Micellization of cetyltrimethylammonium bromide: Effect of small chain bola electrolytes

Article abstract of DOI:10.1021/jp4108427

Sodium dicarboxylates (or Bola salts) with methylene spacers 0, 2, 4, 6, 8, and 10 were studied in aqueous solution to investigate their influence on the micellization of cetyltrimethylammonium bromide (CTAB). Since bolas with spacer length a??12 are known not to micellize in general, the herein used sodium dicarboxylates were treated as 2:1 amphiphilic electrolytes which reduced surface tension of water (except sodium oxalate with zero spacer) without self-association. Their concentration dependent conductance was also linear without breaks. The bolas affected the micellization of CTAB but acted like salts to decrease its CMC. Their combinations did not form bilayer aggregates as found in vesicles. Nevertheless, they synergistically interacted with CTAB at the air/water interface as revealed from Rosen's thermodynamic model. Hydrodynamic radius (R_h), Zeta-potential (??), and electrical double layer behavior of bola interacted CTAB micelles were assessed. From SANS measurements, micelle shape, shape parameters, aggregation number (N _agg), surface charge of the bola influenced CTAB micelles were also determined. NMR study as well supported the non-mixing of bolas with the CTAB micelles. They interacted in solution like amphiphilic electrolytes to influence the surface and micelle forming properties of CTAB. ? 2014 American Chemical Society.

Full text of DOI:10.1021/jp4108427

Article
pubs.acs.org/JPCB
Micellization of Cetyltrimethylammonium Bromide: Effect of Small
Chain Bola Electrolytes
Animesh Pan,† Pallabi Sil,† Sounak Dutta,‡ Prasanta Kumar Das,‡ Subhash Chandra Bhattacharya,†
Animesh Kumar Rakshit,§ Vinod Kumar Aswal,∥ and Satya Priya Moulik*,†
§
†Centre for Surface Science and Indian Society for Surface Science & Technology, Department of Chemistry, Jadavpur University,
Kolkata 700 032, India
‡Department of Biological Chemistry, Indian Association for Cultivation of Science, Kolkata 700 032, India
∥Solid State Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
S
* Supporting Information
ABSTRACT: Sodium dicarboxylates (or Bola salts) with methy-
lene spacers 0, 2, 4, 6, 8, and 10 were studied in aqueous solution
to investigate their influence on the micellization of cetyltrime-
thylammonium bromide (CTAB). Since bolas with spacer length
≤12 are known not to micellize in general, the herein used
sodium dicarboxylates were treated as 2:1 amphiphilic electrolytes
which reduced surface tension of water (except sodium oxalate
with zero spacer) without self-association. Their concentration
dependent conductance was also linear without breaks. The bolas
affected the micellization of CTAB but acted like salts to decrease
its CMC. Their combinations did not form bilayer aggregates as
found in vesicles. Nevertheless, they synergistically interacted with
CTAB at the air/water interface as revealed from Rosen’s ther-
modynamic model. Hydrodynamic radius (Rh), Zeta-potential (ζ), and electrical double layer behavior of bola interacted CTAB
micelles were assessed. From SANS measurements, micelle shape, shape parameters, aggregation number (Nagg), surface charge
of the bola influenced CTAB micelles were also determined. NMR study as well supported the non-mixing of bolas with the
CTAB micelles. They interacted in solution like “amphiphilic electrolytes” to influence the surface and micelle forming properties
of CTAB.
at ≥308 K. These bolaforms also self-associate in the presence
of 10 mM NaBr at 303 K.17 Bolas with methylene spacers
higher than 10 are reported to form mixed micelles with
conventional surfactants, as well as form elongated aggregates,
vesicles, tubes, and fibers.18−21 Long chain mono- and dicarbox-
ylic acids can form stable vesicles in combination with alkyl-
trimethylammonium amphiphiles,22 but such studies with small
chain bola fatty acids and their salts are not found in the liter-
ature. Bhattacharya et al.23 reported bilayer or vesicle forma-
tion of bolaphile−amphiphile ion pairs between C6H4
1. INTRODUCTION
Cetyltrimethylammonium bromide (CTAB) with a 16 methy-
lene chain and a quaternary ammonium cationic headgroup
micellizes at ∼1 mM at 25 °C in aqueous medium. Like other
ionic surfactants, its micellization gets reduced in the presence
of added salt by the electrostatic screening of the headgroup
charge.1,2 It may form mixed micelles with nonionic and other
cationic surfactants.3,4 With anionic surfactants it may form
amphiphilic ion pairs, which at 1:1 molar ratio can form coacer-
vates imparting turbidity in solution;5,6 even precipitation may
result as a consequence.7 At uneven ratios (one is fairly greater
that the other), formation of mixed compartmentalized entities
between the two is also possible.8,9
Amphiphiles having two functional headgroups at both ends
of their chain are called bola amphiphiles; if the headgroups are
ionic, then they are called bola electrolytes.10,11 It is known that
bola electrolytes or “bolas” having methylene spacers ≥12 can
form micelles.12−14 Lower bolas do not self-associate in general,
but in the presence of salt, they may form micelles.15,16 Thus,
Br−Me3N+−(CH2)10−N+Me3Br− and Br−Me3N+−(CH2)10−
COO−Na+ do not form micelles in the temperature range of
298−308 K, whereas Br−Me3N+−(CH2)10−OH forms micelles
+
(O(CH2)10COO−)2 and n-C16H33NMe3 . They also studied
the morphology and bilayer properties of vesicles formed from
ion-paired amphiphiles of cationic gemini surfactants with pal-
mitic acids in aqueous medium.24 Similar and comparable other
systems were also investigated by them25,26 and others.27,28
This has motivated us to study the bulk and interfacial behavior
of small chain bola salts (sodium dicarboxylates) and their
effect on the self-aggregation of CTAB in aqueous solution.
Received: November 4, 2013
Revised: January 30, 2014
Published: February 20, 2014
© 2014 American Chemical Society
3041
dx.doi.org/10.1021/jp4108427 | J. Phys. Chem. B 2014, 118, 3041−3052

