Micelle Formation in Liquid Ammonia

Article abstract of DOI:10.1021/acs.joc.5b00830

Perfluorinated long chain alkyl amides aggregate in liquid ammonia with increasing concentration which reflects micelle-type formation based on changes in ¹⁹F NMR chemical shifts. The critical micelle concentrations (cmc) decrease with increasing chain length and give Kleven parameters A = 0.18 and B = 0.19. The micelles catalyze the ammonolysis of esters in liquid ammonia. The corresponding perfluorinated long chain alkyl carboxylates form ion pairs in liquid ammonia, but the equilibrium dissociation constants indicate favorable interactions between the chains in addition to the electrostatic forces. These perfluorinated carboxylates form micelles in aqueous solution, and their cmc's generate a Kleven B-value = 0.52 compared with 0.30 for the analogous alkyl carboxylates. The differences in hydrophobicity of CH₂ and CF₂ units in water and liquid ammonia are discussed, as is the possible relevance to life forms in liquid ammonia.

Full text of DOI:10.1021/acs.joc.5b00830

Article
pubs.acs.org/joc
Micelle Formation in Liquid Ammonia
Joseph M. Griffin, John H. Atherton, and Michael I. Page*
IPOS, The Page Laboratories, Department of Chemical and Biological Sciences, The University of Huddersfield, Queensgate,
Huddersfield, HD1 3DH, United Kingdom
S
* Supporting Information
ABSTRACT: Perfluorinated long chain alkyl amides aggregate in liquid ammonia
with increasing concentration which reflects micelle-type formation based on
changes in 19F NMR chemical shifts. The critical micelle concentrations (cmc)
decrease with increasing chain length and give Kleven parameters A = 0.18 and B =
0.19. The micelles catalyze the ammonolysis of esters in liquid ammonia. The
corresponding perfluorinated long chain alkyl carboxylates form ion pairs in liquid
ammonia, but the equilibrium dissociation constants indicate favorable interactions
between the chains in addition to the electrostatic forces. These perfluorinated
carboxylates form micelles in aqueous solution, and their cmc’s generate a Kleven B-
value = 0.52 compared with 0.30 for the analogous alkyl carboxylates. The
differences in hydrophobicity of CH2 and CF2 units in water and liquid ammonia
are discussed, as is the possible relevance to life forms in liquid ammonia.
The critical micelle concentration (cmc) of surfactants is greatly
influenced by the nature of solvent, in particular its polarity/
dielectric constant so that, generally, the cmc decreases with
increasing solvent polarity/higher dielectric constant. In the
extreme, as the solvent polarity is reduced, solvent−surfactant
tail interactions are so favorable and interaction between head
group−solvent interactions are so unfavorable that reverse
micelle formation is promoted.8
INTRODUCTION
■
It is a commonly held assumption that if life exists beyond
Earth it is carbon-based with water as the preferred solvent and
utilizes fuels and light as sources of energy to sustain itself.1
Other suppositions are that life originates quickly if it has the
opportunity to do so, but the evolution of multicellularity and
intelligence takes a long time and, maybe, is an improbable
event so that any extra-terrestrial life is probably unicellular and
microbial.2 Although the definition of life is not easy,3 one
characteristic seems to be that it is liquid based with a self-
organizing and bounded local environment not at equilibrium
with its surroundings. In fact, the formation of cells appears to
be the basis of all living systems characterized by cell
membranes formed from the aggregation of monomeric
amphiphiles.4 A major distinction between living and nonliving
systems is the presence of biomembranes which establish a
boundary enabling different solute concentrations within and
outside the cell. The cells of all living organisms on Earth use a
selectively permeable membrane to preserve the high free
energy state of the system and to encapsulate and confine high
concentrations of interacting solutes.
