Aggregation of double-tailed ionic liquid 1,3-dioctylimidazolium bromide and the interaction with triblock copolymer F127

Article abstract of DOI:10.1021/jp405838v

The aggregation of ionic liquid-based double-tailed surfactant, 1,3-dioctylimidazolium bromide ([Doim]Br) and its interaction with pluronic copolymer F127 were systematically investigated by nuclear magnetic resonance (NMR), surface tension, dynamic light scattering (DLS), and isothermal titration calorimetry (ITC). It was found that the [Doim]Br aggregates are composed with the alkyl chains embedded in the micellar core and with the imidazolium rings parallel and staggered on the hydrophilic layer of micelles, which was generally different from the single-tailed IL [omim]Br. The hydrogen bonding between protons attached to the imidazolium rings and anion Br^- was enhanced upon the aggregation of [Doim]Br. The aggregation of F127 was promoted by addition of [Doim]Br, which was more efficient than the single-tailed surfactant. At lower [Doim]Br content, [Doim]Br monomers embedded deeply into F127 micelle core. At higher [Doim]Br concentrations, the F127 micelles were disassociated, and then F127 chains penetrated into [Doim]Br micelle. In addition, the microstructure of F127/[Doim]Br complex can also be tuned by temperature. ? 2013 American Chemical Society.

Full text of DOI:10.1021/jp405838v

Article
pubs.acs.org/JPCB
Aggregation of Double-Tailed Ionic Liquid 1,3-Dioctylimidazolium
Bromide and the Interaction with Triblock Copolymer F127
Lingling Ge, Qi Wang, Duo Wei, Xiaohong Zhang, and Rong Guo*
School of Chemistry and Chemical Engineering, Yangzhou University, Jiangsu Province 225002 People’s Republic of China
ABSTRACT: The aggregation of ionic liquid-based double-tailed surfactant, 1,3-dioctylimidazolium bromide ([Doim]Br) and
its interaction with pluronic copolymer F127 were systematically investigated by nuclear magnetic resonance (NMR), surface
tension, dynamic light scattering (DLS), and isothermal titration calorimetry (ITC). It was found that the [Doim]Br aggregates
are composed with the alkyl chains embedded in the micellar core and with the imidazolium rings parallel and staggered on the
hydrophilic layer of micelles, which was generally different from the single-tailed IL [omim]Br. The hydrogen bonding between
protons attached to the imidazolium rings and anion Br− was enhanced upon the aggregation of [Doim]Br. The aggregation of
F127 was promoted by addition of [Doim]Br, which was more efficient than the single-tailed surfactant. At lower [Doim]Br
content, [Doim]Br monomers embedded deeply into F127 micelle core. At higher [Doim]Br concentrations, the F127 micelles
were disassociated, and then F127 chains penetrated into [Doim]Br micelle. In addition, the microstructure of F127/[Doim]Br
complex can also be tuned by temperature.
[Cl−], [Br−], [BF4 ], and [PF6 ]. All the results above showed
that the cation, as well as ion pair, could interact with water. It
was also reported that the ionic hydration, the cation−anion
association, and the aggregation of ILs offset each other.2c In
addition, the alkyl chain length plays an important role in the
aggregation of ILs. It is found that long alkyl chains hinder the
interaction between the rings and water, which is favorable to
the aggregation.2c,10 Gemini imidazolium surfactants processing
two headgroups and two alkyl chains have lower critical
micellization concentration (cmc), and show the tendency to
form premicellar aggregation at sufficiently low concentration.11
Scientific research developed from the beginning with the
basic physical properties, as well as their aggregation behaviors
in the pure surface active IL systems,2 to very recently the
mixed systems of ILs with small molecules,2a,3 synthetic
polymers,4 and biomacromolecules.5 Investigations on the
−
−
INTRODUCTION
■
Ionic liquids (ILs) have entered into researchers’ sight since the
1940s. They are classified as organic salts. The steric hindrance
of the huge ion disfavors their formation of crystals and
subsequently lowers the melting points. The resultant large
liquid range and negligible vapor pressure, as well as their
thermal stability and nonflammability make ILs green replace-
ments for the traditional volatile organic solvents in chemical
procedures.1 Surface active ILs generate specific self-assembled
structures in aqueous solution. The aggregates can act as a
novel micellar catalyst and perform enhanced template action
for the synthesis of nanostructured materials.2 Hence, the
investigation on the self-assembling of ILs is of great interest.
