Asymmetric sodium benzenesulfonate Gemini surfactant: Synthesis, properties and application

Article abstract of DOI:10.1016/j.molliq.2019.04.109

An asymmetric sodium benzenesulfonate Gemini surfactant (ASBGS) designed and prepared by esterification and sulfonation reaction using 4- (dodecyl amide) phenol (4-DAP), dodecanol maleic acid monoester (DMM) and NaHSO₃. The structure of the ASBGS characterized by FT-IR and ¹H NMR. The surface tension, emulsifying force and aggregation property of the ASBGS tested. The results showed that the critical micelle concentration and γ_CMC of ASBGS were 0.8 × 10^?4 mol/L and 25.1 mN/m, respectively. Thermodynamic parameters results showed that the micellization of this surfactant was a spontaneous process. DLS and TEM results showed that the concentration of 10 CMC surfactant solution aggregated into vesicles, and the average particle size was 85 nm. Then ASBGS was used to emulsify rapeseed oil for lubricating collagen fiber. The emulsification time for rapeseed oil was 283 s, showing that ASBGS has an excellent emulsifying ability. In addition, we prepared collagen fiber lubricating materials (CFLMs) by blending different dosages of the ASBGS with rapeseed oil collagen fiber lubricating material (ROCLM), its emulsifying ability increased from 47.37% to 87.37% in cold water.

Full text of DOI:10.1016/j.molliq.2019.04.109

Accepted Manuscript
Asymmetric sodium benzenesulfonate Gemini surfactant:
Synthesis, properties and application
Bin Lyu, Yajin Yu, Dangge Gao, Yuefeng Wang, Jianzhong Ma
PII:
S0167-7322(18)36102-6
DOI:
https://doi.org/10.1016/j.molliq.2019.04.109
MOLLIQ 10832
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
22 November 2018
6 April 2019
21 April 2019
Please cite this article as: B. Lyu, Y. Yu, D. Gao, et al., Asymmetric sodium
benzenesulfonate Gemini surfactant: Synthesis, properties and application, Journal of
Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.04.109
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ACCEPTED MANUSCRIPT
Asymmetric sodium benzenesulfonate Gemini surfactant: synthesis,
properties and application
Bin Lyu , Yajin Yu , Dangge Gao , Yuefeng Wang , Jianzhong Ma
a, b
a, b
a, b
a, b
a, b
a College of Bioresources Chemical and Materials Engineering, Shannxi University
of Science & Technology, Xi’an 710021;
b National Demonstration Center for Experimental Light Chemistry Engineering
Education (Shaanxi University of Science & Technology), Xi'an 710021, China

Corresponding author. Tel.: 13991372196. E-mail address: xianyanglvbin@163.com.

ACCEPTED MANUSCRIPT
ABSTRACT: An asymmetric sodium benzenesulfonate Gemini surfactant (ASBGS)
designed and prepared by esterification and sulfonation reaction using 4- (dodecyl
amide) phenol (4-DAP), dodecanol maleic acid monoester (DMM) and NaHSO . The
3
1
structure of the ASBGS characterized by FT-IR and H NMR. The surface tension,
emulsifying force and aggregation property of the ASBGS tested. The results showed
-
4
that the critical micelle concentration and γCMC of ASBGS were 0.8×10 mol/L and
2
5.1 mN/m, respectively. Thermodynamic parameters results showed that the
micellization of this surfactant was a spontaneous process. DLS and TEM results
showed that the concentration of 10 CMC surfactant solution aggregated into vesicles,
and the average particle size was 85 nm. Then ASBGS was used to emulsify rapeseed
oil for lubricating collagen fiber. The emulsification time for rapeseed oil was 283 s,
showing that ASBGS has an excellent emulsifying ability. In addition, we prepared
collagen fiber lubricating materials (CFLMs) by blending different dosages of the
ASBGS with rapeseed oil collagen fiber lubricating material (ROCLM), its
emulsifying ability increased from 47.37% to 87.37% in cold water.
Key word: sodium benzenesulfonate, gemini surfactant, collagen fiber lubricating
material, emulsifying ability
1
Introduction
A surfactant is composed of a non-polar lipophilic group and a polar hydrophilic
group, allows the surfactant molecules to have both hydrophilic and lipophilic
[
1-2]
properties
concentration is very low
monosodium glutamate”, used in various industrial production areas
conventional surfactants have the disadvantages of low surface activity, poor
. Liquid surface tension can significantly reduce when the surfactant
[
3-4]
. It was known as a classic of the latest “industrial
[
5-7]
. The

