On the Self-Aggregation and Fluorescence Quenching Aptitude of Surfactant Ionic Liquids
The aggregation behavior in aqueous solution of a number of ionic liquids was investigated at ambient conditions by using three techniques: fluorescence, interfacial tension, and (1)H NMR spectroscopy. For the first time, the fluorescence quenching effect has been used for the determination of critical micelle concentrations. This study focuses on the following ionic liquids: [Cnmpy]Cl (1-alkyl-3-methylpyridinium chlorides) with different linear alkyl chain lengths (n=4, 10, 12, 14, 16, or 18), [C12mpip]Br (1-dodecyl-1-methylpiperidinium bromide), [C12mpy]Br (1-dodecyl-3-methylpyridinium bromide), and [C12mpyrr]Br (1-dodecyl-1-methylpyrrolidinium bromide). Both the influence of the alkyl side-chain length and the type of ring in the cation (head) on the CMC were investigated. A comparison of the self-aggregation behavior of ionic liquids based on 1-alkyl-3-methylpyridinium and 1-alkyl-3-methylpyridinium cations is provided. It was observed that 1-alkyl-3-methylpyridinium ionic liquids could be used as quenchers for some fluorescence probes (fluorophores). As a consequence, a simple and convenient method to probe early evidence of aggregate formation was established.
On the Self-Aggregation and Fluorescence Quenching Aptitude of Surfactant Ionic Liquids
Natalia V. Plechkova,
Jose´ N. Canongia Lopes,
Kenneth R. Seddon,
and Luı´s Paulo N. Rebelo*
Instituto de Tecnologia Quı´mica e Biolo´gica, ITQB 2, UniVersidade NoVa de Lisboa, Apartado 127,
2780-901 Oeiras, Portugal, The QUILL Centre, The Queen’s UniVersity of Belfast, Stranmillis Road,
Belfast BT9 5AG, U.K., and CQFM, Departamento de Engenharia Quı´mica e Biolo´gica and Centro de
Quı´mica Estrutural, Instituto Superior Te´cnico, 1049-001 Lisboa, Portugal
ReceiVed: March 12, 2008
The aggregation behavior in aqueous solution of a number of ionic liquids was investigated at ambient con-
ditions by using three techniques: ﬂuorescence, interfacial tension, and
H NMR spectroscopy. For the ﬁrst
time, the ﬂuorescence quenching effect has been used for the determination of critical micelle concentrations.
This study focuses on the following ionic liquids: [C
mpy]Cl (1-alkyl-3-methylpyridinium chlorides) with
different linear alkyl chain lengths (n ) 4, 10, 12, 14, 16, or 18), [C
mpy]Br (1-dodecyl-3-methylpyridinium bromide), and [C
rolidinium bromide). Both the inﬂuence of the alkyl side-chain length and the type of ring in the cation
(head) on the CMC were investigated. A comparison of the self-aggregation behavior of ionic liquids based
on 1-alkyl-3-methylpyridinium and 1-alkyl-3-methylpyridinium cations is provided. It was observed that 1-alkyl-
3-methylpyridinium ionic liquids could be used as quenchers for some ﬂuorescence probes (ﬂuorophores).
As a consequence, a simple and convenient method to probe early evidence of aggregate formation was
Considering the growing number of reported investigations
in which the dual nature of ionic liquids
has been studied, it
seems that their roles in both surfactant science and its
applications are promising. Surveying the recent literature, three
possible directions for the development and applications of ionic
liquids can be identiﬁed:
In the ﬁrst group of publications, the amphiphilic nature of
some cations, for example, [C
, leading to aggregation
phenomena in aqueous solutions (and in some cases to org-
anization into micelles displaying surfactant behavior) was
With the possibility of the ﬁne-tuning of the ionic
liquids’ hydrophobicity by changing the alkyl chain length and/
or the nature and size of the counterion (anion), one can affect
both the structure and the delicate dynamics of these micellar
aggregates. Second, impressive solvation abilities toward the
dissolution of a series of different common surfactants,
or ionic liquids
in neat ionic liquids
have been demonstrated. Third, ionic liquids can be added as a
cosurfactant or hydrotrope to aqueous solutions of common
surfactants, thus affecting the surface activity and the critical
micelle concentration (CMC) of these solutions.
