Self-aggregation of synthesized novel bolaforms and their mixtures with sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) in aqueous medium

Article abstract of DOI:10.1021/jp910527q

Bolaforms B₁, B₂, and B₃ of the formulas, Br^-Me₃N⁺(CH₂)₁₀N ⁺Me₃Br^-, Br^-Me₃N ⁺(CH₂)₁₀OH, and Br^-Me ₃N⁺(CH₂)₁₀COO^-Na ⁺, respectively, were synthesized, and their properties in the bulk as well as at the air/aqueous NaBr (10 mM) solution interface have been studied. Their interactions with sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) also have been investigated. Tensiometry, conductometry, spectrophotometry, and microcalorimetry techniques were used for characterization and estimation. Both pure bolaforms and their mixtures with SDS and CTAB have been found to self-aggregate, forming micelles in solution. The mixed systems of bolaform and SDS have been observed to form both micelles and vesicles. Their mutual interactions were synergistic, which at the interface was more spontaneous than in the bulk. The interfacial and bulk compositions of the mixed binary systems (bolaform and SDS or CTAB) with their associated interaction parameters have been estimated from the Rosen interaction model and the regular solution theory of Rubingh, respectively. The formed vesicles have been found to entrap the water-soluble dye, bromophenol blue, and the dye solubilized vesicles of B₁-SDS and B₂-SDS completely eluted out of the sephadex column proving their formation. A rough estimation of the size and polydispersity index of the formed micelles and vesicles has been made from DLS measurements.

Full text of DOI:10.1021/jp910527q

J. Phys. Chem. B 2010, 114, 7499–7508
7499
Self-Aggregation of Synthesized Novel Bolaforms and Their Mixtures with Sodium Dodecyl
Sulfate (SDS) and Cetyltrimethylammonium Bromide (CTAB) in Aqueous Medium
Kajari Maiti,† Debolina Mitra,† Rajendra N. Mitra,‡ Amiya K. Panda,§ Prasanta K. Das,‡
Animesh K. Rakshit,*,| and Satya P. Moulik*,†
Centre for Surface Science, Department of Chemistry, JadaVpur UniVersity, Kolkata 700 032, India,
Department of Biological Chemistry, Indian Association for CultiVation of Science, Kolkata 700 032, India,
Department of Chemistry, UniVersity of North Bengal, Darjeeling 734 013, India, and Department of Natural
Sciences, West Bengal UniVersity of Technology, Kolkata 700 064, India
ReceiVed: NoVember 4, 2009; ReVised Manuscript ReceiVed: April 13, 2010
Bolaforms B1, B2, and B3 of the formulas, Br-Me3N+(CH2)10N+Me3Br-, Br-Me3N+(CH2)10OH, and
Br-Me3N+(CH2)10COO-Na+, respectively, were synthesized, and their properties in the bulk as well as at the
air/aqueous NaBr (10 mM) solution interface have been studied. Their interactions with sodium dodecyl
sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) also have been investigated. Tensiometry,
conductometry, spectrophotometry, and microcalorimetry techniques were used for characterization and
estimation. Both pure bolaforms and their mixtures with SDS and CTAB have been found to self-aggregate,
forming micelles in solution. The mixed systems of bolaform and SDS have been observed to form both
micelles and vesicles. Their mutual interactions were synergistic, which at the interface was more spontaneous
than in the bulk. The interfacial and bulk compositions of the mixed binary systems (bolaform and SDS or
CTAB) with their associated interaction parameters have been estimated from the Rosen interaction model
and the regular solution theory of Rubingh, respectively. The formed vesicles have been found to entrap the
water-soluble dye, bromophenol blue, and the dye solubilized vesicles of B1-SDS and B2-SDS completely
eluted out of the sephadex column proving their formation. A rough estimation of the size and polydispersity
index of the formed micelles and vesicles has been made from DLS measurements.
Introduction
are gaining attention where one headgroup remains attached to
the surfaces of electrodes, polyelectrolytes, nanoparticles, etc.,
and the other end functions for solubilization and/or interaction.11
They may also produce mesoporous structures.12 More recently,
scientists are using bolaforms for extraction of metal ions,13 to
induce ion-pair formation in aqueous solution,14 and as photo-
sensitive molecules.15
In the last two decades, model archaeal lipid molecules have
been synthesized and studied.16-19 The synthesis of bolaforms,
with two ammonium head groups, and a single hydrophobic
chain has been fairly explored.16-20 Their tensiometric behaviors
and spectral features in aqueous medium have suggested that
bolaforms with a spacer of less than 12 methylene21,22 groups
behave as simple electrolytes in aqueous solution whereas those
with spacers g16 methylene groups generally form micelles.23,24
Usually, bolaforms with double hydrocarbon chains can easily
form globular vesicles in aqueous solution.20 Vesicle formation
by single chain bolaforms has been also reported.25
Along with single bolaform systems, their mixed systems with
conventional surfactants have attracted the attention of scienti-
sts.16,26-29 Vesicle formation of the mixed systems of bolaforms
and conventional surfactants of similar charges has been
reported.29 However, studies on the mixed systems of bolaforms
and oppositely charged conventional surfactants are still
limited.16,26 It is known that the conventional ion-pair am-
phiphiles (IPA) or catanionics (formed between two oppositely
charged surfactants) have very low solubility, and they easily
form bilayer vesicles.30 Vesicles in aqueous solutions have
importance in the areas of fundamental biophysical chemistry
and industries;27,28 they have application prospects in pharma-
ceutics, catalysis, microreactors,31,32 etc. Compared to the natural
Bolaamphiphiles/bolaforms (or simply bolas) consist of a
hydrophobic chain with two hydrophilic head groups at its
extremities.1,2 Due to their special chemical structure, they are
capable of self-organizing themselves in an aqueous/aquo-salt
environment to form spheres, disks, large cylinders, monola-
mellar membranes, vesicles,3 etc. The morphology of the
aggregates is largely dependent on (i) the length of the spacer
(hydrophobic chain joining the two polar head groups in the
molecule) and (ii) its rigidity/flexibility.4 The presence of a long
spacer leads to a micelle because of easier folding, whereas short
spacers lead to vesicle formation.5 Further, the presence of a
foreign substance like salt, sugar, etc. in solution also affects
the micelle formation.
The bolaforms can mimic the lipids of the cytoplasmic
membranes of the archaebacteria that can cope with extreme
conditions6,7 (such as high temperature, pressure, pH, etc.). A
literature survey reveals that scientists have been trying to
synthesize vesicle forming lipid anlogues.5,6 It is considered that
the mixed bolaform monolayers can be used as model membrane
mimetic systems.8,9 Because of the slower rate of fusion of their
monolamellar membranes, bolaforms are considered useful in
pharmaceutical preparation of liposomes, and it is already in
practice.10 Uses of anchored bolaforms on smooth solid surfaces
* Authors for correspondence. A.K.R.: phone, +91 33 2321 0731; e-mail,
akrakshi@yahoo.co.in. S.P.M.: phone, +91 33 2414 6411; fax, +91 33
2414 6266; e-mail, spmcss@yahoo.com.
† Jadavpur University.
‡ Indian Association for Cultivation of Science.
§ University of North Bengal.
| West Bengal University of Technology.
10.1021/jp910527q  2010 American Chemical Society
Published on Web 05/17/2010

7500 J. Phys. Chem. B, Vol. 114, No. 22, 2010
Maiti et al.
(lipid) and synthetic vesicles (composed of conventional cat-
anionic surfactants), the pure catanionic and mixed bolaform
vesicles are more advantageous because they form easily and
are very stable.30,33
was added freshly prepared trimethyl amine. The test tube was
sealed and then heated at ∼323 K for 12 h. The insoluble
product was separated by filtration, washed with hexane, and
then dissolved in water. The formed acidic solution was then
neutralized by NaOH addition. It was then lyophilized to obtain
the desired white product of N-(10-carboxydecyl)-N,N,N-trim-
ethylammonium bromide. The product yield was 9 mmol
(∼80%).
