Synthesis and solution properties of cationic gemini surfactants with long unsaturated tails

Article abstract of DOI:10.1016/j.susc.2010.03.033

Gemini surfactants 22:1-s-22:1, where s = 2 and 6 methylene groups and 22:1 refer to erucyl dimethylammonium bromide chains, together with the monomeric surfactant erucyl bis-(hydroxyethyl) methylammonium bromide (EHAB) which has the same long unsaturated tails with gemini surfactants, were synthesized and characterized and solution properties of these surfactants were investigated using surface tension, conductivity and viscosity measurement. It has been found that the critical micelle concentration (cmc) values of 22:1-2-22:1 and 22:1-6-22:1 are 15.4 and 8.3 μM, respectively, less than the cmc value of EHAB (38 μM). On the other hand, the surface tension of 22:1-2-22:1 and 22:1-6-22:1 at cmc are 40.9 and 42.4, respectively, greater than the surface tension of EHAB (30.9 at cmc). Both 22:1-2-22:1 and 22:1-6-22:1 have nearly the same value of a₀ (the minimum head group areas per surfactant molecule at the aqueous solution/air interface), which is almost the half value of a₀ corresponding to EHAB. On the other hand, the ionization degree α of micelles of both 22:1-2-22:1 and 22:1-6-22:1 is approximately twice the value of α corresponding to micelles of EHAB. Though 22:1-2-22:1 has more similarity with 22:1-6-22:1 rather than EHAB as presented above, 22:1-2-22:1 in water cannot enhance the viscosity of the solution significantly in the presence of salt. In contrast, both 22:1-6-22:1 and EHAB in water can give rise to highly viscoelastic or gel-like solutions even at the high temperature in the presence of salt. In particular, 22:1-6-22:1 has proved to be a more efficient candidate for high temperature rheology-control applications than EHAB. The effect of salt upon the viscosity of 22:1-6-22:1 in aqueous solution is significant. The most proper ratio of 22:1-6-22:1/NaSal for enhancing the viscosity of solution has been proved to be 0.7.

Full text of DOI:10.1016/j.susc.2010.03.033

Surface Science 604 (2010) 1173–1178
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Surface Science
journal homepage: www.elsevier.com/ locate/susc
Synthesis and solution properties of cationic gemini surfactants with long
unsaturated tails
⁎
Huazhen Li, Haiyang Yang , Yunfei Yan, Qin Wang, Pingsheng He
CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui, 230026, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Gemini surfactants 22:1-s-22:1, where s=2 and
6 methylene groups and 22:1 refer to erucyl
Received 15 January 2010
Accepted 26 March 2010
Available online 7 April 2010
dimethylammonium bromide chains, together with the monomeric surfactant erucyl bis-(hydroxyethyl)
methylammonium bromide (EHAB) which has the same long unsaturated tails with gemini surfactants, were
synthesized and characterized and solution properties of these surfactants were investigated using surface
tension, conductivity and viscosity measurement. It has been found that the critical micelle concentration
(cmc) values of 22:1-2-22:1 and 22:1-6-22:1 are 15.4 and 8.3 μM, respectively, less than the cmc value of
EHAB (38 μM). On the other hand, the surface tension of 22:1-2-22:1 and 22:1-6-22:1 at cmc are 40.9 and
42.4, respectively, greater than the surface tension of EHAB (30.9 at cmc). Both 22:1-2-22:1 and 22:1-6-22:1
have nearly the same value of a0 (the minimum head group areas per surfactant molecule at the aqueous
solution/air interface), which is almost the half value of a0 corresponding to EHAB. On the other hand, the
ionization degree α of micelles of both 22:1-2-22:1 and 22:1-6-22:1 is approximately twice the value of α
corresponding to micelles of EHAB. Though 22:1-2-22:1 has more similarity with 22:1-6-22:1 rather than
EHAB as presented above, 22:1-2-22:1 in water cannot enhance the viscosity of the solution significantly in
the presence of salt. In contrast, both 22:1-6-22:1 and EHAB in water can give rise to highly viscoelastic or
gel-like solutions even at the high temperature in the presence of salt. In particular, 22:1-6-22:1 has proved
to be a more efficient candidate for high temperature rheology-control applications than EHAB. The effect of
salt upon the viscosity of 22:1-6-22:1 in aqueous solution is significant. The most proper ratio of 22:1-6-
22:1/NaSal for enhancing the viscosity of solution has been proved to be 0.7.
