Zwitterionic and mesoionic liquids: Molecular aggregation in 3-methylsydnone

Article abstract of DOI:10.1007/s11426-010-4062-4

Ionic liquids are green solvents with interesting properties: displaying low melting points and high boiling points. They offer a new approach applicable in many instances. Nevertheless, the presence of free ions can be a matter in some cases, e.g. for the study of nucleophilic reactions, in electrochemistry, and in each situation where there is a competition between counter ions, as in micellization of ionic surfactants. Neutral compounds having formal unit electrical charges of opposite sign, and the same physical properties than ionic liquids would be a nice alternative to these latter solvents. There are two classes of chemical compounds having these characteristics: zwitterionic liquids (with no uncharged canonical representation) and mesoionic liquids (in which the negative and the positive charges are delocalized). In that last class we have chosen to work with 3-methylsydnone in order to examine, in this aprotic solvent, if it was possible to observe aggregation of surfactants in the same manner as in water. With all kinds of surfactants studied (ionic, zwitterionic and mesoionic) we have been able to demonstrate the formation of direct micelles: hydrogen bonding is thus not mandatory for molecular aggregation. Comparison of the behavior in water and in formamide showed that solvophobic interactions were qualitatively comparable but with a lower intensity.

Full text of DOI:10.1007/s11426-010-4062-4

SCIENCE CHINA
Chemistry
•
ARTICLES •
September 2010 Vol.53 No.9: 2063–2069
doi: 10.1007/s11426-010-4062-4
Zwitterionic and mesoionic liquids:
Molecular aggregation in 3-methylsydnone
*
CASSEL Stéphanie, RICO-LATTES Isabelle & LATTES Armand
Laboratoire des IMRCP, UMR 5623, Université Paul Sabatier, 118 route de Narbonne, 31062 Toulouse, France
Received April 30, 2010; accepted June 25, 2010
Ionic liquids are green solvents with interesting properties: displaying low melting points and high boiling points. They offer a
new approach applicable in many instances. Nevertheless, the presence of free ions can be a matter in some cases, e.g. for the
study of nucleophilic reactions, in electrochemistry, and in each situation where there is a competition between counter ions, as
in micellization of ionic surfactants. Neutral compounds having formal unit electrical charges of opposite sign, and the same
physical properties than ionic liquids would be a nice alternative to these latter solvents. There are two classes of chemical
compounds having these characteristics: zwitterionic liquids (with no uncharged canonical representation) and mesoionic liq-
uids (in which the negative and the positive charges are delocalized). In that last class we have chosen to work with
3
-methylsydnone in order to examine, in this aprotic solvent, if it was possible to observe aggregation of surfactants in the
same manner as in water. With all kinds of surfactants studied (ionic, zwitterionic and mesoionic) we have been able to dem-
onstrate the formation of direct micelles: hydrogen bonding is thus not mandatory for molecular aggregation. Comparison of
the behavior in water and in formamide showed that solvophobic interactions were qualitatively comparable but with a lower
intensity.
ionic liquids, zwitterionic liquids, mesoionic liquids, 3-methylsydnone, molecular aggregation, micellization, solvophobicity
1
Introduction
ing points and high boiling points.
Ionic liquids offer numerous advantages over conven-
tional organic solvents for carrying out organic reactions.
For example: (1) Product recovery is made easier, catalysts
can be recycled, Ionic liquids can be reused, rates of reac-
tions are often enhanced and selectivity is frequently in-
creased. (2) The main interest lies in their peculiar proper-
ties: absence of flammability, lack of measurable vapor
pressure (which can be an advantage for the reduction of
VOC for example but a drawback for product separation),
good ability to dissolve organic or organometallic com-
pounds, and even some inorganic salts. (3) They are eco-
nomical solvents!
Ionic liquids, salts that are liquid near ambient conditions [1]
represent a relatively new class of materials with unique
properties. They have a wide range of potential applications,
for example as solvents for reactions and material process-
ing [2], as extraction media [3], or as working fluid for
chemical devices [4]. The interest in ionic liquids as green
solvents has grown enormously in the last few years [5].
They are convenient and economical solvents due to their
efficiencies and recyclability. But as some of them are vis-
cous, not very biodegradable and even toxic, they have to be
carefully selected according to the desired application. They
also display unique physicochemical properties: low melt-
Ionic liquids are only composed of ions. The most com-
mon classes are alkylammonium salts, alkylpyridinium salts
and N,N′-dialkylimidazolium salts. Recently, tunable aryl
alkyl ionic liquids (TAAILs) have been synthesized [6] and,
*
Corresponding author (email: lattes@chimie.ups-tlse.fr)
©
Science China Press and Springer-Verlag Berlin Heidelberg 2010
chem.scichina.com
www.springerlink.com