Many reports2−4,5a,6 elucidated the roles of different factors on
the aggregation such as the type of cations, the nature of
counterions, and the alkyl chain length. Headley et al.7,10 have
revealed the effect of counterions on the hydration of
imidazolium based on NMR measurements. Wang et al.9
applied the quantum chemical calculations to study the
interaction between water and ILs with different counterions
Received: June 13, 2013
Revised: November 11, 2013
Published: November 14, 2013
© 2013 American Chemical Society
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interaction between surface active ILs and amphiphilic block
copolymer, such as triblock copolymer poly(ethylene oxide)−
poly(propylene oxide)−poly(ethylene oxide) (PEO−PPO−
PEO), are of great interest. Because the block copolymer
would affect the aggregation behavior of ILs by forming the IL/
copolymer complex driven mainly by the hydrophobic force
and the electrostatic interaction. As reported by Singh et al.,3
poly(ethylene oxide) and its derives reduced the spontaneity of
the aggregation of ILs. On the other hand, the presence of
surface active ILs in the mixed systems of ILs/copolymer can
also realize the control of the microstructure and subsequently
the properties of copolymer solutions by friendly physical
methods.12,13 Studies in the mixed system of copolymer P123
and IL 1-pentyl-3-methylimidazolium tetrafluoroborate
([pmim][BF4]) were performed by Adhikari et al.4a It has
been found that the IL surfactant penetrated into the
hydrophobic core of P123 micelles at lower concentrations
and then invaded both the core and corona region, reducing the
difference between the two regions. The presence of the ion
pairs of ILs and their electrostatic repulsion may be responsible
for the expansion of the polymer core. The same phenomena
was observed in the system of copolymer P104 and 1-butyl-3-
methyl-imidazolium bromide ([bmin]Br).4b In our group, the
interaction mechanism and the binding mode of the copolymer
with traditional nonionic surfactant Triton X-10014 and
functional nonionic surfactant, the hydrophilic modified anti-
inflammatory drug ibuprofen,15 have been systematically
studied. The complexes with various microstructures were
observed. More interestingly, the microstructures of these
complexes were found to be turned by temperature and the
concentration of surfactant.
Scheme 1. Molecular Structure of [Doim]Br
1H NMR: 10.822 ppm (S, 1H, C2−H), 7.239 ppm (S, 2H,
C4, C5−H), 4.368 ppm (T, 4H, C6−H), 1.927−1.275 ppm
(M, 24H, C7−H), 0.874 ppm (T, 6H, C8−H). 13C NMR:
137.085 ppm (C2), 122.132 ppm (C4,5), 50.145 ppm (C6, 2),
31.681 ppm (C7, 2), 30.360 ppm (C8, 2), 29.048 ppm (C9, 2),
28.968 ppm (C10, 2), 26.263 ppm (C11, 2), 22.582 ppm (C12,
2), 14.060 ppm (C13, 2). Element analysis (% in mass)
calculated: C, 61.11; H, 9.99; N, 7.5. Found: C, 60.46; H, 9.79;
N, 7.26. HPLC purity: 97.2%.
Surface Tension Measurement. Isothermal surface
tension measurements were carried out by the DCAT 11
surface tension device (Data physics Instruments GmbH,
German) using the platinum plate mode. The surface tension
was determined with a single-measurement mode. The results
were accurate within 0.1 mN/m. Before each measurement,
distilled water was used as a calibration and the value was 72.0
0.1 mN/m. The temperature was controlled by water bath,
and the uncertainty was within 0.1 °C.
Nuclear Magnetic Resonance (1H NMR). The NMR
experiments were conducted on a Bruker Avance 600
spectrometer with a 1H frequency of 600.13 MHz. The
temperature was kept constant within 0.1 °C by a Bruker
BCU-05 temperature control unit. The samples were allowed
to equilibrate at the desired temperature for at least 10 min
before measurement. A total of 32 accumulations was generally
acquired. A stock solution of 0.003 M DSS in D2O in a capillary
was inserted into the sample tube as a reference, which was
used to eliminate the temperature-induced shifts. D2O was used
as solvent. The temperature ranged from 5 to 40 °C, and the
interval was 5 °C. The data at [Doim]Br concentration higher
than 50 mM could not be obtained at a temperature lower than
15 °C, due to the limitation of solubility in D2O. The error of
chemical shifts value obtained was within 0.01 ppm.
Two-dimensional hydrogen nuclear Overhauser effect spec-
troscopy (2D NOESY) experiments were performed at 30 °C
with a standard three-pulse sequence.16 A mixing time of 600
ms and 32 scans for each of the 1024 t1 increments were
recorded.
Dynamic Light Scatter (DLS). The DLS experiments have
been described in detail in ref 17. In this study, all solutions
were filtered through a Millipore filter with 0.22 μm pore size.
In addition, all the samples were heated from lower to higher
temperature to eliminate the difference caused by heating and
cooling during the temperature effect study.18 The temperature
was controlled by pumping the water bath to the sample
holder, and the uncertainty of the temperature was 0.1 °C.
