Synthesis and surface activities of organic solvent-soluble fluorinated surfactants

Article abstract of DOI:10.1016/j.jfluchem.2009.05.006

A variety of fluorinated surfactants soluble in organic solvent were prepared, including C₈F₁₇SO₂NHC_nH_2n+1 (n = 2, 4, 6, 8, 10), C₈F₁₇SO₂NHR (R = C₆H₁₁, C₆H₅), C₈F₁₇SO₂N(C_nH_2n+1 )₂ (n = 1, 2, 3, 4) and C₈F₁₇SO₂NH(CH₂)_nN HO₂SC₈F₁₇ (n = 6, 10). Their surface activities in various organic solvents were determined by surface tension measurement. The results showed that these fluorinated surfactants can reduce the surface tension of both polar and non-polar organic solvents. In general, organic solvents with strong polarity or long alkyl chain are beneficial to increase the surface activity of these polar fluorinated surfactants. By comparing fluorinated surfactants with the same fluorocarbon segment and connecting group, C₈F₁₇SO₂N(C_nH_2n+1 )₂ (n = 1, 2, 3, 4) showed lower surface activity in organic solvents than C₈F₁₇SO₂NHC_nH_2n+1 (n = 2, 4, 6, 8) with an equal carbon number of the solvophilic group. Through surface tension vs. concentration curves given for N-octyl perfluorooctanesulfonamide in various organic solvents, a break point like the critical micelle concentration of ordinary surfactants in aqueous solutions was observed, and the effect of the different types of organic solvents on adsorption and aggregation behavior was also studied.

Full text of DOI:10.1016/j.jfluchem.2009.05.006

Journal of Fluorine Chemistry 130 (2009) 674–681
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Synthesis and surface activities of organic solvent-soluble fluorinated surfactants
Guo-Li Li a,b, Li-Qiang Zheng a, , Jin-Xin Xiao
*
c,**
a Key Laboratory for Colloid and Interface Chemistry of Education Ministry, Shandong University, Jinan 250100, PR China
b School of Light Chemical and Environmental Engineering, Shandong Institute of Light Industry, Jinan 250353, PR China
c Beijing FLUOBON Surfactant Institute, Beijing 100080, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
A
variety of fluorinated surfactants soluble in organic solvent were prepared, including
Received 11 February 2009
Received in revised form 5 May 2009
Accepted 7 May 2009
C8F17SO2NHCnH2n+1 (n = 2, 4, 6, 8, 10), C8F17SO2NHR (R = C6H11, C6H5), C8F17SO2N(CnH2n+1 2 (n = 1, 2,
)
3, 4) and C8F17SO2NH(CH2)nNHO2SC8F17 (n = 6, 10). Their surface activities in various organic solvents
were determined by surface tension measurement. The results showed that these fluorinated surfactants
can reduce the surface tension of both polar and non-polar organic solvents. In general, organic solvents
with strong polarity or long alkyl chain are beneficial to increase the surface activity of these polar
fluorinated surfactants. By comparing fluorinated surfactants with the same fluorocarbon segment and
Available online 18 May 2009
Keywords:
Fluorinated surfactants
Fluorocarbon
connecting group, C8F17SO2N(CnH2n+1)2 (n = 1, 2, 3, 4) showed lower surface activity in organic solvents
than C8F17SO2NHCnH2n+1 (n = 2, 4, 6, 8) with an equal carbon number of the solvophilic group. Through
surface tension vs. concentration curves given for N-octyl perfluorooctanesulfonamide in various organic
solvents, a break point like the critical micelle concentration of ordinary surfactants in aqueous solutions
was observed, and the effect of the different types of organic solvents on adsorption and aggregation
behavior was also studied.
Organic solvent-soluble surfactant
Surface activity
Adsorption
ß 2009 Elsevier B.V. All rights reserved.
1. Introduction
results in a higher surface energy and an increase of surface tension
of the organic solvent [4]. Therefore, structural requirements for a
Fluorinated surfactants, containing fluorocarbon chains as
hydrophobic groups, have a number of special properties such
as chemical inertness, thermal stability, high surface activity and
water and oil repellence that offer advantages over hydrocarbon
surfactants [1]. Among all surfactants, fluorinated surfactants are
most effective in reducing the surface tension of aqueous solutions
[2]. Their outstanding chemical and thermal stability expands their
applications to extreme conditions which are too severe for
hydrocarbon surfactants [3].
Utilization of surfactants is most frequently implemented in
aqueous systems. However, there are many cases of non-aqueous
systems, such as the coating industry, paint industry, petroleum
exploitation, and extraction process and micelle catalysis. Con-
ventional surfactants used in aqueous systems that combine two
basically different parts, at least one hydrophobic and one
hydrophilic group, are not as effective in organic solvents as in
water. It is known that conventional surfactants orient with the
hydrophilic group away from organic solvent surface, which
surfactant in organic solvents should be different from those of
conventional surfactants.
