Introduction

Fluorinated surfactants are more surface active than ordinary hydrocarbon surfactants. They have been extensively investigated and utilized as emulsifying agents for emulsion polymerization, agents for improving wax leveling, flame retardants, surface modifiers for textiles and surfactants for vesicles or cumulative membranes due to their versatile properties, such as thermal stability, chemical inertness, high surface activity as well as water and oil repellence [1, 2]. These properties are affected by the hydrophobic fluorocarbon moieties existing in the molecular structures. It has been reported that their surface tensions are lower than common hydrocarbon surfactants and they form micelles at low concentrations [3].

Two well-known fluorinated surfactants are the ammonium salt of perfluorooctanoate (PFOA) and perfluorooctane sulfonate (C8F17SO3X, where X = K, Na, H). PFOA plays a key role in the emulsion or dispersion polymerization process to help incorporate hydrophobic monomers into the latex polymer, especially fluorinated monomers such as tetrafluoroethylene and other C2–C3 alkenes [4].

Fluorinated surfactants do have shortcomings, such as low solubility in water, high price and environmental concerns that need to be overcome [5]. Recently, PFOA has been found to be persistent, toxic and bioaccumulable because it is difficult to degrade under enzymatic or metabolic decomposition. Its accumulation in the environment has shown some negative associations with human health [6–8].

Since fluorinated surfactants are important for preparing high performance fluorinated materials [9], extensive research on synthesis and screening of novel fluorocarbon surfactants with a good environmental profile has been performed. Many strategies for synthesizing non-bioaccumulable alternatives to PFOA and related perfluorinated surfactants have been reported [10–13].

One efficient method is to ‘dilute’ the long carbon–fluorine chains with hydrocarbon chains and manufacture hybrid surfactants which possess both fluorocarbon and hydrocarbon chains in their molecule. The incorporation of hydrocarbon chains can improve the short comings mentioned previously and still keep the high surface tension lowering ability at same time [14].

Guo et al. firstly reported new ‘double tail’ type hybrid surfactants composed of hydrocarbon and fluorocarbon chains, both attached to a sulfate group at the head end. Surface tension lowering ability of these surfactants was as high as that for fluorinated surfactants. However, the surfactants were so unstable and had to be stored in a desiccator at −25 °C [15].

Subsequently, Kondo et al. introduced -C6H4CO- or -(CH2)2- groups between a fluorocarbon chain and a hydrocarbon chain in a molecule to successfully synthesize sulfonate-type hybrid surfactants. These surfactants are reported to be very stable in the presence of moisture [16, 17]. The Kondo group also synthesized three novel double-tailed anionic fluorocarbon surfactants with the introduction of two oxyethylene units between the hydrophobic and hydrophilic groups as a spacer to enhance the hydrophilicity of the surfactants [18]. Recently, the synthesis of gemini-type hybrid fluorinated surfactants with good water-solubility was reported, in which a hydrocarbon group was connected with two fluorinated parts using the Grignard reaction [14]. Unfortunately the overall yield of products is a little low (17–20 wt%).

In our previous work, two environmentally friendly succinic acid monofluoroalkyl sulfonate surfactants were synthesized, in which oxyethylene units were introduced between the hydrophobic and hydrophilic groups as spacers to enhance the hydrophilicity of the surfactants and their chemical stability [19]. Considering that the ability of surfactants having two fluoroalkyl chains against flocculation-redispersion is superior to that of surfactants having one fluoroalkyl chain, this article reports the synthesis of four environmentally friendly succinic acid double-tailed sulfonate fluorinated surfactants (FEOS*-1, 2, 4, 6). Their hydrophilicity and surface active properties in terms of Krafft points, surface tension, and critical micellar concentration (CMC) are presented and discussed.

