Synthesis, Surface Activities and Toluene Solubilization by Amine-oxide Gemini Surfactants

Article abstract of DOI:10.1007/s11743-016-1828-5

A series of amine-oxide gemini surfactants featuring amide groups [N, N’-dimethyl-N, N’-bis(2-alkylamideethyl)-ethylenediamine oxide (alkyl?=?C₁₁H₂₃, C₁₃H₂₇, C₁₅H₃₁, C₁₇H₃₅)] have been synthesized via a three-step synthetic route, and their chemical structures were confirmed by mass spectra, FTIR and ¹H-NMR spectra. The surface activities of these compounds have been measured. The results show that these synthesized amine-oxide gemini surfactants reduced the surface tension of water to a minimum value of approximately 26.91?mN?m^?1 at a concentration of 2.92?×?10^?5mol?L^?1. Furthermore, their critical micelle concentration (CMC) values and solubilization of toluene decrease with an increase of the hydrophobic chain length from 12 to 18. Isoelectric point measurements revealed that their pI values range from 4.0 to 10.5.

Full text of DOI:10.1007/s11743-016-1828-5

J Surfact Deterg
DOI 10.1007/s11743-016-1828-5
ORIGINAL ARTICLE
Synthesis, Surface Activities and Toluene Solubilization
by Amine-oxide Gemini Surfactants
Hongqin Liu1 Jun Hu1 Baocai Xu1 Tingting Zhao1 Guoying Shi1
Guiju Zhang1
•
•
•
•
•
Received: 3 July 2015 / Accepted: 15 April 2016
Ó AOCS 2016
Abstract A series of amine-oxide gemini surfactants fea-
turing amide groups [N, N’-dimethyl-N, N’-bis(2-alky-
lamideethyl)-ethylenediamine oxide (alkyl = C11H23,
C13H27, C15H31, C17H35)] have been synthesized via a
three-step synthetic route, and their chemical structures
were confirmed by mass spectra, FTIR and 1H-NMR
spectra. The surface activities of these compounds have
been measured. The results show that these synthesized
amine-oxide gemini surfactants reduced the surface tension
named as gemini surfactants [1]. Since then, the gemini
surfactants have evoked considerable interest due to
their unusual surface and bulk properties [2, 3]. The
surface activity of gemini surfactants is greatly
enhanced with the separation between the polar head
groups strongly decreasing with aggregation [4]. Com-
pared with the corresponding monomeric surfactants,
gemini surfactants exhibit unique physicochemical
properties, such as lower critical micelle concentration
(CMC) values, spontaneous formation of vesicles or
wormlike micelles even at relatively low concentrations,
unusual aggregation morphologies, greater efficiency in
decreasing the surface tension of water and the inter-
facial tension of the oil–water interface, lower Krafft
temperature and good water solubility in the case of
ionic gemini surfactants, better emulsification, better
wetting, thickening and lime-soap dispersing properties
[5–8]. Because of these unique physicochemical prop-
erties, gemini surfactants have been developed not only
as environmentally friendly materials, but also as
functional organic materials [9].
of water to
a minimum value of approximately
26.91 mN m-1 at a concentration of 2.92 9 10-5mol L-1
.
Furthermore, their critical micelle concentration (CMC)
values and solubilization of toluene decrease with an
increase of the hydrophobic chain length from 12 to 18.
Isoelectric point measurements revealed that their pI values
range from 4.0 to 10.5.
Keywords Amine-oxide gemini surfactant Á Synthesis Á
Surface activities Á Solublization Á Isoelectric point
Introduction
The formula of amine oxides is R1R2R3N ? O, con-
sisting of
a hydrophilic head (i.e., N ? O) and
In 1991, amphiphiles formed by connecting a pair of
conventional surfactant molecules via a covalent linkage
(referred to as a spacer) were reported, which were later
hydrophobic tail (long carbon chain). Amine oxides, which
are nonionic at neutral or alkaline pH, turn cationic due to
the protonation of the N–O bond at low pH. So, some
researchers also consider them as zwitterionic surfactants
[10], zwitterionic/cationic surfactants [11] or nonionic/ca-
tionic surfactants. Amine oxides possess outstanding per-
formance abilities, such as good biodegradation [12], a low
cloud point, strong hard water resistance, high solubiliza-
tion capacity, good lather generation and stability, low skin
irritation, and good compatibility with other surfactants.
