Synthesis, surface properties and cytotoxicity evaluation of nonionic urethane fluorinated surfactants with double short fluoroalkyl chains

Journal Pre-proof Synthesis, surface properties and cytotoxicity evaluation of nonionic urethane fluorinated surfactants with double short fluoroalkyl chains Yichao Shen, Yong Jin, Shuangquan Lai, Liangjie Shi, Weining Du, Rong Zhou PII:

S0167-7322(19)33798-5

DOI:

https://doi.org/10.1016/j.molliq.2019.111851

Reference:

MOLLIQ 111851

To appear in:

Journal of Molecular Liquids

Received Date: 8 July 2019 Revised Date:

11 September 2019

Accepted Date: 28 September 2019

Please cite this article as: Y. Shen, Y. Jin, S. Lai, L. Shi, W. Du, R. Zhou, Synthesis, surface properties and cytotoxicity evaluation of nonionic urethane fluorinated surfactants with double short fluoroalkyl chains, Journal of Molecular Liquids (2019), doi: https://doi.org/10.1016/j.molliq.2019.111851. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

1

Synthesis, surface properties and cytotoxicity evaluation of nonionic urethane

2

fluorinated surfactants with double short fluoroalkyl chains

3 a,b

4

Yichao Shen

5

Rong Zhou a,b

, Yong Jin

a,b,

*, Shuangquan Lai

a,b

, Liangjie Shi

a,b

, Weining Du

a,b

,

6 7

a

8

Sichuan, Peoples R China

9

b

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

Sichuan Univ, Key Lab Leather Chem & Engn, Minist Educ, Chengdu 610065,

Sichuan Univ, Natl Engn Lab Clean Technol Leather Manufacture, Chengdu 610065,

Sichuan, Peoples R China *E-mail: [email protected].

1

1

Abstract

2

Developing suitable alternatives to long fluoroalkyl chain surfactants has drawn

3

considerable attention on account of the concerns over the environmental safety and

4

human health. However, low surface activity, poor water solubility and complex

5

preparation process of most currently reported alternatives restrict their widespread

6

applications. Herein, a series of nonionic urethane fluorinated surfactants (FmEGnFm)

7

were synthesized by one-pot method using poly(ethylene glycol) (PEG), isophorone

8

diisocyanate (IPDI) and short chain fluorinated alcohol as raw materials. The

9

surfactant molecule is composed of two short fluoroalkyl chains connected to a PEG

10

molecule via two IPDI spacers. Benefit from the special molecular structure design,

11

these fluorinated surfactants displayed high surface activities, which could reduce the

12

surface tensions of 17.8-28.7 mN/m and had low critical micelle concentrations of

13

0.17-0.98 mmol/L. These fluorinated surfactants showed good salt and pH resistance.

14

Furthermore, contact angle and emulsifying experiments demonstrated that these

15

fluorinated surfactants possessed excellent wetting and emulsifying properties at an

16

extremely low concentration of 0.1wt.%. More importantly, the cytotoxicity

17

experiment verified that these fluorinated surfactants had no obvious cytotoxicity. The

18

ideal properties coupled with a simple and green preparation process make this

19

strategy a new avenue to fabricate sustainable alternatives to long fluoroalkyl chain

20

surfactants.

21 22

Keywords: Nonionic urethane fluorinated surfactants; Double short fluoroalkyl

23

chains; Surface activity; Wetting properties; Emulsifying properties; Cytotoxicity

24 25 26

1. Introduction

27

Fluorinated surfactants consist of two parts: polar hydrophilic head and fluoroalkyl

28

tail while the latter is both hydrophobic and lipophobic simultaneously. In contrast to

29

the hydrocarbon surfactants, fluorinated surfactants exhibit unmatched properties such

30

as excellent surface activity, thermal and chemical stability [1]. Based on these unique

31

properties, fluorinated surfactants play important roles in numerous practical 2

1

applications, including firefighting foam, pigment additive, emulsifying agent, etc

2

[2-4]. Perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic sulfonates (PFOS)

3

are two typical fluorinated surfactants and have been most commonly produced and

4

used for years [5, 6], which usually can reduce the surface tension to as low as 15-20

5

mN/m [7, 8]. However, a lot of environmental and toxicity studies have indicated that

6

these long fluoroalkyl chain surfactants are serious toxic, obvious persistent and

7

highly bioaccumulative in the environment [9-11]. As a consequence, the United

8

States Environmental Protection Agency and European governments introduced

9

related laws and regulations to restrict the productions and uses of these long

10

fluoroalkyl chain surfactants, although they have optimal surface activities in majority

11

of practical applications [12, 13]. Subsequently, perfluorooctane sulfonate (PFOS) and

12

perfluorooctanoic acid (PFOA) were attributed to persistent organic pollutants (POPs)

13

under the Stockholm Convention [14, 15].