The Journal of Physical Chemistry B
Article
We may herein add that CTAB can undergo a postmicellar
transition at almost 0.15 mol/kg.29 In this study we have studied
mixed CTAB and bola at a total concentration of 0.05 mol/kg
(in SANS experiments); in the other experiments, the concen-
trations were kept much smaller. Our objective was to study the
properties of CTAB in bola environment close to its CMC, and
not at higher concentration to avoid the post micellar tran-
sition. Conductometric, tensiometric, DLS, SANS, and NMR
techniques were used in this endeavour. Since the bola salts
used were nonaggregating in the studied range of concen-
tration, they were expected to behave as “amphiphilic electro-
lytes” to affect the micellization of CTAB. The interaction pro-
duced neither coacervates nor bilayer aggregates, i.e., vesicles.
extent of counterion binding of micelle (β) was determined by
the slope ratio method (ratio between the slopes of the post
and the pre CMC linear courses) described and discussed
in the literature31 and cited by others.4,32 Reproducibility of
results were checked from two to three repeat experiments.
2.2.2. Tensiometry. Tensiometric measurements were taken
in a calibrated du Nouy tensiometer (Kruss, Germany). Con-
̈
̈
ductivity of water (6 mL) was taken in a double-wall jacketed
container placed in a thermostatted water-bath at 303 0.1 K
into which a stock solution of (bola or bola mixed with CTAB)
of desired concentration was added in aliquots with a Hamilton
microsyringe as required (allowing ∼10 min for mixing and
thermal equilibration). The surface tension was then measured,
the details of which can be found elsewhere.33 Surface tension
(γ) values determined were accurate within 0.1 mN m−1.
Experiments were duplicated to check reproducibility.
2. EXPERIMENTAL SECTION
2.1. Materials. Cetyltrimethylammonium bromide (CTAB)
and dodecyltrimethylammonium bromide (DTAB) were ob-
tained from Sigma Aldrich (USA). They were 99% pure and
were used as received. Sodium oxalate and sodium succinate
with 99% purity were obtained from Merck, India, and were
used as received. Other dicarboxylic acids, e.g., adipic acid,
suberic acid, sebacic acid, and dodecane dioic acid, were pur-
chased from Sigma Aldrich (USA), and these acids were neu-
tralized by NaOH addition and then lyophilized to obtain the
desired product of sodium salt of different dicarboxylates. 1,6-
Diphenyl-1,3,5-hexatriene (DPH) and Rhodamine6G (R6G)
are obtained from Sigma Aldrich (USA). Double-distilled
conductivity water (specific conductance, κ = 2−3 μS cm−1 at
303 K) was used in the experiments and D2O was also pur-
chased from Sigma-Aldrich (USA).
2.2.3. Dynamic Light Scattering (DLS). A Nano ZS Zetasizer
(Malvern, UK) was employed for dynamic light scattering
measurements at 90° scattering angle with a He−Ne laser (λ =
632.8 nm). All experimental solutions were filtered 2−3 times
through membrane filters (with pore size 0.25 μm) to remove
dust particles. The mean values of triplicate experimental results
are reported. For determination of zeta potential, 1.5 mL of
10 mM micellar solution was placed in the electrophoretic cell.
2.2.4. Small Angle Neutron Scattering (SANS). Small angle
neutron scattering experiments were performed with a SANS
diffractometer at the Dhruva reactor, Bhabha Atomic Research
Centre, Trombay, Mumbai (India). The diffractometer makes
use of a beryllium oxide filtered beam with a mean wavelength
(λ = 5.2 Å). The angular distribution of scattered neutrons was
recorded using a one-dimensional position sensitive detector
(PSD). The accessible wave vector transfer (q = 4π sin θ/λ,
where 2θ is the scattering angle) and the range of the
diffractometer is 0.017−0.35 Å−1. The PSD allows simultaneous
recording of data over the full q range. The samples were held
in a quartz sample holder of 0.5 cm thickness. The experimental
temperature was kept fixed at 303 K. Measured SANS data
were corrected and normalized to absolute units (as cross-
section per unit volume) using standard procedure.34
2.1.1. Characterization of the Amphiphilic Dicarboxylates.
1
Adipic Acid Disodium Salt (bola-4). H NMR (500 MHz,
D2O, rt) δ = 1.45−1.47 (m, 4H), 2.10 (s, 4H); MS (ESI): m/z
calcd for C6H8Na2O4: 190.0218 [M+]; found: 190.9377
[M+ +1]. Anal Calc (%) for C6H8Na2O4: C, 37.91; H, 4.24;
Found: C, 37.93; H, 4.28.
1
Suberic Acid Disodium Salt (bola-6). H NMR (500 MHz,
D2O, rt) δ = 1.23 (s, 4H), 1.47 (s, 4H), 2.07−2.12 (t, 4H); MS
(ESI): m/z calcd for C8H12Na2O4: 218.0531 [M+]; found:
219.1232 [M+ +1]. Anal Calc (%) for C8H12Na2O4: C, 44.04;
H, 5.54; Found: C, 44.09; H, 5.51.
2.2.5. Nuclear Magnetic Resonance (NMR). NMR spectra
were recorded in an Avance (Bruker) spectrometer operating at
500 MHz. Chemical shifts were determined using D2O as inter-
1
Sebacic Acid Disodium Salt (bola-8). H NMR (500 MHz,
D2O, rt) δ = 1.22 (s, 8H), 1.43−1.48 (m, 4H), 2.06−2.11
(t, 4H); MS (ESI): m/z calcd for C10H16Na2O4: 246.0844
[M+]; found: 247.1073 [M+ +1]. Anal Calc (%) for
C10H16Na2O4: C, 48.78; H, 6.55; Found: C, 48.73; H, 6.58.
1
nal locking agent for H and 2D-NOESY NMR. Tetramethylsi-
lane (TMS) was used as an internal chemical shift reference.
2.2.6. Mass Spectra and Elemental Analysis. High resolu-
tion mass spectrometric (HRMS) data were acquired by elec-
trospray ionization (ESI) techniques, both in the positive and
in the negative mode in a Q-tof Micro-Quadruple mass spec-
trophotometer (Micromass, U.K.), and the element analyses
were done in a Perkin-Elmer CHN Analyzer.
2.2.7. Absorbance and Fluorescence Measurements.
Absorption spectra were taken using a SHIMADZU, UV-
1601 (Japan) spectrophotometer at desired temperature using
quartz cuvetts of 1 cm path length and steady-state fluorescence
measurements were performed using a Perkin-Elmer LS 55
(USA) fluorescence spectrophotometer with an attachment of
Fluorescence Peltier System PTP-1 using glass cell of 1 cm path
length.
1
Dodecanedioic Acid Disodium Salt (bola-10). H NMR
(500 MHz, D2O, rt) δ = 1.23 (s, 12H), 1.45−1.50 (m, 4H),
2.08−2.13 (t, 4H); MS (ESI): m/z calcd for C12H20Na2O4:
274.1157 [M+]; found: 275.1141 [M+ +1]. Anal Calc (%) for
C12H20Na2O4: C, 52.55; H, 7.35; Found: C, 52.52; H, 7.39.
The above characteristic results have confirmed the high
purity of the herein synthesized bola electrolytes.
2.2. Methods. 2.2.1. Conductometry. Conductometry
experiments were done with a (EUTECH, Singapore) conduc-
tometer using a dip-type cell of cell constant = 1 cm−1. All
measurements were taken in a double-walled glass container at
303 K maintained by a Hahntech, DC-2006 circulating bath
with an accuracy of 0.1 K. Concentrated surfactant solution
was progressively added to 6 mL of water using a Hamilton
microsyringe. Results were graphically processed by plotting
specific conductance (κ) vs Ctotal to evaluate CMC from the
inflection point.30 The micellar ionization degree and hence the
Steady-state fluorescence anisotropy (rss) measurements
were taken with a polarization filter having the “L-format” con-
figuration to measure the rss (fluorescence anisotropy) of the
probe DPH. The anisotropy relation is expressed as35
3042
dx.doi.org/10.1021/jp4108427 | J. Phys. Chem. B 2014, 118, 3041−3052

The Journal of Physical Chemistry B
Article
Figure 1. (A) Equivalent conductance (Λ) vs concentration plots of pure bolas like bola-0, bola-6, and bola-10 as well as that of CTAB. (B)
Tensiometric profiles of pure bolas with different spacers. (Inset: tensiometric profile of CTAB.)
IVV − GIVH
IVV − 2GIVH
behavior of a diammonium bola salt with 12 methylene groups
rSS
=
as the spacer. We have observed that (CH2)10(Me3N+Br−)2 and
Br−Me3N+-(CH2)10-COO−Na+ did not form micelles in aque-
ous medium in the temperature range of 293−308 K, but in the
presence of 10 mM NaBr they showed self-aggregation.17 Addi-
tion of salt NaBr up to 10 mM did not help to form micelles of
the herein studied bolas with spacer 10, and the question of
self-assembly of lower bolas did not arise.
(1)
where IVV and IVH are the vertically and horizontally polarized
emission intensities, respectively, resulting from vertically
polarized excitation of the probe. The expressions for the G
factor IHV and IHH are similarly the vertically and horizontally
polarized emissions resulting from horizontally polarized exci-
tation, respectively. The anisotropy values were averaged over
an integration time of 20 s and a maximum number of three
measurements were taken for each sample. The anisotropy
values of the probe in micellar media as presented in this work
are the mean value of three individual determinations.
Direct support to the above solution behavior of bolas was
obtained from DLS measurements. The studied solutions of
bolas in water up to 0.2 M produced no scattering. Their as-
sembly formation in aqueous medium was not supported. They
behaved as strong electrolytes (amphiphilic electrolytes) in
solution and influenced the micellization properties of CTAB
discussed in detail in the following section.
3. RESULTS AND DISCUSSION
3.2. CTAB−Bola Electrolyte Mixed Systems. We have
made a detailed conductometric and tensiometric study of the
mixed anionic bolas and cationic CTAB at various mole ratios
between 0.1 and 0.9. Findings on the mixed oxalate (bola-0)
and CTAB are presented in Figure 2A. The response of con-
ductance on the CMC formation was not found at mole ratios
up to 1:7, i.e., 0.125 (XCTAB):0.875 (Xbola). Similar were the
observations with bola-2, 4, and 6; responses of bola-8 and 10
started after 1:9 ratios, i.e., 0.1 (XCTAB):0.9 (Xbola). Con-
ductance results on bola-6 and 10 are shown in Figure 2B and C,
respectively. Surface tension responded to all the studied ratios
to produce CMC. It was observed that γ gradually decreased
with formation of convex type curves until it reached a break
point (CMC point), and the curves became flat thereafter.
Results with bola 0, 6, and 10 are presented in Figure 2D, E,
and F, respectively. The CMC values of bola-6 are presented in
Table 1; the rest (for bolas 0, 2, 4, 8, and 10) are shown in SI
Tables 1−5.
Results presented in Figure 2 and Table 1 (also SI Tables 1−5)
revealed that, with increased mole fraction of CTAB in the
mixed systems, the CMC increased. CMC values by the two
methods were close; their mean values were used in data
processing and analysis. At mole fraction 0.5 the decrease in
surface tension by the bolas up to CMC was ∼40 dyn cm−1;
comparable values of γ were also observed at other mole
fractions. The CTAB-bola-10 at 5:1 mol ratio showed two
breaks in the tensiometry plot (Figure 2F inset). Two breaks in
tensiometry were reported20 on the mixed systems of BPHTAB
3.1. Solution Properties of the Bolas. The bola
electrolytes (dicarboxylates) with spacers 0−10 are soluble in
water, but those with spacer number >10 are sparingly soluble
and herein not considered. Conductance and surface tension
methods were used to examine their bulk and interfacial pro-
perties. Conductance of bolas continued to smoothly change
without break as illustrated in Figure 1A. Both specific and
equivalent conductance plots supported the conclusion. Equiv-
alent conductance plot of CTAB produced a break indicating
micelle formation.
Surface tension (γ) of solutions (except oxalate) decreased
with increasing [bola] but also did not produce breaks to
witness formation of critical micelle concentration or CMC.
Higher bolas than oxalate at equal concentration decreased γ
more, indicating their higher surface activity (Figure 1B) because
of increasing number of methylene groups, i.e., increasing
hydrophobicity. In the inset of Figure 1B the tensiometric profile
of CTAB is also presented to make a contrasting comparison
with the bolas.
The concentration ratios between two successive bolas, i.e.,
bola-2/bola-4, bola-4/bola-6, bola-6/bola-8, and bola-8/bola-10
at a constant surface tension, e.g., γ = 70 mN m−1, were 1.7, 2.9,
3.0, and 1.2, respectively. According to Traube’s rule36 the
ratios should have been 9.0. Hence, the surface activities of the
bolas were different from single headed normal amphiphiles.
According to Brown et al.37 diammonium salts with ten carbon
atoms between the ionic groups behave like normal electrolytes
and did not form micelles. Yiv et al.12 also observed similar
3043
dx.doi.org/10.1021/jp4108427 | J. Phys. Chem. B 2014, 118, 3041−3052