The aggregation of amphiphiles in liquid solution with
increasing concentration forms monolayers at the interface of
the liquid, micelles, and bilayers to give vesicles.5 Water is
usually assumed to be the universal solvent for life as we know
it, as its properties are ideal for the environmental conditions
on Earth, in particular the hydrophobicity and hydrophilicity of
amphiphilic compounds that favor aggregation in water. In
general, the essential condition for solvophobicity is that
solvent−solvent interactions be much stronger than solute−
solvent ones.6 The formation of aggregates such as micelles in
nonaqueous media and those of reverse micelles from common
surfactants in nonpolar, organic solvents are well documented.7
The possibility of life based on liquid ammonia has a long
history,9,10 and it has been suggested that metabolism in liquid
ammonia is conceivable.11 Liquid ammonia has some properties
similar to water such as the ability to dissolve a diverse range of
compounds, and it is excellent at solubilizing polar molecules
but not long chained, nonpolar hydrocarbons;12 so surfactant
molecules in liquid ammonia may behave in an analogous
manner to that in water. The boiling point of liquid ammonia is
−33 °C;13 it acts like a typical dipolar aprotic solvent at 25 °C,
and at a pressure of 10 bar, and can be used to study a wide
variety of chemical reactions over a range of temperatures.14
The dielectric constant of liquid ammonia is only 16.0 at 25
°C,15 but many salts and organic compounds have good
solubility.16,17 However, some properties of liquid ammonia are
very different from those of water. Although ammonia is a good
H-bond acceptor and its lone pair can strongly solvate
cations,18 it is not a good hydrogen bond donor.19 The
solvation of anions is weak,20 and anionic nucleophiles in liquid
ammonia are therefore more reactive than in water.21 The
parameters used to characterize these properties of liquid
ammonia, such as its normalized donor number,22 and its
autoprotolysis constant highlight significant differences from
Received: April 14, 2015
© XXXX American Chemical Society
A
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water.23 Salt solutions in liquid ammonia generally have greater
electrical conductivity than an aqueous solution of the same
salt.24 Liquid ammonia is about 7-fold less viscous than water at
20 °C,25 so rates of diffusion are greater in liquid ammonia.
Although at atmospheric pressure the temperature range over
which ammonia is liquid is only 45 °C, at higher pressures it is
greater than that of water; for example, at 60 bar it is 175 °C.26
Liquid ammonia could present an opportunity for microbial life
on numerous colder bodies in the solar system and perhaps in
Jovian-type planets where the extreme pressures would make
ammonia liquid even at higher temperatures.27
NMR chemical shifts of 16.9 mM perfluorononanamide in
liquid ammonia at 25 °C, relative to ammonium trifluoroacetate
as an internal standard at δ = −75.51, are for the ω terminal
CF3 triplet −82.2 ppm, with JFF 10.6 Hz, and for the α CF2
triplet at −120.0 ppm, with JFF 12.2 Hz. However, these
chemical shifts vary with the concentration of the perfluori-
nated alkyl amide in liquid ammonia. The chemical shift of the
terminal CF3 group in perfluorononanamide is relatively
constant at low concentrations (9−32 mM) but above this
concentration decreases with increasing concentration of the
surfactant (Figure 1). This behavior is typical of micelle
Herein, it is demonstrated that some surfactants form micelle
type structures in liquid ammonia presumably with “ammono-
philic” head groups exposed to the ammonia solvent while the
“ammonophobic” tails orientate themselves away from the bulk
ammonia environment. Comparisons are made with analogous
data in aqueous solution. There is also evidence of micelle type
catalyzed reactions in liquid ammonia.
