that the substituted
spacer can form hydrogen bonds with water more readily, thereby
reducing the unfavorable interactions between the hydrocarbon
tails and water molecules. However, in the present case CMC value
is increased where both s and the number of hydroxyl groups on a
spacer group are increased for Gemini-2 with respect to Gemini-1.
The increase in CMC with increasing s giving a maximum at a value
of s apparently equal to 5 has been reported [29] for a series of gem-
ini surfactants, 10-s-10,2Br− with s = 2, 3, 4 and 6. Also, Rodriguez
et al. [38] have noticed a continuous increase in CMC with increas-
ing s for a series of gemini surfactants, 12-s-12,2Br− with spacer
groups of s = 3, 4 and 5. Based on these reports, our suggestion is
that two opposing effects work for the change in CMC from Gemini-
1 to Gemini-2. With increasing the number of carbon atoms of a
spacer group CMC can increase, but on the other hand with increas-
ing the number of hydroxyl groups CMC decreases. As a result of
these opposing effects, there is only a slight increase in CMC of
Gemini-2 in comparison to Gemini-1. One can also conclude that
the effect of increased number of spacer carbon atoms on CMC is
more pronounced than that of increase in the number of substituted
hydroxyl groups in a spacer group.
against the ET(30) values corresponding to various dioxane–water
mixtures is given elsewhere [22] and also provided as supple-
mentary material (see Supplementary material, Fig. S1). The lower
micropolarity value in case of Gemini-2 as compared to that
of Gemini-1 micelles is in accordance with the fact that the
probe molecules are more protected from the exposure to the
water molecules in the former micelles because of greater extent
of hydrogen bonding interactions between substituted hydroxyl
groups on a spacer group and water molecules. Although, the dif-
ference between the micropolarities of Gemini-1 and Gemini-2 at
their micellar aggregates at CMCs is small, but it is expected to be
very high at their postmicellar aggregates as the difference between
the TICT peak maxima is significantly large (∼12 nm).
Since fluorescence anisotropy of a fluorophore is intimately
connected with the viscosity of the microenvironment around it,
microviscosity is often estimated from a comparison of the fluo-
rescence anisotropy of a fluorescent probe in an environment with
those of the probe in different environments of known viscosi-
ties [24,35,45–47]. With a similar intention, we have attempted
the fluorescence anisotropy measurements of DMASBT in differ-
ent percentages (w/w) of glycerol in glycerol–water mixtures and
compared the values with the anisotropy values of DMASBT in
various concentrations of Gemini-1 and Gemini-2. Details of the
method of determination of microviscosity are explained else-
where [23,24]. The steady-state fluorescence anisotropy values
(r) of DMASBT in solutions of various concentrations of Gemini-
1 and Gemini-2 are given in Table 2. The chosen concentrations
are their premicellar concentrations and various CMCs (CMC, CMCꢀ
and CMCꢀꢀ) obtained from fluorescence measurements (Table 1).
Correlating these anisotropy values (Table 2) with the anisotropy
values of DMASBT in various glycerol–water mixtures [24] (see
Supplementary material, Fig. S2) followed by correlating with the
viscosity of the mixtures [48] (see Supplementary material, Fig. S3),
the found microviscosity values (ꢄ) are also tabulated in Table 2. The
average microviscosity of premicellar aggregates of Gemini-1 and
Gemini-2 are 10.1 0.3 and 12.1 0.3, respectively. For micellar
aggregates, average values are found to be 33.7 0.3 and 43.5 0.3,
respectively. Therefore, one can correlate the growth of micellar
aggregates through the formation of premicellar aggregates with a
continuous increase in the microviscosity of environment around
probe. It is to be noted that the microviscosity of environment of
Gemini-2 where DMASBT molecules reside is greater than that of
Gemini-1.
The mean micellar aggregation number (Nagg) of two geminis at
5 mM concentration in aqueous medium have been estimated by
the usual method of steady-state fluorescence quenching of pyrene
(3 ␮M) by cetylpyridinium chloride (0–0.09 mM) [39–41]. The val-
ues of Nagg of Gemini-1 and Gemini-2 are found to be 27 and 23,
respectively. The lower Nagg of Gemini-2 as compared to that of
Gemini-1 is in accordance with the higher CMC of the former than
that of the later.