Keywords:
Surface structure
Morphology
Roughness and topography
Surface tension
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
sively studied gemini surfactant is 12-s-12. The electron micrographs
of a 12-2-12 solution have shown that 12-2-12 can form entangled
Surfactants containing two hydrophilic and two hydrophobic
groups (so-called gemini surfactants) have been the focus of
considerable research interest since the early 1990s. Several compre-
hensive reviews have documented the solution behavior of gemini
compounds [1–3]. The most interesting properties of gemini surfac-
tants are doubtless the much lower cmc values, the stronger efficiency
at decreasing the surface tension of water [4,5] and the interfacial
tension of oil/solution interfaces [6,7], and the stronger adsorption at
solid/solution interfaces than for the corresponding conventional
surfactants [8,9]. In particular, gemini surfactants are capable of giving
rise to linear and branched worm-like micelles as well as closed ring
micelles at fairly low concentration. These structures bring about
interesting rheological properties to the solution. The gemini
surfactants can be generally referred to as m-s-m (trimer surfactant
[1] is not included here), where m and s are the number of carbon
atoms of the alkyl chain and the spacer, respectively. The most inten-
worm-like micelles which may be several micrometers long [10–12].
In contrast, the corresponding monomeric surfactant dodecyltri-
methylammonium bromide (DTAB) remains spherical micelles even
at fairly high concentration [13]. With the increasing of s, the struc-
tures of the 12-s-12 series in solution change from elongated micelles
to spherical micelles and then to vesicles [11]. The structures of the
16-s-16 series in solution are much more complicated. In fact, the
various structures such as worm-like micelles, vesicles and spherical
micelles have been found to coexist at a certain concentration. With
the increasing of s, the structures of the 16-s-16 series in the solution
change from vesicles or elongated micelles to elongated micelles and
then to spherical micelles [14–16]. Recently, the gemini surfactants
with m greater than 16 have been investigated. It has been found that
the structures and therefore the rheological properties of the series m-
s-m are greatly associated with both m and s. The viscosity mea-
surement has shown that both 22-4-22 and 18-3-18 are more efficient
in enhancing the viscosity of the solution and 22-4-22 in particular.
Comparatively, both 22-3-22 and 18-2-18 are less efficient in
enhancing the viscosity of the solution. Of peculiar interest is that
the structure of 18-4-18 cannot exist steady in solution. In fact, the
⁎
Corresponding author.
E-mail address: yhy@ustc.edu.cn (H. Yang).
0039-6028/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2010.03.033

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H. Li et al. / Surface Science 604 (2010) 1173–1178
structure of 18-4-18 changes from worm-like micelles to spherical
micelles simply by aging solution with two hours [17].