Previous investigations have shown that fluorinated surfactants
consisting of solvophobic and solvophilic segments in the molecule
could be adsorbed at organic solvent/air interfaces as monomole-
cular films, thus depressing the surface tension of the organic
solvent. Such investigations have thrown light on the study of
fluorinated surfactants in organic solvents [5–12]. Several fluori-
nated surfactants such as F[CF(CF3)CF2O]nCF(CF3)–COAr, (Ar = aryl
group, n = 1–4) [13], F[CF(CF3)CF2O]nCF(CF3)–C6H5 (n = 1, 2) [14],
semifluorinated diblock copolymers based on methyl methacrylate
and 1H, 1H, 2H, 2H perfluoroalkyl methacrylate [15], fluoroalkyl
end-capped diacetone (N-1,1-dimethyl-3-oxobutylacrylamide) oli-
gomers [16], fluoroalkyl end-capped cooligomers containing poly-
dimethylsiloxaneandpolyoxyethylenesegments[17]werefoundto
be effective in reducing the surface tension of m-xylene, while their
surface tension curves exhibited a clear break point like the critical
micelle concentration of ordinary surfactants in aqueous solutions.
These findings suggest that these fluorinated surfactants can form
self-assembled molecular aggregates in aromatic solvents. Also
some semifluorinated alkanes which possess no charged or polar
groups were found to aggregate in either hydrocarbon or
fluorocarbon solvents when the incompatibility between the
semifluorinated alkanes and solvents is sufficiently strong [18–21].
* Corresponding author. Tel.: +86 531 88366062; fax: +86 531 88564750.
** Corresponding author. Tel.: +86 10 62561871; fax: +86 10 62561871.
E-mail addresses: lqzheng@sdu.edu.cn (L.-Q. Zheng), xiaojinxin@pku.edu.cn
(J.-X. Xiao).
0022-1139/$ – see front matter ß 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2009.05.006

G.-L. Li et al. / Journal of Fluorine Chemistry 130 (2009) 674–681
675
Surface activities of fluorinated surfactants at the air/organic
2.2. The choice of organic solvent
solvent interface have been studied since the 1950s [5]. However,
rather little work has been reported in recent decades [22–27]. N-
Alkyl perfluorooctanesulfonamides have been widely used in
fabrics and papers, fire retardants, anticorrosion agents, and many
other commercial formulations [5,28]. To our knowledge, there is
little information available in the literature about the surface
activity of derivations of N-alkyl perfluoralkylsulfonamides.
In this work, we synthesized a series of N-alkyl perfluoralk-
ylsulfonamides as organic solvent-soluble fluorinated surfactants
in which straight, branched and cyclic alkyls and phenyl acted as
solvophilic segment and single or double fluorocarbon chains as
solvophobic segment. Surface activities of these surfactants in
organic solvents including aliphatic, aromatic and non-protonic
polar solvents were determined by surface tension measurement.
Also, the relationship between surface activity and molecular
structure of the organic solvent-soluble fluorinated surfactants as
well as the type and structure of organic solvents was discussed.
The surface adsorption state and surface adsorbed layer of
fluorinated surfactants were also discussed.
Compared with aqueous solutions, various organic solvents
used to dissolve surfactant may enlarge the research system and
present diversified properties. Considering the extensive applica-
tion and the solvent cost, a wide variety of organic solvents
including alkanes (n-dodecane, n-tetradecane, n-hexadecane and
liquid paraffin), cyclanes (cyclohexane), aromatic hydrocarbons
(toluene and m-xylene), polar solvents (ethyl acetate, 2-butanone,
nitromethane, DMF, and DMSO) were selected for further
investigation.
2.3. Surface activity of surfactants in organic solvent
Surface activity results of each synthesized fluorinated surfac-
tant in saturated solutions of various solvents are presented in
Table 10 (see supporting information) and Table 1. In this work, the
effectiveness of the fluorinated surfactants was expressed by the
maximum reduction of surface tension of the organic solvents, Dg
(1)
solution is
.
Dg
¼
gsolvent
ꢀ
gsolution
2. Results and discussion
where gsolvent is the surface tension of pure solvent, and g
2.1. Surfactant structure
the surface tension of the saturated solution. Most of surfactant
solutions became saturated at concentration lower than 0.05 M.
The lower the total concentration at which a small amount of
precipitate was observed to appear in solution, the lower the
solubility was. And at a same total concentration, more precipitate
existed in solution, the lower solubility of surfactant was.
However, for some solvents in which surfactants were too soluble,
the effectiveness of surfactants was calculated on the basis of a
0.05 M solution (footnote a). For example, the solution of
surfactants in DMF remained homogeneous even at 0.2 M. The
results in Table 10 (see supporting information) should be quite
comparable because only a small concentration of an effective
surfactant can give the maximum possible surface tension
lowering of solvents [11]. Besides, the surface tension of a solution
containing 0.05 M of surfactant amounts to the value which is very
close to the surface tension of a saturated solution.