Moreover, the solubility and aggregation of amineoxide
surfactants will not be affected by the presence of divalent
& Baocai Xu
xubaoc@btbu.edu.cn
1
Beijing Key Laboratory of Flavor Chemistry, School of Food
and Chemical Engineering, Beijing Higher Institution
Engineering Research Center of Food Additives and
Ingredients, Beijing Technology and Business University,
No. 11 Fucheng Road, Beijing 100048, People’s Republic of
China
123

J Surfact Deterg
metal ions, such as Cu2?, Mg2?, Ca2? and Fe2? [10]. Their
aggregates possess a high dispersing power toward several
water-insoluble organic substances, even in the absence of
a co-surfactant [10]. To date, scientists have done a lot of
research about nonionic gemini surfactants, which is
mainly focused on sugar-based [13–15] or ethoxylated [16,
17] gemini surfactants. Amineoxide gemini surfactants
have seldom been studied.
Synthesis of N,N0-bis(2-
Alkylamideethyl)ethylenediamines (1a–d)
N,N0-bis(2-alkylamideethyl)ethylenediamines were syn-
thesized according to the synthetic route described by Ming
Zhou et al. [20]. Triethylenetetramine (0.12 mol), fatty
acid (dodecanoic, hexadecanoic, tetradecanoic and
octadecanoic) methyl esters (0.2 mol), toluene solvent
(30 mL) and potassium hydroxide (0.005 mol) as a catalyst
were introduced into a 250-mL round-bottom flask equip-
ped with a stirrer and a condenser. The reaction mixture
was stirred and heated under reflux for 20 h, and then was
cooled down to room temperature. Subsequently, excess
solvent and materials were vacuum-stripped in the rotary
evaporator. The solid products were crystallized twice from
a diethyl ether–absolute ethanol mixture (1:1, v/v) and
dried in a vacuum oven. The yield values of 1a, 1b, 1c and
1d were 84.6, 80.9, 81.3 and 75.5 %, respectively.
In addition, studies report that the insertion of an amide
bond in the surfactant molecule, especially when placed
between the hydrocarbon chain and the polar head group,
will not only make surfactants degradable, but also increase
the surfactant aggregation tendencies as compared to the
surfactants having no amide bond [8, 18, 19].
In this paper, in order to obtain gemini surfactants with
better properties, a series of amineoxide gemini surfactants
featuring amide groups have been designed and synthe-
sized. At the same time, the surface activity, isoelectric
point and solubilization of toluene by these compounds
were studied.
Synthesis of N,N0-bis(2-Alkylamideethyl-
methyl)ethylenediamines (2a–d)
Experimental Procedures
N,N0-bis (2-alkylamideethyl-methyl)ethylenediamines were
synthesized with HCHO/HCOOH reductive methylation of
amines (1a–d), i.e. the Eschweiler–Clark reaction [21, 22].
First, 0.05 mol of N,N0-bis(2-alkylamideethyl)ethylenedi-
amine and 40 mL of isopropanol were placed in a 250-mL
three-necked flask, and then HCOOH (0.25 mol) and 36 %
formaldehyde solution (0.20 mol) were sequentially added
dropwise into the flask under agitation. The reaction mixture
was heated at reflux for 8 h with stirring. After that, the
resulting solution was basified with 5 % sodium hydroxide
solution and a white solid separated out. The solid was
collected by suction filtration and washed with a few mil-
liliters of cold water. Finally, the solid products were dried.
The yield values of 2a, 2b, 2c and 2d were 78.4, 81.7, 80.3
and 79.8 %, respectively.
These four surfactants were synthesized via a three-step
synthetic route, shown in Scheme 1.
