Novel gemini micelles from dimeric surfactants with oxyethylene spacer chain. Small angle neutron scattering and fluorescence studies

Article abstract of DOI:10.1021/jp980836c

Three new gemini surfactants containing mono-, di-, and trioxyethylene spacer chains have been synthesized. Small angle neutron scattering (SANS) cross sections from the micellar aggregates of these dimeric amphiphiles Br^-, n-C₁₆H₃₃NMe₂⁺-CH₂(CH ₂OCH₂)_pCH₂-N⁺Me ₂-n-C₁₆H₃₃, Br^-, (where p = 1, 2, and 3) in aqueous media (D₂O) have been measured. The data have been analyzed using the Hayter and Penfold model for macro-ion solution to compute the interparticle structure factor S(Q) taking into account the screened Coulomb interactions between the dimeric micelles. The SANS analysis showed that the micellar morphology depends on both the nature and the length of the spacer unit. Detailed analysis of the data further indicates that the introduction of oxyethylene spacer is not sufficient enough to prevent looping of the spacer chain. Thus the average separation between the dimethylammonium ion headgroups is considerably lower than is expected from a fully extended conformation of the spacer chain. The micelles from these surfactants have also been characterized in terms of their critical micelle concentrations (cmc), microviscosities, and micropolarities on the basis of the information provided by micelle-solubilized fluorescent probes. These results indicate little difference in their micellar properties such as cmc, microviscosity, and micropolarity.

Full text of DOI:10.1021/jp980836c

6152
J. Phys. Chem. B 1998, 102, 6152-6160
ARTICLES
Novel Gemini Micelles from Dimeric Surfactants with Oxyethylene Spacer Chain. Small
Angle Neutron Scattering and Fluorescence Studies
Soma De,† Vinod K. Aswal,‡ Prem S. Goyal,‡ and Santanu Bhattacharya*,†
Department of Organic Chemistry, Indian Institute of Science, Bangalore 560 012, India, and
Solid State Physics DiVision, Bhabha Atomic Research Centre, Bombay 400 085, India
ReceiVed: January 22, 1998; In Final Form: May 4, 1998
Three new gemini surfactants containing mono-, di-, and trioxyethylene spacer chains have been synthesized.
Small angle neutron scattering (SANS) cross sections from the micellar aggregates of these dimeric amphiphiles
+
Br-, n-C16H33NMe2 -CH2(CH2OCH2)pCH2-N+Me2-n-C16H33, Br-, (where p ) 1, 2, and 3) in aqueous media
(D2O) have been measured. The data have been analyzed using the Hayter and Penfold model for macro-ion
solution to compute the interparticle structure factor S(Q) taking into account the screened Coulomb interactions
between the dimeric micelles. The SANS analysis showed that the micellar morphology depends on both
the nature and the length of the spacer unit. Detailed analysis of the data further indicates that the introduction
of oxyethylene spacer is not sufficient enough to prevent looping of the spacer chain. Thus the average
separation between the dimethylammonium ion headgroups is considerably lower than is expected from a
fully extended conformation of the spacer chain. The micelles from these surfactants have also been
characterized in terms of their critical micelle concentrations (cmc), microviscosities, and micropolarities on
the basis of the information provided by micelle-solubilized fluorescent probes. These results indicate little
difference in their micellar properties such as cmc, microviscosity, and micropolarity.
Introduction
continuing interest in the understanding of the aggregation
behavior from amphiphilic molecules of varying architectures,4
we developed a range of new surfactant systems. Various
physical methods are currently available to examine the nature
of aggregation. Among these, the neutron scattering technique
offers ample opportunity to study the aggregate organizations
formed from different surfactants.5-8 This is especially useful
with micellar solutions of dimeric surfactants where the effect
of spacer chain looping can be conveniently examined by this
noninvasive technique. On the basis of the above reasoning,
we already examined the small angle neutron scattering (SANS)
spectra of the micelles formed from dimeric surfactants, 16-m-
16, 2Br-, 1, where, m ) 3, 4, 5, 6, 8, 10, and 12, and their
mixed systems with CTAB.9,10 These studies clearly indicate
that the extent of aggregate growth, and the variations of shapes
of the dimeric micelles and mixed micelles depend primarily
on the spacer chain length (m-value) in 1.
Dimeric or gemini surfactants, consisting of two hydrophobic
chains and two polar headgroups covalently attached through a
spacer, e.g., 1 (16-m-16, 2Br-) are of current interest1 owing
to their unusual properties, such as low critical micellar concen-
trations (cmc), high viscoelasticities, and enhanced abilities for
lowering the oil-water interfacial tension.2 Recently, it has
been shown that aqueous dispersions of dimeric surfactants, 16-
m-16, 2Br- with m-value e 4, have high propensity for micellar
growth3 and generally produce threadlike micelles. In contrast,
the ones with m g 5 generate either ellipsoidal or spherical
micelles. Since the polar headgroups are connected within a
gemini surfactant by a covalent linkage, the separation between
the polar headgroups within a dimeric unit depends on both
the nature (rigid vs flexible) and the length of the spacer chain.
In aqueous solution, the two covalently connected positively
charged -NMe2+ headgroups within a 16-m-16, 2Br- unit tend
to maintain a critical distance between them to minimize the
Coulombic repulsion. But since such situation creates rather
an unfavorable contact of the hydrophobic (CH2)m spacer chain
with the bulk water, a separation based on the compromise of
these two opposing tendencies results. One can therefore
anticipate that a replacement of CH2 by oxygen in selected
positions of the spacer chain would affect its looping in a
different fashion as compared to the dimeric surfactants that
contain polymethylene spacer. As a consequence, this might
enhance the tendency of the spacer chain containing oxyethylene
Attempts to correlate the aggregate properties with that of
the molecular features of surfactant are important for developing
new surfactant systems for specific applications. Owing to our
* To whom the correspondence should be addressed. Fax: 91-080-334-
1683. E-mail: sb@orgchem.iisc.ernet.in. Also at the Chemical Biology Unit,
Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Ban-
galore 560 064, India.
† Indian Institute of Science.
‡ Bhabha Atomic Research Centre.