The Journal of Physical Chemistry B
Article
gas constant (R), and temperature (T) in the relation, ΔG0m =
(1 + β)RT ln XCMC, where XCMC is the mole fraction
concentration of surfactant at CMC. In the studied systems
conductance did not show breaks up to the mole ratio 1:7
(XCTAB = 0.125). Systems with XCTAB = 0.167 to 1.0 produced
β which increased with increasing XCTAB ; with increasing
spacer the magnitude of β decreased. Charge neutralization of
CTAB micelles by the increasing proportion of bola caused
reduction in β, i.e., increase in the micelle ionization degree.
The spontaneity of micellization increased with increasing
proportion of CTAB in the mixture (i.e., ΔG0m continued to
decrease). Spontaneity also increased with increasing spacer
length. In Table 1 (main text and also SI Tables 1−5), the
CMC values as well as magnitudes of other physicochemical
properties for different CTAB-bola electrolyte mixed systems
are presented.
In Figure 3, CMC and β are plotted against the spacer
number (n) of bola for their mixtures at XCTAB = 0.167(1:5),
Figure 2. Conductometric and tensiometric profiles of bolas of spacers
of 0, 6, and 10 mixed with CTAB at different CTAB:bola mole ratios
at 303 K. Insets of (A), (B), and (C) are conductometric plots of 1:9
(CTAB:bola) mole ratio and inset of (F) is the tensiometric plot for
5:1 (CTAB:bola) mole ratio.
Figure 3. Plot of CMC and β vs spacer number in bola for CTAB-bola
mixed system of different ratios 1:5, 1:1, and 5:1 at 303 K.
0.5(1:1), and 0.833(5:1). β linearly decreased with spacer
number (correlation coefficient = 0.984, 0.995, and 0.975,
respectively). CMC showed a maximum at n = 2, and then
declined; with n the CMCs fitted to three degree polynomials
of the form: CMC = a1 + a2n + a3n2 + a4n3 ; where a1, a2, a3,
and a4 are the constants. Similar results were also found for
other ratios (not illustrated). The magnitude of β per spacer in
the bola found from the slopes of the lines were 0.01, 0.01, and
0.02 corresponding to XCTAB 0.167, 0.500, and 0.833, respec-
tively; they were nearly equal.
(biphenyl-4,4′-bis(oxyhexamethylenetrimethyl-ammonium bro-
mide)) with SDS and C9H19COONa. DLS and SANS studies
(subsequently discussed in sections 3.4 and 3.5) could not pro-
vide support to the above tensiometric observation in relation
to the configuration status in the bulk.
Energetics of micellization of CTAB-bola systems were deter-
mined following the procedure described in the literature.33
The standard Gibbs free energy of micellization (ΔG0m) was
calculated using β (fraction of counterion bound to micelle),
Table 1. CMC, β, −ΔG0m, Amin, Γmax, and −ΔGa0d for Mixed CTAB + bola-6 in Aqueous Medium in Different Mole Ratios
a
at 303 K
CMC/mM
ratios (CTAB:bola)
XCTAB
Cond.
ST
Mean
β
−ΔG0m
Amin
106 Γmax
−ΔG0ad
0:1
1:9
1:7
1:5
1:3
1:1
3:1
5:1
7:1
9:1
1:0
0
0.1
0.159
0.160
0.173
0.181
0.240
0.391
0.457
0.579
0.612
1.020
0.159
0.160
0.170
0.184
0.239
0.395
0.457
0.579
0.626
0.970
1.12
1.20
1.19
1.04
0.94
0.91
0.87
0.77
0.97
0.55
1.36
1.38
1.39
1.60
1.76
1.83
1.91
2.17
1.72
3.01
0.125
0.167
0.25
0.5
0.168
0.188
0.238
0.399
0.457
0.579
0.641
0.930
0.084
0.101
0.257
0.316
0.345
0.392
0.459
0.682
34.1
34.4
38.4
38.6
39.0
39.5
41.2
45.6
63.7
60.5
61.8
61.0
60.4
58.2
64.9
58.4
0.75
0.833
0.875
0.9
1
a
ΔGm0 , Amin, Γmax, and ΔGa0d are expressed in kJ mol−1, nm2 /molecule, mol m−2, and kJ mol−1, respectively. Their respective standard deviations are
4, 6, 5, 3%. Standard deviations in CMC = 4%, and in β = 5%.
3044
dx.doi.org/10.1021/jp4108427 | J. Phys. Chem. B 2014, 118, 3041−3052

The Journal of Physical Chemistry B
Article
Interestingly, CMC produced by bola-0 with no hydrophobic
spacer was lower than bola-2. It was a unique property. We
considered that bolas did not form mixed micelles with CTAB;
they acted like surface active salts in solution and decreased
CMC of CTAB micelles with increasing spacer length. This was
supported from the study of Yu et al.38 on the effects of organic
salts (C6H5COONa and p-C6H4(COONa)2) on the aggrega-
tion of cationic gemini surfactants. They have shown that the
hydrophobic inorganic salts were more effective to promote
aggregation of geminis than normal inorganic salts. The bola
class salt p-C6H4(COONa)2 promoted aggregate formation
through reduction of the electrostatic repulsion among the
gemini headgroups because it stayed at the micelle surface
rather than penetrating into the hydrophobic core like
C6H5COONa. Similar was our consideration for bolas: they
could not penetrate the core because of two oppositely placed
ionic headgroups. The “reverse wicket-like” configuration
modeled in Figure 4B (II) is discussed in section 3.4. This pro-
position fitted with the scheme of Yu et al. (scheme 1 lower).38
Amphiphiles with a single hydrophilic ionic headgroup are
known to easily penetrate into the hydrophobic core or region
to form mixed micelles or microemulsions (and reverse micelles).
Amphiphiles with two ionic headgroups as bolas (being more
hydrophilic) do not fit in this line. We have been trying with
the bolas to prepare microemulsions, but so far with little
success.
3.3. Interfacial Properties of Mixed (CTAB-bola)
Systems. Interfacial adsorption of CTAB−bola combinations
was computed from the Gibbs adsorption equation.39 Surface
excess (Γmax) values at CMC were found from the slopes of the
γ vs log C plots.
1
dγ
dlog C
Γmax = −
lim
C→cmc
2.303rRT
(2)
Both CTAB and bolas were surface active; the micelle
formation of CTAB was influenced by the bolas, so interfacial
adsorption where both species were adsorbed also. The r in
eq 2 was composition dependent.40 Its value was found from
the relation, r = ∑ rixi, where ri and xi correspond to the
i
number of species formed from the i-th component and the
mole fraction of the component, respectively. Species formed
from CTAB in solution by dissociation are two (CTA+ + Br−),
and those formed from a bola are three (−OOC − (CH2)n −
COO− + 2Na +). Hence at 1:1 ratio, r = 2 × 0.5 + 3 × 0.5 = 2.5,
at 5:1, r = 2 × 0.833 + 3 × 0.167 = 2.17, etc. In getting the
slope (d γ/d log C), a two-degree polynomial equation of the
form γ = a + b log C + c log C2 was considered whose slope at
C = CMC was used to get Γmax
.
39,41 In many literature reports,
arbitrary straight lines are drawn through the CMC point
42
whose slopes are taken to get incorrect Γmax
.
The minimum
area of exclusion of the surfactant molecules at the air−water
interface was calculated from the relation
1018
Amin
=
nm2/molecule
N Γmax
(3)
A
where NA is the Avogadro number.
The data in Table 1 and also in SI Tables 1−5 show that the
Amin of CTAB (0.55 nm2/molecule) increased with increasing
proportion of bola, and finally at XCTAB = 0.1, Amin of the mixed
system nearly doubled (∼1.20 nm2/molecule). The phenom-
enon was independent of the size of the bola, which was an
interesting observation. The studied bolas did not form micelles
but they were surface active. This supported flat orientation of
small bolas (up to spacer 4), and higher bolas had “bent” or
“wicket-like” conformation.
By comparison of the area per molecule (Amin) of “one-
headed” C12H25Me3N+ (0.55 nm2/molecule), C12H25Bu3N+
(0.73 nm2/molecule), with the corresponding “two-headed”
bolas, C12H25 Me6N+ (1.07 nm2/molecule) and C12H25Bu6 N+
(1.46 nm2/molecule), Menger et al.15 also suggested a “wicket-
like” orientation of bolas at the interface (Amin values of
“one-headed” species were half the Amin values of the “two-
headed” bolas). Yan et al.19 have reported Amin values for their
Figure 4. Suggested structures of CTAB micelle with electrical double
layer in the absence and presence of bola electrolytes.
a
Table 2. CMC, β, −ΔG0m, Amin, Γmax, and −ΔGa0d for (1:1) Mixed CTAB+bola in Aqueous Medium at 303 K
CMC/mM
systems (CTAB:bola)
Cond.
ST
Mean
β
−ΔG0m
Amin
106 Γmax
−ΔG0ad
Bola-0
Bola-2
Bola-4
Bola-6
Bola-8
Bola-10
0.371
0.413
0.337
0.238
0.149
0.064
0.296
0.356
0.291
0.240
0.146
0.086
0.324
0.384
0.314
0.239
0.147
0.075
0.321
0.301
0.282
0.257
0.226
0.215
36.9
38.3
38.4
38.4
36.8
40.6
0.81
0.81
0.84
0.94
0.99
1.18
2.04
1.90
1.97
1.76
1.67
1.41
53.1
54.2
53.9
61.8
54.9
47.8
a
Units and standard deviations of the parameters are as in Table 1.
3045
dx.doi.org/10.1021/jp4108427 | J. Phys. Chem. B 2014, 118, 3041−3052

