Studies on the synthesis, surface activity and the ability to form pH-regulated wormlike micelles with surfactant containing carboxyl group

Article abstract of DOI:10.1016/j.molliq.2020.113182

In this article, a series of pH regulated surfactants with different hydrophobic chain length and carboxyl group molecular structure positions were designed and synthesized. The molecular structure of pH regulated surfactants was analyzed by mass spectrometry and ¹H NMR spectroscopy. The results of the surface activity test show that the pH regulated surfactants have lower surface tension, which indicates that it is easier to adsorb directionally at the gas-liquid interface and to aggregate in solution. Both inorganic and organic counterions can improve the viscosity of the system to some extent, but the viscosity-increasing ability of organic counterions is much higher than that of inorganic counterions. The results of rheology and dynamic light scattering show the transition from spherical micelles to wormlike micelles was observed when pH increased from 5 to 8 in the Docos-13-enoylamino-acetic acid (Gly-22)/Trimethylstearylammonium chioride (ODAC) system. The results of Cryo-transmission electron microscopy verify this result directly. Also, the experimental results show that the Gly-22/ODAC system has excellent pH cycle regulation performance, which can significantly reduce the application cost of the system.

Full text of DOI:10.1016/j.molliq.2020.113182

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Studies on the synthesis, surface activity and the ability to form
pH-regulated wormlike micelles with surfactant containing
carboxyl group
Zhihu Yan, Feng Qian, Haonan Sun, Xia Lu, Yu Li, Haibin Lv,
Caili Dai, Minglian Jiao
PII:
S0167-7322(20)30743-1
DOI:
https://doi.org/10.1016/j.molliq.2020.113182
MOLLIQ 113182
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
4 February 2020
16 April 2020
17 April 2020
Please cite this article as: Z. Yan, F. Qian, H. Sun, et al., Studies on the synthesis,
surface activity and the ability to form pH-regulated wormlike micelles with surfactant
containing carboxyl group, Journal of Molecular Liquids (2020), https://doi.org/10.1016/
j.molliq.2020.113182
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© 2020 Published by Elsevier.

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Studies On the Synthesis, Surface Activity and the Ability to
Form pH-regulated Wormlike Micelles with Surfactant
Containing Carboxyl Group
Zhihu Yan*,1,2,3, Feng Qian1, Haonan Sun1, Xia Lu1, Yu Li1, Haibin Lv1, Caili Dai4*, Minglian Jiao1*
1School of Geomatics and Marine Information, Jiangsu Ocean University, Lianyungang, 222005, China.
2Jiangsu Key Laboratory of Marine Bioresources and Environment, Lianyungang, 222005, China.
3Marine Resources Development Institute of Jiangsu, Jiangsu Ocean University, Lianyungang 222005, China
4School of Petroleum Engineering, State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China),
Qingdao, 266580, China.
E-mail: zhihuyan@126.com, zhihuyan@jou.edu.cn;
ABSTRACT
In this article, a series of pH regulated surfactants with different hydrophobic
chain length and carboxyl group molecular structure positions were designed and
synthesized. The molecular structure of pH regulated surfactants was analyzed by
1
mass spectrometry and H NMR spectroscopy. The results of the surface activity test
show that the pH regulated surfactants have lower surface tension, which indicates
that it is easier to adsorb directionally at the gas-liquid interface and to aggregate in
solution. Both inorganic and organic counterions can improve the viscosity of the
system to some extent, but the viscosity-increasing ability of organic counterions is
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much higher than that of inorganic counterions. The results of rheology and dynamic
light scattering show the transition from spherical micelles to wormlike micelles was
observed when pH increased from 5 to 8 in the Docos-13-enoylamino-acetic acid
(Gly-22)/ Trimethylstearylammonium chioride (ODAC) system. The results of
Cryo-transmission electron microscopy verify this result directly. Also, the
experimental results show that the Gly-22/ODAC system has excellent pH cycle
regulation performance, which can significantly reduce the application cost of the
system.
Keywords: pH regulated surfactant; Surface activity; Wormlike micelles; Molecular
self-assembly.
