Bilayers and wormlike micelles at high pH in fatty acid soap systems

Journal of Colloid and Interface Science 465 (2016) 304–310

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis

Bilayers and wormlike micelles at high pH in fatty acid soap systems Wenlong Xu, Huizhong Liu, Aixin Song, Jingcheng Hao ⇑ Key Laboratory for Colloid and Interface Chemistry & Key Laboratory of Special Aggregated Materials, Shandong University, Ministry of Education, Jinan 250100, PR China

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 16 October 2015 Revised 1 December 2015 Accepted 1 December 2015 Available online 8 December 2015 Keywords: Fatty acids Counterions Wormlike micelles pH Regulating effect

a b s t r a c t Bilayers at high pH in the fatty acid systems of palmitic acid/KOH/H2O, palmitic acid/CsOH/H2O, stearic acid/KOH/H2O and stearic acid/CsOH/H2O can form spontaneously (Xu et al., 2014, 2015). In this work, the bilayers can still be observed at 25 °C with an increase of the concentration of fatty acids. We found that wormlike micelles can also be prepared in the fatty acid soap systems at high pH, even though the temperature was increased to be 50 °C. The viscoelasticity, apparent viscosity, yield stress of the bilayers were determined by the rheological measurements. Wormlike micelles were identified by cryogenic transmission electron microscopy (cryo-TEM) and emphasized by the rheological characterizations, which are in accordance with the Maxwell fluids with good fit of Cole–Cole plots. The phase transition temperature was determined by differential scanning calorimetry (DSC) and the transition process was recorded. The regulating role of counterions of fatty acids were discussed by (CH3)4N+, (C2H5)4N+, (C3H7)4N+, and (C4H9)4N+ as comparison, concluding that counterions with appropriate hydrated radius were the vital factor in the formation wormlike micelles. Ó 2015 Elsevier Inc. All rights reserved.

1. Introduction As a kind of the cheapest and most common surfactants, fatty acid (FA) soaps consisting of protonated and deprotonated fatty acid molecules, are used in various manufacturing industries and our daily life. Though they have been used for a long history, the microstructures in the fatty acid soap solutions were widely studied since the first discovery of fatty acid vesicles using the freezefracture transmission electron microscopy (FF-TEM) technique [1]. The microstructure of the solution has an important influence in their application. Comparing with micelles, bilayers possess higher apparent viscosity, better foam stability [2–4] and emulsification ⇑ Corresponding author. E-mail address: [email protected] (J. Hao). http://dx.doi.org/10.1016/j.jcis.2015.12.006 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.

properties [3]. Therefore, the investigation of phase behavior and microstructure can provide choices for the application of the fatty acid soap system. The study of phase behavior in the fatty acid soap system can track back to 1940s [5–7]. With the development of electron microscopy technique, the microstructures in solution were observed. Spherical micelles and bilayers (including vesicles, planar lamella and tubes) are the most common structures in the fatty acid soap systems [8–11]. Besides, a few systems consisting of fibers [12,13] and wormlike micelles [14,15] were also discovered. Hydrogen bond, which is highly sensitive to temperature, plays an important role in the formation of fatty acid aggregates [16–18]. Microstructures in fatty acid soap solution are mostly dependent of temperature and phase transition can be easily induced by temperature [9,19]. Due to the rather low Krafft point of fatty acid, precipitate