H
N
NH2
CnH2n+1COOCH3
+
H2N
N
H
toluene
KOH
O
H
N
H
N
CnH2n+1
(1a-d)
H2n+1Cn
N
H
N
H
O
Eschweiler-Clark HCOOH / HCHO
O
Synthesis of Amineoxide Gemini Surfactants (3a–d)
H
N
N
CnH2n+1
H2n+1Cn
N
H
N
(2a-d)
(3a-d)
N,N0-bis(2-alkylamideethyl-methyl)ethylenediamines (2a–
d; 0.05 mol) and distilled water (30 mL) were introduced
into a 250-mL three-necked flask. While stirring, 30 %
hydrogen peroxide solution (0.15 mol) was added dropwise
into the flask. The reaction mixture was stirred for 8.5 h at
70–75 °C. After the reaction, the mixture was condensed
by vacuum distillation to give the crude product, which
was recrystallized from ether to give the amine-oxide
gemini surfactants. The yield values of 3a, 3b, 3c and 3d
were 78.3, 74.9, 79.1 and 81.7 %, respectively.
O
H2O2
O
O-
N+
H
N
CnH2n+1
H2n+1Cn
N
N+
O-
H
O
Scheme 1 Synthesis of amine-oxide gemini surfactants, where
n = 11 for a, n = 13 for b, n = 15 for c and n = 17 for d
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J Surfact Deterg
Analytical Methods
Results and Discussion
The content of these synthesized amine-oxide gemini sur-
factants was estimated by potentiometric titration (EN
1791-1997). In a 250-mL beaker, 0.5 g (m1) of sample was
dissolved in 50 mL of acetic acid. Then a composite non-
aqueous pH electrode was inserted into the beaker, which
was placed on a potentiometric titrimeter (Metrohm848).
After that, the sample was titrated with the perchloric acid
standard volumetric solution (c = 0.1 mol L-1 in acetic
acid) until slightly beyond the potential jump, and the
volume of perchloric acid solution necessary to reach the
equivalence point was recorded as V1. In another 250-mL
beaker, 5 g (m2) of sample was dissolved in 50 mL of
acetic anhydride, and was titrated the same way. The vol-
ume of perchloric acid solution was recorded as V2.
Infrared (IR) spectra were recorded on a Nicolet islo
Fourier transform (FT)-IR spectrometer. Mass spectra
(MS) analyses were performed on the AB SCIEX
API3200LC/MS/MS spectrometer. Using CDCl3 as sol-
vent, proton nuclear magnetic resonance ( 1H-NMR)
spectra were acquired on a Bruker AV-300 NMR
spectrometer.
The Contents of Amineoxide Gemini Surfactants
The contents of amineoxide gemini surfactants were cal-
culated by the following equation:
ꢀ
ꢁ
V1 V2
À
c  M
A ð%Þ ¼
Â
 100 %
ð1Þ
m1 m2
2000
where m1 = 0.5 g, m2 = 5 g, M is the mean molecular
weight of the amineoxide surfactant and c is the concen-
tration of perchloric acid standard volumetric solution; i.e.,
c = 0.1 mol L-1. The contents of 3a, 3b, 3c and 3d were
92.90, 90.31, 91.17 and 90.81 %, respectively.
Spectroscopic Characterization of Amine-Oxide
Gemini Surfactants
These synthesized compounds were characterized by their
1
MS, IR and H-NMR spectra, the details of which were as
follows. In all cases, the assigned structures of these
1
compounds were confirmed with MS, IR and H-NMR.
3a: MS m/z: 571.0 [M?1] ?; IR (KBr, v, cm-1):
At 25 °C, the surface tension of aqueous solutions was
3321.00, 3282.92 (N–H), 3000–2800 (C–H), 1642.17
1
(C=O), 1545.68 (N–H), 969.53 (N–O); H-NMR [CDCl3,
measured within a concentration range of 10-7–10-2
-
mol L-1 by the Wilhelmy plate method, which was carried
out on a Dataphysics tensiometer DCAT11. The pH values
of aqueous solutions of 3a, 3b, 3c and 3d were in the ranges
of 7.28–8.31, 7.41–8.51, 7.02–8.73 and 7.57–8.85,
respectively.