In our previous work,
we reported the role of the alkyl chain
length, the concentration, and the nature of the anion on the
aggregation behavior of the ionic compounds belonging to the
mim]X (X ) Cl, [PF
] or [NTf
]) family. In the current
contribution, we also examine the inﬂuence of different cationic
ring types on the aggregation behavior by using aqueous sol-
utions of [C
Y]Br (Y ) mpyrr, mpy, or mpip).
Recently, a few studies on the ﬂuorescence behavior of
imidazolium and pyrrolidinum ionic liquids were published.
Here, we show a unique characteristic of some ionic liquids,
namely, in the case of the 1-alkyl-3-methylpyridinium family:
besides their ability to act as surfactants, they present also a
quencher aptitude for the most commonly used ﬂuorescence
probes for micellar characterization (ﬂuorophores). Probably,
this characteristic has its origin at the pyridinium ring (head),
for it is known that other pyridinium-containing cations can also
act as quenchers.
2. Experimental Section
Ionic Liquids. The 1-alkyl-3-methylpyridinum chlorides,
mpy]Cl (n ) 4, 10, 12, 14, 16, or 18), 1-dodecyl-1-
methylpyrrolidinum bromide, 1-dodecyl-3-methylpyridinium
bromide, and 1-dodecyl-1-methylpiperidinium bromide
Y]Br; Y ) mpyrr, mpy, or mpip; see Scheme 1) were
synthesized by the reaction of one mole equivalent of the amine
3-methylpyridine, 1-methylpyrrolidine, or 1-methylpiperidine
* Correspondingauthors.E-mail: email@example.com@itqb.unl.pt.
Universidade Nova de Lisboa.
The Queen’s University of Belfast.
Departamento de Engenharia Quı´mica e Biolo´gica, Instituto Superior
Centro de Quı´mica Estrutural, Instituto Superior Te´cnico.
SCHEME 1: Cations of Ionic Liquids Discussed: (A)
J. Phys. Chem. B 2008, 112, 8645–8650 8645
10.1021/jp802179j CCC: $40.75 2008 American Chemical Society
Published on Web 07/01/2008
with an excess of the appropriate haloalkane (1.3 mol equiva-
lents). This excess also allows for the reactants to be stirred
without additional solvent at 70 °C; the progress of reactions
was monitored by
H NMR spectroscopy in CDCl
completion of the reaction, there was no evidence for the
presence of unreacted amine. The ionic liquids were puriﬁed
with ethyl ethanoate. The volume of ethyl ethanoate used for
the recrystallization was approximately half that of the halide
salt. The ethyl ethanoate was decanted, followed by the addition
of fresh ethyl ethanoate, and this step was repeated ﬁve times.
The remaining ethyl ethanoate was removed in vacuo, and the
ionic liquids were dried in vacuo (0.1 Pa) to remove any small
traces of volatile compounds at moderate temperatures (60-80
°C) for typically 72 h. The detailed syntheses and spectroscopic
(NMR, MS) and thermophysical (DSC, TGA) characterization
will be reported elsewhere.
All chemicals were purchased from
Sigma-Aldrich; the more volatile liquids were puriﬁed by
distillation under vacuum before use. For the NMR experiments,
-trichloromethane (D+0.03%, Euriso-top) was used:
C NMR analyses showed no major impurities in the ionic
liquids as prepared above by using a Bruker Avance spectrom-
eter DPX 300.
Chemicals for IFT, Fluorescence, and NMR Measure-
ments. Doubly distilled deionized water was obtained from a
Millipore Milli-Q water puriﬁcation system (Millipore). Both
for the interfacial tension (IFT) and ﬂuorescence measurements,
mpy]Cl stock solutions were prepared in either 1.74 × 10
M pyrene or slightly less than saturated anthracene aqueous
solution, and all studied solutions were prepared from the
stock solutions, diluting with the same pyrene or anthracene
aqueous solution. Pyrene (Fluka, Germany, 99%) was recrystal-
lized from benzene. Anthracene (Fluka, Germany, puriss, for
scintillation) and ethanenitrile (Merck, Germany, gradient grade)
were used without further puriﬁcation. For the NMR experi-
O (Cambridge Isotope Laboratory, Andover, MA, D,
99.9%) was used.