W. Xu et al. / Journal of Colloid and Interface Science 465 (2016) 304–310

is usually obtained at room temperature [9]. Fatty acid soap system using organic amine as counterions can decrease Krafft point of fatty acid effectively [3,20–22]. In literatures, bilayers are mostly formed near the pKa of the fatty acid, in which the amounts of the protonated and deprotonated molecules are almost the same. Hydrogen bonds are easy to form and the distance between the molecules can be largely shortened. According to the packing parameter theory [23], p = v/(al), the average area (a) of polar headgroup decreases and the packing parameter (p) increases, resulting in the formation of bilayers [24,25]. In our previous work, bilayers at high pH were discovered in the palmitic acid (PA)/KOH/H2O, PA/CsOH/H2O, stearic acid (SA)/KOH/H2O, and SA/CsOH/H2O systems, which is against the traditional recognition of hydrogen bonds theory [10,26]. We proposed the assumed mechanism that counterions with appropriate hydrated radius could screen the electrostatic repulsion and played the vital role in the formation of bilayers. The inspiration of the work was from our previous work [26]. Through the same sample preparing method, wormlike micelles were obtained when increasing the concentration of fatty acid above 50 mM at 50 °C. The binary diagrams of the four systems were delineated at high concentration of fatty acids and alkalis at 25 and 50 °C, respectively. With the increase of alkali concentration, we found phase transition from separation phase to bilayers phase to separation phase at 25 °C and transparent solutions from low viscosity to high viscosity at 50 °C. Conductivity values also proved the phase transition. Rheological characterizations demonstrated the viscoelasticity, yield stress, and apparent viscosity of the bilayers samples at 25 °C and wormlike micelles at 50 °C. The phase transition temperature was determined by differential scanning calorimetry (DSC) and the transition process was recorded. Other counterions were used to demonstrate the importance of hydrated radius of the counterions. Considering the high viscosity of the wormlike micelles at high temperature, we hope our work can provide some potential applications in the field of oil exploration. 2. Materials and methods 2.1. Chemicals and materials Palmitic acid (PA, analytical reagent) were purchased from Sinopharm Chemical Reagent Co., Ltd. Stearic acid (SA, >98 wt%), CsOH (>99 wt%), (CH3)4NOH (25 wt% aqueous solution), (C2H5)4 NOH (25 wt% aqueous solution), (C3H7)4NOH (25 wt% aqueous solution), (C4H9)4NOH (40 wt% aqueous solution), KCl (>99 wt%), and CsCl (>99 wt%) were purchased from J&K Scientific Co., Ltd. (China). KOH (>96 wt%) were purchased from Aladdin Chemistry Co., Ltd. Ultrapure water was used with a resistivity of 18.25 MX cm from a UPH-IV ultrapure water purifier. Other agents were of analytical purity. 2.2. Phase behavior study The samples were prepared as the following procedures: a series of concentration of fatty acids and alkalis were added into tubes and the total volume of the samples was 5 mL by supplying ultrapure water. The concentration of fatty acids was changed from 50 to 100 mM, and that of the alkalis were changed from 200 to 1000 mM. The homogeneous solutions were obtained by ultrasonic treatment at 50 °C and the rough binary phase diagrams at 50 °C were delineated according to the viscosity of the solutions. Transfer these samples into an incubator at 25 °C for several days and the binary phase diagrams at 25 °C were delineated by visual observations with the help of conductivity measurements.

305

2.3. Conductivity and pH measurements The conductivity measurements were performed on a DDSJ308A (China) conductivity meter with a DJS-10C glass electrode at 25 and 50 °C. The values of pH were determined on a PHS-3C pH meter (China) with an E-201-C glass electrode at 25 and 50 °C. The two-phase samples were detected under stirring. 2.4. Cryogenic transmission electron microscopy (cryo-TEM) observations A drop of sample solution (
4 lL) was dropped on a grid in a high humidity environment (>90%). The excess sample was blotted up by two pieces of blotting paper, leaving a thin film sprawling on the grid. Then the grid was plunged into liquid ethane which was frozen by liquid nitrogen. The vitrified sample was transferred into a sample holder (Gatan 626), and observed on a JEOL JEM-1400 TEM (120 kV) at about 174 °C. The images were recorded on a Gatan multiscan CCD. 2.5. Rheological characterizations The rheological experiments were operated on a HAAKE RheoStress 6000 rheometer with a cone-plate system (C35/1°Ti L07116). In oscillatory measurements, an amplitude sweep at a fixed frequency of 1 Hz was performed prior to the following frequency sweep in order to ensure the selected stress was in the linear viscoelastic region. In the frequency sweep procedure, the shear stress was set at 1 Pa for the bilayers samples and 5 Pa for the wormlike micelle samples. 2.6. Differential scanning calorimetry (DSC) measurements The phase-transition temperature was measured on a DSC-Q10 (TA Instruments, New Castle, PA, USA). The measuring range was from 25 to 50 °C at a rate of 5 °C/min. 3. Results and discussion 3.1. Bilayers at 25 °C Through the same method of preparing samples, bilayers were still obtained when increasing the concentration of fatty acid from 20 mM to 50 mM at 25 °C. With the addition of alkalis, the sample appearance changes from separation phase to homogeneous phase, and finally separation phase, which is shown in Fig. 1. According to the previous work [10,26], the homogeneous phase belongs to bilayers phase. The bilayers phase samples are opaque, while the supernate of the separation phase samples are clear solution and the words on the background can be clearly identified through the supernate. The binary phase diagrams of the four systems containing different concentration of FAs and alkalis were delineated to demonstrate their phase behaviors. As shown in Fig. S1, all the phase diagrams are divided into two parts, containing expansive bilayers phase and separation phase. The pH and conductivity values of the four systems are shown in Fig. 2. The pH value increases rapidly and reaches a plateau near 14. However, the increase of conductivity exhibits some obvious differences. Taking SA/KOH/H2O system as an example, the conductivity of the solutions increases initially with the addition of KOH. After reaching a certain KOH concentration of 150 mM, the conductivity value decreases to near zero sharply, corresponding to the bilayers phase. The decrease of conductivity is due to the binding of the counterions on the bilayers and it is more obvious than the situation of the low concentration of SA [26]. That is easy

