Aggregation behavior of long-chain n -aryl imidazolium bromide in aqueous solution

Article abstract of DOI:10.1021/la104719v

The aggregation behavior of three long-chain N-aryl imidazolium ionic liquids (ILs), 1-(2,4,6-trimethylphenyl)-3-alkylimidazolium bromide [C _npim]Br (n = 10, 12, and 14), in aqueous solutions was systematically explored by surface tension, elec

Full text of DOI:10.1021/la104719v

pubs.acs.org/Langmuir
© 2011 American Chemical Society
Aggregation Behavior of Long-Chain N-Aryl Imidazolium Bromide
in Aqueous Solution
Lijuan Shi,† Na Li,‡ Han Yan,† Yan’an Gao,† and Liqiang Zheng*,†
†Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education,
Jinan 250100, China, and ‡College of Chemistry, Chemical Engineering and Materials Science,
Shandong Normal University, Jinan 250014, China
Received September 13, 2010
The aggregation behavior of three long-chain N-aryl imidazolium ionic liquids (ILs), 1-(2,4,6-trimethylphenyl)-3-
alkylimidazolium bromide [Cnpim]Br (n = 10, 12, and 14), in aqueous solutions was systematically explored by surface
tension, electrical conductivity, and 1H NMR. A lower critical micelle concentration (cmc) for the N-aryl imidazolium
ILs is observed compared with that for 1,3-dialkylimidazolium ILs [Cnmim]Br, indicating that the incorporation of the
2,4,6-trimethylphenyl group into a headgroup favors micellization. The enhanced π-π interactions among the adjacent
2,4,6-trimethylphenyl groups weaken the steric hindrance of headgroups and thus lead to a dense arrangement of
[Cnpim]Br molecules at the air-water interface. An analysis of the 1H NMR spectra revealed that the introduced 2,4,6-
trimethylphenyl group may slightly bend into the hydrophobic regions upon micellization. The micelle formation
process for [Cnpim]Br (n = 10, 12, and 14) was found to be enthalpy-driven in the investigated temperature range, which
is attributed to the strong electrostatic self-repulsion of the headgroups and the counterions as well as the π-π
interactions among headgroups. Strong, stable fluorescence properties are presented by the new N-aryl imidazolium
ILs, indicating their potential application in the field of photochemistry.
Introduction
crystals (ILCs) with extended aromatic cores, where gratifying
properties of the novel ILCs, such as high thermal stabilities and
strong fluorescence behaviors were presented. Then a series of
aryl alkyl imidazolium-based ILs were designed by Ahrens et al.,10
who demonstrated that the combination of sp3 alkyl and sp2
aryl substituents at the nitrogen atoms of the imidazolium core
caused a remarkable modification of the physicochemical proper-
ties (e.g., lower melting points, higher decomposition tempera-
tures) compared with those of current 1,3-dialkylimidazolium
ILs. Most importantly, strong electronic and steric effects of the
substituents on the aromatic ring of the imidazolium cations were
also observed, indicating that the characteristics of N-aryl ILs can
be further tuned because various substitution patterns with dif-
ferent functions are easy to graft onto the aromatic ring. Owing to
the introduced π system and the possibility to interact with other
substances through π-π interactions, the application value of the
new tunable aryl alkyl ionic liquids (TAAILs) in the areas of
extraction separation, catalytic reaction, and self-assembled ma-
terials is significantly enhanced.9,10
Ionic liquids (ILs), as a class of environmentally friendly solvents,
have significantly developed recently because of their extraordin-
ary properties such as nonvolatility, nonflammability, high sta-
bility, high ionic conductivity, and easy recyclability.1,2 Numerous
ILs based on imidazolium, pyridinium, and quaternary ammo-
nium cations with a variety of anions such as halides, PF6-, BF4
,
-
(CF3SO3)2N-, and CF3SO3- have been successively synthesized
and widely utilized in organic synthesis, catalysis, and the pre-
paration of nanostructured materials.3-5 Recently, task-specific
ionic liquids have emerged, where functional groups are intro-
duced as a part of the N-alkyl substituent to impart a particular
capability to the ILs.6,7 Even so, the development of ILs is gen-
erally confined to N-alkyl ILs (e.g., 1, 3-dialkylimidazolium salts),
which restricts the variation of characteristics and applications
of ILs.8
Currently, attempts have been focused on the design and syn-
thesis of N-aryl imidazolium ILs. The incorporation of aryl sub-
stituents into the nitrogen atoms of the heterocycle will signifi-
cantly tune the properties of the cations, such as the size, polarity,
and charge distribution, whichallows a far greater variation of the
IL characteristics than imagined in current systems. Kouwer et al.9
first synthesized a series of imidazolium-based ionic liquid
Because of their unique chemical and physical properties, ILs
have been extensively studied and widely used in the field of
colloid and interface science in recent years. The aggregation
behavior of imidazolium ILs in aqueous solution has been
intensively explored.11-14 Aggregation behavior in aqueous solu-
tions of three imidazolium ILs, 1-butyl-3-methylimidazolium
*Corresponding author. E-mail: lqzheng@sdu.edu.cn. Phone: þ86-531-
88366062. Fax: þ86-531-88564750.
(1) Rogers, R. D.; Seddon, K. R. Science 2003, 302, 792–793.