S1089-5647(98)00836-0 CCC: $15.00 © 1998 American Chemical Society
Published on Web 07/17/1998

Novel Gemini Micelles from Dimeric Surfactants
J. Phys. Chem. B, Vol. 102, No. 32, 1998 6153
to fold in association with water molecules present at the stern
layer region. In other words, additional hydration at the level
of spacer chain should mitigate the Coulombic repulsion
between the two cationic -NMe2+ centers. However, dimeric
biscationic surfactants with spacer containing oxyethylene
groups are hitherto not developed. Only recently, Rosen et al.
has reported a group of dimeric, anionic surfactants with two
carboxylate groups prepared from diepoxy compounds.11 In this
paper, we introduce new dimeric surfactants with three different
oxyethylene spacers, i.e., n-C16H33N+Me2(CH2)(CH2OCH2)p-
(CH2)N+Me2n-C16H33, 2Br-, 2, where p ) 1, 2, 3, and the
results of our SANS studies on these dimeric cationic surfac-
tants. The hydrocarbon chain length in these surfactants is equal
to that present in the corresponding monomeric, cationic sur-
factant, cetyltrimethylammonium bromide (CTAB). To under-
stand the role of “wettable” spacer chain in determining the
morphologies of dimeric surfactant micelles, we have performed
SANS experiments employing the above gemini micelles. The
variation of concentration and the effect of temperature on the
neutron cross sections were also looked upon. We have also
examined the mixed micelles of known compositions of CTAB
and various dimeric surfactants, 16-CH2-(CH2OCH2)p-CH2-
16, 2Br-, 2. Finally, to rationalize the information obtained
from SANS studies with other micellar properties, we have
measured their critical micellar concentrations, microviscosities,
and micropolarities.
individual diol in a dropwise manner over a period of 1 h at 0
°C. Then the mixtures were stirred with concomitant heating
at 50-60 °C for 12 h. The reaction mixture was then cooled
to ambient temperature and dissolved in CHCl3 (25 mL). Water
was added slowly, and the mixture was transferred into a
separatory funnel. The organic layer was separated out after
washing with water, NaHCO3, and again with water and finally
passed through a bed of anhydrous Na2SO4. The resulting
CHCl3 solution was concentrated and loaded on a silica gel
column (60-120 mesh) and was eluted with hexane to give a
colorless liquid (yield 80-90%). All the dibromides gave
1
satisfactory H NMR spectra.
Bis(2-bromoethyl) Ether (4a). 1H NMR (200 MHz, CDCl3)
δ 3.48 (t, 4H), 3.84 (t, 4H).
1,2-Bis(2-bromoethoxy)ethane (4b). 1H NMR (200 MHz,
CDCl3) δ 3.48 (t, 4H), 3.69 (s, 4H), 3.84 (t, 4H).
Bis(5-bromoethoxyethyl) Ether (4c). 1H NMR (200 MHz,
CDCl3) δ 3.45 (t, 4H), 3.65 (s, 8H), 3.81 (t, 4H).
Synthesis of Bis(quaternary ammonium) Surfactants (2a-
2c). The bis(quaternary ammonium) surfactants 2a-2c were
synthesized as described in the following.
All the surfactants 2a-2c were obtained by refluxing the
corresponding R,w-dibromoalkoxyalkanes with N,n-hexadecyl-
N,N-dimethylamine in dry ethanol at ∼80 °C for 72-96 h. At
the end of this period, solvent was removed under vacuum from
the reaction mixture, and the solid thus obtained was recrystal-
lized several times from 10 mL of acetone containing 2 drops
of methanol to obtain solid surfactants as pure product as
determined from 1H NMR spectra and elemental analysis. The
overall yields of the surfactants ranged from 50 to 60%.
Bis(hexadecyl dimethylammonium)diethyl Ether (2a). Mp
230 °C (softening), 245-250 °C; 1H NMR (200 MHz, CDCl3)
δ 0.88 (t, 6H, alkyl chain 2 × 1 CH3), 1.18-1.32 (s + br m,
52H, alkyl chain 2 × 13 CH2), 1.58-1.72 (br m, 4H, alkyl
chain 2 × 1 CH2CH2N+), 3.42 (s, 12H, 2 × 2 N+CH3), 3.54
(crude t, 4H, alkyl chain 2 × 1 CH2N+), 3.99 (br s, 4H,
spacer chain 2 × 1 CH2N+), 4.31 (br s, 4H, spacer chain 2 ×
1 OCH2CH2N+). C, H, N analysis, Calcd for C40H86N2OBr2,
0.5 H2O: C 61.62, H 10.99, N 3.59. Found: C 61.33, H 11.24,
N 3.48.
Bis(hexadecyl dimethylammonium)diethoxyethane (2b). Mp
200 °C (softening), 215-220 °C; 1H NMR (200 MHz, CDCl3)
δ 0.88 (t, 6H, alkyl chain 2 × 1 CH3), 1.25-1.34 (s + br m,
52H, alkyl chain 2 × 13 CH2), 1.65-1.74 (br s, 4H, alkyl chain
2 × 1 CH2CH2N+), 3.44 (s, 12H, 2 × 2 N+CH3), 3.50-3.63
(m, 4H, alkyl chain 2 × 1 CH2N+), 3.97 (br s, 4H, spacer chain
2 × 1 CH2N+), 4.16 (br s, 4H, spacer chain 1 × 1 OCH2CH2O),
4.33 (br s, 4H, spacer chain 2 × 1 OCH2CH2N+). C,H,N
analysis, Calcd for C42H90N2O2Br2, 1.0 H2O: C 60.56, H 11.13,
N 3.36. Found: C 60.51, H 11.27, N 3.03.
Experimental Section
Materials. All the reagents and solvents were highest grade
available commercially and used purified, dried, or freshly
distilled as required. Cetyltrimethylammonium bromide (CTAB),
n-hexadecyl bromide, phosphorus tribromide, diethylene glycol,
triethylene glycol, tetraethylene glycol, and pyrene were pur-
chased from Aldrich Chemical Co. N,n-Hexadecyl-N,N-di-
methylamine was obtained by refluxing n-hexadecyl bromide
with dimethylamine (Merck, 40% aqueous solution) in dry
ethanol at 80 °C for 24 h. The dimeric surfactants 16-m-16,
2Br-, 1, were prepared as described.9a Steam-distilled water
was used for all physical measurements.