Experimental

Materials

1H,1H,7H-Dodecafluoroheptanol, 1H,1H,5H-octafluoropentanol, 1H,1H,3H-tetra-fluoropropanol, and 1H,1H-trifluoroethanol were purchased from China Fluoro Technology Co. Ltd. (Shandong China). Maleic anhydride was obtained from the Tianjin Chemical Reagent Factory (Tianjin China). Sodium hydrogen sulfite was purchased from the Tianjin Kermel Chemical Reagent Co. Ltd. (Tianjin China). Ethanol was provided by the Tianjin Guangcheng Chemical Reagent Co. Ltd. (Tianjin China). P-toluenesulfonic acid was purchased from the Beijing Yili Fine Chemicals Co Ltd. (Beijing China). The raw materials (AR grade) mentioned above were used as received. 1H,1H,7H-Dodecafluoroheptyloxy ethanol, 1-(1H,1H,5H-octafluoropentyloxy) ethanol, 1-(1H,1H,3H-tetrafluoropropoxy) ethanol and 1-(1H,1H-trifluoroethyoxy) ethanol were self-made at our Key Laboratory of Fluorine Chemistry and Chemical Materials, whose purity is up to 92 % and above (by GC). 1,1,2-Trichloro-1,2,2-trifluoroethane and PFOA were also prepared in our group.

Synthesis of Succinic Acid Double-Tailed Sulfonate Fluorinated Surfactants

The process used to synthesize the succinic acid double-tailed sulfonate fluorinated surfactants is shown in Scheme 1.

Scheme 1
scheme 1

Synthetic procedure of the succinic acid double-tailed sulfonate fluorinated surfactants

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A mixture of maleic anhydride (0.5 mol), fluoroalkyloxy ethanol (1.1 mol), and a moderate amount of p-toluenesulfonic acid (0.5–0.7 wt%) as a catalyst were put into a bottle equipped with Dean-stark trap and the reaction was performed at 120 °C for 18 h. Given the reaction is an esterification reaction, suitable amounts of water-carrying agents were added to remove the water produced in the reaction process. The proper water-carrying agents (cyclohexane, benzene, toluene and xylene) were chosen depending on the boiling points of fluoroalkyloxy alcohols. After the reaction, the water-carrying agents and the unreacted fluoroalkyloxy alcohols were removed by the vacuum distillation at 160 °C. The vacuum distillation temperature was adjusted accordingly by the boiling points of fluoroalkyloxy alcohols. The bis[2-(2-fluoroalkyloxy ethyl)] maleate product was obtained as a yellow liquid (yields: 74–83 %). Then, an aqueous solution of sodium hydrogen sulfite (0.55 mol) was added dropwise to the bis[2-(2-fluoroalkyloxy ethyl)] maleate with continuous stirring at 104 °C for 8 h. The coarse products FEOS*-n (n = 1, 2, 4, 6) obtained were washed several times with ethanol to filter the unreacted sodium hydrogen sulfite. The white lardaceous solid products FEOS*-n (n = 1, 2, 4, 6) were collected after removing unreacted ester by washing with 1,1,2-trichloro-1,2,2-trifluoroethane. The overall yield was 67–71 %. The 1H nuclear magnetic resonance (NMR) spectra and the purity of fluoroalkyloxy ethanol measured with gas chromatography were given in Figure S1, Figure S2 and Table S1.

A Bio-Rad FTS165 was used to measure FT-IR spectra of the succinic acid double-tailed sulfonate fluorinated surfactants. 1H-NMR spectra and 19F NMR of the synthesized fluorinated surfactants and intermediate products were obtained using a Bruker Avance III-400 MHz instrument with deuterated chloroform (CDCl3) or deuteroxide (D2O) as solvents and TMS as an internal standard.

A Pryris Diamond TG/DTA (Perkin-Elmer Co., USA) was used to evaluate the thermal stabilities of the fluorinated surfactants. The samples were heated from 37 to 400 °C at a rate of 10 °C/min.

Molar conductivity (Λm) of the prepared fluorinated surfactants was used to determine critical micelle concentrations at 25.0 ± 0.2 °C using a Metrohm 712 conductimeter apparatus with an absolute accuracy up to ±3 %.

