Understanding Differential Interaction of Protic and Aprotic Ionic Liquids inside Molecular Confinement

Article abstract of DOI:10.1021/acs.jpcb.7b07945

Considering the contemporary interests of water-free reverse micelles (RMs) in the field of organic reaction medium and potential drug delivery carrier, we synthesized two different classes of ionic liquids (ILs), protic N-methyl-2-pyrrolidonium hexanoate, [NMP][Hex], and aprotic choline hexanoate, [Chl][Hex], and subsequently incorporated them in a mixture of polyoxyethylene (20) sorbitan monooleate (Tween-80) and cyclohexane. In order to understand the differential nature of interinterionic interaction of two ILs, we performed DFT calculations on pure ILs to correlate with experimental results. The formation of IL-in-oil RMs was confirmed from phase behavior and DLS studies. Interestingly, [NMP][Hex]-based systems showed a larger monophasic region and droplet size along with higher shear viscosity compared to [Chl][Hex]-based systems. Stronger interaction between [NMP]⁺ and Tween-80 due to their protic nature might be the driving force for such observations which supported the resonance stabilization energy [E⁽²⁾] and charge population analysis by NBO calculation. Smaller E⁽²⁾ values along with lesser NBO charges on atoms involved in H-bonding in pure [NMP][Hex] than [Chl][Hex] corroborated with the experimental observations. This primary hypothesis was further confirmed from FTIR and time-resolved fluorescence studies. These systems showed efficient thermal stability. Taking all of the results together, we anticipate that these RMs could be used as efficient delivery systems and for nanomaterial synthesis.

Full text of DOI:10.1021/acs.jpcb.7b07945

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Article
Understanding Differential Interaction of Protic and
Aprotic Ionic Liquids Inside Molecular Confinement
Somenath Panda, Kaushik Kundu, Akhil Pratap Singh, Sanjib Senapati, and Ramesh L Gardas
J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b07945 • Publication Date (Web): 27 Sep 2017
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Understanding Differential Interaction of Protic and Aprotic Ionic Liquids
inside Molecular Confinement
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†
†
Somenath Panda,1 Kaushik Kundu,2, 3 Akhil Pratap Singh,1,2 Sanjib Senapati,2,* and
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Ramesh L. Gardas1*
1 Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India.
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Department of Biotechnology, Bhupat and Jyoti Mehta School of Biosciences, Indian
Institute of Technology Madras, Chennai 600036, India.
3 Present address: Department of Inorganic and Physical Chemistry, Indian Institute of
Science, Bangalore 560012, India.
†
S. P. and K. K have contributed equally to this work.
*Corresponding Authors Information:
Eꢀmail: sanjibs@iitm.ac.in (SS)
Eꢀmail: gardas@iitm.ac.in (RLG)
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ABSTARCT
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Considering the contemporary interests of waterꢀfree reverse micelles (RMs) in the field of
organic reaction medium and potential drug delivery carrier, we synthesized two different
classes of ionic liquids (ILs), protic Nꢀmethylꢀ2ꢀpyrrolidonium hexanoate, [NMP][Hex] and
aprotic choline hexanoate, [Chl][Hex] and subsequently incorporated in mixture of
polyoxyethylene(20)sorbitan monooleate (Tweenꢀ80) and cyclohexane. In order to
understand the differential nature of interꢀinterionic interaction of two ILs, we performed
DFT calculations on pure ILs to correlate with experimental results. The formation of ILꢀinꢀ
oil RMs was confirmed from phase behaviour and DLS studies. Interestingly, [NMP][Hex]ꢀ
based systems showed larger monophasic region and droplet size along with higher shear
viscosity compared to [Chl][Hex]ꢀbased systems. Stronger interaction between [NMP]+ and
Tweenꢀ80 due to their protic nature might be the driving force for such observations which
supported from the resonance stabilisation energy [E(2)] and charge population analysis by
NBO calculation. Smaller E(2) values along with lesser NBO charges on atoms involved in Hꢀ
bonding in pure [NMP][Hex] than [Chl][Hex] corroborated with the experimental
observations. This primary hypothesis was further confirmed from FTIR and timeꢀresolved
fluorescence studies. These systems showed efficient thermal stability. Taken together the
results, we anticipate that these RMs could use as efficient delivery systems and nanomaterial
synthesis.
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1. INTRODUCTION
Microemulsions or reverse micelles are formed when amphiphilic molecules selfꢀaggregates
in a nonpolar solvent leading to a transparent and thermodynamically stable system, often in
combination with coꢀsurfactant. Due to their unique properties such as low viscosity, very
low interfacial tension, high solubilisation capacity etc., microemulsions have been widely
applied in different fields such as in pharmaceutical, cosmetic and agrochemical formulations
and nanoreactors for nanomaterials synthesis.1–3 Most of the studies on conventional
microemulsions have used water as the polar component. However, in recent years, attempts
were made to prepare and study waterless/nonꢀaqueous reverse micelles (RMs), where other
polar solvents like methanol, acetonitrile, glycerol, dimethylformamide (DMF), formamide,
ethylene glycol etc. were exploited. The first study of nonꢀaqueous RMs with ethylene glycol,
lecithin and decane were reported by Friberg and Podzimek in 1984.4 Later, Laia et al., and
Falcone et al. studied the properties of nonꢀaqueous RMs using different polar solvents.5,6
These nonꢀaqueous RMs have distinct advantages over aqueous ones such as: larger stable
regions; large variety of surfactant usability and applicability as good reaction media,
specifically for the reactants which react with water.7 These earlier studies soon attracted
researchers attention towards the formulation of room temperature ionic liquids (RTILs)
based nonꢀaqueous RMs. Nonꢀaqueous ionic liquid (IL) RMs provide hydrophobic or
hydrophilic nanoꢀdomains and thus their applications can be extended to fields of reaction
and separation or extraction media. For example, synthesis of polymer gels, nanoparticles,
and openꢀcell porous materials have been reported surfactant ILꢀbased RMs in some earlier
studies.8 ILs are considered suitable solvents for various chemical processes due to their
unique properties like negligible vapour pressure and tunability in structure by modifying
cation and anion moieties. In addition, ILs exhibit excellent chemical and thermal stability,
wide polarity and are recyclable.9–11 They broadly divided in to protic (PILs) and aprotic
(AILs) classes. Our group also involved in synthesis of different types of PILs and APILs.12ꢀ
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Despite of having many useful properties(arising from protic nature) and potential
applicability, literature on PILs are still scarce compared to their AIL counterparts.15,16 PILs
are formed by the transfer of proton from acid to base and hence they consist of protonꢀdonor
and ꢀ acceptor sites which are responsible for building extended three dimensional hydrogen
bond networks as in the case of water.17 In recent years, considerable progress in the
structureꢀproperty relationships of ILs has been achieved with better understanding of the
intermolecular forces. The elucidation of the difference between the interactions of PILs and
those of AILs is important for understanding the unique properties of PILs. A number of
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reports were published on the differential behaviour on the interionic and ionꢀsolvent
interactions of protic and aprotic ILs with water both by theoretically and experimentally.18ꢀ20
In the last few years, the use of both type of ILs has attracted a great deal of attention as
suitable solvents for amphiphile selfꢀassembly from the scientific community around the
globe.
