Synthesis and Characterization of Novel Betaine-type Fluoroether Surfactants

doi:10.6023/A24120365

Abstract

Betaine-type fluorinated surfactants have some superior properties, which make them have a wide application prospect. These surfactants combine the properties of amphoteric and fluorinated surfactants. Among them, PFOS-AB (betaine derived from perfluorooctane sulfonic acid) is the most effective surfactant due to its high surface activity. However, PFOS (perfluorooctane sulfonic acid) and its derivatives have been identified as having potential adverse effects on the environment and human health. As a result, they are severely restricted by the Stockholm Convention. Manufacturers and users of PFOS are under increasing pressure to switch to environmentally friendly alternatives. Unfortunately, this remains a challenging problem. In this study, we focused on novel betaine-type fluoroether surfactants, which have one or a dual chain structure composed of biodegradable hydrophobic chains. In the synthesis of betaine fluorinated surfactants, the fluorinated chains of C361 (R₁=CF₃CF₂CF₂OCHFCF₂SCH₂CH₂-) and X2 (R₂=CF₃OCF₂OCF₂CF₂OCHFCF₂SCH₂CH₂-) were selected. These new fluorinated surfactants are expected to be safer, more biodegradable and high-performance. Additionally, the surface tension and interfacial tension were measured; the critical micelle concentration (cmc), the limiting molecular areas (A_min) and the fluorine content were obtained; and the foam properties as well as the wettability on polytetrafluoroethylene (PTFE) and glass were also tested. By comparing the traditional PFOS-AB and the currently popular 6:2 FTAB (betaine derived from perfluorohexyl ethyl sulfonic acid), we found that the surface activity of D361-B (Betaine derived from fluoroether alkyl containing two R₁ chains) is better than that of PFOS-AB, and the surface activity of X2-B (Betaine derived from fluoroether alkyl containing one R₂ chains) and DX2-B (Betaine derived from fluoroether alkyl containing two R₂ chains) is better than that of 6:2 FTAB. In particular, the surface tension of D361-B reached 16.8 and 21.1 mN•m⁻¹ at low concentrations of 0.1 and 0.01 g•L⁻¹, respectively, and its fluorine content was lower than that of similar products. The surface tension of X2-B is comparable to that of PFOS-AB, and the spreading and foam properties are also outstanding. In conclusion, this study provides guidance for the development of a new generation of sustainable surfactants with the potential to replace PFOS.

Key words: fluoroether surfactant, betaine-type, surface tension, critical micelle concentration, alternative for PFOS

Cite this article

Junyi Li, Meiwei Huang, Yunwen Zhang, Can Zou, Chao Liu, Yong Guo. Synthesis and Characterization of Novel Betaine-type Fluoroether Surfactants[J]. Acta Chimica Sinica, 2025, 83(1): 36-44.

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Fig. & Tab. 19

Figure 1. Representative betaine type fluorosurfactants

Figure 2. Synthesis route of single-chain betaine surfactants C361-B and X2-B

Figure 3. Synthesis route of double-chain betaine surfactants D361-B and DX2-B

Compound	Fluorocarbon number	Fluorine content/%	γ_cmc/(mN•m⁻¹)		cmc/(g•L⁻¹)	A_min/nm²	Ref.
C6-Betaine	5	45.8	22.3	11.3	5.2	0.44	[6c]
C361-B	5	35.8	21.2	3.2	1.7	0.58	—
C72-Betaine	6	45.9	17.1	2.6	1.3	0.58	[6c]
X2-B	6	38.2	17.4	1.1	0.7	0.72	—
C8-Betaine	7	51.2	18.2	0.7	0.4	0.46	[6c]

Table 1. Surface activity and other data of different compounds

Figure 4. The surface tension of different compounds varies with concentration

Compound	Fluorine content/%	Solubility^a	1 g•L⁻¹	0.1 g•L⁻¹	0.01 g•L⁻¹
PFOS-AB	48.2	微溶	16.9	17.8	26.2
6:2 FTAB	43.3	微溶	17.6	20.1	41.9
C361-B	35.8	可溶	26.2	43.0	61.0
X2-B	38.2	微溶	17.3	17.1	37.0
D361-B	41.5	微溶	不溶	16.8	21.1
DX2-B	42.2	微溶	不溶	19.4	32.0

Table 2. The solubility of compounds and surface tension data under different concentrations

Compound	α	t_FLS75%/s	t_FVS50%/s	F/(mL•s)
C361-B	3.4	21.0	3301	5.6×10⁵
X2-B	3.9	25.9	6881	13.4×10⁵

Table 3. Foam performance parameters of compounds

Figure 5. The foam morphology of (a) 1 g?L?1 C361-B and (b) 1 g?L?1 X2-B surfactant solutions at different times

Figure 6. Contact angle of surfactant solution and deionized water on different surfaces: (a1) 1 g?L?1 C361-B on PTFE surface; (a2) 1 g?L?1 C361-B on the glass surface; (b1) 1 g?L?1 X2-B on the PTFE surface; (b2) 1 g?L?1 X2-B on the glass surface; (c1) Deionized water on the PTFE surface; (c2) Deionized water on the glass surface

Surface	C361-B	X2-B	Water
PTFE	52.9°	22.8°	116.6°
Glass	34.6°	28.1°	18.8°

Table 4. Contact angles of compounds on different surfaces

Compound	Cyclohexane surface tension/(mN•m⁻¹)	Interfacial tension in cyclohexane/ (mN•m⁻¹)	Spreading coefficient S_w/o/(mN•m⁻¹)	n-Heptane surface tension/(mN•m⁻¹)	Interfacial tension in n-heptane/ (mN•m⁻¹)	Spreading coefficient S_w/o/(mN•m⁻¹)
C361-B	25.3	4.1	－4.9	21.6	6.0	－10.6
X2-B	25.3	3.0	5	21.6	2.4	1.9

Table 5. Compound interfacial tension at different water-oil interfaces

Figure 7. 1H NMR (400 MHz) of C361-B

Figure 8. 19F NMR (376 MHz) of C361-B

Figure 9. 1H NMR (400 MHz) of X2-B

Figure 10. 19F NMR (376 MHz) of X2-B

Figure 11. 1H NMR (400 MHz) of D361-B

Figure 12. 19F NMR (376 MHz) of D361-B

Figure 13. 1H NMR (400 MHz) of DX2-B

Figure 14. 19F NMR (376 MHz) of DX2-B

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