In this study, we report synthesis and interfacial behaviors
of three bolaforms having different head groups with identical
spacer structures. They are decane-1,10-bis(trimethylammonium
bromide) (B1), N-(10-hydroxydecyl)-N,N,N-trimethlyammonium
bromide (B2), and N-(10-carboxydecyl)-N,N,N-trimethylammo-
nium bromide (B3). The physicochemical properties of these
bolaforms and their mixed systems with sodium dodecyl sulfate
(SDS) as well as with cetyltrimethylammonium bromide
(CTAB) at the air/solution interface and in the bulk (in salt
solution) have been studied and discussed. It may be mentioned
that this type of bolaform has been rarely studied in the past.22
With ten methylene spacer groups in the molecules they have
been found to self-aggregate in aqueous solution, preferably in
salt (NaBr) solution. Their interacted products with the con-
ventional surfactants SDS and CTAB can also form vesicles in
addition to micelles.
Characterization of Bolaforms. This section comprises
spectroscopic, NMR, mass spectral, and elemental analytical
results for characterization of the synthesized bolaforms.
B1. 1H NMR (300 MHz, D2O): δ/ppm ) 3.12-3.18 (m, 4H),
2.99 (s, 18H), 1.62 (br, 4H), 1.19 (br, 12H). Anal. Calcd for
C16H38Br2N2 (2 mol % crystal water): C, 42.30; H, 9.32; N,
6.17. Found: C, 42.05; H, 9.01; N, 6.03. ESI-MS: m/z: 258.2383
(M+), m/z (calculated): 258.3024 (M+).
B2. 1H NMR (300 MHz, D2O): δ/ppm ) 3.49-3.54 (t, 2H),
3.19-3.25 (m, 2H), 3.02 (s, 9H), 1.69-1.72 (br, 2H), 1.44-1.48
(m, 2H), 1.23-1.28 (br, 12H). Anal. Calcd for C13H30BrNO
(1.33 mol % crystal water): C, 48.75; H, 10.28; N, 4.37. Found:
C, 48.56; H, 9.64; N, 4.41. ESI-MS: m/z: 216.1716 (M+), m/z
(calculated): 216.2232 (M+).
Experimental Section
B3. 1H NMR (300 MHz, D2O): δ/ppm ) 3.08-3.14 (m, 2H),
2.91 (s, 9H), 1.98-2.03 (t, 2H), 1.59-1.62 (br, 2H), 1.34-1.38
(m, 2H), 1.12-1.17 (br, 12H). Anal. Calcd for C14H29BrNNaO2
(4 mol % crystal water): C, 40.19; H, 8.91; N, 3.35. Found: C,
41.31; H, 7.83; N, 3.16. ESI-MS: m/z: 244.1164 [M - Na+ -
Br- + 1]+, m/z (calculated): 243.2198 (M - Na+ - Br-).
Materials. SDS (99%) and CTAB (99%) were products of
Sigma and used as received. C10TAB (decyltrimethylammonium
bromide) was a 99% pure product of Alfa Aeser (USA).
10-bromodecanol, 11-bromoundecanoic acid, and 1,6-diphenyl-
1,3,5-hexatriene (DPH) were purchased from Sigma, Fluka, and
Hi Media, respectively. Tetrahydrofuran (THF, GLC purity
>99.9%), trimethylamine, and ethyl acetate were obtained from
Spectrochem (India). Bromophenol blue, Na2S2O3, HgO, NaBr,
hexane, CCl4, MeOH, and EtOH were analytical grade products
from SRL (India).
The above characteristic results have confirmed the high
purity of the herein synthesized bolaforms.
Preparation of Catanionic Solution. Stock solutions of
bolaforms, SDS and CTAB were prepared in doubly distilled
water (specific conductance, κ ) 2-4 µS cm-1 at 298 K). The
bolaform-SDS or bolaform-CTAB combinations of different
molar ratios at a constant [NaBr] ) 10 mM were prepared26 in
stoppered test tubes and were equilibrated for two weeks in a
water bath at 298 K.
Synthesis of the Bolaforms. 1. B1 [Br-Me3N+-(CH2)10-N+
Me3Br-]. A 250 mL three-necked round-bottom (RB) flask
(wrapped with aluminum foil to exclude light) was fitted with
a mechanical stirrer and a reflux condenser. A 28.3 mmol sample
of HgO was suspended in 100 mL of freshly dried CCl4 through
a pressure equalizer funnel, to it was added 37.7 mmol of 11-
bromoundecanoic acid, and the mixture was heated to reflux at
388 K for 0.5 h. After the reflux, 42.2 mmol of Br2 in 75 mL
of CCl4 was added dropwise for 15 min to the mixture. Another
neck of the RB flask was fitted with a glass tube that was
immersed in a beaker containing 10% (w/v) Na2S2O3. After the
reaction, the mixture was cooled in an ice-bath resulting in
precipitation and the cooling was continued for 30 min. The
material was filtered and the reddish filtrate was vacuum distilled
at 453 K, resulting in the formation of a black-colored 1,10-
dibromodecane solution. Freshly prepared trimethylamine was
added to 28.7 mmol of the 1,10-dibromodecane in a 100 mL
RB flask and stirred for 4 h. TLC performed in 10% ethyl acetate
in hexane, and the product appeared at 0.7Rf. The material was
concentrated and dried under vacuum. The obtained decane-
1,10-bis(trimethylammonium bromide) was crystallized from
methanol/ether and filtered and dried. The overall yield was 20
mmol (∼51%).
Methods. NMR, Mass Spectra, Elemental Analysis. The 1H
NMR spectra of the bolaforms were taken in an AVANCE 300
MHz (BRUKER) spectrometer; observed chemical shift values
(δ) are reported in parts per million (ppm). The elemental
analyses were done in a Perkin-Elmer CHN Analyzer. High-
resolution mass- spectrometric (HRMS) data were acquired by
electrospray ionization (ESI) techniques, both in the positive
and in the negative modes in a Q-tof Micro-Quadruple mass
spectrophotometer (Micromass, U.K.).
Tensiometry. Surface tension was measured by the platinum
ring detachment technique with a du Nou¨y tensiometer (Kru¨ss,
Germany). A concentrated solution of pure bolaforms or their
mixtures with either SDS or CTAB (in water/10 mM NaBr
solution, as the case may be) was added to a known volume of
aqueous solutions or aqueous NaBr solutions (10 mM). After
thorough mixing and temperature equilibration at 298 K (using
a Neslab RTE-100, temperature controlled circulator water bath
of accuracy ) (0.1 K), the surface tension was measured with
an accuracy of (0.1 mN m-1. Further details of measurements
were described in our earlier publication.34
2. B2 [Br-Me3N+-(CH2)10-OH]. A 10 mmol sample of 10-
bromodecanol was taken in a test tube and to it was added
freshly prepared trimethyl amine. The tube was then sealed and
heated at 313 K for 12 h. The resulting white precipitate was
washed with hexane and crystallized from methanol/ether. The
yield of the product N-(10-hydroxydecyl)-N,N,N-trimethylam-
monium bromide was 8.7 mmol (∼87%).