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The effect of ILs on the aggregation properties, critical micellar concentration (cmc) and
hydration behaviour of different kind of surfactants have been reported in several studies and
found to be important for fine tuning the surfactant properties for specific applications.21ꢀ23
Yue et al. had employed aprotic 1ꢀbutylꢀ3ꢀmethylimidazolium tetrafluoroborate ([Bmim]BF4)
and protic ethylammonium nitrate (EAN) to investigate the aggregation behaviour of
phytosterolethoxylate surfactant (BPSꢀ10), which showed higher effectiveness of EAN over
[Bmim]BF4 in promoting nonꢀionic BPSꢀ10 aggregation.24 Later, the aggregation behaviour
of the alkyl triphenylphosphonium bromides, CnTPB, in aprotic and protic ILs was also
investigated, where a stronger solvophobic effect between CnTPB and protic EAN evidenced
than aprotic [Bmim][BF4].25 It can be inferred from these reports that higher protonation
ability of EAN is the extra driving force for surfactant molecules to form ordered aggregates.
However, this has to be further verified for different selfꢀassembling systems. From this point
of view, it is important to understand the nature of differential behaviour of PILs and APILs
inside ILꢀinꢀoil RMs. A significant number of studies are already performed on the formation
of AILs based RMs26ꢀ27,whereas PIL based RMs are scarcely reported. The use of PILs in
RMs predominantly involved EAN and propylammonium nitrate (PAN) till now.28ꢀ31 Very
recently, Kusano et al. formulated waterꢀinꢀIL RMs in an organic solventꢀfree system by
employing a mixture of imidazoliumꢀbased AIL and PILs.32 However, still there is a scope to
compare their differential nature of interactions inside the nanoscopic confinement of RMs
through a systematic investigation, since this is an emerging field of research, where many
issues are still unclear and highly debated.
In view of this limited research on PILꢀinꢀoil nonꢀaqueous RMs compared to the
conventional water and AILꢀinꢀoil RMs, herein we investigated the formation and
characterization of a series of PIL and APILꢀinꢀoil RMs with following specific objectives. It
is noteworthy to mention here that the two ILs, which we have used, bear the same anionic
part, whereas the cationic part differs in presence of exchangeable protons (e.g., Nꢀmethylꢀ2ꢀ
pyrrolidonium hexanoate, [NMP][Hex] and choline hexanoate, [Chl][Hex]). On the basis of
this motivation, the [NMP][Hex] and [Chl][Hex] are employed as a replacement for water as
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the polar phase to explore their effects on aggregation behaviours of polyoxyethylene (20)
sorbitan monooleate (Tweenꢀ80) in cyclohexane (Cy). As in most liquids, Hꢀbonding leads to
a preferred orientation among the nearestꢀneighbour species which in turn dictates important
aspects of liquid behaviour such as solvation, dynamics and macroscopic physical
properties.33 We would like to demonstrate here that how the Hꢀbond direction and strength
of two different family of ILs can be related to macroscopic physicochemical properties of
Tweenꢀ80ꢀbased RMs, which are generally regarded as nonꢀtoxic and nonꢀirritant systems.34
Considering the fact that research activities involving ILs in RMs have significant
ambiguities regarding the mode of interaction between ILs and surfactant, it is essential to
study this aspect by employing varieties of ILs. The majority of the studies in nonꢀaqueous IL
based RMs utilized Triton Xꢀ100 (TXꢀ100) as the surfactant component.35–36 In this report,
the microstructural, microenvironmental, and rheological properties along with the
temperature dependent droplet sizes and viscosities of [NMP][Hex] and [Chl][Hex]ꢀbased
RMs are examined and analyzed in light of our current understanding on hydrogen bonding
and bulk liquid nanostructures in these liquids, which may give different perspective about
the origins of macroscopic behaviour.
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2. EXPERIMENTAL SECTION
2.1. Materials: Polyoxyethylene (20) sorbitan monooleate, Tweenꢀ80 (CAS No. 9005ꢀ65ꢀ6,
SigmaꢀAldrich), 8ꢀAnilinoꢀ1ꢀnaphthalenesulfonic acid, ANS (≥ 97%, CAS No. 82ꢀ76ꢀ8,
SigmaꢀAldrich) and cyclohexane (Cy) were purchased from SigmaꢀAldrich (anhydrous grade,
99.5 %, CAS No. 110ꢀ82ꢀ7) and used without further purification.