3.3. Micropolarity and microviscosity of environment around
DMASBT in various aggregates
3.4. Degree of micellar ionization (˛)
The micropolarity is expressed in equivalent scale of ET(30)
which is an empirical solvent polarity parameter comparing the
fluorescence behavior of the probe molecule in microheteroge-
neous systems to that in a mixture of homogeneous solvents
(dioxane–water) of varying compositions [20,21,24,35,42–44].
Details of the method of determination of micropolarity have
been given elsewhere [20,21,24]. The micropolarity of environ-
ment around DMASBT in micellar aggregates of gemini surfactants
at their CMCs expressed in equivalent scale of ET(30) have been
calculated. But, the micropolarity of micellar aggregates in post-
micellar solutions could not be calculated by this method as
electrostatic interactions between the dipole of DMASBT and sur-
face charges contribute to the stabilization of TICT states in these
aggregates. The Stokes-shift values of DMASBT at 0.72 mM of
Gemini-1 and 0.79 mM of Gemini-2 are found to be 4863 cm−1
The degree of micellar ionization, ˛ value has been calculated
from the ratio of the slopes of the two straight lines above and
below the CMC in the plot of ꢁ versus surfactant concentration
shown by Fig. 2 [29]. The ˛ along with the CMC (already mentioned
above) values thus obtained are presented in Table 3. The ˛ value of
Gemini-2 is lower than that of Gemini-1. It is reported that a polar
protic solvent can act as a good hydrating agent for an anion through
the formation of hydrogen bonds [49]. Therefore, a spacer group
of a Gemini-2 surfactant molecule having two hydroxyl groups is
expected to be a better hydrating agent for the counter ion, Br−
compared to that of a Gemini-1 surfactant molecule with only one
substituted hydroxyl group in it. This could be the possible reason
that Gemini-2 micellar surface will bind more number of Br− ions
as compared to the micellar surface of Gemini-1, which leads to
the lower value of ˛ of the former than the later. This explanation
has also been supported by the fluorescence quantum yield val-
ues of DMASBT in the micellar aggregates of geminis at their CMCs.
The quantum yield of DMASBT in 0.72 mM of Gemini-1 is found
to be 0.31, whereas the same in 0.79 mM of Gemini-2 is only 0.04.
These two concentrations (0.72 mM and 0.79 mM) as mentioned
ab
ab
(ꢂ
= 408 nm and ꢂflmax = 509 nm) and 4744 cm−1 (ꢂ
=
max
max
410 nm and ꢂflmax = 509 nm), respectively. The micropolarity val-
ues calculated comparing the Stokes-shift of DMASBT in micellar
systems to that in dioxane–water mixtures of varying composi-
tions [20,21,24,35,42–44] are found to be 45.8 0.5 and 44.3 0.5
for Gemini-1 and Gemini-2, respectively. The plot of Stokes-shift

12
A.K. Tiwari et al. / Journal of Photochemistry and Photobiology A: Chemistry 223 (2011) 6–13
Table 2
Steady-state fluorescence anisotropy (r) and microviscosity (ꢄ) of environment around the DMASBT molecule in premicellar and micellar aggregates of Gemini-1 and Gemini-2
at 25 ◦C.
[Gemini-1] (mM)
r
ꢄ (cP)
8.1 0.3
12.1 0.3
19.9 0.3
47.5 0.3
[Gemini-2] (mM)
r
ꢄ (cP)
8.1 0.3
12.1 0.3
16.0 0.3
27.8 0.3
35.7 0.3
67.0 0.3
0.002 (Ag1)
0.100 (Ag2)
0.720 (CMC)
2.000 (CMCꢀ)
0.21 0.01
0.05 (Ag1)
0.07 (Ag2)
0.20 (Ag3)
0.79 (CMC)
0.98 (CMCꢀ)
2.00 (CMCꢀꢀ)
0.21 0.01
0.24 0.01
0.26 0.01
0.30 0.01
0.23 0.01
0.25 0.01
0.27 0.01
0.28 0.01
0.31 0.01
Table 3
CMC, degree of micellar ionization (˛) and ꢅG◦m of Gemini-1 and Gemini-2 at 25 ◦C.