A tensiometer JK99B (Shanghai Zhongchen Digital Technique Equipment Limited Company, China) was used to measure the surface and interfacial tension of these fluorosurfactant solutions at 25.0 ± 0.2 °C by the Ring method according to the previous procedure [20].

Critical micelle temperatures (also known as Krafft points) of the synthesized fluorinated surfactants were determined by measuring the electrical conductivity at given high concentrations with a CM-60G(DKK-TOA conductivity meter). The samples were placed in a thermostatted bath with gently heating.

The surface excess concentration at the air/water interface (Γmax) was calculated using the Gibbs adsorption isotherm (Eq. 1) [21]:

$$\varGamma_{\hbox{max} } = - \left( {\frac{1}{4.606RT}} \right)\left( {\frac{\partial \gamma }{\partial \lg C}} \right)_{T}$$
(1)

where R is the gas constant (8.31 J mol−1 K−1), T is the absolute temperature and γ is the surface tension of the synthesized fluorinated surfactant aqueous solution, C is the concentration of the synthesized fluorinated surfactant aqueous solution.

The area per molecule at the interface provides information on the degree of packing and the orientation of the adsorbed surfactant molecule when compared with the dimensions of the molecule as obtained by use of molecular models. From the surface excess concentration, the area per molecule at the gas–liquid interface \(a_{m}^{s}\), in square ångstroms is calculated from Eq. 2 [21]:

$$a_{m}^{s} = \frac{{10^{14} }}{{N_{A} \varGamma_{\hbox{max} } }}$$
(2)

where N A  = Avogadro’s number and Гmax is in mol/cm2. The standard free energy of micelle formation is calculated from Eq. 3 [21]

$$\varDelta G = RT\ln \left( {\frac{\text{CMC}}{55.5}} \right)$$
(3)

where CMC is the critical micelle concentration expressed in molar units.

Results and Discussion

Chemical Structure of Novel Succinic Acid Double-Tailed Sulfonate Fluorinated Surfactants

In this work, four environmentally friendly succinic acid double-tailed sulfonate fluorinated surfactants were synthesized from maleic anhydride, fluoroalkyl alcohols, namely sodium bis[2-(2-1H,1H,7H-dodecafluoroheptyloxy) ethyl]-2-sulfosuccinate surfactant (FEOS*-6), sodium bis[2-(2-1H,1H,5H-octafluoropentyloxy) ethyl]-2-sulfosuccinate surfactant (FEOS*-4), sodium bis[2-(2-1H,1H,3H-tetrafluoropropoxy) ethyl] -2-sulfosuccinate surfactant (FEOS*-2), and sodium bis[2-(2-1H,1H-trifluoroethyoxy) ethyl]-2-sulfosuccinate surfactant (FEOS*-1). The structure of the FEOS*-6 was characterized by FT-IR, 1H NMR and 19F NMR and the results are shown in Figs. 1, 2, 3, 4. And the FT-IR, 1H-NMR and 19F-NMR spectra of FEOS*-4, FEOS*-2 and FEOS*-1 are given in the Supplementary Material (see Figure S3).

Fig. 1
figure 1

1H-NMR spectra of sodium bis[2-(2-1H,1H,7H-dodecafluoroheptyloxy) ethyl] maleate

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Fig. 2
figure 2

1H-NMR spectra of sodium bis[2-(2-1H,1H,7H-dodecafluoroheptyloxy) ethyl]-2-sulfosuccinate surfactant FEOS*-6

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Fig. 3
figure 3

19F-NMR spectra of sodium bis[2-(2-1H,1H,7H-dodecafluoroheptyloxy) ethyl]-2-sulfosuccinate surfactant FEOS*-6

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Fig. 4
figure 4

FT-IR spectra of sodium bis[2-(2-1H,1H,7H-dodecafluoroheptyloxy) ethyl]-2-sulfosuccinate surfactant FEOS*-6