The Journal of Physical Chemistry B
Article
Table 3. Interfacial Parameters Obtained from Rosen’s Mixed Monolayer Model for Three Different Molar Ratios of
a
CTAB:bola at 303 K
systems
(XσCTAB
)
(XσCTAB
)
(βσR)55 (βσR)65
(f σCTAB
)
(f σCTAB
)
65
(fσbola (fσbola
) )
55 65
55
65
55
1:3 ratio
−8.97(−6.82)
−11.20(−8.69)
−14.45(−16.49)
1:1 ratio
Bola-6
Bola-8
Bola-10
0.63(0.82)
0.67(0.72)
0.60(0.60)
1.02(1.01)
1.02(1.02)
1.03(1.03)
1.16(1.09)
1.20(1.16)
1.25(1.25)
Bola-6
Bola-8
Bola-10
0.76(0.83)
0.69(0.72)
0.63(0.68)
−9.77(−8.82)
−12.56(−11.85)
−14.28(−11.32)
3:1 ratio
0.087(0.110)
0.043(0.052)
0.028(0.059)
0.087(0.110)
0.043(0.052)
0.028(0.059)
Bola-6
Bola-8
Bola-10
0.82(0.85)
0.73(0.76)
0.69(0.66)
−9.10(−9.70)
−12.33(−11.79)
−15.70(−13.29)
1.11(1.09)
1.16(1.14)
1.21(1.19)
1.01(1.00)
1.02(1.02)
1.02(1.02)
a
The interfacial parameters were calculated from the following relations A, B, and C:
(Xσ)2ln(α1Cmix/XσC10)
= 1
(1 − Xσ)2ln[(1 − α1)Cmix/(1 − Xσ)C20]
(A)
where Cmix, C01, and C20 are the concentrations of the mixture, component 1, and component 2, respectively, at a fixed γ; α1 is the stoichiometric mole
fraction of component 1 in solution and Xσ is its interfacial composition in mole fraction.
ln(α1Cmax/XσC10)
βRσ(interaction parameter) =
(1 − Xσ)2
(B)
f1σ (activity coefficient of component 1)
= exp[βRσ(1 − Xσ)2],
and f2σ (activity coefficient of component 2)
= exp[βRσ(Xσ)2]
(C)
Results were iteratively calculated in a computer.
cationic (trimethylammonium) bolas Ia (phenyl-1,4-bis-
(oxyhexyltrimethylammonium bromide)), Ib (phenyl-1,4-bis-
(oxydecyltrimethylammonium bromide)), and Ic (biphenyl-
4,4′-bis(oxyhexyltrimethylammonium bromide)) as 1.66, 3.67,
and 1.76 nm2/molecule, respectively. These values were also
reasonably higher in comparison with the Amin of a −N+Me3
headgroup (∼0.50 nm2/molecule) reported in the literature.43
The bola Ia with n = 6 and a phenyl ring in the middle had a
CMC = 11 mM, whereas Ib with n = 10 and a middle phenyl
ring had a CMC = 2.9 mM; Ic with n = 6 and a middle diphenyl
ring evidenced a CMC = 6.8 mM at 303 K, which indicates that
the hydrophobicity of a phenyl ring ≅ the hydrophobicity of
two mehylene groups. The presence of single and double
phenyl rings in the middle of the bolas produced CMC
although n values were 6 and 10, well below the CMC forming
capability (n ≥ 12) of alkyl chain containing bolas.19 Mono-
and biphenyl rings in the middle added the requisite
hydrophobicity to the compounds to form micelles. In
Table 2 the above-mentioned physicochemical parameters of
herein studied bolas at 1:1 (mol/mol) CTAB:bola ratio are
presented. In overall comparison, bola-10 behaved adequately
differently with respect to ΔG0m and Amin from the rest; the
differences in the other parameters were small.
Both bola electrolytes and CTAB (although did not form
mixed micelles in solution) were simultaneously adsorbed at
the air/solution interface; there, they underwent mutual
interaction. Surface compositions of the amphiphiles (bola
and CTAB) were determined from Rosen’s rationale (RR)44
and are listed in Table 3. Additionally, the surface activity
coefficients of the components and the interaction parameters
were also evaluated. The constant levels of surface tension
required for the above estimation were arbitrarily chosen at γ =
55 and 65 mN m−1. The final forms of the relations used are
presented in the footnote of the table.
The results in Table 3 revealed that (i) the interfacial com-
position (IC) of CTAB increased with stoichimetric com-
position (SC) in the presence of bolas (the IC values > SC
values), i.e., for bolas, IC values < SC values; and (ii) the
increasing presence of hydrophobicity of bolas increased its IC,
and caused a decrease of IC of CTAB. The IC vs SC plots for
bola-6, 8, and 10 are presented in Figure SI 1(A, B).
In the Rosen mixed monolayer treatment, only bola-6, bola-
8, and bola-10 were considered. Lower bolas with spacers 0, 2,
and 4 were very weakly surface active and comparison at γ = 55
and 65 mN m−1 was not possible. In all proportions of CTAB
and bola, mole fractions of the components at the air/water
3046
dx.doi.org/10.1021/jp4108427 | J. Phys. Chem. B 2014, 118, 3041−3052