Dimeric Surfactants. The dimeric surfactants, 2, were
synthesized as indicated in the following (Scheme 1).
SCHEME 1a
Bis(hexadecyl dimethylammonium)bis(ethoxyethyl) Ether (2c).
1
Mp 205 °C (softening), 235-240 °C; H NMR (200 MHz,
CDCl3) δ 0.88 (t, 6H, alkyl chain 2 × 1 CH3), 1.25-1.34 (s +
br m, 52H, alkyl chain 2 × 13 CH2), 1.72 (br s, 4H, alkyl chain
2 × 1 CH2CH2N+), 3.44 (s, 12H, 2 × 2 N+CH3), 3.54-3.63
(crude t, 8H, alkyl chain 2 × 1 CH2N+ and spacer chain 1 ×
2 CH2N+), 3.99 (br s, 8H, spacer chain 1 × 4 OCH2CH2O),
4.33 (br s, 4H, spacer chain 1 × 2 OCH2CH2N+). C,H,N
analysis, Calcd for C44H94N2O3Br2: C 61.52, H 11.03, N 3.26.
Found: C 61.62, H 11.33, N 3.13.
a (a) PBr3, ∼80-90% yield; (b) C16H33NMe2 (3.0 equiv), dry EtOH,
reflux, 72-96 h, 70-90% yield.
Methods. Descriptions of analytical and spectroscopic
instruments have been published previously.9a
Synthesis of Dibromides (4a-4c). The dibromo precursors
(4a-4c) were synthesized by the addition of PBr3 to the
Fluorescence Measurements. Steady-state fluorescence
measurements were performed in a Hitachi F-4500 fluorescence

6154 J. Phys. Chem. B, Vol. 102, No. 32, 1998
De et al.
spectrophotometer equipped with a thermostated water-circulat-
ing bath (Julabo model F10). All the measurements were carried
out at 30 °C and using a 3 cm3 cell. Pyrene, a fluorescence
probe the spectral signature of which changes with the formation
of the micelle, was chosen as a probe for the cmc (critical
micellar concentration) determinations. Excitation wavelength
was fixed at 337 nm, and the emission spectra for the range
360-410 nm was examined. Bandwidths were fixed at 5 nm
for both the emission and the excitation. Surfactant solutions
of different compositions in water were doped with pyrene. The
cmc values were determined from the plots of the concentration
of the surfactant vs the ratio (I1/I3) of the intensities of the first
(I1) and the third (I3) vibronic peaks in the fluorescence emission
spectra due to pyrene solubilized in respective micellar ag-
gregates.12 The micropolarities of the surfactant micelles were
characterized by the value of I1/I3.
(∆Q/Q) of about 15% at Q ) 0.05 Å-1. The angular distribution
of the scattered neutrons is recorded using a one-dimensional
position-sensitive detector. The accessible wave vector transfer,
1
Q ()4π sin /2φ/λ, where λ is the wavelength of the incident
neutrons and φ is the scattering angle), range of this instrument
is between 0.02 and 0.3 Å-1. The wavelength is λ ) 0.52 nm.
The SANS data from the above instrument are normalized to
absolute scale by comparing the data against those from Porasil
A11.16
The solutions were held in a 0.5 cm path-length UV grade
quartz sample holder with tight-fitting Teflon stoppers, sealed
with Parafilm. In most of the measurements, the surfactant
concentration (c ) 50 mM) and the sample temperature (30 (
0.1 °C) were kept fixed. The effect of different concentra-
tion on the SANS distribution was also studied for 16-CH2-
(CH2OCH2)2-CH2-16, 2Br-, 2b, micellar sample for concentra-
tion in the range of 30-70 mM at 40 °C. The effect of
temperature was also investigated for this surfactant system at
a fixed surfactant concentration of c ) 50 mM. In the mixed
micellization studies, where each of the above gemini surfactants
was separately mixed with CTAB, the percentages of dimeric
surfactants in the micelles of binary surfactant mixtures were
23.1 mol %.
Data Treatment. Scattering intensities from the surfactant
solutions were corrected for detector background, empty cell
scattering, and sample transmission. The resulting corrected
intensities were normalized to absolute cross-section units, and
thus dΣ/dΩ vs Q was obtained. This absolute calibration has
an estimated uncertainty of 10%. In figures, the Q-axis has
been plotted in log scale to indicate the peak position clearly.
The experimental points are fitted using a nonlinear least-squares
routine as described below. Comparisons between the experi-
mental and the calculated cross sections are shown in Figures
1-4.
Determination of Microviscosities (ηj). The fluorescence
anisotropies (r) of 1,6-diphenyl-1,3,5-hexatriene (DPH), as
sensed by DPH doped in micelles, were measured from the
intensities obtained at 0-0°, 0-90°, 90-0°, and 90-90° angle
settings of the excitation and emission polarization accessories,
respectively. The temperature of the cuvette containing the
sample was maintained at 30 °C with the aid of a thermostated
temperature controlling water-circulating bath (Julabo model
F10). The fluorescence intensities of the emitted light polarized
parallel (I|) and perpendicular (I ) to the exciting light were
recorded and were corrected for the scattered light intensity,
which was determined independently for an unlabeled reference
suspension by the same procedure. The fluorescence anisotropy
(r) for each micellar sample at 30 °C was calculated according
to r ) (I| - G‚I )/(I| + 2‚G‚I ), where G is the grating
correction factor. Microviscosities (ηj) of the individual micellar
systems in which the fluorophore was placed were calculated
from the anisotropy values.13 The measurements were done at
a fixed 50 mM concentration for 2a-2c and at 100 mM
concentration of CTAB.
SANS Analysis
1. Calculation of the Scattering Intensity. The coherent
differential scattering cross-section, (dΣ/dΩ), derived by Hayter
and Penfold17 and Chen,14 can be reduced to eq 1 for an
assembly of monodisperse, uniform ellipsoidal micelles
Determination of the Compositions of the Mixed Micelles.
The composition of the mixed micelles was calculated according
to Clint’s theory, as described previously,10 which assumes an
ideal mixing of the surfactant components.
Small Angle Neutron Scattering (SANS) Measurements.