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Figure 1 shows the 1H-NMR spectra of bis[2-(2-fluoroalkyloxy ethyl)] maleate (CDCl3), 1H NMR: δ = 5.88 ppm to δ = 6.05 ppm (m, 2H, a), δ = 3.87 ppm (m, 4H, b), δ = 4.01 ppm (m, 4H, c), δ = 4.35 ppm (m, 4H, d), δ = 6.29 (m, 4H, e) for Ha (CF2)6CH b2 OCH c2 CH d2 OOCCH e2  = CH e2 COOCH d2 CH c2 OCH b2 (CF2)6Ha; and Fig. 2 shows the 1H-NMR spectra of FEOS*-6 (D2O), 1H NMR: δ = 5.90 ppm to δ = 6.23 ppm (m, 2H, a), δ = 3.75 ppm to δ = 3.85 ppm (m, 4H, b), δ = 3.87 ppm to δ = 3.95 ppm (m, 4H, c), δ = 4.11 ppm to δ = 4.30 ppm (m, 5H, d and f), δ = 3.05 ppm (m,1H, e) for Ha (CF2)6CH b2 OCH c2 CH d2 OOCCHe (SO3Na) CH f2 COOCH d2 CH c2 OCH b2 (CF2)6Ha. The strong peaks at 4.76 ppm is attributed to the D2O.

Figures 3 and 4 show the 19F-NMR and FT-IR spectra of sodium bis[2-(2-1H,1H,7H-dodecafluoroheptyloxy) ethyl]-2-sulfosuccinate surfactant FEOS*-6, respectively. 19F-NMR (D2O): δ = −120.77 ppm (p, 4F, a), δ = −122.87 ppm (p, 4, b), δ = −123.12 ppm (p, 4F, c), δ = −124.43 ppm (p, 4F, d), δ = −130.67 ppm (p, 4F, e), δ = −139.29 ppm (p, 4F, f) for HCF a2 CF b2 CF c2 CF d2 CF e2 CF f2 CH2OCH2CH2OOCCH (SO3Na) CH2COOCH2CH2OCH2CF f2 CF e2 CF d2 CF c2 CF b2 CF a2 H; IR (KBr): 1738(ν C=O), 1637(ν OC=O), 1400(ν SO2-O), 1201(ν CF), 1049(ν COC).

Thermal Stabilities of Novel Succinic Acid Double-Tailed Sulfonate Fluorinated Surfactants

Figure 5 shows the TGA curves of the synthesized fluorinated surfactants. Four novel fluorinated surfactants show different levels of thermal stability respectively. The basic weight of the fluorine surfactants did not change under 200 °C, while FEOS*-1 began to decompose when the temperature reached 235 °C. With a further temperature increase, FEOS*-1 lost most of its weight (about 83 %) at 430 °C, which verified the surfactant possessed high-temperature resistance. The compounds FEOS*-2 and FEOS*-4 have high thermal decomposition temperatures, and began to decompose when the temperature reached about 248 °C. FEOS*-2 had broken down 86 % at 438 °C, and the compound of FEOS*-4 showed 89 % decomposition when the temperature reached 432 °C. Therefore, the usage temperature of FEOS*-2 and FEOS*-4 should be controlled below 240 °C. As for product FEOS*-6, the quality of compounds did not change before 248 °C. However, it began to decompose, when the temperature heated to 445 °C (the surfactant had only 23 % left). From the data above, it could be concluded that all four double-tailed fluorine surfactants have thermal-stable properties and could be applied at relatively high temperature. [22, 23]

Fig. 5
figure 5

Thermal gravimetric analysis (TGA) thermograms of the succinic acid double-tailed sulfonate fluorinated surfactants FEOS*-1 (a), FEOS*-2 (b), FEOS*-4 (c) and FEOS*-6 (d)

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Surface Properties of Novel Succinic Acid Double-Tailed Sulfonate Fluorinated Surfactants

To investigate the surface properties of the double-tailed fluorinated surfactants, the CMC of FEOS*-6, FEOS*-4, FEOS*-2 and FEOS*-1 were measured by conductivity and are given in Fig. 6. The break points in the curves correspond to the CMC of the prepared fluorinated surfactants. The values of CMC for FEOS*-6, FEOS*-4, FEOS*-2 and FEOS*-1 were 0.0719, 5.42, 80.4 and 143.4 mmol L−1, respectively. The CMC values decreased significantly with increasing -CF2- in the hydrophobic chain. FEOS*-6 exhibited the lowest CMC value, indicating it had better surface properties than the others.

