Small angle neutron scattering and viscosity studies of micellar solutions of bis-cationic surfactants containing hydroxyethyl methyl quaternary ammonium head groups

Article abstract of DOI:10.1039/b402767c

The effect of head group polarity and spacer chain length of bis-cationic surfactants on shape, size and rheology of surfactant aggregates in aqueous solution has been investigated through small angle neutron scattering (SANS) and viscosity measurements. A series of bis-cationic surfactants C ₁₂H₂₅N⁺(CH₃)(C₂H ₄OH)-(CH₂)_s-N⁺(CH ₃)(C₂H₄OH)C₁₂H₂₅ 2Br ^-, referred to as 12-S-12 MEA, where two highly polar quaternary amine centers are covalently connected through polymethylene spacers at head group level, has been synthesized and characterized. Critical micellar concentration of these surfactants in aqueous solution determined by tensiometry and conductometry was observed to be 10-100 times lower than conventional bis-cationic surfactants [C₁₂H₂₅N⁺(CH ₃)₂-(CH₂)_s-N⁺(CH ₃)₂C₁₂H₂₅ 2Br^-, referred to as 12-S-12 DMA]. The extent of aggregate growths and variation in shape and size of micelles was observed to be strongly dependent upon head group polarity, spacer chain length and experimental conditions such as concentration and temperature.

Full text of DOI:10.1039/b402767c

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R E S E A R C H P A P E R
Small angle neutron scattering and viscosity studies of micellar
solutions of bis-cationic surfactants containing hydroxyethyl methyl
quaternary ammonium head groups
Mahendra Borse,a Vikas Sharma,a V. K. Aswal,b Naveen K. Pokhriyal,c Jayant V. Joshi,c
Prem S. Goyalc and Surekha Devi*a
a
Department of Chemistry, Faculty of Science, M.S. University of Baroda, Vadodara 390 002,
Gujarat, India. E-mail: surekha_devi@yahoo.com; Fax: þ91-265-2795569/2791891;
Tel: þ91-265-2795552
b
Solid State Physics Division, Bhabha Atomic Research Centre, Trombay,
Mumbai 400 085, India
Inter University Consortium for DAE Facilities, Bhabha Atomic Research Centre,
Trombay, Mumbai 400 085, India
c
Received 23rd February 2004, Accepted13th April 2004
F|rst published as an AdvanceArticle on the web 21st May 2004
The effect of head group polarity and spacer chain length of bis-cationic surfactants on shape, size and rheology
of surfactant aggregates in aqueous solution has been investigated through small angle neutron scattering
(SANS) and viscosity measurements. A series of bis-cationic surfactants C12H25Nþ(CH3)(C2H4OH)–
(CH2)s–Nþ(CH3)(C2H4OH)C12H25 2Brꢀ, referred to as 12-s-12 MEA, where two highly polar quaternary
amine centers are covalently connected through polymethylene spacers at head group level, has been
synthesized and characterized. Critical micellar concentration of these surfactants in aqueous solution
determined by tensiometry and conductometry was observed to be 10–100 times lower than conventional
bis-cationic surfactants [C12H25Nþ(CH3)2–(CH2)s–Nþ(CH3)2C12H25 2Brꢀ, referred to as 12-s-12 DMA]. The
extent of aggregate growths and variation in shape and size of micelles was observed to be strongly dependent
upon head group polarity, spacer chain length and experimental conditions such as concentration and
temperature.
polarity along with spacer length on physicochemical proper-
1. Introduction
ties and microstructure of bis-cationic surfactant aggregates.
We report the synthesis, characterization and physicochemical
properties of a series of novel 12-s-12 MEA bis-cationic surfac-
tants, where the polarity of head groups of conventional 12-s-
12 DMA bis-cationic surfactants has been altered by replacing
–N(CH3)2 head groups by –N(CH3)(C2H4OH) groups. Small-
angle neutron scattering and viscosity measurements have been
used to study the microstructure and rheology of these novel
series of bis-cationic surfactants.
