Study on foam extinguishing agents based on hydrocarbon and perfluorinated branched short-chain fluorocarbon surfactants mixed system

Article abstract of DOI:10.1007/s11696-021-01788-4

Aqueous film-forming foam extinguishing agent (AFFF) plays an important role in fighting pool fires, but the environmental problems caused by them are also increasingly apparent. The key component of traditional AFFF is C8 fluorocarbon surfactant, which is harmful to the ecological environment and human health. In this paper, a perfluorinated branched short-chain fluorocarbon anionic surfactant with high surface activity, simple synthesis route and easy industrial scale-up was reported. The surface activity and aggregation behavior of the mixed solution of this fluorocarbon surfactant and cetyltrimethylammonium bromide (CTAB) were studied by surface tension meter and transmission electron microscope. The results showed that CTAB and anionic fluorocarbon surfactant had the significant synergistic effect, and many vesicles could be observed to nestle together at the concentration of 0.0406?mol/L. Subsequently, three environmental friendly AFFF formulations (F-1, F-2, F-3) were designed with the mixed system of this fluorocarbon surfactant, CTAB and dodecyl betaine (BS-12) as foaming agent. The foam properties confirmed that F-1 possessed excellent foam properties, the initial foam height h₀ was 115.34?mm, 25% drainage time was 182.4?s, extinguishing time was 31?s, and burn-back time was 720?s. This type of fire extinguishing agent had potential for fire protection applications.

Full text of DOI:10.1007/s11696-021-01788-4

Chemical Papers
https://doi.org/10.1007/s11696-021-01788-4
ORIGINAL PAPER
Study on foam extinguishing agents based on hydrocarbon
and perfuorinated branched short‑chain fuorocarbon surfactants
mixed system
Ya‑Wen Yang1 · Jia‑Qing Fang1 · Min Sha4 · Ding Zhang1,2 · Ren‑Ming Pan1 · Biao Jiang1,3
Received: 3 March 2021 / Accepted: 10 July 2021
© Institute of Chemistry, Slovak Academy of Sciences 2021
Abstract
Aqueous flm-forming foam extinguishing agent (AFFF) plays an important role in fghting pool fres, but the environmental
problems caused by them are also increasingly apparent. The key component of traditional AFFF is C8 fuorocarbon sur-
factant, which is harmful to the ecological environment and human health. In this paper, a perfuorinated branched short-chain
fuorocarbon anionic surfactant with high surface activity, simple synthesis route and easy industrial scale-up was reported.
The surface activity and aggregation behavior of the mixed solution of this fuorocarbon surfactant and cetyltrimethylam-
monium bromide (CTAB) were studied by surface tension meter and transmission electron microscope. The results showed
that CTAB and anionic fuorocarbon surfactant had the signifcant synergistic efect, and many vesicles could be observed
to nestle together at the concentration of 0.0406 mol/L. Subsequently, three environmental friendly AFFF formulations
(F-1, F-2, F-3) were designed with the mixed system of this fuorocarbon surfactant, CTAB and dodecyl betaine (BS-12) as
foaming agent. The foam properties confrmed that F-1 possessed excellent foam properties, the initial foam height h0 was
115.34 mm, 25% drainage time was 182.4 s, extinguishing time was 31 s, and burn-back time was 720 s. This type of fre
extinguishing agent had potential for fre protection applications.
Keywords Fluorocarbon surfactant · Environment friendly · Fire extinguishing foam · Multiphase system · Foam properties
Introduction
chemical stability and excellent hydrophobic, oleophobic
and antifouling properties (Verdia et al. 2016; Szymczyk
2013; Sheng et al. 2019). Like conventional surfactants,
fuorinated surfactants can also be divided into anionic, cati-
onic, nonionic and amphoteric surfactants according to the
diferent types of ions generated by hydrophilic groups. At
present, the production of fuorinated surfactants is mainly
concentrated in 3 M and DuPont companies in the USA,
ICL companies in the UK, Atochem companies in France,
etc. (Aihara 1988).
Fluorocarbon surfactants are widely used in fre protection,
detergents, wetting agents and other industrial felds because
of their high surface activity, high thermal stability, high
* Ding Zhang
zhangdingkathy@163.com
1
2
3
School of Chemical Engineering 1, Nanjing University
of Science and Technology, Nanjing 210094,
People’s Republic of China
It was widely known that fuorocarbon surfactants were
the core components of AFFF, and perfuorooctane sul-
fonate (PFOS), and its derivatives were the most com-
monly used. Unfortunately, PFOS was ofcially listed in
the controlled list of persistent organic pollutants (POPs)
by the United Nations Environment Programme (UNEP)
due to its biological accumulation, persistence and the
possibility of long-distance environmental migration
(Kishi and Arai 2008; Jia et al. 2019; Zhang et al. 2019b).
On November 11, 2014, Stockholm Convention on per-
sistent organic pollutants came into force in China, which
Key Laboratory of Organofuorine Chemistry 2, Shanghai
Institute of Organic Chemistry, Chinese Academy Sciences,
Shanghai 200032, People’s Republic of China
CAS Key Laboratory of Synthetic Chemistry of Natural
Substances 3, Shanghai Institute of Organic Chemistry,
Chinese Academy of Sciences, 345 Lingling Road,
Shanghai 200032, People’s Republic of China
4
School of Management Science & Engineering, Nanjing
University of Finance & Economics, Nanjing 210046,
Jiangsu, People’s Republic of China
Vol.:(0123456789)
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Chemical Papers
implied that China would gradually reduce and eventually
stop using AFFF with PFOS as the main component (Jia
2015; Houde et al. 2006; Prescher et al. 1985; Key et al.