1. Introduction
In the past decade, the fabrication of smart responsive materials has been a hot
research area in the field of soft materials1. Smart responsive materials can
dynamically change their structural and functional requirements in response to
environmental changes. In these studies, the surfactant is one of the hot research
objects, which has the characteristic of changing the structure spontaneously under the
stimulation of the environment and then inducing the change of the macroscopic
properties such as viscosity and elasticity of the solution2. At the same time, the
molecular structure of the surfactant remains intact, so that it is recycled. The external
stimuli that regulate changes in the molecular structure of surfactants are known as
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“switches”. The main types of the switch are light, carbon dioxide, temperature,
magnetic, pH and so on3.
Up to now, extensive and in-depth research has carried out on the synthesis and
application of regulated surfactant. Although some progress has been made, the
shortcomings still exist. The light-regulated surfactant can adjust the wavelength and
duration of light precisely, but it has some disadvantages such as longer response time,
incomplete change of molecular structure, reduced light transmittance, environmental
limitation and high requirement of equipment4. The pollution degree of
redox-regulated surfactant is small, but the complex molecular structure, the
expensive ferrocene derivatives, and the difficulty of viscosity reduction limit the
application of this kind of regulated surfactant in the practical production5. The CO2
regulated surfactant has little pollution to the environment, but the system has poor
temperature resistance, slow response period, complicated and expensive ventilation
equipment6. The temperature regulated surfactant has little pollution to the system.
However, it is challenging to realize precise control and narrow temperature control
window. In contrast, the stable, biocompatibility, highly surface-active, and
inexpensive pH regulated surfactant has the potential to achieve large-scale industrial
applications7.
pH regulated surfactant is a class of molecules that can respond to pH
environment changes in the system. These surfactants mainly through cis-trans,
electrostatic repulsion, protonation, and other ways to cause changes in physical and
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chemical properties of surfactant. Hongsheng Lu8 et al. studied the wormlike micelle
system with pH regulation ability, which was formed by N, N-dimethyloleamide alone.
By changing the pH value, the micelles can change from spherical micelles to vesicles
in a reversible way. Wang9 et al. designed and synthesized a series of single-chain
surfactant C18N3 with multi-amino groups, and studied the aggregation behavior of
C18N3 in aqueous solution under different pH conditions. With the increase in pH
value, the surface tension of C18N3 solution also increases, and the aggregation also
changes from spherical micelles to lamellar micelles. Yiyang Lin10 et al. synthesized a
single chain pH-responsive surfactant N-dodecyl phosphate (DPA), which changes its
degree of ionization by adjusting pH value. The morphology of aggregates in the
mixture of CTAB and DPA has been investigated. With decreasing in pH value, the
self-assembly structure of the system has many changes, such as spherical micelles,
rod micelles, wormlike micelles, lamellar liquid crystals, and vesicles.
Because carboxyl groups are environmentally friendly, responsive, and
salt-tolerant, selecting it as pH regulating groups can achieve excellent
physicochemical properties of pH regulated surfactant11. However, the pH regulated
surfactant containing carboxyl group has not been deeply investigated. Therefore, we
designed and synthesized a series of pH regulated surfactants with different
hydrophobic chain length and carboxyl group molecular structure positions. The
structures and abbreviations of pH regulated surfactants are depicted in Scheme 1.
Second, the molecular structures of pH regulated surfactants were analyzed by mass
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1
spectrometry and H NMR spectroscopy. Besides, the effect of different types of
inorganic and organic counterions on the apparent viscosity of the mixed system was
investigated12, 13. Finally, the morphological changes of aggregates formed by
self-assembly of surfactant in a mixed system with different pH values were studied
utilizing rheology, dynamic light scattering, and Cryo-Transmission Electron
Microscopy.
2. Experiment and methods
2.1. Materials and synthesis method
Dichloromethane, toluene, ethyl acetate, acetone, thionyl dichloride, sodium
hydroxide, anhydrous magnesium sulfate were purchased from Sinopharm Chemical
Reagent Co., Ltd. Palmitic acid, stearic acid, erucic acid, benzene, iminodiacetic acid,
glycine, dl-glutamate, dimethyl sulfoxide were purchased from Shanghai Jingchun
Biochemical Technology Co., Ltd.
0.5 mol fatty acid and 150 ml benzene were added into a 500 ml three-necked
flask, stirred and dissolved in a water bath to 30℃. 1 mol thionyl dichloride was
slowly added in by a constant pressure funnel in 30 minutes, then the condensation
reflux reaction was carried out at 70~90°C for 3~4 hours. Then a vacuum distillation
process was used to remove the excess thionyl dichloride and benzene to obtain the
fatty acyl chloride. Add 1 mol amino acid to 500 ml flask, add 50 ml 20% sodium
hydroxide solution, then add 100 ml anhydrous acetone, stirred thoroughly until the
mixture was homogeneous. Then slowly adding fatty acyl chloride to a container.
