Synthesis and Properties of Novel Acrylic Fluorinated Surfactants

Abstract

Branched fluorinated surfactants with creatively introduced acrylate in the hydrophilic group were designed and prepared by adopting perfluoro-2-methyl-2-pentene as the raw substrate. These new compounds showed excellent surface properties, and the surface tension of their aqueous solution at 25 °C could be below 20.00 mN/m at the critical micelle concentration. Compared with similar structures we have synthesized previously, these synthesized compounds exhibit a great improvement with regard to their molecular arrangement at the gas–liquid interface, their polymerizability, and the antibacterial properties of their polymer form, which can provide new ideas in the work to replace perfluorooctane sulfonate/perfluorooctanoic acid.

1. Introduction

Fluorinated surfactants were first developed by 3M in the US in the 1950s through electrolytic fluorination under the trade name "Fluorad" [1]. Their unique properties, e.g., their high surface activity and high thermal and chemical stability at low concentrations compare to non-fluorinated ones [2,3], means they have been widely applied in industrial processes and for consumer uses [4,5,6,7]. The most common commercially produced fluorinated surfactants are perfluorooctane sulfonate (PFOS)/perfluorooctanoic acid (PFOA) and their derivatives. Their widespread use, disposal, and high stability (they cannot be broken down readily either abiotically or biotically in the environment) have resulted in the widespread presence of per- and polyfluoroalkyl substances (PFASs) in the environment. Commercial productions have shifted toward short-chain alternatives and new fluorinated moieties such as per- and polyfluorinated ethers because perfluoroalkyl substances with chain lengths of C8 or longer have shown great potential to be persistent, toxic, and bioaccumulable [8,9].

Perflur-C6-based fluorine-containing additives do not contain perflur-C8 components and have not been classified as persistent organic pollutants (POPs) previously, but their environmental safety is still controversial. Compared with long-chain analogs, short-chain substitutes with higher water solubility, higher saturated vapor pressure, and lower adsorbability are more likely to migrate in the environment [10,11]. Perfluorohexane sulfonic acid (PFHS) and its related compounds were submitted to the POPs review committee as POP candidates in 2017 [12], and they are now listed in Annex A of the Stockholm Convention without specific exemptions. Substances that potentially degrade to PFHS are also considered as their related compounds [13].

Although other chemical structural compounds with straight-chain perfluorinated hexyl have not been listed in Annex A of the Stockholm Convention, they are the subject of great concern currently. Finding more suitable and reliable compounds and effective preparation methods is very urgent. Our group has carried out numerous studies on preparation processes, structure–activity relationships, and applications using hexafluoropropylene dimer as the raw material [14,15,16]. The surface tension of this kind of branched fluorinated surfactant at the critical micelle concentration (CMC) is usually around 19 mN/m–20 mN/m, while some compounds with excellent surface activity can reduce the surface tension to below 18 mN/m [16].

To the best of our knowledge, most studies are still focused on reducing the toxicity of fluorinate hydrophobic fragments without paying enough attention to the innovative design of hydrophilic groups. For example, the hydrophilic groups of cationic, amphoteric, and nonionic surfactants are usually quaternary ammonium salt, betaine, and poly-oligomeric ethoxylated alcohol, respectively [9,17]. Designing and synthesizing suitable hydrophilic structures for specific hydrophobic groups to achieve better structural matching are very important tasks in the future for research into PFOS/PFOA alternatives.

2. Materials and Methods

In view of the facts mentioned above, this study proposed a novel hydrophilic group to matching the branched hydrophobic fluorinated tails (Scheme 1). The traditional hydrophilic group structure of quaternary ammonium salt was modified to contain an acrylate structure using an ionic bond or a chemical bond. The existing acrylate structure connected via an ionic bond or chemical bond causes the branched fluorinated surfactants to display significantly different surface activities, thus demonstrating greater application prospects than previous works [14,15,16], such as more stable foams in fire-fighting agents, polymerizable emulsifiers in emulsion polymerization, and antibacterial additives in coatings.