The samples were thermostated at the desired temperature for
at least 10 min before measurements. Measurements were
carried out at a scattering angle of 90°. Experiment duration
was 5 min, and each experiment was repeated two times or
more.
In the present contribution, a new kind of double-tail
surfactant, 1,3-dioctylimidazolium bromide ([Doim]Br), was
synthesized, and the aggregation behavior of this novel ionic
liquid-based surfactant was investigated. In addition, the
interaction mechanism between [Doim]Br and F127 as well
as the microstructure change of the complex was systematically
studied.
EXPERIMENTS SECTION
■
Materials and Sample Preparation. Pluronic F127 of
average composition EO97PO68EO97 was obtained from Sigma
Corp. and used without further purification; 2-dimethyl-2-
silapentane-5-sulfonate sodium (DSS, >97%) was a Sigma
product. Deuterium oxide (99.9%) was from Qing Dao Teng
Long Corp., and 1-bromooctane (99%) was purchased from
Shanghai Chemical Corp.
Ionic liquid [Doim]Br was synthesized by two steps. First,
metal sodium (7.6 g) was added to the anhydrous ethanol
(about 100 mL) gradually to form ethanol−sodium ethylate
stock solution. Imidazole (20.0 g) was then put into the stock
solution and subsequently 1-bromooctane (59.6 mL) was
added when solid imidazole disappeared. The reaction solution
was stirred at 65 °C for 20 h, and then a yellow liquid was
obtained. The liquid was washed by water and extracted by
anhydrous ethyl ether, which were repeated two times or more.
The anhydrous ethyl ether was removed by rotary evaporator,
and finally 1-octylimidazole was gained (>90%). Second, 1-
octylimidazole (6.5 mL) and 1-bromooctane (6.9 mL) were
mixed and stirred at 100 °C for 20 h with the protection of
nitrogen. After the yellow viscous liquid was washed by light
petroleum and the light petroleum was wiped off by a rotary
evaporator, the product [Doim]Br was gained (Scheme 1).
Isothermal Titration Calorimetry (ITC). The calorimetry
experiments were performed using a titration microcalorimeter
(Microcal VP-ITC, North Hampton, MA) at temperatures of
10
0.1 and 30 0.1 °C.14 A computer-controlled syringe
(250 μL) was filled with the [Doim]Br solution at higher
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concentrations, which was titrated into a 1.438 mL sample cell
containing purified water or 20 mg/mL F127 aqueous solution.
The volume of each injection was 5 μL, and the time delay
between successive injections was 360 s. The stirring speed was
maintained at 502 rpm to ensure thorough mixing.
Conductivity Measurement. The conductivities of the
[Doim]Br solutions were carried out using a DDS-11A digital
conductivity meter (Shanghai Tianda Precision Instrument Co.
Ltd.) calibrated by KCl aqueous solution.
with F127 suppresses the self-aggregation of [Doim]Br. As
reported in ref 18, the aggregation behavior of the copolymer
strongly depends on temperature, mainly because the hydration
state of the PPO blocks is temperature sensitive. The
hydrophobicity of the PPO blocks will increase upon heating
due to the disrupted hydrogen bond between ether oxygen and
water. The reported critical micellization temperature (cmt) for
20 mg/mL F127 aqueous solution is about 25 °C.14 In the
present study, the surface tension curve shifts to higher [Doim]
Br concentration as the temperature rises from 15 to 25 °C.
This shift of surface tension curves and the increased cmc value
of [Doim]Br indicate that the interaction between [Doim]Br
and F127 is enhanced with rising temperature, from which we
can deduce that the hydrophobic interaction between PPO
blocks and the alkyl chains of [Doim]Br is probably one of the
main driving forces. At temperatures higher than the cmt, no
further difference of surface tension is observed, which suggests
that the temperature-enhanced interaction is saturated.
RESULTS AND DISCUSSION
Surface Tension Measurement. Figure 1 shows the
surface tension curve of 1,3-dioctylimidazolium bromide
■
NMR Measurement. Figure 2 shows the plot of the
chemical shift variation Δδ of [Doim]Br protons. Inter alia, Δδ
Figure 1. Surface tension in the system [Doim]Br/F127/H2O at
various temperatures. The concentration of F127 is 20 mg/mL. The
surface tension of [Doim]Br aqueous solution at 25 °C is also
presented. The temperature uncertainty is within 0.1 °C.
Figure 2. [Doim]Br concentration dependence of Δδ for [Doim]Br
protons at 25 0.1 °C. Δδ = δobs − δori, where δori is the chemical shift
value at the lowest [Doim]Br concentration under study.