Small angle neutron scattering (SANS) is a well-established
technique to investigate the structural aspects of materials on a
length scale of 10-1000 Å.14 As reported earlier,5-7 these
measurements provide useful information pertaining to the
shapes of various self-organizing systems in a noninvasive
manner. Recently, we examined the role of spacer chain length
(m) on the morphology of dimeric micelles using the SANS
experiments.9 To explore whether the incorporation of “O” in
place of some -CH2-’s within spacer chain has any role in
dictating the micellar morphology, we have now carried out
similar studies of the dimeric surfactant system 16-CH2-
(CH2OCH2)p-CH2-16, 2Br-, where p ) 1, 2, 3. We have also
compared various micellar parameters from these newly devel-
oped surfactants with the corresponding dimeric surfactant
(16-m-16, 2Br-, 1) devoid of any oxygen atom in the spacer.
dΣ/dΩ ) n(Fm - Fs)2Vm [ F2(Q) +
2
2
F(Q) {S(Q) - 1}] + B (1)
where n denotes the number density of the micelles, Fm and Fs
are, respectively, the scattering length densities of the micelle
and the solvent, and Vm is the volume of the micelle. F(Q) is
the single particle form factor, and S(Q) is the interparticle
structure factor. B is a constant term that represents the
incoherent background. For the analysis, we assume the
micelles to be monodisperse prolate ellipsoids, (a ) c * b),
where a sphere is a special case of that. The aggregation number
N for the micelle is related to the micellar volume Vm by the
relation N ) Vm/V, where V is the volume of the individual
surfactant molecule. Then the eq 1 can be rewritten as
All the final solutions used in the neutron-scattering experi-
ments were prepared in D2O (at least 99.5 atom % D pure).
This provides a good contrast between the micelle and the
solvent in a SANS experiment. Neutron-scattering measure-
ments were performed on the 7.0 m (source-to-detector distance)
SANS instrument at the CIRUS Reactor, Trombay.15 In this
setup, the sample-to-detector distance is 1.8 m for all the runs.
This spectrometer uses a BeO filtered beam and has a resolution
dΣ/dΩ ) 1/N(c - cm)Vm (Fm - Fs)2[ F2(Q) +
2
2
F(Q) {S(Q) - 1}] + B (2)
where (c - cm) ) nN, cm is the cmc, and c is the surfactant
concentration.
The volume of the mixed micelle was calculated as described
in our previous paper10 by eq 3

Novel Gemini Micelles from Dimeric Surfactants
Vm ) N(x1VCTAB + (1 - x1)V16-CH -p-CH -16
J. Phys. Chem. B, Vol. 102, No. 32, 1998 6155
)
2
2
) (NCTABVCTAB + N16-CH -p-CH -16V16-CH -p-CH -16
)
2
2
2
2
(3)
where VCTAB and V16-CH -p-CH -16 are the volumes of CTAB
2
2
andgeminisurfactant,respectively. N)(NCTAB +N16-CH -p-CH -16),
2
2
N
CTAB, and N16-CH -p-CH -16 are the aggregation numbers of
2 2
CTAB and gemini surfactant in the mixed micelle, respectively.
The scattering length density of mixed micelle is obtained
from eq 4
Fm ) (x1FCTAB + (1 - x1)F16-CH -p-CH -16
)
(4)
2
2
where FCTAB and F16-CH -p-CH -16 are the scattering length
2
2
densities due to CTAB and 16-CH2-p-CH2-16, 2Br- surfactants,
respectively, and x1 is the mole fraction of CTAB in the mixed
micelle.
For an ellipsoidal particle
Figure 1. SANS distributions from dimeric micellar systems 2a-2c
at specified concentration (50 mM) at 30 °C. The lines shown are
theoretical fits, and the solid marks are experimentally determined data
points. The SANS distributions for p ) 2 and 3 are shifted vertically
by 5 and 10 units, respectively.
1
F2(Q) ) [F(Q, µ)]2 dµ
(5)
∫
0
and
1
F2(Q) 2 ) [ F(Q, µ) dµ]2
(6)
∫
we assumed the present model, which was also used in the
previous reports.9b
0
where µ is the cosine of the angle between the axis of resolution
and Q.
The data in Figure 1 (corresponding to different p ) 1, 2,
and 3) were first analyzed in terms of eq 2. N, a, and R were
taken as the parameters of the fit, and V was calculated using
Tanford’s formula: V ) (1052 + 53.8 + 73 × p) Å3, where
1052 Å3 is the volume of two tails with the headgroups, 53.8
Å3 is the volume of two -(CH2)- units, and 73 Å3 is the
volume of a ethylene oxide (-CH2-O-CH2-) group.18 The
contribution of scattering from bound counterions is very small
because of small volume of counterions and relatively poor
contrast. Hence the volume of the counterion Br- is not
considered for their contributions to the scattering. The solid
lines in Figure 1 are the calculated curves. The major axis b
(3NV/4πa2) was obtained from a knowledge of the above
parameters. The values of N, R, a, b, and V are given in Table
1. The axial ratios of 2a-2c are also compared with that of
16-m-16, 2Br- in Table 1. The effects of concentration and
temperature on size parameters for 2b are shown in Figures 2
and 3. The data in Figure 4 corresponds to mixed micelles
composed of 100 mM CTAB with 30 mM 2b and 16-6-16, 2Br-
respectively.
The form factor, F(Q, µ) is given by the eq 7, i.e.
F(Q, µ) ) 3(sin w - w cos w)/w3
(7)
where w ) Q[a2µ2 + b2(1 - µ2)]1/2 and a and b are, respectively,
the semiminor and semimajor axes of the ellipsoid of revolution.
2. Structure Factor for Interacting Micelles. The inter-
particle structure factor S(Q) depends on the spatial distribution
of micelles. In the following analysis, we have calculated S(Q)
using mean spherical approximation as developed by Hayter
and Penfold.17b This theory is applicable if there is no angular
correlation between the particles. This assumption is quite
reasonable for charged micelles especially when the surfactant
concentration is low and the ratio of the axes is not much greater
than unity. Strong electrostatic repulsions prohibit close
proximity between two micelles. It may be mentioned that
satisfactory data analysis procedures for ellipsoidal particles have
not been developed. Though the approximation of treating
ellipsoid as a sphere has been often used in the literature,14 its
consequences on size parameters are not fully understood. The
intermicellar interaction is modeled via a screened Coulomb
potential, and S(Q) is calculated in mean spherical approxima-
tion. On the basis of such consideration in the following, the
ellipsoidal micelle is approximated by an equivalent sphere of
Results and Discussion
Molecular Features and Solubilities of New Geminis.