(2) Welton, T. Chem. Rev. 1999, 99, 2071–2084.
(10) Ahrens, S.; Peritz, A.; Strassner, T. Angew. Chem., Int. Ed. 2009, 48, 7908–
7910.
(3) Song, C. E. Chem. Commun. 2004, 1033–1043.
(11) Goodchild, I.; Collier, L.; Millar, S. L.; Prokes, I.; Lord, J. C. D.; Butts,
C. P.; Bowers, J.; Webster, J. R. P.; Heenan, R. K. J. Colloid Interface Sci. 2007,
307, 455–468.
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2004, 20, 2191–2198.
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1618 DOI: 10.1021/la104719v
Published on Web 01/19/2011
Langmuir 2011, 27(5), 1618–1625

Shi et al.
Article
tetrafluoroborate ([C4mim]BF4), 1-octyl-3-methylimidazolium
chloride ([C8mim]Cl), and 1-octyl-3-methylimidazolium iodide
([C8mim]I), has been explored by Bowers et al.12 Our group
systematically investigated the surface properties and aggregation
behavior of aqueous solutions of long-chain 1,3-dialkylimidazo-
lium ILs CnmimBr (n = 10, 12, 14, and 16) and 1-dodecyl-3-
methylimidazolium tetrafluoroborate ([C12mim]BF4).13,14 In ad-
dition, much research has focused on the effect of structural
change (especially the long hydrocarbon chain) on the aggrega-
tion behavior of ILs in water. For example, Firestone and co-
workers15 described the self-aggregation of thiophene-tailed imi-
dazolium ILs in aqueous solutions. Our group synthesized a class
of fluorescent carbazole-tailed imidazolium ILs and studied their
aggregation behavior in water, where the incorporation of a
carbazole moiety improved the ability of imidazolium ILs to
form micelles.16 However, few investigations have considered the
effect of change in the polar head on the aggregation behavior of
ILs in water. Yoshida et al.17 reported that methylation at the
2-position of the imidazolium ring induces an increase in surface
tension. For traditional cationic surfactants (e. g., cetyltrialk-
ylammonium halides), the incorporation of an aromatic group
into the headgroup produces interesting consequences, such as a
lowercmc and equilibrium constant for micelle formation(K).18,19
The effect of the incorporated aromatic group on molecular
aggregation, on one hand, is unfavorable because the large size
of the aromatic group increases the steric hindrance among
molecules.20 On the other hand, the incorporation of an aromatic
group could produce π-π interactions among the adjacent
aromatic groups. This π-π interaction could influence the
molecules to pack more compactly and then reduce the disad-
vantageous influence of steric hindrance to micellization.21-23 In
addition, the enhanced hydrophobicity derived from the incor-
poration of aromatic groups can also favor micelle formation.16,24
To expand the application of the N-aryl imidazolium ILs in the
field of colloid and interface science, the aggregation behavior of
three long-chain N-aryl imidazolium ILs in which the 2,4,6-
trimethylphenyl group was grafted to the imidazolium ring was
explored in the present work. The effect of the incorporation of the
N-aryl moiety in the headgroup on micellization was investigated by
surface tension, electrical conductivity, and 1H NMR spectrometry.
Compared with 1-alkyl-3-methylimidazolium salts, the incor-
poration of an aryl group has a significant influence on the surface
activity, thermodynamics, and mechanism of micelle formation. In
addition, the N-aryl imidazolium ILs have strong fluorescence
properties that are due to the introduction of the aryl moiety.
Chart 1. Chemical Structure of the ILs [Cnpim]Br (n = 10, 12, and
14)
were purchased from Aladdin Chemical Reagent Co., Ltd. D2O
(99.96%) and CDCl3 (99.96%) were obtained form Sigma-
Aldrich. Glyoxal (40%), MeOH (99.5%), NH4Cl (99.5%), formal-
dehyde solution (37%), KOH (82%), CH2Cl2 (99.5%), H3PO4
(85%), tetrahydrofuran (99%), and toluene (99.5%) were pur-
chased from Beijing Chemical Reagent Co. Triply distilled water
was used throughout the experiments. The molecular structure of
the ILs used in this work is depicted in Chart 1.
Synthesis of 1-(2,4,6-Trimethylphenyl)imidazole. 1-(2,4,6-
Trimethylphenyl)imidazole was synthesized following a pre-
viously reported procedure.10 Briefly, a mixture of aqueous gly-
oxal (0.1 m) and 2,4,6-trimethylaniline (0.1 m) in MeOH was
stirred at room temperature until a yellow precipitate formed.
After the addition of NH4Cl (0.2 m), formaldehyde solution
(37%, 0.21 m), and H3PO4 (85%, 14 mL), the solution was
refluxed for 9 h. The majority of the solvent (ca. 85%) was
removed in vacuo, and then the pH of the solution was adjusted
to 9 with KOH. The product was extracted with CH2Cl2, and then
the solvent was removed in vacuo. The final product was purified
by recrystallization using tetrahydrofuran at least four times. The
purity of 2,4,6-trimethylaniline was ascertained by the 1H NMR
spectrum in CDCl3.