14

In view of the potential environmental pollution and bioaccumulation of long

15

fluoroalkyl chain surfactants, there is an urgent demand to develop novel fluorinated

16

surfactants as alternatives to these conventional long fluoroalkyl chain surfactants

17

[16]. Numerous evidences have manifested that the shorter chain fluorinated

18

compounds containing less than seven CFn groups have less toxicity and lower

19

bioaccumulation [17, 18]. Therefore, a booming interest in the synthesis of short

20

fluoroalkyl chain surfactants has been mentioned, especially since the 3M Company

21

produced novel fluorinated compounds based on perfluorobutane sulfonyl [19].

22

Bodduri et al. synthesized a series of perfluorobutyl substituted disodium

23

alkanesulfonates derivatives whose lowest surface tension could reach 26 mN/m [20].

24

Schuster et al. synthesized carbohydrate-based branched fluorinated amphiphiles with

25

short fluoroalkyl chains[21]. Unfortunately, most of the short fluoroalkyl chain

26

surfactants reported show worse surface activities compared to the long fluoroalkyl

27

chain counterparts, since the surface tension and critical micelle concentration (cmc)

28

of fluorinated surfactants based on conventional molecular structure design increase

29

with shortening the fluoroalkyl chain length.

30

In recent years, special molecular structure surfactants such as Gemini [22, 23], 3

1

bolaform[24] and multi-chain[25, 26] surfactants have been extensively synthesized

2

in the research of hydrocarbon surfactants and these special molecular structure

3

surfactants have been verified that they can more effectively enhance the surface

4

activity compared to conventional hydrocarbon surfactants. To the best of our

5

knowledge, there are relatively few studies on fluorinated surfactants with special

6

molecular structure. Kateb et al. synthesized a kind of semi-fluorinated gemini

7

surfactants with two side bromine groups, and investigated the relationship between

8

the number of methylene units in the spacer group and surface activity [27].

9

Yoshimura et al. reported a partially fluorinated cationic gemini surfactant and found

10

that the length and number of fluoroalkyl chains had significant impacts on the

11

equilibrium and dynamic surface tension [28]. However, many methods for

12

synthesizing fluorinated surfactants expose obvious disadvantages, including complex

13

preparation process as well as consumption of large amounts of organic solvents.

14

Therefore, we attempt to design novel fluorinated surfactants via simple and green

15

preparation process and expect to obtain ideal surface activity. Polyurethane structure

16

has excellent structural controllability, and urethane bond can act as a ‘weak’

17

degradable point. Thus, it is potential to take advantage of polyurethane structure to

18

develop novel fluorinated surfactants with double short fluoroalkyl chains as effective

19

alternatives to long fluoroalkyl chain surfactants.

20

In this paper, we report a facile one-pot synthesis of a series of nonionic urethane

21

fluorinated surfactants (FmEGnFm) by connecting two short fluoroalkyl chains to a

22

polyethylene glycol molecule with isophorone diisocyanate served as the spacer. No

23

organic solvent was used in the preparation process, and the raw materials involved in

24

the reaction are commercially available, which will be easy to facilitate large-scale

25

production and application in the industry. In addition, urethane bond is also

26

considered as a ‘weak’ point for the initial degradation of the surfactant molecule and

27

the components after initial degradation are regarded as low bioaccumulation or good

28

biocompatible [29, 30], which makes these fluorinated surfactants more

29

environmentally sustainable. Furthermore, physicochemical properties such as

30

equilibrium surface tension, critical micelle concentration, interfacial tension, wetting 4

1

and emulsifying properties were systematically investigated. The motivation of

2

present work is to find novel efficient ecofriendly fluorinated surfactants with

3

outstanding surface activity, which could be used as alternative products to

4

conventional long fluoroalkyl chain surfactants, and achieve sustainable fluorine

5

production. More importantly, the strategy utilizing polyurethane structure to

6

introduce two short fluoroalkyl chains into the molecule opens a new feasible and

7

green avenue to fabricate novel fluorinated surfactants.

8 9 10 11 12

2. Materials and methods 2.1. Materials

13

Poly(ethylene glycol) with number average molecular weight of 600 g/mol (PEG13,

14

AR) and 800 g/mol (PEG17, AR) respectively, sodium chloride (NaCl), ammonium

15

hydroxide (NH3·H2O), sodium hydroxide (NaOH) and toluene were supplied by

16

Kelong chemical Co., Ltd (Chengdu, China). Isophorone diisocyanate (IPDI, AR),

17

1,1,1,3,3,3-hexafluoropropan-2-ol (F6, 99.5%), 2,2,3,3,3-pentafluoropropan-1-ol (F5,

18

98%),

19

3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctan-1-ol (F13, >98%) and perfluorooctanoic

20

acid (PFOA, 98%) were all purchased from Aladdin Chemistry Co., Ltd (Shanghai,

21

China). Bismuth neodecanoate catalyst (AC-83) was supplied by Haoyi Chemical

22

Technology Co., Ltd (Guangzhou, China). Mouse fibroblast (L929) cells were

23

purchased from Procell Life Science Co., Ltd (Wuhan, China). Before the

24

experiments, PEG13 and PEG17 were dried under vacuum at 80 °C. All other reagents

25

and solvents were used as received. The distilled water was used in the experiments.