Though the gemini surfactants with the saturated alkyl chains
have been studied in detail, less attention has been devoted to the
gemini surfactants with the long unsaturated chains [18]. The
limitation of using monomer surfactants with the long saturated
chains is that they usually have a high Krafft temperature. The Krafft
temperatures of the corresponding gemini surfactants are even
greater than the monomer surfactants. For example, the Krafft
temperature for 16-2-16 is 45 °C whereas the Krafft temperature of
the corresponding monomer surfactants cetyltrimethy lammonium
bromide (CTAB) is 24 °C [19]. The Krafft temperatures for 22-s-22
with the long saturated chains, as presented by Han et al., can be even
as high as 80–90 °C[17]. In contrast, the Krafft temperature for the
monomer surfactants with the long unsaturated chains is usually very
low (b0 °C), due to the hydrophilicity of the cis-double bond of the
tails [20]. Another great advantage for the monomer surfactants with
the long unsaturated chains such as erucyl bis-(hydroxyethyl)
methylammonium chloride (EHAC) and erucyl trimethylammonium
chloride (ETAC) is their excellent efficiency in enhancing the viscosity
of solution [21]. In fact, EHAC in the presence of salt can impart
viscoelastic properties to solution even at fairly low concentration and
at the high temperature. The viscoelastic fluid made of EHAC, known
as the clean fracturing fluid [22], can be used efficiently for oil and gas
recovery. The applications of the anionic monomer surfactants with
the long unsaturated chains such as sodium oleoylsarcosine (SOS) for
oil and gas recovery have also been reported [23].
The salt dissolved in solution acts as a role to screening the repulsive
electrostatic interaction between headgroups of surfactants in solution.
If the monomer surfactants are covalently linked via a spacer to form
gemini surfactants, the repulsive electrostatic interaction between
headgroups of surfactants will decrease significantly. In other word,
the covalently linked spacer is more efficiency in decreasing the
repulsive electrostatic interaction between headgroups of surfactants
than salt. As being pointed out by Candau et al., DTAB remains spherical
micelles even at fairly high concentration. However, the corresponding
gemini surfactants 12-s-12 can form worm-like micelles which may be
several micrometers long [13]. In the present study the surfactants with
the long unsaturated tails are covalently linked via a spacer to form
gemini surfactants. Our experimental results have shown that gemini
surfactants with the proper number s of the spacer (s=6) are more
efficient in imparting viscoelastic properties to solution than the
corresponding monomer surfactants with the long unsaturated tails.
The other interesting properties of these gemini surfactants are also
presented and compared with the corresponding conventional mono-
mer surfactants.
Scheme 1. General structure of the 22:1-s-22:1 gemini surfactants.
72 h. Upon cooling, a white solid was recovered by filtration and it was
re-crystallized three times from a mixture of ethyl alcohol and ethyl
acetate. The purified gemini salts were normally stored at −20 °C.
The EHAB surfactant was synthesized by a reaction of 1 equivalent
erucyl bromide (0.1 mol) with 2 equivalents of N,N,N-bis(2-hydro-
xyethyl) methylamine (0.2 mol) in 50 mL ethyl acetate under reflux for
24 h. The crude product was re-crystallized three times from ethyl acetate.
The structures of all synthesized compounds were confirmed by 1H
NMR spectroscopy (CDCl3, 400 MHz Bruker) and the purity was
verified by surface tension measurements. The gemini compounds and
erucyl bromide were found to be pure, there being no evidence of
starting materials or mono-quaternary intermediate in the proton
NMR spectra. For example, chemical shift and the integrated spectra of
22:1-6-22:1 gave the expected proton contents: 5.39 (t, 4H, –HC ),
3.70 (t, 4H, spacer N–CH2), 3.48 (m, 4H, alkyl chain N–CH2), 3.38 (s,
12H, headgroup N–CH3), 2.00 (m, 12H, spacer β-CH2 and allylic), 1.72
(m, 8H, central CH2 in spacer and alkyl chain β-CH2 to N), 1.28–1.38 (m,
60H, other CH2 in alkyl chain), 0.88 (t, 6H, alkyl–CH3). Elemental
analysis: calculated for C54H110N2Br2 (%): C, 68.47, H, 11.70, N, 2.96.
Found: C, 68.22, H, 11.64, N, 2.94. Chemical shift and the integrated
spectra of EHAB gave the expected proton contents:5.36 (t, 2H, –HC ),
4.48 (s, 2H, –OH), 4.11 (m, 4H, –OCH2), 3.74 (t, 4H, O–C–CH2–N), 3.54
(t, 2H,–CH2 in the alkyl chain to N), 3.33 (s, 3H, N–CH3), 1.96 (m, 4H, C–
CH2), 1.75 (m, 2H, alkyl chain –CH2 to N), 1.24–1.34 (m, 30H, other CH2
in alkyl chain), 0.88 (t, 3H, alkyl–CH3).