Details about the experimental techniques and instrumentation
for IFT, ﬂuorescence, and
H NMR spectroscopy are found in
the Supporting Information.
Ab Initio Calculations. The molecular geometry and charge
distribution of isolated 1,1-dimethylpyrrolidinium and 1,1-
dimethylpiperidinium cations were obtained by quantum chemi-
cal (ab initio) calculations. These were performed by using the
Gaussian 03 program
at the HF/6-31G(d) level of theory for
geometry optimization and the MP2/cc-pVTZ-f level for single-
point energy and electronic density calculations, as is current
practice in the development of force-ﬁeld parameters for ions
present in ionic liquids.
The point-charge assignment was done
by using the CHelpG algorithm.
3. Results and Discussion
Self-Aggregation Assessment by Surface Tension. The sur-
face tension of aqueous solutions of 1-alkyl-3-methylpyridinium
mpy]Cl, (n ) 10, 12, 14, 16, or 18) was measured,
see Figure 1a. The results of the IFT for the aqueous solutions as
a function of the total concentration of [C
mpy]Cl were used to
determine the CMC and to study adsorption parameters: the
efﬁciency of adsorption, pC
, (deﬁned as the negative logarithm
to the base 10 of the concentration of amphiphilic molecules
required to reduce the surface tension of the pure solvent by 20
), the effectiveness of the surface tension reduction, Π
(deﬁned as Π
, where γ
is surface tension of the
pure solvent (water) and γ
the surface tension of the solution
at the CMC), and the minimum area per amphiphilic molecule at
the interface, a
Micellisation. If aggregation phenomena occur, then, as the
concentration of an ionic liquid increases, the surface tension
of the solution initially decreases and then becomes almost
constant. The CMC is determined as the intersection of two
linearly extrapolated lines. It is obvious that the decrease in the
CMC of aqueous solutions of [C
mpy]Cl is a consequence of
the growth of the alkyl chain. The experimental values for the
break points in the IFT measurement are presented in Table 1
(along with other CMC results from different techniques). Figure
1b summarizes the CMC values obtained with the IFT meth-
odology plotted as a function of the number of carbon atoms,
n, in the cationic side chain of [C
mpy]Cl, n ) 10-18. For
comparison purposes, data obtained for the [C
with similar hydrocarbon chain lengths are depicted.
expected slightly higher hydrophobicity of the [C
in comparison with the [C
mim] cation is probably the reason
for the lower CMC values of the former.
Adsorption. The lowering of the surface tension values, γ,
is a consequence of the increased concentration of the ionic
Figure 1. (a) Monitoring the self-aggregation of [C
mpy]Cl by using
IFT for different chain lengths: n ) (/) 10, ())12, (∆)14, ()16, (O)18.
(b) CMC values for [C
mpy]Cl (2) and [C
mim]Cl (b) as a function
of n. (c) Efﬁciency of absorption pC
([) and minimum area per ionic
liquid molecule, a
(b) as a function of n [C
TABLE 1: CMC (in mM) of [C
mpy]Cl (n ) 10-18),
Y]Br, (Y ) mpyrr, mpy, mpip, and mim) Measured by
IFT, Fluorescence Quenching of Pyrene, Fluor, and
Ionic Liquid IFT ﬂuor
13 13.5 12.5
3.1 3.1 3.2
0.8 0.77 0.9
0.3 0.23 0.25
9, 10, 12
The experimental errors for all used techniques are e5%.
Result obtained by using anthracene as a ﬂuorophore.
From ref 9.
CMCs obtained by using conductivity, volume, and ﬂuorescence
From ref 7. CMC obtained by using conductivity
8646 J. Phys. Chem. B, Vol. 112, No. 29, 2008 Blesic et al.
liquid at the air-water surface. Because of the almost invariant
condition of the surface for concentrations greater than that of
the CMC, the chemical potential of the ionic liquid changes
Although the adsorption efﬁciency, pC
increases with increasing alkyl chain length (Figure 1c), the
effectiveness of the surface tension reduction, Π
slightly. They follow tendencies published for both [C
and classical cationic surfactants.