2.2. Synthesis and characterization of the ionic liquids (ILs): Analytical grade chemicals
were used for all synthesis. Nꢀmethylpyrrolidinone (≥ 99%, CAS No. 872ꢀ50ꢀ4, Sigmaꢀ
Aldrich), choline hydroxide solution (46 wt. % in H2O, CAS No. 123ꢀ41ꢀ1, Aldrich), formic
acid (≥ 95%, CAS No. 64ꢀ18ꢀ6, SigmaꢀAldrich) and hexanoic acid (≥ 99%, CAS No. 142ꢀ62ꢀ
1, SigmaꢀAldrich) were used without further purification. The ILs was synthesized by adding
equimolar amount of acid to neutralize the respective bases. For example, 11.616 g hexanoic
acid (0.1 mol) was added drop wise from a pressure equalizing dropping funnel to 9.9 g of Nꢀ
methylꢀ2ꢀpyrrolidone (NMP) (0.1 mol) kept in a 250 mL single necked round bottomed flask
in an ice bath with vigorous stirring. After complete addition, the ice bath was removed and
stirring was continued for 12 h in room temperature (∼27 °C). The obtained ILs were washed
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several times with diethylether to remove any excess reactants. Then synthesised ILs were
dried for 6 h under vacuum using a rotary evaporator. To minimise the water content the ILs
were dried further in vacuum at 70 °C for 24 h. On the other hand, hexanoic acid (0.1 mol)
was added drop wise into an aqueous solution of choline hydroxide (0.1 mol) with continuous
stirring for 12 h at room temperature (∼27 °C). Similar drying procedure as described above
was followed. The synthesized ILs were characterized by 1H NMR and 13C NMR
spectroscopy recorded on a Bruker AM 500 spectrometer and shown in Supplementary
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Information (Figs. S1). HꢀNMR (500 MHz, CDCl3) for [NMP][For] δ(ppm): 2.32 (m, 4H)
2.93 (s, 3H) 3.34 (t, 2H) 6ꢀ8 (broad NH+ and HCOOꢀ); for [NMP][Hex] δ(ppm): 0.89 (t, 3H)
1.31 (m, 4H) 1.63 (q, 2H) 2.01 (m, 2H) 2.35 (m, 4H) 2.83 (s, 3H) 3.38(t, 2H) ( no broad peak
1
for NH+); and HꢀNMR (500 MHz, DMSOꢀd6) of [Chl][Hex] δ(ppm): 0.74 (t, 3H) 1.15 (m,
4H) 1.41 (m, 2H) 1.92 (q, 2H) 3.2 (s, 9H) 3.52 (t, 2H) 3.91(s, 2H).
2.3. Experimental methods: The phase diagram of the ternary ILs/Tweenꢀ80/Cy mixture was
determined by observing the physical states of the equilibrated mixture of synthesized ILs,
Tweenꢀ80 and Cy. During the experiment, the IL was first mixed with the surfactantꢀoil
solution at a series of weight ratios of Tweenꢀ80 and Cy, and then the mixtures were kept at
303 K for several minutes. After each addition of IL, the resulting mixture was thoroughly
mixed with a vortex shaker through visual observation and equilibrated. The phase
boundaries were taken when the mixture turned turbid from transparent condition and
compositions of different components (wt %) of systems were evaluated.Anton Paar Lovis
2000ME instrument were used for viscosity measurements. The instrument measures the time
taken by a rolling ball moving through a capillary filled with the sample to measure the
viscosity. A builtꢀin peltier device was used to maintain the temperature within accuracy of
0.02K. The sample to be measured (approx. 0.5ml) was filled in the capillary tube in bubble
free condition. The calibration of the instrument were verified by measuring double distilled
water (resistivity 18mꢁ·m) and atmospheric air and comparing their values with reported
ones. A DLS instrument (Malvern Zetasizer nanoꢀseries) was employed to measure the
hydrodynamic diameter of RM nanodroplets. This instrument is equipped with a
thermostatted sample chamber and uses a 4 mW HeꢀNe laser (λ = 632.8 nm) for irradiation
and collecting the scattered photons at 90° scattering angle. The software provided with the
instrument were used to process the scattering intensity data to obtain the hydrodynamic
diameter (dh) and the size distribution of the scatterer. The dh of the nanoꢀdroplet is estimated
from the StokesꢀEinstein equation, which is defined as;
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dꢀ =
ꢂ
(1)
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Where, kB is the Boltzmann constant, η is the viscosity (as obtained from viscometer), D is
the translational diffusion coefficient, and T is the temperature. DLS experiments were
performed three times for each samples with at least 15 runs per measurement. Prior to
measurement, to remove possible dust particles, the RM solutions were filtered through a
0.22 ꢂm pore size membrane (MILLEX(R)ꢀGP. Fourier transform infrared (FTIR) spectra
were recorded in a JASCO FT/IRꢀ4100 spectrometer (transmission mode) in the 400ꢀ4000
cmꢀ1 window using CaF2 window with 100 number scans and 4 cmꢀ1 spectral resolution. For
the experiments performed on the different IR modes, the cyclohexane spectrum was used as
the background. The steadyꢀstate fluorescence spectra were performed in a Fluoromaxꢀ4
spectrofluorimeter. During recording the emission spectra, excitation wavelength was fixed at
360 nm and the excitation/emission slit widths was maintained at 2/2 nm. Water was
circulated through a jacketed cuvette holder from a JULABO refrigerated bath (JULABO,
Germany) to maintain proper temperature. Fluorescence lifetime measurements were
performed using a Horiba JobinꢀYvon time correlated singleꢀphoton counting (TCSPC)
lifetime instrument. For excitation source, a 370 nm nanoꢀLED was used. The instrumental
full width halfꢀmaximum of the 370 nm LED was ∼1.1 ns, including the detector response.