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solubility in cold water, poor solubilization, poor synergistic effect, and difficulty in
forming micelles, which cannot meet the higher demands of current industrial
[
8]
applications . Therefore, some new surfactants with special structures and functions
[
9]
have been developed successively . Gemini surfactant is a special new surfactant,
which contains two hydrophilic groups and two hydrophobic groups. Compared with
conventional surfactants, Gemini surfactant has many excellent properties, including
higher surface activity, lower critical micelle concentration (CMC) and better
compatibility with other surfactants, better water solubility, wettability and
[
10-11]
emulsifiability
. Consequently, a large number of Gemini surfactants have been
[
12-14]
prepared to reveal the relationship between structure and properties
.
Among them, alkyl benzene sulfonate Gemini surfactant is a very important
anionic Gemini surfactant, which has high surface activity, good water solubility,
wettability, dispersibility. Good performances make alkyl benzene sulfonate Gemini
[15-18]
[19]
surfactant become a research hotspot
. Du et al.
synthesized an alkylbenzene
sulfonate Gemini surfactant with flexible methylene as a connecting group. Its CMC
was about two orders of magnitude lower than that of conventional alkylbenzene
[
20]
sulfonate, and its surface activity was good. Wang et al.
synthesized anionic
sulfonate Gemini surfactants 8-s-8 (SO ) and 12-s-12 (SO ) (s = 3~6). Its CMC of
3
2
3 2
8
-s-8 (SO ) was two orders of magnitude larger than the 12-s-12 (SO ) , while the
3 2 3 2
[
21]
influence of the spacer length on the CMC was relatively small. Li et al.
synthesized a new sulfonate Gemini surfactant (3C -DS) and investigated the
1
0
interfacial activity and emulsifying properties of 3C -DS and different surfactants
10

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with different mass ratios. The results showed that the interfacial tension of OP-10,
PS and HABS can be significantly reduced by adding 3C -DS, and the interfacial
1
0
-3
tension of the composite system [m (3C -DS: m (PS)=2:8] was 2.8×10 mN/m, and
10
the emulsification rate was 90%. Therefore, compared with traditional surfactants, the
alkylbenzene sulfonate Gemini surfactants are more widely used due to their excellent
properties, such as emulsifiers, foam stabilizers, thickeners, and other additives.
Gemini surfactants can be structurally classified into symmetric Gemini
surfactants and asymmetric Gemini surfactants. Among them, the asymmetric Gemini
surfactant structure is a novel surfactant, which contains different polar groups,
[
22]
different hydrophobic segments, and various linking groups . Due to its structurally
rich and controllable factors, asymmetric Gemini surfactants have superior
performance, for example, better surface activity, better water solubility, higher
[
7]
hydrophilicity, lower critical micelle concentration, good foaming, etc. Wang
prepared three asymmetric sulfosuccinate diester Gemini surfactants and all three
surfactants can significantly reduce the surface tension and have a good emulsifying
ability, good permeability, and certain foaming ability.
Rapeseed oil is one of the world’s largest vegetable oils and is one of the first
vegetable oils used to lubricate collagen fiber materials. However, natural rapeseed oil
is easily oxidized due to a large amount of unsaturated double bonds, which limited
[
23]
the application of rapeseed oil in collagen fiber lubricating materials . Therefore, it
was usually necessary to chemically modify the rapeseed oil to prepare rapeseed oil
collagen fiber lubricating material (ROCLM). However, the traditional ROCLM

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needed to be emulsified by hot water before lubricating collagen fiber, which was
complex and time-consuming. Therefore, in order to emulsify the traditional ROCLM
in cold water and simplify the operation steps, we proposed a strategy to introduce
Gemini surfactant as an emulsifier into the ROCLM to obtain a new collagen fiber
lubricating material (CFLM). And the obtained CFLM may have better water
solubility and better emulsifying ability in cold water.
Based on the above background, an asymmetric sodium benzenesulfonate
Gemini surfactant (ASBGS) was synthesized and its structure as showed in Figure 1a.
Two sulfonic acid groups (-SO Na) made it emulsifying properties better. The basic
3
performances of ASBGS were measured. Due to the similar hydrophilic group in
structure and commonly used surfactant, sodium dodecylbenzene sulfonate (SDBS)
was used as a control sample. Then different dosages of surfactants (ASBGS and
SDBS) were introduced into the traditional ROCLM to obtain CFLMs, and their
emulsifying ability in cold water was investigated by R-value.
Figure 1 Structural formula of ASBGS (a) and SDBS (b)
Note: R is an alkyl long chain of dodecanoic acid and R is an alkyl long chain of
1
2
dodecanol.
2
2
Experimental
.1 Materials