([Doim]Br) in the absence and presence of F127. The surface
tension in the absence of F127 starts to decrease from about
64.0 mN/m to about 25.9 mN/m at a [Doim]Br concentration
of 8.0 mM, where the surface tension remains constant. The
concentration of 8.0 mM at the break point is ascribed to the
critical micellar concentration (cmc) of [Doim]Br. The surface
tension of single chain surfactant 1-methyl-3-octylimidazolium
bromide ([omim]Br) has been measured by Wang et al.2e The
cmc of [omim]Br is about 120 mM, which is about 15 times
higher than that of [Doim]Br, and the plateau surface tension
value is as high as 35.0 mN/m. These results indicate that
[Doim]Br, which has the double symmetrical aliphatic chains,
has remarkably higher surface activity. As for the curves in the
presence of F127, the initial surface tension value at the lowest
concentration of [Doim]Br investigated is only about 38 mN/
m, which is close to the value in the system F127/H2O,
indicating that the air/water surface is occupied mostly by F127
chains. The surface tension decreases with further addition of
[Doim]Br due to the adsorption of [Doim]Br molecules onto
the surface. The final plateau surface tension values are the
same as that in pure [Doim]Br solutions, which indicates that
most of the F127 chains on the surface are replaced by [Doim]
Br molecules that are of higher surface activity. It should be
noted that the cmc value of [Doim]Br in the presence of F127
gets much higher, indicating that the interaction of [Doim]Br
= δobs − δori, where δori is the chemical shift value at the lowest
[Doim]Br concentration under study and δobs is the observed
chemical shift at higher [Doim]Br concentrations. No signal for
H2 is observed due to the deuteration. It is worthy to mention
that the protons at the same position on the two symmetrical
alkyl chains have the same chemical shift value, which further
indicates that the chemical environment of the two alkyl chains
is uniform. From Figure 2 we can find that all the Δδ values
keep constant at lower [Doim]Br concentrations. Obvious
changes of Δδ are observed at [Doim]Br concentrations higher
than 8.0 mM, which is defined as the cmc of [Doim]Br. The
cmc value agrees with that determined by surface tension
measurement. An abrupt increase of protons H4,5 on the
imidazolium ring as well as nearby protons H6, is observed,
whereas Δδ values of protons H8−12 and H13 on the alkyl
chains decrease. In addition, no obvious variation of Δδ for the
proton H7 is observed.
The double-tailed ILs exist as monomers in diluted solutions,
and both the cations and anions are highly dissociated and
hydrated. As the monomers transfer from water to aggregates
above the cmc, the double tails are immersed into the
hydrophobic micellar core, and the hydrophilic imidozalium
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rings stay on the surface of the micelle and remain hydrated.
Along with the cations, anion Br− will also be transferred from
water to the micellar surface, resulting in increased local
concentration and hence the enhanced cation−anion associa-
tion.2c,8,19 The conductivity profile of the [Doim]Br solution is
shown in Figure 3. Two linear regions are obtained. and the
conformation will also occur upon aggregation and contribute
to the upfield shift of alkyl protons.2a The chemical shift change
of proton H7 in the vicinity of polar head groups is not obvious,
as they locate at the interface between the hydrophilic layer and
hydrophobic core where the effects mentioned above offset
each other.3
For the mixed system F127/[Doim]Br/H2O, the effect of
[Doim]Br on the temperature-induced aggregation of F127 is
investigated by the chemical shift change of PO−CH3 protons
(Figure 4).14,23 The chemical shift curve of pure F127 aqueous
Figure 3. Electric conductivity (κ) of [Doim]Br solution as a function
1
Figure 4. (A) Temperature-dependent H NMR chemical shifts of
of concentration at 25
vertical marks.
0.1 °C. The cmc values are denoted by
PO−CH3 protons in 20 mg/mL F1 27 aqueous solution at different
[Doim]Br concentrations. (B) [Doim]Br concentration dependence
of the cmt of F127. The temperature uncertainty is 0.1 °C.