26 27 28

2.2. Synthesis of nonionic urethane fluorinated surfactants (FmEGnFm)

3,3,4,4,5,5,6,6,6-nonafluorohexan-1-ol

(F9,

98%),

29

The nonionic urethane fluorinated surfactants (FmEGnFm, where m = 5, 6, 9, or 13

30

represents the fluorine atoms number of 5, 6, 9, or 13, and n = 13, or 17 represents the

31

EG groups of 13, or 17, respectively) were one-pot synthesized as shown in Fig. 1 and

32

the composite formula is listed in Table S1. In brief, IPDI (0.1 mol) was added into a 5

1

200 mL three-neck flask equipped with a thermometer and a mechanical stirrer. Then

2

fluoroalkyl alcohol (0.1 mol) was added drop-wisely into the flask. After the addition

3

of 0.2 g AC-83 as a catalyst, the temperature was maintained at 80 °C for 8 h. Finally,

4

PEG (0.05 mol) was added and allowed to react for another 10 h to yield the target

5

product in the form of yellow viscous liquid.

6 7 8 9 10

Fig. 1. Synthetic route of nonionic urethane fluorinated surfactants (FmEGnFm). 2.3. Structure characterization

11

Fourier transform infrared (FTIR) was performed with a Nicolet 6700

12

spectrophotometer in KBr pellets. Proton nuclear magnetic resonance (1H NMR) and

13

fluorine nuclear magnetic resonance (19F NMR) were determined on an

14

AV11-400MHz spectrometer with deuterium chloroform (CDCl3) as a solvent. Gel

15

permeation chromatography (GPC) was carried out with a PL-GPC-220

16

chromatograph using tetrahydrofuran (THF) as the eluent. Matrix-assisted laser 6

1

desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) was

2

performed using an Autoflex-III mass spectrometer.

3 4 5

2.4. Surface tension measurement

6

The equilibrium surface tension of surfactant aqueous solutions was measured by a

7

BZY-1 automatic tensiometer. The aqueous solutions of surfactants were prepared in a

8

wide range of concentrations before the tests. All tests were operated at 25 °C. Three

9

replicate measurements were carried out, and the surface tension value was

10

determined until the average standard deviation was less than 0.3 mN/m.

11

The amount of surfactant molecules adsorbed at the air-liquid interface per unit

12

area (Γmax) and the average minimum area (Amin) per surfactant molecule are

13

calculated according to the Gibb’s surface adsorption equation [31, 32]. The standard

14

free energy of adsorption (∆G ) and micelle formation (∆G ) is also quantified [33,

15

34].

16

Γ  = −

17

A !" = 1/(N% · Γ  )

18

∆G = RTln(++.+)

(3)

19

∆G = ∆G − π- - /Γ

(4)

 .

(d /d ) 

(1) (2)



20

Where T is absolute temperature, 298.15 K; R denotes the ideal gas constant, 8.314

21

J·mol−1K−1; γ represents the surface tension, mN/m; C is the concentration of the

22

surfactant in aqueous solution, mol/L; NA is Avogadro constant, 6.023 × 1023 mol−1,

23

respectively. Furthermore, π- - denotes the surface pressure at the cmc (π- - =

24

γ − γ- - , where γ and γ- - are the surface tensions of water and the surfactant

25

solution at the cmc, respectively).

26 27

2.5. Interfacial tension measurement

28 29

The interfacial tension between 0.1wt % F6EG13F6, F5EG13F5, F9EG13F9,

30

F13EG13F13 and PFOA surfactant aqueous solution and toluene was also measured at 7

1

25 °C using a BZY-1 automatic tensiometer.

2 3

2.6. Fluorescence measurement

4 5

Fluorescent probe method was used to determine the critical micelle concentration

6

(cmc). A fixed concentration of pyrene (1.0  ×  10-6 M) in methanol was added to

7

volumetric flasks. The various concentrations of surfactants were configured

8

(F6EG13F6: 3×10-5, 6×10-5, 9×10-5, 3×10-4, 9×10-4, 2×10-3, 4×10-3 mol/L; F5EG13F5:

9

3×10-5, 8×10-5, 2×10-4, 4×10-4, 10-3, 3×10-3, 6×10-3 mol/L; F9EG13F9: 3×10-5, 6×10-5,

10

10-4, 2×10-4, 6×10-4, 10-3, 3×10-3 mol/L; F13EG13F13: 3×10-5, 5×10-5, 8×10-5, 10-4,

11

2×10-4, 3×10-4, 4×10-4, 6×10-4, 10-3 mol/L). The methanol was allowed to evaporate

12

off in air before the surfactant solutions were added. The fluorescence measurements

13

were performed using an F-7000 fluorescence spectrophotometer and the spectra were

14

recorded from 350 to 500 nm.