RESULTS AND DISCUSSION
■
A g g r e g a t i o n o f P e r fl u o r o a l k y l A m i d e s
CF3(CF2)nCONH2 in Liquid Ammonia. Some properties of
fluorinated surfactants are significantly different from those of
their hydrocarbon analogues. The surface tensions of aqueous
solutions of fluorocarbon surfactants at concentrations above
their cmc are much lower than those of analogous hydrocarbon
surfactants.28 The small size of the fluorine atom renders
fluorocarbon chains more “rigid” than a hydrocarbon chain.29
The enhanced hydrophobicity of fluorocarbon chains is
reflected in the fact that each CF2 group has a similar effect
on the cmc in water as does 1.6 CH2 groups.30 Fluorocarbons
pack less densely on surfaces31 giving rise to poorer van der
Waals interactions with water. The free energy of hydration of
hydrophobic solutes/surfaces in water is much more dependent
on Lennard−Jones interactions than on electrostatic inter-
actions, which may not be the case for nonaqueous polar
solvents.32 The energy required for cavity formation in liquids
is an important factor in determining solubility and aggregation,
and liquid ammonia has a much lower cohesive energy density
(0.83 GJ/m3 at 298 K and 10 bar) than water (2.3 GJ/m3 at
298 K and 1 bar).33 Liquid ammonia is a good H-bond
acceptor, and therefore it was conjectured that perfluorinated
alkyl amides may form stabilized head groups in micelles or
similar aggregates.
It was noted during the synthesis of perfluorinated alkyl
amides that they showed a large degree of “frothing and
foaming” in liquid ammonia, typical of surfactant/micelle
systems and a qualitative indicator that these compounds have
surface active properties in ammonia and their potential to
aggregate into micelles was promising. Amides are well solvated
by ammonia with its relatively high normalized donor number
(DNN) of 1.52,22 and long chain amides are more soluble in
liquid ammonia than their analogous carboxylate anions. For
example, perfluorododecanoic acid is insoluble in liquid
ammonia at 5 mM, whereas perfluorododecanamide is soluble
up to 40 mM.
Figure 1. 19F-NMR CF3 chemical shift as a function of concentration
for perfluorononamide in liquid ammonia at 25 °C.
formation and aggregation.33 It is also observed with other long
chain perfluorinated alkyl amides in liquid ammonia, but not for
trifluoroacetamide, which is not expected to aggregate, and for
which the 19F NMR chemical shift for CF3 remains invariant at
δ = −76.858 0.002 up to 0.25 M.
These changes in chemical shift of the terminal −CF3 group
are consistent with a change in its environment and compatible
with a free monomer dominating at low concentrations, but
buried deep within a micelle aggregate in a nonpolar
fluoroalkane-type environment at higher concentrations. In
aqueous solutions of perfluoroheptanoate the 19F NMR
chemical shifts also decrease with increasing concentration
but the peaks became broader because the rapid exchange of
the fluorocarbon surfactant molecules between the monomer
species and micelles with an estimated mean lifetime of
10−6 s.34 This peak broadening is not observed with
perfluorinated alkyl amides in liquid ammonia which is perhaps
indicative of a much slower exchange rate and more structured
and/or larger aggregates.
A mass action approximation can be used to relate the
observed chemical shift δobs and the total surfactant molar
concentration c to the molar concentration at which micelles or
aggregates begin to form, loosely defined as the cmc, and the
chemical shift of the free surfactant monomer δm and the
chemical shift of the micellar or aggregated phase δa (eq 1).36
δobs = δa + (cmc/c)(δm − δ )
(1)
a
The aggregation of fluorinated surfactants in water has been
detected by using the sensitivity of 19F NMR chemical shifts to
the environment,34,35 and so this method was used to
investigate the behavior of perfluorinated alkyl amides
CF3(CF2)nCONH2 in liquid ammonia for which the terminal
(ω)-CF3 group, its adjacent −CF2− group, and the α carbon
−CF2− group are readily distinguishable. For example, the 19F
A plot of δobs against 1/c for perfluorononanamide in liquid
ammonia consists of two linear segments that intersect at 1/c =
1/cmc (Figure 2), and these derived cmc values for various
perfluorinated alkyl amides are given in Table 1. As expected, at
low concentrations of surfactant (c < cmc) δobs is independent
of concentration and is identified with δm, the free monomer
B
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adjacent to the amide carbonyl and the CF2 next to the terminal
CF3, yielded similar results (Figures 4 and 5). The relative
Figure 2. 19F NMR chemical shift of the ω terminal CF3 group as a
function of the reciprocal of concentration for perfluorononanamide in
liquid ammonia at 25 °C.