From surface tension data, by assuming for these low-
concentration regimes a monolayer structure at the surface, we
calculated the minimum area per ionic liquid molecule, a
using the well-known Gibbs equation.
Values for a
are listed in Table 2
. The dependence of the minimum
area per amphiphilic molecule versus the number of carbon
atoms in the alkyl chain, n (up to 16 carbon atoms), for
mpy]Cl is linear and can be described by a
) 1.906 -
0.0695n (Figure 1c). A similar dependence for [C
and the corresponding equation is a
- 0.062n. A comparison of the equations for these two families
suggests a higher degree of packing of adsorbed [C
molecules. A slightly lower surface tension in the plateau region
reported in our previous paper
mim]Cl in comparison
mpy]Cl (Figure 1a) also leads to the same conclusion.
It is well-known that surface tension measurements are a very
sensitive test for the presence of impurities, usually being
unreacted surface active compounds (long chain chloroalkane
in the case of chloride-based ionic liquids that are usually used
in excess during the synthesis). The absence of a minimum
around the CMC conﬁrms the high purity of the ionic liquids
used in this study.
CMC Determination by Changes in the Fluorescence of
Added Probes. A widely used method for the determination
of the aggregation of amphiphilic molecules (surfactants,
polymers, and so forth) is the comparison of the intensities of
the ﬁrst, I1, and third, I3, vibronic bands of the pyrene emission
spectrum. The ratio I3/I1 is a function of the polarity of the
pyrene environment, and it increases with decreasing solvent
Obviously, the method cannot be applied when the ﬂuores-
cence of pyrene is quenched by the surfactant itself. This is
found to be the case for the 1-alkyl-3-methylpyridinium ionic
liquids, in analogy with other cases containing the same common
pyridinium ring (head).
In fact, it was witnessed that the
ﬂuorescence of both pyrene and anthracene in water vanished
(indistinguishable from the noise level) for all [C
liquids used in this study, at concentrations for which aggrega-
tion is expected. Therefore, it seems that once in the aggregate,
the close contact between the ﬂuorophore probe and the 1-alkyl-
3-methylpyridinium group results in an efﬁcient static ﬂuores-
cence quenching. In order to verify that the 1-alkyl-3-
methylpyridinium group is able to quench the ﬂuorescence of
these probes, Stern-Volmer quenching constants, K
, for the
steady-state ﬂuorescence quenching were determined from the
variation of ﬂuorescence intensity in the absence (I
presence (I) of low concentrations, c, of 1-alkyl-3-methylpyri-
dinium in ethanenitrile. In order to quantify the quenching effect
of the monomer (preventing aggregation), we have chosen the
short chain [C
mpy]Cl homologue to perform the experiments,
and (for c < 0.6 mM) a linear Stern-Volmer plot, (I
/I) ) 1
, was obtained (Figure 2
a). From the slopes of the
Stern-Volmer plots, values of K
) (0.3 ( 0.04) and (0.07
( 0.005) mM
for pyrene and anthracene, respectively, were
obtained. For the quenching of an excited state, we have K
is the ﬂuorescence lifetime of the excited
state and k
is the rate constant for bimolecular quenching. By
taking as reasonable approximations the published values of τ
of pyrene and anthracene in polar aerated solvents, respectively
18.9 ns (19.0 ns without O
) and 4.2 ns (5.3 ns without O
we calculated the values of the bimolecular quenching constant
(pyrene) ) (1.58 ( 0.23) × 10
thracene) ) (1.67 ( 0.12) × 10
. This demon
strates that within the experimental error, the quenching is
diffusion controlled (k
) 1.9 × 10
quenching mechanism probably involves electron transfer, but
this is out of the scope of the current work.
The fact that the [C
mpy]Cl family may act as ﬂuorescence
quenchers opens the possibility of determining CMCs by
detecting the surfactant concentration for which the quenching
deviates from the normal Stern-Volmer equation, that is, from
a slope comparable to that which occurs in homogeneous media.
The onset of micellization, deﬁned as the CMC, can be
recognized as a pronounced break-point in the dependence of
/I versus the concentration of [C
mpy]Cl in aqueous solution.