Isothermal Titration Calorimetry. An Omega ITC micro-
calorimeter of Microcal Inc. was used for the microcalorimetric
measurements at 298 K. A Neslab RTE-100 water bath
employed to circulate constant-temperature water at several
degrees lower than that in the calorimetric cell surroundings
wherein the internal device maintained the experimental tem-
perature with an accuracy of (0.01 K. Concentrated surfactant
3. B3 [Br-Me3N+-(CH2)10-COO-Na+]. A 11.3 mmol sample
of 11-bromoundecanoic acid was taken in a test tube, and to it

Bolaforms and Their Mixtures with SDS and CTAB
J. Phys. Chem. B, Vol. 114, No. 22, 2010 7501
(SDS, CTAB, bolaforms, and their mixtures) solutions (about
20 times their critical micelle concentration) were added in
installments at equal time intervals (210 s) into 1.352 mL of
10 mM NaBr solution taken in the calorimeter cell under
constant stirring (300 rpm) condition. The reference cell
contained 1.6 mL of 10 mM NaBr solution. The heat of dilution
of the amphiphile or mixed amphiphile solution at each step of
addition was stored in the instrument software. The enthalpy
of micellization was the difference between the initial and final
asymptotes in the observed enthalpogram. For further details
of the measurement protocol, we refer to our previous
publications.34,35
Conductometry. Conductance measurements were taken in
a Jenway conductometer with an accuracy of (0.5% using a
dip-type conductance cell of cell constant 0.1 cm-1. The
temperature of the solution (10 mM NaBr) was maintained at
298 K with a Neslab RTE-100 circulator water bath with an
accuracy of (0.1 K.34
Figure 1. Determination of CMC of B2 in aqueous medium. Main
diagram: b, conductometry; 2, spectrophotometry at 308 K. CMC
points are indicated by broken arrows. Inset A: tensiometric plots of
B2 in aqueous medium. Inset B: tensiometric profiles of B1 and B3 in
aqueous medium at 308 K.
Results and Discussion
Spectrophotometry. Absorption spectra of pure and mixed
bolaform solutions at 298 K were recorded with a UV-vis 1601
spectrophotometer (Shimadzu) using a quartz cell of path length
1 cm.
1. Physicochemical Behavior of Pure Bolaforms. A. Self-
Assembly Formation in Aqueous Medium. The aqueous
solubility of bolaforms followed the order B1 ≈ B3 . B2. B1
and B3 have strong hydrophilic groups at both ends of the
methylene chain to make them water-soluble, whereas B2 has a
single hydroxyl group at one end that causes reduction in
solubility. All three bolaforms have a common trimethylam-
monium end group.
Dye Solubilization. In this process, solubilization of a water
insoluble, neutral dye, DPH,36 into micellar and vesicular
solutions was studied. Five microliters of THF solution of DPH
(0.075 mM) was added both to 4 mL of 10 mM aquo-NaBr
solution and to 4 mL of pure THF, and the absorption spectra
of the prepared solutions were recorded after 24 h in a Shimadzu
UV-vis 1601 spectrophotometer at 298 K. Various concentra-
tions of mixed bolaform systems with a particular [bolaform]/
[SDS] or [bolaform]/[CTAB] ratio and containing same con-
centrations of NaBr and DPH were also prepared and their
absorption spectra were also recorded at the same temperature
after 24 h of ageing. The analyzed results helped to understand
the solubilization behavior of the self-aggregated assemblies of
mixed bolaform-SDS and bolaform-CTAB systems.
Dye Entrapment/Encapsulation. The vesicular solutions of
mixed bolaform-SDS and bolaform-CTAB systems of dif-
ferent concentrations were prepared in 0.5 mM aqueous bro-
mophenol blue solution and aged at 298 K for 1 day prior to
the experiment. A 0.35 mL aliquot of the mixture in each case
was added slowly on the top of a 1 × 25 cm Sephadex G-50
(AR grade, Sigma) gel column, and then eluted with 10 mM
NaBr solution at a very slow rate (0.25 mL min-1). The released
fractions were collected in separate test tubes, and their optical
densities were measured in a Shimadzu UV-vis 1601 spectro-
photometer at 592 nm. The elution process was carried out until
the absorbances of the collected samples became practically
zero.26,37
Dynamic Light Scattering. A BI-200SM Goniometer (Ver.
2.0) of Brookhaven Instruments Co., Hottsville, NY, was used
for dynamic light scattering (DLS) measurements at 298 K.
Light of 632 nm from a He-Ne laser (22 mW) was used as the
incident beam. The measurements were conducted at a scattering
angle of 90°. All the solutions were filtered several times through
a cellulose acetate membrane filter of pore size 0.45 µm before
the measurements. The diffusion coefficients (D) of the dispersed
colloidal bolaforms were determined in the instrument and the
apparent hydrodynamic radii (Rh) were deduced from Stokes-
Einstein equation Rh ) kB T/6πηD for spherical particles,
where kB and η represent the Boltzmann constant and the solvent
viscosity, respectively, and T is the absolute temperature. For
further details, we refer to our earlier publication.38
The tensiometric results for B1 and B3 in aqueous solution
showed no break (although surface tension (γ) values for both
decreased with concentration), implying the absence of self-
aggregation (i.e., micelle formation) in aqueous medium up to
reasonably high concentrations,22,23 or if there is self-aggregation,
it must be greater than 100 mM (Figure 1, inset B). The
conductometric and spectrophotometric experiments also sup-
ported the tensiometric results of no transitions in their
concentration dependent courses. For bolaforms, the spacer,
nature of the head groups, solvent, and the temperature are
factors responsible for self-assembly.4 They were not conducive
for B1 and B3. Similar results were also reported earlier.22-24
B2 also did not show any concentration dependent transition at
temperatures below 308 K, but sharp breaks were obtained at
higher temperatures (T g 308 K) in tensiometric, conducto-
metric, and spectrophotometric experiments (Figure 1, main
diagram and inset A). The CMC values realized by the three
methods were comparable. This led us to conclude that self-
assembly of B1 and B3 in aqueous medium was impracticable
under the present conditions of measurements whereas B2
formed micelles at T g 308 K. Comparable temperature
dependent micellization processes for triblock copolymers
[poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene
oxide)] of PPO/PEO ratios (0.19-1.79) are found in the
literature.36 The average CMC of B2 increased with temperature
(2.55 and 6.00 mM at 308 and 313 K, respectively). The above
apparent irregular results may be attempted to rationalize in the
following way. The nonpolar part of the amphiphile remained
surrounded by “water-cage” (“iceberg”) by way of hydrophobic
hydration (HH). At higher temperature (>303 K) the caging
effect or HH became weak, allowing the related B2 molecules
to self-aggregate with a CMC of 2.55 mM. At 313 K, the CMC
increased (6.0 mM), although a lower value was expected. The
temperature effect on factors like water structure, kinetic energy,
headgroup desolvation, etc. might have concertedly overridden
the decreased HH contribution and reversed the direction of
the CMC change. Further studies are required to shed more light
on this matter. However, since ionic surfactants show a

7502 J. Phys. Chem. B, Vol. 114, No. 22, 2010
Maiti et al.
Figure 3. Representative normal and derivative plots of tensiometric
profile of equimolar mixture of B2 and SDS at 298 K. Symbols: 1,
normal tensiometric plot; 2, second derivative plot; 3, third derivative
plot. CMCs (mM): CMC (1), 0.147; CMC (2), 0.139; CMC (3), 0.137;
CMCave, 0.141. CVCs (mM): CVC (1), 1.60; CVC (2), 1.51; CVC (3),
1.56; CVCave, 1.56.
Figure 2. Tensiometric profiles of bolaforms in 10 mM NaBr solution
at 298 K. Main plot: 0, B1; ∆, B2; O, B3. CMC points are indicated by
broken arrows. Inset: conductometric determination of CMC of C10TAB
in 10 mM NaBr solution at 298 K.
minimum in the CMC-temperature plot, the increase in CMC
with increasing temperature is possible.39 The results also do
not corroborate with the existing knowledge that in bolaforms,
the length of spacers equivalent to sixteen or more carbon atoms
is required for them to self-associate.23,24 The above observations
warrant further research. The fluctuating γ value as a function
of concentration for B2 at temperatures g308 K (Figure 1, inset
A) up to the CMC point was an unexpected behavior.22 It
implied that the adsorption of B2 at the air/water interface did
not reach a stable state until the amphiphile concentration
attained CMC.