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Dodecanoic acid, Guangdong Chemical Reagent Engineering Technology
Research and Development Center (≥98.5%); p-aminophenol, Shanghai Shanpu
Chemical Co., Ltd (≥99%); dodecanol, Tianjin Fuchen Chemical Reagent Factory
(
≥99%); maleic anhydride (MAH), Hedong District, Tianjin Hongyan reagent plant
≥99.5%); dimethylsulfoxide (DMSO) (≥99.5%), anhydrous sodium acetate (≥99.0%),
(
ethyl acetate (≥99.5%), Tianjin Tianli Chemical Reagent Co., Ltd. All the materials
are analytical pure.
2
.2 Synthesis of asymmetric sodium benzenesulfonate Gemini surfactant
2
.2.1 Synthesis of 4- (dodecyl amide) phenol
With dodecanoic acid and p-aminophenol as raw materials, ethyl acetate as
catalyst (the dosage was 0.5% of dodecanoic acid quality), the mixture of sodium
bisulfate and sodium borohydride as antioxidant (the dosage of dodecanoic acid 0.4%,
wherein m (sodium bisulfate): m (sodium borohydride) = 3:2) for reaction. The
reaction formula was shown in Scheme 1. The reactants were weighed according to
the ratio of n (dodecanoic acid): n (p-aminophenol) = 1:1.05. The p-aminophenol was
dissolved with DMSO. All reactants were added into the 250 mL hydrothermal reactor
(Xi'an Zhiyan Instrument Equipment Co., Ltd.) and put it into the oven for a reaction.
The reaction temperature was 160 ℃ and the reaction time was 5 h. The crude
product 4-DAP was obtained. The product was extracted with ethyl acetate and
distilled water, then washed with distilled water for 3 to 4 times to remove the solvent
DMSO; the ethyl acetate phase was rotary evaporated and then vacuum drying for
several hours; the dried product was recrystallized to obtain the purified product.
-1
-1
-1
FT-IR: 3311 cm (phenolic hydroxyl), 1653 cm (amide linkage), 1548 cm
-1
-1
-1
(
secondary amine), 1247 cm (cyano), 1515 cm and 1467 cm (aromatic
-1
1
hydrocarbons), and 832 cm (benzene ring). H NMR (CDCl , ppm): δ0.854 (3H,
3

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CH ), δ1.057 (18H, CH (CH ) ), δ1.240 (2H, CH -C=O), δ4.388 (1H, Ar-OH),
3
3
2 9
2
δ6.653 (1H, -NH-), δ7.343 (2H, Ar-H).
O
O
N2
50~160℃
R1 C OH + NH2
OH
R1 C NH
OH
1
Scheme 1 Reaction formula of 4-DAP
Note: R is an alkyl long chain of dodecanoic acid and R is an alkyl long chain of dodecanol.
1
2
2
.2.2 Synthesis of dodecanol maleic acid monoester
In a 250 mL four-necked flask equipped with condensate, agitator and
temperature control device, then passed into N , the reaction was carried out with
2
MAH and dodecanol as raw materials and anhydrous sodium acetate as the catalyst
(
accounting for 0.1% of the mass fraction of the system). The reaction formula was
shown in Scheme 2. The reactants were weighed according to the ratio of n (MAH): n
dodecanol) = 1:1. The reaction temperature was 85℃ and the reaction time was 4 h.
(
The crude product DMM was obtained. Finally, the product was recrystallized to
obtain the purified product.
-1
-1
FT-IR: 1716 cm (ester carbonyl), 1646 cm (carbon-carbon double bond), 1292
-1
-1
1
cm and 1166 cm (ether bond). H NMR (CDCl , ppm): δ0.901 (3H, CH ), δ1.283
3
3
(
18H, CH (CH ) ), δ1.725 (2H, CH -C=O), δ4.310 (2H, CH -O), δ6.411 (2H,
3
2 9
2
2
-CH=CH-), δ7.264 (1H, -COOH).
O
O
catalyst
R2 CH OH +
O
R2 CH O C
CH CH COOH
2
2
O
Scheme 2 Reaction formula of DMM
Note: R and R are the same as scheme 1.
1
2
2
.2.3 Synthesis of 4- (dodecyl amide) phenyl maleic acid diester