1997). Therefore, it was very important to develop new
fuorocarbon surfactants with less environmental pollu-
tion, no bioaccumulation and biodegradation. Reducing
the number of carbon atoms of perfuorinated radicals
from C8–C10 to C4–C6 and branching perfluorocarbon
chain were two efective methods at present (Zhang et al.
2019a; Jiang et al. 2018; Vinogradov et al. 2016; Lin et al.
2018a). Perfuorinated short-chain surfactant was a new
type of fuorocarbon surfactant. The number of carbon
atoms in the fuorocarbon chain was 6, while the num-
ber of carbon atoms in the straight chain was 5, and the
perfuorocarbon chain was branched, which satisfed the
requirements of current environmental protection (Lin
et al. 2018b). In addition, a large number of studies sug-
gested that this type of fuorocarbon surfactant possessed
excellent surface properties, so it was an environmental
friendly and efcient fuorocarbon surfactant (Wang et al.
2017; Howell et al. 2004). Surprisingly, these studies were
mainly focused on theory, but the further application per-
formance was very few.
This paper provided a synthetic route for the perfuori-
nated branched short-chain fluorocarbon anionic sur-
1
factant, and its structure was characterized by HNMR.
Because the combination of fuorocarbon surfactant and
proper hydrocarbon surfactant could reduce the production
cost and improve the performance, the surface activity,
adsorption property and aggregation behavior of the mixed
solution of benzene ring side chain anionic fuorocarbon
surfactant and CTAB were studied. Herein, three kinds
of new environment-friendly AFFF formulations (F-1,
F-2, F-3) were designed by adopting the perfuorinated
branched short-chain fluorocarbon anionic surfactant,
CTAB and dodecyl betaine as foaming agents. Through
the study of foaming, foam stability, drainage, spreading,
fre extinguishing and burn-back testing, it was known that
F-1 had the potential to replace the traditional AFFF.
Experimental
Synthesis of anionic fuorocarbon surfactant
with benzene ring side chain (Sha et al. 2015)
High chemical stability implied excellent chemical inert-
ness. However, fuorocarbon surfactants were compatible
with other surfactants and could be used in almost all formu-
lation systems. This made fuorocarbon surfactants played an
irreplaceable role in the development of fre extinguishing
agent. As a new type of extinguishing agent, it had attracted
more and more attention. Typical examples included fuorine
protein foam fre extinguishing agent, light water foam fre
extinguishing agent and anti-resume dry powder fre extin-
guishing agent, etc. (Liu et al. 2008). Currently, the widely
used AFFF still adopted the formulas of decades ago, which
had serious ecological damage and environmental pollution
problems. In view of this, the foam industry introduced fu-
orine-free fre foam, completely eliminating fuorocarbon
surfactants and perfuorooctane sulfonate in the solution, and
relying solely on hydrocarbon surfactants and other addi-
tives. However, commercial fuorine-free products did not
meet the requirements of the current US MilSpec, includ-
ing fre extinguishing performance requirements for ship fre
protection (Hinnant et al. 2020). Sheng et al. reported the
spreadability of fuorine-free foam and fuorine-containing
foam and concluded that fuorine-containing foam spread
faster than fuorine-free foam at the same foam fow rate
(Sheng et al. 2018a). Yu et al. studied the foam stability of
fuorine-free foam and fuorine-containing foam and con-
cluded that fuorine-containing foam flm was more stable
than fuorine-free foam flm (Yu et al. 2020). Therefore,
searching for PFOS alternatives was of great signifcance to
the development of alternative technologies for traditional
AFFF.
20 g hexafuoropropylene dimer (D2), 7.6 g methyl 4-bro-
momethylbenzoate, 2.906 g anhydrous potassium fuoride,
0.784 g tetrabutylammonium bromide and 50.7 ml dry N-
N dimethylacetamide (DMAc) were placed in a closed
tube, heated to 75 °C and stirred for two days. The mixed
solution was cooled to room temperature, poured into
ethyl ether, washed with water and salt water, dried with
anhydrous sodium sulfate, fltered and concentrated under
reduced pressure. The product was purifed by column
chromatography (petroleum ether/ethyl acetate = 20:1).
2.11 g of compound 1 was dissolved in 50 ml methanol,
then sodium hydroxide solution was added in ice water
bath, and the mixture was moved to 60 °C water bath and
stirred for 4 h. After the reaction, water was added to the
system, the mixed solution was washed with ether, and the
pH value of the solution was adjusted to 3 with 6 mol/L
HCl solution. The aqueous solution was extracted with
ether, dried on anhydrous sodium sulfate, fltered and con-
centrated under reduced pressure to obtain compound 2.
In a 50-ml round-bottomed fask, 1.00 g compound 2
and 10 ml methanol were added, stirred until completely
dissolved, and then 0.1 mol/L potassium hydroxide aque-
ous solution was added slowly until the pH value of the
system was 7. The solvent was distilled under reduced
pressure to obtain a white solid, washed with acetone and
then vacuum dried to obtain benzene ring side chain ani-
onic fuorocarbon surfactant. Figure 1 shows the synthetic
route of anionic fuorocarbon surfactant with benzene ring
side chain.