Surfactant molecules above critical micellar concentration
(cmc) in aqueous solution are known to form variety of micro-
structures such as spherical, ellipsoidal, vesicular, rod-like and
thread-like.1–9 The microstructure of surfactant aggregates
depends on the molecular architecture of the surfactant and
solution conditions, such as concentration and tempera-
ture.1–7 The bis-cationic surfactants where two long chains of
ter-amines are covalently attached through the polymethylene
spacer chain at the head group level, have recently been well
studied.10–15 The physicochemical properties of bis-cationic
surfactant molecules primarily depend upon the structure of
molecules under consideration, as seen in more than 10 times
decrease in cmc, when two DTAB molecules are covalently
connected through polymethylene spacer chain at the head
group level.8,11 Moreover, the microstructure of micelles and
physicochemical properties of bis-cationic surfactants depend
on the nature and length of the spacer chain which has been
examined for different types of spacers such as polymethylene,
polyoxyethylene, and aromatic rings in the molecules.12–18
Zana and co-workers9,11 studied micellar solutions of conven-
tional bis-cationic surfactants 12-s-12 DMA through cryo-
transmission electron microscopy. They have reported that
surfactants with spacer chain length 2 and 3 show long
thread-like micelles, while nearly spherical micelles are formed
with spacer chain length 4 and 6. SANS studies of alkanediyl-
a,o-bis(alkyldimethylammonium bromide) bis-cationic surfac-
tants containing –N(CH3)2 head groups and C10 , C16 alkyl
chain lengths have been extensively reported.12–15,19 However,
this is a first attempt to examine the effect of variation in head
2. Experimental
2.1 Materials
n-Dodecyl bromide, a,o-dibromoalkane and 2(methylamino)-
ethanol were purchased from Lancaster Chemical Company,
Morecambe, England. N,n-dodecyl hydroxyethylmethyl
amine was prepared by refluxing n-dodecyl bromide with
2(methylamino)ethanol in dry ethanol at 80 ꢁC for 15 h. All
the reagents and solvents used were of AR grade. The identity
of intermediate compound was confirmed by elemental, FTIR
and 1H NMR analysis. Triple distilled deionized water was
used for all physicochemical measurements.
2.2 Synthesis of dimeric surfactants
The dimeric surfactants were synthesized by refluxing 2.2
moles of dodecyl hydroxyethylmethyl amine in dry acetone
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
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ꢀ1
with 1.0 mole of a,o-dibromo alkane for 70 h, at 60 ꢁC.
was 0.018–0.30 A and is given as 4psiny/l, where l is the
wavelength of the incident neutron and 2y is the scattering
angle. The detector allows a simultaneous recording of data
over the full Q range. The wavelength of incident neutron (l)
was 0.52 nm. The solutions were held in a 0.5 cm path length
UV grade quartz sample holder with tight fitting teflon stop-
pers sealed with para-film. In most of the measurements the
surfactant concentration was 100 mM and the sample tem-
perature was 30 ꢂ 0.1 ꢁC. The effect of the concentration on
the microstructure of 12-4-12 MEA bis-cationic surfactant in
aqueous solution at 30 ꢁC was studied in the concentration
range of 50 to 100 mM. The effect of temperature variations
on the microstructure of the micelle was investigated for
12-4-12 MEA surfactant solution of 100 mM concentration
at 30, 40 and 60 ꢁC. The measured scattering intensities of
neutron were corrected for background, empty cell scattering
and sample transmission. The intensities then were normalized
to absolute cross-section units. Thus plots of dS/dO vs. Q were
obtained. The uncertainty in the measured scattering intensi-
ties is estimated to be <10%. The experimental points are fitted
using a nonlinear least square method.
˚
The solvent from reaction mixture was removed under
vacuum and the crude white solid thus obtained was purified
by washing with hexane/ethyl acetate mixture and recrystal-
lized from acetone/methanol mixture for at least three times
to obtain pure compound. The overall yield of the surfactant
was observed to be 70–80%. The identity and purity of the final
product was confirmed by TLC, elemental, FTIR and 1H
NMR analysis.
2.3 Conductance measurements
The critical micellar concentration of the surfactants was mea-
sured through conductance measurements at 30.0 ꢂ 0.1 ꢁC,
using Digital Conductivity Meter (Equip-Tronics, Model
No.-664). The conductance of different solutions was measured
on addition of an aliquot of a known concentration of a sur-
factant solution to a given volume of the thermostated solvent.
The average degree of ionization (aave) of a micelle was taken
as the ratio of the values of dk/dC above and below the cmc.