The photomultiplier tube (PMT) voltage was set as 950V. The pulse repetition rate was set to
1 MHz. The instrument response function was collected using Ludox AS40 colloidal silica.
IBH (DASꢀ6) software was used to analyze the decay data. The fitting is considered to be
good when the symmetrical distribution of the residuals gives a value of χ2, 0.99 ≤ χ2 ≤ 1.2.
The average fluorescence lifetime (τavg) values were calculated by following equation:
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ꢆ∑ꢉ
ꢇꢊꢋ αꢇτ
ꢈꢇ ꢌ⁄ꢆ∑ꢉꢇꢊꢋ
ꢌ
αꢇτꢇ
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=
(2)
Rheological measurement of RM samples was executed by Anton Paar’s MCR (Modular
Compact Rheometer) 102, with a CP 40 Cone and Plate configuration (radius 40 mm and
angle = 1°) in dynamic mode. To measure rheological properties, a small portion of sample
was placed on the smooth plate of rheometer and the coneꢀplate geometry was allowed to
compress slowly to the set truncation gap (0.08 mm). The temperature of system was
controlled to 303 K using PꢀPTD 200/AIR PlateꢀPeltier temperature device. Equilibrium
process runs for 10 minutes before the experiment starts. The storage and loss modulus
measurements were performed from 1 to 100 rad sꢀ1 frequency sweeps with 50% strain.
Steadyꢀshear experiments were performed in the range of shear rate from 100 to 1000 sꢀ1. The
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viscosity curve was extrapolated to the vertical coordinate to obtain the zero shear viscosity.
Prior to rheological measurements, all samples were equilibriated at the experimental
temperature for at least 15 min. The surfactant solution was fixed at a concentration of 0.1
mol dmꢀ3 for all the experiments if not mentioned otherwise.
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2.4. Computational details: Gaussian 09 program was used for all theoretical calculations
containing the ionꢀpairs of ILs of interest.37 Stable optimized structures of ILs were
deadlocked at B3LYP/6ꢀ311+G(d,p) theoretical level.38,39 Herein, NBO5.G code40 was
employed to compute the natural bond orbital (NBO) charges of discrete atoms of optimized
ILs due to its more reliability compared to the estimation of Mulliken charges.41
3. RESULTS AND DISCUSSION
In the present report, two important class of newly synthesized ILs, namely aprotic
[Chl][Hex] and protic [NMP][Hex], were used instead of water as polar phase to form a nonꢀ
aqueous RMs stabilized by the nonꢀionic Tweenꢀ80 in cyclohexane (Cy). We have
specifically focused on the determination of different degree of Hꢀbonding interaction
between the ion pairs of two ILs (protic and aprotic) theoretically, which believed to be the
primary governing factor in controlling the microstructure of the formulated ILꢀbased RMs.
In view of these, we optimized the structures of two ILs considering all possible
configurations using the B3LYP method in conjunction with 6ꢀ311+G(d,p) basis set, which
have been shown in Fig. 1. The interionic interactions between the ion pairs of [NMP][Hex]
and [Chl][Hex] could be characterized by intermolecular hydrogen bonds of O···HꢀN and
O···HꢀO type that formed mainly between the electronegative oxygen atoms of the
carboxylate anion and the NꢀH and OꢀH bonds of the [NMP]+ and [Chl]+, respectively. In
both ILs, two O atoms of [Hex]ꢀ act as proton acceptors, whereas the NꢀH bonds of [NMP]+
and OꢀH bonds of [Chl]+ act as proton donors. It is clearly evident from the Fig. 1 that the
interionic distance is much shorter in [Chl][Hex] (O···HꢀO = 1.733 Å) compared to
[NMP][Hex] (O···HꢀN = 1.824 Å). The Hꢀbonds are the explicit structural features in these
ion pairs. Along with these structural features, we also determined the resonance stabilisation
energy, E(2) from secondꢀorder perturbation theory analysis obtained after the NBO
calculations. The stabilization energy was measured using the following equation:42
ꢒ
ꢑꢆꢎ,ꢏꢌ
ꢍꢆꢈꢌ = ∆ꢍꢎꢏ = ꢐꢎ
(3)
ꢓ ꢕꢓ
ꢔ
ꢖ
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where, qi, εi , εj and Fij are designated as the occupancy of the donor orbital, energies of the
orbitals and the off diagonal NBO Fock matrix element, respectively. Therefore, the
estimated energy is interrelated to the degree of covalency, and subsequently with Hꢀbonding
strength.42 Earlier, Weinhold and coꢀworkers have shown a direct relation between the NBO
charge transfer from acceptor to donor and the Hꢀbond interaction strength,42 which can
further be connected to the energies of the charged Hꢀbonded complex.43 The E(2) values for
major Hꢀbonds in two ILs are listed in Table 1, which mainly involve an oxygen loneꢀpair of
electrons n(O) and nitrogenꢀhydrogen or oxygenꢀhydrogen antiꢀbond σ*(N–H or OꢀH) that
are important in stabilizing these ILs. The NBO analysis suggest strong orbital
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Table 1. Second order perturbation theory analysis of Fock matrix in NBO basis of ionic
liquids calculated by B3LYP/6ꢀ311+G(d,p) level of theory.