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During the process, the constant pressure funnel was used to add 20% sodium
hydroxide water solution to control the pH value of the system in the range of 8~10.
The system temperature was set at 0~10°C by an ice-water bath. After the reaction,
the acetone was removed by spin-evaporation. A large number of white crystalline
solid was precipitated after the pH value of the system was adjusted to 1~2 using a
hydrochloric acid solution. Add the white crystal to the flask, dissolve in ethanol, heat
to 40°C, and then add 20% sodium hydroxide. The precipitated crystal was dried to
obtain the final product.
In section 3.3, the full and abbreviated names of the organic counterions are as
follows: Octyltrimethylammonium chloride (OTAC), Decyltrimethylammonium
chloride
Dodecyltrimethylammonium chloride (LTAC), Didodecyldimethylammonium
bromide (DDAB), Benzenemethanaminium (LBAC),
(DTAC),
Decyltrimethylammonium
bromide
(DTAB),
Dodecyldimethylethylammonium bromide (DDAB), Dodecyl Trimethyl Ammonium
Bromide (LTAB), Didodecyl dimethyl ammonium chloride (DDAC),
Trimethyltetradecylaminium·bromide
chloride (MTAC), Tetradecyl dimethyl benzyl ammonium chloride (TDBAC),
Hexadecyltrimethylammonium chloride (CTAC),
Benzylhexadecyldimethylammonium chloride (BHDAC), Cetyltrimethylammonium
bromide (CTAB), Dihexadecyldimethylammonium bromide (DDTAB),
(MTAB),
Trimethyltetradecylammonium
Cetethyldimonium bromide (HDEAB), Dimethyl distearylammonium chloride
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(DTDAC),
Trimethylstearylammonium
chioride
(ODAC),
Dimethyldioctadecylammonium
bromide
(DTDAB),
Stearyldimethylbenzylammonium chloride (SDBAC) , Trimethylstearylammonium
Bromide (TSAB).
2.2. Molecular structure verification
The molecular structure of the synthesized pH regulated surfactants were
1
analyzed by liquid chromatography-mass spectrometry (Agilent 6510, USA) and H
NMR spectrum (Bruker III 400, Switzerland). The results are depicted in
Supplementary Material.
2.3. Krafft point
The data of Krafft point (Tk) was obtained by cooling 1 wt% surfactant solution
in an ice salt bath to -4℃. If the solution is clear, Tk of the surfactant was recorded as
0℃. If the solution is cloudy, slowly heating up and recording the temperature of the
solution from cloudy to clear. The average value was obtained after repeated three
times. During the measurements, the solutions were not buffered and the pH was
around 9.
2.4. Surface tension test
The surface tension was measured by the surface tension meter (Dataphysics,
China). A series of surfactant solutions were prepared and the surface tension was
measured by the Wilhelmy plate method at 25°C. To eliminate the influence of metal
ions on the results, self-made ultrapure water (PURELAB Option-Q, China) was used
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to prepare samples, and resin-based storage bottles and test tubes were used to store
samples. Besides, the average surface tension was measured three times at each
concentration. During the measurements, the solutions were not buffered and the pH
was around 9.
2.5. Rheological property test
The HAAKE MARS 60 rheometer was used to carry out the rheological
experiment. The geometric fixture used as a parallel plate with a diameter of 50 mm.
A bottom-mounted rheometer motor applies a strain on the sample through a lower
plate, and a torque sensor was connected to the upper plate in contact with the sample
to measure the resulting stress. By lowering the upper plate to extrude the sample, the
sample can be uniformly distributed on the lower plate to form a regular cylindrical
geometry with a slight expansion. The thickness of each sample was about 2 mm to
ensure sufficient time for the sample to relax. The linear viscoelastic region was
determined by a 1 Hz stress scan. The temperature was set at 70℃ using an
environmental control panel.