306

W. Xu et al. / Journal of Colloid and Interface Science 465 (2016) 304–310

Fig. 1. The appearance of the typical samples in the 50 mM SA/KOH/H2O system with different concentrations of KOH at 25 °C.

14

150

14

150

PA/KOH/H2O

SA/KOH/H2O

600

800

0 1000

κ

0

200

0 1000

150

SA/CsOH/H2O

14

pH

75

50

11

25

pH

12

pH

13

-1

κ

κ / mS•cm

pH

800

100

PA/CsOH/H2O

13

600

cKOH / mM

cKOH / mM

14

400

κ

100

12

-1

400

pH

8

κ / mS•cm

200

-1

50

κ

0

κ / mS•cm

10

pH 8

100

pH

50

-1

10

12

κ / mS•cm

100

pH

12

50

11 10

10 0

200

400

600

800

0 1000

cCsOH / mM

0

200

400

600

0 1000

cCsOH / mM

Fig. 2. Phase diagrams of the four systems including conductivity and pH value at 25 °C. cfatty

to understand because the viscoelasticity of 50 mM SA/KOH/H2O sample is higher than that of 20 mM sample, which is demonstrated by rheological characterization in Fig. S2. However, shaking these bilayers samples several times, the viscosity decreases and the conductivity value increases obviously, indicating the destruction of the microstructures in the solutions. When the concentration of KOH reaches 600 mM, these samples are homogeneous opaque solutions as soon as preparation. It was interesting that regular precipitates (Fig. 1) were formed when leaving them alone without any operation for several hours. We assume that the precipitates destroy the binding of the counterions and release part of them into the solution, resulting in the rapid increase of the conductivity value. The viscoelasticity of the typical bilayers samples was demonstrated by rheological measurements. From Fig. 3, we can see that all the samples exhibit the similar features: Both the elastic modulus (G0 ) and the viscous modulus (G00 ) are independent of the sweep frequency (f), G0 is almost an order of magnitude larger than G00 , exhibiting the elasticity dominant properties. With the increase

800

acid

= 50 mM.

of the SA and KOH concentration, the viscoelasticity of the sample increases. The yield stress values of the samples are almost the same, locating near 10 Pa (Fig. S3a) and the apparent viscosity of the above samples is almost overlapped in the steady shear rheogram (Fig. S3b), meaning the similar apparent viscosity of these samples. 3.2. Wormlike micelles at 50 °C When the samples were heated to 50 °C, the appearance of the samples took great changes. For all the samples, they exhibit homogeneous transparent appearance (Fig. 4, upper row), and the words on the background can be clearly seen. The viscosity of the samples increases with the addition of KOH, when the concentration of alkali reaches a certain value, the viscoelasticity of the solution is enough strong to support its own gravity when inverting the tube. In the bottom row in Fig. 4, the samples with lower viscosity possess excellent foamability and large amount of bubbles can exist on the surface of the solutions after shaking

307

W. Xu et al. / Journal of Colloid and Interface Science 465 (2016) 304–310 5

a

10

4

10

3

10

SA:KOH=50:400 G' SA:KOH=50:400 G''

SA:KOH=80:400 G' SA:KOH=80:400 G'' SA:KOH=100:400 G' SA:KOH=100:400 G''