From Table 10 (see supporting information), it can be seen that
the synthesized fluorinated surfactants can reduce the surface
tension of both polar and non-polar organic solvents. In all cases,
the surface tension depression is smaller than 21 mN/m. However,
common fluorinated surfactants can lower the surface tension of
water by more than 50 mN/m. Surfactants are not as effective in
organic solvents as in water because of two reasons. First, the
initial surface tension of organic solvents, 19–30 mN/m for most of
hydrocarbon solvents, is much lower than that of pure water
The nature of common surfactants is based on the chemical
antipathy of the surfactant head and tail, and on their opposite
sympathy for water molecules. The situation is different in organic
solvents. Surfactants which operate in organic solvent are
generally composed of a solvophobic group and a solvophilic
group within the same molecule. The hydrocarbon tail, hydro-
phobic in water, may be the solvophilic group providing solubility
in organic solvents and the fluorinated alkyl tail may be the
solvophobic group. Such surfactants are expected to adsorb at the
organic solvent/air surface to form a monolayer in which the
fluorocarbon segment of the surfactant tilts away from the surface.
Thus their surface tension is expected to be reduced to a low value
as a result of low cohesive energy density of perfluorocarbons [29].
In this work, perfluorooctanesulfonyl fluoride was used as the raw
material in view of its cheapness. Three series of fluorinated
surfactants were designed as following:
(1) RF–Q–RH. where RF = C8F17, Q = SO2NH, RH = (CH2)nH (n = 2, 4, 6,
8, 10), C6H11, C6H5.
(2) RF–Q–(RH)2. where RF = C8F17, Q = SO2N, RH = (CH2)nH (n = 1, 2,
3, 4).
(3) RF–Q–RH–Q–RF. where RF = C8F17
, Q = SO2NH, RH = (CH2)n
(n = 6, 10).
Table 1
Surface tension of saturated solution of surfactant containing double fluorocarbon chains in various organic solvents at 25 8C.
Surfactant
RH
Solvent
gsolution (mN mꢀ1
)
Dg
C8F17SO2NH–RH–NHO2SC8F17
(CH2)6
Toluene
24.6
24.3
24.0
23.7
22.0
19.5
20.9
24.7
24.9
26.6a
24.1
28.4a
20.9
2.1
m-xylene
DMSO
2.7
17.5
10.8
12.6
2.8
Nitromethane
DMF
2-butanone
Ethyl acetate
Toluene
1.4
(CH2)10
2.0
m-xylene
DMSO
2.1
14.9
10.4
6.2
Nitromethane
DMF
2-butanone
1.4
RH, the solvophilic group of fluorinated surfactant.
a
Minimum surface tension at concentration 0.05 mol lꢀ1
.

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G.-L. Li et al. / Journal of Fluorine Chemistry 130 (2009) 674–681
(72 mN/m, 25 8C). Second, the free energy cost of transferring a –
CF2– group from the pure fluorocarbon to an alkane solution is
approximately 1.4 kJ/mol, whereas the cost of transferring a –CF2–
from fluorocarbon to water is 6.5 kJ/mol [30].
As expected, thereisa close relationship between the structureof
the solvophobic and solvophilic constituents of surfactant mole-
cules, the property of organic solvents, and the surface activity of
surfactants. That is why a fluorinated surfactant cannot have the
same optimum structure for all organic solvents [14]. In order to
clarify the effect of surfactant structure on the decrease of surface
tension of various organic solvents, Figs. 1–3 were drawn according
to corresponding results in Table 10 (see supporting information).
Fig. 2. Relationship between the decrease of surface tension of organic solvents and
Fig.1. Relationshipbetweenthedecrease ofsurfacetension oforganic solventsand the
carbonatomnumbersof single alkyl chain ofC8F17SO2NHCnH2n+1 at25 8C. (A) alkanes;
the carbon atom numbers of double alkyl chains of C8F17SO2N (CnH2n+1
(A) alkanes; (B) cyclohexane and aromatic solvents; (C) polar solvents.
gsolution indicates the effectiveness of the surfactant in lowering
)2 at 25 8C.
(B) cyclohexane and aromatic solvents; (C) polar solvents. Dg
indicates the effectiveness of the surfactant in lowering the surface tension of the
solvent; n, the carbon atom numbers of single alkyl of C8F17SO2NHCnH2n+1
=
gsolvent
ꢀ
gsolution
Dg
=
gsolvent
ꢀ
the surface tension of the solvent; n, the carbon atom numbers of solvophilic
.
segment of C8F17SO2N (CnH2n+1)2.