Synthesis of [Cnpim]Br (n = 10, 12, and 14). For the syn-
thesis of [C10pim]Br, (2,4,6-trimethylphenyl)imidazole (0.1 mol,
18.6 g) and an excess amount of 1-bromodecane (0.11 mol, 24.3 g)
were mixed in dry toluene (200 mL) in a 500 mL round-bottomed
flask and then refluxed at 110 °C under a nitrogen atmosphere for
48 h. The obtained product was cooled to room temperature and
purified by recrystallization in fresh diethyl ether at least four
times. The final product was dried in vacuo for 48 h. The purity of
[C10pim]Br was ascertained by the 1H NMR spectrum in CDCl3.
[C12pim]Br and [C14pim]Br were synthesized following the synthe-
sis procedure for [C10pim]Br.
Apparatus and Procedures. Surface tension measurements
were carried out on a model JYW-200B tensiometer (Chengde
Dahua Instrument Co., Ltd., accuracy(0.1 mN/m) using the ring
method. Temperature was controlled at 25 ( 0.1 °C using a thermo-
static bath. All measurements were repeated until the values were
reproducible.
Specific conductivity measurements on the aqueous solutions
were performed using a low-frequency conductivity analyzer
(model DDS-307, Shanghai Precision & Scientific Instrument
Co., Ltd., accuracy (1%).
Experimental Section
1H NMR spectra were run on a Bruker Avance 400 spectrom-
eter equipped with a pulse field gradient module (Z axis) using a
5 mm BBO probe. The instrument was operated at a frequency of
400.13 MHz at 25 ( 0.1 °C. The observed chemical shifts (δobs) of
the discrete protons of the ILs were examined as a function of
concentration below and above the cmc. All of the samples were
dissolved inD2O, and chemicalshiftswere referencedtothe center
of the HDO signal (4.700 ppm). Two-dimensional nuclear over-
hauser effect spectroscopy (2D NOESY) was performed with the
standard NOESY pulse sequence.25 A relaxation delay of 2 s was
used between scans. A sine apodization function (ssb = 2) was
applied in both dimensions before Fourier transformation. The
mixing time was chosen to be 800 ms.
Materials. 2,4,6-Trimethylaniline (98%), 1-bromodecane
(98%), 1-bromododecane (98%), and 1-bromotetradecane (98%)
(15) Burns, C. T.; Lee, S.; Seifert, S.; Firestone, M. A. Polym. Adv. Technol.
2008, 19, 1369–1382.
(16) Dong, B.; Gao, Y. A.; Su, Y. J.; Zheng, L. Q.; Xu, J. K.; Inoue, T. J. Phys.
Chem. B 2010, 114, 340–348.
(17) Yoshida, Y.; Baba, O.; Larriba, C.; Saito, G. J. Phys. Chem. B 2007, 111,
12204–12210.
ꢀ
(18) E1 Seoud, O. A.; Blasko, A.; Bunton, C. A. Langmuir 1994, 10, 653–657.
(19) Okano, L. T.; E1 Seoud, O. A.; Halstead, T. K. Colloid Polym. Sci. 1997,
275, 138–145.
ꢀ_
(20) Rozycka-Roszak, B. J. Colloid Interface Sci. 1990, 140, 538–540.
(21) Li, Y. J.; Reeve, J.; Wang, Y. L.; Thomas, R. K.; Wang, J. B.; Yan, H. K.
J. Phys. Chem. B 2005, 109, 16070–10674.
(22) Yao, J. H.; Mya, K. Y.; Li, X.; Parameswaran, M.; Xu, Q. H.; Loh, K. P.;
Chen, Z. K. J. Phys. Chem. B 2008, 112, 749–755.
(23) Chen, Q. B.; Liang, X. D.; Wang, S. L.; Xu, S. H.; Liu, H. L.; Hu, Y.
The fluorescence excitation and emission spectra were carried
out using a PerkinElmer LS-55 spectrofluorometer(PE Company)
J. Colloid Interface Sci. 2007, 314, 651–658.
(24) Fan, Y. R.; Li, Y. J.; Yuan, G. C.; Wang, Y. L.; Wang, J. B.; Han, C. C.;
Yan, H. K.; Li, Z. X.; Thomas, R. K. Langmuir 2005, 21, 3814–3820.
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Resonance in One and Two Dimensions; Oxford University Press: New York, 1987.
Langmuir 2011, 27(5), 1618–1625
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Article
Shi et al.
equipped with a thermostatted cell holder at 25 ( 0.1 °C. The slit
widths were fixed at 10 and 3 nm for the excitation and emission,
respectively.
responsible for a decreased cmc. Because of the hydrophobic
effect, the 2,4,6-trimethylphenyl group has a weak tendency to
move away from the micellar interface, which means that the
2,4,6-trimethylphenyl group plays a role similar to that of the
second hydrophobic chain when micelles are formed20 and thus
the cmc becomes smaller. This result is in agreement with the con-
clusion confirmed by 1H NMR spectra and 2D NOESY spectra
as shown below. The γcmc value for [Cnpim]Br is also lower than
that of [Cnmim]Br, suggesting that the former possesses a greater
ability to reduce the surface tension. Herein, the surface activity of
the novel N-aryl imidazolium ILs [Cnpim]Br is somewhat superior
to that of N-alkyl imidazolium ILs [Cnmim]Br and the 2,4,6-
trimethylphenyl group plays an important role in micelle forma-
tion.