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Chemical Papers
Fig.1 Synthesis route of anionic
fuorocarbon surfactant with
benzene ring side chain
Surface tension measurements
Zou 2003). Therefore, the new perfluorinated branched
short-chain fluorocarbon anionic surfactant, CTAB and
BS-12 were selected as foaming agents.
The surface tension was usually used to characterize the
surface activity of the aqueous solution of surfactant and
its mixed system. In this paper, the surface tensions of all
surfactants and their mixed system aqueous solutions were
measured by the platinum plate method with SFZL-A2
series automatic surface tension tester of Shanghai Yingnuo
Precision Instrument Co., Ltd. at 25 °C. The average value
of each concentration of aqueous solution was taken after
three measurements.
Selection of foam stabilizer
Commonly used surfactants were foaming agents as well as
foam stabilizers. But for foam extinguishing agents, the sta-
bility of foaming agents was not enough to satisfy the high
stability required by fre extinguishing, so foam stabilizer
was needed. This paper selected one of the conventional
industrial felds of sodium carboxymethyl cellulose, xanthan
gum and coconut diethanol amide (6501) as the foam stabi-
lizer (Sheng et al. 2018b; Duanmu et al. 2011).
Transmission electron micrographs (TEM)
In order to study the morphology of vesicles in the mixed
solution of benzene ring side chain anionic fuorocarbon
surfactant and CTAB, the samples were rebalanced in a
controlled incubator for two weeks, then stained negatively
with 2 wt% phosphotungstic acid and fnally observed by
JEM-1011 transmission electron microscope at 100 kV.
Selection of other components
Common cosolvents mainly included high boiling point sol-
vents such as ethylene glycol, ethylene glycol butyl ether,
diethylene glycol monobutyl ether, propylene glycol methyl
ether, propylene glycol methyl ether acetate, among which
ethylene glycol was an excellent antifreeze agent. In this
paper, diethylene glycol monobutyl ether was selected as
cosolvent and ethylene glycol as antifreeze agent. In addi-
tion, urea as a burn-back agent not only played a burn-back
role, but also could further improve the stability of foam and
had certain solubilizing efect.
Optical microscope
A certain amount of blistering mixed solution was dripped
onto the glass slide, and the foam coarsening process was
observed under optical microscope. A foam coarsening
image was taken every minute to analyze the foam size dis-
tribution and foam coarsening kinetics.
Determination of foam extinguishing agent formulation
According to the selected components, determine the basic
Formulation design
formula of fre extinguishing agent, as shown in Table 1.
Selection of foaming agent
Determination of component concentration of foam
extinguishing agent
According to the relevant literature (Wang et al. 2017), the
mixed system of benzene ring side chain anionic fuorocar-
bon surfactant/CTAB could produce signifcant synergistic
efect. In addition, amphoteric surfactants had certain bac-
tericidal, fungicidal, foam stability and foaming properties
and could be compatible with various surfactants (Lan and
AFFF was an efcient fre extinguishing agent developed in
foreign countries in 1960s. The foam solution of this foam
extinguishing agent could be mixed with water and could be
divided into two kinds of 3% AFFF and 6% AFFF according
to their mixing ratios (Sontake and Wagh 2014). Since 3%
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Chemical Papers
Table 1 Basic formula of foam extinguishing agent
to dodecyl betaine; the anionic fluorocarbon surfactant
−0.8%"referred to the benzene ring side chain anion fuo-
rocarbon surfactant, and the mass concentration in the foam
concentrate was 0.8%; "DB" referred to diethylene glycol
monobutyl ether; "6501-3%" referred to the type of hydro-
carbon surfactant was coconut diethanol amide, the mass
concentration in the foam concentrate was 3%; all concen-
trations in the table referred to the mass concentration, and
the unit was "%".
Components
Name of substance
Foaming agent
Benzene ring side chain anionic fuorocarbon
surfactant, BS-12, CTAB
Foam stabilizer
Other additives
Sodium carboxymethyl cellulose or xanthan gum
or 6501
Diethylene glycol monobutyl ether, urea, ethylene
glycol
AFFF were widely used in the market, this work focuses
on 3% types. According to the volume concentration of the
concentrated solution in the diluted foam solution, the con-
centration of each main component in the foam concentrate
used in this paper could be determined by formula 1:
Determination of foaming properties
There are many ways to generate foam: one is the traditional
air inhalation, that is, the foam production point is located
at the sprinkler head. The sprinkler is specially designed,
which can inhale the surrounding air and mix with foam
liquid. The other is compressed air, that is, a certain propor-
tion of compressed air is mixed into the foam mixture, and
foam is produced after impact and mixing. In this paper,
Ross–Miles method was used to evaluate the foaming per-
formance of the solution. Then, the foaming properties were
characterized by initial foam height h0 (Sheng et al. 2016a).
The specifc operation steps were as follows: 10 ml solution
was poured into 100-ml measuring cylinder with stopper and
shook for 1 min, and then the foaming height of diferent
solutions was recorded, and each experiment was repeated
at least three times (Xiao et al. 2008).
(
)
CFoam solution
CFoam concentrate solution
=
∕
× 100%
(1)
3%
where CFoam concentrate solution and CFoam solution referred to the
concentration of surfactant in foam concentrate before and
after dilution, respectively.
The concentration of surfactants in the foam solution
depended on the critical micelle concentration (CMC) of the
surfactant. Most surfactants had the lowest surface tension
after reaching the CMC, and their properties were gradu-
ally stable. Finally, the dosage of each surfactant should be
considered to avoid waste.