G.-L. Li et al. / Journal of Fluorine Chemistry 130 (2009) 674–681
677
can be seen from Fig. 1A that in n-alkane (dodecane, tetradecane
and hexadecane) solutions, surface activity increases with
increasing alkyl chain length of the surfactants. Only those
C8F17SO2NHCnH2n+1 with enough long carbon chain and sufficient
solubility can exhibit surface activity in n-alkane solutions. For
example, in n-hexadecane, such a surfactant has the highest
surface tension reduction when the solvophilic group is n-decyl
(Dg = 6.2 mN/m). In liquid paraffin, the surfactants with a long
alkyl chain, such as N-hexyl, N-octyl and N-decyl perfluoroocta-
nesulfonamide exhibit surface activity whereas surfactants with a
short alkyl chain cannot decrease the surface tension of liquid
paraffin. The differences in surface activity of surfactants are based
on the high solubility of N-alkyl perfluorooctanesulfonamide with
a long alkyl chain, leading to the adsorption of a high concentration
of perfluorocarbon chains at the organic solvent/air interface. Thus,
it is not surprising to find that N-ethyl perfluorooctanesulfonamide
has little surface activity due to the short alkyl chain. However, in
n-tetradecane and n-dodecane, the highest surface tension
decrease can be obtained by N-octyl perfluorooctanesulfonamide
to 5.7 mN/m and 3.9 mN/m, respectively. A possible explanation is
that a long alkyl chain favors sufficient solubility, yet a long alkyl
chain arranged in zigzag conformation is easier to bend than a
short alkyl chain. Because of the increasing steric hindrance
between the bending long alkyl chains, a long alkyl chain maybe
unfavorable for high packing density of fluorocarbon chains at the
interface. Moreover, it can also be inferred from Fig. 1A that
C8F17SO2NHCnH2n+1 show higher surface activity in long chain n-
alkane solvents than that in short chain n-alkane solvents. For
example, the Dg of N-octyl perfluorooctanesulfonamide, one
surfactant of C8F17SO2NHCnH2n+1 is 5.5 mN/m in n-hexadecane
which is higher than the Dg in n-dodecane. The reason for this
trend is quite obvious. Long chain alkane solvents have the
contribution to maximize the mutual immiscibility and incompat-
ibility between the fluorocarbon chain and solvent, to promote
fluorocarbon chain escaping from the bulk of organic solvent, and
thus to increase adsorption [21]. In general, the surface tension
decrease of n-alkane depends mostly on the length of the
solvophilic group of C8F17SO2NHCnH2n+1, which exhibits a great
decrease when the length of the single alkyl is long enough.
In cyclohexane, as can be seen from Fig. 1B, the values of Dg for
most of C8F17SO2NHCnH2n+1 (n = 2, 6, 8, 10) are all small (near
2 mN/m), so the surface activities do not obviously change with an
increase of the alkyl chain because of limited solubility. In
particular, Dg of C8F17SO2NHCnH2n+1 (n = 4) is a little higher
(3 mN/m) than the other four surfactants, and its solubility is a
little higher too. In the case of aromatic solvents, such as toluene
and m-xylene, Dg of C8F17SO2NHCnH2n+1 (n = 2, 4, 6, 8, 10)
increases, then decreases a bit and increases again with an increase
of alkyl chain length. This trend might be ascribed to different
solubility of surfactants, and N-butyl perfluorooctanesulfonamide
has a higher solubility than the other four surfactants. As discussed
above, a long alkyl chain in solvophilic group favors sufficient
solubility in n-alkane solvents with ten or more carbon atoms, such
as n-dodecane, n-tetradecane and n-hexadecane. But for cyclohex-
ane and aromatic solvents, the number of carbon atoms is six. Thus
compared to N-butyl perfluorooctanesulfonamide, other similar
surfactants with shorter (n = 2) or longer alkyl chains (n > 6) as the
solvophilic group have too low a solubility to impart the desired
orientation of the surfactant molecule at the interface, resulting in
low surface activity.
Fig. 3. Relationship between the decrease of surface tension of organic solvents and
the structure of solvophilic group of C8F17SO2NH–RH (RH = n-hexyl, cyclohexyl,
phenyl) at 25 8C. (A) alkanes; (B) cyclohexane and aromatic solvents; (C) polar
solvents. Dg
=
gsolvent
ꢀ
gsolution indicate the effectiveness of the surfactant in
lowering the surface tension of the solvent; n-Hexyl, cyclohexyl and phenyl, the
solvophilic segment of C8F17SO2NH–RH.
As shown in Fig. 1C, surfactant with ethyl solvophilic group
exhibits the highest surface tension reduction of DMSO. Moreover,
the surface activity in DMSO gradually decreases with the increase
of the length of alkyl chain. This variation of surface activity may be
affected by the result of change of solubility. In strong polar DMSO
solvent, the longer the alkyl chain, the lower the solubility of
2.3.1. Effect of the length of the solvophilic group
2.3.1.1. Single alkyl solvophilic group structure. Fig. 1 shows the
relationship between the decreases of surface tension of organic
solvents (Dg) and the carbon number (n) of C8F17SO2NHCnH2n+1. It

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G.-L. Li et al. / Journal of Fluorine Chemistry 130 (2009) 674–681
surfactants. During the preparation of solutions, it was found that
the solution of N-butyl perfluorooctanesulfonamide or N-ethyl
perfluorooctanesulfonamide in DMSO was homogeneous at
0.05 M, but the solution of other longer alkyl perfluorooctane-
sulfonamides at the same concentration became supersaturated.
