Facile synthesis, surface activity, wettability and ultrahigh foaming properties of novel nonionic Gemini fluorocarbon surfactants

Journal of Molecular Liquids 302 (2020) 112469

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Facile synthesis, surface activity, wettability and ultrahigh foaming properties of novel nonionic Gemini fluorocarbon surfactants Cai-Lian Chen, Yi-Fan Liao, Feng Lu, Yu-Sen Zheng, Ying-Ying Peng, Chong-Wei Ding, Qing-Xiao Tong ⁎ Department of Chemistry and Key Laboratory for Preparation and Application of Ordered Structural Material of Guangdong Province, Shantou University, Guangdong 515063, PR China

a r t i c l e

i n f o

Article history: Received 21 May 2019 Received in revised form 27 December 2019 Accepted 5 January 2020 Available online 10 January 2020 Keywords: Gemini fluorocarbon surfactants Spacer Surface properties Wettability Foaming characterization

a b s t r a c t Two nonionic Gemini fluorocarbon surfactants containing ecofriendly ester-bonded, named as MN-2C9F19 and EN-2C9F19 were designed and synthesized by one-step esterification. Their surface properties, wettability, thermal properties, and foaming characterization were systematically studied. Two surfactants exhibited high surface activity in aqueous solution, with surface tension (γcmc) of 14.20 mN/m for MN-2C9F19 and 13.71 mN/m for EN2C9F19. Moreover, MN-2C9F19 and EN-2C9F19 showed high decomposition temperature (Td) of up to 165.4 °C and 176.7 °C respectively. Although the structural difference between methyl and ethyl group is negligible, their wettability and foaming properties are quite different. EN-2C9F19 could wet the low surface energy PTEF plate completely at low concentration (0.6 mM) but MN-2C9F19 needed higher concentration (1.2 mM). Also, EN2C9F19 exhibited much better foaming property with the foam integrated value F up to 4,370,500 mL·s while MN-2C9F19 could reach up to 2,732,580 mL·s. This maybe ascribe to lower surface tension and higher viscosity of EN-2C9F19 solution. Furthermore, two surfactants showed stronger foaming properties at high temperatures than the conventional foam surfactant SDBS even at room temperature. This work provided an easy synthesis route to develop high-performance foaming agents at high temperatures. © 2020 Elsevier B.V. All rights reserved.

1. Introduction Foam fluids are dispersions of gas bubbles in a continuous liquid phase which also are metastable systems [1,2]. And it has been widely applied in various industrial fields, such as enhanced oil and gas recovery, cosmetics, food process, wastewater technologies and aqueous film-forming foams [3–7]. In most cases, surfactants are used adsorbing at the bubble surface to generate foams. Foam properties not only are related to bubble decay time, wettability, and mean bubble size but also with the dynamic properties of the surfactant adsorption layers like viscosity and surface activity [8,9]. At present, sodium dodecane sulfonate (SDS) and sodium dodecylbenzene sulfonate (SDBS) take possession of foam agent business, however, with the weaknesses of low surface activity, big wetting angle, and short-term stability [10,11]. To address these shortcomings, one strategy is to come up with converting a traditional single hydrocarbon hydrophobic chain into a fluorocarbon hydrophobic chain which is characterized by weak intermolecular interactions and strong intramolecular bonds. As a result, fluorocarbon surfactants exhibit high surface activity, strong chemical, and thermal stability. For example, Mamadou Oumar's group studied a new fluorocarbon/hydrocarbon hybrid cationic surfactant S-F8H10 which exhibited very low surface tension 15.6 mN/m [12]. Qing You, ⁎ Corresponding author. E-mail address: [email protected] (Q.-X. Tong).

https://doi.org/10.1016/j.molliq.2020.112469 0167-7322/© 2020 Elsevier B.V. All rights reserved.