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The reaction was carried out with the purified 4-DAP and DMM as raw materials,
and zinc oxide as the catalyst (accounting for 0.2% of the mass fraction of the system).
The reaction formula was shown in Scheme 3. The reactants were weighed according
to the ratio of n (4-DAP): n (DMM) = 1:1. The reactions were added into the 250 mL
hydrothermal reactor and put it into the oven for a reaction. The reaction temperature
was 140 ℃ and the reaction time was 5 h. The crude product 4- (dodecyl amide)
phenyl maleic acid diester (4-DAPMD) was obtained. It was extracted with ethyl
acetate and then the ethyl acetate phase was rotary evaporated to obtain the purified
product.
-1
-1
-1
FT-IR: 1729 cm (ester carbonyl), 1645 cm (amide linkage), 1162 cm (ether
-1
-1
-1
bond), 1548 cm (secondary amine), 1216 cm (cyano), 1457 cm (aromatic
-1
1
hydrocarbons), and 832 cm (benzene ring). H NMR (DMSO, ppm): δ0.857 (6H,
CH ), δ1.081 (36H, CH (CH ) ), δ1.243 (2H, CH -C=O), δ4.347 (2H, CH -O),
3
3
2 9
2
2
δ6.361 (2H, -CH=CH-), δ6.689 (1H, -NH-), δ7.354 (2H, Ar-H).
OH
O
C
O
C
O
C
O
CH
CH
OCH2 R2
CH
CH
OCH2 R2
OH
+
C
O
C
O
R1
HN
C
O
R1
HN
Scheme 3 Reaction formula of 4-DAPMD
Note: R and R are the same as scheme 1.
1
2
2
.2.4 Synthesis of sodium benzenesulfonate Gemini surfactant
The pH value of the diester product system was adjusted to 7 by the sodium
hydroxide solution with a mass fraction of 30%. Phase transfer catalyst was
dodecanol maleate monoester salt, which was synthesized by DMM and sodium
hydroxide (n (DMM): n (sodium hydroxide) = 1:1). Next, the catalyst was added to
the diester product system and stirred uniform. Then, 30% of sodium bisulfate

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solution was slowly added to carry out sulfonation of the reaction system. Wherein
the quality of sodium bisulfate was weighed by the ratio of n (4-DAPMD): n (sodium
bisulfate) = 1:2.2). The reaction formula was shown in Scheme 4. The reaction
temperature was 150 ℃ and the time was 6 h. The crude product ASBGS was
obtained, which was extracted with distilled water and heated in a water bath at 80 ℃
for 30 min. The insoluble matter was removed by filtration, and the filtrate was
collected. The filtrate was evaporated to crystallize, and crystals were collected and
washed three times with absolute ethanol to remove excess phase transfer catalyst.
Finally, the product obtained was a purified product.
-1
-1
-1
FT-IR: 3472 cm (phenolic hydroxyl), 1716 cm (ester carbonyl), 1603 cm
-1
-1
-1
-1
(
amide linkage), 1409 cm and 1133 cm (ether bond), 1047 cm and 624 cm
1
(
sulfonate group). H NMR (D O, ppm): δ0.791 (6H, CH ), δ1.189 (36H, CH (CH ) ),
2
3
3
2 9
δ2.030 (2H, CH -C=O), δ2.770 (1H, -CH-), δ3.807 (2H, -CH -O), δ6.841 (1H, -NH-),
2
2
δ7.208 (2H, Ar-H).
O
C
O
C
O
O
C
O
CH2 CH
OCH2 R2
O
C
CH
CH
OCH2 R2
NaHSO3
SO3Na
SO Na
3
C
O
R1
HN
C
O
HN
R1
Scheme 4 Reaction formula of ASBGS
Note: R and R are the same as scheme 1.
1
2
2
.2.5 Preparation of collagen fiber lubricating materials
The CFLMs were prepared by the surfactants (ASBGS, SDBS) and ROCLM.
The amount of surfactant showed in Table 1. The reaction vessel was a three-necked
flask equipped with a stirrer, a condensing and a temperature control device. The
reaction needed to be stirred at 85 C for 1 h. The specific experimental scheme
showed in Table 1.

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Table 1 Experimental scheme for preparation of CFLMs
CFLMs types
1
0
2
3
1
4
2
5
4
Mass fraction (%)
0.5
Note: 1 represents the original CFLMs which was not added with surfactants (ASBGS,
SDBS); 2 to 5 respectively represent the mass fraction of surfactants (ASBGS, SDBS) in the
CFLMs of 0.5%, 1%, 2%, 4%.
2
.3 Surface tension measurement
With reference to QB/T1323-1991, the surface tension measurement was
conducted using a ring-pull liquid film method on XJZ-200 automatic surface tension
meter (Chengde Jinjian Testing Instrument Co., Ltd) at 298.15 ± 0.3 K. The surface
tension meter calibrated by measuring the surface tension of pure water of known
concentration. The prepared Gemini surfactant could be dissolved at room
temperature (25 ℃) by more than 90%, and the higher temperature (~ 40 ℃) can
completely dissolve it in water. Therefore, a series of different concentrations of
aqueous surfactant solutions prepared at a certain temperature (~ 40 ℃) and their
surface tensions were measured separately. And the surface tension values measured
three times and the recorded values taken from the average of these values. Then drew
the curve that the surface tension followed the concentration changed (γ-c curve).
Critical micelle concentration (CMC) determined by the inflection point of the γ-c
curve. At this time, the concentration was the critical micelle concentration, and the
corresponding surface tension was the surface tension corresponding to the critical
micelle concentration (CMC).
The surface activity at C , the surfactant concentration required to reduce the
20