The maximum excess surface concentration (Γmax) and the area
occupied by a single amphiphile molecule at the air-water inter-
face (Amin), obtained from the Gibbs adsorption isotherm, can
reflect the surface arrangement of surfactants at the air/liquid
interface.29 A greater value of Γmax or a smaller value of Amin
means a denserarrangement of surfactant moleculesat the surface
of the solution. It is clearly seen from Table 1 that [Cnpim]Br has a
significantly larger Γmax value and a correspondingly smaller Amin
value than [Cnmim]Br, indicating a denser arrangement of
[Cnpim]Br molecules at the air-water interface compared with
that of [Cnmim]Br. This result is also causedby the more dispersed
charge of the cations, which could weaken the electrostatic
repulsion between headgroups. Moreover, the incorporation of
a 2,4,6-trimethylphenyl group could produce π-π interactions
among the adjacent [Cnpim]Br molecules. This π-π interaction
could attract the molecules to pack more compactly in the
air-water interface monolayer.23,24
Results and Discussion
Surface Properties and Micellization. Surface tension mea-
surements were performed to evaluate the surface activities of the
three ILs in aqueous solutions. Figure 1 depicts the surface
tension (γ) versus concentration (C) plots for the aqueous solu-
tions of [Cnpim]Br (n = 10, 12, and 14) at 25 °C. The surface
tension decreases initially with increasing concentration of
[Cnpim]Br, suggesting that the IL molecules are adsorbed at the
air/solution interface.26 Then a plateau appears in the (γ-C) plot,
indicating that the micelles have been formed, and the critical
micelle concentration (cmc) is determined from the break point of
the plot. The values of the cmc and γcmc (surface tension at cmc)
for the three ILs are listed in Table 1, together with the data for
1-alkyl-3-methylimidazolium bromide, [Cnmim]Br (n = 10, 12,
and 14).
Changes in the headgroup of the imidazolium ILs have an
effect on their surface activities. As shown in Table 1, the cmc of
[Cnpim]Br is much lower than that of [Cnmim]Br although they
have the same alkyl chains, implying that the incorporated 2,4,6-
trimethylphenyl group favors molecular aggregation. This is
probably because the more delocalized charge of the cationic
head reduces the electrostatic repulsion of the headgroups and
thus facilitates micelle formation.27 A similar result is obtained
for [Cnmim]Br compared with traditional cationic surfactants
(e.g., alkyltrimethylammonium bromides), where a lower cmc for
[Cnmim]Br is due to the delocalized positive charge on the
imidazolium ring.28 The enhanced hydrophobicity derived from
the incorporation of 2,4,6-trimethylphenyl groups can also be
The correlation between the cmc and the number of carbon
atoms in the hydrocarbon chain (NC) for [Cnpim]Br is shown in
Figure 2. The log cmc of [Cnpim]Br decreases linearly with the
increase in NC, following the empirical eq 130
log cmc ¼ A - BNc
ð1Þ
where A and B are empirical constants. The value of A is related to
the contribution of the polar head to micelle formation, and the
value of B indicates the average contribution to the micelle
formation by each methylene in the hydrophobic chain. Micelle
formation is favored by smaller values of A or larger values of B.31
The values of A and B for [Cnpim]Br calculated from Figure 2 are
0.89 and 0.29, respectively, which are different from those for
[Cnmim]Br, where A = 1.05 and B = 0.28.13 This result indicates
that the formation of micelles is much easier for [Cnpim]Br than
for [Cnmim]Br in aqueous solution because of the incorporation
of the 2,4,6-trimethylphenyl group.
Figure 1. Surface tension of [Cnpim]Br aqueous solutions as a
function of their concentrations. [C10pim]Br (2), [C12pim]Br (b),
and [C14pim]Br (9) at 25 °C.
Thermodynamic Analysis of Micelle Formation. The elec-
trical conductivity of the IL aqueous solutions at different
Table 1. Surface Properties and Micellization Parameters of [Cnpim]Br (n = 10, 12, and 14) and [Cnmim]Br (n = 10, 12, and 14) in Aqueous
Solutions at 25 °C
cmc (mM)
determined
from surface tension
determined from
electrical conductivity
determined from
1H NMR
2
Amin (A )
Γ
max (μmol/m2)
˚
ILs
γ
cmc (mN/m)
[C10pim]Br
[C12pim]Br
[C14pim]Br
[C10mim]Bra
[C12mim]Bra
[C14mim]Brb
9.30
2.34
0.61
29.3
10.9
2.8
9.31
2.60
0.56
32.9
8.5
9.20
1.98
38.3
38.4
38.3
39.7
39.4
39.2
1.90
2.09
2.21
1.72
1.91
1.96
87.4
79.4
75.1
96.7
86.8
84.7
31
11
2.6
a Reported in refs 13 and 32. b Reported in ref 14.
1620 DOI: 10.1021/la104719v
Langmuir 2011, 27(5), 1618–1625

Shi et al.