Figure 4. Conductometric and microcalorimetric titration of B1 by
SDS at 298 K. Break points are shown by solid arrows. Main diagram:
conductometry (b) and microcalorimetry (O). Inset: conductometric
(9) and microcalorimetric (0) titration of B3 by CTAB at 298 K.
B. Self-Aggregation in 10 mM NaBr Solution. Salts gener-
ally have a CMC decreasing effect,39 the ionic amphiphiles are
perceptibly affected; the nonionics and zwitterionics are weakly
affected. A large CMC reduction of hexadecyltrimethylammo-
nium bromide and its analogues in the presence of salt has been
reported in literature.40 All the bolaforms were found to form
micelles in 10 mM NaBr solution at 298 K. Their tensiometric
profiles were smooth. The average CMCs of B1, B2, and B3 by
different methods were 66.7, 5.12, and 63.5 mM, respectively,
at 298 K (Figure 2, main plot). In 10 mM NaBr, the analogous
C10TAB formed CMC at 54.0 mM at 298 K (Figure 2, inset).
From the CMC formation point of view, B1 and B3 were more
hydrophilic than C10TAB. Like in aqueous medium, the CMC
formed in salt medium by conductometry and spectrometry
fairly agreed with tensiometry (e.g., in 10 mM NaBr, the CMC
values for B2 obtained by tensiometry, conductometry, and
spectrophotometry at 298 K were 5.15, 5.10, and 5.11 mM,
respectively). It may be added that along with the transition
points in the plot (Figures 1 and 2) to locate CMCs, the
procedure of second and third derivatives of the measured
concentration dependent physical properties were also used.41
All these results were in close agreements. A typical illustration
is presented in Figure 3 for equimolar mixtures of B2 and SDS
to witness applicability of the derivative plots both for CMC
and for CVC (subsequently discussed in section 3). The reported
average CMC and CVC values were, thus, fairly accurate.
2. Interaction of Bolaforms with SDS and CTAB. The
interaction between the bolaforms and the conventional surfac-
tants, SDS and CTAB were studied at 298 K by conductometry
and microcalorimetry. The aqueous solutions of B1 and B2 (0.5
mM) were titrated with SDS, and B3 (0.5 mM) was titrated with
both SDS and CTAB since B3 was zwitterionic. The conduc-
tometric and microcalorimetric results were comparable (Figure
4, main plot). The results are summarized in Table 1. As per
expectation, while titrating with SDS, B1 produced the first break
TABLE 1: Results of Conductometric and
Microcalorimetric Titration of Bolaformsa by SDS and
CTAB at 298 K
break point molar ratios
method
[SDS]/[B1] [SDS]/[B2] [SDS]/[B3] [CTAB]/[B3]
conductometry
microcalorimetry 1.92, 5.27 1.10, 5.27 1.08, 5.90
mean 1.97, 5.42 1.10, 5.29 0.99, 5.71
2.01, 5.57 1.09, 5.30 0.90, 5.52
1.00
0.92
0.96
a Concentrations of bolaforms used ) 0.5 mM.
at 2:1 mole ratio ([SDS]/[B1]), whereas both B2 and B3 produced
the first breaks at about 1:1 mole ratio. Likewise, the calorimetric
and conductometric titrations of B3 with CTAB produced breaks
at 1:1 mole ratio (Figure 4, inset). The products were totally or
partially charged or neutralized species. It is seen in Figure 4
(main plot) that for B1-SDS, after the first break (at [SDS]/
[B1] ≈ 2), a second break appeared at a mole ratio of 5.42.
Similar second breaks were also observed in both B3-SDS and
B2-SDS systems (Table 1). The second break stood for
association of the formed ion pairs in solution with excess SDS
to yield mixed micelles. For B3-CTAB systems, only a single
break at 1:1 mole ratio was observed. The second breaks
occurred at SDS to bolaform mole ratios of 5.42, 5.3, and 5.5
for B1, B2, and B3, respectively; they corresponded to the [SDS]
of 2.71, 2.65, and 2.75 mM, respectively. The observed CMCs
of SDS and CTAB at 298 K in the presence of 10 mM NaBr
were at 6.76 and 0.87 mM, respectively. The CMCmix values
were much lower than the CMC of SDS, indicating the
formation of mixed micelles. B3 (0.5 mM) required ∼0.5 mM
CTAB solution for the neutralization of its carboxylate end. The
product was a quaternary salt having appreciable solubility not
to form mixed micelle easily even by the addition of excess
CTAB.

Bolaforms and Their Mixtures with SDS and CTAB
J. Phys. Chem. B, Vol. 114, No. 22, 2010 7503
2 ; for
For B1-SDS mixtures, then, n ) 2RB + 2RSDS
)
1
B2-SDS mixtures, n ) 2RB + 2RSDS ) 2; and for B3-SDS
2
or B3-CTAB mixtures, n ) 3R + 2RSDS or 3R + 2RCTAB
B3
B3
) RB + 2. In this scheme, the contribution of the added salt (10
3
mM NaBr) was not considered.
The Γmax values are in Tables 2-4. For B1-SDS mixed
systems, the order of Γmax obtained in terms of RB is 0.333 >
1
0.667 > 0.2 > 0.75 > pure SDS > pure B1. Increase in Γmax
suggested that the packing at the air/water interface for the
binary systems was enhanced more than that of the pure
components. Tomasic et al.44 reported a higher Γmax for the
equimolar mixture of CTAB and SDS systems than for the pure
species. For B2-SDS systems, the 1:1 ratio (charge-neutralized
composition) produced the minimum Γmax. It was even less than
that of SDS, indicating loose packing of the ion pairs at the
interface. The equimolar combination of B3-SDS was moder-
ately larger than the pure components. On the other hand, the
B3-CTAB system showed a maximum Γmax at 1:1 ratio with
the following order: 0.5 > 0.667 > 0.75 > 0.333 > pure CATB
≈ pure B3.
Figure 5. Determination of CMC of mixed bolaform-SDS systems at
298 K. CMCmix and CVC are indicated by broken and bent arrows,
respectively. Main diagram: conductometry (0) and spectrophotometry (∆).
Inset A: tensiometry (O). Inset B: microcalorimetry (3). Systems: 0,
B2-SDS (3:1); ∆, B2-SDS (1:1); O, B2-SDS (3:1); 3, B2-SDS (2:3).
3. Behavior of Bolaforms Mixed with Either SDS or
CTAB in 10 mM NaBr Solution. A. Interfacial and Bulk
Phenomena. The bolaforms formed clear solutions when mixed
with oppositely charged surfactants in varied molar ratios in 10
mM aqueous NaBr solution. Throughout the measurements, [NaBr]
was maintained at 10 mM. In the literature, oppositely as well as
similarly charged surfactants of widely varied alkyl chains as in
bolaforms can be found.16,26 Herein we use surfactants of mild to
moderately varied alkyl chains to interact with the bolaforms. The
tensiometric profiles of mixed B1-SDS and B2-SDS systems
produced two transitions at 298 K (Figure 5, inset A). Formation
of two types of aggregates was envisaged.26 The first break
The minimum area of exclusion per surfactant molecule at
the air/solution interface, Amin (nm2 molecule-1) was obtained
from the relation45
Amin ) 1018/NAΓmax
(3)
where NA is Avogadro’s number. The Amin values are in Tables
SDS
2-4. The A
min
value was less than that of the bolaforms. The
min
CTAB
A
≈ ABm3in condition implied that both B3 and CTAB had
comparable interfacial occupancies.