Figure 4. 19F NMR chemical shift of α CF2 group as a function of the
reciprocal of the concentration of perfluorodecanamide in liquid
ammonia at 25 °C.
Table 1. Cmc Values of Perfluoroalkyl Amides
CF3(CF2)nCONH2 in Liquid Ammonia at 25°C
surfactant
n + 1
cmc (mM)
perfluoroheptanamide
perfluorooctanamide
perfluorononanamide
perfluorodecanamide
6
7
8
9
104
66
42
27
chemical shift. At higher concentrations (c > cmc), extrap-
olation to 1/c = 0 gives the chemical shift of the aggregate, δa.
The apparent cmc values for the perfluoroalkyl amides vary
as expected with chain length; they decrease with increasing
chain length. Klevens rule is an empirical formula37 that
proposes a linear relationship between log(cmc) and surfactant
carbon chain length n (eq 2), and it is interesting to note that
this applies to perfluorinated alkyl amides in liquid ammonia
(Figure 3). The Kleven parameters A and B are 0.18 and 0.19,
Figure 5. Chemical shift of CF2 group adjacent to the ω CF3 as a
function of the reciprocal of the concentration for perfluorodecana-
mide in liquid ammonia at 25 °C.
changes in chemical shift on going from monomer to aggregate
are given by δm − δa and are all positive (Table 2) as there is a
Table 2. 19F Chemical Shifts of Perfluorodecanamide
Monomer and Aggregate Peaks in Liquid Ammonia at 25°C
CF group
δm
δa
δm − δa
α CF2
−120.06
−127.44
−82.21
−120.20
−127.77
−82.63
0.14
0.33
0.42
CF2(CF3)
ω CF3
decrease in chemical shift on going from monomer to
aggregate. The monomers are surrounded by a relatively
polar ammonia environment whereas those in the aggregated
micelle are surrounded by neighboring fluorine atoms from the
adjacent hydrophobic chains. The magnitude of δm − δa
increases with increasing distance from the amide head
group. This is compatible with micelle formation, as it suggests
that the further along the chain then the deeper the fluorinated
chain is buried into the micelle structure. Similar changes in δm
− δa that are dependent on the position of the fluoro group in
the chain are seen for the other perfluoro alkyl amides, with the
magnitude increasing with increasing chain length, which again
Figure 3. Plot of log(cmc) as a function of the number of carbons in
the hydrophobic tail for a series of perfluorinated alkyl amides in liquid
ammonia at 25 °C.
respectively, and are discussed and are later compared with
parameters obtained for perfluoroalkyl carboxylic acids in water.
log(cmc) = A − Bn
(2)
As well as using the changes in 19F NMR chemical shifts of
the terminal CF3 group of the perfluoro alkyl amides to identify
aggregation, those of other residues, particularly the α CF2
C
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is compatible with greater shifts being seen the deeper the
reporter group is buried inside the micelle. A simple model
envisages the head group amides H-bonded to each other and
solvated by liquid ammonia with the fluorocarbon chains
packed into an aggregate (Scheme 1).
Scheme 2
this is reflected in conductivity data. For example, whereas the
conductivity of ammonium chloride in water increases linearly
with concentration, that in liquid ammonia is nonlinear (Figure
7). The observed conductivity κ can be related to the molar
Scheme 1
Conductivity of Ionic Surfactants in Liquid Ammonia.