ACCEPTED MANUSCRIPT
[
16]
surface tension of water by 20 mN/m , as well the surface tension at CMC (γCMC
)
also determined by the same curve.
2
.4 Conductivity measurement
The conductivity of the surfactant solution measured by a conductivity meter
DDJS-308A (Shanghai Instrument and Electric Scientific Instrument Co., Ltd., an
accuracy of 1%). The conductivity meter calibrated by measuring the conductivity of
the solution with a known concentration of potassium chloride. The electrode of the
conductivity meter washed with distilled water to remove impurities adsorbed on the
surface, and finally immersed in the calibration solution. After the calibration, the
conductivity of the surfactant solutions at a temperature of 298.15±0.3, 308.15±0.3,
[
24]
3
18.15±0.3, 328.15±0.3 K measured
.
2
.5 Emulsification measurement
A certain volume of the surfactant solution thoroughly mixed with the liquid
paraffin wax to form an emulsion. The emulsification performance of the surfactant
[
25]
evaluated according to the time required to separate 10 mL of water . In this study,
another major objective was to improve the emulsifying property in cold water for
CFLMs by combining Gemini surfactant with the ROCLM. Therefore, when we
examined the emulsifying property of the surfactant, the ROCLM selected instead of
the liquid paraffin wax, thereby detecting the emulsifying effect of the surfactant on
the grease. The specific operation was as follows:
Firstly, a surfactant solution with a mass fraction of 0.1% prepared with ultrapure
water. Then, adding 40 mL of the surfactant solution and 40 mL of the ROCLM to

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1
00 mL graduated cylinder with stopper. Putting it into a 40 ℃ water bath constant
temperature 5 min. Turning the cylinder up and down five times, then standing for
another minute, repeated the same operation five times, and recorded the results with
a stopwatch. At this point, the water and oil phase gradually separated. The water
phase appeared slowly, and until the height of the water phase reached to 10 mL.
Finally, recording the time required for separation.
2
.6 Dynamic Light Scattering (DLS)
The DLS measurement was recorded using a Zetasizer Nano-ZS90 (Malvern
Instrument Ltd., Malvern, UK) at a 90° scattering angle. The concentration of the
surfactant solution configured was 10 CMC. The solution ultrasonically dispersed
uniformly and then measured, and the temperature controlled at 25.0±0.1 ℃. Each
sample repeatedly measured 3 times and the average value was taken.
2
.7 Transmission Electron Microscope (TEM)
TEM images obtained by a negative-staining method using a Tecnai G2 F20
S-TWIN transmission electron microscope (FEI Company, US). Phospho-tungstic
acid solution (1 wt %) used as a dyeing agent. The copper mesh placed on one drop of
the sample solution for 5 minutes, and the excess surfactant solution wiped off with
filter paper to form a thin liquid film on the copper mesh. Afterward, a small drop of
phospho-tungstic acid solution added to the surface of the thin liquid film and the
excess solution was wiped off, and then the samples dried in the air. After drying, the
aggregated morphologies of the samples observed using the TEM.
2
.8 Emulsifying property in cold water

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As the oil density is smaller than the water, so the emulsified oil will float on the
upper emulsion. The emulsified CFLMs and water evenly distributed in the lower
layer of the test tube. The height of the lower layer emulsion can express as the
emulsification capability of the CFLMs in cold water. The higher the height indicates
the better the emulsification capability of the CFLMs in cold water. Therefore, in this
study, different types of CFLMs were used. The CFLMs were diluted 4 times with
normal temperature water and mixed well. Then standing for 4 h, the oil film
observed. After the end of standing, the h-value and the H-value measured. Finally,
the R-value calculated according to the formula (1):
h
H
R 
(1)
Where R is the emulsion stability [%], h is the emulsified emulsion height [cm], and
H is the total emulsion height [cm].
3
3
Results and discussion
.1 Surface tension
70
60
50
40
30
20
-4
0.8×10 mol/L
γCMC
C
2
0
CMC
0
2
4
6
8
10
-
4
C(×10 mol/L)
Figure 2 Plot of surface tension versus molar concentration of ASBGS aqueous
solution.
The CMC refers to the concentration that the surfactant solution begins to form a