300 MHz, d ppm]: 0.852–0.896 [t, 6H, CH3C- of two long
carbon chains], 1.249 [br, methylene in two long carbon
chains, C(CH2)8C-], 1.591[m, 4H, C-C8-CH2-C-],
2.191–2.252 [t, 4H, -CH2(C=O)N-], 3.248 [s, 6H, CH3-
N?], 3.428 [t, 4H, N?-CH2-C-N(C=O)-], 3.539 [t, 4H, -
CH2N(C=O)-], 3.695 [t, 4H, N?-C–CH2-N?].
3b: MS m/z: 627.0 [M ? 1] ?; IR (KBr, v, cm-1):
3333.35 (N–H), 3000–2800 (C–H), 1642.86 (C=O),
1547.66 (N–H), 963.19 (N–O); 1H-NMR [CDCl3,
300 MHz, d ppm]: 0.857–0.901 [t, 6H, CH3C- of two long
carbon chains], 1.252 [br, methylene in two long carbon
chains, C(CH2)10C-], 1.599 [m, 4H, C-C10-CH2-C-],
2.183–2.252 [t, 4H, -CH2(C=O)N-], 3.241 [s, 6H, CH3-
N?], 3.402 [t, 4H, N?-CH2-C-N(C=O)-], 3.497 [t, 4H, -
CH2N(C=O)-], 3.763 [t, 4H, N?-C–CH2-N?].
3c: MS m/z: 682.9 [M?1] ?; IR (KBr, v, cm-1): 3308.99
(N–H), 3000–2800 (C–H), 1650.22 (C=O), 1598.89 (N–H),
969.92 (N–O); 1H-NMR [CDCl3, 300 MHz, d ppm]:
0.875–0.891 [t, 6H, CH3C- of two long carbon chains],
1.251 [br, methylene in two long carbon chains,
C(CH2)12C-], 1.610 [m, 4H, C-C12-CH2-C-], 2.192–2.250
[t, 4H, -CH2(C=O)N-], 3.192 [s, 6H, CH3-N?], 3.405 [t,
4H, N?-CH2-C-N(C=O)-], 3.506 [t, 4H, -CH2N(C=O)-],
3.661 [t, 4H, N?-C–CH2-N?].
3d: MS m/z: 739.2 [M?1] ?; IR (KBr, v, cm-1):
3311.47 (N–H), 3000–2800 (C–H), 1652.69 (C=O),
1599.83 (N–H), 968.76 (N–O); 1H-NMR [CDCl3,
300 MHz, d ppm]: 0.865–0.879 [t, 6H, CH3C- of two long
The solubilization of toluene by these synthesized
amine-oxide gemini surfactants was measured. Aqueous
solutions of the synthesized surfactants covering suit-
able concentrations above the CMC were prepared in high-
purity water [23]. A volume range of 0–50 lL of toluene
was pipetted into 10 mL of surfactant solutions (10-3
-
mol L-1). The pH values of 3a, 3b, 3c and 3d solutions
were 7.98, 8.17, 8.29 and 8.42, respectively. After oscil-
lating at 50 °C in a thermostated oscillator for 1 h, the
absorbance values of the micellar solutions were recorded
at the wavelength of 500 nm on a SHIMADZUUV-3600
UV–VIS-NIR spectrophotometer at 25 °C. The measured
absorbance values were plotted as a function of the volume
of toluene, and the maximum amount of toluene solubi-
lized in the micelle was estimated from the break point in
the resulting curve.
Isoelectric points (pI) were measured to determine the
form in the aqueous solution with various pH values and
the pH at which amine oxide molecules carry no net
electrical charge based on the measurement of the pH value
and the electrical conductivity. The pH values were
adjusted with acetic acid and high-purity water and the
conductivities were measured in various pH values.
123

J Surfact Deterg
be noted that Menger has reported that Eqs. (2) and (3)
assume that the interface is already saturated when the
surface tension first begins its precipitous linear decline
[25].
DGHm, the standard free energy of micellization per mole
of monomer unit, is calculated according to the following
equation [26]:
carbon chains], 1.251[br, methylene in two long carbon
chains, C(CH2)14C-], 1.592 [m, 4H, C-C14-CH2-C-],
2.168–2.247 [t, 4H, -CH2(C = O)N-], 3.217[s, 6H, CH3-
N?], 3.396 [t, 4H, N?-CH2-C-N(C=O)-], 3.486 [t, 4H, -
CH2N(C=O)-], 3.666 [t, 4H, N?-C–CH2-N?].