Typical graphs of this type for [C
mpy]Cl (n ) 12, 14, 16, or
18) in aqueous solution by using pyrene as a ﬂuorescence probe
are shown in Figure 2b. The values of CMCs for all systems
are presented in Table 1. In order to apply this new method, it
is necessary that the CMC of certain compound-quencher
combinations is not greater than the concentration at which
complete quenching of ﬂuorescence probes occurs. It means
that this method requires an appropriate match between the
ﬂuorescence probe and the quencher. One should note here the
very good agreement between values determined by using
distinct methodologies. Having a good agreement between this
new method and two well-established methods for the onset of
aggregation, surface tension and
H NMR (see next section), is
important. This avoids potentially fallacious interpretations
which are a common occurrence when novel methods are used
without the proof of concept.
Self-Aggregation Assessment by
H NMR res
onances for the protons of [C
mpy]Cl undergo reasonable shifts
as a function of the [C
mpy]Cl concentration, namely, the pro
tons of the ring and the protons of the terminal CH
Figure 3 shows the evolution of the chemical shifts (δ, ppm) in
H NMR spectra for the protons of the terminal CH
as a function of the reciprocal concentration (logarithmic scale)
Once again, in agreement with the previous methodologies,
the chemical shift shows a distinctive decrease for all n > 12,
indicating the change in the environment for these molecules
as a function of their concentration, related to the self-aggre-
gation in small micellar-type aggregates.
Self-Aggregation for Other Types of Cationic RingssAs-
sessment by Surface Tension and Ab Initio Calculations. The
inﬂuence of the type of ring in the cation on the aggregation
behavior was also investigated. The underlying inﬂuence of this
effect is very complex because head groups have opposing
TABLE 2: Efﬁciency of Adsorption, pC
, Effectiveness of
Surface Tension Reduction, Π
, and Minimum Area per
Ionic Liquid Molecule at the Interface Air/Liquid, a
mpy]Cl (n ) 10-18)
ionic liquid pC
,/ mN m
1.62 27.9 1.21
2.22 27.9 1.08
2.82 28.0 0.92
3.39 27.9 0.80
3.87 27.8 0.76
Self-Aggregation of Surfactant ILs J. Phys. Chem. B, Vol. 112, No. 29, 2008 8647
tendencies to keep close to minimize hydrocarbon-water
contacts and to repel as a result of electrostatic repulsion,
hydration, and steric hindrance.
Three ionic liquids with the
same anion and the same alkyl chain length but with different
types of hydrocarbon rings have been analyzed. Figure 4 shows
the surface tension curves for [C
Y]Br, (Y ) mpyrr, mpy, and
mpip). Surprisingly, two very similar curves with almost the
same CMC values were obtained for [C
mpip]Br. Probably, in the case of [C
mpip]Br, two effects
compensate each other. On the one hand, the higher hydropho-
bicity of the [C
cation and stronger bound anion, Br
in comparison with the [C
cation, where the positive
charge is delocalized, would give a lower CMC. On the other
hand, because these two molecules have a different geometry
and volume, we can expect that the more space-demanding
cation (we are comparing boat and/or chair struc
tures of a cyclic six-membered ring with a planar aromatic
molecule) has a higher CMC because of steric hindrance. One
can also notice that the CMC value for [C
mpy]Br (10 mM) is
lower than that for [C
mpy]Cl (13 mM). The slightly lower
values obtained for [C
mpy]Br are expected because it is known
that the binding of anionic counterions to cationic micelles
increases in the order F
It is even more difﬁcult to explain the signiﬁcantly lower
CMC and surface tension in the plateau region of the ﬁve-
membered heterocyclic ring of [C
mpyrr]Br in comparison with
the six-membered heterocyclic ring of [C
further studies, we are unable to speculate whether this is only
the effect of their different volume and geometry and consequent
head packing in the micelles or in the monolayers. However, a
complementary interpretation can be developed if one compares
the geometry and internal charge distribution of the two isolated
The necessary data were calculated ab initio (cf. Table S1 of
the Supporting Information), and different conclusions can be
drawn from. First, the geometry (bond lengths and angles)
around the nitrogen atom are very similar in both cases
(N-C(methyl) and N-C(methylene) distances of 149 and 151
pm, respectively, and angles close to 109.5°). This can be seen
in Figure 5, where the most stable conformation of each cation
is represented. In fact, the chair conformation of the piperidinium
cation can be thought as an envelope conformation (like that
of the pyrrolidinium cation) with an extra ﬂap. Because that
extra ﬂap occupies a position opposite to the nitrogen atom,
the inﬂuence of the former on the latter is very limited from a
geometrical perspective. Also, the charge distribution in the two
cations is quite different because of different hyper-conjugation
effects on the ﬁve- and six-membered rings (cf. Figure 5).