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large number of micelles, which is also an important measure of the activity of the
surfactant active parameters. When CMC reached, the nature of the solution will
change significantly. The smaller the CMC is, the higher the efficiency of the
[
26]
surfactant is
. The CMC usually determined by the turning point of the surface
tension-concentration curve (γ-c curve). The surface tension at the CMC (γCMC) is one
of the measures of the decrease in surface tension, which means the maximum extent
that the surface tension can reduce. It can use as one of the indicators of measuring
[
16]
surfactant activity advantages and disadvantages at the gas/liquid interface
. C20
refers to the surfactant concentration at which the surface tension of pure solvent
[
27]
reduced by 20 mN/m . The γCMC and C also determined from the same γ-c curve.
20
The γ-c curve of the synthesized ASBGS showed in Figure 2. The surface tension
decreased as the concentration of ASBGS increased. At the CMC, micelles formed
and the surface tension reached a platform.
The values of the CMC, γCMC, and C of the ASBGS and data corresponding to
20
[
16]
the conventional analog SDBS
summarized in Table 2. It was obvious that the
surface tension of ASBGS at the CMC (γCMC = 25.1 mN/m) was lower compared with
the SDBS (γCMC = 39.0 mN/m), indicating that the ASBGS adsorbed strongly at the
[
14]
air/water interface and demonstrated effective surface activities . The value of C20
-4
(
C = 0.4×10 mol/L) indicated that the efficiency of ASBGS in reducing the surface
20
tension of water increased by three orders of magnitude compared with SDBS (C =
2
0
-4
2
6.0×10 mol/L).

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Table 2 Surface active parameters of ASBGS and SDBS
-
4
-4
Surfactants
ASBGS
CMC (10 mol/L)
γCMC (mN/m) C (10 mol/L)
CMC/C20
2.0
20
0.8
25.1
39.0
0.4
[
16]
SDBS
16.3
26.0
0.6
In addition, the CMC/C20 ratio indicates whether the surfactant has a better
[
28]
affinity for micellization or adsorption at the air/water interface
. The value of
CMC/C20 (CMC/C20 = 2.0) indicated the relationship that the micellization and
surface adsorption of ASBGS was one order of magnitude higher than the value of
SDBS (CMC/C = 0.6). That was to say, the ASBGS adsorbed more easily at the
2
0
air/water interface compared with SDBS. In summary, the ASBGS offered high
efficiency in reducing the surface tension of water and showed high surface activity
[
29]
.
3
.2 Thermodynamic parameters
In order to obtain the thermodynamic parameters of the micellization of ASBGS
aqueous solution, the micellization behavior of the aqueous surfactant solution was
determined by conductivity at different temperatures. The electrical conductivity (k)
versus concentration (c) plots for the aqueous solutions of the surfactant showed in
Figure 3. The corresponding thermodynamics parameters of micellization and CMC
values at different temperatures listed in Table 3. Obviously, the CMC obtained from
surface tension and conductivity measurement was consistent for the surfactant.
In each diagram, two straight lines with different slopes obtained and intersected
at CMC points. It was observed that the CMC values of ASBGS increased with the
increase of temperature (Figure 3). Generally speaking, the influence of temperature

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on ionic surfactant CMC mainly includes two aspects. A higher temperature reduces
the hydration degree of the hydrophilic cephalic base, which is conducive to the
[
24, 30]
formation of micelles and the reduction of CMC
. On the contrary, the ordered
structure of water molecules around hydrocarbon chains may be destroyed, which
may promote the dissolution of surfactant monomers and hinder the micellization of
[
31]
surfactant monomers . For ASBGS, the latter factor dominated the micellization in
the studied temperature range.
65
60
55
50
45
40
35
30
298.15K
308.15K
318.15K
328.15K
0.06
0.07
0.08
0.09
0.10
0.11
0.12
c(mol/L)
Figure 3 Conductivity versus concentration plot of ASBGS aqueous solutions at
different temperatures.
According to the phase separation model, the thermodynamic parameters of
0
0
micellization calculated. The standard Gibbs free energy (ΔG mic), entropy (ΔH mic)
0
and enthalpy (ΔS mic) of micellization of the synthesized ASBGS could obtain from
[
30, 32]
the following Eqs
.
0
mic
0