3. SANS data analysis
In SANS one measures the differential scattering cross-section
per unit volume (dS/dO) as a function of the scattering vector
Q. For a system of charged interacting micelles dS/dO is given
by24–27
2.4 Surface tension measurements
hD
E
i
dS
dO
2
2
2
¼ nðrm ꢀ rsÞ V2 FðQÞ þ hFðQÞi ðSðQÞ ꢀ 1Þ þ B
The reduction in the surface tension of water with respect to
surfactant concentration was measured on du-Nouy tensi-
¨
ometer (Winson and Co. Kolkata, India). The cmc values were
obtained from inflection point in the plots of the surface
tension (g) against log concentration (C).
ð1Þ
where n denotes the number density of the micelles, rm and rs
are, respectively, the scattering length densities of the micelle
and the solvent and V is the volume of the micelle. The aggre-
gation number N of the micelle is related to the micellar
volume V by the relation V ¼ Nv, where v is the volume of
the surfactant monomer. The volume of the 12-s-12 MEA
monomer including the head groups was calculated by using
2.5 Viscosity measurements
Viscometry can be used to obtain gross structural information
of the surfactant solutions.20–23 The absolute viscosity values of
aqueous surfactant solutions were determined as a function of
shear rate, temperature, surfactant concentrations and spacer
chain length, using Brook Field DV-III digital cone and plate
rheometer. The diameter of the cone was 40 mm and the cone
angle was 0.8ꢁ. To ensure the reproducibility of the measure-
ments, great care was exercised when the sample was intro-
duced into the rheometer to avoid shear effect on the
solution. Measurements were repeated (2–3 times) on fresh
solutions and were found to be reproducible within 5%. The
measuring device was equipped with a Peltier Plate tempera-
ture unit for good temperature control over an extended time.
3
˚
an equation v ¼ (947 þ 26.9 s) A , derived from Tanford’s for-
mula. F(Q) is the single particle form factor and S(Q) is the
interparticle structure factor. B is a constant term that repre-
sents the incoherent scattering background, which is mainly
due to hydrogen in the sample. The single particle form factor
F(Q) has been calculated by treating the micelle as a prolate
ellipsoid.28 For such an ellipsoidal micelle
1
Z
h
i
ꢀ
ꢁ
2
F2ðQÞ ¼
FðQ; mÞ dm
ð2Þ
ð3Þ
0
2
3
2
1
Z
2.6 SANS measurements
2
4
5
hFðQÞi ¼
FðQ; mÞdm
Small angle neutron scattering is a powerful technique for
studying the shape and size of surfactant aggregates in aqueous
solutions.24 Small-angle neutron scattering measurements for
12-s-12 MEA bis-cationic surfactants, where s ¼ 4, 6, 8 and
10 were carried out using SANS diffractometer at Dhruva
reactor, Bhabha Atomic Research Center (BARC), Mumbai,
India. The surfactant solutions used for SANS studies were
prepared in D2O, obtained from Heavy Water Division of
BARC. The use of D2O instead of H2O provides very good
contrast between surfactant aggregates and solvent in a SANS
experiment. The sample-to-detector distance was 1.85 m for all
the runs. The spectrometer makes use of a BeO filtered beam
0
3ðsin x ꢀ x cos xÞ
FðQ; mÞ ¼
ð4Þ
ð5Þ
x3
1=2
x ¼ Q½a2ð1 ꢀ m2Þ þ b2m2Þꢃ
where a and b are, respectively, the semiminor and semimajor
axes of the ellipsoidal micelle and m is the cosine of the angle
between the axis of resolution and the wave vector transfer Q.
In general, micellar solutions of ionic surfactants show a
correlation peak in the SANS distribution.27,28 The peak arises
because of the corresponding peak in the interparticle structure
factor S(Q) and indicates the presence of electrostatic inter-
actions between the micelles. S(Q) specifies the correlation
between the centers of different micelles and it is Fourier
transform of the radial distribution function g(r) for the mass
ꢀ1
˚
and has a resolution (DQ/Q) of about 30% at Q ¼ 0.05 A
.