break of the conductivity curve originates from the micellization
of [Doim]Br. The cmc value is in good agreement with the
values determined by surface tension and NMR measure-
ments.20 The slope of the linear region above the cmc is smaller
than that below the cmc, indicating an effective loss of ionic
charges owing to the binding of counterions on the micellar
surface. This phenomenon verifies the enhanced cation−anion
association. As reported by Singh et al.,3 the protons on the
imidazolium ring, e.g., H2 and H4,5 including proton H6 on
the side chains, are acidic due to the vicinity of electronegative
nitrogen. They tend to form hydrogen bonding with the anion
Br− and water. The enhanced hydrogen bonding between the
protons H4,5,6 and Br− leads to a downfield shift.21 What is
more, the protons lying outside the shielding cone of the
imidazolium ring experience a downfield shift, because the
imidazolium ring is conventionally aromatic and capable of
sustaining a ring current.2c,21,22 It has been confirmed that the
imidazolium rings arrange in a stack way with the rings parallel
and staggered.2c,21,22 Thus, the proton H4,5 and H6 lying
outside the shielding cone would undergo a downfield shift
upon the micellization as the ion pairs get closer. The enhanced
hydrogen bonding as well as the ring current is verified by the
increased chemical shift of proton H4,5, as well as H6 (Figure
2). It should be noted that the extent of increase in chemical
shift upon the micellization of double-tailed [Doim]Br is much
higher than that of single-tailed [omim]Br.2c This phenomenon
is attributed to more significantly enhanced hydrogen bonding
upon micellization, because the initial contact between [Doim]
Br monomers and solvent is much weaker due to the steric
hindrance of two long hydrophobic side chains.10 As for the
alkyl protons H8−13, the dehydration and subsequently
interrupted deshielding effect should be the main reason for
the upfield shift upon aggregation. In addition, a partial
structural change of the alkyl chain from trans to gauche
solution agrees with the result reported.14 The initially higher
constant δ value at 1.17 ppm indicates highly hydrated PO
blocks due to the deshielding effect from the hydrogen bond
between ether oxygen and water, and the polymers exist as
water-swollen random coils.23 An abrupt decrease of δ value
corresponds to the temperature-induced breakdown of the
hydrogen bond and subsequently the aggregation of the PO
block to form the hydrophobic core of the copolymer micelle.
The first inflection of the chemical shift curve is ascribed to the
critical micellization temperature (cmt) and the value obtained
for 20.0 mg/mL F127 aqueous solution is about 26 °C, which
agrees well with the value reported.14 Upon addition of
[Doim]Br, the initial δ value remains at about 1.17 ppm,
suggesting the unchanged hydration degree of the F127
random coil. What is more, all the δ curves exhibit the same
manner at all the concentrations of [Doim]Br investigated.
Similar phenomena are observed in the mixed system F127/
TX-100, but only within the lower surfactant concentration
region (<9.42 mM).14 Within the higher TX-100 concentration
region (27.85−95.85 mM), the increased initial δ values at
lower temperatures as well as the reincrease of the δ value at
higher temperatures are reported. The increased initial δ value
is ascribed to the stretching effect on F127 unimers induced by
the adsorption of TX-100 micelles. The reincrease of the δ
value is caused by the microstructural rearrangement of the
F127/TX-100 complex. In the present system F127/[Doim]Br,
neither of these two phenomena is observed. The main reason
is the smaller size of [Doim]Br aggregates, which will be further
demonstrated in the Dynamic Light Scatter section. In addition,
the final constant δ value of each curve keeps decreasing with
increasing [Doim]Br content, from about 1.14 ppm to about
1.03 ppm, whereas the lowest δ value in the system F127/TX-
100 is as high as about 1.12 ppm. The decrease in δ value
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mainly originates from the improved hydrophobic interaction
between PPO blocks and [Doim]Br. The lower δ values imply
that the double-tailed surfactant is more efficient than the
single-tailed surfactant in complexation with F127. The cmt
values in the mixed systems of F127/[Doim]Br can be obtained
from the curves in Figure 4A. The value keeps decreasing with
increasing amount of [Doim]Br till 50.50 mM (Figure 4B),
after which the value tends to be constant, indicating that the
interaction between F127 and [Doim]Br is saturated. The
reduced cmt value indicates that the addition of [Doim]Br
promotes the aggregation of F127.
curve happens to be the same as the cmt values of F127 in the
mixed system. This phenomenon indicates that [Doim]Br
molecules are solubilized into F127 micelles driven by the
hydrophobic force. Proton H13 then experiences a more
hydrophilic environment because the hydrophobicity of the
PPO core is lower than that of the premicellar core of
[Doim]Br.24 At higher concentrations of [Doim]Br, the δ
values of H13 in the mixed system exhibit a linear increase with
rising temperature. It is probably because the PPO chains of
F127 gradually penetrate into [Doim]Br micelles due to
improved hydrophobic properties of the PPO chain.24 It should
be noted that the data at 5 and 10 °C are missed at higher
[Doim]Br concentrations, because the system becomes turbid
in D2O unlike in H2O. Similar phenomena have been
reported,25 and the reason is the different structure of D2O
and H2O. Compared with the cases of the alkyl protons, two
main differences are observed in the δ value of imidazolium ring
protons H4,5 (Figure 5B). The first is that the higher the
chemical shift, the higher the concentration of [Doim]Br. The
reasons are the enhanced cation−anion association, the
improved binding with water, and also the ring stacking in
the micelles as discussed above. Second, the δ value of proton
H4,5 shows a decrease with increased temperature, which is
caused by the prominent dehydration of the imidazolium ring
upon heating.