The parameter, pC20 is a measure of the surface activity45 of
corresponded to the critical mixed micelle concentration (CMCmix
)
a surfactant (pC20
)
-log C20, where C20 denotes the
whereas the second represented the critical vesicle concentration
(CVC).26,42 The locations of the breaks depended on the bolaform:
SDS mole ratio. In the case of B1-SDS, two transitions occurred
up to RB ) 0.40. For the B2-SDS system, two transitions appeared
up to R1B ) 0.75. The systems B3-SDS and B3-CTAB only
produced2a single transition at all the mole ratio compositions. The
first transition (CMCmix) values followed a sigmoidal trend with
concentration of the surfactant in mol dm-3 needed to reduce
the surface tension of pure solvent by 20 mN m-1). The higher
the pC20 value, greater is the surface activity of the surfactant.
According to this rationale and results in Tables 2-4, the herein
studied systems produced the following efficiency order: CTAB
> SDS ≈ B2 > B1 > B3. All the mixed bolaform systems with
SDS were more surface active than their parent components.
The B3-CTAB mixed systems were less surface active than
CTAB but fairly greater surface active than B3.
RB in the mixture. The results of the three systems are presented
3
in Table 2-4. The transition points presented and discussed above
were the average values obtained by tensiometry, microcalorimetry
(Figure 5, inset B), conductometry (Figure 5, main plot), and
spectrophotometry (Figure 5, main plot). The interfacial adsorptions
of the bolaforms and their combinations with SDS and CTAB were
estimated using the Gibbs adsorption equation.41 Thus,
Microcalorimetric results (Figure 5, inset B) produced sharp
breaks for the mixed bolaform systems, and the results were in
good agreement with tensiometry. The sigmoidal part for the
second transition evidenced vesicle formation. The derived
parameters are in Tables 2 and 3. The enthalpy values for vesicle
formation were all negative. The magnitudes for the B1-SDS
system were apparently greater than the B2-SDS system. The
composition dependent enthalpies for micelle and vesicle
formation for both the systems were comparable. The scope
for a detailed energetic study on the vesicle formation remains.
B. Energetics of Formation of Self-Assembled Species in
10 mM Aqueous NaBr Solution. The energetics of the
assembled species formed in solution were assessed in the light
of the pseudophase model. The standard Gibbs’ free energies
of micellization for both pure and mixed species were calculated
from the relation41,45,46
-1
2.303nRT
dγ
d log C
Γmax
)
lim
(1)
(
)
CfCMC
where Γmax (in mol m-2) is the maximum surface excess at CMC,
C stands for the total concentration of the pure or mixed surfactant
in M, n is the effective number of chemical species (ions) present
in solution, R is the universal gas constant, and T is the absolute
temperature. The value of n in the binary mixtures of surfactants
herein considered was obtained from the following rationale. For
B1, B2, and B3, the number of species formed by way of dissociation
were 2, 2, and 3, respectively. The SDS and CTAB produced two
species each. The n in a binary mixture of surfactants with
stoichiometric mole fractions RA and RB producing a and b number
of species, respectively can be found from the general formula43
∆G0m ) (1 + ꢀ)RT ln xT
(4)
where xT is the average transition point values (CMC or CMCmix
of the surfactant systems in mole fraction unit. In this calcula-
)
n ) aRA + bRB
(2)

7504 J. Phys. Chem. B, Vol. 114, No. 22, 2010
Maiti et al.
TABLE 2: Thermodynamic Parameters of B1-SDS Mixed Systemsa,b,c in 10 mM NaBr Medium at 298 K
(CMCave
(CVCave
)
)
(-∆H0cal
)
m
m
RB
106 Γmax (Amin
)
(-∆H0cal)v [pC20]
-∆G0m (-∆G0ad
)
XMB (XσB )
fMB (fσB )
fMSDS (fσSDS
)
-ꢀM (-ꢀσ)
-∆GME
v
1
1
1
1
1
0
(6.76)m
1.57
(1.06)
2.25
(2.37)m
[3.17]
(12.0)m
(3.93)v
[4.05]
22.3
(41.0)
28.4
0.2
(0.575)m
(0.814)v
0.39
(0.40)
0.004
(0.029)
0.109
(0.209)
14.6
(9.81)
8.32
9.69
8.49
7.70
(1.65)
(44.0)
0.333
0.667
0.75
1
(0.354)m
(0.864)v
2.62
(1.42)
(-15.0)m
(13.6)v
[4.35]
29.6
(44.2)
0.42
(0.43)
0.004
(0.035)
0.056
(0.149)
16.3
(10.3)
(0.630)m
(0.884)m
(66.7)m
2.46
(1.52)
(1.53)m
28.2
(43.5)
0.46
(0.49)
0.014
(0.085)
0.045
(0.103)
14.7
(9.48)
[4.02]
[4.67]
[3.08]
2.08
(1.79)
27.4
(43.4)
0.47
0.024
0.048
13.7
(0.50)
(0.106)
(0.106)
(8.98)
0.94
(3.99)
16.7
(58.4)
a CMC/CVC, Γmax, Amin, and (∆H0cal, ∆G0m, ∆G0ad, and ∆GEM) are expressed in mM, mol m-2, nm2 molecule-1, and kJ mol-1 units,
respectively. b Both CMC and ∆Hc0al have two values for a single binary composition at different RB . The first transition represents critical
1
concentration for mixed micelle formation (CMCmix) and the second one represents critical concentration for vesicle formation (CVC). c In
columns 2 and 4, micelle and vesicle parameters are presented with subscript m and v, respectively.
TABLE 3: Thermodynamic Parameters of B2-SDS Mixed Systemsa,b,c in 10 mM NaBr Medium at 298 K
(CMCave
(CVCave
)
)
(-∆H0cal
)
m
m
RB
106 Γmax (Amin
)
(-∆H0cal)v [pC20]
-∆G0m (-∆G0ad
)
XMB (XσB )
fMB (fσB )
fMSDS (fσSDS
)
-ꢀM (-ꢀσ)
-∆GME
v
2
2
2
2
2
0
(6.76)m
1.56
(2.37)m
22.3
(1.06)
(41.0)
[3.17]
0.333
0.4
(0.139)m
(0.735)v
2.27
(0.73)
32.0
(47.5)
0.49
(0.44)
0.018
(0.021)
0.025
(0.091)
15.4
(12.4)
9.28
8.16
9.04
9.05
8.99
7.86
(-3.99)v
[4.45]
(0.219)m
(1.01)v
1.79
(0.93)
30.8
(50.4)
0.5
(0.45)
0.034
(0.024)
0.034
(0.083)
13.5
(12.3)
[4.39]
0.5
(0.153)m
(1.60)v
1.42
(1.17)
(8.45)m
(4.87)v
[4.43]
(1.56)m
(1.29)v
[4.38]
(3.91)m
(1.01)v
[4.32]
31.7
(52.7)
0.51
(0.47)
0.029
(0.016)
0.022
(0.039)
14.7
(14.7)
0.667
0.75
0.833
1
(0.151)m
(0.374)v
2.07
(0.80)
31.8
(47.9)
0.53
(0.49)
0.037
(0.039)
0.015
(0.050)
14.9
(12.5)
(0.154)m
(1.58)v
2.27
(0.73)
31.7
(46.8)
0.55
(0.5)
0.041
(0.055)
0.008
(0.055)
15.8
(11.6)
(0.242)m
(5.12)m
1.72
(0.96)
30.6
(51.6)
0.56
(0.52)
0.07
(0.044)
0.014
(0.025)
13.7
(13.6)
[4.46]
0.66
23.0
(2.50)
(61.0)
[3.15]
a CMC/CVC, Γmax, Amin, and (∆H0cal, ∆Gm0 , ∆G0ad and ∆GME ) are expressed in mM, mol m-2, nm2 molecule-1, and kJ mol-1 units, respectively.
b Both CMC and ∆Hc0al have two values for a single binary composition at different RB . The first transition represents critical concentration for
1
mixed micelle formation (CMCmix) and the second one represents critical concentration for vesicle formation (CVC). c In columns 2 and 4,
micelle and vesicle parameters are presented with subscript m and v, respectively.
tion, the contribution of ꢀ (fraction of counterions bound to the
micellar species) was neglected since it was small (<5%). For
normalization of results, in the calculations of ∆G0m of pure SDS
and CTAB (in 10 mM NaBr solution), their ꢀ values were also
not used.
found to be exothermic and dependent on the system composi-
tion (Tables 2 and 3). The enthalpy changes for the mixed
micelle formation for both B1-SDS and B2-SDS systems with
respect to their stoichoimetric ratios fitted to tertiary polynomial
equations with R2 ) 1. The mixed vesicle formation was less
spontaneous than mixed micelle formation.