In addition, the concentration of antifreeze glycol was
set at 20%, the concentration of cosolvent diethylene glycol
monobutyl ether was set as 5%, the concentration of burn-
back agent urea was set as 5%, and the rest components were
water.
Determination of foam stability
The stability of foam is the main performance of foam fuid.
Foam fuid is a bubble dispersing system composed of gas.
It has high surface free energy and is thermodynamically
unstable. However, some measures can be taken to change
the conditions and enhance the stability of foam to meet the
application requirements. It is generally accepted that the
mechanism of foam decay is the loss of liquid in the foam
and the difusion of gas through the liquid membrane.
The method of determining foam stability was to record
the initial foam height h0 after the foam solution was foamed
and let it stand for 5 min and then recorded the foam height
h5 (mm) again. According to formula 2, R5 was used to
characterize the foam stability of surfactant (Azira et al.
The foam solution used in this study was a solution
diluted with foam concentrate and water at a volume ratio
of 3:97.
Test conditions for foam concentrate
To sum up, this paper designed a set of fuorocarbon foam
test conditions based on hydrocarbon and fuorocarbon ani-
onic surfactants as shown in Table 2.
It should be pointed out that in the table, "CTAB" referred
to cetyltrimethylammonium bromide; "BS-12" referred
Table 2 Test conditions for fuorinated foam
Serial number CTAB (%) BS-12 (%) Anionic fuorocarbon surfactant-0.8% Glycol (%) DB (%) Urea (%) Foam stabilizer
Water (%)
3.28
F-1
5.92
56
Anionic fuorocarbon surfactant-0.8% 20
5
5
Sodium carboxy-
methyl cellu-
lose-4%
F-2
F-3
5.92
2.96
56
10
Anionic fuorocarbon surfactant-0.8% 20
5
5
5
5
6501-3%
Xanthan gum-0.3% 56.341
4.28
Anionic fuorocarbon sur-
factant-0.399%
20
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Chemical Papers
2008; Lunkenheimer and Malysa 2003a; Lunkenheimer and
Malysa 2003b; Sebastiani et al. 2019; Tamura et al. 1995):
(
)
/
h5
R5 =
× 100%
(2)
h0
Determination of drainage performance
The characteristic parameters of foam drainage, such as the
drainage rate parameter, the maximum drainage rate and the
50% drainage time, are all related to the mass fraction of
the foam solution. The change in the mass fraction of the
foam solution has a great infuence on the foam drainage
process (Lin et al. 2007). Essentially, foaming is the fow of
liquid through the voids between bubbles. The fow is driven
by capillary action and gravity and is resisted by viscous
damping (Bikerman 2013; Weaire et al. 1997; Bhakta and
Ruckenstein 1997). Usually, a foam is composed of flms
(between bubbles), channels (also known as the platform
boundary at the intersection of three layers of flms) and
vertices (also known as vertices or nodes at the intersection
of four layers of flms) (Sheng et al. 2018c).
Fig.2 The device used in foam spreading experiments
The test method of drainage performance was as follows:
after the above foaming process was completed, started the
timer and recorded the time of 2.5 ml liquid discharge at the
bottom as the 25% drainage time (Xiao et al. 2008).
Determination of spreading performance
Theoretically, whether the aqueous foam solution (above)
can spread on the surface of the fuel (below) is determined
by three forces, namely the surface tension of the aqueous
foam solution, the surface tension of the fuel and the interfa-
cial tension between the aqueous foam solution and the fuel.
The spreading coefcient can be calculated by formula 3:
Fig.3 Study on the device used in foam extinguishing experiment
of the foam solution was calculated using the MATLAB
S = ꢀL − ꢀU − ꢀLU
(3)
image processing technology (Sheng et al. 2018d).
where S is the spreading coefcient; γU is the surface tension
of the foam aqueous solution; γL is the surface tension of the
fuel; γLU is the interfacial tension between the foam aqueous
solution and the fuel.
Determination of fre extinguishing performance
The extinguishing mechanism of AFFF is mainly due to its
good cooling, covering and sufocating efects. AFFF can
quickly form a aqueous flm on the surface of combustible
liquid to isolate the heat feedback of fame and inhibit the
generation of combustible steam, thus efectively preventing
the further combustion of fuel (Xu et al. 2020).
When S is greater than zero, the foam solution (above)
can be spread on the fuel (below) surface; when S is less than
or equal to zero, the foam solution (above) cannot be spread
on the fuel (below) surface (Woodman et al. 1978; Harkins
and Films 1922; He et al. 2019).
Figure 3 is a device for testing foam extinguishing perfor-
mance. In brief, the following methods were tested: n-hep-
tane (50 mL) was poured into a glass and ignited. After 60 s
of pre-burning, supplying foam was started until the fame
was completely extinguished. The fre extinguishing time
was recorded, and the fre extinguishing performance of the
As shown in Fig. 2, the spreading performance of foam
solution was tested by the following methods: The foam
solution spread to the surface of cyclohexane through a
foam release tube, and then a camera was used to record
the spreading of the foam solution on the surface of the
cyclohexane at diferent times. Finally, the spreading area
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Chemical Papers
foam was analyzed by the fre extinguishing time (Sheng
for 1 min, the 20 mL n-heptane could be placed in the posi-
tive center of the oil pool to adjust the position of the burn-
back tank. Then ignite the burn-back tank, and adjust the
position of the burn-back tank continuously during this
period. Finally, the burn-back time of diferent stages was
recorded until the entire fuel surface was completely burned
(Yu et al. 2020).
et al. 2016b).