et al. introduced nonionic fluorocarbon surfactants NPFOA (N(diethylene glycol) perfluorooctane amide). The study demonstrated that NPFOA has superior surface activity and excellent foam performance that foam integrated value could attain 108,102 mL s [13]. Another strategy is using Gemini surfactants, the amphiphilic molecules, including two hydrophilic head groups and two hydrophobic tails connected by a spacer at the two head groups. Compared with single-chain surfactant's analogs, Gemini surfactants show exceptional properties like high surface reduction efficiency, low critical micelle concentration (cmc), unusual rheological property, excellent wetting ability, and excellent dispersion stabilization [14,15]. Recently, the combination of Gemini surfactants and fluorocarbon surfactants has attracted much attention. For instance, the semifluorinated Gemini quaternary ammonium acrylic surfactant C8F17·NH has surface tension 22.7 mN/m and great antimicrobial activity against Candida albicans with MIC values 7 μmol/L [16]. The surface tension of Gemini surfactants GF6Hn were between 18.9 and 24.4 mN/m, surfactants with higher surface-active properties always have shorter spacer (n ≤ 6) [17]. In Wang's group, the surface tension was found to be 20.27 mN/m for 2C6 FC3-Sul [18]. However, those fluorocarbon Gemini surfactants have a high surface tension which is N18 mN/m. In addition, their foam property and wettability were seldom reported. In this work, taking full advantage of both Gemini surfactants and fluorocarbon Gemini surfactants, we designed and synthesized two ester-bonded Gemini surfactants (Scheme 1) were confirmed by 1H NMR, 19F NMR, and

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Scheme 1. Synthesis of non-ionic Gemini fluorocarbon surfactants: (a) MN-2C9F19 and (b) EN-2C9F19.

elemental analysis. The strategy is introducing ecofriendly ester group to potentially enhance the biodegradability and water solubility of Gemini surfactants [19–23]. And the spacer of the tertiary amino group decreases surfactant hydrophobicity and strengthens the spacer flexibility. The effect of the different tertiary amino spacers in the ester-bonded Gemini surfactants was investigated by studying surface activities, wettability, thermal properties, and foam characterization.

2. Experimental 2.1. Materials and synthesis Nonadecafluorodecanoic acid was purchased from Alfa Aesar. NMethyldiethanolamine and N-Ethyldiethanolamine were provided by Aldrich. Dichloromethane and toluene were obtained from Guangdong Guanghua Science and Technology Company. Redistilled water was used for the measurements. Ascertaining of the Gemini surfactant structures were obtained by 1H NMR and 19F NMR using AVANCE 400/ 471 MHz-Bruker in dimethyl-d6 sulfoxide, elemental analysis realized with Vario EL III CHNS, those characterizations are showed in ESI. These two surfactants were synthesized via one-step by esterification (as depicted in Scheme 1).

2.2. Surface tension measurements We measured the surface tensionγusing a JK99C automatic surface tensiometer (made in China) with a platinum‑iridium ring method at 25.0 ± 0.1 °C. The tensiometer was calibrated byof surfactant solution. Each sample was observed measuring deionized water (surface tension 71.33 mN/m) before each set of measurements. The tests were repeated at least twice times to ensure good reproducibility [24]. The value of the standard deviation for each set was b0.52 mN/m. 2.3. Contact angle measurements The poly(tetrafluoroethylene) (PTFE) plate was tested and characterized through the AFM. The static contact angle was measured by a drop shape analysis system (JCY-3, Shanghai) on a PTFE plate at a constant temperature (25 ± 0.5 °C). The good quality PTFE plate was chosen and washed several times with acetone and deionized water. All the surfactant solutions repeated several times (N10 for each concentration) and the average of those values was taken [25].

2.4. Thermal properties Under the nitrogen atmosphere, the thermal stabilization of two fluorocarbon surfactants were investigated by thermal gravimetric analyses (TGA). The sample was heated to 300 °C using a heating rate of 10 °C/min [26].