Changes in conductivity with concentration are often used as
analytical tools to identify aggregation, and so such measure-
ments of ionic solutes in liquid ammonia were performed but at
−15 °C due to the pressure limit on the conductivity cell. The
change in conductivity with the concentration of ionic
surfactants such as perfluorinated carboxylates, sodium dodecyl
sulfate (SDS), and Aerosol OT (dioctyl sodium sulfosuccinate)
in liquid ammonia are all nonlinear; however, they do not show
the distinct break expected if aggregation occurs (Figure 6).
Figure 7. Conductivity as a function of the concentration of
ammonium chloride in liquid ammonia at −15 °C; the continuous
line is calculated using eq 3.
conductance Λ using the mass balance, and the equilibrium
constant for dissociation of the ion-pair formation Kdiss (eq 3)
and the derived values are given in Table 3.
2
−
−Kdiss
+
Kdiss − (4 × Kdiss × [A ]tot)
κ = λ + Λ
(3)
2
Although there is no strong evidence for aggregation of the
ionic surfactants, there does appear to be some favorable
interactions between the perfluorinated alkyl chains in liquid
ammonia. As the chain length of the perfluorinated alkyl
Table 3. Equilibrium Constants Kdiss, Kip, and Molar
Conductance Values ε for Ion Pairing of Surfactants and
Salts in Liquid Ammonia at −15°C
103·Kdiss
Kip
Salt/surfactant
(M)
(M−1
)
Λ (Sm−1M−1
)
Figure 6. Conductivity as a function of the concentration of
perfluorooctanoic acid in liquid ammonia at −15 °C; the continuous
line is calculated using eq 3.
NH4Cl
TBAB
31.09
15.8
6.06
5.90
12.9
9.18
6.89
4.18
2.36
0.26
1.43
1.49
32.2
63.3
9.82
17.3
23.0
17.6
28.1
19.8
21.5
23.2
24.0
51.0
35.2
49.3
Aerosol OT
165.0
169.5
77.5
Although this behavior is similar to that observed for a weak
electrolyte, such as acetic acid in water, the data for the
surfactants in liquid ammonia do not fit with Ostwald’s dilution
law or Kohlrausch’s law for fully ionized species (see
Supporting Information). The data appear more compatible
with ion-pair formation in liquid ammonia which is only
moderately polar compared with water (the dielectric constant
of ammonia = 16.7 compared with 80 for water at 25 °C). Ion-
pair formation (Scheme 2) in liquid ammonia is thus expected
to be much more favorable compared with that in water, and
Sodium dodecyl sulfate
Trifluoroacetic acid (C2)
Perfluorooctanoic acid (C8)
Perfluorononanoic acid (C9)
Perfluorodecanoic acid (C10)
Perfluoroundecanoic acid (C11)
Perfluorododecanoic acid (C12)
Pefluorooctanesulfonate (Et4N)
108.9
145.1
239.2
423.7
3914
699.3
671.1
Decyltrimethyl ammonium iodide
(DTAI)
D
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carboxylate surfactants increases, Kdiss decreases (50-fold from
C2 to C12) (Table 3) with the most dramatic change occurring
between C11 and C12. This indicates that more than just
electrostatic interactions between the head groups and
counterion contribute to the energetics of ion pairing. It may
suggest that the longer chained ionic perfluoro surfactants are
forming higher aggregates such as dimers or trimers, although
not forming typical larger micelle type structures. A possible
reason why the ionized perfluorinated carboxylates, unlike the
analogous perfluorinated amides, do not show significant
aggregation/micelle formation in liquid ammonia is the
expected greater repulsion between the similarly charged head
groups due to the low dielectric constant of ammonia.