Surface Activity and Surface Property Parameters
DGHm ¼ RT lnðCMCÞ ¼ 2:303RT logðCMCÞ
It is in reference to the standard state of the unit mole
ð4Þ
The variation in surface tension versus the concentration of
these surfactant aqueous solutions is plotted in Fig. 1.
The CMC values of these surfactant aqueous solutions
are determined from the break points of the surface tension
versus the logarithm of the concentration curves at 25 °C
[6]. The CMC values and the minimum surface tension
(cCMC) values are listed in Table 1.
The maximum surface excess concentration (Cmax) is an
effective measure of the adsorption at the air–water inter-
face. The concentration of the surfactant is always more at
the surface than that in the bulk [6]. The maximum surface
excess and the minimum surface area (Amin) per surfactant
molecule at the air––water interface are calculated from the
Gibbs adsorption equations [24]:
fraction for the nonionic surfactants.
The values of Cmax, Amin and DGHm are also summarized
in Table 1.
From the surface tension isotherms of these amineoxide
gemini surfactants (Fig. 1), we can see that there was no
minimum at the CMC. This suggested that the purity of
these synthesized gemini surfactants is very high [27]. As
is seen in Table 1, the ccmc values of these surfactants are
between 26.91 and 31.00 mN m-1. Compared with some
surfactants that have been reported, the ccmc values of
which are usually greater than 30 mN m-1 [28], these
values of ccmc are lower. This indicates that their surface
activities are higher and suggests that these gemini sur-
factants aggregated easily, possibly because of the spacer
group in the gemini surfactants. Due to the existence of the
spacer group, the hydrophobic interaction is enhanced and
electrostatic repulsion is reduced [29]. In addition, it is
clear that the value of ccmc increases as the hydrocarbon
chain length increases (Table 1), which is contrary to the
reported results of the influence of the alkyl chain for most
conventional surfactants. It may be due to the formation of
pre-micellar aggregates at concentrations below the CMC
as the alkyl chain length increased [13].
ꢀ
ꢁꢀ
ꢁ
ꢀ
ꢁꢀ
ꢁ
À1
dc
d ln c
À1
dc
d log c
Cmax
¼
¼
¼
ð2Þ
ð3Þ
nRT
2:303nRT
1
Amin
NACmax
where dc/dlogc is the slope of the surface tension/log
concentration plot at each concentration, R is the gas
constant, NA is Avogadro’s number, T is the absolute
temperature, n is the number of solute species, which
constitute the surfactants adsorbed at the interface. For our
surfactant solutions, in which amine-oxide gemini surfac-
tants exist as nonionic forms, we can set n = 1. It should
From Table 1, we can also see that the CMC values of
these gemini surfactants increase with the hydrophobicity
of the alkyl chain increasing. Notably, the CMC value of a
conventional surfactant generally decreases as the
hydrophobic alkyl chain length increases. However, for our
gemini surfactants, the CMC values have not followed this
normal decreasing tendency. This anomalous trend also has
been reported and studied by some researchers [2, 3, 30].