These phenomena may support an explanation that the solubility
of N-alkyl perfluorooctanesulfonamides with longer alkyl chain is
too small to obtain high surface activity in DMSO solution.
Known as the ‘‘all-purpose dissolvent’’, DMF has excellent
dissolving ability for C8F17SO2NHCnH2n+1. The solution of these
surfactants in DMF was still homogeneous even at 0.2 M. But the
excessive solvophilic property of surfactants increases the con-
centration to obtain minimum surface tension of organic solvents,
and may prevent the close packing of fluorocarbon chains adsorbed
at the interface. Consequently, it is well understood that Dg of all
surfactants in DMF is less than that in DMSO. Moreover, surfactant
with the n-decyl solvophilic group exhibits the highest surface
tension reduction of DMF.
2.3.2. Comparison of single, double and cyclic alkyl and phenyl
structure of the solvophilic segment in the surfactant molecule
It can be seen from Figs. 1 and 2 the single alkyl solvophilic
group is more effective in lowering surface tension of organic
solvents discussed in this paper than the double alkyl structure
when the total carbon number of the solvophilic segment are
equal. That is, the Dg is n-octyl > dibutyl, n-hexyl > dipropyl, n-
propyl > diethyl
(except
for
DMF
solution)
and
n-
ethyl > dimethyl. This can be explained in part by the branching
chain effect of the solvophilic segment which increases the steric
hindrance between the surfactant molecules, leading to a lower
density of fluorocarbon chains at the organic solvent/air interface.
That is why only diethyl as the solvophilic segment among
C8F17SO2N(CnH2n+1)2 shows surface activity in n-alkane solutions.
Fig. 3 shows the surface activity of C8F17SO2NH–RH (RH = n-
hexyl, cyclohexyl, phenyl) in different organic solvents. It can be
found that N-cyclohexyl perfluorooctanesulfonamide exhibits
the highest surface tension reduction of cyclohexane solution
owing to the similar polarity of solvophilic group with solvent
molecule (see Fig. 3B). Likewise, N-phenyl perfluorooctanesulfo-
namide has the best surface activity in toluene and m-xylene
solution (see Fig. 3B), and N-hexyl perfluorooctanesulfonamide
can effectively decrease the surface tension of n-dodecane, n-
tetradecane and n-hexadecane (see Fig. 3A). It can also be found
that N-cyclohexyl perfluorooctanesulfonamide is only effective in
decreasing the surface tension of n-hexadecane. As for surfac-
tants with phenyl or dipropyl as the solvophilic segment, they are
all not effective in decreasing the surface tension of n-alkane
solvents. This different ability in lowering the surface tension of
n-alkane solvents might be not only caused by insufficient
solubility, but also due to the increased steric hindrance of
cyclohexyl, phenyl and dipropyl groups in surfactant molecules
as shown in Fig. 4.
2.3.1.2. Double alkyl solvophilic groups structure. Fig. 2 shows the
relationship between the decrease of surface tension of organic
solvents
(Dg) and the carbon atom numbers (n) of C8F17
SO2N(CnH2n+1)2. Among the four surfactants with double alkyl as
solvophilic groups, only N,N-diethyl perfluorooctanesulfonamide
can show surface activity in n-alkane and liquid paraffin solutions
(see Fig. 2A). These results are not surprising because N,N-diethyl
perfluorooctanesulfonamide has not only sufficient solubility but
also low steric hindrance. But surfactant with the dimethyl
solvophilic groups has such poor solubility in long chain alkane
solvents that the molecule cannot effectively be inserted into the
bulk of solvents, resulting in a limited surface activity. For
surfactants with dibutyl or dipropyl solvophilic groups, low
surface activity may be caused by higher steric hindrance, and
thus lower density of fluorocarbon chains packed at the interface.
In toluene and m-xylene solutions, it can be seen from Fig. 2B
that the surface activity sequence of C8F17SO2N(CnH2n+1)2 is N,N-
dipropyl perfluoroctanesulfonamide, N,N-dibutyl perfluoroocta-
nesulfonamide, N,N-diethyl perfluorooctanesulfonamide and
N,N-dimethyl perfluorooctanesulfonamide in terms of Dg value
from high to low. This is because the solubility of the surfactants
is controlled by the structural similarity between the solvopho-
bic portion of the solute molecule and the solvent molecules.
Among the surfactants with the dialkyl solvophilic groups, the
dipropyl structure is similar to the six-membered ring of the
solvents, so it shows higher solubility. In cyclohexane, poor
solubility of N,N-dimethyl perfluorooctanesulfonamide limits
the maximum surface tension decrease, while surfactants with
the dipropyl or dibutyl solvophilic groups have relatively higher
surface activity.