Scheme 3
Micelle Catalyzed Ammonolysis of Esters. Changes in
organic reactivity in aqueous solutions of micelles38 have been
used to explore the structure of micelles themselves and as
models for simple cells.39 The interior of micelles in water and
the head group/solvent interface act as pseudophases distinct
from the bulk solvent and can accelerate or inhibit reactions,
depending on the mechanism and the rate constants and
reactant concentrations in the two regions.40 It is therefore of
interest to explore organic reactivity in liquid ammonia in the
presence of aggregates/micelles. The solvolysis of a series of
alkyl benzoates and alkyl phenylacetates and aryl benzoates in
liquid ammonia gives the corresponding amide and alcohol/
phenol.41 The observed pseudo-first-order rate constant for the
ammonolysis of propargyl benzoate in liquid ammonia at 25 °C
was studied over a range of perfluorononanamide concen-
trations and showed no change in the rate up to 38 mM
surfactant but then increases at higher concentrations
(Supporting Information). The rate increase coincides with
concentrations around the cmc obtained from the NMR
studies, 42 mM. There is a 14-fold increase in the rate in the
presence of 90 mM perfluorononanamide, but trifluoroaceta-
mide has no effect. The modest rate enhancement seen in the
presence of the surfactant is typical of the magnitude seen in
aqueous micelle catalyzed reactions and further evidence of
aggregation of the perfluoroalkyl amides in liquid ammonia.
The rate limiting step in the uncatalysed solvolysis reaction
involves late formation of the zwitterionic tetrahedral
intermediate or a reaction of the neutral tetrahedral
intermediate with little C−OR bond fission in the transition
state.40 Presumably, this transition state is stabilized at the
micelle−ammonia interface either because of increased polarity
of the region or due to direct H-bond interaction between the
amide and the tetrahedral intermediate (Scheme 3).
Figure 8. Conductivity of perfluorooctanoic acid as a function of
concentration in water at 25 °C.
perfluorooctanoate is 9.3 mM, similar to the value obtained
from surface tension.29,42 The cmc values of other perfluoralkyl
−
carboxylates CF3(CF2)nCO2 in water were similarly obtained
using conductivity data (Table 4). As expected, the longer the
Table 4. Cmc’s of Perfluoralkyl Carboxylates
−
CF3(CF2)nCO2 in Water at 25°C
Surfactant
cmc (mM)
Surfactant
cmc (mM)
Aggregation of Perfluoralkyl Carboxylates
a
a
CF3(CF2)5COO−H+
CF3(CF2)6COO−H+
CF3(CF2)7COO−H+
CF3(CF2)8COO−H+
CF3(CF2)9COO−H+
CF3(CF2)10COO−H+
25
CF3(CF2)5COO−Na+
CF3(CF2)6COO−Na+
CF3(CF2)7COO−Na+
CF3(CF2)8COO−Na+
CF3(CF2)9COO−Na+
CF3(CF2)10COO−Na+
83
−
CF3(CF2)nCO2 in Water. In order to compare the data
a
9.3
3.4
35
obtained for the aggregation of perfluoralkyl amides
11.8
3.1
CF3(CF2)nCONH2 in liquid ammonia, the behavior of the
−
1.1
analogous perfluoralkyl carboxylates CF3(CF2)nCO2 in water
0.23
0.11
0.55
0.2
was examined using conductivity. Perfluorooctanoic acid is a
strong acid with a pKa of ∼0 and hence is fully ionized in water
behaving as a strong electrolyte; a 5 mM solution in water
generates a pH of 2.4. The change in conductivity of increasing
concentrations of perfluorooctanoic acid in water at 25 °C
shows the typical behavior of aggregation to form micelles
(Figure 8).
As the concentration of the perfluorooctanoic acid surfactant
is increased, an inflection in the conductivity isotherm is
observed at the point of micellization when the thermodynami-
cally stable aggregates form, and this concentration can be
assigned as the cmc of the surfactant in water. The cmc of
a
W. Guo, T. Brown and B. Fung, J. Phys. Chem. 1991, 95, 1829−1836.
hydrophobic tail of the surfactant, the lower the cmc value
which follows (Figure 9) the empirical Klevens rule36 (eq 2).