The microenvironment the change of [Doim]Br protons
upon complexation with F127 is also monitored. As shown in
Figure 5A (hollow symbols), the lower the chemical shift of the
Nuclear Overhauser effect spectroscopy (NOESY) is usually
applied to provide information on the arrangement of
molecules in space with the distance of the protons within
0.5 nm. The contour plots of the 2D NOESY spectra for the
[Doim]Br/F127/D2O system are shown in Figure 6. The
concentrations of [Doim]Br are chosen to be lower and higher
than cmc, and the F127 concentration is fixed. As shown in
Figure 6, the cross peaks of H(8−12) with the nearby protons
H7 and H13 are observed in both systems, and they mainly
originate from intramolecular NOE.26 By comparison, in the
spectra at different concentrations of [Doim]Br (Figure 6A,B),
the cross-peak between protons H(8−12) and H6 appears at a
concentration higher than the cmc, which reflects their
decreasing distance and subsequently verifies the aggregation
of [Doim]Br monomers. Former investigations on the system
of the single-tailed IL [omim]Br by Zhao et al.2c showed that
alkyl chains of [omim]Br extended to the other two
imidazolium rings verified by the cross peak between protons
of the terminal methyl and the rings. In the present manuscript,
1
Figure 5. Temperature-dependent H NMR chemical shifts of H13
(A) and H4,5 (B) signals of [Doim]Br aqueous solution in the
absence (hollow symbols) and presence (filled symbols) of 20 mg/mL
F127 at different [Doim]Br concentrations. The temperature
uncertainty is 0.1 °C.
terminal alkyl proton H13, the higher the concentration, which
indicates that proton H13 transfers from the bulk to the
hydrophobic micellar core. In addition, the δ values of H13
keep decreasing with rising temperature at all [Doim]Br
concentrations investigated, which originates from the dis-
turbance of the water structure around the alkyl chains of
[Doim]Br by heating. In the mixed system F127/[Doim]Br/
D2O, an “S” shape is observed at lower concentrations of
[Doim]Br. The temperature at the first intersection of the δ
Figure 6. 2D NOESY spectra in the system [Doim]Br/F127/D2O with [Doim]Br concentrations of 4.38 mM (A) and 60.22 mM (B) at 30 0.1
°C. The concentration of F127 is fixed at 20 mg/mL.
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no cross peak between protons H13 and H4,5 is observed,
suggesting that the alkyl chains of [Doim]Br mainly embed in
the core of [Doim]Br micelles apart from the imidazolium
rings. F127 mainly exists as micelles at 30 °C.15a Thus, the
disappearance of the cross peaks between EO−CH2 protons
and PO−CH3 protons at higher [Doim]Br concentrations (the
ellipse line, Figure 6) suggests the decomposition of F127
micelles. In addition, the cross peak of H13 and PO−CH3
protons is observed at both [Doim]Br concentrations
investigated, demonstrating their shorter distance regardless
of the aggregation state of F127 and [Doim]Br. As a
conclusion, [Doim]Br monomers embed deeply into the
F127 micelle core at lower [Doim]Br content. The F127
micelles are disassociated at higher concentrations accompanied
by F127 chains penetrating into [Doim]Br micelles.
mainly contributed by the dehydration of the rings, which is
also verified by a faster decrease in the chemical shift of proton
H4,5 compared to H13 (Figure 5).
Figure 8 shows the temperature induced change of relaxation
time distribution in the mixed system F127/[Doim]Br/H2O.
DLS Measurement. Figure 7A shows the relaxation time
distribution (τA(τ)) versus log(τ/ms) in the system [Doim]Br/
Figure 8. Relaxation time distributions at various temperatures for 20
mg/mL F127 aqueous solutions with increasing [Doim]Br concen-
trations (mM): (A)17 no [Doim]Br; (B) 1.00; (C) 23.58; (D) 100.23.
The temperature uncertainty is within 0.1 °C.