2

1

10 -1 10

G', G'' / Pa

G', G'' / Pa

10

10

0

10

5

10

4

10

3

10

2

10

1

b

SA:KOH=100:200 G' SA:KOH=100:200 G''

SA:KOH=100:400 G' SA:KOH=100:400 G'' SA:KOH=100:600 G' SA:KOH=100:600 G''

0

1

10

10 -1 10

0

10

10

1

f / Hz

f / Hz

Fig. 3. Viscoelasticity of the typical bilayers samples in the SA/KOH/H2O system. (a) cKOH = 400 mM with an increase of cSA and (b) cSA = 100 mM with an increase of cKOH.

typical samples are shown in Fig. 7a. G0 is lower than G00 at low frequency and higher than G00 at high frequency, which exhibits a good fit with the single-relaxation-time Maxwell model. Equations are as follows [27]:

G0 ¼ G0 x2 t 2R =ð1 þ x2 t 2R Þ

ð1Þ

G00 ¼ G0 =ð1 þ x2 t2R Þ

ð2Þ

where G0, x and tR is the high frequency plateau modulus, angular frequency (x = 2pf) and relaxation time, respectively. G0 and tR can be obtained from the critical angular frequency x⁄ and modulus G⁄ at which the curve of G0 and G00 intersects according to Eqs. (3) and (4):

tR ¼ 1=x

ð3Þ

G0 ¼ 2G

ð4Þ

The Cole–Cole plot of G00 versus G0 exhibits the semicircle of a Maxwell fluid. From Eqs. (1) and (2), the following equation can be obtained: Fig. 4. The appearance of the typical samples in the 50 mM SA/KOH/H2O system at 50 °C. The photographs in the upper row were taken under white background and those in the bottom row were taken under black background.

the samples. With the increase of the sample viscosity, the amount of the bubbles on the surface decreases and it increases in the bulk solution. For the samples with rather high viscosity, the bubbles can be stored in the solution for several days. The rough binary phase diagrams of the four systems are shown in Fig. 5, in which the phase boundaries were simply delineated according to the viscosity of the samples. The samples are low viscous solutions at low concentration of alkali, whose appearance is similar with water (Phase I). With the increase of alkali, the viscosity increases obviously (Phase II). Interestingly, the samples possess rather high viscoelasticity with further increase of alkali in Phase III and exhibit flow birefringence in the crossed polarizers, which is the typical features of wormlike micelles [27]. Conductivity data can also support the phase transition with increasing temperature. Comparing with the conductivity values at 25 °C, those at 50 °C increase monotonically with the addition of alkali for all the four systems, which are shown in Fig. S4. The pH values show no obvious changes except for the temperature effect. Cryo-TEM is the most direct method to confirm the wormlike micelles. As shown in Fig. 6, wormlike micelles can be clearly observed from the cryo-TEM images. The rheological data is a powerful tool to support the discovery of wormlike micelles. The frequency sweep rheograms of the

2

G002 þ ðG0  G0 =2Þ ¼ ðG0 =2Þ2

ð5Þ

From Fig. 7b, we can observe the good correspondence with the Maxwell model, indicating the existence of wormlike micelles [28]. The frequency sweep rheograms and the Cole–Cole plots of the typical samples in the PA/KOH/H2O, PA/CsOH/H2O and SA/CsOH/ H2O systems are shown in Fig. S5, in which all of them correspond to the Maxwell model. From Fig. 8a, we can see that with the increase of KOH, the apparent viscosity of the samples increase obviously. The sample with cKOH = 200 mM possesses rather low viscosity and the viscosity curve is parallel with x-axis at high shear rate. The apparent viscosity of the sample with cKOH = 500 mM is an order of magnitude larger than that of the sample with cKOH = 200 mM. When the concentration of KOH is above than 800 mM, the viscosity curves exhibit shear plateau at low shear rate and shear thinning at high shear rate. The shear plateau attributes to the network of the wormlike micelles and the shear thinning is caused by the alignment of the wormlike chain in the shear flow at high shear rate [28]. The apparent viscosity also increases with the increase of cSA (Fig. 8b). The shear plateau appears when cSA reaching 50 mM. The yield stress of the viscoelastic samples increases with the increase of cSA and cKOH, ranging from 10 Pa to 40 Pa, which is shown in Fig. S6. The apparent viscosity and the yield stress data of the other three systems are shown in Fig. S7, exhibiting the similar properties with those in the SA/KOH/H2O systems.