For the hydronium salts of the perfluoralkyl carboxylates, the
constant A = 1.38 and reflects the specific nature of the
hydrophilic head group and its interactions with the solvent,
which shows the relative effect on the cmc of each additional
−CF2 unit. For the corresponding sodium salts, A = 2.31 and B
= 0.54. These A value for the analogous alkyl carboxylates is
E
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claim that life processes could take place in this solvent, as it is
difficult to envisage how such perfluorinated systems could
readily be formed. The micelles catalyze the ammonolysis of
esters in liquid ammonia. The corresponding perfluorinated
long chain alkyl carboxylates form ion pairs in liquid ammonia,
but the equilibrium dissociation constants indicate favorable
interactions between the chains in addition to the electrostatic
forces. These perfluorinated carboxylates form micelles in
aqueous solution, and their cmc’s generate a Kleven B-value =
0.52 compared with 0.30 for the analogous alkyl carboxylates.
There are significant differences in solvophobicity of CH2 and
CF2 units in water and liquid ammonia. The free-energy of
transfer of a CF2 group from liquid ammonia to its aggregate is
less than half the value of the corresponding transfer from
water. The effect of introducing extra −CF2− groups into the
fluorinated surfactant chain has a greater effect on the cmc due
to the greater hydrophobicity of fluorocarbons: one −CF2−
group is approximately equivalent to 1.73 −CH2− units.
Figure 9. Klevens plot of log(cmc) (M) as a function of the number of
carbon atoms in the hydrophobic tail for a series of hydronium salts of
perfluorinated carboxylates in water at 25 °C.
1.80, whereas the B-values for the perfluoralkyl carboxylates
(0.52 0.02) are significantly greater than the 0.30 seen for the
alkyl derivatives.43
EXPERIMENTAL SECTION
■
Synthesis. Decyl Trimethylammonium Iodide (DTAI). N,N-
Dimethyldecylamine (2 mL, 0.00841 mol) was reacted with excess
methyl iodide (5 mL, 0.0807 mol) at room temperature for 1 h. The
white solid product was washed repeatedly with dichloromethane and
filtered under vacuum. Excess solvent was removed by oven drying to
constant mass (90 °C, 72 h). Mass (yield %) = 2.41 g (87%). GC-MS
analysis showed no methyl iodide or N,N-dimethyldecylamine present.
1H NMR (D2O) δ ppm 0.90 (3H, t), 1.3 (14H, m), 1.81 (2H, t), 3.10
(s, 9H), 3.30 (2H, t). ESI-MS m/z 200.2375.
Perfluorinated Amides. Long chained perfluorinated carboxylic acid
methyl ester (2.0 g, 0.005 mol) was reacted in liquid ammonia (20
mL) at room temperature for 24 h. Ammonia was allowed to
evaporate, and the white powdered product was solubilized in
dichloromethane and washed with water. After drying and rotary
evaporation to remove the solid, perfluorinated amide was oven-dried
to constant mass (90 °C, 48 h). Mass amide (yield %): C7 = 1.54 g
(88%); C8 = 1.82 g (89%); C9 = 2.28 g (93%);C10 = 1.82 g (86%).
GC-MS showed purity was >99%. ESI-MS: perfluoroheptanamide m/z
364.0006[M + H]+; perfluorooctanamide m/z 413.9974[M + H]+;
perfluorononanamide m/z 481.0214[M + NH4]+; perfluoro-
decanamide m/z 513.9908[M + H]+. 19F NMR (DMF with
trifluoroacetic acid as the internal standard set to δ = −75.5100
ppm) δ ppm: perfluoroheptanamide −80.4 (3F, t), −118.4 (2F, t),
−121.3 to −122.3 (6F, m), −125.7 (2F, m); perfluorooctanamide
−80.6 (3F, t), −118.3 (2F, t), −121.3 to −122.3 (8F, m), −125.7 (2F,
m); perfluorononanamide −80.8 (3F, t), 118.6 (2F, t), −121.4−122.4
(10F, m), −125.8 (2F, m); perfluorodecanamide −80.8 (3F, t),
−118.6 (2F, t), −121.5 to −122.5 (12F, m), −125.9 ppm (2F, m).