RT ln X
（2）
（3）

G
cmc
0

ln X
0
2
cmc

RT

H
mic

T

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0
0

H  G
0
mic
mic
mic

（4）
S
T
Where R is the gas constant [8.314 J/(mol·k)]; T is the absolute temperature [K]; Xcmc
is the CMC expressed as mole fraction, Xcmc = CMC/55.4, 55.4 is the number of
moles of 1L water at 298.15 K. All the calculated parameters were listed in Table 3.
Table 3 Micellar thermodynamic parameters of ASBGS at different temperatures
-
3
0
0
CMC (×10
mol/L)
ΔG
ΔH
0
mic
mic
T(K)
-TΔS (kJ/mol)
mic
(kJ/mol)
-33.22
-34.27
-35.35
-36.43
(kJ/mol)
-1.74
-1.39
-0.97
-1.03
0
2
98.15
08.15
18.15
28.15
0.084
0.086
0.087
0.088
-31.48
-32.88
-34.38
-35.40
3
3
3
In the range of 298.15~328.15K, the values of ΔG
found to be negative for
mic
the ASBGS system, indicating that the micellization process of the surfactant in
aqueous solution was spontaneous (Table 3). The micellar solution formed was a
[
33]
thermodynamically stable system
. At the same time, it could see from the table
0
that the ΔG
values decreased with increasing temperature, indicating that the
mic
[
34-35]
increase of temperature not conducive to the formation of micelles
. In other
0
words, value of |ΔG mic| increased gradually with the increase of temperature. Due to
the increase of temperature destroyed the hydrogen bond between the surfactant and
[
36]
0
water molecule, which makes the stability of micelles higher . The ΔH
was also
mic
negative, indicating that micellization was an exothermic process. And within the
0
temperature range studied, the ΔH
of ABSGS did not change much, indicating that
mic
there was no significant change in the environment around the hydrophobic chain of
ABSGS molecules during the temperature changed. Obviously, the main contribution
0
0
to ΔG
during micelle formation came from ΔS mic. Therefore, the micellization
mic
process of ABSGS was an entropy-driven process over the temperature range studied

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[
37]
0
.
However, positive ΔS
value indicated that the process of forming micelles
mic
would increase the degree of disorder of the system. The main reason for this might
be when a small amount of surfactant dissolved in an aqueous solution, parts of the
hydrogen bonding structures between the water molecules rearranged and a new
structure (different from water structure) formed around the hydrophobic hydrocarbon
[
38]
chains. That is, the so-called “iceberg structure”
. When the concentration of
surfactant reached to the critical micelle concentration, the hydrophobic hydrocarbon
chains moved closer and associated with each other, which led to the destruction of
the water molecules around the hydrophobic carbon chains, thus strengthen the chaos
[
39]
0
of surfactant solution
. As seen from Table 3, |-TΔS mic| value increased with
temperature increasing, indicating that the increase of temperature increased the
molecular motion of surfactant. In other words, the increase in temperature
strengthened the chaos of the system.
3
.3 Emulsification
Emulsification means that under certain conditions, the two immiscible liquids
can be formed with certain stability of the role of a liquid-liquid dispersion system.
Only after the addition of an emulsifier to emulsify, thereby obtain a stable emulsion.
[
40]
The emulsifier is mostly surfactant
. The emulsion is a heterogeneous dispersion
system formed by dispersing a liquid in another immiscible liquid. The surfactant is
added as an emulsifier to reduce the interfacial tension and adsorbs at the interface to
form an interfacial film that can prevent or delay the aggregation and condensation
caused by the mutual collision of the dispersion liquid beads. The stronger the effect
of surfactant adsorption at the interface, the stronger the interfacial film formed, and
the more stable the emulsion was.
We investigated the emulsifying ability of ASBGS for ROCLM (Table 4), which

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qualitatively evaluated according to the time required to separate 10 mL of the water
phase. The longer separation time of the water phase was, and the higher emulsion
power was. The ASBGS had the separation time of 283 s for ROCLM. The results
showed that the ASBGS had a better emulsifying ability for ROCLM than SDBS. In
other words, the ASBGS has a good emulsifying effect on grease. The main reason is
that Gemini surfactants have two hydrophobic chains in their molecules, which are
closely linked by linkers and increase the hydrophobic interaction. Thus, Gemini
surfactants are more likely to encapsulate oil-soluble groups than conventional
surfactants containing only one hydrophobic group, so which emulsifying properties
are greater than conventional surfactants. And Gemini surfactants have good affinity
for hydrophobic substances (oils, solids, etc.), which can greatly reduce the surface
[
41]
tension between water and oil and therefore have good emulsifying properties
That is, the ASBGS emulsified the ROCLM better than SDBS.
Table 4 Results of the emulsification force testing
.
Surfactants
emulsification capacity (s)
ASBGS
SDBS
283
176
3
.4 Aggregation property
ASBGS
SDBS
40
30
20
10
0
10
100
1000
Diameter (nm)
Figure 4 Apparent particle size distributions of the ASBGS and SDBS