Determination of burn‑back performance
The burn-back time refers to the time from the ignition of
the burn-back tank to the ignition of the fuel surface. The
burn-back time of foam depends not only on the components
of the fre extinguishing agent, but also on the percentage of
gases in the foam.
Results and discussion
Figure 4 is a device used to study the foam burn-back
test. After the abovementioned bubble supply was fnished
Characterization of anionic fuorocarbon surfactant
with benzene ring side chain
The anionic fuorocarbon surfactant with benzene ring side
chain was synthesized from compound 2 and potassium
hydroxide solution by acid–base neutralization reaction. The
structure of anionic fuorocarbon surfactant and compound 2
were characterized by 1HNMR. The 1HNMR spectra of the
anionic fuorocarbon surfactant and compound 2 are given
in Fig. 5. The signal at δ=13.07 ppm was the characteristic
absorption peak of –COOH, and the signal at δ=3.66 ppm
c
was the characteristic absorption peak of –CH2 , the signal
at δ = 7.29 ppm was the characteristic absorption peak of
–CHb. Due to the infuence of adjacent groups, the signal at
δ=7.84 ppm was the characteristic absorption peak of –CHa.
By analyzing the signal at δ=13.07 ppm, it could be con-
frmed that the functional group –COOH had disappeared,
that was to say –COOH and KOH were well neutralized and
anionic fuorocarbon surfactant was prepared.
Fig.4 The device used in the foam burn-back experiment
Fig.5 1HNMR spectra of
benzene ring side chain anionic
fuorocarbon surfactant and
compound 2
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Chemical Papers
1HNMR(CD3OD, 300 MHz)δ:3.66(s, C6H4CH2C6F13,
2H), 7.29(d, J = 8.1 Hz, Ar–H, 2H), 7.84(d, J = 8.1 Hz,
Ar–H, 2H); 19FNMR(CD3OD, 282 MHz)δ: − 63.68 to
− 63.90(m,6F), − 82.05(t, J = 13.7 Hz,3F), 106.90 to
−107.25(m, 2F), −124.10 to −124.35(m, 2F).
Table 3 Surface tension, CMC, Γmax and Amin of benzene ring side
chain anionic fuorocarbon surfactant/CTAB with diferent mole frac-
tion at 25 °C
Mole ratio
1:0
1:2
1:5
1:10
0:1
ꢀCMC(mN∕m)
20.40 22.38
22.39
21.47
24.30
CMC(mmol∕L) 30
0.03175 0.0099125 0.00991211 0.0856
Surface tension, foaming, foam stability and critical
micelle concentration of the mixed system
Γmax(10−10 mol/ 1.29 14.89
6.08
3.1497
2.9
cm2)
Amin(nm2)
1.287 0.11156 0.273
0.527
0.5725
Through the measurement of surface tension meter, we
could observe that the surface tension of the benzene ring
side chain anionic surfactant was 20.4 mN/m.
interaction included electrostatic attraction between heter-
ogenous ions and hydrophobic interaction between hydro-
carbon groups, which made the surfactant molecules in
the solution easier to gather and form micelles, and the
surfactant molecules in the surface adsorption layer were
arranged more closely and the surface energy was lower
(Ghosh et al. 2011). In addition, fuorine was the most nega-
tive electronic element. The negative charge around fuorine
atom and the electron cloud densely covered in the fuorine
chain made the hydrophobicity of CF–CH chain stronger
than that of CH–CH chain. When the mixed ratio was 1:2,
the CMC was lower than that of the benzene ring side chain
anionic fuorocarbon surfactant, close to that of CTAB, but
its surface tension was higher than that of benzene side chain
anionic fuorocarbon surfactant. According to the viewpoint
of molecular motion, the possibility (probability) of free
movement of the longer hydrocarbon chain in the system
was greater, because there was a "blank" around the top,
which allowed the single bond to rotate freely. And then the
longer hydrocarbon chain might bend and cover the shorter
fuorocarbon chain, which made the density of the outermost
fuorocarbon chain relatively decrease and eventually led to
the increase of surface tension (Wang et al. 2017).
