Since the enormous potential of fluorochemical cleaning products were highlighted by 3M company in the 1950s, fluorosurfactants
have been extensively exploited over 200 applications in clothing fabrics, electroplating, fire fighting foams, food packaging,
petroleum, textile, along with multibillion dollar industries [1, 2]. Aqueous film forming foams (AFFFs), an extinguishing agents to put
out polar (alcohols, ketones, esters) or non-polar (hydrocarbons) solvent fire, is one of the most important applications of fluorinated
surfactants. The very low apparent density of AFFFs allows them to be spread at the surface of burning liquids [3, 4]. The evaporation
of the water resulting from the heat reduces the intensity of the fire and the foam generates a water film at the surface of the solvent
which prevents the emission of flammable vapors. In addition, after extinction, the foam prevents the risk of fire burnback [4]. AFFFs
are often found where there are large volumes of flammable liquids and the potential for a fire exists. For example, AFFFs are found at
military bases, fire departments, and airports [5,6]. Fluorinated surfactants have outstanding chemical and thermal stabilities, and play
a crucial role in the formation of the water film at the surface of the solvent.
Featuring the most stable C-F single bond [7], perfluoroalkyl substances recently have awakened the concern in human health (the
presence and persistency in fetuses, newborn babies, human milk, and human blood, etc.) and social environment [8]. Among of them,
fluorinated surfactants that chain lengths of C8 or longer have been revealed more possible to be bioaccumulative and potently toxic
[9]. According to the PFOA Stewardship Program, both industrial and academic sectors have set about the deploitation of short-chain
based fluorosurfactants (Rf < C6-7) to mitigate their persistent nature of pollution [10].
Hexafluoropropylene dimer containing only C5 main chain is a representative template to synthesize various branched fluorinated
surfactants [11-13]. The short chain length makes it is an idea material to preparing non-bioaccumulable alternatives to PFOA/PFOS.
On the other hand, hyperbranched hydrocarbon surfactants possess fluorocarbon-like low surface energies than conventional straight
chain ones [14], and branched fluorinated surfactants show more efficient at a relatively low concentration than common linear
fluorosurfactants [15, 16]. As such, the import of branched chain is a valid strategy to develop surfactants with high performance. So
far, to the best of our knowledge, most studies just focused on the preparation of new compound derived from hexafluoropropylene
oligomer [11-13,15-17], no report has discussed the application of this kind of branched fluorinated surfactants in the AFFFs field.
However, years of employing AFFFs in a variety of situations has resulted in these fire-fighting foam components being directly
released to the environment and the contamination of groundwater [5].
In view of those facts mentioned above, this study proposed a novel serious of fluorosurfactants with branched short fluorinated tails
as hydrophobic groups, ammonium oxide as polar groups and benzene ring as space units, respectively (Scheme. 1). Four steps led to
the compound 4. First, methyl 4-(bromomethyl)benzoate was changed to compound 1 by a nucleophilic substitution reaction. Second,
saponification of the ester 1 followed by acidification led to the carboxylic acid 2. Then, conversion of the acid 2 into the amide 3 was
carried out by treating with EDCI and appropriate diamine. Finally, the title compound 4 was obtained by oxidation with hydrogen
peroxide. All the chemicals, instruments used in this work, the experimental details and the key spectra are presented in the Supporting
information.
The surface or interface tension of surfactants in individual form or mixed system was tested by the Wilhelmy plate method using a
Kruss K100 tensiometer at 25 °C. All the values were the average of three-run measurements. The change trend of surface tension for
4a~c in aqueous solution upon various concentrations is presented in Fig. 1A. All the CMC of 4a-c are below 1.0 × 10-4 mol/L and the
surface tension at CMC (γCMC) are below 20 mN/m. Compounds 4b and 4c showed better surface activity than 4a, for instance, the
CMC of 4b reduced from 9.97×10-5 mol/L to 5.51×10-5 mol/L at 298 K, along with the simultaneous decrease of γCMC from 19.31
mN/m to 18.70 mN/m. All the values of surface properties of compounds 4a-c are lower than that of sodium perfluorooctanoate (about
24.7 mN/m at the CMC of 3.1 × 10-2 mol/L) [18]. Comparisons of surface properties with several fluorinated surfactants (Fig. 1B and
synthetic procedure of FO1 see Supporting documents) synthesized from hexafluoropropylene dimmer were summarized in Table 1.
The results showed that the compounds we synthesized in this work exhibited the best ability and efficiency to reduce the surface
tension of water (The γCMC of compound 4a, FCS and FAS, are 19.31 mN/m, 19.68 mN/m, and 21.39 mN/m, respectively, meanwhile,
the CMC values of them are 9.97×10-5 mol/L, 2.50×10-3 mol/L, 1.04×10-4 mol/L, respectively).
It is generally acknowledged that lower interfacial molecular areas indicate improved interfacial packing ability [19], the abilities of
surfactants to form packed layers on water can be evaluated by the values of ACMC (minimal area per molecule). The occupied area per
molecule (ACMC) was derived from the static surface tension vs. log(c) curves through the following equation [20, 21]:
1
(1)
ACMC
NAmax
where NA is the Avogadro’s number and max is the surface excess concentration as defined by:
1
d
d lg c
(2)
max
logcc
CMC
2.303RT
where R is a gas constant, T is the absolute temperature.
As seen in Table 1, compound 4a possessed much lower CMC (9.97×10-5 mol/L) and ACMC (27 Å2/mol) than FO1, suggesting that
the rigid spacer unit (aromatic spacer) would promote micellization and packing.
AFFFs usually contain fluorinated surfactants, hydrocarbon surfactants, organic solvents, and so on. SDS/AOS/APG/LAB (Fig. 1B)
are the most frequently used hydrocarbon surfactants in foam concentrates, determining the formation of foams as well as the reduction