At lower concentrations of [Doim]Br (Figure 8B), the
relaxation distribution is bimodal at 5 °C. The faster mode
corresponds to the F127 unimers. The slow mode is attributed
to the clusters consisting of the hydrophobic components in the
system.27 Different from the case of the F127/H2O system
(Figure 8A),14 the slow mode dominates the distribution curve
because of the extra hydrophobic components from [Doim]Br
as described above. The slow mode dramatically diminishes as
the temperature rises to 20 °C accompanied by the appearance
of a middle mode corresponding to the complex F127/
[Doim]Br. The middle mode shifts to faster time and becomes
prominent, accompanying the disappearance of both slow and
fast modes. The formed F127/[Doim]Br complex solubilizes
the extra hydrophobic components, leading to a dramatic
decrease of the slow mode. The Rh value of the complex
decreases from about 12.5 nm at 20 °C to about 5.9 nm at 40
°C. All the Rh values are smaller than the corresponding ones in
F127 aqueous solutions.14 This phenomenon might be
attributed to the electro elastic repulsion between the
imidazolium rings at the surface of the complexes, which
inhibits the growth of the complex.28 At a higher concentration
of [Doim]Br (Figure 8C), a single mode is observed within all
the temperature range investigated and turns out to be broad
and unsymmetric at higher temperatures. The reported micellar
cluster with F127 chains wrapping around several micelles as
described in the F127/TX-100 system is not observed in the
present system. The hydration radius Rh of the micelle of
nonionic surfactant TX-100 is reported to be about 4.5 nm at
25 °C.14 For IL-based surfactant [Doim]Br, the Rh value is
around 1.5 nm, which is about one-third that of the TX-100
micelle. Thus, the stretching effect of [Doim]Br micelles on
PPO chains and subsequently the formation of a micellar
cluster are not as significant as that in the TX-100/F127
Figure 7. Variation of the relaxation time distributions for pure
[Doim]Br aqueous solutions with varied concentrations at 20 °C (A)
and varied temperatures at a [Doim]Br concentration of 97.35 mM
(B). The temperature uncertainty is within 0.1 °C.
H2O at various [Doim]Br concentrations. A slow mode at
around 1.3 ms is observed at lower [Doim]Br concentrations
(<cmc) with a hydrodynamic radius (Rh) of about 100 nm.
This slow mode diminishes gradually with increasing
concentration of [Doim]Br, and a faster mode appears at
around 0.01 ms with a Rh of about 1.5 nm, which corresponds
to the [Doim]Br micelle. Referring to the synthesis process of
[Doim]Br, the larger aggregates probably come from the
hydrophobic impurity. Higher integral area of the resonance
peaks corresponding to the alkyl protons in the NMR spectrum
also verifies this impurity (data were not shown). As the
micelles are formed, the impurity is gradually solubilized into
the hydrophobic core of the micelles, leading to the
disappearance of the slow mode, which is similar to the case
in F127 aqueous solution reported.14 Figure 7B shows the
temperature effect on the size of [Doim]Br micelles. The
relaxation time distribution moves to faster time with increasing
temperature, and the corresponding Rh value decreases from
2.58 to 0.75 nm. Two contrary effects on the micellization of
surfactant are caused by the increase of temperature. They are
the decrease in the hydration degree of the hydrophilic
headgroups favorable to the micellization, and the disruption of
the water structures surrounding the hydrophobic chains
weakening the hydrophobic effect and subsequently unfavor-
able to the micellization.2d Due to highly solvated imidazolium
ring at the surface of the micelle, the significant shrink might be
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system. This conclusion is also verified by the unchanged
chemical shift of proton PO−CH3 (Figure 4A). The
coexistence of [Doim]Br micelles and F127/[Doim]Br
complex leads to one broad relaxation time distribution. At a
[Doim]Br concentration of 100.33 mM, the amount of free
[Doim]Br micelles increases. Thus, the distribution of free
[Doim]Br micelles at higher temperatures becomes separate
from that of the F127/[Doim]Br complex (Figure 8D).
ITC Measurement. The isothermal titration calorimetry
(ITC) is a vital method to determine the cmc and the
micellization enthalpy (ΔHmic).25,29 The values of micellization
enthalpy (ΔHmic) are determined from the enthalpy difference
between the two linear segments of the enthalpy curves, as
illustrated in Figure 9.29,30 The observed enthalpy curves are
are sigmoidal in shape and are subdivided into two
concentration regions separated by a transition region
associated with micelle formation. The micellization process
is exothermic at temperatures below 25 °C and then it becomes
endothermic at higher temperatures. This phenomenon is
similar to the case in the system nonylglucoside/H2O,31 which
stems from the temperature dependence of the dilution
enthalpies of concentrated micelles. The dependence of the
derived ΔHmic values on temperature is presented in Figure
10B. A linear decrease of ΔHmic on temperature is observed.