2.6. Dynamic light scattering test
A NanoBrookOmni laser particle size analyzer was used to conduct a dynamic
light scattering test. To ensure the stability of the test light system, it was necessary to
preheat the instrument 30 minutes before the experiment. The sample was filtered by
0.04 μm filter and put into the test instrument. The observation grating was adjusted
to 400, the wavelength is 532 nm and the scattering angle is 90°. The CONTIN
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algorithm was used to process the experimental results. The temperature was set at
70℃.
2.7. Cryo-Transmission Electron Microscopy
Sample preparation shall be carried out in an environmental control system
(T=25°C, humidity=90%). The copper net was clamped with tweezers and inserted
into the chamber of the environmental control system. 5 μL sample was moved by a
pipette gun and dropped on the copper net. The solution forms a thin film on the
copper net, quickly insert the copper mesh into the liquid ethane to freeze the sample.
The samples were observed in Jeol JEM-2100UHR by using Gatan 626. The image
acquisition and storage system was Gatan 832 CCD camera.
3. Results and discussion.
3.1. Krafft point
Tk is the temperature at which the hydrated crystals, monomers, and micelles of
ionic surfactants coexist. It is the lowest temperature at which ionic surfactants can be
micellized, and is the lower limit of the temperature at which it can be applied. Tk of
the pH regulated surfactant is shown in Table 1. As can be seen from Table 1, Tk
decreases with the increase of carbon number in the hydrophobic alkyl chain of the
surfactant molecule. That is consistent with the Tk of conventional ionic surfactants.
Under the same length of hydrophobic tail chain, Tk of the Gly series surfactant is the
highest, the Imi series surfactant is in the middle, and the Glu series surfactant is the
lowest. It is generally believed that the ionic type, the head structure and the length of
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the hydrophobic alkyl chain of the surfactant molecule all have an important effect on
the solubility of the surfactant and Tk, among which the head structure is the main
factor. The result shows that the existence of the amide group and dicarboxylic group
lead to the highest strength of the non-covalent bond between the Glu series surfactant
and water molecules, so that the lowest Tk can be obtained.
3.2. Surface activity
The variation curves of the surface tension of surfactant with concentration at
25°C are shown in Fig. 1 ~ Fig. 3. When the concentration of the surfactant is very
low, the changing trend of surface tension is very tiny. With the concentration of
surfactant increasing, the surface tension drops sharply. When the concentration of the
surfactant continues to increase to a certain value, the surface tension reaches a stable
platform. The concentration at the breakpoint of the curve is the critical micelle
concentration (CMC), and the surface tension at this concentration is the critical
surface tension (γcmc). Combining with Gibbs adsorption isotherm formula14, two
physical quantities which can reflect the compactness of molecules on the gas-liquid
interface can be calculated15: the max saturated adsorption (Γmax) and the minimum
molecular occupied area (Amin).


1

 max

(1)
(2)


2.303nRT logC

T
1
Amin

NA  Γmax
Where R stands for gas constant (8.314 J·mol-1·K-1), T for absolute temperature.
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A value of n=3 for the Gly and Imi series surfactants, and n=2 for Glu series
surfactants16. Where NA stands for Avogadro constant (6.022×1023 mol-1).
In general, the smaller the CMC, the higher the efficiency of reducing surface
tension. The lower the γcmc, the stronger the ability to reduce the solvent surface
tension. The surface activity parameters of the pH regulated surfactants are listed in
Table 2. As can be seen, with the increase of the length of the hydrophobic tail chain,
the value of CMC and γcmc of the pH regulated surfactants gradually decreased. That
because with the increase of the length of the hydrophobic chain, the hydrophobic
property of the surfactant is enhanced, and the self-aggregation occurs more
efficiently at the gas-liquid interface, which leads to the gradual decrease the values of
CMC and γcmc. As the length of hydrophobic tails increases, Γmax increases and Amin
decreases, which is caused by the enhancement of hydrophobic forces. The result is
that the surfactant molecules are more tightly packed, and the packing density is
increasing.