2.5. Foaming characterization Handshaking: foam samples were generated by handshaking of plastic cylindrical containers (26 mm internal diameter, 115 mm height) filled with 5–6 mL of surfactant solution. Each sample was observed three times at 25 °C [1]. Foaming parameters: we used waring blender to test foaming parameters. Orderly, a 100 mL surfactant aqueous solution was injected into the stirring container. Then the rotational speed was kept 3000 rpm for 1 min equably. Meanwhile, the thermostatic water bath was used to maintain a constant system temperature. Recording foam volume V, half-life period t1/2 (the time when the foam volume reached half of the original) and foam integrated value F [27], which can be calculated according to the following equation: F ¼ V  t1=2

ð1Þ

2.6. Viscosity measurement Viscosity measurement was prepared and performed for all the synthesized surfactants of aqueous solutions at 5 cmc. The viscosity was obtained using the RST-SST Rheometer (made in America). Furthermore, the shear rate range was used between 100 and 1000 s−1. The temperature was kept at the desired level during the measure the samples, which were accurately controlled within 25 ± 0.1 °C /70 ± 0.1 °C [28,29]. 3. Results and discussion 3.1. Surface properties The surface performance of the MN-2C9F19 and EN-2C9F19 was studied by surface tension measurements. Fig. 1 shows the relationship between static surface tension and log C of the Gemini surfactant solutions. It shows that surface tension declines with increasing concentration. When surfactant molecules reach adsorption saturation, the densest surfactant monomers are aligned at the air/liquid interface and then the surface tension remains constant. Meanwhile, a welldefined cmc and γcmc value (the maximal ability of the surfactant to reduce surface tension of water close to the cmc value) can be obtained from the transition point. Breaks at the cmc don't appear any shoulders in Fig. 1 which demonstrates surfactant purity [30]. The cmc and γcmc are listed in Table 1. Because fluorine is more electronegative and has smaller polarizability than hydrogen, the hydrophobic effect of one CF2 group is far greater than one CH2 group (CF2 = 1.5 CH2), two synthesized Gemini surfactants have good surface activity [31–33]. MN-2C9F19 exhibits cmc of 0.47 mM and low γcmc of 14.20 mN/m. Compared with the MN-2C9F19, EN-2C9F19 shows a lower cmc of 0.38 mM and a relatively low γcmc of 13.71 mN/m. This can be ascribed to the different spacer of the two surfactants, the ethyl group is slightly more hydrophobic than the methyl group. It is

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the Gibbs adsorption isotherm equation [30]. The head group area is occupied by a surfactant single molecule at the air/liquid interface (Amin). All the formulas are as follows: Π cmc ¼ γ 0 −γ cmc

ð2Þ 

1 dγ Γ max ¼ − 2:30nRT d logC  Amin ¼

Fig. 1. Variation of surface tension as a function of the logarithm of the concentration of the nonionic Gemini fluorocarbon surfactants at 25 °C.

Table 1 Surface property parameters of MN-2C9F19, EN-2C9F19 aqueous solutions at 25 °C. Name MN-2C9F19 EN-2C9F19

(mM)

Γcmc (mN/m)

Πcmc (mN/m)

C20 (mM)

Γmax (μmol·m−2)

Amin (nm2)

0.47 0.38

14.20 13.71

57.13 57.62

0.011 0.014

4.04 4.48

0.41 0.37

cmc

favorable to reinforce hydrophobic interaction from EN-2C9F19 chains having stronger intramolecular bonds and weaker intermolecular interactions [34–36]. Another reason is that the ethyl group can balance the intramolecular interaction between hydrophobic fluorocarbon chains effectually. So, EN-2C9F19 has better surface activity. Obviously, their surface activities are quite good compared to nonionic alkylgluronamide foam agent 14b (γcmc = 29.9 mN/m, cmc = 0.13 mM) and single-chain nonionic fluorocarbon foam agent NPFOA (γcmc = 17.83 mN/m, cmc = 1.54 mM) [13,37]. Related surface parameters are summarized in Table 1. The efficiency of surface tension reduction (ΠCMC) indicates its ability to reduce surface tension of deionized water [38]. C20 represents the surfactant concentration of water surface tension reduced by 20 mN/m [39]. The surface excess maximum concentration (Γmax) can be calculated from