According to the literature [13, 29, 31, 32], this may be
because the aggregations of self-coiling or pre-micellar,
such as dimer and trimer, which were assumed to show
little or no significant surface activity, were formed at
concentrations below the CMC as the chain length of the
surfactants increases. Maybe another reason is the possible
occurrence of a large extent of cation and H3O? com-
plexation with the increasing length of the hydrophobic
3a (R=C11H23)
3b (R=C13H27)
70
3c (R=C15H31)
60
50
40
30
20
3d (R=C17H35)
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0.01
0.1
concentration (mol/L)
alkyl chain [33]. In addition, as is seen in Table 1, Amin
,
which is related to the degree of packing of surfactant
molecules adsorbed at the air–water interface [34],
increased upon increasing the hydrophobic alkyl chain
Fig. 1 The variation in surface tension versus the logarithm of the
concentration for 3a (filled square), 3b (filled circle), 3c (filled
triangle) and 3d (filled diamond) at 25 °C
123

J Surfact Deterg
Table 1 CMC, ccmc and surface property parameters for gemini surfactants at 25 °C
Compound
CMC (mol L-1
)
ccmc (mN m-1
)
Cmax (mol cm-2
)
Amin (nm-2 mol)
DGHm (KJ mol-1
)
3a (R = C11H23
)
2.92 9 10-5
5.60 9 10-5
6.95 9 10-5
2.00 9 10-4
26.91
27.60
29.57
31.00
8.69 9 10-10
4.88 9 10-10
2.20 9 10-10
1.97 9 10-10
0.186
0.330
0.731
0.817
-25.88
-24.27
-23.73
-21.11
3b (R = C13H27
3c (R = C15H31
3d (R = C17H35
)
)
)
(
), 3b ( ), 3c ( ) and 3d ( ) at 25
length. Thus, we suspect that gemini surfactant molecules
with a short carbon chain length pack more closely at the
air–water interface, due to stronger interactions between
the hydrocarbon chains [32].
1.5
3a(R=C11H23)
3b(R=C13H27)
3c(R=C15H31)
3d(R=C17H35)
The negative values of DGHm indicate that at 25 °C, the
micellization was spontaneous. It also implies that these
compounds have an excellent ability to absorb at the air–
water interface and to form micelles in solution [35]. In
addition, the value of DGmH increases with the increased
alkyl chain length, suggesting that the interaction between
the alkyl chains was the driving force for micellization or
adsorption [27, 35].
From the above analysis and comparison, one can
safely conclude that an increase in the hydrophobicity of
the alkyl chain decreases the efficiency of the surfactant,
perhaps because the hydrophobic chain was long enough
to curl up, making it more difficult to form the micelles.
In addition, from Fig. 1, we can see that the tensiometric
plots of 3a and 3b are different than 3c and 3d. It may be
due to a difference in degree of self-coiling or pre-mi-
cellization of the surfactant molecules when the number
of carbon atoms in the hydrophobic chains is more than
14 [30].
0.0
0
1
2
3
4
5
6
toluene volume /mL
Fig. 2 The variation in absorbance versus the volume of toluene for
3a (filled square), 3b (filled circle), 3c (filled triangle) and 3d (filled
diamond)
Table 2 The solubilization of amine-oxide gemini surfactant to
toluene
Gemini surfactants
Toluene volumes
V0 (mL)
X
-1
(mL mol
)
(g g-1
)
The Solubilization of Toluene by Amineoxide
Gemini Surfactants
3a (R = C11H23
3b (R = C13H27
3c (R = C15H31
3d (R = C17H35
)
0.282
0.294
0.298
0.310
30,984
29,823
29,409
28,168
42.729
40.619
37.808
36.398
)
)
Toluene is one of the most representative volatile organic
compounds (VOCs), which are a source of serious envi-
ronmental issues [36]. Among the developed techniques
available to control VOC emissions, the absorption
method is one of the most economical technologies [37].
The enhancement of VOC solubility in water can be
achieved in the presence of surfactants [38]. To determine
the suitability of this application, the solubilization of
toluene by these amineoxide gemini surfactants was
studied.
)
V0 Â q
Xðg Á gÀ1Þ ¼
ð6Þ
ms
where V0, V, C, q, and ms are the maximum volume of
solubilization of toluene, the volume of surfactant solution,
the concentration of the surfactant solution, solubilization
density and surfactant mass, respectively. The toluene
volumes are determined from the breaks in the curves
(Fig. 2) and the efficiencies of solubilization are listed in
Table 2.
From Table 2, we can see that X (mL mol-1) is between
28,168 and 38,964. Compared with some reported results
[39], these amine-oxide gemini surfactants exhibit a good
toluene solubilizing capacity. Solubilization is the process
The absorbance of the micellar solution as a function of
the volume of toluene added is shown in Fig. 2.