The Journal of Physical Chemistry B
Article
interface were different from the stoichiometric compositions in
Table 4. Physicochemical Parameters of Aggregates of
3:1
1:1
solution. Thus, [XCTAB monolayer
]
≈ [XCTAB soln
] ] >
3:1 , [XCTAB monolayer
CTAB+Bola Mixture from DLS and Anisotropy
a
1:3
[XCTAB soln
]
1:1 , and [XCTAB monolayer 1:3 . The corre-
]
≫ [XCTAB]soln
Measurement at 303 K at 10 mM
sponding compositions of bolas in the monolayer and in
solution followed from above. It was found that with increasing
spacer length, the proportion of CTAB at the interface
decreased. Higher bolas were surface active enough to compete
with CTAB in the formation of Gibbs monolayer (GM);
increasing spacer length increased the proportion of bola in the
GM, but the fraction of CTAB always remained > bola. There
was no marked difference between the results found from the
two γ values 55 and 65 mN m−1. The activity coefficients of
both components in each set were nearly equivalent, but their
magnitudes for the mole ratios 3:1 and 1:3 were like ideal
mixtures, which for 1:1 mol ratio were highly nonideal. The
interaction parameters βσR were all negative with fairly high
values. The components showed strong synergistic interaction,
which increased with increasing spacer length in the bola.
Average values of βσR for bola-6, bola-8, and bola-10 were −9.3,
−12.0, and −14.8, respectively, at 303 K. In a previous study,17
we presented interactions of different types of bolas with
oppositely charged conventional surfactants like SDS and
CTAB. Since they formed mixed micelles, both Rubing’s
regular solution theory and Rosen’s model were used to get
information in the solution and at the interface. There, inter-
actions of bola (Br−Me3N+ −(CH2)10 − COO−Na+) with
CTAB and SDS were also synergistic with fairly large βσR values
−17.0 and −6.72 with CTAB and SDS, respectively. At 1:1 mol
b
Rh/
zeta potential
(z/mV)
% charge
systems CTAB:bola
rss
nm
neutralized
CTAB
Bola-0
-
0.069
0.070
0.071
0.066
0.064
0.066
0.067
0.064
0.065
0.062
0.062
0.064
2.69
2.74
2.66
2.78
2.84
2.50
2.62
2.80
2.36
2.81
2.51
2.69
2.81
32.9
10.2
12.4
16.1
11.9
12.7
10.6
9.46
8.60
6.81
5.32
3.38
4.65
0
69
62
51
64
61
68
71
74
79
84
90
86
1:1
2:1
1:1
2:1
1:1
2:1
1:1
2:1
1:1
2:1
1:1
2:1
Bola-2
Bola-4
Bola-6
Bola-8
Bola-10
0.066
a
b
100(ζCTAB − ζbola)/ζCTAB. Standard deviation in rss, Rh, and ζ are 2,
6, and 4%, respectively.
few minor inconsistencies (experimental artifacts) was
observed. The degree of neutralization of the double layer
charge increased with increasing molecular size (length) of the
bolas. Higher bolas by their mass and mobility restrictions had
greater probability of residence in the electrical double layer.
Interestingly, the average Rh values of the micelles at both mole
ratios 1:1 and 2:1 were close (the values were independent of
the spacer length; the averages at 1:1 and 2:1 mol ratios were
2.72 and 2.62 nm, respectively). They were closely comparable
with those of pure CTAB micelle (Rh = 2.69 nm), which
potentially supported the absence of mixed micelles.
ratio, the interfacial composition of CTAB or SDS with respect
1:1
to the solution was [XCTAB or XSDS monolayer > [XCTAB or
]
XSDS]soln
1:1 . The activity coefficients at 1:1 mol ratio were small;
σ
σ
[fbola monolayer ≪ [f CTAB or f SDS]monolayer for both pairs.
]
3.4. Zeta Potential (Electrical Double Layer Behavior)
And Hydrodynamic Radius. Bola was expected to affect the
size and charge of the CTAB micelle and consequently its zeta
potential. DLS measurements provided the micelle size and ζ
potential. The measured ζ-potential of the CTAB micelles was
+32.9 mV at 303 K. Addition of bola with different spacers
reduced ζ from +32.9 to +4.65 mV. The results at two mixing
ratios, 1:1 and 2:1 (mol/mol) of CTAB:bola at an overall con-
centration of 10 mM, are presented in Table 4 (a graphical
representation is given in Supporting Information Figure SI 2).
An initially large depression of ζ of CTAB micelle by oxalate
followed by a continuous moderate decline over and above that
of oxalate by higher bolas was observed. The ζ-potential of 1:1
mol proportion of CTAB:bola was higher than that of 2:1
excepting bola-10. At 1:1 molar proportion, the concentration
of the micelle was lower than that at 2:1. Stoichiometrically,
in the first CTAB molecules were expected to be totally
neutralized by one equivalent of bola, and one equivalent of
non-neutralized carboxylate anion in bola remained free. In the
second, total charge neutralization with a neutral condition
was the expectation. Both were not found: at 1:1 proportion
ζ-potential was considerably reduced compared to pure CTAB
micelle, and at 2:1, it was reduced further but did not become
zero or very small. ζ decreased with increased spacer length; the
Stern-layer (in the electrical double layer of CTAB micelles)
did not accommodate enough bolas for steric and other
accommodative restrictions. By consideration of ζ ∝ surface
charge (net charge of the diffuse or Gouy−Chapman double
layer), the % neutralization of charge relative to CTAB micelle
is tabulated in column 6 (Table 4) as an overall estimation (see
footnote of the table). 60−90% neutralization of charge with a
Therefore, bolaforms did not form mixed micelles in solu-
tion; they influenced the interfacial adsorption, i.e., Γmax, and
hence Amin. Formation of mixed micelles would expect to
expand the micelle size for their residence in the micelle phase.
Weak influence of bolas on Rh at both 1:1 and 2:1 CTAB:bola
mole ratios suggested their noninclusion in the micelle interior.
They accumulated in the diffuse Gouy−Chapman double layer,
and reduced its thickness to make ζ decline with slight variation
in Rh. It is pictorially presented in Figure 4A.
Figure 4B is a comparative presentation of CTAB micelle and
its bola interacted form together with a hypothetical assembly
containing bolas in a “reverse wicket-like” conformation placed
in the micelle interior to make the mixed micelle expand.
According to the results and the above discussion, model (II) in
Figure 4B is impracticable whereas model (I) ≡ model (III)
(with respect to size). Model (III) is also equivalent to the
scheme 1 (lower) of Yu et al.38 Conclusion section provides
additional reasoning against the formation of mixed micelles
between bola and CTAB.
3.5. Microstructures of (CTAB− and DTAB−Bola)
Mixed Systems. Quantitative estimation of aggregate mor-
phology, Nagg, micelle charge can be obtained from model
fitting of SANS spectra. In addition to CTAB in the SANS
experiments we have also studied DTAB (dodecyltrimethy-
lammonium bromide) to get additional information on a lower
homologue of the alkyl trimethylammonium bromide series.
Both CTAB and DTAB aggregates with or without bolas were
modeled as charged prolate ellipsoids interacting through a
screened Coulomb potential.29,45 SANS data were analyzed
using this model wherein the differential scattering cross
3047
dx.doi.org/10.1021/jp4108427 | J. Phys. Chem. B 2014, 118, 3041−3052

The Journal of Physical Chemistry B
Article
section per unit volume (dΣ/dΩ) as a function of scattering
vector q can be written as
Table 5. Parameters Obtained from the SANS Analysis for
1:1 Mixtures of CTAB, DTAB, and Different Bola
a,b
Electrolytes at 303 K
dΣ
dΩ
2
= n(ρ − ρ)2V2[ F(q)2 + F(q) (S(q) − 1)] + B
̅
m
s
aggregation
semimajor
semiminor
fractional
systems
CTAB
number (Nagg
)
axis (a/Å)
axis (b/Å)
charge (α)
(4)
152
142
137
118
97
42.58
42.37
40.90
39.07
34.85
26.45
28.67
27.32
26.21
26.83
21.84
21.17
21.17
20.11
19.25
15.98
15.17
15.51
15.27
14.47
0.14
0.13
0.13
0.12
0.11
0.27
0.18
0.18
0.15
where n denotes the number density of micelles, ρm and ρs are
̅
CTAB:Bola-0
CTAB:Bola-2
CTAB:Bola-6
CTAB:Bola-10
DTAB
the scattering length densities of the micelle and the solvent,
respectively, and V is the volume. F(q) is the single particle
form factor, and S(q) is the interparticle structure factor. B is a
constant term which represents the incoherent scattering back-
ground coming mainly from the hydrogen atoms present in the
sample, and F(q) is calculated using the relations:
63
DTAB:Bola-0
DTAB:Bola-2
DTAB:Bola-6
DTAB:Bola-10
61
61
57
3(sin x − xcos x)
F(q, μ) =
52
0.11
x3
(5)
a
b
Concentration of all CTAB and DTAB were 50 mM. Standard
1/2
x = q[a2μ2 + b2(1 − μ2)]
deviation in Nagg, a, b, and α are 6, 3, 3, and 2%, respectively.
(6)
where a and b are the semimajor and semiminor axes of an
ellipsoid micelle (a > b = c), respectively, and μ is the cosine of
the angle between the directions of a and the wave vector q.
The interparticle structure factor S(q) has been calculated using
expressions derived by Hayter and Penfold from the Ornstein−
Zernike equation making the rescaled mean spherical approxi-
mation (RMSA).34,46 The fractional charges of the semimajor
and semiminor axes are used as the variables in the fit. For a
better fit, a polydispersity in the semimajor axis is necessary,
and is considered using the Schultz distribution;47,48 a poly-
dispersity parameter z = 35 was used in the process. The
micelle aggregation number was calculated from the relation
Nagg = 4πba2/v where v is the surfactant monomer volume
(found from Tanford formula49). Experimental data were found
to fit well with the model: the derived parameters from the fit
(presented in Figure 5) are tabulated in Table 5. Recently, from
than the CMC of CTAB) produced some interesting results
(Figure 5). It was found that mixed species of CTAB and bola
were not formed (since all the spectra nicely overlapped with
the parent component). The same was true for DTAB. The
derived results are presented in Table 5.
Nagg, a, b, and α of CTAB micelles declined with interaction
with bolas with increasing number of the spacer. The de-
pendence was nicely linear with spacer number (Figure SI 3).
Electrostatic interactions between the cationic CTAB and the
anionic bola molecules helped easier formation of micelles.
Micelle size decreased by lowering the solvent polarity by the
bolas (solvophobic effect), like nonpolar water-miscible
solvents in general.33 Empirical relation of the following type
correlated the results.
Thus,
obs
agg
CTAB
agg
N
= N
− 5m(COO−) − 4.5p(CH2)
(7)
−
where m(COO ) and p(CH2) represent the number of car-
boxylate and methylene groups present in the bolaform, respec-
tively.
The SANS results of DTAB−bola at 1:1 mol ratio are also
presented in Figure 5B and Table 5. Nagg values found for
DTAB were smaller than CTAB and reasonably tallied with
literature values (of course, because of higher concentration of
the surfactants required in SANS experiments, Nagg were on the
higher side51). Nagg also decreased with increasing spacer length
in bola; a 36% decrease in Nagg of CTAB micelle and 16% of
DTAB micelle were caused by bola-10. The effect of spacer
length of bola on the semimajor axis of micelles was appreciable
on CTAB and moderate on DTAB, whereas the fraction of
the dissociated counterions increased in both cases. Smaller
micelles with lesser charge density increased counterion disso-
ciation. A SANS study of the effect of neutral sugars (50 mM)
on nonionic surfactant C12E10 showed high Nagg. Addition of
sugar increased as well as decreased Nagg depending on the con-
centration. The semiminor axis remained essentially constant,
but the semimajor axis changed.52 The correlation found for
CTAB (eq 7) was not observed for DTAB. The above results
suggested a preference of bolas near the cationic micelles to
influence the overall charge and the potential of the electrical
double layer. The fractional charge of pure CTAB micelle at
higher concentration was 0.14, which was reduced by bola-0 (a
salt without a spacer) to 0.13, and was reduced onward by bolas
with increasing spacers up to 0.11 by bola-10. The amphiphilic
Figure 5. SANS spectra for (A) pure CTAB; (B) DTAB and mixtures
with different bola electrolytes at 303 K (solid lines show the fits to the
data). [CTAB] = 50 mM and [DTAB] = 50 mM.
SANS measurements Liu et al.50 reported that, unlike alkyl-
trimethylammonium halides, micelles were small and spherical
with hydrolyzable counterions of all valences (phosphate, oxa-
late, and carbonate), and remain spherical even in the presence
of added salt by way of strong binding that “prevents the
screening of repulsions between adjacent headgroups necessary
for sphere−cylinder transformation”. In the present study, 1:1
mol ratio of CTAB or DTAB and bolas (0, 2, 6, 10) was used,
and SANS results were fitted to the prolate geometry36,47 of the
alkytrimethylammonium bromide micelles. The presence of
sufficient Br− (50 mM) and bola ions (100 mM) in the system
prevented spherical geometry of the micelles.
SANS studies on the characterization of CTAB−bola mixed
systems at 1:1 (mol/mol) mixtures at 50:50 mM (much higher
3048
dx.doi.org/10.1021/jp4108427 | J. Phys. Chem. B 2014, 118, 3041−3052