1 NA Γ max

 T

ð3Þ

 ð4Þ

where γ0 is the surface tension of deionized water (experimental value 71.33 mN/m), R is the gas constant (8.314 J·mol−1·K−1), T is the absolute temperature in Kelvin, (dγ/dlogC)T is the slope of the linear fitting of the surface tension and Log C curve before cmc. For nonionic surfactants n = 1. NA is Avogadro's (6.02 × 1023 mol−1). Hydrocarbon/fluorocarbon hybrid surfactants are strong associative surfactants and their mutual hydrophobicity induces the creation of nonconventional interactions, which make for high surface activity. As a result, two nonionic fluorocarbon surfactants possess prominent ΠCMC (~57 mN/m) and C20 (~0.01 mM) that manifest they can effectively reduce the water surface tension. In addition, the Γmax value of EN-2C9F19 is obviously higher than that of MN-2C9F19 and the Amin value of EN-2C9F19 is slightly lower than that of MN-2C9F19. This result reveals that EN-2C9F19 has higher packing density at the air/liquid interface compared to MN-2C9F19. With the hydrophobicity of the nonionic fluorocarbon surfactant increasing, the cmc values decrease. Further, fluorocarbon surfactant monomers prefer adsorption at the air/liquid interface and hence make a stronger intermolecular interaction between the fluorocarbon hydrophobic tails, leading to smaller Amin and bigger Γmax [40]. 3.2. Wetting properties on the PTFE surface Numerous studies reported that the PTFE plate has superbly low surface energy and worst wettability [41–43]. To further evaluate PTFE surface smoothness, we utilized atomic force microscopy (AFM) to observe the surface topography. Fig. 2a depicts AFM image of surface topography at the PTFE plate used for this investigation. Fig. 2b depicts the zaxis profile of the surface by the inset lines got from Fig. 2a. As a result, the PTFE is quite smooth with the surface roughness (Ra) of 49.3 nm and the root-mean-square roughness (RMS) of 63.3 nm. In other words, the PTFE plate can be used as a substrate to measure wetting performance accurately.

Fig. 2. (a) AFM image of surface topography at PTFE plate; (b) The profile of the surface about the z-axis.

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Wettability by surfactant solutions play an important role in many daily application areas, such as coating, flotation, lubrication, agriculture, painting, washing industry [44–48]. The static contact angle is widely used to test surfactant wettability. Fig. 3 depicts the wetting pictures of the contact angle on the PTFE plate. The pure water contact angle is 107.06°. With the concentration of surfactant aqueous solution increasing, MN-2C9F19 and EN-2C9F19 can cut down on water contact angle to 6.06° and 5.09°respectively, which reflects almost complete wetting on the PTEF (contact angle b10°). By comparing with nearterm new fluorocarbon surfactant PFPF-B [49], PFPF-B had a large contact angle on the PTFE plate and just could wet parafilm absolutely, which means two as-synthesized surfactants have better wettability. Since the structure of the spacer is correlated to wettability, EN-2C9F19 can wet the low surface energy PTEF plate completely at low concentration (0.6 mM), while MN-2C9F19 need higher concentration (1.2 mM). The result indicates that EN-2C9F19 has much better wetting property, which is consistent with the surface activity. The spreading coefficient S is assigned to explain the wettability [50]. S ¼ γsg −γsl −γ lg