Figure 6 shows the relationship between surface tension
and concentration of benzene ring side chain anionic fuo-
rocarbon surfactant/CTAB with diferent mole fraction at
25 °C. The surface tension and CMC of benzene ring side
chain anionic fuorocarbon surfactant/CTAB with difer-
ent mole fraction at 25 °C are listed in Table 3. It could be
seen from the fgure that the surface tension of the anionic
fuorocarbon surfactant with benzene ring side chain was
lower than that of CTAB, but the CMC value was higher,
which indicated that the ability of reducing the surface ten-
sion of aqueous solution by the anionic fuorocarbon sur-
factant with benzene ring side chain was stronger than that
of CTAB, and the efciency was lower. For the study of
the mixed system, we were surprised to fnd that the CMC
was 0.00991211 mmol/L when the mixed ratio of benzene
ring side chain anionic fuorocarbon surfactant and CTAB
reached 1:10, which was 103 times lower than that of ben-
zene ring side chain anionic fuorocarbon surfactant and 10
times lower than that of CTAB. It was due to the strong
interaction between anionic and cationic surfactants. The
Figure 7 shows the foaming and foam stability of aque-
ous solutions of CTAB and benzene ring side chain anionic
fuorocarbon surfactant at the optimal compound ratio. Fig-
ure 8 shows the initial foam height of solutions with difer-
ent concentrations of CTAB/benzene ring side chain anionic
fuorocarbon surfactant at the optimal mixed ratio. Figure 9
shows the foam height of solutions with diferent concentra-
tions of CTAB/benzene ring side chain anionic fuorocarbon
surfactant at the optimal compound ratio after fve minutes. It
could be seen from the fgure that the h0 value of the benzene
ring side chain anionic fuorocarbon surfactant/CTAB had
no obvious change in the CTAB with higher concentration,
which indicated that the molecular number at the liquid gas
interface was saturated when the concentration of CTAB was
high. For most surfactants, the surface tension almost did not
change with the increase of the concentration after the concen-
tration reached CMC, but the foaming property of them was
not completely consistent with the concentration after reach-
ing CMC. The foaming performance of the benzene ring side
Fig.6 The relationship between surface tension and concentration of
benzene ring side chain anionic fuorocarbon surfactant/CTAB with
diferent mole fraction at 25 °C
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Chemical Papers
Fig.7 The curve of foam height
and R5 with CTAB concentra-
tion under the optimum mixing
ratio
Fig.8 The initial foam height of CTAB/benzene ring side chain anionic fuorocarbon surfactant with diferent concentrations under the optimum
mixture ratio (the unit of the number indicated in the fgure was ml, and the scale of the measuring cylinder used was 1 ml=2 mm)
chain anionic fuorocarbon surfactant/CTAB still improved
with the increase of the concentration after reaching CMC. It
should be noted that the diference between h0 and h5 curves
was very small, especially for CTAB with high concentra-
tion. Besides, with the increase of CTAB concentration, R5
tended to stabilize after a small foating. The reason was that
the CTAB molecules were arranged more tightly at the liquid
air interface as the concentration of solution increased. This
change improved the foaming property and the foam stability
of surfactant solution.
Aggregation behavior of aqueous solution
in the mixed system
According to Gibbs adsorption equation (Zhou and
Rosen 2003; Rosen 2004) (formulas 4, 5), the saturated
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Chemical Papers
Fig.9 The foam height of CTAB/benzene ring side chain anionic fuorocarbon surfactant after fve minutes under the optimum mixture ratio
adsorption capacity (Γmax) and the minimum area (Amin) of
single surfactant and binary surfactant on air/water inter-
face were calculated.
state of saturated adsorption on the surface of the aqueous
solution.
Figure 10 shows the TEM negative staining images of
vesicles in the mixed system of benzene ring side chain
anionic fuorocarbon surfactant/CTAB at the optimum
mixture ratio. It could be seen that the change trend of
vesicles was consistent with that observed in TEM images.
Generally speaking, the change of concentration would
afect the number of vesicles, but almost had no efect
on the size of vesicles (Zhai et al. 2005). The reason for
this phenomenon was that with the increased of concentra-
tion, the aggregation of vesicles led to the change of vesi-
cle diameter. Obviously, many vesicles nestled together
to form a beaded distribution at the concentration of
0.0406 mol/L.
(
)(
)
/
−1
ꢀꢁ
Γmax
=
∕
(4)
2.303nRT
ꢀlg c
T
/
(
)
1
Amin
=
(5)
ΓmaxNA
where R was 8.314 J/mol−1 K−1; T was the thermodynamic
/
temperature (K); ꢀꢁ ꢀlg c was the maximum slope; NA was
Avogadro constant, n=2 for pure surfactant and 1 for cation/
anion mixed system (Zhao and Xiao 1995; Rosen 1989);
higher Γmax or lower Amin value meant that the surfactant
molecules were more densely arranged at the air/water inter-
face. The most direct result of surfactant adsorption on the
solution surface was to reduce the surface tension of aque-
ous solution. Generally speaking, the greater the surface
adsorption capacity, the more the surface tension decreased.
It could be seen from Table 3 that compared with single
surfactant, the Γmax of binary surfactant mixture was higher,
while Amin was lower, which indicated that the molecules
of the mixed system were more densely arranged at the air/
water interface. By comparing the Γmax, Amin, CMC and ꢀCMC
at the mixed ratios of 1:2 and 1:10, although the molecular
arrangement of the mixed system was more compact when
the molar ratio was 1:2, its CMC and ꢀCMC were higher than
that of 1:10. The reason was that the ability of surfactant to
reduce the surface tension of the aqueous solution was not
only related to the amount of surface adsorption, but also
mainly depended on the outermost atom or clusters in the
Fig.10 TEM negative staining images of vesicles in the mixed sys-
tem of benzene ring side chain anionic fuorocarbon surfactant/CTAB
with diferent concentrations (a=0.0406 mol/L; b=0.0203 mol/L)
under the optimum mixture ratio
1 3

Chemical Papers
Fig.11 Optical microscope pictures of the foam coarsening process of the aqueous solution of diferent concentrations of the mixed system
under the optimum mixture ratio
Table 4 Surface tension of three
AFFF formulations
Optical microscope pictures of the foam coarsening
process of the mixed system aqueous solution
Foam sample
Surface
tension
(mN/m)
Figure 11 shows the optical microscope images of the foam
coarsening process of aqueous solutions of diferent concen-
trations at the optimum mixture ratio. From the diagram, we
could see that the morphology of low-concentration foam
was much larger than that of high concentration at the begin-
ning (1 min). Due to the pressure diference between bub-
bles of diferent sizes, bubbles grew and merged with time.