308

W. Xu et al. / Journal of Colloid and Interface Science 465 (2016) 304–310

1000

1000

a

b

III

III

800

cCsOH / mM

cKOH / mM

800

II

600

400

I 200 50

60

70

80

600

II 400

I 90

200 50

100

60

cPA / mM 1000

90

100

d

III

600

cCsOH / mM

800

II

400

III

600

II

400

I 200 50

80

cPA / mM 1000

c

800

cKOH / mM

70

60

70

80

90

I

200 50

100

60

70

80

90

100

cSA / mM

cSA / mM

Fig. 5. The binary phase diagrams of the four systems at 50 °C. I: Low viscous solution (like water); II: middle viscous solution; III: wormlike micelles (the solution can support its own gravity when inverting the tube).

Fig. 6. Cryo-TEM images of the 50 mM SA/1000 mM KOH/H2O samples.

2

10

a 1

10

0

b

G' G''

10

1

10

0

-1

10

-2

10

-1

f / Hz

4

3

2

⏐η∗⏐

10

5

G'' / Pa

10

2

η∗ / Pa•s

G', G'' / Pa

10

10

0

10

-1

1

0

2

4

6

8

G' / Pa

Fig. 7. Frequency sweep rheograms (a) and the Cole-Cole plot (b) of the sample with 50 mM SA/1000 mM KOH/H2O.

309

W. Xu et al. / Journal of Colloid and Interface Science 465 (2016) 304–310

10

4

10

2

10

0

10

-2

10

-4

10

a

-3

10

-2

10

-1

10 •

0

10

1

10

b

2

10

η / Pa•s

η / Pa•s

4

10

SA:KOH=100:200 SA:KOH=100:500 SA:KOH=100:800 SA:KOH=100:1000

2

0

10

10

SA:KOH=20:1000 SA:KOH=50:1000 SA:KOH=80:1000 SA:KOH=100:1000

-2 -3

10

-2

10

-1

0

10

10 •

10

1

10

2

-1

γ/s

-1

γ/s

Fig. 8. The steady shear rheographs of the typical samples in SA/KOH/H2O system at 50 °C. (a) cSA = 100 mM with an increase of cKOH and (b) cKOH = 1000 mM with an increase of cSA.

3.3. Transition process between separation phase and wormlike micelles The phase transition temperature is determined by DSC measurements, which is shown in Fig. 9. We can clearly see that the phase transition temperature increases with the increase of KOH concentration, ranging from 35.74 °C to 43.55 °C. The phase transition process is recorded by the photographs in Fig. 10. The temperature raising process is shown in the upper row in Fig. 10. The regular precipitate changes from opaque to transparent in several minutes (Fig. 10a–d), however, homogeneous solution cannot be obtained with long time. With the help of shaking (Fig. 10e), a homogeneous solution can be formed after a day (Fig. 10f). From the bottom row of Fig. 10, we can see that cooling the homogeneous solution at 25 °C from 50 °C water bath, the sample becomes completely white in 20 min (Fig. 10i). Aging for 7 h, regular precipitate was obtained (Fig. 10j). 3.4. The regulating effect of counterions In the previous work, we concluded that counterions played the vital role in the formation of bilayers in the fatty acid soap systems [10]. Herein, we applied the organic alkalis, (CH3)4NOH, (C2H5)4 NOH, (C3H7)4NOH, (C4H9)4NOH, for comparison. Only clear solutions with low viscosity can be obtained at high pH both at 25 and 50 °C, indicating no bilayers and wormlike micelles are

35.74

Heat Flow / a.u.

38.12

39.06

cKOH / mM

200 400 600 800 1000

25

30

40.92

43.55 35

40

45

50

T / °C Fig. 9. The phase transition temperature of the samples in 100 mM SA/KOH/H2O system.