It is also clear from Table 2 that, the order of CMC and γcmc values is
Glu>Imi>Gly when the hydrophobic tail chain length is the same. The possible
explanation for this phenomenon is due to the combined effect of steric hindrance and
electrostatic repulsion. From the point of view of steric hindrance, because the
headgroup of the Gly series surfactant only contains one hydroxyl group, its
headgroup has the least steric hindrance effect, so it has the most efficient and the
strongest ability to reduce the surface tension. For both the Imi series and the Glu
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series surfactant, it have the same number of the hydroxyl group on the headgroup of
the surfactant. However the number of methylene groups on the headgroup of the Glu
series surfactant is more than the Imi series surfactant, which leads to the maximum
steric resistance among three, so the Glu series surfactant has the least efficient and
the weakest ability to reduce the surface tension. From the point of view of
electrostatic repulsion, due to the least charge on the head group of the Gly series
surfactant, the electrostatic repulsion between the head group is also the least, which
makes it easier to aggregate. When the length of the hydrophobic tail chain is the
same but the molecular structure of the headgroup is different, for the Gly series
surfactant, the value of Γmax is the largest and the value of Amin is the smallest. This
phenomenon can still be explained by the theory of the steric hindrance effect and
electrostatic repulsion effect, which will not be repeated here. Due to the strongest
surface activity, Gly-22 was chosen as the surfactant for constructing a pH-regulated
wormlike micelles system.
3.3. Effect of counterions on the apparent viscosity of the system
The viscosity of the mixed solution with a total mass concentration of 4 wt%
(including 2 wt% of Gly-22 and 2 wt% of counter-ion) was measured by rheometer at
70°C and 170 s-1 shear rate to determine the best counterion. The mixed solutions
were not buffered and the pH was around 9.
Counterion can associate with pH regulated surfactants by hydrophobic and
electrostatic interactions to form spherical, rod, wormlike and branched wormlike
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micelles. The type of counter ion has a significant influence on the micelle
morphology and then affects the viscosity increasing effect of the system. Fig. 4 and
Fig. 5 show the effect of inorganic and organic counterions on the apparent viscosity
of the system, respectively.
It can be seen from Fig. 4 and Fig. 5 that both inorganic and organic counterions
can improve the viscosity of the system to some extent, but the viscosity-increasing
ability of organic counterions is much higher than that of inorganic counterions17. A
mean field treatment of the growth process for neutral of highly-screened micelles
gives a prediction for the average contour length L of the wormlike micelles18.
L ~ 1/2 exp[Ec kB T]
(3)
In the above formula, φ is the surfactant volume fraction, T is the temperature, Ec
is the end-cap energy required to form two hemispherical end-caps as a result of rod
scission. The primary mechanism of increasing L by adding inorganic salt is that
inorganic counterion can screen the electrostatic repulsion between the charged
headgroups of Gly-22, which results in a reduction of the optimal molecular area at
the hydrocarbon–water interface, leading to an increase in Ec. Consequently, L,
which increases exponentially with Ec, increases dramatically. The mechanism of the
micelle growth promoted by organic counterion can be clearly explained by the
critical packing parameter P19. According to P=V/aL, where V is the volume of the
lipophilic chain having maximum effective length L, and a is the effective area per
molecule at the surfactant–water interface. For P<1/3, it is predicted that the
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surfactants would assemble into spherical aggregates; for 1/3<P<1/2, wormlike
micelles are expected; whereas for P>1/2, lamellar structures should form
spontaneously.
On the one hand, when adding organic counterion with the opposite charge to
Gly-22 aqueous solution, the hydrophilic head part of organic counterion neutralizes
the head charge of Gly-22, which makes the electrostatic repulsion between the
molecules of Gly-22 weaken and reduces the value of a. On the other hand, the
hydrophobic tails of organic counterion were inserted into the hydrophobic barrier of
micelles, which increased the value of V. The value of P increases with the two
factors, which promotes the formation of wormlike micelles and explains why organic
counterion is more effective than inorganic counterion in promoting viscosity increase.
Also, as can be seen from Fig. 5, the viscosity of the mixed system increases with the
increase of the hydrophobic tail chain length of organic counterion. As the
hydrophobic tail chain of the organic counterion grows, the value of V increases. L is
the maximum length of the hydrophobic chain, due to none of the hydrophobic tail
chain of organic counterions is longer than that of Gly-22, L remains unchanged. Both
of these factors contribute to an increase in the value of P. Therefore, ODAC was
chosen as the counterion of the pH regulated wormlike micelles.