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The aggregate size distributions of the ASBGS and SDBS studied at the
concentration of 10 CMC by DLS and TEM measurements at 25 ℃, separately.
Figure 4 showed the size distribution of the ASBGS and SDBS in solution as
observed at the concentration of 10 CMC by DLS. It could be seen from Figure 4 that
there was both only one peak in DLS, the average peak diameter of the ASBGS was
8
0 nm and the SDBS was 200 nm. This indicated that the concentration of 10 CMC of
ASBGS and SDBS aggregated in aqueous solution may be the vesicles. And the
vesicle was a mono-dispersed system, which indicated that its particle size
distribution was more uniform.
Figure 5 TEM images of negatively-stained aggregates formed in the ASBGS (a) and
SDBS (b) solution at a concentration of 10 CMC.
To further elucidate the conclusion of DLS, the aggregation morphologies of the
ASBGS and SDBS at the concentration of 10 CMC tested by TEM measurement.
Figure 5 showed the presence of micelle and vesicle. Figure 5a was the TEM image of
the ASBGS. As could see from Figure 5a, it could observe that the size of the vesicles
formed by the ASBGS in aqueous solution (the concentration of 10 CMC) was about
8
0-90 nm. The size of vesicles provided by TEM was consistent with the result of
DLS. The majority shapes of the emulsion were spherical or spherical-like. In the

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ASBGS studied, the aromatic rings in the head may participate in the π-π interaction
between molecules, thereby affecting the formation of vesicles, as observed by Wang
[
42]
.
And Figure 5b was the TEM image of the SDBS. As could be seen from Figure
5
b, the size of the vesicles formed by the SDBS in aqueous solution (the
concentration of 10 CMC) was about 200± nm. The size of vesicles provided by TEM
was not consistent very well with the result of DLS.
3
.5 Emulsifying property in cold water
The emulsifying ability of CFLMs in cold water could qualitatively characterize
by measuring its R-value. The higher the R-value, the better the emulsifying property.
The experimental results showed that with the increase of the ASBGS, the
emulsifying property of the CFLMs increased gradually (Table 5). Since the prepared
surfactant is a dark brown solid powder, the addition of too much amount will affect
the color and viscosity of the CFLMs, so the amount of the surfactant added was only
4
%. Although only 4% was added, the emulsifying capacity of the CFLMs was still
improved from 47.37% to 87.37%. The R-value of CFLMs prepared by introducing
SDBS ranged from 47.37% to 54.24%. Although the R-value increased, the growth
rate was small, indicating that SBDS contributed little to the emulsification ability of
CFLMs in cold water.
In other words, the addition of ASBGS could improve the emulsification of
CFLM. And its emulsifying ability in cold water increased from 47.37% to 87.37%
(Table 5). The experimental results further proved that Gemini surfactants have
excellent emulsification and compounding properties.

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Table 5 Results of the emulsifying ability of CFLMs in cold water
dosage of surfactants
0
0.5%
1%
2%
4%
R (ASBGS) (%)
R (SDBS) (%)
6
4
3.16
8.14
75.79
49.61
78.95
51.16
87.37
54.26
4
7.37
4
Conclusions
An asymmetric sodium benzenesulfonate Gemini surfactant was synthesized.
The resulting Gemini surfactant was more effective at reducing surface tension than
SDBS. The CMC of the Gemini surfactant determined from surface tension and
conductivity measurements was at least two orders of magnitude lower than the CMC
of the SDBS. And the micellization of this Gemini surfactant was a spontaneous
process. In most cases, 10 CMC Gemini surfactant in aqueous solution presented an
aggregate having an apparent hydrodynamic diameter of about 80 nm. The Gemini
surfactant has excellent emulsifying ability, and can significantly improve the
emulsification ability of the rapeseed oil collagen fiber lubricating materials in cold
water. The application of the Gemini surfactant in collagen fiber lubricating materials
not only simplifies the operation procedure but also saves energy. The Gemini
surfactant with excellent emulsifying ability in cold water has a certain application
prospect in the fields of the leather industry and tertiary oil recovery.
Acknowledgements
This work was supported by the key research project of Shaanxi Province
[
2017GY-187]; the National Natural Science Foundation Project [21878182].
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