2.2. Methods
Surface tension was determined using a Krüss (Model K20T) tension
meter, applying the Wilhelmy plate technique. Experimental tempera-
tures were maintained at 25 0.05 °C using a Haake (Model F3) cir-
culating water bath. Solutions of all surfactants were prepared in a
0.01 N KBr solution, stirred and equilibrated overnight.
The electric conductivity of the gemini surfactant solution was
measured by Model DDS-IIA conductometer. Gemini surfactant was
carefully weighed and put into double distilled water with a concen-
tration of 0.15%. Experimental temperatures were maintained at 25 °C
using a circulating water bath. The apparent cmc values of the sur-
factants have been taken as the concentration corresponding to the
intercept of the two linear segments of the k vs. C plots.
Rheology of gemini surfactant solutions was measured by Haake Mars
II stress controlled rheometer. A couette geometry with a cup of 40 mm
diameter and a bob of 38 mm diameter and 80 mm length was used. The
cell was heated by a reservoir of fluid circulating from Phoenix high
temperature bath. The sample was equilibrated for at least 20 min at each
temperature prior to conducting experiments. For the steady-shear
experiments, an equilibration time of 120 s was given at each shear stress.
2. Experimental section
2.1. Materials
Erucic acid, a commercial product from Sipo Oil Chemical Corpora-
tion (Mianyang, China), was re-crystallized three times in methyl
alcohol at −16 °C [24] and then converted to methyl erucate according
to a procedure previously described [25]. Having purified by fraction-
ation in the Todd column, methyl erucate was converted to erucyl
alcohol in 90% yield with sodium and amyl alcohol [18]. Erucyl alcohol
was additionally purified by low temperature crystallization twice
before use. The purity of erucyl alcohol was checked by GC and the
content of erucyl alcohol was found to be about 98.8%.
Erucyl bromide was synthesized and purified according to a
procedure presented by Zana and Talmon [10]. The 22:1-s-22:1
surfactants, as indicated in Scheme 1, were synthesized by a reaction
of 1 equivalent of N, N, N′, N′-tetramethylalkylenediamine (Aldrich)
(0.1 mol) with 2.4 equivalents of 1-bromo-cis-13-dococene (0.24 mol)
in an ethyl alcohol–ethyl acetate mixture (1:9, 500 mL) under reflux for
3. Results and discussion
3.1. Surface tension
Fig. 1 shows a plot of surface tension (γ) vs. log surfactant
concentration for the 22:1-2-22:1, 22:1-6-22:1 and EHAB at 25 °C

H. Li et al. / Surface Science 604 (2010) 1173–1178
1175
where NA is the Avogadro number. Γ can be calculated from the Gibbs
adsorption isotherm relation
ꢀ
ꢁ
1
dγ
Γ = −
ð2Þ
2:30nRT d logC
T
where the parameters R and T have their usual meaning and n is the
number of species at the interface whose concentration changes with
surfactant concentration. It has been set at 2 or 3 in aqueous media by
investigators. In the presence of a swamping electrolyte it has a value
of one and there is no change in the ionic strength of the aqueous
solution with increasing surfactant concentration, thus minimizing
any change in the effective charge on the headgroups [27]. The results
are shown in Table 1. It is already known that the cross-sectional areas
of an aliphatic chain oriented perpendicular to the interface and of a –
CH2– group lying flat in the interface are approximately about 0.2 and
0.07 nm2, respectively [18]. Considering the linkage of the chain has
little flexibility and lies flat with a fairly linear conformation at the air-
solution interface. We believe that a0 of 22:1-6-22:1 should be
0.28 nm2 greater than that of 22:1-2-22:1. This suggests that a0
should calculate assuming n=3 in Eq. (2). From Table 1 it can be seen
that both 22:1-2-22:1 and 22:1-6-22:1 have nearly the same value of
a0, which is almost the half value of a0 corresponding monomeric
surfactants. This similar phenomenon is also reported for gemini
surfactants with the saturated tails. For instance, the value of a0 for
12-3-12 is less than that for DTAB [5]. The reason is that the
electrostatic repulsion between the headgroups is suppressed by the
spacer of the gemini surfactants. This indicates that the strong
correlation of orientation between the gemini surfactants occurs in
the air–water interface. As a result, the a0 of either 22:1-6-22:1 or
22:1-2-22:1 is less than that of EHAB.