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CASSEL Stéphanie, et al. Sci China Chem September (2010) Vol.53 No.9
owing to their properties, authors present them as “the next
generation of Ionic Liquids”.
is operative too with ionic liquids themselves: an appropri-
ate substance may be added to finely tune the physical
properties of the system).
In many cases, the presence of ions does not play an im-
portant role on the course of reactions. However, in some
cases, ions may be an extra constraint. For example: (1) In
electrochemistry, where addition of salts is generally needed
to generate carrier ions. With ionic liquids, not only carrier
ions, but also ions of the ionic liquids migrate along with
potential gradient. (2) In the study of ionic surfactants, for
which a rather small quantity of the surfactant (and, of
course, a small quantity of the counter ions) is dissolved in a
liquid which is composed of different ions in enormous
quantity! The surfactant studied is then not the original salt
but a new surfactant resulting from the exchange of its
counter ions with those of the solvent. (3) In reactivity, as in
nucleophilic reactions, where there is a competition between
the nucleophile and the anion of the ionic liquid, etc.
For all these reasons, it would be interesting to work with
compounds having the same fascinating properties than
conventional I.L. (low melting point and high boiling point),
but without the presence of free ions. In our laboratory we
are interested in the study of surfactants in the wide way;
therefore our work will be oriented in the field of “ion-free
I.L.”. The first class of compounds answering that condition
can be represented by zwitterionic liquids.
(3) Increasing the length of the hydrocarbon spacer be-
tween cation and anion generally lowers the melting point
of the zwitterion.
Betaines are a subclass of zwitterionic compounds. A
betaine is any neutral chemical compound with both a posi-
tively charged cationic functional group: such as an ammo-
nium or a phosphonium ion which bears no hydrogen atom -
and a negatively charged functional group (such as car-
boxylate group which may not be adjacent to the cationic
site). A betaine thus may be a specific type of zwitterions;
trimethylglycine is a typical example of betaines.
Mesoionic liquids
Mesoionic compounds are a subclass of betaines. They are
essentially dipolar five- (or possibly six-) membered het-
erocyclic compounds in which both the negative and the
positive charge are delocalized, and for which a totally co-
valent structure cannot be written. Therefore they cannot be
represented satisfactorily by any sole polar structure. The
formal positive charge is associated with the ring atoms, and
the formal negative charge is associated with ring atoms or
an exocyclic nitrogen or a chalcogen atom [7]. The most
representative members of this family are five membered
mesomeric betaines: sydnones and munchnones.
Zwitterionic liquids
Zwitterionic compounds are chemicals that have a total net
charge of zero and are thus electrically neutral. They carry
formal electrical charges of opposite sign localized on dif-
ferent atoms and then can be considered as inner salts [7].
So amino acids exist mostly as zwitterions in a certain range
of pH and the pH at which the average charge is zero is
called the molecule’s isoelectric point.
With zwitterionic compounds, cations and anions are
tethered covalently. Another example of zwitterionic liquids
is imidazolium sulfonate.
Sydnones constitute a well-defined class of mesoionic
compounds: they are the most important and significant
mesoionic compounds because they possess different types
of physiological activity, depending on the substituents in the
heterocyclic ring. Sydnones find their use as anti-inflammatory,
antimicrobial, anticancer, and analgesic agents. They were
first prepared in 1935 by Earl and Mackney [9].
Owing these interesting properties we have chosen to
work with the most simple sydnone: 3-methylsydnone,
which possesses the same interesting properties than ionic
liquids, and we decided to study the aggregation behavior of
surfactants in this compound.
Many zwitterionic compounds are solid at room tem-
perature, but:
(
1) It is possible, by structural transformation, to obtain
liquids; for example, a new family of zwitterionic liquids
has been described arising from a phase transition of am-
monium salts containing an ether bond which is effective
for lowering the melting point [8].
(
2) For some of them they turned liquid at room tem-
perature by adding an appropriate compound (such strategy