Figure 2. (a) Stern-Volmer relation for [C
mpy]Cl in ethanenitrile solutions of anthracene (0) and pyrene (O). (b) Monitoring the self-aggregation
mpy]Cl in aqueous solution by using the ﬂuorescence quenching technique for pyrene for different alkyl side-chain lengths of the ionic
liquid: n ) (])12, (∆)14, (0)16, and (O)18.
Figure 3. Monitoring the self-aggregation of [C
mpy]Cl by using
NMR spectroscopy. δ is the observed chemical shift, and c is the ionic
liquid concentration. Results for protons of the terminal CH
different chain lengths: n ) (])12, (∆)14, (0)16, and (O)18.
Figure 4. - Monitoring the self-aggregation of [C
Y]Br by using the
IFT technique: (])[C
mpy]Br, and (O)[C
8648 J. Phys. Chem. B, Vol. 112, No. 29, 2008 Blesic et al.
Taken together, these conclusions mean that the headgroup
of the anion will interact with the two cations at a similar
position (the most positive charge is centered around the
geometrically similar nitrogen atoms) but with different mag-
nitude. In fact, the larger charge density on the nitrogen of the
piperidinium cation will lead to weaker interactions because this
atom (surrounded by four carbon atoms) cannot be approached
by the anion. On the other hand, the pyrolidinium cation has
greater charge densities in the 2,6-methylene groups (taken as
the sum of the charge of the ortho carbon atom plus its two
hydrogen atoms) where most of the interactions with the anion
will take place. This fact can compensate for the lower charge
density of its nitrogen atom and lead to an effectively higher
counteranion binding, and consequently, to a lower CMC, as
experimentally measured. These observations have to be
examined in deeper detail by using other classes of ionic liquids
and other methods.
We propose a novel methodology that can be used to ﬁnd
out whether ionic liquids are capable of forming aggregates in
aqueous solutions. This technique is based on the ﬂuorescence
quenching effect that speciﬁc ionic liquids provoke in common
ﬂuorophores. This procedure was successfully tested against two
well-established methodologies. Additional classes of ionic
liquids can now be checked for their micellar behavior by using
this new technique.
Quenching of ﬂuorescence has found wide utility in bio-
The current new result, connected to aggre
gation phenomena in ionic-liquids-containing systems, may ﬁnd
application in other ﬁelds too.
It was found that pyridinium ionic liquids are more biode-
gradable than imidazolium ionic liquids and biodegradation rates
increase with longer alkyl chain length.
The ability of pyri
dinium ionic liquids to act as a quencher for some ﬂuorophores
can be used as a very sensitive method for following their
degradation rates or generally concentration change. This
method can be used in broad concentration range and in the
presence of many other species in solution, because quenching
requires speciﬁc contact pyridinium nucleus-ﬂuorophore.
Besides the length of the alkyl chain and the nature of the
anion, both the structure of the headgroup and the point-charge
density distribution are parameters which play important roles
in the geometry and packing of micelles, in particular by
controlling the magnitude of the steric repulsions between the
head groups. However, the issue of the inﬂuence of the head-
group on the aggregation behavior deserves further investi-
Acknowledgment. This work was supported by the Fundac¸a˜o
para a Cieˆncia e Tecnologia (FC&T), Portugal (Projects POCTI/
QUI/35413/2000 and POCI/QUI/57716/2004). M.B. thanks
FC&T for a Ph.D. Grant SFRH/BD/13763/2003) and Marie
Curie Fellowships for Early Stage Research Training (EST
No505613). K.R.S. thanks the EPSRC (Portfolio Partnership
Scheme, Grant no. EP/D029538/1).
Supporting Information Available: Experimental methods
and detailed ab initio results. This material is available free of
charge via the Internet at http://pubs.acs.org.
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