in addition to the determination of micropolarity and microvis-
cosity of environment. A progressive change in the location of
probe molecule and its nature during the process of aggregation
is reflected by the change in its fluorescence properties, which
have been explored for the characterization of aggregates espe-
cially multiple premicellar aggregates. It has been shown that the
effect of increase in spacer chain length on CMC is more pro-
nounced than that of increased number of hydroxyl substitution of
a spacer group. The occurrence of premicellar aggregation even in
the case of gemini surfactants with C12 hydrocarbon tails could be
induced by intermolecular hydrogen bonding interactions. Microp-
olarity of environment around probe molecules increases on going
from premicellar to micellar aggregates. The microenvironment of
Gemini-2 micellar aggregates is less polar than that of Gemini-1
due to the protection of probe molecules to be in contact with
water molecules as a result of greater extent of hydrogen bond-
ing interactions between the hydroxyl groups on spacer group in
Gemini-2 and water molecules. Because of greater extent of hydra-
tion of counter ions, Br− by hydroxyl groups of a spacer group in
Gemini-2 compared to that in Gemini-1, the ˛ value of the for-
mer is less than that of the later. The growth of micellar aggregates
has been demonstrated by a continuous increase in microviscos-
ity of aggregates on going from premicellar to micellar aggregates.
The microviscosity of environment around DMASBT in Gemini-2 is
higher than that of Gemini-1. A rearrangement process of postmi-
cellar aggregates represents a phase where there are readjustments
of the sizes/shapes of micelles itself before approaching complete
micellization.
Surfactant
CMC (mM)
˛
ꢅGm◦ (kJ mol−1
)
Gemini-1
Gemini-2
0.67 0.02
0.78 0.02
0.21 0.01
0.18 0.01
−72.7
−73.1
above are CMCs of Gemini-1 and Gemini-2, respectively found from
fluorescence measurements. The significantly lower value of quan-
tum yield in case of Gemini-2 as compared to that in Gemini-1 is
because of greater extent of fluorescence quenching of DMASBT by
Br− ions in the former. Since the concentration of Br− ions is higher
on the surface of Gemini-2 micellar aggregates as compared to
that on Gemini-1, DMASBT molecules being present on the surface
experience more fluorescence quenching in the former than that
in the later. Quenching of fluorescent probes by Br− ions present
on the micellar surface containing positively charged headgroups
have been reported in the literature [50,51]. Reports on increase
in ˛ value with increasing number of s in a series of gemini sur-
factants of type 12-s-12,2Br− are available in the literature [38].
Therefore, in the present study observed concomitant decrease in
˛ of Gemini-2 could be because of predominant effect of greater
extent of hydration of Br− ions over the effect of increasing num-
ber of s in the spacer. It has also been reported [38] for the gemini
surfactants of type, 12-s-12,2Br− that the average distance between
two cationic centers of headgroups becomes larger with increasing
the spacer chain length at least in case of short chain spacer groups.
Therefore, the average electrostatic repulsion between the posi-
tively charged headgroups is lowered with increasing the spacer
chain length which results in an increase in ˛ value. Thus, in our
case the decrease in the ˛ value of Gemini-2 in comparison to that
of Gemini-1 further supporting the dominating effect of number of
substituted hydroxyl groups on a spacer group.
Acknowledgements
SKS acknowledges the University Grants Commission for finan-
cial supports under major research project (33-257/2007(SR))
and special assistance programme (F.540/14/DRS/2007 (SAP-I))
and the Council of Scientific and Industrial Research, Govern-
ment of India for financial supports under major research project
(01(2213)/08/EMR-II).
The standard Gibbs free energy of micellization per monomer
molecule, ꢅGm◦ has been calculated [52], at a given temperature
using Eq. (1):
ꢅGm◦ = RT(3 − 2˛) ln XCMC
(1)
where the parameters R and T have their usual meaning, XCMC is
the mole fraction of surfactant at CMC, ˛ is the degree of micel-
lar ionization and the partial dissociation of counter ions from the
micelles is accounted by the factor (3-2˛). Data in Table 3 indi-
cate that ꢅGm◦ value of Gemini-1 is almost same as Gemini-2 could
be because of same reason (two opposing effect) for which the
difference between the CMCs of two geminis is very small.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jphotochem.2011.07.009.
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