Article
Figure 2. Logarithmic plots of cmc against the number of carbon
atoms in the alkyl chain (NC) for [Cnpim]Br at 25 °C.
temperatures was measured to characterize the thermody-
namics of micelle formation. The temperature dependence of
electrical conductivity as a function of concentration for
[Cnpim]Br (n = 10, 12, and 14) is shown in Figure 3. As the
temperature increases, the cmc value of [Cnpim]Br increases,
which is similar to the case for [Cnmim]Br.32 The effect of
temperature on the cmc of an ionic surfactant mainly includes
two aspects.33 A higher temperature reduces the degree of
hydration of the hydrophilic headgroups, which facilitates
micelle formation. Additionally, the water structures sur-
rounding the hydrophobic chains are destroyed as the tem-
perature increases, which hinders micelle formation. Because
the cmc increases with temperature, it is clear that the latter
effect plays a crucial role in micelle formation in these systems.
The degree of counterion binding (β) can also be obtained from
the electrical conductivity versus concentration plots on the basis
of the mixed electrolyte model of micellar solution developed by
Shanks and Franses.34 The values of β for [Cnpim]Br at different
temperatures are summarized in Table 2. Compared with
[Cnmim]Br, with the same alkyl chain lengths (e.g., β = 0.77
([C12mim]Br)),32 the present ILs, [Cnpim]Br, have a much lower
value of β. The possible reason is that the interaction between the
anion and hydrophilic group is reduced because of the larger size
of the hydrophilic groups in the headgroup.20,35
As is well established in the thermodynamics of micelle forma-
Figure 3. Specific conductivity as a function of concentration
at different temperatures for (a) [C10pim]Br, (b) [C12pim]Br, and
(c) [C14pim]Br.
tion, the standard Gibbs free energy of micelle formation (ΔGm0 ic
)
for ionic surfactants is given by eq 236
ΔG0mic ¼ ð1 þ βÞRT ln χcmc
ð2Þ
micelle formation (ΔH0mic) can be derived by applying the Gibbs-
where χcmc is the cmc in terms of the mole fraction. Once ΔG0mic as
a function of temperature is known, the standard enthalpy of
Helmholtz equation:
"
#
DðΔG0mic=TÞ
Dð1=TÞ
¼ ΔHm0 ic
ð3Þ
(26) Adamson, A. W., Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; Wiley-
Interscience: New York, 1997.
(27) Miki, K.; Westh, P.; Nishikawa, K.; Koga, Y. J. Phys. Chem. B 2005, 109,
9014–9019.
(28) Zhang, H. C.; Li, K.; Liang, H. J.; Wang, J. J. Colloids Surf., A 2008, 329,
75–81.
(29) Jaycock, M. J.; Parfitt, G. D. Chemistry of Interfaces; John Wiley and Sons:
New York, 1981.
Then the standard entropy of micelle formation (ΔS0mic) is obtained
through the following relation:
ΔHm0 ic - ΔG0mic
ΔSm0 ic
¼
ð4Þ
T
(30) Shinoda, K. Colloidal Surfactants: Some Physicochemical Properties;
Academic Press: New York, 1963; Chapter 1.
(31) Li, N.; Zhang, S. H.; Zheng, L. Q.; Dong, B.; Li, X. W.; Yu, L. Phys. Chem.
The thermodynamic parameters for the ILs [Cnpim]Br (n = 10,
12, and 14) were thus obtained and are listed in Table 2.
Chem. Phys. 2008, 10, 1–3.
The plots of ΔGm0 ic, ΔHm0 ic, and -TΔSm0 ic as a function of tem-
perature for [Cnpim]Br (n = 10, 12, and 14) are shown in Figure 4.
In the range of temperatures investigated, the values of ΔG0mic for
the three ILs are negative and the negative ΔG0mic values are
mainly contributed by ΔH0mic, suggesting that the micelle forma-
tion for the ILs inaqueous solution is enthalpy-driven. It is known
(32) Inoue, T.; Ebina, H.; Dong, B.; Zheng, L. Q. J. Colloid Interface Sci. 2007,
314, 236–241.
(33) Hunter, R. J. Foundations of Colloid Science; Oxford University Press:
New York, 1989; Vol. 1.
(34) Shanks, P. C.; Franses, E. I. J. Phys. Chem. 1992, 96, 1794–1805.
(35) Shimizu, S.; Pires, P. A. R.; El Seoud, O. A. Langmuir 2004, 20, 9551–9559.
(36) Stodghill, S. P.; Smith, A. E.; O’Haver, J. H. Langmuir 2004, 20, 11387–
11392.
Langmuir 2011, 27(5), 1618–1625
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Shi et al.