Fig. 1. Surface tension plots for EHAB, 22:1-2-22:1 and 22:1-6-22:1 gemini surfactants.
with 0.01 N KBr in the subphase. Table 1 lists the cmc, γcmc (surface
tension at cmc), and a0 (minimum area per surfactant molecule at the
aqueous solution/air interface) properties of these surfactants
obtained from these plots. It is already known that the most
interesting properties of gemini surfactants are doubtless the much
lower cmc values and the stronger efficiency at decreasing the
surface tension of water. In fact, the cmc of the gemini surfactants
may be lower than those of the corresponding monomeric surfac-
tants by one order of magnitude and more. For instance, the cmc of
12-2-12 and DTAB are 0.81 mM and 15 mM, respectively [26]. From
Fig. 1 and Table 1 it can be seen that the cmc values of both 22:1-2-
22:1 and 22:1-6-22:1 are less than that of the corresponding
monomeric suractant EHAB, but not as significant as imagined. The
possible interpretation is that the monomeric surfactants with the
long unsaturated chains such as EHAB already have a strong
tendency to associate in solution. As a result, even converting these
monomeric surfactants to the gemini surfactants by synthesis, the
increase of the efficiency of association for the corresponding gemini
surfactants in the solution is not significant. Viscosity measurement
also verifies this assumption as indicated in the next subsection. Of
peculiar interest is that the surface tension of both 22:1-2-22:1 and
22:1-6-22:1 at cmc is greater than that of EHAB, i.e., the gemini
surfactants with the long unsaturated tails are less efficient at
decreasing the surface tension of water than corresponding mono-
meric surfactants. The minimum head group areas per surfactant
molecule at the aqueous solution/air interface, a0, can be determined
from the surface excess concentration, Γ, according to the equation
3.2. Conductivity
Fig. 2 shows the specific conductance as a function of surfactant
concentration for EHAB, the 22:1-2-22:1 and 22:1-6-22:1 at 25 °C. The
cmc value can be determined from the intercept of the two linear
segments of the phk vs. C plots. The result is shown in Table 1. From
Table 1 it can be seen that the cmc value obtained by specific con-
ductance is systematic larger than that obtained by surface tension,
similar to the results presented by Li et al. [18]. A possible interpretation
is that, using surface tension measurement to determine cmc, a 0.01 N
KBr is dissolved in surfactant solution. The dissolved KBr will encourage
a0 = ðNAΓÞ−1
ð1Þ
Table 1
Surface tension properties for the 22:1-s-22:1 and EHAB surfactants.
Surfactants STa
SCb
γcmc
a0
(nm2 molecule−1
a0
(nm2molecule−1
)
α
c
d
(μM) (μM) (mNm−1
)
)
22:1-2-22:1 15.4 23.1 40.9
22:6-2-22:1 8.3 33.8 42.4
0.22
0.25
0.41
0.66
0.75
1.23
0.79
0.83
0.46
EHAB
38.0 52.0 30.9
a
Obtained using surface tension.
b
c
Obtained using specific conductance.
Calculated assuming n=1 in Eq. 2.
Calculated assuming n=3 in Eq. 2.