The angular distribution of the scattered neutrons was
recorded using one-dimensional position-sensitive detector
(PSD). The range of wave vector transfer Q of the instrument
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 3 5 0 8 – 3 5 1 4
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centers of the micelles. We have calculated S(Q) as derived by
Hayter and Penfold25,26 from the Ornstein–Zernike equation
and using the rescaled mean spherical approximation. The
micelle is assumed to be a rigid equivalent sphere of diameter
s ¼ 2(a2b)1/3 interacting through a screened Coulomb poten-
tial, which is given by
195 ꢂ 2 ꢁC; C, H, N analysis. Calculated for C34H74N2O2Br2
was C: 58.10; H: 10.61; N: 3.98%. Found C: 58.20; H: 10.75;
1
N: 4.01%. H NMR analysis, d 0.84 ppm (t, 6H 2CH3 alkyl
chain), 1.25–1.40 ppm (br m, 36 H, 18CH2 alkyl chain), 1.75
ppm (m, 4H, 2CH2 alkyl chain), 2.1 ppm (m, 4H, 2CH2 spacer
chain), 3.25 ppm (s, 6H, 2NþCH3), 3.62 ppm (t, 12 H,
2 ꢄ Nþ(CH2)3), 3.82 ppm (t, 4H, 2CH2-OH) and 4.18 ppm
(s, 2H, 2OH).
exp½ꢀkðr ꢀ sÞꢃ
uðrÞ ¼ u0s
; r > s
ð6Þ
r
where k is the Debye–Huckel inverse screening length and is
calculated by
¨
Hexanediyl-1,6-N-N0-bis(dodecyl hyroxyethyl methyl ammo-
nium bromide). Represented as (12-6-12 MEA) M.P.
206 ꢂ 2 ꢁC; C, H, N analysis. Calculated for C36H78N2O2Br2
was C: 59.16; H: 10.75; N: 3.83%. Found C: 59.49; H: 10.86;
ꢂ
ꢃ
1=2
8pNAe2I
103ekBT
k ¼
ð7Þ
1
N: 4.00%. H NMR analysis, d 0.84 ppm (t, 6 H, 2CH3 alkyl
chain), 1.25 ppm (br m, 40 H, 20 CH2 alkyl chain), 1.65
ppm (m, 4 H, 2CH2 spacer chain), 1.95 ppm (m, 4 H, 2CH2
spacer chain), 3.25 ppm (s, 6 H, 2NþCH3), 3.59 ppm (t, 12
H, 2Nþ(CH2)3), 3.75 ppm (t, 4 H, 2CH2-OH), 4.15 ppm (s, 2
H, 2OH).
where I is the ionic strength of the micellar solution and is
determined from the CMC and dissociated counterions of
the micelles. The fractional charge a ( ¼ Z/N, where Z is the
micellar charge) is the charge per surfactant molecule in the
micelle. The contact potential u0 is given by
Z2e2
Octanediyl-1,8-N-N0-bis(dodecyl hydroxyethyl methyl ammo-
nium bromide). Represented as (12-8-12 MEA)M.P.193 ꢂ 2 ꢁC;
C, H, N analysis. Calculated for C38H82N2O2Br2 was C: 60.14;
u0 ¼
ð8Þ
2
pee0sð2 þ ksÞ
where e is the dielectric constant of the solvent medium, e0 is
the permittivity of free space and e is the electronic charge.
Although micelles may produce polydisperse systems, we
have assumed them as monodisperse for the simplicity of the
calculation and to limit the number of unknown parameters
in the analysis. The dimensions of the micelle, aggregation
number and the fractional charge have been determined from
the analysis. The semi-major axis (b), semi-minor axis (a) and
the fractional charge (a) are the parameters optimized by
means of a nonlinear least squre fitting program while analyz-
ing the SANS data. The aggregation number is calculated by
using the relation N ¼ 4pa2b/3v, where v is the volume of
the surfactant monomer.
1
H: 10.89; N: 3.69%. Found C: 60.32; H: 11.00; N: 3.73%. H
NMR analysis, d 0.81 ppm (t, 6 H, 2CH3 alkyl chain), 1.17
ppm (br m, 40 H, 20CH2 alkyl chain), 1.35 ppm (t, 8 H,
4CH2 spacer chain), 1.74 ppm (m, 4 H, 2CH2 spacer chain),
3.20 ppm (s, 6 H, 2NþCH3), 3.59 ppm (t, 12 H, 2Nþ
(CH2)3), 3.93 ppm (t, 4 H, 2CH2-OH), 3.99 ppm (s, 2 H, 2OH).
Decanediyl-1,10-N-N0-bis(dodecyl
hydroxyethyl
methyl
ammonium bromide). Represented as (12-10-12 MEA) M.P.