The energetic and related parameters are presented in Tables
2-4. They evidenced that the free energy changes for the mixed
micellar systems of B1-SDS and B2-SDS systems were more
spontaneous than the pure systems; for the B3-CTAB system,
they were comparable with pure CTAB and more spontaneous
than B3. Except for two cases (RB and RB ) 0.333) the enthalpy
The standard free energy of interfacial adsorption (∆G0ad) at
CMC or CMCmix was estimated from the relation,46
πCMC
∆G0ad ) ∆G0m -
(5)
1
2
(
)
Γmax
changes for mixed micelle and mixed vesicle formation were

Bolaforms and Their Mixtures with SDS and CTAB
J. Phys. Chem. B, Vol. 114, No. 22, 2010 7505
TABLE 4: Thermodynamic Parameters of B3-CTAB and B3-SDS Mixed Systemsa in 10 mM NaBr Medium at 298 K
RB
CMCave
106 Γmax (Amin
)
pC20
-∆G0m ( -∆G0ad
)
XMB (XσB )
fMB (fσB )
fMSDS (fσSDS
)
-ꢀM (-ꢀσ)
-∆GME
3
3
3
3
3
B3-CTAB
0
0.87
1.02
1.23
1.87
2.69
63.5
1.20
(1.39)
1.04
(1.58)
1.56
(1.06)
1.34
(1.24)
1.08
(1.54)
1.18
4.90
4.00
4.47
4.38
4.58
2.06
27.4
(56.3)
27.0
(59.0)
26.6
(47.6)
25.5
(51.0)
24.6
(56.2)
16.8
0.333
0.5
0.15
(0.41)
0.19
0.035
(0.001)
0.050
0.901
(0.026)
0.849
4.61
(21.6)
4.54
1.20
1.16
(0.41)
0.19
(0.43)
0.18
(0.003)
0.100
(0.005)
0.176
(0.057)
0.883
(0.046)
0.920
(17.0)
3.46
(16.6)
2.58
0.667
0.75
1
0.12
-0.89
(0.45)
(0.002)
(0.017)
(20.1)
(1.40)
(45.5)
B3-SDS
0.5
5.25
1.20
3.47
23.0
0.34
0.121
0.571
4.84
2.52
(1.08)
(38.9)
(0.32)
(0.045)
(0.503)
(6.72)
a CMC/CVC, Γmax, Amin, and (∆H0cal, ∆G0m, ∆G0ad, and ∆GEM) are expressed in mM, mol m-2, nm2 molecule-1, and kJ mol-1 units,
respectively.
where πCMC ( γ0 - γCMC or γ0
-
γCMC ) is the surface
micelle concentrations of surfactants 1 and 2, respectively. The
above equation is a modified form of Clint’s equation53 for ideal
mixture wherein the activity coefficients (‘f ’ terms) are taken to
be unity so that 1/CMCmix ) R1/ CMC1 + R2/CMC2.
mix
pressure at CMC or CMCmix; γ0, γCMC, and γCMC are the
surface tension values of 10 mM NaBr solution, pure aminx d mixed
surfactant solutions, respectively, at their respective transitions.
The change in free energies due to self-aggregation in the
bulk micelle formations and other transitions suggested that they
were much less spontaneous than amphiphile adsorption at the
interface, i.e., ∆Ga0d , ∆Gm0 . The order of magnitudes of ∆G0ad
for the B1-SDS mixed system was pure B1 > mixed B1-SDS
> pure SDS. Effectively, the ∆G0ad values only mildly varied
with solution composition in the mixed surfactant systems.
Similar was the observations on B2-SDS and B3-SDS systems.
The B3-CTAB mixed systems showed ∆G0ad values to be more
negative than pure B3 compared to other binaries. For
B3-CTAB systems, there was no composition dependent trend
in ∆G0ad.
The average CMCmix values herein observed were lower than
those calculated from the above relation of Clint. The negative
deviation from the ideal equation followed the order B2-SDS
> B1-SDS > B3-SDS > B3-CTAB at a given mole fraction
composition. The mixed systems were all synergistic.
The micellar compositions (XMB and XMSDS or XMCTAB), the activity
i
coefficients (fBM and fMSDS or fMCTAB) of the binary components and
the interactioni parameter ꢀM (synergistic or antagonistic) has
been calculated from RM,49 and the relations used for the
estimations can be found in the literature.47,48,54-56 These are
not presented to save space; only the results are shown in Tables
2-4.
C. Assessment of Mutual Interaction of Bolaforms with
SDS and CTAB in the Micelle and at the Interface. The
compositions of mixed surfactants in micelles in the bulk as well
as in the adsorbed monolayer at the interface are usually different
from their stoichiometric compositions.47,48 The interaction between
the species in the bulk and at the interface can be reasonably studied
in the light of regular solution theory (RST) of Rubingh49 and the
theory of Rosen,50,51 respectively. The RST, based on thermody-
namic principle, is capable of predicting the synergistic as well as
antagonistic interaction between the amphiphiles in mixed micelles,
and the micellar composition along with their activity coefficients.
The theory of Rosen provides similar information on the interaction
at the air/solution interface. Rubingh’s theory of interaction between
binary mixtures of bolaform with conventional surfactant in solution
has been used by different workers.16,52 Their interactions at the
interface as schemed in Rosen’s theory has scarcely (if ever) done
in the past. The theories are simple and reasonably informative on
the mixed interacting amphiphile systems. We have used the
Rubingh and Rosen models to investigate the mixed systems of
bolaforms with both SDS and CTAB. The Rubingh’s model (RM)
deals with mixed systems based on nonideality and several
reasonable approximations.49-51 The CMCmix of a binary mixture
of surfactants with activity coefficients f1 and f2, respectively, can
be expressed by the following relation,
Using Rosen’s rationale (RR)50,51 and the measured surface
tension of mixed surfactants, the composition of bolaforms and
SDS or CTAB in the mixed monolayer, their surface activity
coefficients (fBσ and fSσDS or fCσTAB), and the interaction parameter
i
(ꢀσ) were calculated. The constant level of the surface tension
(γ) was arbitrarily chosen at γ ) 38 mN m-1 sufficiently below
their respective CMCs.54,55 The mole fraction of the components
in the mixed monolayer (XσB and XSσDS or XCσTAB) were evaluated
i
using the molar solution concentrations of the individual
surfactants and their mixtures required to have the same value
of γ (38 mN m-1). The data treatment rationale is comparable
with Rubingh’s treatment.49 The results by the RR model are
also presented in Tables 2-4.
The XMB values in B1-SDS combinations varied between 0.39
1
and 0.47; thus, SDS contributed more in the mixed micellar
phase than B1. For the B2-SDS systems, B2 dominated in the
mixed micellar phase (XMB varied from 0.49 to 0.6) whereas in
2
the 1:1 B3-SDS system, SDS again contributed more than the
bolaform, B3 (XMSDS ) 0.66). On the other hand, in the B3-CTAB
system, XMB , XCMTAB, the former ranged only between 0.15 and
3
0.19. The varied physicochemistry of the mixed systems were
thus realized.