Compound 1 was prepared via four steps according to the literature [15] and was then changed into the ionic paired structural compound 3 (FIS) and the chemical bond structural compound 5 (FBS) via two steps, respectively. For compound 3, first, quaternary ammonium salt compound 2 underwent quaternarization with iodoethane [15]. Then, conversion of the quaternary ammonium salt 2 into the ionic paired compound 3 was carried out via ion exchange and esterification in turn. For compound 5, first, quaternary ammonium salt compound 4 underwent quaternarization with bromoethanol. Then, esterification of the alcohol 4 led to the chemical bond structural compound 5. All of the chemicals and instruments used in this work, the experimental details, and the key spectra are presented in the Supplementary Materials.

3. Results and Discussion

3.1. Surface Activities

The surface or interface tension of surfactants in individual form was tested via the Wilhelmy plate method using a Kruss K100 tensiometer at 25 °C. All values were the average of three measurements. The change trend of surface tension for FIS and FBS in aqueous solutions with various concentrations is presented in Figure 1. The CMC of FIS is 6.57 × 10⁻⁴ mol/L and the surface tension at the CMC (γ_CMC) is 23.24 mN/m; for FBS, these values are 1.15 × 10⁻³ mol/L and 19.02 mN/m, respectively. The values of the surface properties of FIS and FBS are better than that of sodium perfluorooctanoate (about 24.7 mN/m at the CMC of 3.1 × 10⁻² mol/L) [18].

The ability to reduce surface tension is slightly different between FBS and FIS, which can be explained by calculating the surface occupied area per molecule (A_CMC). A_CMC was derived from the static surface tension vs. log_C curves through the following equation [19,20]:

$$A_{CMC}={\displaystyle \frac{1}{N_A{\mathsf{\Gamma }}_{\max }}}$$

(1)

where N_A is Avogadro’s number and Γ_max is the surface excess concentration as defined by

$${\mathsf{\Gamma }}_{\max }=-{\displaystyle \frac{1}{2.303RT}}{\log }_{c\to c_{CMC}}\left({\displaystyle \frac{d\gamma }{d\lg c}}\right)$$

(2)

where R is a gas constant and T is the absolute temperature.

The calculated A_CMC values of FBS and FIS are listed in

Table 1. Static surface and interface properties of different water solutions at 25 °C.

Entry	Systems	CMC (mol/L)	γCMC (mN/m)	ACMC (Å2/mol)
1	FIS	6.57 × 10−4	23.24	2.46
2	FBS	1.15 × 10−3	19.02	1.65
3 a	FCS	2.50 × 10−3	19.68	52
4 a	FAS	1.04 × 10−4	21.39	35
5 b	FO1	1.73 × 10−2	19.93	49
6 b	FO2	9.97 × 10−5	19.31	27

^a Values are taken from reference [15]. ^b Values are taken from reference [14].

.

From Figure 1 and Table 1, it can be seen that FBS showed better surface activities than FIS, caused by the chemical bond structure, which causes the compounds to have a more stable arrangement at the air–water interface, and thus the A_CMC of FBS is 1.65 Ų/mol, while the A_CMC of FIS is 2.46 Ų/mol.

3.2. Foaming Properties

Comparisons of surface properties with the same hydrophobic fluorinated tail structure compounds (see Figure 2) are summarized in Table 1. The A_CMC values of FCS/FAS/FO1/FO2 were larger than those of FIS and FBS, meaning that the technique of introducing an acrylate group into the hydrophilic head group can effectively improve the compact arrangement of molecules at the gas–liquid interface. This causes these fluorosurfactants to have improved application performance in fire-fighting foams.

The foaming ratio and eluting time are important indexes used to evaluate the foaming properties of surfactants, representing the foaming ability and foam stability of surfactants, respectively. In this work, we chose a simple method to evaluate the foam properties of the new hydrophilic structure [21]. FCS and FIS were both prepared as 1 wt% aqueous solutions, and 10 mL of these aqueous solutions was added to a 100 mL measuring cylinder with a lid. Then, the cylinder was shaken violently for 20 s to cause the solution to foam fully. From Figure 3A, it can be seen that the initial foam height (4 s after shaking) of FIS was about 85 mL, while the initial foam height of FCS was about 80 mL. This means that the foaming ratios of FIS and FCS are 8.5 times and 8 times, respectively. Meanwhile, the eluting time can be assessed using the time taken for the liquid to precipitate. Figure 3B shows that, at the bottom of both cylinders, about 8 mL of the solution is present at 135 s (after shaking for 115 s), which means that the foam stability of FIS and FCS is almost the same. The above test results show that the more compact arrangement of FIS molecules at the gas–liquid interface improves the foaming ability and has no impact on the foam’s stability.