The capacity of solubilization (X) is calculated from the
following equations:
1000 Â V0
XðmL Á molÀ1Þ ¼
ð5Þ
C Â V
123

J Surfact Deterg
3.6
3.4
3.2
3.0
2.8
2.6
2.4
2.2
Fig. 3 The pH-conductivity
curve of 3a (filled square), 3b
(filled circle), 3c (filled triangle)
and 3d (filled diamond)
3a
(
R=C
H
11 23
)
3.2
2.8
2.4
2.0
1.6
1.2
3b
(
R=C
H
13 27
)
2
4
6
8
10
12
2
4
6
8
10
12
pH
pH
3.2
2.8
2.4
2.0
1.6
1.2
3.2
2.8
2.4
2.0
1.6
1.2
3d
(
R=C
H
)
3c
(
R=C
H
)
17 35
15 31
2
4
6
8
10
12
2
4
6
8
10
12
pH
pH
of incorporating the solubilizate into or onto the micelles
while the surfactant concentration in solution is above the
CMC. The capacity of solubilization is influenced by the
CMC. From Fig. 1, the CMC values increase with an
increase of the alkyl chain length from 12 to 18 carbons. As
expected, in aqueous solutions at the same concentration,
the solubilization volumes of toluene decrease with the
hydrophobic chain length increasing, because at the same
surfactant concentration, the lower the CMC, the more
micelles will be formed to solubilize more components.
Isoelectric points measurements revealed that the pI of
them is pH 4.0–10.5. It is very significant that their surface
activities are higher and that they are highly effective
aqueous surfactants. Studies have shown that the values of
ccmc and CMC increase with an increase of the
hydrophobic alkyl chain from 12 to 18. It may be due to the
self-coiling or the formation of pre-micellar aggregates at
concentrations below the CMC as the alkyl chain length
increased. Furthermore, these compounds exhibit a good
toluene solubilizing capacity, which decreases with the
hydrophobic chain length increasing. Further work will
focus on more detailed studies such as Zeta potentials,
aggregation number and rheological properties.
Isoelectric Points
The relationship between pH and conductivity is plotted in
Fig. 3.
Acknowledgments We gratefully acknowledge the Science and
Technology Program Key Project of Beijing Municipal Commission
of Education (KZ201510011010), the transformation of scientific
and technological achievements-promotion plan project (PXM
2015_014213_000049), the financial support of the Funding Project
for the Cultivation of Excellent Talents in Beijing City
(2013D005003000010), the National Natural Science Foundation of
China (21176004, 21376008 and 21203005) and projects in the
National Science & Technology Pillar Program during the Twelfth
Five-year Plan Period (2013BAC01B04 and 2014BAE03B01).
Amineoxide surfactant molecules carry no net electrical
charge and show as a class of zwitterionic species in a range
of pH, i.e. pI. Below the pI, amine-oxide surfactants mole-
cules show cationic properties (N–OH?); above the pI they
show anionic properties [40, 41]. From Fig. 3, the pI of
these amine-oxide gemini surfactants is pH 4.0–10.5, in
which the surfactant molecules exist in inner-salt com-
pounds. When the pH is below 4.0 or above 10.5, they show
a high conductivity, suggesting that the strong ions have
been formed due to the protonation of the N–O amino group.
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Hongqin Liu received a Ph. D. in Environmental Engineering from
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Jun Hu is a postgraduate student in Applied Chemistry in Beijing
Technology and Business University. His research interests are
synthesis and properties of surfactants.
123

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Baocai Xu received a Ph. D. in Applied Chemistry from Beijing
Institute of Technology. He is a professor at Beijing Key Laboratory
of Flavor Chemistry, Beijing Higher Institution Engineering Research
Center of Food Additives and Ingredients, with a research field in
synthesis and properties of surfactants.
Guoying Shi is a postgraduate student in Applied Chemistry in
Beijing Technology and Business University. Her research interests
are synthesis and properties of surfactants.
Guiju Zhang received a Ph. D. in Organic Chemistry from Technical
Institute of Physics and Chemistry of the Chinese Academy of
Sciences. She is an associate professor with a research field in
synthesis and properties of surfactants.
Tingting Zhao is a postgraduate student in Applied Chemistry in
Beijing Technology and Business University. Her research interests
are synthesis and properties of surfactants.
123