However, there were many phenomena in the bubble mor-
phology at diferent concentrations. From 1 to 20 min, the
foam morphology of aqueous solution of high-concentration
mixed system changed little, while 20 min still had a large
amount of liquid trapped in the bubble, and the bubbles
were round. With the passage of time, the amount of foam
in aqueous solution of low-concentration mixed system was
slightly reduced and was looser than that in high concentra-
tion. The dispersion of foam was related to the stability of
foam. Under the action of the background light, some of the
liquid in the foam flm and the platform glass evaporated
into steam. The evaporated vapor accumulated on the slide
and aggregated into droplets (Sheng et al. 2020). To sum up,
the high-concentration mixed system aqueous solution was
more suitable for AFFF than the low-concentration mixed
system aqueous solution.
F-1
F-2
F-3
19.18
27.10
21.39
Table 5 H0, h5 and R5 of three aqueous flm-forming foam fre extin-
guishing agent formulations
Foam sample
h0 (mm)
h5 (mm)
R5 (%)
F-1
F-2
F-3
110
30
126
108
26
123.34
98.2
86.67
97.89
be seen from Table 4 that the surface tension of F-2 was
the highest and the surface tension of F-1 was the lowest.
According to the defnition and calculation of the spread-
ing coefcient S, when the S was greater than zero, the
foam solution could be spread on the fuel surface. The
fuel used in this paper was cyclohexane. The surface ten-
sion test showed that the surface tension of cyclohexane
was 25.24 mN/m, while the surface tension of F-2 was
27.10 mN/m. Therefore, F-2 could not be spread on the
surface of cyclohexane. From Table 5 and Fig. 12, we
could see that F-1 and F-3 possessed better foaming prop-
erties, and F-2 had poor foaming capacity. In terms of
foam stability, R5 of F-1 and F-3 was close to 100%, while
R5 of F-2 was still poor, only 86.67%, so F-2 was not suit-
able for AFFF.
Surface tension, foaming property and foam
stability of the formula
Table 4 shows the surface tension of three AFFF formula-
tions. Table 5 shows the h0, h5 and R5 of the three AFFF
formulations. Figure 12 is a histogram of three foam
extinguishing agent formulations h0, h5 and R5. It could
1 3

Chemical Papers
Fig.13 25% Drainage time of three foam fre extinguishing agent for-
Fig.12 H0, h5 and R5 of three foam extinguishing agent formulations
mulations
Drainage performance
Spreading performance
The extinguishing efect of AFFF was shared by the aque-
ous flm and foam layer foating on the oil surface. Proper
drainage time could provide a stable foam layer, and could
continuously form an aqueous flm to isolate oxygen and
achieve efcient extinguishment of fammable liquid fres.
Table 6 shows the 25% drainage time of the three AFFF
formulations. Figure 13 is the histogram of the 25% drain-
age time of three foam extinguishing agent formulations.
It could be seen from Table 6 and Fig. 13 that the 25%
drainage time of F-1 was the longest, followed by F-3, and
the 25% drainage time of F-2 was the shortest. According
to the standard technical requirements of GB15308-2006
foam extinguishing agent, the drainage time of the foam
extinguishing agent should be greater than 2.5 min, and
the 25% drainage time for F-1 was 182.4 s, while the 25%
drainage time of the other two foam extinguishing agents
was lower than that of the national standard (GB15308-
2006), so only F-1 was considered after performance
evaluation.
Figure 14 is the numerical simulation of the foam spreading
process (the red circle represents the culture dish bound-
ary). Figure 15 is the binary image after digitizing the foam
spreading process using MATLAB image processing tech-
nology. Figure 16 is the curve of the foam spreading area
of foam extinguishing agent F-1 over time. In Fig. 15, it
could be seen that there was no rapid spreading stage in
the foam spreading process. From the beginning, the foam
slowly spread on the surface of cyclohexane in a noncir-
cular manner until the boundary of the foam layer was
exposed to the edge of the dish and the foam spread was
blocked. According to the defnition and calculation method
of spreading coefcient, foam solution could form aqueous
flm on the surface of fuel when foam coefcient was posi-
tive (Sheng et al. 2018d). By analyzing the surface tension
of the formula, it was known that the surface tension of F-1
was only 19.14 mN/m. Therefore, there was a clear aqueous
flm formed on the surface of the cyclohexane. It could be
seen from Fig. 16 that the foam spreading area raised with
the increase of time, and the foam spreading rate decreased
with the increase of time. This was due to the diferent vis-
cosity of foam solution resulting in diferent sizes of resist-
ance encountered during foam spreading. The greater the
resistance encountered during foam spreading, the greater
the vertical height of foam required to spread in the hori-
zontal direction.