The Journal of Physical Chemistry B
Article
Figure 6. 1H NMR spectra (top) of 10 mM of CTAB, and (middle) 10 mM 1:1 mol ratio of CTAB:bola-10 system. 2D-NOESY spectra (bottom) of
CTAB-bola-10 mixed system at 1:1 molar ratio in D2O.
bola electrolytes were accommodated more in the double layer
to make the observed continued reduction. A similar decrease
in the surface charge of micelles by bolas with increasing
spacers was also found from zeta potential measurements.
The ionization degrees of the counterions at the aggregation
points (0.08−0.32 mM) of the globular micelles at 1:1 (mol/mol)
composition were found to be 0.68−0.78 (Table 2), whereas
those found from SANS at concentrations >200-fold are only
0.11−0.14. At swamping concentrations, the anionic bola
electrolytes caused the counterions to be much less dissociated
from the prolate micelles. The counterion dissociation degree
found from zeta potential measurements at 50-fold concen-
tration was in the range of 0.20−0.40. Results by SANS and
electrokinetic measurements are nearly comparable; lower
concentration in the latter yielded 10−30% higher degree of
dissociation.
3.6. NMR Spectroscopy. NMR is a powerful and reliable
method for investigating surfactant aggregates.53,54 Both 1D
and 2D NMR techniques can be successfully applied to study
static (micelle size and shape, degree of association, structure)
as well as dynamic (molecular mobility, kinetics of micelle
formation) properties of surfactant systems. 2D-NOESY
measurement is a potential way to measure dipolar interactions
between protons for understanding three-dimensional structure.55,56
3049
dx.doi.org/10.1021/jp4108427 | J. Phys. Chem. B 2014, 118, 3041−3052

The Journal of Physical Chemistry B
Article
NMR study can lend information of interactions in micelle, reverse
micelle, and microemulsion systems;57,58 employment of NMR
method to explore mixed surfactant systems has also been
reported.59 1H-NMR experiments were performed to understand
the intermolecular interaction in the mixed CTAB−bola system.
Results on their mixed solution at 1:1 molar ratio of CTAB:Bola-10
at 10 mM are documented in Figure 6.
Proton signals of the system did not show any chemical shift
compared to their individual proton signals in the pure state.
The existence of a noninteracting individual structure in the
mixed CTAB−bola system was evidenced. 2D-NOESY experi-
ments were then performed to get information on the
interaction between protons of the herein mixed systems. As
per structural identification (top of Figure 6), information on
the interaction of δ-H of CTAB and n-H of bola with other
protons in the mixed system was restricted because of overlap
of these resonance peaks, and no cross peaks in 2D-NOESY
spectra of CTAB−bola mixed systems at 1:1 mol ratio was
observed (bottom of Figure 6). We have thus inferred non-
incorporation of bolas in the CTAB micelles. The surface active
bola electrolytes coexisted and interacted with CTAB at the air/
solution interface and did not participate in the formation of
mixed micelles.
3.7. Possibility of Vesicle Formation. Dimeric surfac-
tants, geminis, dimeric lipids, and bolaamphiphiles are used
with monomeric amphiphiles to form vesicular species in
aqueous medium. Thus, vesicular aggregation of dicarboxylates
(with spacer numbers 8 and 10) by interaction with CTAB
might be a possibility. We have studied the fluorescence
anisotropy (rss) of DPH (1,6-diphenyl-1,3,5-hexatriene) probe
in the bola−CTAB mixtures of 1:1 and 1:2 molar ratios. The
nonpolar DPH molecule became solubilized in the nonpolar
(hydrocarbon) part of the aggregates wherein the rotational
diffusion was affected, and hence the fluorescence anisotropy
(rss) was also affected. It is known that for micelles the rss values
are <0.14, whereas for bilayer aggregates or vesicles rss > 0.14.60
Therefore, the rss values can be used for understanding the
nature of the aggregates, micelles, or vesicle. In Table 4, column
3, the rss values of the bola−CTAB interacted products are
presented. The values fall in the range of 0.062−0.071 and are
<0.14. Hence, the assemblies are micelles, not vesicles. For
more support, dialysis of water-soluble dye Rhodamine 6G
(R6G) taken in the bola−CTAB mixed systems at 1:1 and 1:2
molar ratio is done against the same mixed solutions of bola
and CTAB but without dye. In the case of vesicles, the dye
entrapped in the interior or core aqueous solution should not
pass out of the membrane; only the dye present in the outside
solution would come into the dialyzed solution. Thus, the
absorbance and fluorescence intensities should decrease for
vesicles and will become zero for micelles. The latter was
observed in our experiments. The fluorescence anisotropy
experimental findings were corroborated. Another single but
potential test could be the packing parameters of the produced
species. Such calculations and the results are presented in the
Supporting Information. All the valus were <0.33 signifying the
presence of micelles, not bilayers or vesicles, for which Pc
should be >0.5.
CTAB is reduced by the bolas like salts; additionally, by their
nonpolarity the aggregation number of CTAB micelles is
reduced. The bolas are considered as “amphiphilic electrolytes”.
Two ionic headgroups restrict them to penetrate into the
CTAB micelles in contrast with amphiphiles having a single
headgroup that can insert the nonpolar tail into the micelle core
to form mixed entities. This is corroborated by our scant
success in preparing microemulsions using bolas of varied
spacer lengths. Yu et al.38 also proposed the same reason for the
penetration of C6H5COONa into the core of cationic micelles
of gemini surfactants, for which the bola-like species p-C6H4
(COONa)2 could not, and remained in the peripherial region.
Further, binary surfactants forming mixed micelles always cause
a decrease in the CMCmix with increasing proportion of the
lower CMC forming component in the composition. This is
reversed with increasing proportion of the higher CMC
forming component.3 In the studied bola−CTAB mixture,
with increasing proportion of bola the CMC of the mixed
system decreases. By the above rationale, the increase for the
CMC of the bola (if any) should be much higher than that
of CTAB. This is the physicochemical confirmation that the
studied bolas do not form mixed micelles (they act like
electrolytes and continuously decrease the CMC of CTAB). In
this context, the possibility of bilayer type vesicular aggregation
between dicarboxylates (with spacer numbers 8 and 10) and
alkyltrimethyl ammonium salts like CTAB has been examined.
The studied bolas (dicarboxylates with spacer 8 and 10) did
not form either mixed micelles or bilayer aggregates upon
interaction with CTAB. The calculation of packing parameter,
determination of DPH anisotropy, and dialysis of R6G dye and
DLS studies unequivocally supported the formation of micelles,
not vesicles, of bola−CTAB interacting products in solution.
At the air/water interface, the bola and CTAB undergo
synergistic interaction. At different mixing ratios, the relative
interfacial concentration of CTAB is higher than that of bola. At
a fixed mixing ratio, the relative proportion of the latter at the
interface increases with the length of its spacer. SANS study
supports prolate geometry of CTAB and DTAB micelles. Effect
of bolas on the size and charge of CTAB micelles is more
prominent than that of DTAB. A much higher concentration of
[CTAB + bola] = 50 mM (at 1:1 mol/mol) in SANS experi-
ments makes the ionization degree of the prolate micelles 10−
15%, which from zeta potential measurements (at [CTAB +
bola] = 10 mM) is 20−40%. At CMC points (0.08−0.32 mM),
the ionization degrees are expectedly much higher, 68−78%.
The concentration of the amphihilic electrolytes has an effect
on the ionization degree of the formed CTAB micelles.
ASSOCIATED CONTENT
■
S
* Supporting Information
Table (SI 1-SI 5) of various physicochemical parameters of
CTAB with bola-0, 2, 4, 8, and 10. Dependence of IC
(interfacial composition) on SC (stoichiometric composition)
of mixed CTAB-bola systems at 303 K is presented in Figure SI 1.
Figure SI 2 illustrates dependence of zeta potential on spacer
number of bola at 1:1, 2:1 (CTAB:bola). Figure SI 3 depicts the
variation of SANS parameters with spacer number of bolas in
CTAB-bola mixed systems. Text SIT presents calculation of
packing parameters. This material is available free of charge via
the Internet at http://pubs.acs.org.
CONCLUSION
■
The studied surface active bolaforms do not self-aggregate in
aqueous medium. Those with spacer number ≤4 lie flat at the
interface, and the rest having spacers ≥4−10 assume “wicket-
like” conformation (their Amin ≡ 2Amin of CTAB). The CMC of
3050
dx.doi.org/10.1021/jp4108427 | J. Phys. Chem. B 2014, 118, 3041−3052