The fMB values in the mixed micelles of B1-SDS hardly varied
1
with RB and that of SDS decreased with increasing RB (range
1
1
0.109-0.045). For the B2-SDS system, the fMB for the mixed
2
R1
f1CMC1
R2
1
micelles were also small (varied between 0.07 and 0.041). The
)
+
(6)
CMCmix
f2CMC2
activity coefficients of SDS in the mixed B2-SDS system did
not vary much (0.008-0.034). The fMB value for the B3-CTAB
3
where R1 and R2 are the mole fraction of surfactants 1 and 2,
respectively in the solution; CMC1 and CMC2 are the critical
mixed system increased with increasing RB (fitted to a secondary
3
polynomial equation with R2 ) 0.985) whereas those for SDS

7506 J. Phys. Chem. B, Vol. 114, No. 22, 2010
Maiti et al.
∆GME ) ∆G0m, mix - XBM∆Gm0 ,B - XSMDS∆Gm0 ,SDS (7)
were close to unity, though less than unity for both components.
i
i
The fMB value (0.121) in the 1:1 B3-SDS system was smaller
3
than that of SDS (0.571).
The results are presented in Tables 2-4. The ∆GME for both
B1-SDS and B2-SDS were fairly negative compared with those
The mole fraction of all the bolaforms in the mixed
monolayers (XσB ) mildly increased with increasing RB. The
i
for B3-SDS and B3-CTAB systems (at RB ) 0.75, the value
3
bolaforms (B1 and B2) and SDS almost equally contributed to
the mixed monolayer formation. The B3 and CTAB also
contributed nearly equally to the mixed monolayer, the contri-
bution of SDS in the B3-SDS system was much greater. The
activity coefficients of all the bolaforms in the mixed monolayer
were small and mildly increased with RB for the B1-SDS
was found to be positive). The synergistic interaction thus
followed the order B1-SDS ≈ B2-SDS . B3-SDS >
B3-CTAB. This order of ∆GME was more or less obeyed also
for ꢀM, as discussed earlier.
4. Dynamic Light Scattering Results. The preliminary DLS
results on bolaform-SDS and bolaform-CTAB mixed micelles
and vesicles produced first hand information on their hydrody-
namic dimensions and polydispersities.58 The system wise
magnitudes of the mean diameter (d) and the PDI (polydispersity
index) are as follows:
(1) For the 1:2 B1-SDS system: at 0.50 mM (>CMCmix), d
) 220 nm, PDI ) 0.273; at 1.30 mM (>CVC), d ) 325 nm,
PDI ) 0.300.
(2) For the 1:1 B2-SDS system: at 0.50 mM (>CMCmix), d
) 186 nm, PDI ) 0.317; at 2.40 mM (>CVC), d ) 325 nm,
PDI ) 0.280.
(3) For the 1:1 B3-SDS system: at 6.20 mM (>CMCmix), d
) 216 nm, PDI ) 0.390.
system. No such ordering in the value of fσB was observed for
2
the B2-SDS system. However, the fσB values in the mixed
3
B3-CTAB monolayer were practically nonvariant. For the
B1-SDS and B3-CTAB systems, the maximum fSσDS values
arose at the charge- neutralized condition.
D. Synergism in Mixed Micelle Formation. The signs and
magnitudes of ꢀM and ꢀσ tell about the types of interaction and
their extents. The necessary conditions for synergistic intera-
ction47,48,51,54 are (1) ꢀM and ꢀσ must be negative, and (2) |ꢀM|
> |ln(CMC1/CMC2)|and |ꢀσ| > |ln(C01/C02)|. Here CMC1 and CMC2
are the CMCs of component 1 and component 2, respectively,
and C01 and C02 are the molar concentrations of the first and
second surfactants, respectively, in the bulk phase required to
produce the same γ in both the systems. The ꢀM and ꢀσ values
found in the mixed bolaform-SDS and mixed bolaform-CTAB
systems were negative. The second condition of greater
magnitude of ꢀM values than the ln(CMC ratio) or ꢀσ values
than the ln(concentration ratio) at equal γ also was in favor of
synergism. Similar synergistic interactions are also found in the
literature.54,55 According to Rosen et al.,54 |ꢀM| > |ꢀσ| should be
the condition for synergism. In this study, this condition was
found to hold for B1-SDS and B2-SDS systems; both B3-SDS
and B3-CTAB evidenced |ꢀM| < |ꢀσ|. Clint56 has shown that
the above-mentioned Rosen54 criterion cannot be a generalization
for synergism. The essential condition has to be ꢀ < 0, which
was true for all the mixed systems herein studied. The ꢀM values
for B1-SDS were fitted to a secondary polynomial equation
(4) For the 1:1 B3-CTAB system: at 1.85 mM (>CMCmix),
d ) 225 nm, PDI ) 0.346.
In the above, the system concentrations were greater than
their corresponding CMCmix and CVC values. At the studied
bola:SDS and bola:CTAB ratios, both mixed micelles and
vesicles were formed for systems 1 and 2 whereas systems 3
and 4 produced only mixed micelles. The results evidenced the
types of sizes that the assemblies offered. Their polydispersities
were reasonable and comparable. The vesicle sizes were
obviously larger than the mixed micelles. More studies are
required for confirmation and elaboration. From TEM measure-
ments, Yan et al.16,26 have reported vesicle dimensions in the
range 10-100 nm for mixed cationic bolaform-SDS and
anionic bolaform-DTAB systems. The presently studied sys-
tems in the solution state have produced larger dimensions than
the solvent removed samples examined by the above authors.
Complementary studies by DLS and TEM are therefore required
for better information on vesicle morphology and size.
5. Dye Solubilization and Entrapment. The structural
changes (formation of micelles and vesicles) of the mixed
bolaform and SDS or CTAB with increasing concentration can
be supported from dye solubilization and dye entrapment
experiments.26,36 The neutral dye, DPH, was found to show
spectral characteristics with a shoulder at 271 nm and three peak
structures at 340, 355, and 375 nm in THF (dipole moment )
1.75D; dielectric constant ) 7.58) (Figure 6, inset). In 10 mM
aqueous NaBr solution DPH spectra showed a much reduced
shoulder without the peaks at higher wavelengths. Increasing
addition of the B2-SDS mixture at 3:1 mol ratio evidenced the
appearance of three characteristic peaks at 0.99 mM (fairly
above its average CMCmix ) 0.154 mM, where solubilization
of DPH in the interior of the mixed micelle occurred). At
B2-SDS concentrations of 1.79, 2.99, and 3.63 mM, the
structural features of DPH became prominent with increasing
absorbances (Figure 6, main diagram). The polarity of the
aqueous salt solution was higher than that of THF, the interior
of the micelles, and the molecular membrane of the vesicle,26
where the dye molecules were located with comparable mi-
croenvironments.
with RB (R2 ) 0.969) showing a maximum at the charge
1
neutralization ratio, the ꢀM values with RB for the B2-SDS
2
system did not produce a proper trend. The 1:1 B3-SDS system
yielded a fair negative ꢀM value (-4.84). The B3-CTAB
combinations yielded mixed CMCs lower than B3 but greater
than CTAB. Their negative ꢀM values (ranged between -2.58
and -4.61) also suggested fair synergism in the mixed micelles.
The values fitted to a secondary polynomial equation with
respect to RB , yielding R2 ) 0.999. Synergistic interaction of
3
cationic bolaform with SDS and sodium laurate16 produced ꢀM
values -10.9 and -7.2, respectively. Such interaction between
a nonionic bolaform with two aza-crown ether head groups and
SDS52 yielded ꢀM ) -7.6. The interactions were stronger for
the herein studied systems.