15 16

2.7. Effect of pH and electrolyte on the surface activity

17 18

Solid NaCl was used to adjust the salt content of surfactant solution from 0 wt % to

19

2 wt %. The studied pH range was between 3 and 11, which was adjusted with 0.1M

20

HCl or NH3·H2O using a PHS-3C+ Acidity Meter (calibrated before use). Then, the

21

equilibrium surface tension of the surfactant aqueous solution was measured

22

according to the above mentioned method.

23 24

2.8. Contact angle measurement

25 26

The contact angle of F5EG13F5, F6EG13F6, F9EG13F9, F13EG13F13 and PFOA

27

surfactant aqueous solution (0.1wt %) on low energy solid surface was measured on a

28

DSA30 contact angle goniometer at 25 °C, respectively. The polytetrafluoroethylene

29

(PTFE) and paraffin film were used as base plates.

30 8

1

2.9. Emulsifying property

2 3

The aqueous solutions of F5EG13F5, F6EG13F6, F9EG13F9, F13EG13F13 and PFOA

4

surfactants (0.1wt %) was mixed with an equal volume of toluene to prepare emulsion,

5

respectively. Then the mixed solution was homogenized using a T18 Basic

6

Ultra-Turrax for 3 min at 10000 rpm. The optical microscope images were taken using

7

a DM2000 optical microscope at 1 h after homogenization. The mean diameter was

8

counted with Image-Pro Plus 6.0 software (Media Cybernetics) by analysis of more

9

than 500 emulsion droplets.

10 11

2.10. Cytotoxicity test

12 13

The

cytotoxicity

of

fluorinated

surfactants

was

assessed

via

MTT

14

(3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay on mouse

15

fibroblast (L929) cells. Briefly, the mouse fibroblast (L929) cells were cultured in a

16

growth

17

penicillin-streptomycin at 37 °C in a 5% CO2 atmosphere for 24 h. The cells were

18

seeded in a 96-well plate at a density of about 5 × 104 cells/well.

medium

supplemented

with

89%

DMEM,

10%

FBS

and

1%

19

The L929 cells were incubated with the fluorinated surfactants (0, 2.5, 5, 10, 25,

20

and 50 µg/ml) for 24 h. Following incubation with the fluorinated surfactants for 24 h,

21

200 µL of MTT (0.5 mg/ml) were added and incubated for 4 h. Then 150 µL of

22

dimethyl sulfoxide (DMSO) were added to each well. After 10 min of oscillation, the

23

absorbance at 570 nm was recorded by a multifunctional microplate reader (Thermo

24

Fisher Scientific, Multlskan Mk3). Cells cultured without any surfactant were set as

25

the blank control. Each experiment was repeated four times. Morphological cell

26

images at different surfactant concentrations were photographed by an inverted

27

biological microscope (Motic, AE31).

28 29 30

The cell viability was determined according to the following equation: Cell viability (%) = (At/Ac)×100%

(5)

Where At is the absorbance of cells treated with different surfactant concentrations 9

1

at 570 nm and Ac is the absorbance of the blank control at 570 nm.

2 3 4 5 6 7 8

3. Results and discussion

9

3.1. Synthesis and characterization of nonionic urethane fluorinated surfactants

10

(FmEGnFm)

11 12

The nonionic urethane fluorinated surfactants (FmEGnFm) were synthesized by

13

one-pot method using isophorone diisocyanate (IPDI), short chain fluorinated alcohol,

14

and poly(ethylene glycol) (PEG) as raw materials. The coupling reaction of

15

isocyanate and hydroxyl groups is mainly involved during the synthesis process.

16

The characterization methods involving FTIR, 1H NMR,

19

F NMR, TOF-MS and

17

GPC analyses were employed to identify the structure of the resultant fluorinated

18

surfactants. The FTIR spectra of F5EG13F5, F6EG13F6, F9EG13F9, and F13EG13F13 are

19

shown in Fig. S1. The peaks at 3328 cm-1 and 1535 cm-1 correspond to the stretching

20

vibration of N-H group. The characteristic absorption at 1717 cm-1 can be attributed to

21

the stretching vibration of C=O group. What’s more, no absorption peak at 2272 cm-1

22

and the appearance of the characteristic vibrations at 1717 cm-1, 3328 cm-1 and 1535

23

cm-1 in the FTIR spectra suggest that -NCO groups from IPDI units were all

24

consumed and transferred to the urethane linkages during the reaction [35]. The 1H

25

NMR and

26

S2(a), the characteristic signals at 0.88-1.89 ppm and 3.60-3.69 ppm are ascribed to

27

the protons of trimethylcyclohexyl frame from IPDI units and -CH2-CH2-O- of PEG

28

blocks, respectively. The chemical shifts at 4.31-4.38 ppm and 2.31-2.97 ppm are

29

assigned to the protons of –NHCO- and RFCH2CH2-, respectively. From the 19F NMR

30

spectrum, four obvious characteristic peaks are observed. Peaks at -81 ppm, -114 ppm,

31

-124 ppm, and -126 ppm can be attributed to -CF2CF2CF2CF3, -CF2CF2CF2CF3,

32

-CF2CF2CF2CF3, and -CF2CF2CF2CF3 groups, respectively. Fig. S2(c) shows the GPC

19

F NMR spectra of F9EG13F9 are exhibited in Fig. S2. As shown in Fig.