CASSEL Stéphanie, et al. Sci China Chem September (2010) Vol.53 No.9
2065
Table 1 Various physicochemical parameters of water, formamide (at 25 °C) and 3-methylsydnone (at 40 °C)
1
Solvent
Water
Formamide
-Methylsydnone
mp (°C)
bp (°C/mm Hg)
100/760


/
o
 (D)
1.8
 (mN/m)
72


/ /
3
(mN/m²)
0
4
80
2750
1700
1340
210/760
109.5
144
3.7
58.5
57

2
3
36
91/5.10
7.3
3
-methylsydnone: A new mesoionic liquid solvent
In general, the Krafft point is lower in water than in other
solvent (Table 2).
There is still controversy about micellization in non aqueous
polar solvents. Compared to aqueous media, relatively few
studies on aggregation of amphiphilic molecules have been
carried out in non aqueous structured liquids.
Moreover, even if there are some aprotic ionic liquids
known to allow micelle formation, nearly all studies to date
on micellization in non aqueous polar solvents have been
carried out in protic solvents such as hydrazine [10], ethyl-
ammonium nitrate [11], glycols [12] and formamide [13]. A
convenient sydnone would be a very interesting aprotic me-
dium for such studies, but precautions have to be taken as
sydnones in general have proven to be reprotoxic com-
pounds.
We have shown that for SDS it is 39 °C and for CTAB
1
7 °C higher in formamide compared to water (Table 2). In
our opinion, Evans et al. did not observe aggregation be-
cause they worked at a too low temperature. We have been
able to detect liquid crystalline phases in the binary system
hexadecylpyridinium bromide/3-methylsydnone by optical
microscopy and small angle X-ray diffraction: we observed
two phases, the first one at 45.4 °C and the second at 55 °C
[18]. Our conclusion was: surfactant self aggregation can
thus occur in 3-methylsydnone and therefore in an aprotic
solvent.
Our purpose here is to demonstrate that molecular
aggregation is a general behavior of surfactants, to prove it
by studying the micellization of different families of surfac-
tants by different means, and to compare the results ob-
tained in 3-methylsydnone with those obtained in water and
formamide. Following that idea, molecular aggregation of
ionic, zwitterionic and mesoionic amphiphiles was studied
in 3-methylsydnone.
It can be seen from the various physicochemical charac-
teristics of 3-methylsydnone (Table 1) that this compound
corresponds to a structured polar medium which could be
exploited to investigate molecular aggregation [14]. The
highly polar nature of 3-methylsydnone is shown by its high
relative dielectric constant 

/
( = molar volume) is also elevated, indicating the
structured nature of this solvent. Although it is aprotic,
-methylsydnone does have properties favoring the aggre-
o
and dipole moment . The
1

/ /
3
3
2
Experimental
gation of amphiphiles.
Nevertheless, Evans et al. [15] have studied the variation
of the surface tension and the conductance of tetrade-
cylpyridinium nitrate and hexadecylpyridinium bromide as
well as the diffusive interfacial transport (DIT) of dihexa-
decyldimethylammonium acetate in 3-methylsydnone. These
authors did not observe any discernable modification in the
surface tension vs. concentration for neither pyridinium
surfactants nor liquid crystalline phases with DIT method
2
.1 General
Formamide (Aldrich 99% spectrophotometric grade) was
kept over molecular sieves (3 Å) and used as supplied. The
water content (< 1% for solvents and 0.05% for surfactants)
was measured by the Karl-Fischer method). Surface ten-
sions were measured using a Prolabo tensiometer (Tensiomat
N°3) using the ring detachment method. Microscopic ex-
aminations and X-ray scattering experiments have been
described elsewhere [14]. 3-Methylsydnone melts at 36 °C,
but displays surfusion, therefore measurements below that
temperature were determined in the superfused solvent.
[
16]. Evans et al. concluded: “These results suggest that
surfactant self aggregation processes do not occur in
-methylsydnone and probably not in other aprotic, polar,
3
high cohesive energy density solvents. Hydrogen bonding –
leading to solvent structure – must play a decisive role in
amphiphilic self assembly”.
But, (1) all these authors experiments with pyridinium
surfactants have been carried out at 40 °C; (2) with the dou-
ble-chained surfactant, the Krafft temperature in water is
2
.2 Synthesis of 3-methylsydnone
3
-Methylsydnone was synthesized using the procedure de-
scribed by Earl and Mackney [9].
3
4 °C.
All their experiments have been carried out at tempera-
Table 2 Krafft points (°C)
tures above the Krafft point “in water”, but this parameter is
different in non aqueous media! The Krafft point can be
defined as the temperature of fusion of solvated surfactants:
this temperature will depend on the nature of solvent [17].
SDS
CTAB
26
Water
16
55
Formamide
43