Table 2. Critical Micelle Concentration (cmc), Degree of Counterion Binding (β), and Thermodynamic Parameters of Micelle Formation for
[Cnpim]Br (n = 10, 12, and 14) in Aqueous Solutions at Various Temperatures
ILs
T (°C)
cmc (mM)
β
ΔG0m (kJ/mol)
ΔH0m (kJ/mol)
-TΔS0m (kJ/mol)
[C10pim]Br
25
30
35
40
45
25
30
35
40
45
25
30
35
40
45
9.8
0.48
0.47
0.46
0.44
0.43
0.54
0.52
0.50
0.47
0.43
0.50
0.47
0.44
0.40
0.38
-31.69
-31.79
-31.81
-31.60
-31.55
-38.02
-38.07
-38.05
-37.27
-37.15
-42.74
-42.51
-42.18
-41.79
-41.41
-32.3
-33.45
-34.56
-35.69
-36.84
-33.75
-43.56
-59.25
-65.64
-69.77
-55.06
-59.68
-63.62
-66.62
-68.76
0.61
1.66
2.96
4.09
5.29
-4.27
5.49
21.20
28.37
32.62
12.32
17.17
21.14
24.83
27.35
10.4
11.2
12.1
13.3
2.61
2.67
2.77
2.84
3.00
0.550
0.574
0.597
0.624
0.651
[C12pim]Br
[C14pim]Br
from the 2,4,6-trimethylphenyl group may be another reason for
the large negative ΔH0mic, as observed for carbazole-tailed imida-
zolium ILs and phenyl-tailed surfactants.16,21,24 Herein, it can be
demonstrated that the 2,4,6-trimethylphenyl group introduced
into the headgroup played an important role in micelle formation
by the ILs.
1H NMR Spectra. It is known that prominent changes in
both shielding and relaxation are presented in the NMR spectra
during the aggregation of surfactants.40-42 This means that
proton signal changes can be obtained in both the hydrophobic
chain and the hydrophilic group of a surfactant upon micelliza-
tion. The proton assignments and 1H NMR spectra for [C10pim]Br
and [C12pim]Br at different concentrations (below and above the
cmc) in D2O are shown in Figure 5a,b, respectively. (1H NMR
spectra for [C14pim]Br were uninterpretable owing to its low cmc.)
Discernible proton signals of the surfactant molecules in the 1H
NMR spectra undergo obvious changes upon micellization. In
particular, some small multiplets unite into broad ones (Hj, Hk),
which is powerful evidence of the formation of thermodynami-
cally stable self-assemblies.43 The variationof the chemical shift of
protons of [C10pim]Br, Δδ (= δobs - δmon), as a function of the
reciprocal concentration is shown in Figure 6 as an example.
Small changes in the chemical shifts are observed below the cmc,
whereas above the cmc the chemical shifts change significantly.
The sudden change in chemical shifts at the cmc may be caused by
an abrupt change in the environment of the headgroup when
micelles are formed; precisely, the configuration of the headgroup
is adjusted to fit the critical packing parameter required for
micelle formation.44 A similar result was obtained for a chiral
long-chain imidazolium IL, where the headgroup of the IL bends
into the hydrophobic region during micellization.45
Figure 4. Thermodynamic parameters of aggregate formation as
a function of temperature for (a) [C10pim]Br, (b) [C12pim]Br, and
(c) [C14pim]Br in water. Squares, circles, and triangles correspond
to ΔG0mic, ΔH0mic, and -TΔSm0 ic, respectively.
that the enthalpy change of micellization is mainly contributed by
hydrophobic and electrostatic interactions.37,38 During micelliza-
tion, the hydrocarbon chain of the surfactant monomer transfers
from the aqueous environment to the micelle, accompanied by the
release of solvating water molecules; this process is expected to be
exothermic.39 Electrostatic interactions are divided into self-
repulsion of the headgroups and the counterions (exothermic)
and attraction between the headgroup and the counterion (endo-
thermic).35 As mentioned above, the degree of counterion binding
(β) of the ILs [Cnpim]Br is rather low, suggesting that the self-
repulsion of the headgroups and the counterions is stronger than
the attraction between the headgroups and the counterions. That
is, the contribution of electrostatic interactions should also be
exothermic. Meanwhile, the effect of π-π interactions arising
Protons inthe present N-aryl imidazolium ILs could bedevided
into three classes: imidazole protons (Ha, Hb; no signal for Hc is
observed because of the deuteration) and protons in the 2,4,6-
trimethylphenyl group (Hd-Hh) and in the alkyl chain (Hi-Hl). It
is clear in Figure 6 that protons of Hb and Hi move downfield
€
€
(40) Lindman, B.; Soderman, O.; Wennerstrom, H. In Surfactant Solutions:
New Methods of Investigation; Zana, R., Ed.; Marcel Dekker: New York, 1987;
Chapter 6.
(41) Nakagawa, T.; Tokiwa, F. In Surface and Colloid Science; Matijevic, E., Ed.;
Wiley: New York, 1976; Vol. 9, p 69.
(42) Haigh, C. W.; Mallion, R. B. Prog. Nucl. Magn. Reson. Spectrosc. 1980, 13,
303–344.
(37) Myers, D. Surfaces, Interfaces, and Colloids: Principles and Applications,
2nd ed.; Wiley-VCH: New York, 1999.
(43) Singh, T.; Kumar, A. J. Phys. Chem. B 2007, 111, 7843–7851.
(44) Li, N.; Zhang, S. H.; Zheng, L. Q.; Inoue, T. Langmuir 2009, 25, 10473–
10482.
(38) Muller, N. Langmuir 1993, 9, 96–100.