From the earlier studies,9 we found that the length and the nature
of the spacer chain connecting the two headgroups mainly
determine the micellar morphology in the case of the dimeric
surfactants (16-m-16, 2Br-). SANS studies further indicated
that the dimeric surfactants, 16-m-16, 2Br- form disk or
cylindrical micelles for m e 4 and ellipsoidal or spherical
micelles for m g 5. With the replacement of some -CH2-’s
in the spacer chain with -O-, one would anticipate some
changes in the conformation of the latter. In addition the
presence of -O- in the spacer chain will make the same
relatively more hydrophilic. Another point of worth consider-
ation is that the C-O-C bond angle (115.8°) is higher than
the C-C-C (112.2°) bond angle when the spacer chain remains
in an extended s-trans conformation. This would render the
radius R ) (a2b)1/3
. In this analysis, the only unknown
parameters in dΣ/dΩ are the effective monomer charge, R, and
the aggregation number, N. The ellipsoidal micelles, in general,
could have polydispersity in their sizes. However, we have
assumed them to be monodisperse for the simplicity of the
calculation and limit the number of unknown parameters in the
analysis.
Here, it should be worthwhile to mention that the analysis
assuming core + shell model of micelle would probably be more
accurate. However, this will also increase the number of
unknown parameters in the fitting, such as thickness of the shell,
number of “wet” methylene units, etc. Therefore, for simplicity,

6156 J. Phys. Chem. B, Vol. 102, No. 32, 1998
De et al.
TABLE 1: Effect of Spacer Chain in Micellar Systems, 2a-2c, on Q Value.a The b/a Values of 16-m-16, 2Br- (m ) 5, 8, and
10)b Are Given for Comparison
aggregtion
no. N
effective monomer
monomer
semiminor
axis a ) c (Å)
semimajor
axis b (Å)
entry no.
surfactant
charge R
vol. V (Å3)
b/a
1
2
3
4
5
6
2a
16-5-16
2b
16-8-16
2c
16-10-16
220
238
165
72
226
83
0.062
0.11
0.066
0.29
0.058
0.2
1179
1187
1252
1267
1325
1321
25.6
24.2
24.8
22.8
25.8
22.6
94.3
115.0
80.2
42.0
107.6
45.2
3.68
4.75
3.23
1.82
4.17
1.95
a All the SANS spectra were taken at 30 °C using 50 mM 16-CH2-(CH2OCH2)p-CH2-16, 2Br- and 16-m-16, 2Br- micelles. b Taken from ref
9b.
Figure 4. SANS distributions from pure CTAB and the mixed micelles
of 30 mM 2b and 16-6-16, 2Br- dimeric systems with 100 mM
CTAB: (a) 100 mM CTAB (b), (b) 160 mM CTAB (O), (c) 100 mM
CTAB + 30 mM 16-6-16, 2Br- (0), and 100 mM CTAB + 30 mM
2b (4) at 30 °C. The lines shown are theoretical fits, and the solid
marks are experimentally determined data points.
Figure 2. SANS distributions from dimeric micelles 2b at different
concentrations: 30 mM (4), 50 mM (0), and 70 mM (O) at 40 °C.
comparable with that of 16-5-16, 2Br-, 16-8-16, 2Br-, and 16-
10-16, 2Br-, respectively, as obtained from the molecular
modeling studies (INSIGHT II 2.3.5 package, Biosym. Tech-
nologies), most of the time we have compared their micellar
properties against those of 16-m-16, 2Br-, m ) 5, 8, 10.
Neutron Cross Sections. We first describe the results of
the measurements of neutron cross sections from the micellar
solutions of dimeric surfactants 16-CH2-(CH2OCH2)p-CH2-
16, 2Br- in D2O as a function of p values at a fixed surfactant
concentration (50 mM) and temperature (30 °C). Measurements
have covered Q ranges from 0.02 to 0.16 Å-1
. SANS
distributions for 50 mM 16-CH2-(CH2OCH2)p-CH2-16, 2Br-,
p ) 1, 2, and 3, show well-defined peaks characteristic of
dispersions of charged particles as is the case with pure 50 mM
16-m-16, 2Br- solution. This peak arises because of a corre-
sponding peak in the interparticle structure factor S(Q). Usually,
this peak occurs at Qm ∼ 2π/d, where d is the average distance
between the micelles. Since the Qm was found to differ with
p, one can easily conclude that the number density (n) of
micelles is not the same in above samples even when they have
identical surfactant concentration. The above observations
further imply that the aggregation number of the micelle, N,
depends on the length and the nature of the spacer unit. It is,
however, not apparent from this that the micelles are spherical.
Consequently, in the following analysis, we assume them to be
prolate ellipsoids, (a ) c * b), sphere being a special case of
that.
Figure 3. SANS distributions from dimeric micelles 2b at various
temperatures: 30 °C (0), 40 °C (4), 50 °C (O), and 60 °C (b).
chain little more flexible. Keeping these aspects in mind, we
have synthesized three new gemini surfactants having variable
number of oxygens in the spacer chain.