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CASSEL Stéphanie, et al. Sci China Chem September (2010) Vol.53 No.9
1
)
Preparation of N-nitrososarcosine
mol of sarcosine in alkaline solution (pH 10) were re-
[20] involving a liquid-liquid phase transfer reaction be-
tween an aqueous solution of sodium azide and an alkyl
halide in the absence of organic solvent. Previous work in
our laboratory having shown that formamide can be profita-
bly employed instead of water as solvent in various reac-
tions [21], we therefore used formamide in place of water.
The alkyl bromide (8 mmol) was added to a 25% sodium
2
fluxed for 15 min to remove methylamine. The solution was
cooled to 0 °C, and 2 mol of sodium nitrate dissolved in 80
mL of water were then added dropwise. The mixture was
acidified with hydrochloric acid to pH 2. The reaction mix-
ture was left to stand for 3 d under stirring at room tem-
perature. The solution was extracted six times with diethyl
4
and evaporating the ether, a
pale yellow liquid, N-nitrososarcosine, was obtained (yield
8%; IR 3500 (OH), 1750 (CO), 1470 cm (NN=O)).
®
azide (16 mmol) solution in formamide. Aliquat 336 (0.4
mmol) was then added to the mixture which was heated to
ether. After drying over MgSO
1
00 °C for 6 h under stirring. After extraction with diethyl
1
ether and distillation, the products were identified by ele-
mental analysis, NMR and IR spectroscopy. Boiling points
were in agreement with literature data (yields: 98%–99%).
N-Heptylazide: bp 75 °C/21 mbar. N-Decylazide: bp
1
Preparation of 3-methylsydnone
× 101 mol of N-nitrososarcosine in 200 mL of freshly
5
6
2 °C/0.02 mbar. N-Dodecylazide: bp 71 °C/0.03 mbar.
distilled acetic anhydride was refluxed for 15 min in a round
bottomed flask. The reaction starts rapidly and was carried
out without heating, but in the dark. The acetic anhydride
was evaporated under vacuum (T < 60 °C) and the residue
distilled under reduced pressure. 3-Methylsydnone was re-
covered as a pale yellow liquid (yield 60%; mp: 36 °C; bp:
3
Results and discussion
3
.1 Ionic surfactants
1 °C/5 × 10 mmHg; IR: 1750 cm (C=O); H NMR
CDCl )  3.91 (3p, s, CH N); 6.35 (1p, s, CH). Anal. calcd
2
1
1
Investigations of the 3-methylsydnone/cetylpyridinium
bromide (CPBr: Ia) and 3-methylsydnone/cetyltrimethyl-
ammonium bromide (CTAB: Ib) binary systems by optical
microscopy and small angle X-ray diffraction gave different
results. The examination of the system with CPBr under
polarizing conditions showed the presence of ordered la-
mellar and hexagonal phases. By contrast, optical micro-
scopic observations have shown only the presence of both
isotropic and lamellar phases in the CTAB/3-methylsydnone
system. X-ray diffraction studies confirmed these results
and the existence of micelles only for the system with
CPBr.
CTAB and CPBr differ only in the nature of their polar
heads which do not interact in the same way as with the
sydnone. This factor, which also depends on the nature of
the solvent, is ill-defined. Polar head/solvent interactions
have to be investigated.
9
(
3
3
3 4 2 3
C 36.00; H 4.02; N 27.99; O 31.97. Found for C H N O : C
3
5.98; H 4.10; N 27.92; O 31.94.
2
.3 Surfactants
Cetylpyridinium bromide (Fluka) was recrystallized four
times from ethyl acetate/acetone (50/50, v/v) and dried in a
vacuum oven at 30 °C. Elemental analysis indicated a purity
99%. Cetyltrimethylammonium bromide (Merck, 99%
minimum purity) was used as supplied.
-Alaninebetaines have been prepared using the method
described by Le Berre and Delacroix [19]: reaction of
acrylic acid with tertiary amines. 2 mmol of the suitable amine
were reacted with 3 mmoles of acrylic acid, at room tem-
perature, in a mixture of 3 mL of diethyl ether and 1 mL of
acetonitrile. After one week a precipitate was recovered by
filtration.
>