For B1-SDS systems, ꢀσ was found to fit to a secondary
polynomial equation (R2 ) 0.992) with respect to RB ; for the
1
B3-CTAB system, a perfect fitting (R2 ) 1) to a tertiary
polynomial equation was found with respect to RB . For the
3
mixed B2-SDS system, the maximum synergism in the mono-
layer arose at the equimolar composition.
E. Excess Free Energy of Mixed Micellization and Mixed
Monolayer Formation. The mutual interaction between the
components of the mixed micelles was analyzed in terms of
the excess free energy of micellization (∆GME ), using the
following relation,57
The vesicles can entrap water-soluble dyes, drugs, etc. into
their interior. We have used the water-soluble dye bromophenol

Bolaforms and Their Mixtures with SDS and CTAB
J. Phys. Chem. B, Vol. 114, No. 22, 2010 7507
for B3 at 1:1 molar ratio), the order of Amin was parallel to the
parent bolaforms, i.e., B1-SDS (1.42 nm2) > B2-SDS (1.17
nm2) > B3-SDS (1.08 nm2) ≈ B3-CTAB (1.06 nm2). The larger
Amin values of the parent bolaforms than of SDS and CTAB
suggested their wicket-like structures at the air/solution interface.
In pure CTAB, repulsive interaction among the CTA+ ions at
the air/solution interface produced a larger Amin value than
expected for a nonionic species. The combinations of bolaforms
with SDS and CTAB produced Amin lower than their parent
components (B1, B2, and B3) because of charge neutralization.
For quantification of results, detailed studies with rational
modeling are warranted.
Bolaforms with large hydrophobic spacers form spherical
micelles by way of folding the segments. In the case of shorter
hydrophobic segments, such “folding up” becomes difficult and
spherical micelles do not form. In this study, we have used
bolaforms having a reasonably low hydrophobic segment. It was
observed that they did not form micelles in the aqueous medium
at around 298 K. It may be mentioned here that the “spacer
length” criterion for the formation of micelle cannot be used
on all kinds of bolaform surfactants.59 However, in the presence
of 10 mM NaBr in the aqueous medium, the herein studied
bolaforms produced micelles. The structures of these micelles
are, therefore, of importance. We believe that in the presence
of electrolyte (10 mM NaBr), the “folding up” of the hydro-
phobic segment is possible to form spherical bolaform micelles
in solution. But, the possibilities of formation of assemblies of
“lamellar” as well as “stretched” conformations are also not
ruled out.60 A conclusive opinion on the assembly geometry
cannot be given at present. To resolve the issue, more detailed
studies are required. SDS and CTAB interacted with the
bolaforms to form IPAs or catanionics, which became more
hydrophobic to form both micelles and vesicles. Among all the
studied combinations, B3-SDS and B3-CTAB formed only
micelles but no vesicles, whereas the pairs B1-SDS and
B2-SDS produced both micelles and vesicles. The B2-SDS
appeared to be more effective in this behavior. The factors like
charge, solubility, and stoichiometry were considered vital for
the types of assemblies to be formed in solution.
Figure 6. Shift of λmax of DPH at different concentrations of B2:SDS
(3:1) at 298 K. [DPH] used ) 0.09 µM. Main diagram: 1-7, mixed
system of concentrations 0, 0.025, 0.09, 0.99, 1.79, 2.99, 3.60 mM,
respectively. Inset: absorption spectra of DPH in THF at 298 K.
Figure 7. Elution profiles of mixed bolaform-SDS systems at 298
K. Systems: 0, B1-SDS (1:2); O, B2-SDS (3:1); ∆, B2-SDS (1:1).
blue for such entrapment in the vesicles’ aqueous interior.
Addition of bolaform-surfactant (SDS or CTAB) mixtures at
different mole ratios with a fixed concentration of the dye (9.4
× 10-8 M) on the top of a Sephadex (G-50) column produced
elution profiles for B1-SDS (1:2), B2-SDS (3:1), and B2-SDS
(1:1) systems as depicted in Figure 7. The profiles for the
B1-SDS and B2-SDS systems appear separated; the elution
process was delayed in the latter. The symmetrical elution
profiles with a single peak and without shoulder meant release
of a single species (the vesicle-entrapped dye) from the column
matrix. Interestingly, the same procedure followed on the
B3-CTAB system failed to elute out the vesicle from the
column. Although this supported vesicle uptake in the Sephadex
matrix, its nonelution remained to be explained.
6. Probable Structures at the Interface and Assemblies
in the Bulk. The studied bolaforms (B1, B2, and B3) and their
combinations with both SDS and CTAB were expected to
produce interesting species at the air/solution interface and in
the bulk. Because of a short spacer (with ten methylene groups),
it was expected (cf. Introduction) that their bending at the
interface and in the aqueous bulk would not arise. But in 10
mM aqueous NaBr solution, the experimental results have
produced interesting structural manifestations that are discussed
below.
In Tables 2-4, the Amin values of the pure bolaforms and
their combinations with SDS and CTAB are presented. The
values followed the order B1 (3.99 nm2) > B2 (2.50 nm2) > B3
(1.40 nm2). This was because in B1, both the hydrophobic groups
are positively charged and hence repulsive force resulted in a
higher area. In B3, hydrophilic groups are oppositely charged
and hence attractive to produce the lowest area. B2 shows the
intermediate value in Amin because one of hydrophilic groups is
neutral. SDS and CTAB produced Amin values 1.06 and 1.39
nm2, respectively. For their mixtures (with SDS for B2 and B3
at 1:1 molar ratio, and for B1 at 1:2 molar ratio, and with CTAB
Conclusions
In aqueous medium, the doubly charged surface-active
bolaforms (B1 and B3) do not show evidence for self-association
within the studied temperature range 298-323 K, but the singly
charged representative (B2) forms micelles at T g 308 K. All
the bolaforms, on the other hand, self-associate in 10 mM NaBr
solution (i.e., in salt environment). The phenomenon is a
contradiction to the existing experiences that bolaforms with
hexadecyl or higher alkyl chain spacers can only form micelles
in aqueous solution. The presently studied compounds have only
decyl alkyl chains in the molecules. They form mixed micelles
in combinations with both SDS and CTAB. The mixed systems
are more surface active than their precursors. At higher
concentrations, the mixed systems also produce vesicles. The
B1-SDS and B2-SDS systems form mixed micelles and
vesicles whereas B3-SDS and B3-CTAB systems form only
mixed micelles. In the mixed micelles, the bolaforms show
synergism with SDS and CTAB. In the B1-SDS and B2-SDS
systems, the two components equally contribute to their ag-
gregates composition. For the B3-SDS and the B3-CTAB
systems, the contribution of B3 in the aggregates is much less
than both SDS and CTAB. However, their interfacial mono-
layers constitute nearly equal proportions of both the compo-
nents (except 1:1 B3-SDS system, where the contribution of

7508 J. Phys. Chem. B, Vol. 114, No. 22, 2010
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SDS is greater than that of B3). Energetically, the interfacial
adsorption is fairly more spontaneous than micelle formation,
as expected. The mixed micelle and vesicle formation processes
were found to be mostly exothermic, which decreased with
increasing proportions of bolaform in the system. The distinct
solubilization of the neutral dye, DPH, in the amphiphilic media
suggested the existence of micelles and vesicles in them. The
elution profile of bromophenol blue containing vesicle solution
was found to be fairly sharp and symmetric; overlap with a
simultaneous elution of the free dye was absent. The B3-SDS
and B3-CTAB systems did not respond to the process for
reasons unknown.
Acknowledgment. K.M. and D.M. are thankful to CSIR,
Govt. of India, for Senior Research Fellowships. P.K.D. is
thankful to the Department of Science and Technology, Govt.
of India, for financial assistance through a Ramanna Fellowship
(No. SR/S1/RFPC-04/2006). S.P.M. thanks the Indian National
Science Academy for an honorary scientist position. A.K.R.
thanks AICTE, New Delhi, for an Emeritus Fellow position.
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