All the new surfactants 2a-2c could be readily dissolved in
D2O and remained optically translucent for several weeks. The
SANS analysis showed that, at a fixed concentration (50 mM)
of the dimeric surfactants, the micellar structures of 2a-2c are
different. Since the spacer chain lengths of 2a-2c are

Novel Gemini Micelles from Dimeric Surfactants
J. Phys. Chem. B, Vol. 102, No. 32, 1998 6157
TABLE 2: Average Separationa between the Two Me2N+
Headgroups within Dimeric Micelles 2a-2c and Comparison
with That Obtained from Molecular Modeling within a
Gemini.b Corresponding Values for 16-m-16, 2Br- (m ) 5, 8,
and 10)c Are Given for Comparison
conformation, and A is the surface area occupied by a surfactant
headgroup at the water/micelle hydrophobic core interface. Both
l and V can be calculated using Tanford’s equations.20 As long
as the hydrophobic tail length is constant, l is likely to remain
the same throughout all the surfactants 2a-2c and V should
increase marginally (relative %) with the change in the length
of the spacer since the length of the two hydrophobic tails has
been kept unchanged. The effective headgroup area A is decided
by the effective charge on the headgroup and by the size of the
headgroup. Therefore, this is an important quantity for the
dimeric surfactants, which is mainly responsible for the change
in the packing parameter. It is clear from Table 1 that the
effective headgroup charge (R) is almost the same for all the
three new dimeric surfactant systems in the corresponding
micellar aggregates. Therefore, the change in A or packing
parameter is expected to be largely due to the alterations in the
conformation of the spacer, which is found to follow a
nonmonotonic dependence. For the dimeric surfactant with
short spacer, i.e. 2a, because of the Coulombic repulsion
between the headgroups, the spacers are stretched to adopt more
extended conformation. The length of the spacer to be stretched
is decided by the charge on the headgroup. In particular, with
16-m-16, 2Br- systems, we found that A approaches a maximum
surface area occupied by its surfactant headgroups at the water/
micelle hydrophobic core interface for m ) 8. Here, for p )
1, i.e., with the incorporation of one oxygen in the spacer, the
length of the spacer chain becomes lesser than that in 16-5-16,
2Br-, thus increasing the Coulombic repulsion between the
headgroups. Then the micelles of 2a should be more elliptical
than those of 16-5-16, 2Br-. But the stabilizing interaction
between the spacer chain and water somewhat neutralizes the
repulsion and gives rise to less ellipsoidal morphology for 2a
compared to 16-5-16, 2Br-. For p ) 2, the length of the spacer
is now longer than the critical distance necessary to minimize
Coulombic repulsion between -NMe2+ groups to maintain the
equilibrium distance. So it can now loop as the Coulombic
repulsion has not increased since the charge on the headgroup
remains the same. It can be understood that owing to the
increase in A, the packing parameter decreases; hence, the
micelles become less ellipsoidal. For p ) 3, A decreases owing
to a folded arrangement of the spacer, thereby increasing the
packing parameter. So, it forms more elliptical micelles. It is
possible that under this situation, the spacer chain is also more
hydrated making the micelles of 2c more “wet”.
avg. separation
between headgroups
in micelles (Å)
calcd distance between the
two Me2N+ headgroups
within a gemini (Å)
entry
no. surfactant
1
2
3
4
5
6
2a
16-5-16
2b
16-8-16
2c
16-10-16
9.45
11.1
10.12
14.4
9.79
13.6
7.64
7.82
11.33
11.72
14.2
14.27
a Obtained assuming it to be equal to x(4πaeff.2/N), where aeff.
)
(a2b)1/3, considering the micelles to be ellipsoidal as found from SANS
data. b Calculated using INSIGHT II 2.3.5 package, Biosym. Technolo-
gies assuming the spacer chain is in a fully extended conformation.
c Taken from ref 9a.
For 2a, which has one oxygen in the spacer, the micelles are
less ellipsoidal compared to 16-5-16, 2Br- micelles9 (Table 1).
For 2b, the aggregation number, N, decreases compared to that
for 2a but again increases for 2c. But, the effective fractional
charge (R) does not differ much. The axial ratio (b/a) is less
for 2b compared to 2a but more with respect to the correspond-
ing surfactant without oxygen 16-8-16, 2Br-; i.e., the micelle
size increases. Similar comparison for 2c shows that the
micelles of 2c are more ellipsoidal compared to either 2a or
2b. Though the lengths of the spacer chain in 2c and 16-10-
16, 2Br- are comparable in their extended conformations, the
micelles of 2c are more ellipsoidal than that of the gemini
surfactant, 16-10-16, 2Br-, in which the spacer chain is devoid
of oxygen. Therefore, one can anticipate a dissimilar spacer
chain conformation in 2c as compared to 16-10-16, 2Br- owing
to large differences in the axial ratios. It seems that the changes
are connected with the increase in the length and the flexibility
of the spacer as well as its accessibility to water.
SANS data (Table 2) show that the incorporation of -O-
in place of a -CH2- decreases the distance between the
headgroups of the gemini surfactant compared to that in 16-m-
16, 2Br-. If the spacer chain looping alone governs the micellar
structure, then an increase in the spacer chain length (and hence
looping) from 2a to 2b should make the micelles of 2b more
spherical than the micelles of 2a. Since the micelles of 2b are
found to be more spherical than those of 2a, the spacer chain
in 2b is not significantly more hydrophilic than that in 2a. Thus
even after incorporation of an additional O in 2b, the spacer in
2b prefers to loop deeper into the micellar interior. However,
in 2c, the hydration of the spacer appears to be significant. In
such situation the cationic headgroups should come considerably
closer although the mutual electrostatic repulsion is probably
compensated by the additional hydration at the trioxyethylene
spacer chain. Thus, owing to combination of looping and
hydration of the spacers in 2a-2c, the situation appears to be
more complex (see below).
Separation between the Two Polar Headgroups within a
Dimeric Surfactant. In aqueous solution, the two covalently
+
connected positively charged -NMe2 headgroups within a
dimeric surfactant unit tend to maintain a critical distance
between them to minimize the electrostatic repulsion. But since
such situation creates a rather undesirable contact of the spacer
chain with the bulk water especially when the spacer chain is
hydrophobic, a separation based on the compromise of these
two opposing tendencies results. In 2a-2c, spacers are
relatively more hydrophilic than 16-m-16, 2Br-. We have
estimated this critical distance on the basis of the SANS data,
Packing Parameter. As stated earlier, the insertion of a
assuming it to be equal to x(4πaeff.2/N), where aeff. ) (a2b)1/3
,
+
spacer unit between two -NMe2 ions at the level of head-
considering the micelles to be ellipsoidal. Applying this, we
obtained a value of aeff. ∼ 7.94 Å for CTAB (m ) 0), which is
in reasonable agreement with the value reported by Zana et al.2
In the same way, we obtained the critical distances between
the two cationic centers for 2a-2c, which came out to be 9.45,
10.12, and 9.79 Å respectively (Table 2). Examination of the
data suggests that the average separation between the cationic
headgroups in these micelles is smaller than that for the
groups imposes additional geometric constraints on the surfac-
tant intramolecular packing. This is reflected in the aggregate
morphology. The packing parameter (P), introduced by Is-
raelachvili et al.,19 is related to aggregate morphology in aqueous
solutions at a concentration higher than the critical micellar
concentration (cmc) by the equation P ) V/l‚A, where V is the
effective volume occupied by the hydrophobic moiety of the
surfactant molecule, l is the critical length in the fully extended

6158 J. Phys. Chem. B, Vol. 102, No. 32, 1998
De et al.