ð5Þ

The surface tension of the PTFE-air, liquid-air, PTFE-liquid is represented by γsg, γlg, and γsl respectively. When the concentration of the surfactant increased, values of γlg and γsl lowered, in this way S increased which makes wettability stronger. EN-2C9F19 is more efficient

in lowering solid-air surface tension thus has more excellent wetting property. The contact angle curve in Fig. 4a shows a platform when PTFEliquid surface adsorption becomes saturated. The cmc value of two fluorocarbon surfactants is lower than the wetting equilibrium, concentration owing to the difference adsorption of liquid-air and PTFE-liquid interface. A convenient method to understand the relative adsorption of surfactants at surfaces was utilized by the Lucassen-Reynders equation [51–53].   d γ lg cosθ dγ lg

¼

Γ sg −Γ sl Γ lg

ð6Þ

where Γlg, Γsg, and Γsl represent interface excess concentrations of liquid-air, PTFE-air, PTFE-liquid surfaces respectively, and some research recorded that Γsg = 0. [54,55] Fig. 4b shows the plot of adhesion tension (γlgcosθ) versus surface tension. The slope values indicate Γsl/ Γlg b 1 by the Lucassen-Reynders equation. It explains the lower adsorption of surfactants at the PTFE-liquid interface than at the liquid-air interface [56]. The Γsl/Γlg ratios of Gemini surfactants MN-2C9F19 and EN-2C9F19 are less than those of single-chain surfactants on the PTFE surface, such as hydrocarbon surfactants SDBS (0.82), CTAB (0.85), TX-100 (0.83) and fluorocarbon surfactants C9F19AM (0.68) and C9F19AE (0.66). [11,57]

Fig. 3. The contact angle of pure water and versus surfactant aqueous solutions on the PTFE plate, 19 (a) MN-2C9F19: (1) 0.01 mM, (2) 0.15 mM, (3) 0.8 mM, (4) 1.5 mM; (b) EN-2C9F19: (1) 0.01 mM, 20 (2) 0.15 mM, (3) 0.6 mM, (4) 1.8 mM.

Fig. 4. (a) Variation in the contact angle with different concentrations at 25 °C; (b) the plot of adhesion tension (γlgcosθ) versus surface tension (γlg).

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We speculate that these may be affected by the adsorption type of surfactant molecules. As illustrated in Fig. 5, Gemini surfactant molecules could be arranged closer than single-chain surfactants at the air-liquid interface, making the direction of tails outward due to stronger intramolecular interaction between the hydrophobic chain. Meanwhile, MN2C9F19 and EN-2C9F19 have more flexible adsorption than single-chain surfactants at PTFE-liquid surface, leading to lower molecular density. Ultimately, two nonionic Gemini surfactants display excellent wetting properties. 3.3. Thermal properties It is well known that fluorocarbon surfactants have excellent thermal stability, such as a high melting point and decomposition temperature (Td) [58]. As seen in Fig. 6, MN-2C9F19 and EN-2C9F19 show high Td of up to 165.4 °C and 176.7 °C respectively at 5% weight loss suggesting they can maintain stable performance at high temperatures. 3.4. Foaming characterization Fig. 6. TGA thermograms of MN-2C9F19 and EN-2C9F19.

Fig. 7 illustrates a series of images showing the long-term evolution of foams created by as-synthesized Gemini fluorocarbon surfactant solutions. The foaming volume elevates rapidly with concentration increasing. When the surfactant concentration was at 1.5 cmc, the solutions not only show superb foam ability visibly but also reveal outstanding foam stability (lifetime of one day) contrast to conventional surfactant foams [2]. Their strong foam stability is characterized by two primary features: mighty foam anti-coalescence and slow bubble coarsening. As Fig. 8a depicts, the foam integrated value and foam volume rises with surfactant concentration increasing before their values appear a platform. Foam parameter results are summarized in Table 2. It shows that the foam integrated value F can achieve incredibly high to 2,732,580 mL·s for MN-2C9F19 and 4,370,500 mL·s for EN-2C9F19, which are not only ten times higher than some commonly used