10

1

spectrum of F9EG13F9, the number average relative molecular mass (Mn) is

2

approximately 1623 g/mol and the polymer dispersity index (PDI) is 1.18. Meanwhile,

3

the relative molecular mass obtained from the MALDI-TOF-MS spectrum is

4

approximately 1572.76 g/mol, as shown in Fig. S2(d). It can be observed that the

5

molecular weight of F9EG13F9 measured by GPC and MALDI-TOF-MS is slightly

6

different, which may be due to different measurement methods, but in general, the

7

measured relative molecular mass is consistent with the theoretically calculated data

8

(the molecular mass was 1573 g/mol). The above characterization results confirm that

9

the target products are successfully obtained.

10 11

3.2. Surface activity properties

12 13

The curves of surface tension as a function of the logarithm concentration of

14

FmEGnFm in aqueous solution are shown in Fig. 3. The surface tension of FmEGnFm

15

decreases sharply with increasing the concentration and then levels off, indicating that

16

the surfactant molecules saturate the air-liquid interface and begin to form aggregates

17

spontaneously in aqueous solution. The break point observed for all the surfactants

18

corresponds to the critical micelle concentration (cmc). And the cmc values and

19

surface tension at cmc (γcmc) of FmEGnFm are summarized in the Table 1.

20

Similar to conventional fluorinated surfactants, the increase of the fluoroalkyl chain

21

length of FmEGnFm leads to a reduction of critical micelle concentration values.

22

Interestingly, the cmc values of these fluorinated surfactants, especially observed in

23

the case of the fluoroalkyl chain length of 6 whose cmc value can reach as low as 0.17

24

mmol/L, are approximately 10-100 times lower than that of perfluorooctanoic acid

25

(PFOA) and perfluorooctane sulfonic acid (PFOS), whose cmc values are 10 mmol/L

26

and 31 mmol/L, respectively [36]. This suggests that such type of fluorinated

27

surfactants can effectively reduce the surface tension at lower concentrations

28

compared to the commercial long fluoroalkyl chain surfactants, which can greatly

29

reduce total consumption of fluorinated surfactants used in actual applications, not

30

only saving economic costs, but also reducing emissions of fluorinated surfactants in 11

1

the environment. The reason of this phenomenon can be explained that there are two

2

fluoroalkyl chains attached to a hydrophilic chain simultaneously, and each

3

fluoroalkyl chain occupies a certain area at the air-liquid interface, that is, one

4

surfactant molecule occupies about twice the area compared to the monofluoroalkyl

5

chain surfactant molecule, which leads to a lower cmc value. To explain this

6

phenomenon more intuitively, F13EG13F13 and PFOA are selected as examples, and

7

the adsorption behavior of two kinds of surfactant molecules at air-liquid interface is

8

illustrated in Fig. 2.

9

10 11 12 13

Fig. 2. Schematic representation of adsorption behavior for F13EG13F13 and PFOA molecules at air-liquid interface.

14

The fluorescence spectroscopy was employed to further investigate the aggregation

15

of these fluorinated surfactants in aqueous solution. The intensity ratio of 373 to 384

16

nm emission peaks (I1/I3) in the fluorescence spectrum of pyrene is a desired index for

17

the polarity around the pyrene [37]. The emission spectra of pyrene in aqueous

18

solution with various concentrations of FmEG13Fm and the variations of I1/I3 values at

19

different concentrations are shown in supporting information (Fig. S3). The cmc

20

values obtained by fluorescence spectroscopy method are also listed in Table 1. As

21

shown in Table 1, the cmc values of F6EG13F6, F5EG13F5, F9EG13F9 and F13EG13F13 12

1

are 0.24 mmol/L, 0.45 mmol/L, 0.19 mmol/L, and 0.13 mmol/L, respectively. The

2

cmc values obtained by fluorescence spectroscopy method clearly agree well with

3

those obtained from the surface tension method, although these cmc values are little

4

lower than those obtained by surface tension method, which is mainly due to the

5

distinct sensitivity of the two test methods on the micelles.

6

As for surface tension, F13EG13F13 shows the lowest surface tension value among

7

these surfactants, which can reach 17.8 mN/m at the critical micelle concentration of

8

0.17 mmol/L, while the surface tension at the cmc of perfluorooctanoic acid (PFOA,

9

C7F15COOH) obtained from the literature is 19.8 mN/m [38]. The result suggests that

10

such type of surfactant with two short fluoroalkyl chains in a molecule has more

11

notably superior effectiveness to reduce the surface tension in comparison with the

12

conventional long fluoroalkyl chain surfactant, which further indicates that these

13

synthesized fluorinated surfactants could be potential effective substitutes to

14

conventional long fluoroalkyl chain surfactants. It can be speculated that there are

15

some possible reasons for this situation (as shown in Fig. 2): (i) hydrophilic chain

16

tightly bonds the two fluoroalkyl chains together and each surfactant molecule

17

contains two fluoroalkyl chains, leading a closer arrangement between the fluoroalkyl

18

chains and reducing the surface tension more effectively; (ii) compared with ionic

19

fluorinated surfactants, there is no strong electrostatic repulsion between the

20

hydrophilic groups of the surfactant molecules, and they can be closely arranged at

21

the air-liquid interface, resulting in a further decrease of surface tension. These

22

synthesized short fluoroalkyl chain nonionic surfactants also show lower surface

23

tensions than those of the commercial long fluoroalkyl chain nonionic surfactants [39].