TABLE 3: Effects of Change in Surfactant (2b) Concentrations on Q Value at 40 °C
entry
no.
concn
(mM)
aggregation
effective monomer
monomer
semiminor
axis a ) c (Å)
semimajor
axis b (Å)
no. N
charge R
vol. V (Å3)
b/a
1
2
3
30
50
70
100
145
179
0.12
0.075
0.07
1252
1252
1252
24.8
24.8
24.8
48.7
70.3
87.1
1.96
2.83
3.51
TABLE 4: Effect of Temperature on Q Value for the
TABLE 5: Dependence of the Critical Micellar
Micellar 2b at a Fixed 50 mM Concentration
Concentrations, Microviscosities,a and Micropolarities of
16-CH2-(CH2OCH2)p-CH2-16, 2Br- Dimeric Micelles as a
Function of p-Value at 30 °C
effective
temp. aggregation monomer monomer semiminor semimajor
charge R vol. (Å3) axis a (Å) axis b (Å) b/a
entry
no.
microviscosity
(°C)
no. N
surfactant
cmc × 105 (M)
(jη)a (P)
I1/I3
30
40
50
60
165
145
109
101
0.066
0.075
0.011
0.116
1252
1252
1252
1252
24.8
24.8
24.8
24.8
80.2
70.3
52.8
49.1
3.23
2.83
2.13
1.98
1
2
3
2a
2b
2c
1.9 ( 0.15
1.7 ( 0.15
2.0 ( 0.15
1.116
1.205
1.146
1.028
1.029
1.024
a Measured at 50 mM concentration for 16-CH2-(CH2OCH2)p-CH2-
16, 2Br- dimeric micelles at 30 °C.
corresponding dimeric surfactant system, 1, containing (CH2)m
spacers, which proves that indeed the stabilizing interaction
between the spacer and water partly overcomes the Coulombic
repulsion, thus decreasing the critical distance between the two
cationic centers.
ellipsoid-to-sphere transition. This notion is also supported by
the concomitant decrease in b/a values upon increase in
temperature.
Critical Micellar Concentrations. The critical micellar
concentration (cmc) data for 16-CH2-(CH2OCH2)p-CH2-16, 2Br-
series have been summarized in Table 5. The respective plots
of the concentrations of the new surfactant solutions vs the ratios
(I1/I3) of the intensities of the first (I1) and the third (I3) vibronic
peaks in the fluorescence emission spectra due to solubilized
pyrene gave apparent breaks related to critical micellar con-
centrations (not shown). In contrast to the dimeric surfactant
containing polymethylene spacer, which showed a sharp break,
these presently described surfactants 2a-2c showed rather broad
breaks in the above plots. This may be due to the tendency of
these micelles to form premicellar aggregates prior to formation
of gemini micellar aggregates. Evidence of premicellar ag-
gregation was also reported by Zana et al.21a for gemini
amphiphilic 8-3-8, 2Br- and 8-6-8, 2Br- systems during the
determination of their cmc by the electrical conductivity method.
Another point to note is that there is little difference in the cmc
values among 2a to 2c. It is possible that the probe pyrene
used in this experiment may not be sensing the differences in
the relative wetnesses of the micelles.
Dependence of Microviscosity (ηj). Another parameter of
considerable interest is the microenvironmental viscosity (mi-
croviscosity, ηj) of the dimeric micellar aggregates. This
parameter can be conveniently estimated by the determination
of the fluorescence polarization of the probe, 1,6-diphenyl-1,3,5-
hexatriene (DPH).13 Table 5 summarizes the microviscosity
estimates of 16-CH2-(CH2OCH2)p-CH2-16, 2Br- micelles at
a fixed concentration of 50 mM as a function of variable p
values. DPH is a fluorescence probe whose emission quantum
yield gets enhanced upon partitioning into a micelle or a bilayer
from water. The microviscosity we have measured agrees for
CTAB (100 mM) with that reported using DPH and other
probes, around 0.56 P for CTAB.22 We have observed that the
microviscosities of 16-m-16, 2Br- systems decreased with the
increase in m values. But, there is not much difference in the
microviscosities of 2a-2c. Though the microviscosity of 2a
is very close to that of 16-5-16, 2Br-, the microviscosities of
2b and 2c, despite having longer spacer length, are very different
from that of 16-m-16, 2Br-, m g 8. They are closer to that of
16-5-16, 2Br-.
Effect of Surfactant Concentration Variation. The effect
of surfactant concentration on SANS distributions is shown in
Figure 2 for 2b at 40 °C. As already indicated, the peak in
dΣ/dΩ occurs at Qm ) 2π/d, where d is the average distance
between the micellar particles. With an increase in concentra-
tion, the interparticle distance decreases and the peak shifts to
lower Q values. The concentration range examined was from
30 to 70 mM. It is seen that the calculated distributions give
the peak positions in dΣ/dΩ with a good correspondence with
experimentally determined points. As the concentration of 2b
is decreased, it is found that the peak in the measured
distribution broadens with significant shifts in the peak position.
The micellar shape changes from b/a ∼ 1.96 to b/a ∼ 3.51 upon
increase in concentration of 2b from 30 to 70 mM (Table 3)
owing to the formation of more prolate ellipsoidal micellar shape
at higher concentration. Increasing N for a given spacer chain
length results in an increase in the axial ratio (b/a); i.e., as N
increases, micellar shape tends to become more prolate el-
lipsoidal. Since the shape of the micelle changes with respect
to concentration, the interactions among charged headgroups
of the dimeric 2b units and water in the Stern layer region of
the micelle appear to play an important role in determining the
micellar shape. The effective fractional charge on the dimeric
units of 2b decreases with increase in concentration.