2.3.3. Comparison of single fluorocarbon chain and double
fluorocarbon chains of the solvophobic segment
As is known, long fluoroalkylated compounds exhibit a strong
repellent property against water or hydrocarbons. Therefore,
compounds containing two fluorinated chains listed in Table 1
were designed and the surface activity was examined in a series of
organic solvents. Effect of the number of solvophobic groups on the
decrease of surface tension of organic solvents can be obtained by
comparing results in Figs. 1–3 and Table 1. Results show that the
most effective surfactant that lowers the surface tension of
nitromethane and DMF is C8F17SO2NHC6H12NHO2SC8F17 among all
synthesized surfactants. Especially, in 2-butanone and ethyl
acetate, surfactants with a single fluorocarbon chain as the
solvophobic group cannot play a role due to excessive solubility
in these solvents. However, the Dg values of C8F17SO2NHC6H12N-
HO2SC8F17 are both higher than 1 mN/m, which suggests that this
fluorocarbon compound can function as a surfactant in these
solvents. Consequently, the use of fluorinated surfactants in
industrial applications may be widened. The discrepancy between
the ability of surfactants containing different fluorinated chains
may reflect the differences in adsorption states. It is believed that
As shown in Fig. 2C, in strong polar organic solvents such as
nitromethane, DMSO and DMF, N,N-diethyl perfluorooctanesulfo-
namide also exhibits higher surface activity than that of other
surfactants with the dialkyl solvophilic groups, which might also
be explained in the same way as in n-alkane and liquid paraffin
solutions.
Fig. 4. Schematic illustration of the adsorption of C8F17SO2NH(CH2)5CH3, C8F17SO2N(CH2 CH2CH3)2 and C8F17SO2NHC6H11 in n-alkane solvent. Bent curves depict alkyl groups,
shaded blocks represent fluorinated chains, and dark dots indicate the connecting group.

G.-L. Li et al. / Journal of Fluorine Chemistry 130 (2009) 674–681
679
an increase of the solvophobic part in surfactant molecules
may decrease the solubility and lead to closer packing at the
surface.
2.4. Adsorption at interface and the adsorbing state of surfactant
molecules
The orientation and packing of fluorinated surfactant molecules
at the organic solvent/air interface prominently depend upon the
molecular structure, solubility and extent of association of the
solute and solvent molecules. As described above, N-octyl
perfluorooctanesulfonamide exhibits remarkably decrease the
surface tension of various organic solvents, so special attention
was paid to the adsorption of N-octyl perfluorooctanesulfonamide
in some representative organic solvents including n-tetradecane,
n-hexadecane, DMF, DMSO and nitromethane. As shown in Fig. 5,
the surface tension curves (g-log c curves) of N-octyl perfluor-
ooctanesulfonamide solutions in five organic solvents at 25 8C, are
similar to those of common surfactants in aqueous solution. The
initial decrease of the surface tension is followed by an abrupt
change in the slope of the surface tension curve. After the break
point, the surface tension of the solutions no longer changes,
suggesting the aggregation of surfactant molecules in the organic
solvent. The intersection in the surface tension curves can be
defined as critical aggregate concentrations (cac), and the
Fig. 5. Surface tension curves of C8F17SO2NHC8H17 in various organic solvents at
25 8C.
2
˚
sectional area of the fluorocarbon chain (28.3 A ) [31]. The
sulfonamide group of the surfactant molecule coexisting at the
organic solvent/air interface increases the steric hindrance
between fluorocarbon chains, thus inevitably increases the area
occupied by a single surfactant molecule at the interface. And it is
reasonable for the high Amin data calculated from N-octyl
perfluorooctanesulfonamide in n-hexadecane, nitromethane and
DMSO. For example, the Amin data (0.87 nm2) of N-octyl
perfluorooctanesulfonamide in nitromethane is basically coin-
cident with the literature data (0.84 nm2) of perfluorinatedoctyl
ethanesulfonate in nitromethane [11]. However, a high Amin data
suggest incompact adsorption of fluorocarbon chains at interface
or incomplete surface coverage by a fluorocarbon film, even
though the fluorinated chains are oriented away from the liquid
surface. When N-octyl perfluorooctanesulfonamide dissolves in
DMF, it is evident that Gmax values are the minimum, cac is the
maximum, and Amin is the maximum (even up to 1.33 nm2). These
results may be due to the excessive solubility of the surfactant in
DMF to prevent the close packing of fluorocarbon chains adsorbed
at the interface. Therefore, the adsorbed molecules fail to form
close-packed condensed monolayer even at cac. At the same time,
the fluorinated groups probably lie flat on the surface when they
are adsorbed.
corresponding surface tension is defined as
gcac. The main
difference between these curves and the analogous curves for
common surfactants in aqueous solutions is that the initial surface
tensions of the pure organic solvents are much lower than that of
pure water.
The values of cac and gcac for C8F17SO2NHC8H17 in various
organic solvents at 25 8C are shown in Table 2. It is evident that the
cac values increase and then decrease with an increase of polarity
of organic solvents, which agrees with the idea that excessive
solubility increases the concentration to obtain minimum surface
tension.