24

Furthermore, the effect of hydrophilic chain length to surface tension was also

25

investigated, it can be found that the change of hydrophilic chain length show little

26

effect on the surface tension. When the fluoroalkyl chain length is the same and the

27

number average molecular weight of PEG increases from 600 to 800, the surface

28

tension changes slightly, only increasing approximately 1.0 mN/m.

29

In general, the surface properties are closely related to the adsorption of surfactant

30

molecules at the air-liquid interface. Therefore, the amount of surfactant molecules 13

1

adsorbed at the air-liquid interface per unit area (Γmax) and the average minimum area

2

(Amin) per surfactant molecule are calculated to further study the adsorption behavior

3

of FmEGnFm. The Γmax and Amin values of FmEGnFm calculated are listed in Table 1. It

4

can be observed that the Amin values increase with the increase of fluoroalkyl chain

5

length of FmEGnFm, revealing that fluorinated surfactant with a longer fluoroalkyl

6

chain length occupies a larger area at the air-liquid interface because of hydrophobic

7

fluoroalkyl chain interactions [40]. In addition, it can be found that although the

8

fluoroalkyl chain length is same, Amin value of F6EGnF6 (45.5 Å2 and 50.5 Å2) is more

9

than that of F5EGnF5 (27.7 Å2 and 31.2 Å2). This unusual phenomenon is guessed that

10

the fluoroalkyl chain of F6EGnF6 is branched, resulting in a larger molecule

11

cross-sectional area at the air-liquid interface.

12

For a deeper understanding of the influence of fluoroalkyl chain length on the

13

micellization

and

adsorption

behaviors,

14

thermodynamics of these fluorinated surfactants were investigated and the related

15

thermodynamic parameters summarized in Table 1. It is obvious that both the △Gmic

16

and △Gads values of all the fluorinated surfactants are negative, suggesting that the

17

micellization and adsorption of these fluorinated surfactants are spontaneous

18

processes. Additionally, both the △Gmic and △Gads values of FmEGnFm (represented in

19

Table 1) become more negative with the growth of fluoroalkyl chain length. The more

20

negative of △Gmic and △Gads means the stronger aggregation and adsorption in

21

aqueous solution. The increase of fluoroalkyl chain length contributes to the

22

adsorption and micelles formation of surfactant molecules because of the

23

enhancement of fluoroalkyl chain interactions. In comparison, the △Gmic and △Gads

24

values of PFOA obtained from literature are -20.64 KJ/mol and -44.49 KJ/mol,

25

respectively [41]. Obviously, F13EGnF13 show a significantly larger standard Gibbs

26

free energy than that of PFOA. From above these results, it can be concluded that

27

these fluorinated surfactants, especially the fluoroalkyl chain length of 6, can

28

effectively adsorb and aggregate in aqueous solution.

29

14

the

micellization

and

adsorption

1 2 3 4 5 6 7 8 9

Fig. 3. Surface tension as a function of the logarithm concentration of (a) FmEG13Fm and (b) FmEG17Fm.

Table 1. Surface properties of nonionic urethane fluorinated surfactants (FmEGnFm) in aqueous solution at 25 °C. cmc[a] cmc[b] γcmc 106 Γmax Amin △Gads △Gmic Surfactant 2 2 (mmol/L) (mmol/L) (mN/m) (mol/m ) (Å ) (KJ/mol) (KJ/mol) F5EG13F5 0.79 0.45 27.7 5.99 27.7 -35.04 -27.64 F5EG17 F5 0.98 28.7 5.33 31.2 -35.25 -27.13 / F6EG13F6 0.63 0.24 26.8 3.65 45.5 -40.60 -28.22 F6EG17 F6 0.89 27.6 3.29 50.5 -40.87 -27.37 / F9EG13 F9 0.40 0.19 21.0 4.12 40.3 -41.73 -29.35 / 22.1 3.60 46.1 -42.08 -28.22 F9EG17 F9 0.63 15

F13EG13 F13 F13EG17 F13

0.17

0.13

17.8

3.74

44.4 -45.96

-31.47

0.25

/

18.1

3.54

46.9 -45.75

-30.52

1

[a]: critical micelle concentration measured by surface tension method;

2

[b]: critical micelle concentration measured by fluorescent probe method.