203 ꢂ 2 ꢁC; C, H, N analysis. Calculated for C40H86N2O2Br2
was C: 61.05; H: 11.02; N: 3.56%. Found C: 61.25; H: 11.10;
1
N: 3.60%. H NMR analysis, d 0.81 ppm (t, 6 H 2CH3 alkyl
chain), 1.30 ppm (m, 40 H, 20CH2 alkyl chain),1.35 ppm (t,
8 H, 4CH2 spacer chain), 1.70 ppm (m, 8 H, 4CH2 spacer
chain), 3.24 ppm (s, 6 H, 2NþCH3), 3.67 ppm (t, 12 H,
2 ꢄ Nþ(CH2)3), 4.05 ppm (t, 4 H, 2CH2-OH), 5.12 ppm (s, 2
H, 2OH).
4. Results and discussion
4.1 Characterization
The structure and purity of synthesized series of bis-cationic
surfactants was confirmed through FTIR, elemental, and H
NMR analysis.
1
The IR spectra of the surfactants were recorded in KBr pel-
lets using Perkin Elmer FTIR spectrophotometer RX, of
a resolution 2 cmꢀ1. The absorption bands were observed at
3401–3656 cmꢀ1 (OH stretching), 2916 cmꢀ1 (CH stretching),
1108 cmꢀ1 (CN stretching), 1084 cmꢀ1 (CO stretching) and
720 cmꢀ1 (CH rocking of long alkyl chain).
4.2 Critical micellar concentration
The cmc values for 12-s-12 MEA novel series of bis-cationic
surfactants were determined by surface tension and conduc-
tance measurements. The cmc values obtained from both the
techniques show a similar trend with spacer chain length and
are given in Table 1. It is interesting to note that cmc data
of the 12-s-12 MEA bis-cationic surfactants are observed to
be 10–100 times lower than the conventional 12-s-12 DMA
bis-cationic surfactants.9–10 For 12-s-12 MEA, hydrogen
bonding with water can take place through oxygen atom of
–C2H4OH groups. This is likely to provide additional hydra-
tion at the head group level resulting in screening of Coulom-
bic forces of repulsion between charged heads and helping
Elemental analysis and 1H NMR spectra of products in
CDCl3 were determined using Perkin Elmer Series II elemental
analyzer and 300 MHZ Bruker NMR spectrophotometer,
respectively.
Butanediyl-1,4,-N,N0-bis(dodecyle
ammonium bromide). Represented as (12-4-12 MEA) M.P.
hydroxyethyl
methyl
Table 1 Critical micellar concentration (cmc), average degree of ionization (aave) and Gibb’s free energy of micellization (DGm) for 12-s-12 MEA
novel series of bis-cationic surfactants, at 30 ꢁCa
Spacer length
(s)
cmc by conductometry
C ꢄ 10ꢀ5
cmc by tensiometry
C ꢄ 10ꢀ5
(DGm)
M
M
(aave
)
kJ molꢀ1
4
6
3.49 ꢂ 0.10 (117 ꢂ 10)
4.49 ꢂ 0.10 (103 ꢂ 10)
3.99 ꢂ 0.15 (83 ꢂ 10)
3.12 ꢂ 0.10 (63 ꢂ 10)
2.78 ꢂ 0.10
4.33 ꢂ 0.10
3.18 ꢂ 0.10
2.59 ꢂ 0.10
0.26 (0.31)
0.31 (0.33)
0.36 (0.45)
0.39 (0.54)
ꢀ32.06
ꢀ30.01
ꢀ29.01
ꢀ29.01
8
10
The values given in brackets are of conventional 12-s-12 DMA bis-cationic surfactants with –Nþ(CH3)2 head groups, taken from ref. 8.
a
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
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12-s-12 MEA surfactants to form aggregates at a lower
concentration than those of 12-s-12 DMA surfactants.
Fig. 1 shows that cmc goes on increasing up to the spacer
chain length of 6 carbon atoms and thereafter decreases with
increase in the spacer length. Similar behavior has also been
reported by Zana and coworkers.8,9 The variation in cmc
values with the spacer chain length can be attributed to the
conformational changes taking place at the polymethylene
spacer. For the spacer chain s ꢅ 6, spacer remains in a fully
extended conformation making it somewhat difficult to be
located at micelle-water interface, whereas for longer spacers
s > 6, spacer tries to form a loop in the hydrophobic core of
the micelle disrupting the micelle geometry. This statement is
well supported by theoretically as well as experimentally calcu-
lated distances (d) between the charged heads of the surfactant
molecule (Table 3). The observed increasing difference between
the theoretical and experimental values of the equilibrium dis-
tance between charged heads shows increased tendency of for-
mation of a loop due to the hydrophobic nature of a spacer.