The Journal of Physical Chemistry B
Article
(14) Zana, R.; Yiv, S.; Kale, K. M. Chemical Relaxation and
Equilibrium Studies of Association in Aqueous Solutions of Bolaform
Detergents. 3. Docosane-1, 22-bis (trimethylammonium bromide). J.
Colloid Interface Sci. 1980, 77, 456−465.
(15) Menger, F. M.; Wrenn, S. Interfacial and Micellar Properties of
Bolaform Electrolytes. J. Phys. Chem. 1974, 78, 1387−1390.
(16) Johnson, J. R.; Fleming, R. Micellar Properties of Bolaform
Electrolytes in Aqueous Solution. J. Phys. Chem. 1975, 79, 2327−2328.
(17) Maiti, K.; Mitra, D.; Mitra, R. N.; Panda, A. K.; Das, P. K.;
Rakshit, A. K.; Moulik, S. P. Self-Aggregation of Synthesized Novel
Bolaforms and Their Mixtures with Sodium Dodecyl Sulfate (SDS)
and Cetyltrimethylammonium Bromide (CTAB) in Aqueous Medium.
J. Phys. Chem. B 2010, 114, 7499−7508.
(18) Yan, Y.; Xiong, W.; Huang, J.; Li, Z.; Li, X.; Li, N.; Fu, H.
Organized Assemblies in Bolaamphiphile/Oppositely Charged Con-
ventional Surfactant Mixed Systems. J. Phys. Chem. B 2005, 109, 357−
364 and the references therein.
(19) Han, F.; He, Xiao; Huang, J.; Li, Z.; Wang, Y.; Fu, H. Surface
Properties and Aggregates in the Mixed Systems of Bolaamphiphiles
and Their Oppositely Charged Conventional Surfactants. J. Phys.
Chem. B 2004, 108, 5256−5262.
(20) Yan, Y.; Xiong, W.; Li, X.; Lu, T.; Huang, J.; Li, Z.; Fu, H.
Molecular Packing Parameter in Bolaamphiphile Solutions: Adjust-
ment of Aggregate Morphology by Modifying the Solution
Conditions. J. Phys. Chem. B 2007, 111, 2225−2230 and the references
therein.
(21) Shimizu, T.; Iwaura, R.; Masuda, M.; Hanada, T.; Yase, K.
Internucleobase-Interaction-Directed Self-Assembly of Nanofibers
from Homo- and Heteroditopic 1,ω-Nucleobase Bolaamphiphiles. J.
Am. Chem. Soc. 2001, 123, 5947−5955 and the references therein.
(22) Caschera, F.; de la Serna, J. B.; Loffler, P. M. G.; Rasmussen, T.
E.; Hanczyc, M. M.; Bagatoli, A.; Monnard, P.-A. Stable Vesicles
Composed of Monocarboxylic or Dicarboxylic Fatty Acids and
Trimethylammonium Amphiphiles. Langmuir 2011, 27, 14078−
14090 and the references therein.
AUTHOR INFORMATION
Corresponding Author
*Phone:+91-33-2414-6411; Fax:+91-33-2414-6266; E-mail:
spmcss@yahoo.com.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
A.P., P.S., and S.D. are thankful to UGC−CSIR, Government of
India, New Delhi, for granting Senior Research Fellowships.
P.K.D. is thankful to the Council of Scientific and Industrial
Research (CSIR), India, for financial assistance (01(2471)/11/
EMR-II). A.K.R. thanks AICTE, New Delhi, for a former
Emeritus Fellow position. S.P.M. thanks Jadavpur University
and Indian National Science Academy for the positions of
Emeritus Professor and Honorary Scientist, respectively.
REFERENCES
■
(1) Moroi, Y. Micelles, Theoretical and Applied Aspects; Plenum Press:
New York, 1992.
(2) Chatterjee, A.; Maiti, S.; Sanyl, S. K.; Moulik, S. P. Micellization
and Related Behaviors of N-Cetyl-N-ethanoyl-N,N-dimethyl and N-
Cetyl-N,N-diethanolyl-N-methyl Ammonium Bromide. Langmuir
2002, 18, 2998−3004.
(3) Haque, Md. E.; Das, A. R.; Rakshit, A. K.; Moulik, S. P. Properties
of Mixed Micelles of Binary Surfactant Combination. Langmuir 1996,
12, 4084−4089.
(4) Basu Ray, G.; Chakraborty, I.; Ghosh, S.; Moulik, S. P.; Palepu, R.
Self-Aggregation of Alkyltrimethylammonium Bromides (C10-, C12-,
C14-, and C16TAB) and Their Binary Mixtures in Aqueous Medium: A
Critical and Comprehensive Assessment of Interfacial Behaviour and
Bulk Properties with Reference to Two Types of Micelle Formation.
Langmuir 2005, 21, 10958−10967.
(5) Maiti, K.; Bhattacharya, S. C.; Moulik, S. P.; Panda, A. K.
Physicochemical Studies on Ion-Pair Amphiphiles: Solution and
Interfacial Properties of Systems Derived From Sodium Dodecylsulfate
and n-Alkyltrimethylammonium Bromide Homologues. J. Chem. Sci.
2010, 122, 1−13.
(6) Mitra, D.; Chakraborty, I.; Bhattacharya, S. C.; Moulik, S. P.
Interfacial and Solution Properties of Tetraalkylammonium Bromides
and Their Sodium Dodecyl Sulfate Interacted Products: A Detailed
Physicochemical Study. Langmuir 2007, 23, 3049−3061 and
references therein.
(7) Kaler, E. W.; Herrington, K. L.; Murthy, A. K.; Zasadazinski, J. A.
N. Phase Behavior and Structures of Mixtures of Anionic and Cationic
Surfactants. J. Phys. Chem. 1992, 96, 6698−6707.
(8) Raghavan, S. R.; Fritz, G.; Kaler, E. W. Wormlike Micelles
Formed by Synergistic Self-Assembly in Mixtures of Anionic and
Cationic Surfactants. Langmuir 2002, 18, 3797−3803.
(9) Shome, A.; Kar, T.; Das, K. P. Spontaneous Formation of
Biocompatable Vesicles in Aqueous Mixtures of Amino Acid-Based
Cationic Surfactants And SDS/SDBS. Chem Phys Chem 2011, 12,
369−378 and references therein.
(10) Fuoss, R. M.; Edelson, D. J. Bolaform Electrolytes. I. Di-(β-
Trimethylammonium Ethyl) Succinate Dibromide and Related
Compounds. J. Am. Chem. Soc. 1951, 73, 269−273.
(11) Fuhrhop, J. -H.; Wang, T. Bolaamphiphiles. Chem. Rev. 2004,
104, 2901−2938.
(12) Yiv, S.; Kale, K. M.; Lang, J.; Zana, R. Chemical Relaxation and
Equilibrium Studies of Association in Aqueous Solutions of Bolaform
Detergents. 1. Dodecane-1,12-Bis(trimethylammonium bromide). J.
Phys. Chem. 1976, 80, 2651−2655.
(13) Yiv, S.; Zana, R. Chemical Relaxation and Equilibrium Studies of
Association in Aqueous Solutions of Bolaform Detergents. 2.
Hexadecane-1,16-Bis (trimethylammonium bromide) and Dodecane-
1,12-Bis (tributylammonium bromide). J. Colloid Interface Sci. 1980,
77, 449−455.
(23) Bhattacharya, S.; De, S.; Subramanian, M. Synthesis and Vesicle
Formation from Hybrid Bolaphile/Amphiphile Ion-Pairs. Evidence of
Membrane Properties Modulation by Molecular Design. J. Org. Chem.
1998, 63, 7640−7651.
(24) Bhattacharya, S.; De, S. Vesicle Formation from Dimeric Ion-
Paired Amphiphiles. Control over Vesicular Thermotropic and Ion-
Transport Properties as a Function of Intra-amphiphilic Headgroup
Separation. Langmuir 1999, 15, 3400−3410.
(25) Bhattacharya, S.; De, S. Vesicle Formation from Dimeric
Surfactants Through Ion-Pairing. Adjustment of Polar Headgroup
Separation Leads to Control Over Vesucular Thermotropic Properties.
J. Chem. Soc., Chem. Commun. 1995, 6, 651−652.
(26) Bhattacharya, S.; De, S. Synthesis and Vesicle Formation from
Dimeric Pseudoglyceryl Lipids with (CH2)m Spacers: Pronounced
m-value Dependence of Thermal Properties, Vesicle Fusion, and
Cholesterol Complexation. Chem.Eur. J. 1999, 2335−2347.
(27) Fukuda, H.; Kawata, K.; Okuda, H.; Regen, S. L. Bilayer-
Forming Ion-Pair Amphiphiles from Single-Chain Surfactants. J. Am.
Chem. Soc. 1990, 112, 1635−1637.
(28) Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadzinski, J. A.
N. Spontaneous Vesicle formation in Aqueous Mixtures of Single-
Tailed Surfactants. Science 1989, 245, 1371−1374.
(29) Quirion, F.; Magid, L. J. Growth and Counterion Binding of
Cetyl Trimethylammonium Bromide Aggregates at 25°C: A Neutron
and Light Scattering Study. J. Phys. Chem. 1986, 90, 5435−5441.
(30) Mukherjee, P.; Mysels, K. J. Critical Micelle Concentration of
Aqueous Surfactant Systems; National Standard Reference Data Service,
Vol. 36; U.S. National Bureau of Standards, Washington, DC, 1971.
(31) Evans, D. F.; Allen, M.; Ninham, B. W.; Fouda, A. Critical
Micelle Concentrations of Alkyltrimethylammonium Bromides in
Water from 25 to 160°C. J. Solution Chem. 1984, 13, 87−101.
(32) Zana, R. Ionization of Cationic Micelles: Effect of the Detergent
Structure. J. Colloid Interface Sci. 1980, 78, 330−337.
3051
dx.doi.org/10.1021/jp4108427 | J. Phys. Chem. B 2014, 118, 3041−3052