ꢀ
(39) Faucompre, B.; Bouzerda, M.; Lindheimer, M.; Douillard, J. M.; Partyka,
(45) Li, X. W.; Gao, Y. A.; Liu, J.; Zheng, L. Q.; Chen, B.; Wu, L. Z.; Tung,
S. J. Therm. Anal. 1994, 41, 1325–1333.
C. H. J. Colloid Interface Sci. 2010, 343, 94–101.
1622 DOI: 10.1021/la104719v
Langmuir 2011, 27(5), 1618–1625

Shi et al.
Article
Figure 7. Two-dimensional NOESY spectra of [C10pim]Br in
D2O at a concentration of 40.0 mM at 25 °C.
immersed in a hydrophobic core and the hydrophilic headgroups
are solvated by interfacial water molecules whose polarity is lower
than that of bulk water. The decrease in the polarity of the micro-
environment upon solvation leads to a downfield shift for the
protons in the hydrophilic region.48 The conformational effects,
resulting from a partial changeover from a gauche to trans con-
formation in the alkyl chains upon micellization, usually make the
protons in the middle of the hydrophobic chain shift downfield.49
That is, in the absence of any other specific interaction that affects
the observed chemical shift, one expects a downfield shift, as can
be seen from Hb and Hi. The upfield shifts of other protons would
be caused by the ring effect originating from the aromatic ring in
the 2,4,6-trimethylphenyl group. It is known that protons located
above and below the plane of the aromatic ring resonate at a
higher field because of the shielding effect.50 As the micelles are
formed, the hydrophobicity of the 2,4,6-trimethylphenyl group
makes it bend into the hydrophobic regions slightly, which causes
the protons on the hydrophobic chains to move into the field of
the ring current and then shift upfield. This result can be further
verified by 2D NOESY. The 2D NOESY spectra of [C10pim]Br in
D2O at a concentration of 40.0 mM are provided in Figure 7,
where a NOE contact between the protons in the 2,4,6-trimethyl-
phenyl group (Hf,g) and the hydrophobic chains (Hk) is presented.
The result suggests that the orientation of the 2,4,6-trimethylphe-
nyl group is favorable in the hydrophobic region and the group
interacts with the hydrophobic chains.
Figure 5. Proton assignments and 1H NMR spectra of (a) [C10pim]Br
and (b) [C12pim]Br at concentrations below and above the cmc in D2O
at 25 °C.
The protons of the 2,4,6-trimethylphenyl group (Hd-Hh) shift
upfield during micellization, which is in agreement with the result
obtained above. As the 2,4,6-trimethylphenyl group bends into
the hydrophobic regions upon micellization, the decrease in the
polarity of the media in the hydrophobic regions could reduce the
deshielding effect on the protons attached to the 2,4,6-trimethyl-
phenyl group, which causes their upfield shifts.51 In addition, the
Figure 6. Variation of chemical shifts for various protons of
[C10pim]Br at different concentrations in D2O at 25 °C.
whereas upfield shifts are exhibited for other protons. The chem-
ical shifts upon micellization are mainly affected by the medium
and conformational effects.46,47 The medium effect is caused by
the movement of surfactant molecules from water to micelles,
where the hydrophobic chains of the surfactant molecules are
(48) Tada, E. B.; Novaki, L. P.; El Seoud, O. A. Langmuir 2001, 17, 652–658.
(49) Persson, B. O.; Drakenberg, T.; Lindman, B. J. Phys. Chem. 1979, 83, 3011–
3015.
(50) Shimizu, S.; El Seoud, O. A. Langmuir 2003, 19, 238–243.
(51) Fendler, J. H.; Fendler, E. J.; Infante, G. A.; Shih, P. S.; Patterson, L. K.
J. Am. Chem. Soc. 1975, 97, 89–95.
(46) Huang, X.; Han, Y. C.; Wang, Y. X.; Wang, Y. L. J. Phys. Chem. B 2007,
111, 12439–12446.
(47) Persson, B. O.; Drakenberg, T.; Lindman, B. J. Phys. Chem. 1976, 80, 2124–
2125.
Langmuir 2011, 27(5), 1618–1625
DOI: 10.1021/la104719v 1623

Article
Shi et al.
Figure 9. Variation of chemical shifts for the terminal proton (Hl)
signals vs the reciprocal concentration of [C10pim]Br (left) and
[C12pim]Br (right) in D2O at 25 °C.
Figure 8. Schematic illustration of the proposed structures of
micelles formed by [Cnpim]Br (n = 10 as an example). Circles
denote the imidazole rings; wavy lines denote the alkyl chains; and
ellipses denote the 2,4,6-trimethylphenyl groups.
diamagnetic anisotropy of the π-π conjugated system formed by
the phenyl ring would bring about a remarkable shielding effect
and thus lead to an upfield shift of the attached protons.16 On the
basis of theoretical calculations, Palomar52 also believes that the
chemical shift of the protons at the aromatic ring shifts downfield
because of the specific interactions with water molecules whereas
the nonspecific polarization effects of the solvent cage provide the
opposite effect. According to the analysis above, a possible
packing structure of molecules in the micelle is depicted in Figure
8. In this staggered arrangement, the 2,4,6-trimethylphenyl group
tends to move awayfrom the micellar interface and then bend into
the hydrophobic region to a small extent.