The overall average degree of ionization (aave) of micelle was
observed to be less than that of conventional bis-cationic sur-
factants and increases with increasing the spacer chain length,
as shown in Table 1. The Gibb’s free energy of micellization
(DGm) was obtained from conductance measurement and is
given in Table 1. The observed more negative DGm value for
12-4-12 MEA shows more favorable micellization than the
surfactant with spacer length of 6, 8 and 10.
Fig. 2 Effect of variation of shear rate on absolute viscosity of
12-s-12 MEA novel series of bis-cationic surfactants at 100 mM and
30 ꢁC. s ¼ 4 (X), 6 (N), 8 (K),10 (˙).
4.4 SANS and micellar solutions
In the SANS experiment, a beam of neutrons is directed upon
the sample under examination and the neutron scattering
intensities in different directions are measured. Since the neu-
trons are scattered by the nuclei in the sample, even isotopes
of the same element can differ in their scattering power. Thus,
by taking aggregates in D2O rather than in H2O, the scattering
densities of various regions can be obtained, since deuterons
and protons differ widely in their respective scattering capaci-
ties. SANS is one of the powerful techniques for getting the
structural information especially on micellar aggregates.24
4.3 Viscosity of micellar solutions
Table 2 shows the viscosity data for 100 mM, 12-s-12 MEA
bis-cationic surfactants with different spacer lengths. Surfac-
tant with spacer length 4 showed very high absolute viscosity
in comparison with the surfactant with spacer lengths 6, 8
and 10, indicating the formation of larger aggregates in aqu-
eous solution. Fig. 2 shows the effect of the shear rate on visc-
osity behavior as a function of spacer length, and it was
observed that all the present surfactants show essentially New-
tonian behavior. Table 2, shows the viscosity at various con-
centrations and temperatures for surfactant with spacer
length of 4 carbon atoms. As expected, the viscosity decreases
with decreasing the surfactant concentration or increasing the
temperature.
Effect of spacer chain length on surfactant aggregates. To
understand the effect of the head group polarity and spacer
chain length on the microstructure of surfactant aggregates
in aqueous solution, SANS measurements were carried out
at 100 mM solution of 12-s-12 MEA surfactant with spacers
(s) ¼ 4, 6, 8 and 10. SANS distributions (Fig. 3) show well
defined correlation peaks irrespective of the spacer chain
length, due to a peak in the inter-micellar structure factor
S(Q). The correlation peaks appears at around Qmax ꢆ 2p/D,
where D is the average distance between the micelles. Since
Table 2 Effect of spacer lengths, concentrations and temperatures on
the absolute viscosity of 12-s-12 MEA novel series of bis-cationic
surfactants in aqueous solutions
Parameter
Z/m PaS
Effect of the spacer chain length(s) on the absolute viscosity of
12-s-12 MEA bis-cationic surfactants, at 100 mM and 30 ꢁC
s ¼ 4
4.60 ꢂ 0.20
s ¼ 6
1.42 ꢂ 0.07
1.47 ꢂ 0.07
1.72 ꢂ 0.09
s ¼ 8
s ¼ 10
Effect of the concentrations on the absolute viscosity for
12-4-12 MEA bis-cationic surfactant, at 30 ꢁC
100 mM
75 mM
50 mM
25 mM
4.60 ꢂ 0.20
1.70 ꢂ 0.09
1.22 ꢂ 0.06
1.07 ꢂ 0.05
Effect of the temperature on the absolute viscosity for 12-4-12
MEA bis-cationic surfactant, at 100 mM
30 ꢁC
35 ꢁC
40 ꢁC
45 ꢁC
4.60 ꢂ 0.20
Fig. 1 Effect of the spacer chain length on the critical micellar con-
centration of 12-s-12 MEA novel series of bis-cationic surfactants at
30 ꢁC. Variation in the cmc values as a function of spacers chain
length, by tensiometry (N) and by conductometry (X).