Fig. 6
figure 6

Variation in the molar conductivity (Λm) with the FEOS*-6 (a), FEOS*-1 (b), FEOS*-2 (c), and FEOS*-4 (d) surfactant SQRT concentration at 25.0 ± 0.2 °C

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The surface tension (γ)-bulk concentration (C) dependencies for prepared fluorinated surfactants are presented in Fig. 7. No minimum was observed on the γ-lgC curves of the aqueous synthesized surfactants solution, indicating that there were no surface-active impurities in the products [24]. The surface tension at the CMC of FEOS*-1, FEOS*-2, FEOS*-4 and FEOS*-6 is 26.50, 31.34, 25.43 and 21.63, respectively. In particular, the surface tension of FEOS*-6 has minimum surface tension, comparable to that of PFOA.

Fig. 7
figure 7

Surface tension plots of the FEOS*-6 (a), FEOS*-1 (b), FEOS*-2 (c), and FEOS*-4 (d) logarithm concentration at 25.0 ± 0.2 °C

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Figure 8 shows the interfacial tension of the prepared fluorinated surfactants in cyclohexane and a similar trend in interfacial tension is found in comparison with that of the surface tension. Compared to the interfacial tension of hydrogenated surfactants, perfluoro-surfactants possess higher interfacial tension and lower interfacial activity. For the prepared fluorinated surfactants, the ethoxyl group enhances the interfacial activity of the prepared fluorinated surfactants, hence widening the range of application.

Fig. 8
figure 8

Interfacial tension plots of the FEOS*-6 (a), FEOS*-4 (b), FEOS*-2 (c), and FEOS*-1 (d) logarithm concentration at 25.0 ± 0.2 °C

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The surface activities of the four novel succinic acid double-tailed sulfonate fluorinated surfactants at 25.0 ± 0.2 °C are summarized in Table 1, together with PFOA and succinic acid monofluoroalkyl sulfonate surfactant (FEOS-1) taken from the previous work [19]. From Table 1, the Krafft temperatures of the four novel succinic acid double-tailed sulfonate fluorinated surfactants are all below 0 °C. The γCMC values of FEOS*-4 and FEOS*-6 were close to those of PFOA and FEOS-1, while the CMC values of the former exhibited a lower CMC value and higher micelle forming ability than the latter due to the double-tail molecular structure. A similar tendency was displayed for the gemini-type surfactants and the corresponding mono-type surfactants prepared by Yoshino et al. [14] Our finding suggest that the double-tailed fluorinated surfactants may be good emulsifiers.

Table 1 The Krafft points (Kp), CMC, γCMC, γjCMC, Гmax, \(a_{m}^{s}\), and ΔG of synthesized surfactants, PFOA and FEOS-1
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Conclusions

In summary, four novel environmentally friendly succinic acid double-tailed sulfonate fluorinated surfactants, FEOS*-n (n = 1, 2, 4, 6), were successfully synthesized. The properties of the synthesized fluorinated surfactants with those of succinic acid monofluoroalkyl sulfonate surfactant and PFOA were tested and all the synthesized fluorinated surfactants showed low Krafft points. The surface tension values of the synthesized succinic acid double-tailed sulfonate fluorinated surfactants at the CMC were similar to those of FEOS-1 and PFOA, while the values of CMC were far less than that of the FEOS-1 and PFOA. In addition, the fluoroalkyl sulfonate surfactants were found to have excellent thermostability with starting equilibrium thermal degradation temperatures of 235 °C, which allows them to be applied at high temperatures. The synthesized novel fluorinated surfactants showed great potential to replace traditional long-chain perfluorinated surfactants due to their outstanding biodegradability. (Have you measured the biodegradability?)