3.3. Coating Applications

The introduction of an acrylate group provides these novel fluorosurfactants with more new application possibilities compared with the traditional fluorinate surfactant. For example, polymerizable emulsifiers are a kind of new emulsifier that do not migrate to the film surface during the film formation process, thus avoiding deleterious effects on the film’s properties [22]. We used FBS as a polymerizable emulsifier to discuss its application in the coating industry.

For emulsion 1, a polymerizable emulsifier named DNS-86 purchased from Guangzhou Shuangjian Trading Co., Ltd, Guangzhou, China, was used as a single emulsifier. Meanwhile, for emulsion 2, FBS was used with DNS-86 with a mass radio of 1:1 as a mixed emulsifier. The other additives and total feed weight of emulsion 1 and emulsion 2 were kept consistent. The preparation details of the two kinds of polyacrylate emulsions and the coating process are presented in the Supporting Information, while the general processes are shown in Figure 4A. Transmission electron microscope (TEM) images of the latexes were obtained by a JEOL JEM 1230 instrument operated at 80 kV. The TEM micrograph reveals that both prepared emulsions have a good dispersity, with nanoscale sizes of less than 100 nm (Figure 4B), indicating that small amounts of FBS did not significantly change the micromorphology of the emulsion.

The contact angle (CA) test is commonly used as a quick and precise tool for the evaluation of the liquid repellence of a coating surface. The measurements were performed on a contact angle goniometer (XG-CAM, Shanghai Xuanyichuangxi Industrial Equioment Co., Ltd., Shanghai, China) by the sessile drop method with a microsyringe at room temperature. We chose a glass slide as a template substrate to perform the water contact angle (WCA) test and hexamethylene contact angle (HCA) test (Figure 5A,B, respectively), and it can be observed that the WCA of the film decreased by only 1% (from 86.63° to 85.42°) with the addition of a small amount of FBS (1.22 wt% in total). Meanwhile, the HCA of the coating improved by over 100% (from 5.02° to 10.13°), caused by the fluorinated groups and quaternary ammonium salt groups of the 1.22 wt% FBS, improving the oleophobic properties of the coating together. Regarding the water repellence of the coatings, the hydrophilic quaternary ammonium salt groups play a dominant role compared with the hydrophobic fluorinated groups.

To assess the remarkable influence of the quaternary ammonium salt groups on the coatings, we further conducted antibacterial tests of the two emulsions by the inhibition zone method according to the literature [23,24]. Figure 6 shows the inhibition zone photos of different bacteria in Petri dishes after bacterial cultures for 96 h; the larger inhibition zone means a better ability to inhibit the corresponding bacteria. From Figure 6, it can be found that the antibacterial zone of emulsion 2 was larger than that of emulsion 1, meaning that the addition of FBS can greatly improve the anti-Staphylococcus aureus activity by approximately 2 times after 96 h and improves the anti-Escherichia coli activity by approximately 1.6 times after 96 h. Unfortunately, both of the latex emulsions showed no anti-Aspergillus niger activity. This phenomenon is mainly due to the presence of the quaternary ammonium salt structure, but the relative amount of FBS in emulsion 2 was too low (below 3 wt‰) that it has not yet acted as an inhibitor against Aspergillus niger.

4. Conclusions

In this work, we successfully synthesized two novel branched fluorosurfactants with multi-functional groups starting from perfluoro-2-methyl-2-pentene. The introduction of an acrylate group into the hydrophilic head group effectively promoted the compact arrangement of molecules at the air–water interface, thus improving the foaming ability of the molecular solutions. The as-obtained compounds not only exhibited better surface properties than those of sodium perfluorooctanoate but also showed remarkable application possibilities compared to the traditional fluorinate surfactant. When FBS was applied as a polymerizable emulsifier, the as-obtained latex emulsion showed much better organic solvent repellence, anti-Staphylococcus aureus activity, and anti-Escherichia coli activity.