N-Dodecylalaninebetaine, yield: 68%, mp 135 °C. Anal.
calcd C 71.52; H 12.35; N 4.90; O 11.20. Found for
C
17
H
35NO
2
: C 71.47; H 12.34; N 5.01; O 11.18.
N-Hexadecylalaninebetaine, yield: 57%, mp 160 °C.
Anal. calcd C 73.84; H 12.68; N 4.10; O 9.36. Found for
: C 73.78; H 12.70; N 4.07; O 9.40.
N-Octadecylalaninebetaine, yield: 55%, mp 179 °C. Anal.
calcd C 74.73; H 12.81; N 3.78; O 8.65. Found for
: C 74.80; H 12.78; N 3.82; O 8.62.
21 2
C H43NO
3.2 Zwitterionic surfactants
This study was devoted to -alaninebetaines (II) synthesized
using the method described by Le Berre and Delacroix [19]:
23 2
C H47NO
2
.4 Synthesis of long chain alkyl azides
+

RN (CH
3
)
2
(CH
IIb
2
)
2
CO2
We employed the method described by Reeves and Bahr
R  C H

IIa

; C H


; C H IIc


1
2
25
16 33
18 37
1
) Nitroso compounds are known to be carcinogenic; therefore all necessary precautions have to be taken during the synthesis.

CASSEL Stéphanie, et al. Sci China Chem September (2010) Vol.53 No.9
2067
A plot of logCMC of the -alaninebetaines vs. the num-
ber of carbon atoms in the hydrophobic tail, in water, for-
mamide, and 3-methylsydnone is shown in Figure 2.
It can be seen that in all these three media, there was a
linear relationship between logCMC and the length of the
hydrocarbon chain. From the slopes we could evaluate the
respective incremental contribution a and b of the polar
heads and CH
2
groups to micellization. The values of these
parameters are listed in Table 3.
In this table we observe that parameter b is essentially
the same in all three solvents. This shows that solvophobic
interactions in formamide and 3-methylsydnone are similar
to those in water. In contrast, parameter a is not the same in
the different solvents.
Figure 1 Plot of surface tension against the logarithm of the concentration
of -alaninebetaines in 3-methylsydnone at 60 °C (temperature above the
highest Krafft point of these compounds).
We were able to observe micellization of -alaninebe-
taines in 3-methylsydnone (Figure 1). As in water and for-
mamide, the critical micelle concentration (CMC) fell with
increasing length of the hydrophobic tail. However, for
equal chain lengths the CMC was almost double compared
to the one observed in formamide.
3
.3 Mesoionic surfactants
We then investigated a series of mesoionic compounds of
original structures, the alkylazides (III):

+
+

RN N =N  RN=N =N
R  C H IIIa ; C H IIIb ; C H IIIc
12 25
The values of the dielectric constant (

/
o
=144 at 40 °C)






7
15
10 21
and the dipole moment ( = 7.3 D) of 3-methylsydnone are

The long-chain azides were synthesized using a method
developed in our laboratory involving a liquid/liquid phase
transfer reaction in formamide [21]. The CMC of these
long-chain alkyl azides were measured in 3-methylsydnone
at 40 °C, which is above the highest of the Krafft point for
these compounds. These alkyl azides are insoluble in water.
Nevertheless, they display surfactant properties in both
formamide and 3-methylsydnone. In a previous study, we
observed surfactant properties of water-insoluble amphi-
philes (the long-chain phosphonium salts) in formamide
higher than those of formamide ( / = 109.5 at 25 °C;  =