Preparation of Liquid Ammonia Surfactant Solutions. The
equipment and methodology for charging the vessel with liquid
ammonia was as previously described.41
The B-values for alkyl chains in water are constant for a
variety of surfactants with different polar head groups; for alkyl-
N-methylpyrrolidinium bromides, B = 0.30, and for alkyl-1-
sulfonates, B = 0.29.36 This reflects that the cmc is roughly
halved with each additional methylene unit added. However,
the effect of introducing extra −CF2− groups into the
fluorinated surfactant chain has a greater effect on the cmc
due to the greater hydrophobicity of fluorocarbons, such that
one −CF2− group is approximately equivalent to 1.73 −CH2−
units; a fluorosurfactant with 8 carbons behaves like a
hydrocarbon surfactant with 14 carbons. It is interesting to
compare these B-values with those for the perfluorinated alkyl
amides in liquid ammonia (Table 5).
Table 5. Klevens B-values for Various Surfactants in Water
and Liquid Ammonia
Surfactant chain type/solvent
Alkyl carboxylates in water
Klevens B-value
0.30
0.52
0.19
Perfluorinated alkyl carboxylates in water
Perfluorinated alkyl amides in liquid ammonia
The B-value of 0.19 obtained for the perfluorinated alkyl
chain in liquid ammonia is much lower than those for both the
simple alkyl and fluorinated chains in water. Effectively, the B-
values are an indication of the free-energy of transfer of the
monomer in the solvent to the aggregate micelle, so the transfer
of a CF2 group from liquid ammonia to its aggregate is less than
half of the corresponding transfer from water. This is due to the
much greater changes in the Lennard−Jones interactions
between the fluorocarbon and water compared with those in
liquid ammonia.31,44
19F NMR in Liquid Ammonia. Liquid ammonia solutions of
surfactants contained a small amount of deuterated standard (<1%)
such as DMSO-d6 for a lock and ammonium trifluoroacetate (<1%) as
the internal standard. High pressure, quick valve NMR tubes were
pressure rated to 14 bar (200 psi) designed specifically for 500 MHz
NMR instruments with a tube length of 7 in., outside diameter (O.D.)
of 5 mm, and wall thickness of 0.38 mm. The top inlet valve of the
pressure tube has a standard Swagelok connection allowing for easy
connection to the Omnifit apparatus and so practical and safe charging
of the NMR tube from the reaction vessel. The tube was cooled in an
ice−water slurry, and the liquid ammonia was transferred carefully
from the reaction vessel (at room temperature) to the chilled tube. It
was then allowed to warm to room temperature before transfer to the
NMR instrument. Spectra were recorded at a regulated temperature of
25 °C on a spectrometer operating at 470.5 MHz. 19F chemical shifts
CONCLUSION
■
Perfluorinated long chain alkyl amides aggregate in liquid
ammonia with increasing concentration, which reflects micelle-
type formation in this solvent. The critical micelle concen-
trations (cmc) decrease with increasing chain length and give
Kleven parameters A = 0.18 and B = 0.19. Although long chain
perfluoroalkyl amides aggregate and form micelles in liquid
ammonia this is probably not good evidence to support the
F
DOI: 10.1021/acs.joc.5b00830
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
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ASSOCIATED CONTENT
* Supporting Information
■
S
Data fit to other models for conductivity and data for micelle
catalysis. The Supporting Information is available free of charge
on the ACS Publications website at DOI: 10.1021/
acs.joc.5b00830.
AUTHOR INFORMATION
Corresponding Author
*E-mail: m.i.page@hud.ac.uk.
■
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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J.G. is grateful for financial support from IPOS.
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