Fig. 10. The heating process (upper row) and the cooling process (bottom row) of the typical sample with different time scale. (a) 0 s, (b) 200 s, (c) 210 s, (d) 220 s, (e) shaking, (f) 1 d, (g) 0 min, (h) 15 min, (i) 20 min, (j) 7 h, (k) 1 d, (l) 3 d. cSA = 50 mM and cKOH = 1000 mM.

formed. The conductivity data at 25 °C support the assumption. From Fig. S8a, one can see that the conductivity value increases rapidly with the increase of organic alkali concentration, which is obviously different from the situation of KOH and CsOH (Fig. 2). However, adding KCl or CsCl into 50 mM SA/50 mM (CH3)4NOH solutions, the situation of low conductivity recovers, which is shown in Fig. S8b, indicating the important role of counterions rather than pH. At 50 °C, the apparent viscosity of the SA/organic alkali/H2O samples is rather low, with a shear plateau at high shear rate, exhibiting the typical feature of Newtonian fluid (Fig. 11a). However, as shown in Fig. 11b, adding KOH into the SA/alkali/ H2O solution, the apparent viscosity increases obviously, especially for (CH3)4NOH and (C2H5)4NOH, the viscosity plateaus at low shear rate reappear. Besides, different organic alkali counterions have great effect in the apparent viscosity. The sample with smaller counterions has larger apparent viscosity. The frequency sweep rheograms and the corresponding Cole–Cole plots of the samples were shown in Fig. S9, exhibiting the Maxwell fluid properties and indicating the wormlike micelle structure. The hydrated radius is in the order of Cs+ (3.29 Å) < K+ (3.31 Å) < Na+ (3.58 Å) < (CH3)4N+ (3.67 Å) < Li+ (3.82 Å) < (C2H5)4N+ (4.00 Å) < (C3H7)4N+ (4.52 Å) < (C4H9)4N+ (4.94 Å) [29,30]. In the

310

W. Xu et al. / Journal of Colloid and Interface Science 465 (2016) 304–310

η / Pa•s

10

10

2

3

10

a

b 1

0

10

η / Pa•s

10

(CH3)4NOH

-2

(C2H5)4NOH

10

-1

(CH3)4NOH (C2H5)4NOH

(C3H7)4NOH

(C3H7)4NOH

(C4H9)4NOH 10

10

-4

10

-3

-2

10

10

-1

0

10 •

γ/s

10

1

2

10

10

3

(C4H9)4NOH

-3

10

-3

-2

10

-1

10

10 •

γ/s

-1

0

10

1

10

2

-1

Fig. 11. The apparent viscosity of the 50 mM SA/50 mM organic alkali/H2O (a) and 50 mM SA/50 mM organic alkali/1000 mM KOH/H2O samples (b) at 50 °C.

previous work [10] of the bilayers at high pH, we assumed that counterions with smaller hydrated radius could penetrate into the stern layer of bilayer and screen the electrostatic repulsion between the deprotonated fatty acid molecules. Therefore, the distance between polar headgroups decreases, resulting in the decrease of a and the increase of p, further the formation of bilayers. Herein, it is the same that only K+ and Cs+ can form bilayers at 25 °C and wormlike micelles at 50 °C, indicating the important role of the hydrated radius of counterions.

4. Conclusions This work is an extension of the previous work [10,26], in which fatty acid bilayers at high pH were discovered in the PA/KOH/H2O, PA/CsOH/H2O, SA/KOH/H2O, and SA/CsOH/H2O systems. In this work, we discovered bilayers at 25 °C and wormlike micelles at 50 °C in the same systems. Preparing the samples with the same method, these different structures were obtained just by increasing the concentration of the fatty acids. The rheological measurements demonstrated the yield stress, viscoelasticity and the apparent viscosity of the samples. The Cole–Cole plots indicated the wormlike micelles fitting well with the Maxwell model. The transition process of the wormlike micelles was recorded and the transition temperature was determined by DSC measurements. The important role of counterions were illustrated using (CH3)4N+, (C2H5)4N+, (C3H7)4N+ and (C4H9)4N+ as comparison. Although a lot of papers have reported the aggregate transition between bilayers and micelles, only a few paper discovered wormlike micelle in the fatty acid soap systems. Comparing with the previous reports [14,15], we discovered both bilayers and wormlike micelles in this system and the other important discovery was counterions played an important role in the formation of bilayers and wormlike micelles. We assumed that counterions with appropriate hydrated radius (K+ and Cs+) could screen the electrostatic repulsion between the fatty acid soap molecules, resulting in the formation of bilayers and wormlike micelles. Furthermore, the wormlike micelles were formed at high temperature, considering the high viscoelasticity of the wormlike micelles at high temperature, we hope our work can provide some potential applications in the field of oil exploration.