3 4

3.3. Effect of pH and electrolyte on the surface tension

5 6

Most fluorinated surfactants described in the literatures are anionic or cationic,

7

whose surface activities are susceptible to the change of pH value and the presence of

8

electrolytes in the surrounding environment [42, 43]. However, a desirable salt and

9

pH resistance of surfactants is essential in practical production and daily applications.

10

Here, the effect of pH and electrolyte on the surface tension (represented by

11

F13EG13F13) was investigated. As shown in Fig. 4(a), when the concentration of

12

F13EG13F13 is below cmc, the surface tension approximately decreases by 2 mN/m

13

with increasing the value of pH from 3 to 5. However, the surface tension changes

14

little with increasing the value of pH when the concentration of F13EG13F13 is larger

15

than cmc. This phenomenon might be attributed to the presence of -NH- groups in

16

surfactant molecule which could be protonated under acid condition. The protonation

17

of -NH- groups slightly enhances the intermolecular electrostatic repulsion, resulting

18

in a loose packaging of the surfactant molecules at the air-liquid interface, and

19

decreases the surface tension in a small extent [44]. Meanwhile, the effect of the

20

addition of sodium chloride on the surface tension was also studied. As shown in Fig.

21

4(b), when the concentration of F13EG13F13 is lower than cmc, the surface tension

22

decreases about 1 mN/m as the concentration of sodium chloride increase from 0 wt. %

23

to 2 wt. %. We guessed that the hydration of salts indirectly increases the effective

24

concentration of the fluorinated surfactant solution, leading to a slight decrease in

25

surface tension. Through the above experiments, it can be concluded that this series of

26

fluorinated surfactants exhibit an outstanding salt and pH resistance, and potentially

27

used in certain special situations. 16

1 2

Fig. 4. Effect of (a) pH value and (b) concentration of sodium chloride on the surface

3

activity of F13EG13F13.

4 5

3.4. Wetting and spreading properties

6 7

Generally, superior wetting and spreading of aqueous solution on the low energy

8

solid surfaces are significant in numerous applications of industrial production and

9

daily life [45, 46]. Surfactants are usually added to the solution to adjust the

10

interfacial property of solid-liquid interface to endow solution good wetting and 17

1

spreading properties over the low energy solid surfaces. Meanwhile, it is well

2

accepted that fluorinated surfactants exhibit better wetting and spreading properties on

3

the low energy solid surfaces than hydrocarbon analogues. The contact angle is

4

regarded as a preferable criterion to characterize the wettability of surfactants aqueous

5

solution on low energy solid surfaces [47]. Fig. 5 shows the dynamic contact angles of

6

0.1wt. % F6EG13F6, F5EG13F5, F9EG13F9, F13EG13F13 and PFOA surfactant aqueous

7

solutions on paraffin and PTFE film, which are typical of hydrophobic low energy

8

solid surfaces. As shown in Fig. 5, the contact angles of these surfactant aqueous

9

solutions decrease rapidly between 0-20 s and then reach an equilibrium value over

10

time both on paraffin and PTFE film surface. Moreover, it can be obtained that the

11

contact angle of F5EG13F5, F9EG13F9, and F13EG13F13 surfactant aqueous solutions can

12

reach approximately 65º on PTFE film and 55º on paraffin film, respectively,

13

indicating they possess outstanding wetting and spreading properties

14

extremely low concentration. Surprisingly, F6EG13F6 surfactant aqueous solution

15

shows a worse wettability compared with other three surfactants aqueous solutions. It

16

is presumably attributed to the fact that the fluoroalkyl chain of F6EG13F6 is branched,

17

which generates a looser arrangement between the fluoroalkyl chains and leads to a

18

poor wettability on the low energy solid surfaces. For comparison, the contact angle

19

of 0.1wt. % PFOA surfactant aqueous solution measured was more than 75º whether

20

on parafilm or PTFE film, showing an unsatisfactory wettability at this very low

21

concentration. The above results indicate that the addition of F5EG13F5, F9EG13F9, and

22

F13EG13F13 at an extremely small amount can impart good wettability and spreading

23

properties to the aqueous droplets on low energy solid surfaces.

24

18

at an

1 2 3 4 5 6 7

Fig. 5. Dynamic contact angle of 0.1wt. % F5EG13F5, 0.1wt. % F6EG13F6, 0.1wt. % F9EG13F9, 0.1wt. % F13EG13F13, 0.1wt. % PFOA and H2O on (a) PTFE and (c) Paraffin film at 25 °C. Contact angle images of 0.1wt. % F5EG13F5, 0.1wt. % F6EG13F6, 0.1wt. % F9EG13F9, 0.1wt. % F13EG13F13, and 0.1wt. % PFOA on (b) PTFE and (d) Paraffin film at 110s.