Interestingly, an upfield shift is observed for the other proton
on the imidazolium ring (Ha). Competition between the medium
effect and the phenyl-group-induced ring-current effect could be
responsible for the result. As the 2,4,6-trimethylphenyl group is
inserted, the aromatic-moiety-induced magnetic deshielding ef-
fect on Ha is more significant when compared to the hydrogen-
bonding interaction between Ha and interfacial water molecules.
When the micelles are formed, the deshielding effect is reduced by
the decreased polarity, which causes an upfield shift for Ha.
The cmc for surfactants can also be obtained from the 1H
NMR chemical shifts.53 On the basis of the fast exchange of the
surfactant molecules on the NMR timescale, the observed chem-
ical shift (δobs) can be expressed as a weighted average of the
Figure 10. (a) Excitation and (b, c) emission spectra of [C10pim]Br
inaqueous solutions with a concentration of 2.0 mmol/L. λem (nm):
(a) 370. λex (nm): (b) 290 and (c) 260.
an example of the variation of chemical shifts (δobs - δmon) versus
1/CT plots for the terminal protons (Hl) of [C10pim]Br and
[C12pim]Br. It can be seen that two straight lines were obtained
and the intersection point corresponds to the cmc (9.20 mM for
[C10pim]Br, 1.98 mM for [C12pim]Br), which is a little lower than
that in H2O because D2O is a more structured liquid.54
Fluorescence Properties. The fluorescence properties of
[Cnpim]Br (n = 10, 12, and 14) in aqueous solution were measured
at various concentrations. Figure 10 shows the excitation and
emission spectra for aqueous solutions of [C10pim]Br at a con-
centration of 2.0 mmol/L. It can be seen that strong fluorescence
peaks are present in both the excitation and emission spectra.
Notably, when excited at different excitation wavelengths, the
emission peak position remains unchanged, suggestingthat the N-
aryl imidazolium ILs have stable, strong fluorescence. It is known
that most traditional imidazolium ILs are not fluorescent.55,56
Thus, the fluorescence of the new ILs is ascribed to the incorpora-
tion of the 2,4,6-trimethylphenyl group. The unique fluorescence
properties of the ILs may extend their application into the field of
photochemistry.
chemical shifts of the monomer (δmon) and the micelle (δmic
)
ꢀ
ꢁ
ꢀ
ꢁ
Cmon
CT
Cmic
CT
δobs ¼ δmon
þ δmic
ð5Þ
where Cmon, Cmic, and CT are the concentrations of surfactant
existing as monomers, micelles, and the total concentration, respec-
tively. It is assumed that the monomer concentration is constant
above the cmc, thus
Conclusions
The aggregation behavior of three N-aryl imidazolium ILs
[Cnpim]Br (n = 10, 12, and 14) in aqueous solutions was inves-
tigated by surface tension, electrical conductivity, and 1H NMR.
The cmc obtained for [Cnpim]Br is much lower than that for
[Cnmim]Br, suggesting that the incorporation of the 2,4,6-trimethyl-
phenyl group facilitates micelle formation. The prominently
enhanced π-π interactions due to the 2,4,6-trimethylphenyl
ꢀ
ꢁ
cmc
δobs ¼ δmic
-
ðδmic - δmon
Þ
ð6Þ
CT
δmon and δmic can be estimated by extrapolation of the plots of
δobs versus CT and δobs versus 1/CT, respectively. Figure 9 shows
(52) Palomar, J.; Ferro, V. R.; Gilarranz, M. A.; Rodriguez, J. J. J. Phys. Chem.
B 2007, 111, 168–180.
(53) Goon, P.; Das, S.; Clemett, C. J.; Tiddy, T.; Kumar, V. V. Langmuir 1997,
13, 5577–5582.
(54) Chang, N. J.; Kaler, E. W. J. Phys. Chem. 1985, 89, 2996–3000.
(55) Paul, A.; Mandal, P. K.; Samanta, A. Chem. Phys. Lett. 2005, 402, 375–379.
(56) Paul, A.; Mandal, P. K.; Samanta, A. J. Phys. Chem. B 2005, 109, 9148–
9153.
1624 DOI: 10.1021/la104719v
Langmuir 2011, 27(5), 1618–1625

Shi et al.
Article
groups cause the [Cnpim]Br molecules to arrange densely at the
air-water interface. The thermodynamic results revealed that the
micelle formation for [Cnpim]Br (n = 10, 12, and 14) is enthalpy-
driven throughout the whole temperature range (25-45 °C),
which is attributed to strong self-repulsion of the headgroups
and counterions and π-π interactions among the 2,4,6-trimethyl-
formed. These N-aryl imidazolium ILs present strong, stable fluo-
rescence properties, which could extend their applications in
optical fields.
Acknowledgment. We thank the National Natural Science
Foundation of China (nos. 20773081 and 50972080) and the
National Basic Research Program (2009CB930101) for financial
support and Dr. Pamela Holt (Shandong University) for proof-
reading the manuscript.
1
phenyl groups in the micelles. The results of H NMR and 2D
NOESY spectra indicate that the 2,4,6-trimethylphenyl group
slightly bends into the hydrophobic region when micelles are
Langmuir 2011, 27(5), 1618–1625
DOI: 10.1021/la104719v 1625