Effect of Temperature. Previously, we have carried out the
SANS experiments to show how micellar size varies with
temperature for polymethylene spacer containing gemini mi-
celles and their mixed systems with single-chain surfactant.
Figure 3 shows the variation of neutron cross sections for 2b
micelles as the temperature is increased. The neutron cross
sections build up at higher Q values as the temperature is
increased. The peak in the measured distribution also broadens
with the increase in temperature.
Table 4 records the information based on the above experi-
mental findings as a function of temperature. Increase in
temperature enhances the degree of ionization and thus effects
a modification of the magnitude of electrostatic repulsion. This
results in a decrease in the aggregation number, N, upon the
increase in temperature. The effective fractional charge per
monomer, however, appears to increase with increase in
temperature. Since a smaller effective charge indicates a more
ellipsoidal morphology, increasing temperature appears to induce
Dependence of Micropolarity (I1/I3). Table 5 also lists the
values of the pyrene intensity ratio for the surfactant micelles
investigated at 30 °C. The measurements were performed at

Novel Gemini Micelles from Dimeric Surfactants
J. Phys. Chem. B, Vol. 102, No. 32, 1998 6159
TABLE 6: Values of Various Parameters for the Mixed Micelles of Pure CTAB and 30 mM 16-6-16, 2Br- and 2b with 100
mM CTAB Obtained from Fits to the Experimental Data
aggregation
effective monomer
monomer vol.
semiminor axis
a ) c (Å)
semimajor axis
concn (mM)
100 mM CTAB
160 mM CTAB
no.a N
charge R
V (Å3)
b (Å)
b/a
160
168
175
0.10
0.09
0.082
560
560
22.0
22.0
22.0
44.5
46.5
61.1
2.02
2.11
2.78
23.1 mol % 16-6-16 in CTAB
560 (CTAB)
1213 (16-6-16)
560 (CTAB)
1252 (2b)
23.1 mol % 2b in CTAB
200
0.07
22.0
71.2
3.24
a N is the equivalent aggregation number of monomers.
surfactant concentrations 2 wt %, where the probe is fully
partitioned in the micelles. These values suggest that though
the micellar shapes are different for 2a-2c owing to different
surfactant packing parameter, there is very little effect on the
micropolarity sensed by pyrene. Since, the value of I1/I3
depends on the chemical composition of the medium surround-
ing the probe, it prompted us to believe that the local
composition at the micelle solubilization site of pyrene is
virtually the same irrespective of the nature of the spacer unit.
Since pyrene solubilizes preferentially in the palisade layer of
the cationic micelles, the variations of I1/I3 with the spacer
unit simply reflect the changes of composition of the palisade
layer. However, notably in general, the I1/I3 values of 2a-2c
are slightly higher than that of the 16-m-16, 2Br- system
presumably owing to presence of oxygen atom in the spacer,
although we do not see any significant rise in the I1/I3 value of
2b and 2c.
Mixed Micellization Studies with CTAB. Since the sur-
factant mixtures bear potential for numerous applications and
also play a pertinent role in other solubilization processes,23 we
considered that it may be worthwhile to examine the mixed
micellization behavior of the presently developed new gemini
surfactants with the commonly used monomeric surfactant,
CTAB. Toward this direction, we already reported the SANS
studies of different mixed micelles composed of dimeric
surfactant, 16-m-16, 2Br- (where m is devoid of any oxygen
atom) and CTAB.10 This allowed the long ellipsoidal micelle
due to 16-m-16, 2Br- to undergo a transition to a more
spherelike micelle in the resulting mixed micelles.
spacer chain (p ) 1, 2, and 3) have been synthesized. For these
dimeric surfactant micelles, the SANS spectra were measured
and the cmc values were determined using an extrinsic
fluorescence probe, pyrene. From the measurements of small-
angle neutron-scattering cross sections, it is shown that the
micellar structures of the 16-CH2-(CH2OCH2)p-CH2-16, 2Br-
system change with p values. The micellar morphology was
found to depend on both the nature and the length of the spacer
chain. The fractional charge is less and the minor axis is high
for these new dimeric micelles compared to that of the 16-m-
16, 2Br- system, which contained only polymethylene spacer.
The presence of three oxygen atoms in the spacer chain of 2c
appears to make the spacer chain loop as the distance between
the cationic dimethylammonium ion centers is shorter in this
case than that in 2b. It is therefore possible that micelles of 2c
are possibly more wet owing to additional ability of the spacer
chain in 2c to assocaite with interfacially located water
molecules. This additional hydration shields should assist in
+
bringing the -NMe2 headgroups closer despite unfavorable
electrostatic consequences. Although fluorescence studies do
not show much dependence of critical micellar concentrations
or micropolarities on the spacer unit, it may be possible that
the microenvironments around the probe molecules may not be
sensing the finer aspects of hydration and looping especially if
the probes are located deeper inside the micellar interior.
Acknowledgment. We thank the Inter University Consor-
tium for financial support of this work. We also thank Dr. B.
A. Dasannacharya for his encouragement and interest in this
work.
The SANS measurements for the mixed micellar solutions
of 2b and 16-6-16, 2Br- with 100 mM CTAB were carried out
at 23.1 mol % of dimeric surfactants at 30 °C in D2O. For
comparison, the data for a pure 100 and 160 mM CTAB
solutions are also shown (Figure 4, Table 6). As the experi-
mental cmc values for the mixed micellar solution matched well
with the theoretical values, we assumed that the dimeric
surfactants 2b and 16-6-16, 2Br- mixed ideally with CTAB.
With the same percentage of dimeric surfactants in the mixed
micelles, the position of maximum intensity (Qm) shifts to a
lower Q value with a concomitant increase in the maximum
intensity for 2b compared to 16-6-16, 2Br-. This is reflected
in the equivalent aggregation number of monomers, the axial
ratio (b/a) of the micelle, and the effective fractional charge
(R) on micelles. This suggests that a lesser number of -CH2-
groups in the spacer chain protrude into the shell when 2b is
incorporated into the mixed micelle. Thus the mixed micelles
of 2b/CTAB are more ellipsoidal in nature than that of 16-6-
16, 2Br-/CTAB.
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