Fig. 2. The specific conductance as a function of surfactant concentration for EHAB,
22:1-2-22:1 and 22:1-6-22:1 at 25 °C.
d

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H. Li et al. / Surface Science 604 (2010) 1173–1178
surfactants to aggregate in solution. As a result, the cmc value obtained
by surface tension is less than that obtained by specific conductance.
The ionization degree of micelles of ionic surfactants can be obtain from
k vs. C plots according to equation
on condition the spacer s is proper. In particular, the effect of m and s
upon the viscosity of solution seems synergetic. For example, the pro-
per s of 22-s-22 for enhancing the viscosity of solution is 4. On the other
hand, the proper s of both 18-s-18 and 16-s-16 for enhancing the
viscosity of solution is 3 [17]. The experimental results present by In
et al. [5] indicate that the gemini surfactants 12-s-12 with s=2 are
more efficient in enhancing the viscosity of solution. In our experiments
the gemini surfactants 22:1-s-22:1 have the long unsaturated chains.
The cis-double bond makes it difficult to assemble the hydrophobic
tail properly. As a result, the monomer surfactants with a cis-double
bond is difficult to crystallize and its Kraft point is very low (b0 °C)
[20]. Obviously, for the gemini surfactants with the long unsaturated
chains such as 22:1-s-22:1, the proper s for enhancing the viscosity of
solution is even greater than 4. This also explains why the gemini
surfactants 22:1-2-22:1 as presented in this paper cannot enhance
the viscosity of solution significantly even in the presence of salt. In this
subsection and the subsequent subsection we will only compare the ef-
ficiency of 22:1-6-22:1 with EHAB in enhancing the viscosity of solution.
It is generally believed that aqueous solutions of some gemini
surfactants with short spacers can have a very high viscosity at
relatively low surfactant concentration whereas the solution of the
corresponding monomer remains low viscous. In fact, the viscosity of
the aqueous solution of 12-2-12 may be 106 times larger than
corresponding monomer surfactants DTAB [13]. From Fig. 3 it can be
seen that the viscosity of the aqueous solution of 22:1-6-22:1 is larger
than that of EHAB, but not significant as in the case of 12-2-12 with
DTAB. The reason is that EHAB have a more tendency to aggregate into
worm-like micelles than DTAB. The critical concentration C⁎, which
was interpreted as the onset of fast micellar growth and of the semi-
dilute regime, is indicated in Fig. 3. It can be seen that C⁎ of 22:1-6-22:1
is less than that of EHAB. The effect of temperature upon the viscosity
of aqueous solution of either 22:1-6-22:1or EHAB is significant. In fact,
the viscosity of aqueous solution of 22:1-6-22:1 becomes the maxi-
mum at 35 °C. On the other hand, the viscosity of the aqueous solution
of EHAB keeps almost the same as the temperature is less than 35 °C and
decreases considerably as the temperature is greater than 35 °C. It
is also noted from Fig. 3b that the viscosity of the aqueous solution of
22:1-6-22:1 becomes the maximum at the certain concentration of
22:1-6-22:1, free from temperature. The similar experimental results
were also reported for the gemini surfactants with the saturated chains
such as 12-2-12 as reported by Kern et al. [28], though the interpreta-
tions are quite different as summarized by Zana [1]. However, molecular
dynamics simulations [29] indicate that the branching of the worm-like
micelles of the gemini surfactants should be responsible for the results.
ꢀ
ꢁ
ꢀ
ꢁ
dk
dc
dk
dc
α =
ð3Þ
=
C N cmc
Cbcmc
The results are shown in Table 1. It can be seen that the ionization
degree α of micelles of the gemini surfactants is almost twice the
value of α corresponding to monomeric surfactant. However, the
ionization degree of micelles of 22:1-2-22:1 is almost the same as that
of 22:1-6-22:1.
3.3. Concentration
Fig. 3 shows the zero shear viscosity versus concentration for EHAB
and 22:1-6-22:1 in aqueous solutions at 25, 35 and 40 °C, respectively.