Acknowledgements This work is financially supported by the NSFC (Grant Nos. 21420102006 and 21273134). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2015.12.006. References [1] J.M. Gebicki, M. Hicks, Nature 243 (1973) 232. [2] B. Novales, A. Riaublanc, L. Navailles, B.H. Houssou, C. Gaillard, F. Nallet, J.P. Douliez, Langmuir 26 (2010) 5329. [3] B. Novales, L. Navailles, M. Axelos, F. Nallet, J.P. Douliez, Langmuir 24 (2008) 62. [4] A.L. Fameau, A. Saint-Jalmes, F. Cousin, B.H. Houssou, B. Novales, L. Navailles, F. Nallet, C. Gaillard, F. Boué, J.P. Douliez, Angew. Chem. Int. Ed. 50 (2011) 8264. [5] R.D. Vold, J.M. Philipson, J. Phys. Chem. 50 (1946) 39. [6] G.H. Smith, J.W. McBain, J. Phys. Chem. 51 (1947) 1189. [7] J.W. Mcbain, W.C. Sierichs, J. Am. Oil Chem. Soc. 25 (1948) 221. [8] W. Xu, A. Song, S. Dong, J. Chen, J. Hao, Langmuir 29 (2013) 12380. [9] W. Xu, X. Wang, Z. Zhong, A. Song, J. Hao, J. Phys. Chem. B 117 (2013) 242. [10] W. Xu, H. Zhang, S. Dong, J. Hao, Langmuir 30 (2014) 11567. [11] A.-L. Fameau, A. Arnould, M. Lehmann, R. von Klitzing, Chem. Commun. 51 (2015) 2907. [12] D. Wang, J. Hao, Langmuir 27 (2011) 1713. [13] Z. Yuan, W. Lu, W. Liu, J. Hao, Soft Matter 4 (2008) 1639. [14] Y. Zhang, H. Yin, Y. Feng, Green Mater. 2 (2014) 95. [15] Y. Zhang, Y. Han, Z. Chu, S. He, J. Zhang, Y. Feng, J. Colloid Interface Sci. 394 (2013) 319. [16] T.H. Haines, Proc. Natl. Acad. Sci. USA 80 (1983) 160. [17] R. Smith, C. Tanford, Proc. Nat. Acad. Sci. USA 70 (1973) 289. [18] F. Caschera, J. Bernardino de la Serna, P.M.G. Löffler, T.E. Rasmussen, M.M. Hanczyc, L.A. Bagatolli, P.-A. Monnard, Langmuir 27 (2011) 14078. [19] W. Xu, H. Gu, X. Zhu, Y. Zhong, L. Jiang, M. Xu, A. Song, J. Hao, Langmuir 31 (2015) 5758. [20] A.-L. Fameau, T. Zemb, Adv. Colloid Interface Sci. 207 (2014) 43. [21] J.-P. Douliez, J. Am. Chem. Soc. 127 (2005) 15694. [22] R. Klein, D. Touraud, W. Kunz, Green Chem. 10 (2008) 433. [23] J.N. Israelachvili, D.J. Mitchell, B.W. Ninham, J. Chem. Soc., Faraday Trans. 2 (72) (1976) 1525. [24] C.L. Apel, D.W. Deamer, M.N. Mautner, Biochim. Biophys. Acta 1559 (2002) 1. [25] J.R. Kanicky, D.O. Shah, Langmuir 19 (2003) 2034. [26] W. Xu, H. Zhang, Y. Zhong, L. Jiang, M. Xu, X. Zhu, J. Hao, J. Phys. Chem. B 119 (2015) 10760. [27] S.R. Raghavan, E.W. Kaler, Langmuir 17 (2001) 300. [28] D. Wang, R. Dong, P. Long, J. Hao, Soft Matter 7 (2011) 10713. [29] E.R. Nightingale Jr., J. Phys. Chem. 63 (1959) 1381. [30] A.G. Volkov, S. Paula, D.W. Deamer, Bioelectrochem. Bioenerg. 42 (1997) 153.