o
3
.4 D). Thus the electrostatic bonds formed by addition of
surfactants molecules in the sydnone will tend to favor sol-
vation of these molecules. During micellization, the energy
required for the desolvation of these amphiphiles will be
higher in 3-methylsydnone than in formamide. For a given
homologous series of surfactants, the CMC can be ex-
pressed in terms of the following relationship:
logCMC  a b

n 1

[
22]. The present results are a further example of the gener-
where a is the contribution of the polar head to micellization,
ality of surfactant behavior in non aqueous solvent systems.
The surface tension results for the alkyl azides in forma-
mide and 3-methylsydnone are shown in Figure 3.
The CMC values are listed in Table 4.
b the increment per CH group, and n+1 the total number of
2
carbon atoms in the hydrophobic tail.
As found with the betaines, the CMCs of the azides for
equal chain length were higher in 3-methylsydnone than in
formamide. Figure 4 shows the plot of the logCMC against
the number of carbon atoms in the chain.
The values of parameters a and b are listed in Table 5. It
Table 3 Values of parameters a and b, representing the contributions of
polar heads and CH groups, respectively, to micellization of betaines in
2
water, formamide and N-methylsydnone
Parameter
Compound
a
b
Water
0.9
0.32
0.33
0.29
Formamide
N-methylsydnone
3.58
3.72
Figure 2 Plot of logarithm of CMC against the number of carbon atoms
in the hydrophobic chain of -alaninebetaines in water (▲), formamide (
■
)
and 3-methylsydnone (◆).

2068
CASSEL Stéphanie, et al. Sci China Chem September (2010) Vol.53 No.9
Figure 3 Plot of surface tension against the logarithm of the concentration of compounds IIIa, IIIb and IIIc in formamide (a) and in 3-methylsydnone (b) at
0 °C.
4
Table 4 CMC values of alkyl azides in formamide and 3-methylsydnone
CMC (mol/L)
Compound
IIIa
IIIb
IIIc


2
2
3
Formamide
0.125
0.400
2.3 × 10
6.6 × 10
6 × 10
2
3
-Methylsydnone
2.5 × 10
tions were comparable in both solvents.
4
Conclusions
In this paper we have shown that synthesized ionic, zwit-
terionic and mesoionic amphiphiles displayed surfactant
properties in 3-methylsydnone, which is a polar and aprotic
solvent. Moreover, we found that solvophobic interactions
in formamide and 3-methylsydnone were comparable to the
hydrophobic interactions encountered in water. Since the
solvophobic interactions seemed identical in these media,
the interactions between the polar heads of the surfactants
and the solvent would tend to have the greatest influence on
micellization. Therefore, we showed that parameter a, rep-
resenting the contribution of the polar head to micellization,
was different in the different media, and also differed be-
tween the zwitterionic and mesoionic surfactants. The val-
ues of a were comparable in formamide and in 3-methyl-
sydnone, both being considerably higher than in water. This
phenomenon can be accounted for by interactions stemming
essentially from solvation. In aqueous medium, micelliza-
tion of surfactants involves mainly the formation of hydro-
gen bonds with water, whereas dipole-dipole interactions
predominate in formamide and in 3-methylsydnone, which
have similarly delocalized electronic structures, these inter-
actions being more difficult to break than hydrogen bonds.
Since the molecular aggregation depends on the nature of
the surfactants, the nature of the solvent and strongly on the
interactions between both, micelles production hinges on
the optimal choice of the surfactant-solvent pair [23].
Figure 4 Plot of logarithm of CMC against the number of carbon atoms
in the hydrocarbon chain of alkyl azides in formamide ( ◆ ) and
3
-methylsydnone (■).
Table 5 Parameters a and b, corresponding to the respective contribu-
tions of polar heads and CH
2
groups to micellization of alkyl azides in
formamide and N-methylsydnone
Parameter
Compound
a
b
Formamide
0.9
1.3
0.25
0.24
N-Methylsydnone
can be seen that parameter a was not the same in the two
solvents, whereas parameter b was identical in formamide
and 3-methylsydnone, indicating that solvophobic interac-

CASSEL Stéphanie, et al. Sci China Chem September (2010) Vol.53 No.9
2069
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