To investigate the adsorbing state of surfactant molecules at the
organic solvent/air interface, the maximum surface excess con-
centration,
Gmax, and the area occupied by a single surfactant
molecule at the organic solvent/air interface, Amin, were both
estimated from the Gibbs adsorption isotherm of nonionic
surfactants [11] and also shown in Table 2.
ꢀ
ꢁ
1
@g
Gmax ¼ ꢀ
(2)
RT
@lnC
T
For conventional surfactants in water, the driving force for
micellization and adsorption derives from the unfavorable contact
between water and the hydrocarbon chain, called the hydrophobic
effect of surfactant. The magnitude of this hydrophobic effect can
be quantified in terms of the standard free energy of transfer of the
hydrocarbon chain from bulk alkane to water. In organic solvents,
the adsorption and aggregation of an organic solvent-soluble
surfactant can be attributed to the solvophobic driving force of the
fluorocarbon chain. And the magnitude of the driving force can be
estimated from the approximate Eq. (4) [30,32].
1014
Gmax ꢁ NoÞ
Amin
¼
(3)
ð
where C is the concentration of surfactant, (
@g
/@
ln C)T is the slope
in the surface tension isotherm when the concentration is near the
cac. R is the gas constant (8.314 J molꢀ1 Kꢀ1), T is the absolute
temperature and No is the Avogadro constant.
The maximum surface excess concentration (Gmax) reflects the
degree of packing and orientation of the adsorbed surfactant
molecules at the interface. As shown in Table 2, the values of Gmax
in n-tetradecane, n-hexadecane, nitromethane and DMSO is higher
than that in DMF, therefore, the area occupied by a single
surfactant molecule at organic solvent/air interface (Amin) of these
four systems is found to be smaller than that at DMF/air interface. A
larger Gmax means that there are more surfactant molecules
adsorbed on the surface of the solution, which also means a lower
surface tension. The lowest value of Amin obtained from n-
tetradecane system is 0.44 nm2, which suggests adsorption of
surfactant with the solvophobic fluorocarbon chain oriented away
from the liquid in a more tilted position, approaching to the cross
u
D
Gagg ¼ ꢀRTln S
(4)
where S is the solubility of the surfactant in organic solvents in
u
mole fraction units and
DGagg is the standard free energy of
u
aggregation. As illustrated in Table 2, the values of
D
Gagg in n-
tetradecane, n-hexadecane, nitromethane and DMSO are higher
than that in DMF, indicative of higher solvophobic driving force of
the fluorocarbon chain in n-tetradecane, n-hexadecane, nitro-
methane and DMSO. However, the driving force of aggregation and
adsorption in DMF are lower than n-alkanes in spite of an increase

680
G.-L. Li et al. / Journal of Fluorine Chemistry 130 (2009) 674–681
Table 2
Surface properties of C8F17SO2NHC8H17 in various organic solvents at 25 8C.
u
Solvent
Dielectric constant [36]
0
cac (mol lꢀ1
)
gcac (mN mꢀ1
)
Gcac (mol cmꢀ2
)
Amin (nm2)
D
GaggðkJ=molÞ
n-tetradecane
n-hexadecane
Nitromethane
DMF
0.0065
0.010
0.031
0.24
19.1
19.8
25.6
20.9
22.8
3.77 ꢃ 10ꢀ10
2.15 ꢃ 10ꢀ10
1.79 ꢃ 10ꢀ10
1.25 ꢃ 10ꢀ10
2.04 ꢃ 10ꢀ10
0.44
0.77
0.87
1.33
0.81
15.81
14.44
15.85
9.89
35.9c
36.7b
46.9a
DMSO
0.0067
18.88
a
20 8C.
25 8C.
30 8C.
b
c
in the polarity and surface tension of the pure organic solvent. That
is, polarity of the organic solvents cannot fully explain the extents
of the spontaneous process of aggregation and adsorption. Instead,
the solvophobic driving force highly depends upon the proper
solubility and extent of association of the solute and solvent
[33]. All the amines including ethylamine (alcohol solution), n-
butylamine, n-hexylamine, n-octylamine, n-decylamine, dimethy-
lamine (alcohol solution), diethylamine, di-n-propylamine, di-n-
butylamine, triethylamine, cyclohexylamine, aniline, 1,6-hexane-
diamine and 1,10-diaminodecane were of AR grade from
Sinopharm Chemical Reagent Beijing Co., Ltd. n-Dodecane, n-
tetradecane and n-hexadecane were products of Haltermann, their
purities were 97%, 99% and 99% respectively. Cyclohexane, toluene,
m-xylene, ethanol, 2-butanone, nitromethane, DMF, DMSO, ethyl
acetate and isopropyl ether were of AR grade and liquid paraffin
was of chemical grade from Sinopharm Chemical Reagent Beijing
Co., Ltd. The water for the experiment was secondary distilled
water.
molecules. It is well worth noting that Eq. (4) is applicable for those
u
systems with a high aggregation number. Hence,
D
Gagg maybe an
estimated value due to an unknown aggregation number of the
surfactant systems at present.