E(2)
E(2)
Donor NBO (i)
Acceptor NBO
[kcal molꢀ1]
[kcal molꢀ1]
(j)
[NMP][Hex]
[Chl][Hex]
LP(1) O19
LP(2) O19
LP(2) O20
LP(1) O23
LP(2) O23
LP(1) O24
LP(2) O24
LP(2) O24
BD*(1) N10ꢀH15
BD*(1) N10ꢀH15
BD*(1) C3ꢀH8
BD*(1) O8ꢀH9
BD*(1) O8ꢀH9
BD*(1) C18ꢀH21
BD*(1) C1ꢀH2
BD*(1) C18ꢀH21
2.03
ꢀ
6.38
ꢀ
0.07
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
6.51
6.37
3.24
1.35
6.01
interactions between the antiꢀbonding orbital of the proton donor σ*(O–H or NꢀH), and the
lone pairs of the proton acceptor, n(O), which is supported by the increased electron density
and the increased secondꢀorder interaction energies in the related localized antiꢀbonding
orbital. The increase of the electron density in σ*(N–H or OꢀH) is a consequence of the
charge transfer from LP(O19) to mainly σ*(N10–H15) or from LP(O23) to mainly σ*(O8–
H9) in [NMP][Hex] and [Chl][Hex], respectively. The stabilization energy E(2) associated
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with hyperconjugative interactions for LP1(O19) → σ*(N10ꢀH15) and LP2(O19) → σ*(N10ꢀ
H15) of [NMP][Hex] are 2.03 and 6.38 kcal/mol. Whereas, for LP1(O23) → σ*(O8ꢀH9) and
LP2(O23) → σ*(O8ꢀH9) of [Chl][Hex], the E(2) values are 6.51 and 6.37 kcal/mol. These
results point out that the hydrogen bond in [Chl][Hex] is much stronger than that of
[NMP][Hex]. This assumption is further strengthen by looking at the O···HꢀC distance in
these two ILs (Fig. 1) along with the value of related to E(2) for n(O)→σ*(C–H) (Table 1).
Considering the proximity of donorꢀacceptor atoms, the obtained stabilization energies falls
under the nominal Hꢀbond range (5ꢀ40 kJ molꢀ1),44 which indicates a significant Hꢀbonding
interactions in case of these ILs.
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FIG. 1. The optimized molecular structures with interionic distances and NBO charges for
ionic liquids (ILs).
Since the formation of hydrogen bonds is followed by charge redistributions, we further
examined the NBO charge distribution of these ILs to gain more insight into the problem. It
is evident that the hydrogen atoms involved in Hꢀbonds gain more positive charges, while the
oxygen atoms acting as electron acceptor have more negative charges. Indeed, the charge of
H15 in [NMP][Hex] is +0.473, while it is +0.508 on H9 of [Chl][Hex]. Consequently, the
NBO charges of O19 and O20 in [NMP][Hex] are ꢀ0.490 and ꢀ0.348, respectively while it is ꢀ
0.836 and ꢀ0.785, respectively on O23 and O24 in [Chl][Hex]. Thus, the NBO charges of
hydrogen and oxygen atoms involved in Hꢀbonds in [Chl][Hex] are larger than the
corresponding values obtained for [NMP][Hex]. These results are consistent with our
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prediction in secondꢀorder perturbation energy E(2) and confirm that interionic hydrogen
bond in [Chl][Hex] is stronger than that of [NMP][Hex].45
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Our next objective is to correlate the computed parameters for two ILs with various physical
properties observed for ILꢀbased nonꢀaqueous RMs. The knowledge of the phase behaviour is
important to ensure an optimal microemulsion system for chemical reaction and separation
processes. The ternary phase diagrams of [NMP][Hex or [Chl][Hex]/Tweenꢀ80/Cy systems at
303 K are shown in Fig. 2A. Two different regions were observed, namely single phase and
biphasic, which marked as “1Φ” and “2Φ”, respectively. Admittedly, the overall “1Φ”
microemulsion region for [NMP][Hex]ꢀbased system was monitored to be larger than that in
the [Chl][Hex]ꢀbased system. This could be attributed to the differential interaction strength
between the electropositive cationic counterpart of ILs and the electronegative oxygen atoms
of polyoxyethylene (POE) units (Tweenꢀ80), leading to the effective solubilisation of ILs
inside the core of the surfactant aggregates and alters rigidity of interfacial layer.46
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FIG. 2. Phase diagrams of the ILs/Tweenꢀ80/cyclohexane ternary systems at 303 K, where
ILs are (A) [NMP][Hex] or [Chl][Hex], and (B) [NMP][For].