3.4. Rheological properties of pH regulated wormlike micelles
The steady-state shear viscosity of the Gly-22/ODAC mixed solution under
different pH conditions varies with the shear rate is shown in Fig. 6. When pH≤5, the
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apparent viscosity of the solution is minimal and does not change with the increase of
shear rate. It shows the characteristic of Newton fluid, which indicates that the
aggregates formed by surfactant in the system are mainly spherical micelles20. As the
pH increases, the solution begins to exhibit shear thinning and non-Newtonian fluid
behavior, indicating the gradual formation of wormlike micelles in the system. Under
the action of shear force, the wormlike micelle solution changes from the entangled
network state to the directional parallel structure state, which leads to the gradual
decrease of solution viscosity21.
The zero shear viscosity (η) of the mixed system can be obtained by
extrapolation under different pH conditions. The results are shown in Fig. 7. When pH
≤5, the zero shear viscosity of the system is very small. With pH value increases, the
zero shear viscosity also increases. The zero shear viscosity of the system can reflect
the strength of the network structure formed by the wormlike micelles, which
indicates that the strength of the network structure formed by aggregates in the system
increases with the increase of pH value. When pH≥8, the zero shear viscosity of the
system is stable, which means that the network structure formed by aggregates in the
system remains unchanged.
From the steady-state shear analysis, with the pH value increases, the strength of
the network structure formed by aggregates in the system increases. In order to verify
the relationship between the strength of the network structure and the pH value, the
dynamic viscoelasticity of the Gly-22/ODAC mixed system under different pH
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conditions was measured. The experimental results are shown in Fig. 8. The plateau
modulus G0 represents the physical entanglement density between the wormlike
micelles. The relaxation time τ represents the complexity of the three-dimensional
network structure22. The effects of the pH value on plateau modulus G0 and relaxation
time τ of the mixed system are shown in Fig. 9 and Fig. 10. As can be seen from the
diagram, both G0 and τ of the mixed system show a monotonous increasing trend with
the pH value increases, which indicates that the density of the physical entanglement
points between the wormlike micelles in the system increases. At the same time, the
strength of the spatial three-dimensional network structure increases23. The results
above consistent with the steady state shear analysis.
3.5. Morphological characteristics of pH regulated wormlike micelles
The dynamic light scattering works like this: large, slow-moving particles exhibit
a slower pattern of light, while small, fast-moving particles exhibit a faster pattern of
light24. Therefore, the particle size and its distribution can be calculated by the
fluctuation and the light intensity correlation function. In order to verify the changes
in the internal structure of the Gly-22/ODAC system, the aggregate structure of the
mixed system was characterized by the dynamic light scattering, and the experimental
results are shown in Fig. 11.
The average fluid mechanics radius of aggregates with different structures is
significantly different in the dynamic light scattering results. By analyzing the results
of the mixed system under different pH conditions, it is found that the average fluid
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mechanics radius of the system is about 2~5 nm at pH=5, which indicates that the
aggregates in the Gly-22/ODAC system were mainly in the form of spherical micelles.
At pH=8, the average fluid mechanics radius of the system is about 30~40 nm, which
indicates that the aggregates existed mainly in the form of wormlike micelles25. The
results show that the aggregation could change from spherical micelles to wormlike
micelles by changing pH conditions.
To verify that pH value can regulate the aggregates morphology from spherical
micelles to wormlike micelles, the microscopic morphology of aggregates under
different pH value conditions was observed employing Cryo-TEM. As shown in Fig.
12, no wormlike micelles is observed at pH=5. When pH=8, a dense entangled
network formed by wormlike micelles can be observed in the picture. When the pH
value of the solution is greater than 8, the carboxyl group on the molecular structure
of Gly-22 is entirely ionized, so the electrostatic attraction between the negatively
charged Gly-22 and the positively charged ODAC is strongest. By reducing the value
of a and increasing the value of V, the formation of a large number of wormlike
micelles can be explained. When the pH value of the solution decreases from 8, the
ionization of Gly-22 decreases gradually, so the electrostatic attraction between
Gly-22 and ODAC decreases gradually, resulting in the transition from wormlike
micelles to spherical micelles.
If the viscosity of the Gly-22/ODAC system is reversible, the system can be
reused, which can significantly reduce fracturing, stimulation and other applications
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in the oilfield or other industrial fields. In order to study the pH cycling performance
of the 2% Gly-22/2% ODAC system, we repeatedly adjust the pH value of the system
between 5 and 8 and measure the viscosity. The experimental results are shown in Fig.