19

1 2

3.5. Emulsifying property

3

Oil-water emulsification is a critical issue for surfactants due to its wide range of

4

applications in numerous fields including oil exploitation, cosmetics, pigment and

5

food industry. Fluorinated surfactants are believed to be more effective in reducing

6

oil-water interfacial tension compared to hydrocarbon surfactants [48]. Herein, four

7

described types of F5EG13F5, F6EG13F6, F9EG13F9, and F13EG13F13 surfactants were

8

used to emulsify toluene-water emulsions, respectively. As a contrast, PFOA was also

9

used as an emulsifier to emulsify toluene-water emulsion. The optical microscope

10

images as well as the mean diameters of emulsions droplets emulsified by above

11

surfactants are shown in Fig. 6. It can be seen that with the increase of fluorocarbon

12

chain length, the emulsifying ability of the surfactants enhances accordingly. As

13

shown in Fig. 6(f), the emulsions emulsified by F9EG13F9 and F13EG13F13 show

14

comparatively small mean droplet sizes with the vast majority of droplets sizes

15

between 5 and 10 µm. In contrast, the toluene-water emulsion emulsified by PFOA

16

shows a worse emulsifying property, whose mean diameter is larger and the size

17

distribution is wider. In order to further investigate emulsifying property of these

18

fluorinated surfactants, the interface tension values between surfactants aqueous

19

solutions and toluene were also measured and the data were measured (the measured

20

values are listed in Table S2). The results demonstrate that F9EG13F9 and F13EG13F13

21

possessing longer fluoroalkyl chain can reduce the oil-water interfacial tension

22

effectively and show an outstanding emulsifying property at an extremely low

23

concentration.

20

1 2

Fig. 6. Optical microscope images of toluene-water emulsions stabilized by 0.1wt. %

3

FmEG13Fm with different fluoroalkyl chain length: (a) F5EG13F5, (b) F6EG13F6, (c)

4

F9EG13F9, (d) F13EG13F13, (e) PFOA. (f) Statistical size distribution of each emulsion

5

counted from more than 500 oil droplets in the corresponding optical microscope

6

images.

7 8

3.6. Cytotoxicity evaluation

9 10

Cytotoxicity is an important test for assessing environmental safety of chemical

11

substances. In this study, F13EG13F13 were selected for cell cytotoxicity tests. The

12

cytotoxicity of F13EG13F13 was evaluated against mouse fibroblast (L929) cells using

13

an MTT assay. The images of cells morphology at different fluorinated surfactant 21

1

concentrations are described in Fig. S4. The MTT assay clearly demonstrates the

2

cytotoxicity of F13EG13F13 with different concentrations to cells and the experiment

3

results are also shown in Fig. 7. Noticeably, the cell viabilities of the surfactant

4

incubated with mouse fibroblast (L929) cells are all above 78% at the concentrations

5

of fluorinated surfactant increases from 2.5 to 50µg/ml, demonstrating that F13EG13F13

6

has not shown any significant cytotoxicity to mouse fibroblast (L929) cells. This

7

result suggests that these synthesized fluorinated surfactants have no significant

8

cytotoxicity.

9

10 11

Fig. 7. Cytotoxicity test of F13EG13F13 with different concentrations (2.5, 5, 10, 25,

12

and 50 µg/mL).

13 14 15 16 17 18 19 22

1

4. Conclusions

2 3

In summary, a series of nonionic urethane fluorinated surfactants (FmEGnFm) were

4

synthesized by one-pot method using poly(ethylene glycol) (PEG), isophorone

5

diisocyanate (IPDI) and short chain fluoroalkyl alcohol as raw materials. These

6

fluorinated surfactants exhibited excellent surface activity. The measurement results

7

showed that these fluorinated surfactants could reduce the surface tensions in the

8

range of 17.8 to 28.7 mN/m and had low critical micelle concentrations from 0.17 to

9

0.98 mmol/L. Furthermore, these fluorinated surfactants showed a desirable salt and

10

pH resistance, which can be applied in certain special situations. The contact angle

11

and emulsifying tests had shown that these fluorinated surfactants exhibit excellent

12

performances at an extremely low concentration of 0.1wt. %, which are more efficient

13

at very low concentrations compared to perfluorooctanoic acid (PFOA). More

14

importantly, these fluorinated surfactants had no significant cytotoxicity. In view of

15

the above excellent performances, these short fluoroalkyl chain fluorinated surfactants

16

(FmEGnFm) are promising as replacements for the long fluoroalkyl chain surfactants.

17 18

Acknowledgements

19 20

This work was financially supported by the National Natural Science Foundation of

21

China (No. 21474065), the Sichuan Province Science and Technology Support Project

22

(No. 2017GZ0422) and the Fundamental Research Funds for the Central Universities.

23

The authors would thank Wang Zhonghui (College of Light Industry, Textile and Food

24

Engineering, Sichuan University) for her great help in FT-IR observation.

25 26

Appendix A. Supplementary data

27 28

Supplementary material

29 30 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

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27

Highlights

1. Nonionic urethane fluorinated surfactants with double short fluoroalkyl chains were synthesized. 2. Surface activities of these fluorinated surfactants are excellent. 3. These fluorinated surfactants show favorable wetting and emulsifying properties. 4. These fluorinated surfactants have no obvious cytotoxicity.

Declaration of competing interest The authors declare no competing financial interest.