To further investigate the influence of chain length of the RTIL anion in formation of the
nonꢀaqueous microemulsion, the phase behaviour was investigated using [NMP][For] as
polar phase. Fig. 2B shows the phase diagram of [NMP][For]/Tweenꢀ80/Cy ternary system at
303 K. The area of single phase region was found to follow the trend; [NMP][Hex] >
[NMP][For]. This may be due to the difference in strength of interaction, which arises due to
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the presence of formyl and hexyl chain in [NMP][For] and [NMP][Hex], respectively. Longer
chain length of [Hex]ꢀ may be responsible for more pronounced penetration because it aids in
aligning the anion of IL with the tail part of Tweenꢀ80.46 This results in more rigid oil/IL
interfacial layer in case of [NMP][Hex]ꢀbased system, which further increases solubilization
capacity of RTIL as well as the singleꢀphase region.36 Although the formation of
RTIL/Tweenꢀ80/Cy microemulsion is indicated by the phase behaviour study, we need to
confirm whether the surfactant molecules are effectively entrapping the RTIL molecules or
not. In view of this, we have conducted DLS measurement to confirm the formation of ILꢀinꢀ
oil (IL/O) microemulsions or RMs. Earlier studies using DLS have shown that the size of
droplets increased regularly up to a certain level with increase in R values (where, R is
defined as the molar ratio of RTILs to Tweenꢀ80) when the IL encapsulated to form IL/O
systems.47,48 The size distribution and variation of size with increasing R for [NMP][Hex] or
[Chl][Hex]/Tweenꢀ80/Cy systems are shown in Fig. S2 and Fig. 3, respectively. In all these
systems, an increase in the droplets size was evidenced with increase in IL content (i.e., R),
which implies that the both ILs are sequestrated by the surfactants constructing ILꢀinꢀCy
RMs. This clearly shows that the RM media consist of discrete spherical and nonꢀinteracting
droplets of ILs stabilized by the Tweenꢀ80,35 which is further confirmed from the monomodal
size distributions of droplets (Fig. S2). In this study, maximum droplet sizes were observed in
the order of 100 nm for these RMs, which are much larger than common traditional aqueous
RMs of droplet sizes less than 40 nm. The larger droplet size may be related to the unique
formation mechanism of nonꢀionic surfactant based IL/O RMs. The effective area of
electrostatic attraction for POE chains of Tweenꢀ80 with ILs is significantly higher compared
to water, which leads to a much extended conformation resulting in an increased size of IL
RMs.48 It is interesting to mention that although both systems show a linear tendency, the
droplet sizes at studied range of R were quite different for two ILs. For example, at R = 0.3
and 0.7 the droplet size is 50 and 63 nm, respectively for [Chl][Hex]ꢀbased and 70 and
108 nm, respectively for [NMP][Hex]ꢀbased RMs. Moreover, the different slopes of the
droplet size’s plot in Fig. 3 could also indicate a differential interaction between ILs and
surfactant in these systems. Due to comparatively weaker Hꢀbonding interaction between ionꢀ
pairs of [NMP][Hex] compared to [Chl][Hex] (as revealed from theoretical NBO
calculations), we might consider the stronger interaction between [NMP]+ and Tweenꢀ80 at
the palisade layer of IL/O system. However, this assumption could not be confirmed due to
the lack of enough evidence at this point. In the subsequent paragraph, we will show how the
rheological properties of such nanostructures are affected after incorporation of ILs through
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systematic variation in the structure and Hꢀbonding capacity of the constituents, such as IL
ions and polar head of Tweenꢀ80.
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In principle, rheological measurements contain information regarding the interꢀdroplet
interactions to monitor microstructural changes in RMs.49 To investigate the effect of
molecular architecture of protic and aprotic ILs on the formation of IL/O RMs, samples were
prepared by adding different amounts of ILs ([Chl][Hex] and[NMP][Hex]) to Tweenꢀ80/Cy
solution with a fixed surfactant concentration.
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FIG. 3. Hydrodynamic diameter of the droplets of [NMP][Hex] or [Chl][Hex]/Tweenꢀ
80/cyclohexane RMs as a function of molar ratio of IL to surfactant (R) at fixed surfactant
concentration of 0.1 mol dmꢀ3 and at 303 K.
The steadyꢀshear measurements were carried out for two nonꢀaqueous ILꢀbased RMs at
different R (= 0.3 and 0.7). The shear rate curve in Fig. 4A shows the change in shear
viscosity (η) as a function of shear rate (γ) for four investigated systems. Overall, the solution
viscosity is found to be independent of the shear rate, and thus shows a typical Newtonian
fluid behaviour.50 This indicate that in the given shear rate, the three dimensional network
structures formed by the IL droplets were apparently intact. This network structure is not
static like a synthetic polymer, instead, a transparent network which repeatedly collapse and
reforms itself. Further, to estimate the rheological characteristics of the two nonꢀaqueous
RMs more specifically, the apparent viscosity (ηa) was estimated from the HerschelꢀBulkely
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equation.51 This equation, though empirical and does not have theoretical corroboration, is
useful for estimation of apparent viscosity of similar sample, which can be defined as;52
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ꢚꢛ
ꢗ = ꢗꢘ + ꢙꢘꢆ ꢌꢝ
(4)
ꢚꢜ
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Where, τ, τa, (dγ/dt) and n are the shear stress, apparent yield stress, shear rate, and HꢀB
index, respectively. Herein, the value of n is considered to be 1. However, the “apparent
viscosity” (ηa) term used here is clearly different from “viscosity” and also have different
dimensions. The systems showed good proportionality between the shear stress and rate (with
R2 values ≥ 0.98) and the apparent viscosity was obtained from the slopes of the shear stress
vs. shear rate plots (Fig. 4B). The ηa values were found to be 3.36 and 4.64 mPa.s for
[Chl][Hex]ꢀbased RMs and 4.29 and 9.50 mPa.s for [NMP][Hex]ꢀbased RMs at R = 0.3 and
0.7, respectively. The increase of the shear viscosity for both systems with increasing IL
content (R) can be correlated with the enlarged droplet size and/or dropletꢀdroplet
interactions.53 It is also worth noticing the subtle differences present between the RMs
containing [Chl][Hex] and [NMP][Hex], which appear to be even more compelling. Very
similar to the DLS results, the ηa of the [NMP][Hex]/Tweenꢀ80/Cy system found to be higher
than that containing [Chl][Hex]. This implies that a stronger network and more steady
internal structure of the inverted micelle is developed for [NMP][Hex] based systems, which
increases the dropletꢀdroplet interactions, causing the droplets to be packed more closely.
Thus, these fluids can resist shear more strongly.54 Similar flow behaviour was earlier
reported for RMs composed of water/polysorbate 80/soybean lecithin/IPM
water/mediumꢀchain glyceride/polysorbate 80/IPM.55,56
and
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FIG. 4. Variation of (A) viscosity (η) and (B) shear stress (τ) versus steady shear rate (γ)
plots for [NMP][Hex] or [Chl][Hex]/Tweenꢀ80/cyclohexane RMs at different molar ratio of
IL to surfactant (R) and at 303 K. The filled spheres and solid lines in (B) denote
experimental data and linear fits to the HerschelꢀBulkely equation, respectively. View in
color for best clarity.