hydrocarbon surfactants, such as SDBS of 174,660 mL·s and SDS of 287,280 mL·s. But also are superior to single-chain fluorocarbon surfactants such as NPFOA of 472,650 mL·s and HPA4 of 108,102 mL·s [59]. Compared with their related single-chain surfactant analogs C9F19AE (536,500 mL·s), they also show exceptional foaming properties [57]. Comprehensively, the excellent foam performance maybe ascribes to their integrated advantages of Gemini surfactant and fluorocarbon surfactant. The effect of the structure on its foaming properties is obvious with slightly different spacers. EN-2C9F19 shows obviously lower surface tension and higher viscosity (as shown in Fig. 9), which caused the foam integrated value of EN-2C9F19 to be bigger than that of MN2C9F19. On the one hand, empirical evidence exhibited a positive correlation between surface tension and foam performance. On the other

Fig. 5. Diagrammatic drawing of adsorption layer: (a) Gemini surfactants at the air-liquid interface; (b) Single-chain surfactants at the air-liquid interface; (c) Gemini surfactants at the PTFE-liquid interface; (d) Single-chain surfactants at the PTFE-liquid interface.

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Fig. 7. A series of images showing the long-term evolution of foams created by Gemini fluorocarbon surfactant solutions: MN-2C9F19 (a) and EN-2C9F19 (b).

Fig. 8. (a) Parameters of foaming performance for MN-2C9F19 and EN-2C9F19 at different concentrations; the dimensional microstructure of bubbles produced by SDS (b), MN-2C9F19 (c) and EN-2C9F19 (d) solution.

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hand, with the increase of surface viscosity, the surface strength of the foam film improved. It was difficult for the foam liquid to flow and discharge the near foam liquid and then promote foam performance. Table 2 shows that the increment of V is lower than that of t1/2 for two Gemini surfactants, suggesting strong foam stability. It can be interpreted from three aspects, energy theory, gas diffusion, and drainage mechanism. For energy theory, low surface tension leads to low surface energy, which is beneficial to foam stability. Moreover, based on gas diffusion theory, the two surfactants form a dense monolayer to prevent gas diffusing through foam film, leading to long-time foam stability. From the last aspect, the slow flow of liquid film can improve the foaming stability for the drainage mechanism, which is tightly connected to the extrusion between bubbles. This aspect can be explained from Laplace's equation as follows: P B −P A ¼

γ R

ð7Þ

where PA and PB represent the fluid pressure in A and B, R is the radius of the bubbles and γ is surface tension. Since the surface tension of the two surfactants is very low to 14 mN/m, indicating PA is different from PB slightly, liquid drainage and bubbles coarsening slow down to maintain foaming stability. As a result, the dimensional microstructure of MN2C9F19 and EN-2C9F19 bubbles were observed to be uniform as shown in Fig. 8c-d. For hydrocarbon surfactants SDS used as foaming agents, its surface tension is much bigger than that of the two fluorocarbon surfactants. Therefore, the difference in pressure A and B is bigger and fluid flow quickly. As depicted in Fig. 8b, the dimensional microstructure of SDS bubbles appear uneven, demonstrating a difference between the fluid pressure. Compared to traditional surfactant SDS, two Gemini surfactants are obviously more suitable for long-term stability foaming agent. The thermostatic water bath was used for controlling system temperature to study foam performance at different temperatures. As described in Table 2, the foaming property of EN-2C9F19 is better than that of MN-2C9F19 at different temperatures. This phenomenon is consistent with the surface activities and thermal properties, because of the higher thermal stability and stronger surface activity of EN-2C9F19. All the foaming parameters decrease with the temperature increasing. This trend is influenced by two factors [60], the higher surface tension and lower viscosity at high temperatures. The plots of viscosity of surfactant aqueous solutions are shown in Fig. 9. Viscosity is related to temperature, the type of substance, and concentration. At low temperature (25 °C), the viscosity of MN-2C9F19 and EN-2C9F19 could reach up to 11 mPa·s and 14 mPa·s. As we know, EN-2C9F19 has one more methylene group than MN-2C9F19. Probably the steric hindrance of the methylene group in EN-2C9F19 makes the perfluorinated chains of molecular unable to rotate flexibly. Thus, when molecules form micelles in solution, perfluorinated chains of EN-2C9F19 are less mobile than MN2C9F19 in a micelle. This results in the micelles less prone to deformation and increases the overall viscosity of the solution consequently. The structural difference makes EN-2C9F19 show higher viscosity, which is one of the reasons for its better foaming performance. At high