13. As can be seen from Fig. 13, when pH=5, the viscosity of the system is
approximately equal to 2 mPa·s. When the pH value increases to 8, the viscosity
increases by 3 orders of magnitude. When the pH value drops to 5, the viscosity of the
system drops back to 2 mPa·s. We repeated the experiment 3 times, and the change of
viscosity is within 1 order of magnitude, and the change trend remained unchanged.
The experimental results show that the Gly-22/ODAC system has excellent pH cycle
regulation performance, which can greatly reduce the application cost of the system in
oil field chemistry and other industrial fields.
4. Conclusion
In this article, the results of the surface activity test show that pH regulated
surfactants have lower surface tension, which indicates that it is easier to adsorb
directionally at the gas-liquid interface and to aggregate in solution. On the one hand,
the hydrophilic head part of organic counterion neutralizes the head charge of Gly-22,
which makes the electrostatic repulsion between the molecules of Gly-22 weaken and
reduces the value of a. On the other hand, the hydrophobic tails of organic counterion
were inserted into the hydrophobic barrier of micelles, which increased the value of V.
The value of P increases with the two factors, which promotes the formation of
wormlike micelles and explains why organic counterion is more effective than
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inorganic counterion in promoting viscosity increase. The results of rheology and
dynamic light scattering show the transition from spherical micelles to wormlike
micelles was observed when the pH value increased from 5 to 8. The results of
Cryo-transmission electron microscopy verify the result directly. Besides, the
experimental results show that the Gly-22/ODAC system has excellent pH cycle
regulation performance, which can significantly reduce the application cost of the
system. It is hoped that the results will be helpful to the application of regulated
surfactants in oil field chemistry and other industrial fields.
Corresponding Author
*E-mail: zhihuyan@126.com, zhihuyan@jou.edu.cn;
Tel: +86-518-85895589. Fax: +86-518-86985587.
Acknowledgments
We gratefully acknowledge the financial support by the National Natural Science
Foundation of China (51904109), the Natural Science Foundation of Jiangsu Province
(BK20181072), the Natural Science Foundation of the Jiangsu Higher Education
Institutions of China (18KJB440003), the Jiangsu Provence Postdoctoral Research
Project Fund (2019K143, 2019Z185) and the Lianyungang Postdoctoral Research
Project Fund (ZKK201918).
Conflict of interest
The authors declare that they have no conflict of interest.
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Scheme 1. Molecular structure and code of pH rugulated surfactants.
Number
1
Abbv.
Molecular structure
O
Gly-16
Gly-18
Gly-22
Imi-16
N
COONa
COONa
COONa
H
O
2
3
4
N
H
O
N
H
O
COONa
COONa
N
N
O
COONa
COONa
5
6
Imi-18
Imi-22
COONa
O
N
COONa
O
COONa
COONa
7
8
9
Glu-16
Glu-18
Glu-22
N
H
COONa
COONa
COONa
O
N
H
O
COONa
N
H
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Fig. 1. The surface tension of the Gly series surfactant.
Fig. 2. The surface tension of the Imi series surfactant.
Fig. 3. The surface tension of the Glu series surfactant.
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Fig. 4. Apparent viscosity of 2 wt% Gly-22/2 wt% inorganic counterion mixed system.
Fig. 5. Apparent viscosity of 2 wt% Gly-22/2 wt% organic counterion mixed system.
Fig. 6. The steady-shear viscosity of the 2 wt% Gly-22/2 wt% ODAC mixed system under
different pH conditions.
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Fig. 7. Zero-shear viscosity of the 2 wt% Gly-22/2 wt% ODAC mixed system under different pH
conditions at 70°C.
Fig. 8. Dynamic viscoelasticity of the 2 wt% Gly-22/2 wt% ODAC mixed system under different
pH conditions at 70°C.
Fig. 9. The G0 of the 2 wt% Gly-22/2 wt% ODAC mixed system under different pH conditions at
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70°C.
Fig. 10. The τ of the 2 wt% Gly-22/2 wt% ODAC mixed system under different pH conditions at
70°C.
Fig. 11. DLS result of the 2 wt% Gly-22/2 wt% ODAC mixed system under different pH
conditions at 70°C.
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Fig. 12. The morphology of micelle formed in the 2 wt% Gly-22/2 wt% ODAC mixed system at
different conditions (a. pH=5，b. pH=8).
Fig. 13. Different cycles of zero-shear viscosity of the 2 wt% Gly-22/2 wt% ODAC mixed system
at 70°C
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