Oscillatory shear (frequency sweep) measurements were also performed to further investigate
viscoelastic properties of these solutions in addition to steadyꢀshear measurements. The
variations of the storage modulus (G’), the loss modulus (G’’), and the complex viscosity
(|η*|) versus oscillation frequency (ω) of the RMs at different R values are shown in Figs.
S3AꢀS3D (Supporting Information). Essential information regarding the fluidity and rigidity
of viscoelastic samples can be obtained from dynamic rheological studies. Two key
parameters are G’, which corresponds to the ability of the deformed materials to store energy,
and G’’, which represents the flow behaviour of the material under applied stress.57 All these
systems show a liquidꢀlike behaviour normally observed in RMs, indicating the loss modulus
is greater than the storage modulus (G’< G’’) at any tested frequency. Also, a differences
between the shear (η) and the dynamic (|η*|) viscosity was observed. The dynamic viscosity
exceeds the shear viscosity, especially at higher frequencies. This variation can be attributed
to intermolecular interactions.58 The |η*| values also found to be higher for [NMP][Hex]ꢀ
based systems compared to [Chl][Hex]ꢀbased RMs. The molecular architectures of
constituents ions of ILs in addition to the strength of intermolecular Hꢀbonding network of
ILs and surfactant have a significant impact on viscosity.33 In this study, two ILs have
common anionic counterpart (Hex]ꢀ) but different cation ([NMP]+ and [Chl]+), in which
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[NMP]+ is protic in nature. These two ILs ( protic and aprotic) primarily differs from each
other in the proficiency of donation and acceptance of proton. The protic IL can build up Hꢀ
bonding network like water, whereas the aprotic IL cannot.15 As a result, higher viscosity
(both apparent as well as complex) obtained for [NMP][Hex]ꢀbased system compared to
[Chl][Hex]ꢀbased system. Next, we resorted to FTIR measurement in the subsequent section
to confirm these postulates.
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Herein, the formation mechanism of the ILs/Tweenꢀ80/Cy microemulsions was studied by
employing FTIR techniques after monitoring the change in vibrations of the bonds of
surfactant head groups that are particularly involved in the interactions with ILs at the droplet
interface. Thus, we focused on the OꢀH stretching frequency of the terminal hydroxyl group
of Tweenꢀ80. Figs. 5A and 5B reveal the CꢀH and OꢀH stretching modes of Tweenꢀ80 in
[NMP][Hex] and [Chl][Hex]ꢀbased RMs, respectively as a function of various amounts of
ILs (indicated by R values) at 303 K. The FTIR spectra of pure [NMP][Hex] and [Chl][Hex]
were subtracted from the spectra of RMs to eliminate the contribution of vibrational bands of
pure ILs. It can be noted that the OꢀH bands are progressively blueꢀshifted with increasing R
for [NMP][Hex]ꢀbased RMs, whereas no noticeable changes are observed for [Chl][Hex]ꢀ
based systems. This behaviour can be rationalized by hypothesizing that in pure Tweenꢀ
80/Cy RMs, the terminal hydroxyl of Tweenꢀ80 is Hꢀbonded with the oxygen atoms of POE
units or terminal hydroxyl groups of other Tweenꢀ80 molecules. Conversely, when
[NMP][Hex] is added to this, the electronegative oxygen atoms of the POE units bind to the
electropositive [NMP]+ ring by weakening the hydrogenꢀbond interaction between the POE
groups of Tweenꢀ80. This leads to the appearance of OꢀH stretching band in the highꢀ
frequency region of the IR spectrum.59 Similar behaviour were also observed earlier for the
bmimBF4ꢀinꢀpꢀxylene RMs.60 So far, it is observed that the microstructures of formulated
nonꢀaqueous RMs differ drastically for two ILs upon confinement and show a considerable
degree of inhomogeneity due to disruption in their hydrogen bonding network. To acquire a
full panorama for these nonꢀionic IL/O RMs, we further attempt to examine the
microenvironment that the both ILs and Tweenꢀ80 molecules experience inside the RM
droplets and to improve the basic understanding about the polarity and Hꢀbond interactions
between the constituents.
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FIG. 5. FTIR spectra of ꢀCH and ꢀOH stretching bands of Tweenꢀ80 in ILs/Tweenꢀ
80/cyclohexane RMs as a function of R at 303 K, where ILs are (A) [NMP][Hex] and (B)
[Chl][Hex]. View in color for best clarity.
In view of this, we have performed steadyꢀstate fluorescence measurement using ANS as a
probe molecule, which is wellꢀknown to extract information of microheterogeneous
systems.61 The ANS concentration remains constant (~10ꢀ6 mol dmꢀ3) throughout the
spectroscopic measurements. The emission spectra of [NMP][Hex] or [Chl][Hex]/Tweenꢀ
80/Cy RMs as a function of R (= 0.3 and 0.7) at 303 K are presented Fig. 6. ANS exhibits
emission maxima at 430, 494 and 489 nm in Cy, [NMP][Hex] and [Chl][Hex], respectively at
excitation wavelength (λexi) of 360 nm (Figs. S4ꢀS5). On addition of Tweenꢀ80 to a solution
of ANS in Cy, the emission maximum of ANS exhibits a marked red shift from 430 to 460
nm, which indicates transfer of the ANS molecules from bulk Cy to the interior of Tweenꢀ80
RMs. The blue shift of probe in ILꢀinꢀoil RMs compared to neat ILs indicates that the
polarity of the RMs is much lower compared to that of bulk ILs. The emission maximum of
ANS displays red shift from 465 to 471 nm and 470 to 472 nm for [Chl][Hex] and
[NMP][Hex]ꢀbased RMs, respectively with increase in R (Table 2). These results suggest that
a significant portion of the probe molecules moved towards the polar IL core of the RMs. At
comparable R, the more redꢀshifted emission maxima for [NMP][Hex]ꢀbased RMs compared
to [Chl][Hex]ꢀbased system indicates that the polarity of the microenvironment of
[NMP][Hex]ꢀbased RMs is superior compared to [Chl][Hex]ꢀbased system. This may be due
to effective penetration of [NMP][Hex] near surfactant head group for its protic nature.62
Thus, it can be concluded that the microenvironment experienced by ANS in the
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[NMP][Hex]ꢀbased system is different from the [Chl][Hex]ꢀbased system. So far, we have
gathered information regarding location of the probe molecule (ANS) inside the [NMP][Hex]
or [Chl][Hex]/Tweenꢀ80/Cy RMs. It will therefore be fascinating to find out how the Hꢀ
bonding interactions between the constituents and droplet size influence the dynamics of the
solute in the nanopools of RMs.