3. Conclusions
In this work, a series of perfluoroalkyl-sulfonamide derivatives
were designed and synthesized. These compounds can reduce the
surface tension of polar and non-polar organic solvents, exhibit
surfactant properties in various organic solvents, and break a new
path for the diversified uses of fluorinated surfactants. The
relationships between surface activity and molecular structure
of surfactants as well as the property of organic solvents are quite
complicated owing to the structural diversity of both the
fluorinated surfactants and the organic solvents. Proper balance
between solvophilic and solvophobic segment can be seen as a
function of solubility, which is of importance to obtain the
maximum decrease of surface tension of organic solvents. In
general, it was found that long chain n-alkane solvents are
beneficial to increase the surface activity of these fluorinated
surfactants because of stronger incompatibility between the
fluorocarbon segment of the surfactant and the solvent molecules.
And polar organic solvent can improve solubility and surface
activity of these polar surfactants. Among these synthesized
4.2. General procedure for the synthesis of fluorocarbon surfactants
[34]
The surfactants C8F17SO2NHR and C8F17SO2NH(R)2 were pre-
pared by the following reactions:
C8F17SO2F þ NH2R þ NðC2H5Þ3 ! C8F17SO2NHR þ ½NHðC2H5Þ3ꢂF
R ¼ ðCH2ÞnHðn ¼ 2; 4; 6; 8; 10Þ; C6H11; C6H5
C8F17SO2F þ NHðRÞ2 þ NðC2H5Þ3 ! C8F17SO2NðRÞ2 þ ½NHðC2H5Þ3ꢂF
R ¼ ðCH2ÞnHðn ¼ 1; 2; 3; 4Þ
The reaction was carried out in a reactor of 500 ml volume
equipped with stirrer, thermometer, reflux condenser and
dropping funnel. Fluorinated surfactants were typically synthe-
sized by reaction of perfluorooctanesulfonyl fluoride (0.05 mol)
with an excess of the respective alkyl amine (0.075 mol) or
dialkyl amine (0.075 mol), with triethylamine (0.075 mol) as a
base. All materials were used after pre-drying by anhydrous
magnesium sulfate. Perfluorooctanesulfonyl fluoride was slowly
dropped into the mixed solution of amine and triethylamine,
controlling the mole ratio of perfluorooctanesulfonyl fluoride to
amine and triethylamine as 1:1.5:1.5. Isopropyl ether (50 ml)
was used as the solvent. The mixture was refluxed for 4 h. Solvent
and excess amines were removed by distillation under reduced
pressure, and the residue was washed with dilute hydrochloric
acid and water successively. The washing times were determined
by monitoring the surface tension of the washing water
(supernatant liquid). Once the surface tension of washing water
remains constant and does not increase with the increasing of
washing times, the desired pure product is obtained. For
example, pure N-butyl perfluorooctanesulfonamide (identified
by Elemental analysis, NMR spectra and IR spectra) was obtained
after seven times of water washing to remove the high surface
active impurity, and the surface tension of washing water
remains about 53 mN/m at 25 8C. Finally, the products were dried
in a vacuum.
surfactants, C8F17SO2N(CnH2n+1 2 (n = 1, 2, 3, 4) shows lower
)
surface activity than C8F17SO2NHCnH2n+1 (n = 2, 4, 6, 8) with an
equal carbon number of the solvophilic group due to enlarging
steric hindrance between surfactant molecules. Similarly,
C8F17SO2NHC6H11 and C8F17SO2NHC6H5 exhibit lower surface
activity than C8F17SO2NHC6H13 in organic solvents (except for
cyclohexane and aromatic solvents).
The surface tension-concentration curves of N-octyl perfluor-
ooctanesulfonamide in various organic solvents exhibit a break
point like the critical micelle concentration of ordinary surfactants
in aqueous solutions. A quiet compact adsorption of fluorocarbon
chains was found at n-tetradecane solution surface, while the
fluorocarbon chains probably lay flat on the DMF solution/air
interface resulting in an incomplete surface coverage by fluor-
ocarbon film.
4. Experiments
4.1. Materials
Perfluorooctanesulfonyl fluoride was purchased from Aldrich,
containing 74% of the linear isomer and 28% of branched isomers

G.-L. Li et al. / Journal of Fluorine Chemistry 130 (2009) 674–681
681
The surfactants C8F17SO2NH(CH2)nNHO2SC8F17 were prepared
Appendix A. Supplementary data
by the following reactions:
2C8F17SO2F þ H2NðCH2ÞnNH2 þ 2NðC2H5Þ3
! C8F17SO2NHðCH2ÞnNHO2SC8F17 þ 2½NHðC2H5Þ3ꢂF
n ¼ 6; 10
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jfluchem.2009.05.006.
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because of the excellent solubility of 1,10-Diaminododecane in it.
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Acknowledgements
This project was financially supported by National Natural
Science Foundation of China (No. 20573007, 20773081) and
Natural Science Foundation of Shandong province (Z20071306).
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