Table 6 The 25% drainage time
Foam sample
25%
of three aqueous flm-forming
foam fre extinguishing agent
formulations
Drainage
time (s)
Fire extinguishing performance
F-1
F-2
F-3
182.4
28
30
Figure 17 shows the fre extinguishing process of F-1, and
Table 7 shows the extinguishing time of F-1. It could be
1 3

Chemical Papers
Fig.14 Numerical simulation of
foam spreading process
Fig.15 Binary images of foam spreading process
Fig.16 The curve of foam
spreading area with time
seen from the table that the fre extinguishing time of F-1
was only 31 s. According to the graph, the fre extinguish-
ing process of foam could be divided into three stages: pre-
combustion stage, foam spreading stage and fame extinction
stage. In the pre-combustion stage, the fame spread rapidly
to the whole fuel area and formed a relatively stable fame
after 60 s; In the foam spreading stage, with the increased of
foam, the foam gradually covered the surface of n-heptane,
and the foam’s inhibitory efect on fuel vapor was enhanced.
However, the increased of total vapor pressure on the liquid
1 3

Chemical Papers
Fig.17 Fire extinguishing process of F-1
Fig.18 Burn-back process of F-1
Table 8 Burn-back time of F-1
Table 7 Fire extinguishing time of F-1
Foam sample
Extinguishing time
31 s
Foam
The frst
stage
The second The third
100%
Reburn-
ing
sample
stage
180 s
stage
F-1
F-1
370 s
170 s
720 s
surface aggravated the evaporation of fuel, resulting in the
accumulation of a large number of fammable gases in the
middle of the fame, which made the fame still strong. In the
stage of fame extinction, the foam had completely covered
the surface of n-heptane, but the foam was still unable to
inhibit the fuel vapor, and continued to increase the amount
of foam, so that the foam layer was thickened, the fame
could not cover the entire fuel surface, and the fre gradually
decreased and eventually extinguished.
of fame reburning, there were many pores on the foam sur-
face, and the fuel vapor spilt from the cracked pores and then
ignited. Due to the decreased of foam content, the foam layer
could not completely inhibit the overfow of fuel vapor, and
the continuous fame started to burn on the surface of the
foam layer, resulting in complete combustion of the entire
oil surface.
Burn‑back performance
Conclusions
Figure 18 shows the burn-back process of F-1, and Table 8
revealed the burn-back time of F-1. It could be seen from the
table that the foam expansion phase of F-1 lasted for 370 s.
At the 550 s, the upper foam completely disappeared and
the foam entered a relatively stable phase. Moreover, the oil
surface was completely burned at the 720 s. It could be seen
from the diagram that the burn-back process of foam could
also be divided into three stages: foam expansion stage,
foam layer attenuation stage and fame reburning stage. The
foam expansion stage was due to the heat radiation of the
fame to the foam, which caused the gas inside the foam to
expand due to heat. Meanwhile, the volume of the foam layer
increased and expanded outward. In the decay stage of the
foam layer, fuel vapor began to overfow from the vicinity
of the foam layer and was ignited by the burn-back tank.
Because the overfowing fuel vapor was directly exposed to
the fame, the upper layer foam rapidly decayed. In the stage
In this paper, a perfuorinated branched short-chain fuo-
rocarbon anionic surfactant with high surface activity was
prepared by adopting industrialized D2 as the starting
material, and then three kinds of new environment-friendly
AFFF formulations (F-1, F-2, F-3) were designed by using
the mixed system of the perfuorinated branched short-chain
fuorocarbon anionic surfactant and CTAB as the foaming
agent. The performances of foam extinguishing agents were
also studied. The conclusions were as follows:
1. The above synthetic route possessed the advantages of
simple operation, mild reaction conditions and low cost.
2. The surface tension of the benzene ring side chain ani-
onic fuorocarbon surfactant was 20.4 mN/m and the
CMC was 0.01 mol/L. Besides, the mixed system of
benzene ring side chain anionic fuorocarbon surfactant
1 3

Chemical Papers
Bikerman J, J. Foams (2013). Springer Science & Business Media,
Russia.
and CTAB would produce synergistic efect. The results
showed that the addition of CTAB could signifcantly
reduce the amount of fuorocarbon surfactant and fur-
ther decrease its surface tension. The optimum mixed
ratio of this fuorocarbon surfactant and CTAB was 1:10.
Meanwhile, there were many vesicles nestling together
to form a bead-like distribution in the TEM images of
the optimum mixed solution with the concentration of
0.0406 mol/L.
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centration mixed system was slightly reduced and was
looser than that in high concentration. Accordingly, the
mixed solution with a concentration of 0.0406 mol/L
was more suitable for AFFF than that with a concen-
tration of 0.0203 mol/L. Moreover, compared with the
other two foam stabilizers, sodium carboxymethyl cel-
lulose had better foam stability and flm-forming proper-
ties in this system.
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BS-12 56%, anionic fuorocarbon surfactant with ben-
zene ring side chain 0.8%, ethylene glycol 20%, dieth-
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boxymethyl cellulose 4%, and water 3.28%. The initial
foam height of F-1 was 115.34 mm; the extinguishing
time was 31 s; the burn-back time was 720 s; the 25%
drainage time was 182.4 s. Obviously, the property of
F-1 was consistent with the standard technical require-
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was superior to that of the traditional AFFF. Therefore,
it has the broad application prospect in oil pool fre
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Acknowledgements This work was supported by the National Natural
Science Foundation of China (No.21908109), the Open Project of Key
Laboratory of Fire Emergency Rescue Equipment of Ministry of Emer-
gency Management of People’s Republic of China (No.2020XFZB05),
the Fundamental Research Funds for the Central Universities
(No.30919011272). And the properties characterization process was
supported by Analysis and Testing Center of Nanjing University of
Science and Technology.
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with CF3CF2CF2C(CF3)2 group. J Fluorine Chem 214:35–41.
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