The Journal of Physical Chemistry B
Article
1
(33) Pan, A.; Naskar, B.; Prameela, G. S. K.; Phani Kumar, B. V. N.;
Mandal, A. B.; Bhattacharya, S. C.; Moulik, S. P. Amphiphile Behavior
in Mixed Solvent Media I: Self-Aggregation and Ion Association of
Sodium Dodecylsulfate in 1,4-Dioxane-Water and Methanol-Water
Media. Langmuir 2012, 28, 13830−13843.
(34) Bhattacharjee, J.; Aswal, V. K.; Hassan, P. A.; Pamu, R.;
Narayanan, J.; Bellare, J. Structural Evolution in Catanionic Mixtures
of Cetylpyridinium Chloride and Sodium Deoxycholate. Soft Matter
2012, 8, 10130−10140.
(35) Pan, A.; Mati, S. S.; Naskar, B.; Bhattacharya, S. C.; Moulik, S. P.
Self-Aggregation of MEGA-9 (N-nonanoyl-N-methyl-D-glucamine) in
Aqueous Medium: Physicochemistry of Interfacial and Solution
Behaviors with Special Reference to Formation Energetics and Micelle
Environment. J. Phys. Chem. B 2013, 117, 7578−7592.
(36) Adamson, A. W. Physical Chemistry of Surfaces; Interscience
(John Wiley), 1963; Chapter II, pp 94−96.
(37) Brown, G. L.; Grieger, P. F.; Kraus, C. A. Properties of
Electrolytic Solutions. XXXVI. Effect of Addition Agents on the
Conductance of Long Chain Salts. J. Am. Chem. Soc. 1949, 71, 95−
101.
(38) Yu, D.; Huang, X.; Deng, M.; Lin, Y.; Jiang, L.; Huang, J.; Wang,
Y. Effect of Inorganic and Organic Salts on Aggregation Behavior of
Cationic Gemini Surfactants. J. Phys. Chem. B 2010, 114, 14955−
14964.
(39) Mukherjee, I.; Moulik, S. P.; Rakshit, A. K. Tensiometric
Determination of Gibbs Surface Excess and Micelle Point: A Critical
Revisit. J. Colloid Interface Sci. 2013, 394, 329−336.
(40) Oh, S.-W.; Na, J.-S.; Ko, J.-S.; Nagadome, S.; Sugihara, G.
Blending Effects on Adsorption and Micellization of Different
Membrane Protein Solubilizers: II. A Thermodynamic Study on a
Mixed System of CHAPS with a Bile salt in pH Phosphate buffer
solution. Colloids Surf., B 2008, 62, 112−124.
(53) Shimizu, S.; Pires, P. A. R.; Seoud, O. A. E. H and 13C NMR
Study on the Aggregation of (2-Acylaminoethyl)Trimethylammonium
Chloride Surfactants in D2O. Langmuir 2003, 19, 9645−9652.
(54) Alonso, B.; Harris, R. K.; Kenwright, A. M. Micellar
Solubilisation: Structural and Conformational Changes Investigated
by H and 13C Liquid-State NMR. J. Colloid Interface Sci. 2002, 251,
1
366−375.
(55) Gjerde, M.; Nerdal, W.; Holand, H. A NOESY NMR Study of
the Interaction Between Sodium Dodecylsulfate and Poly (ethylene
oxide). J. Colloid Interface Sci. 1996, 183, 285−288.
(56) Yuan, H. Z.; Cheng, G. Z.; Zhao, S.; Miao, X. J.; Yu, J. Y.; Shen,
L. F.; Du, Y. R. Conformational Dependence of Triton X-100 on
Environment Studied by 2D NOESY and 1H NMR Relaxation.
Langmuir 2000, 16, 3030−3045.
(57) Shi, Z.; Peterson, R. W.; Wand, A. J. New Reverse Micelle
Surfactant Systems Optimized for High-Resolution NMR Spectrosco-
py of Encapsulated Proteins. Langmuir 2005, 21, 10632−10637.
(58) Kaur, G.; Chiappisi, L.; Prevost, S.; Schweins, R.; Gradzielski,
M.; Mehta, S. K. Probing the Microstructure of Nonionic Micro-
emulsions with Ethyl Oleate by Viscosity, ROSEY, DLS, SANS, and
Cyclic Voltammetry. Langmuir 2012, 28, 10640−10652.
(59) Yuan, H. Z.; Zhao, S.; Cheng, G. Z.; Zhang, L.; Miao, X. J.; Mao,
S. Z.; Yu, J. Y.; Shen, L. F.; Du, Y. R. Mixed Micelles of Triton X-100
and Cetyltrimethylammonium Bromide in Aqueous Solution Studied
1
by H NMR. J. Phys. Chem. B 2001, 105, 4611−4615.
(60) Ghosh, S.; Khatua, D.; Dey, J. Interaction Between Zwitterionic
and Anionic Surfcatnats: Spontaneous Formation of Zwitterionic
Vesicles. Langmuir 2011, 27, 5184−5192.
(41) Sulthana, S. B.; Bhat, S. G. T.; Rakshit, A. K. Studies of the
Effect of Additives on the Surface and Thermodynamic Properties of
Poly(oxyethylene(10)) Lauryl Ether in Aqueous Solution. Langmuir
1997, 13, 4562−4568.
(42) Eastoe, J.; Nave, S.; Downer, A.; Paul, A.; Rankin, A.; Tribe, K.;
Penfold, J. Adsorption of Ionic Surfactants at the Air-Solution
Interface. Langmuir 2000, 16, 4511−4518.
(43) Mukherjee, I.; Mukherjee, S.; Naskar, B.; Ghosh, S.; Moulik, S.
P. Amphiphilic Behavior of Two Phosphonium Based Ionic Liquids. J.
Colloid Interface Sci. 2013, 395, 135−144 and references therein.
(44) Rosen, M. J. Surfactants and Interfacial Phenomenon, 2nd ed.;
Wiley: New York, 1989.
(45) Shrestha, R. G.; Shrestha, L. K.; Aramaki, K. Formation of
Wormlike Micelle in a Mixed Amino-Acid Based Anionic Surfactant
and Cationic Surfactant Systems. J. Colloid Interface Sci. 2007, 311,
276−284.
(46) Sheu, E. Y.; Wu, C. F.; Chen, S. H. Effects of Ion Sizes on the
Aggregation and Surface Charge of Ionic Micelles in 1:1 Electrolyte
Solutions. J. Phys. Chem. 1986, 90, 4179−4187.
(47) Hayter, J. B.; Penfold, J. An Analytic Structure Factor for
Macroion Solutions. Mol. Phys. 1981, 42, 109−118.
(48) Hayter, J. B.; Penfold, J. Determination of Micelle Structure and
Charge by Neutron Small-Angle Scattering. Colloid Polym. Sci. 1983,
261, 1022−1030.
(49) Tanford, C. The Hydrophobic Effect: Formation of Micelles and
Biological Membranes; Wiley: New York, 1980.
(50) Liu, C. K.; Warr, G. G. Resiliently Spherical Micelles of
Alkyltrimethylammonium Surfactants with Multivalent, Hydrolyzable
Counterions. Langmuir 2012, 28, 11007−11016.
(51) Naskar, B.; Dan, A.; Ghosh, S.; Aswal, V. K.; Moulik, S. P.
Revisiting the Self-Aggregation Behavior of Cetyltrimethylammonium
Bromide in Aqueous Sodium Salt Solution with Varied Anions. J. Mol.
Liq. 2012, 170, 1−10.
(52) Sharma, K. S.; Joshi, J. V.; Aswal, V. K.; Goyal, P. S.; Rakshit, A.
K. Small-Angle Neutron Scattering Studies of Non-ionic Surfactant:
Effect of Sugars. Pramana - J. Phys. 2004, 63, 297−302.
3052
dx.doi.org/10.1021/jp4108427 | J. Phys. Chem. B 2014, 118, 3041−3052