2.44 ꢂ 0.12
1.92 ꢂ 0.10
1.60 ꢂ 0.08
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 3 5 0 8 – 3 5 1 4
3511

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interface, as discussed earlier. The observed increase in the
equilibrium distance (d) between the charged heads with spacer
length increases the spontaneous packing curvature restricting
the growth of micelle. The surfactant with shorter spacer of 4
carbon atoms shows larger aggregates in aqueous solution and
high viscosity (Table 2 and 3). Zana et al.9,11 have reported
spherical micelles for 12-s-12 DMA bis-cationic surfactant
with spacer lengths 4 and 6 whereas in the present study the
surfactant with change in head polarity 12-s-12 MEA with
spacer lengths 4 and 6 shows formation of more elongated
ellipsoidal micelles, indicating that the increase in head polar-
ity of surfactant increases the aggregation tendency. The
change in the head group polarity and spacer length of the
bis-cationic surfactant results in a significant change. We have
observed the b/a ratio to decrease from 4.23 to 2.82 with
increasing the spacer length from 4 to 10 indicating that the
spacer length 4 shows higher tendency of aggregation than
larger spacers 6, 8 and 10.
Effect of the concentration on surfactant aggregates. The
SANS data from 12-4-12 MEA bis-cationic surfactants at dif-
ferent concentrations and at 30 ꢁC is illustrated in Fig. 4. The
concentration range examined was 50 to 100 mM. All distribu-
tions were observed to shows well defined correlation peaks
because of the intermicellar structure factor S(Q). The correla-
tion peaks appear at around Qmax ꢆ 2p/D. It was observed
that with increased concentration, there is little shift in the
peak position towards higher Q values with overall increase
in the peak intensity. With increase in the concentration, the
average distance between the micelles decreases indicating an
increase in the number density of micelles. The increase in con-
centration of surfactant helps aggregation growth along the
direction of semi-major axis (b) to form more prolate ellipsoi-
dal micelle (b/a ¼ 4.23). The effective fractional charge was
observed to decrease with increase in the concentration of sur-
factants from 50 to 100 mM. This can be attributed to the
increase in aggregation numbers, which increases the surface
charge density of the micelle and helps to increase the associa-
tion of counterions (Table 4).
Fig. 3 Effect of spacer chain length on the SANS distribution for 12-
s-12 MEA novel series of bis-cationic surfactants at 100 mM concen-
trations and 30 ꢁC. The solid line shows the theoretical fits and the
symbols are experimentally determined values. s ¼ 4 (X), 6 (N), 8
(K), 10 (˙).
Qmax was found to vary with spacers, one can conclude that
the number density (n) of micelles is not the same in the above
sample even when they have identical surfactant concentra-
tion. It is observed from Fig. 3 that the peak positions shift
towards higher Q values with increase in the spacer chain
length from 4 to 6. However, the shift in Q is small with the
change in spacer chain length from 6 to 10. The observed
SANS data were analyzed by taking aggregation number
(N), effective fractional charge (a) and semi-minor axis (a) as
fitting parameters and semi-major axis (b) is calculated by
using equation b ¼ (3Nn/4pa2). The extracted micellar para-
meter values are given in Table 3. The equilibrium distances
between the charged heads (d) calculated for the bis-cationic
surfactants with respect to spacer length are given in Table
3. From the results in Table 3, the aggregation number (N)
was observed to decrease significantly and fractional charge
per monomer was observed to increase with spacer length.
The observed large variation in values for spacer length 4 to
6 rather than for the spacer length 6 to 10, can be attributed
to the conformational changes of spacer at micelle/water
Effect of temperature on surfactant aggregates. Fig. 5 shows
the SANS distribution for 100 mM of 12-4-12 MEA bis-catio-
nic surfactant at 30, 45, and 60 ꢁC. It is observed that the peak
position in SANS shifted to the higher Q values with overall
decrease in the peak intensity and increase in the broadness
Table 3 Effect of the spacer chain length (s) on the micellar parameters of 12-s-12 MEA novel series of bis-cationic surfactants of 100 mM at
30 ꢁCa
Equilibrium distance
Spacer length Aggregation number Effective monomer charge Semi-minor axis, Semi-major axis, between charged heads
˚
a (A)
˚
b (A)
˚
(d) A
(s)
(N)
(a)
b/a
4
6
108 ꢂ 8
56 ꢂ 5
55 ꢂ 5
52 ꢂ 4
0.11 ꢂ 0.01
0.19 ꢂ 0.01
0.18 ꢂ 0.01
0.24 ꢂ 0.01
18.6 ꢂ 0.5
16.4 ꢂ 0.5
17.2 ꢂ 0.5
17.6 ꢂ 0.5
78.6 ꢂ 1.8
55.3 ꢂ 1.4
51.3 ꢂ 1.3
48.7 ꢂ 1.3
7.25 (7.56)
8.23 (10.64)
8.37 (13.74)
8.59 (16.80)
4.23 ꢂ 0.05
3.32 ꢂ 0.05
3.01 ꢂ 0.05
2.82 ꢂ 0.05
8
10
a
The values given in brackets are the theoretically calculated distances between the charged heads (d) of the surfactant molecule.