The difference in heat capacities of surfactants, ΔCp,mic, is
obtained from the slope. The negative value of ΔCp,mic reflects
the hydrophobic segments of surfactant transferring from the
bulk to micelles.32 According to Jolicoeur and Gill et al.,33 the
absolute value of ΔCp,mic is approximatively proportional to the
fraction of the hydrophobic chains transferred. The fraction is
represented by the number of hydrogen atoms (nH) including
those on the terminal methyl group and the intermediate
methylene group.The equation ΔCp,mic = −7.88nH cal/(mol·K)
was therefore approximately built.29a,30,34 The value nH for
double-tailed [Doim]Br is about 9 with the ΔCp,mic value equal
to −0.09 kcal/(mol·K) obtained from Figure 10B. It means that
only about two methylene groups on each octyl chain of
[Doim]Br are embedded into the micellar core besides one
terminal methyl, and the rest of the alkyl chains remain
hydrated. This deduction is confirmed by the remarkable
temperature-induced shrink of [Doim]Br micelles due to
dehydration determined by DLS (Figure 7B). The highly
hydrated state of the alkyl chain in the micelle might originate
from high steric hindrance of the double chains attaching on
position 1,3 of the imidazolium ring.
The enthalpy profile for the titration of concentrated [Doim]
Br solutions into aqueous solutions of F127, and pure water at
10 and 30 °C are shown in Figure 11. At 10 °C (Figure 11A),
Figure 9. ITC curve of concentrated [Doim]Br solution titrated into
water at 40 0.1 °C and the first derivative of the curve calculated
from the interpolated values.
fitted and then differentiated with respect to the concentration
of [Doim]Br. The position of the extremum at differential curve
is taken as the cmc.
The calorimetric titration curves of ΔHobs at different
temperatures are presented in Figure 10. The dilution curves
Figure 11. ITC curve of concentrated [Doim]Br solutions titrated into
F127 aqueous solutions of 20 mM at 10 0.1 °C (A) and 30 0.1 °C
(B). The results of concentrated [Doim]Br solutions titrated into
water are shown for comparison (hollow symbols).
the ΔHobs values in the mixed system start from higher values
and exhibit a more significant increase with rising [Doim]Br
concentration compared to that in [Doim]Br aqueous
solutions. The endothermic effect is caused by the interaction
between [Doim]Br and F127 unimers, which compensates the
exothermic effect originating from the dilution of [Doim]Br
micelles. The more significant increase in the mixed system is
contributed by the dehydration of copolymer unimers, the
Figure 10. ITC curve of concentrated [Doim]Br solutions titrated into
water at different temperatures (A) and the temperature dependence
of the micellization enthalpy (B). The temperature uncertainty is
within 0.1 °C.
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Angew. Chem., Int. Ed. 2000, 39 (21), 3772−3789. (f) Hussey, C. L.;
Barnard, P. A.; Sun, I. W.; Appleby, D.; Hitchcock, P. B.; Seddon, K.
R.; Welton, T.; Zora, J. A. J. Electrochem. Soc. 1991, 138 (9), 2590−
2594.
conformational changes of copolymer, and the interaction
between surfactant and copolymer unimers. At 30 °C (Figure
11B), the ΔHobs value for the first injection increases as high as
5.43 kcal/mol compared to 0.9 kcal/mol of pure [Doim]Br
solutions. F127 unimers have completed the micellization at 30
°C, and hence, the high endothermic value is attributed to the
interaction between F127 micelles and [Doim]Br. It is reported
that the interactions between the copolymer and anion
surfactant lead to the formation of the complex as polymer
micelle/surfactant and further to a disruption of copolymer
micelles.35 The disruption of the F127 micelle would arise,
which is an exothermic effect, resulting in a decrease of ΔHobs
with the concentration of [Doim]Br. At higher [Doim]Br
concentrations, the formation of the complex with a skeleton of
[Doim]Br micelles might lead to a slight rehydration of F127
chains, which leads to an exothermic effect showing a lower
ΔHobs value than in the system [Doim]Br/water.
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The microstructure of [Doim]Br aggregates with the alkyl
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The formation of [Doim]Br micelles was driven mainly by the
hydrophobic interaction, as well as the enhanced hydrogen
bonding between the imidazolium ring and anions. The steric
hindrance of the double tail yields the higher hydrated micelles,
which exhibits more significant temperature dependence on
both the hydration state and the size.
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The interaction model between [Doim]Br and F127 can be
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addition of [Doim]Br promotes the aggregation of F127 to
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the single-tailed surfactant. With an increase of [Doim]Br
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AUTHOR INFORMATION
Corresponding Author
*R. Guo: e-mail, guorong@yzu.edu.cn; fax, (+86)-514-
87311374.
(17) Ge, L.; Zhang, X.; Guo, R. Polymer 2007, 48 (9), 2681−2691.
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9596−9605.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
(22) Bonho
̂
te, P.; Dias, A. P.; Papageorgiou, N.; Kalyanasundaram,
This work was financially supported by the National Nature
Science Foundation of China (No. 21203162, 21073156),
Project Funded by the Priority Academic Program Develop-
ment of Jiangsu Higher Education Institutions, and Natural
Science Fund for Colleges and Universities in Jiangsu Province
(No. 13KJ13050040).
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