22:1-2-22:1 is not taken into account because 22:1-2-22:1 in water
cannot enhance the viscosity of the solution significantly even in the
presence of salt. It is already known that the efficiency of the gemini
surfactants m-s-m in enhancing the viscosity of solution depends on not
only m but also s. In fact, the gemini surfactants with the long saturated
alkyl chain seems more efficient in enhancing the viscosity of solution
3.4. Salt
Fig. 4 shows the viscosity of aqueous 22:1-6-22:1/sodium
salicylate (NaSal) solution as a function of shear rate at 50, 70 and
90 °C, respectively. The solutions contain a fixed 22:1-6-22:1
concentration CD of 30 mM and varying concentrations of NaSal Cs,
with the salt to surfactant molar ratio (Cs/CD) to be 0.6, 0.7, 0.8 and
1.0, respectively. It is well known that organic counterions such as
NaSal are more efficient in promoting worm-like micelle formation
than weakly binding inorganic salt. Salicylate ions not only adsorb on
a Stern layer of micelle surface but also penetrate into micelles,
which is of different from simple inorganic salts such as KBr [30,31].
The excess adsorption and penetration of salicylate ions, however,
induce negaztive net charge of micelles. Simultaneously micelle size
decreases to small micelles because of electrostatic repulsion in
“anionic” micelles. From Fig. 4a it can be seen that 22:1-6-22:1/NaSal
is less efficient in enhancing the viscosity of aqueous solution as Cs/CD
is 1.0. This suggests that the salicylate ions have excessively adsorbed
on such an occasion. It can also be seen from Fig. 4 that 22:1-6-22:1/
NaSal with the ratio of 0.7 is the most efficient in enhancing the
viscosity of the aqueous solution even at different temperatures. Of
Fig. 3. Zero shear viscosities of EHAB and 22:1-6-22:1 as a function of concentration at
different temperatures.

H. Li et al. / Surface Science 604 (2010) 1173–1178
1177
ratio of either 0.6 or 0.8 drops one order of magnitude as the
temperature increases from 50 °C to 90 °C. This suggests that the
most proper ratio of the 22:1-6-22:1/Nasal for enhancing the vis-
cosity of solution has proved to be 0.7.
3.5. Temperature
Fig. 5 shows the viscosity of aqueous 22:1-6-22:1 and EHAB
solution in the presence of NaSal as a function of shear rate at 70, 90
and 110 °C, respectively. The concentration of EHAB is selected to be
60 mM so that both EHAB solution and 22:1-6-22:1 solution (30 mM)
have the same number of the long unsaturated tails. The concentra-
tion of NaSal fixed to be 21 mM in both EHAB solution and 22:1-6-
22:1 solution. As a result, Cs/CD corresponding to EHAB solution is 0.35
whereas Cs/CD corresponding to the 22:1-6-22:1 solution is 0.7. From
Fig. 5b it can be seen that the viscosity of EHAB solution at the lower
shear rate (b10−1 s−1) and high temperature (90 °C) is larger than
105 mPa s. In contrast, the viscosity of erucyl bis-(hydroxyethyl)
methylammonium chloride (EHAC) solution (60 mM) in the presence
of NaSal (30 mM) at the lower shear rate (b10−1 s−1) and high
temperature (85 °C), as presented by Raghavan and Kaler, is less than
105 mPa s[21]. This suggests that EHAB is more efficient in enhancing
the viscosity of the solution than EHAC. It is noted that the proper ratio
of Cs/CD corresponding to EHAC solution as presented by Raghavan
and Kaler is 0.5. In our experiments the proper ratio of Cs/CD
corresponding to EHAB solution is 0.35. A possible interpretation is
that the ionization degree of EHAC is greater than EHAB. As a result, it
needs more salicylate ions penetrate into micelles to screening the
repulsive electrostatic interaction between the headgroups of EHAC.