Table 2 Foaming characterization of two nonionic Gemini fluorocarbon surfactants. Temperature (°C)

30

40

50

Va (mL) Vb (mL) ta1/2 (s) tb1/2 (s) Fa (mL s) Fb (mL s)

470 460 430 362 235 140 500 442.5 410 395 382.5 180 5814 2284 1393 1220 1083 613 8741 5730 3670 1863 926 701 2,732,580 1,050,640 598,990 441,640 254,505 85,820 4,370,500 2,535,525 1,504,700 735,885 354,195 126,180

Va, ta1/2 and Fa from MN-2C9F19 aqueous solution. Vb, tb1/2 and Fb from EN-2C9F19 aqueous solution.

60

70

80

Fig. 9. Plots of viscosity as a function of shear rate using five times cmc aqueous solution at 25 °C/70 °C.

temperature, the viscosity of two Gemini surfactant solutions is constant (about 2.5 mPa·s), which were not noticeably more viscous than water, indicating that the ability of two Gemini surfactants to form of worm-like micelles are weak. As a matter of fact, the viscosity indeed decreases at a high temperature. But viscosity does not decrease with the increasing shear rate at 70 °C. It is because the molecular thermal motion intensifies, resulting in a sharp decrease in viscosity. In other works [62], when the viscosity is lower than 2.5 mPa·s, the change of shear rate will not have an obvious impact on the viscosity. Regardless of the shear rate, reflecting a typical behavior of Newtonian fluids [61]. Those two possibilities may contribute to the reduction of foam performance at high temperatures. Although the temperature is up to 70 °C, the MN-2C9F19 and EN-2C9F19 aqueous solutions still show preeminent foam characterization, which is higher than conventional foam surfactant SDBS even at 25 °C (174,660 mL·s), demonstrating their potential application as excellent foaming agents at high temperature. 4. Conclusions Two new nonionic ester-bonded Gemini fluorocarbon surfactants were synthesized by a one-step reaction, which were confirmed by 1H NMR, 19F NMR, and elemental analysis. They showed excellent surface activity, wettability, and high foaming performance. Although the structural difference between the two surfactants is negligible, EN-2C9F19 showed more outstanding properties. On the one hand, EN-2C9F19 exhibited superior surface activity with a cmc of 0.38 mM and γcmc of 13.71 mN/m. On the other hand, EN-2C9F19 could wet completely on the PTEF plate using low concentration (0.6 mM). In addition, EN2C9F19 has more excellent foaming property with the foam integrated value F up to 4,370,500 mL·s, which is one of the highest values of fluorocarbon surfactants so far. This may be attributed to lower surface tension and higher viscosity. Furthermore, they also showed higher foam integrated value at 70 °C than conventional widely used foam surfactant SDBS even at 25 °C demonstrating their potential application as excellent foaming agents at high temperature. Declaration of competing interest The authors declare that they have no competing interests. Acknowledgment We gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 51673113 and 51973107), the key project of Department of Education of Guangdong Province (No.

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2018KZDXM032) and Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme 2019 (GDUPS 2019). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2020.112469.

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