Table 4 Effect of the variation in concentrations on the micellar parameters of 12-4-12 MEA bis-cationic surfactant at 30 ꢁC
Concentrations
(mM)
Aggregation number
(N)
Effective monomer charge
(a)
Semi-minor axis,
˚
a (A)
Semi-major axis,
˚
b (A)
b/a
100
75
108 ꢂ 8
81 ꢂ 6
63 ꢂ 5
0.11 ꢂ 0.01
0.14 ꢂ 0.01
0.17 ꢂ 0.01
18.6 ꢂ 0.5
18.3 ꢂ 0.5
17.7 ꢂ 0.5
78.6 ꢂ 1.8
60.8 ꢂ 1.5
50.4 ꢂ 1.4
4.23 ꢂ 0.05
3.34 ꢂ 0.05
2.78 ꢂ 0.06
50
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
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Table 5 Effect of temperatures on micellar parameters of 12-4-12
MEA bis-cationic surfactant at 100 mM concentration
Effective Semi-
Aggregation monomer minor
Semi-
major
axis,
Temperature number
T (ꢁC)
charge
(a)
axis,
˚
a (A)
˚
(N)
b (A)
b/a
30
45
60
108 ꢂ 8
71 ꢂ 4
57 ꢂ 5
0.11 ꢂ 0.01 18.6 ꢂ 0.5 78.6 ꢂ 1.8 4.23 ꢂ 0.05
0.15 ꢂ 0.01 18.2 ꢂ 0.5 53.8 ꢂ 1.4 3.02 ꢂ 0.05
0.21 ꢂ 0.01 17.1 ꢂ 0.5 49.0 ꢂ 1.4 2.87 ꢂ 0.06
connected through polymethylene spacers of 4, 6, 8 and 10
at the head group level, has been synthesized and character-
ized. To the best of our knowledge, this is a first report on
the synthesis of surfactants with quaternary amines with
ethanolic groups. The presence of ethanolic groups at the head
groups increases the head polarity of the surfactants, resulting
in reduction of the cmc, average degree of ionization (aave) and
higher tendency of surfactants to aggregate, as compared to
conventional 12-s-12 DMA bis-cationic surfactants containing
–Nþ(CH3)2 head groups. The cmc of the surfactant increases
up to spacer length 6 and then it decreases with further
increase in the spacer length. SANS studies show that the size
of micelles for spacer length 4 is much larger than those of
larger spacer lengths. This is well supported by viscosity mea-
surements as the surfactant with spacer length 4 shows the very
high viscosity than those of spacer 6, 8 and 10. The flow curves
of all the 12-s-12 MEA surfactants show Newtonian behavior.
It has also been found that the size of micelles decreases when
the concentration is decreased or the temperature is increased.
Fig. 4 Effect of variation of concentration on SANS distribution for
12-4-12 MEA bis-cationic surfactant at 30 ꢁC. The solid line shows the
theoretical fits and the symbols are experimentally determined values.
100 mM (X), 75 mM (N), 50 mM (K).
of peak, when temperature increased. Similarly, the correlation
peak Qmax was observed to change with temperature, indicat-
ing the increased number density of micelle changes with tem-
perature. The micellar parameters in these systems are given in
the Table 5. The increase in temperatures enhances the degree
of dissociation of counterions and affects the magnitude of
electrostatic repulsion. This results in decrease of the aggrega-
tion number (N), upon increase of the temperature. The effec-
tive fractional charge was observed to increase with increasing
the temperature. Since the smaller effective charge indicates a
more ellipsoidal morphology, increasing the temperature
appears to induce somewhat ellipsoidal to sphere transition
for 12-4-12 MEA. This notation is also supported by the con-
comitant decrease in b/a ca. 4.23 to ca. 2.87 upon increase in
temperature.
Acknowledgements
We gratefully acknowledge the financial support for this work
from the Department of Science and Technology, New Delhi
and IUC-DAEF Mumbai Center. Initial help extended in the
synthesis of surfactants by Dr Alpesh Patel is gratefully
acknowledged.
5. Conclusion
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