From Fig. 5 it can be seen that the viscosity of EHAB/NaSal solution
drops almost three orders of magnitude as the temperature increases
from 70 °C to 110 °C. The viscosity of EHAB/NaSal solution at 110 °C,
as shown in Fig. 5c, is already less than 104 mPa s. Comparatively,
22:1-6-22:1 is more efficient in enhancing the viscosity of solution,
especially at very high temperature. In particular, the viscosity of
22:1-6-22:1/NaSal solution with the ratio of 0.7 seems insensitive to
temperature. In fact, the viscosity of the 22:1-6-22:1/NaSal solution
with the ratio of 0.7, as indicated in Fig. 5, almost keeps the same as
the temperature increases from 70 °C to 110 °C. It should be
emphasized that 22:1-6-22:1/NaSal solution with the ratio of 0.7
show high viscosity (N107 mPa s) even at 110 °C. As we can see, there
are few, if any, the surfactants which can enhance the viscosity of
aqueous solution efficiently at such a high temperature.
4. Conclusions
The gemini surfactants 22:1-s-22:1, where s=2 and 6 methylene
groups and 22:1 refer to erucyl dimethylammonium bromide chains,
together with the monomeric surfactant erucyl bis-(hydroxyethyl)
methylammonium bromide (EHAB) which has the same long
unsaturated tails with gemini surfactants, were synthesized, charac-
terized and solution properties of these surfactants investigated using
surface tension, conductivity and viscosity measurement. The cmc
values of both 22:1-2-22:1 and 22:1-6-22:1 are less than that of the
corresponding monomeric surfactant EHAB, but not as significant as
reported by others concerning the m-s-m type of the gemini
surfactants and their monomeric surfactants. Both 22:1-2-22:1 and
22:1-6-22:1 have nearly the same value of a0 (the minimum head
group areas per surfactant molecule at the aqueous solution/air
interface), which is almost the half value of a0 corresponding mono-
meric surfactants. On the other hand, the ionization degree α of
micelles of both 22:1-2-22:1 and 22:1-6-22:1 is almost twice value of
α corresponding to monomeric surfactant EHAB. Though 22:1-2-22:1
has more similarity with 22:1-6-22:1 rather than EHAB as presented
above, 22:1-2-22:1 in water cannot enhance the viscosity of solution
significantly in the presence of salt. In contrast, both 22:1-6-22:1 and
Fig. 4. Effect of NaSal/GS 22:1-6-22:1 molar ratio (Cs/CD) on the steady-shear rheology
at 50, 70 and 90 °C, respectively.
peculiar interest is that the viscosity of the 22:1-6-22:1/NaSal
solution with the ratio of 0.7 is insensitive to temperature. In
contrast, the viscosity of the 22:1-6-22:1/NaSal solution with the

1178
H. Li et al. / Surface Science 604 (2010) 1173–1178
EHAB in water can give rise to highly viscoelastic or gel-like solutions
even at the high temperature. In particular, 22:1-6-22:1 has proved to
be a more efficient candidate for high temperature rheology-control
applications than EHAB. The experimental results have shown that the
viscosity of 22:1-6-22:1 solution in the presence of NaSal at 110 °C is
greater than 107 mPa s. In contrast, the corresponding EHAB solution
with the same numbers of the long unsaturated tails in the presence of
NaSal at 110 °C is less than 104 mPa s. The effect of salt (NaSal in our
experiment) upon the viscosity of 22:1-6-22:1 solution is significant.
The most proper ratio of 22:1-6-22:1/Nasal for enhancing the
viscosity of the solution is 0.7. In fact, the viscosity of 22:1-6-22:1/
Nasal with the ratio of 0.7 is almost insensitive to the temperature.
Acknowledgment
The financial support of the Ministry of Science and Technology of
China (2007CB936401) is acknowledged.
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Fig. 5. Effect of temperature on the viscosity of EHAB and GS22:1-s-22:1 solution.