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FIG. 6. Emission spectra of ANS in ILs/Tweenꢀ80/cyclohexane RMs at different R values
and at 303 K.
In order to understand the diffusional dynamics of the ANS in presence of encapsulated ILs
and to get an inꢀdepth knowledge on the influences of the local Hꢀbonding environment on
excited state behaviour, we performed timeꢀresolved fluorescence study. It was reported that
the difference in the lifetime of the fluorophore arises due to differential degrees of solvent
relaxation around the fluorophore and/or the distribution of fluorophore in different distinct
regions of a confined environment.63 The timeꢀresolved decay profiles of ANS (λex = 360 nm)
with varying R in these RMs at their corresponding maxima are illustrated in Figs. 7A and
7B, and the decay parameters are summarized in Table 2. ANS shows biꢀexponential decay in
neat ILs, whereas triꢀexponential decay evidenced inside ILꢀbased RMs. Several distinct
features of dynamics of probe in RTIL should be discussed before explaining the behaviour
of solvent relaxations in [NMP][Hex] or [Chl][Hex]/Tweenꢀ80/Cy RMs.
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The process of solvation in neat RTIL is much different from the solvation process occurring
in conventional polar solvents like methanol or acetonitrile.64 The solvation time in neat
RTILs is much larger (0.1ꢀ10 ns) compared to the solvation time for most conventional
solvents (1ꢀ10 ps).65 As shown by Chapman and Maroncelli the ionic solvation is slower
compared to the pure solvent.66 The solvation in neat RTILs are controlled by motion of the
cations and anions around the photoexcited dipolar molecules. On the other hand, in case of
polar solvents, the solvation happens as a result of reorientation of solvent molecules around
the excited dye. The average lifeꢀtimes of ANS along with different components in neat ILs
are tabulated in Table 2. The τavg of ANS in neat [NMP][Hex] was found to be 3.63 ns with
time constants of 0.81 and 3.87 ns along with relative contribution of the fast and slow
components being 29 and 71%, respectively. Whereas, the values for neat [Chl][Hex] were
2.39 ns with time constants of 0.40 and 2.60 ns along with relative contribution of the fast
and slow components being 40 and 60%, respectively. The diverse features of the dynamics
of bulk [NMP][Hex] and [Chl][Hex] would arise from the variable interionic and ionꢀsolvent
interactions of two ILs, as observed earlier from computational study. Several reports are
available which describes the biphasic nature of solvation process in RTIL.67ꢀ69
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Table 2. SteadyꢀState Emission Maxima and Lifetimes of ANS in neat ionic liquids (ILs),
and ILs/Tweenꢀ80/cyclohexane RMs at 303 K.
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System
λmax
(nm)
τ1/ns (α1)
τ2/ns (α2)
τ3/ns (α3)
τavg/ns
χ2
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489
465
468
471
2.60 (0.60)
0.96 (0.38)
0.74 (0.42)
0.66 (0.45)
0.40 (0.40)
2.95 (0.30)
3.05 (0.32)
2.92 (0.35)
2.39
8.39
7.76
6.87
1.01
1.01
1.08
1.09
[Chl][Hex]
10.79 (0.29)
10.46 (0.24)
9.85 (0.20)
R=0.3_[Chl][Hex]
R=0.7_[Chl][Hex]
R=1.1_[Chl][Hex]
494
470
472
3.87 (0.71)
1.29 (0.37)
0.97 (0.40)
0.81 (0.29)
3.18 (0.15)
3.41 (0.16)
3.63
11.70
11.34
1.06
1.10
1.05
[NMP][Hex]
12.82 (0.46)
13.19 (0.44)
R=0.3_[NMP][Hex]
R=0.7_[NMP][Hex]
FIG. 7. Timeꢀresolved fluorescence intensity decay of ANS in ILs/Tweenꢀ80/cyclohexane
RMs at different R values and at 303 K; where (A) [NMP][Hex], and (B) [Chl][Hex] (inset
shows comparative decay profile for two ILs in RMs at identical R = 0.30). The sharp
profiles in black are the instrument response functions. View in color for best clarity.
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The triꢀexponential nature of solvation dynamics in ILꢀbased RMs with increased time
constants compared to that of neat ILs originates from the presence of different environments
of IL molecules inside the RMs, such as near and apart from surfactants head group. RTILs in
the core of RMs experience less restricted environment as compared to those present near the
head group, i.e., at the interfacial regions. As a result slow lifeꢀtime component arises due to
the presence of RTIL near surfactant head group or at the interfacial region. On the other
hand, the fast component of solvation dynamics implies the presence of RTIL in the nano ILꢀ
pool. At lower R values, most of the RTIL molecules are involved in interacting with other
ILs and surfactant head group through ionꢀdipole interactions and mainly in solvating
surfactant molecules. As the concentration of RTIL in the system increases, they forms a bulk
like pool where they have less restricted movement as compared to the interfacial region.72 At
higher R values, proportion of IL present at the core of RMs increases and subsequent
proportion at interfacial regions decreases. As a consequence, diffusional motion of ions
increases and values of slow and fast components of the solvation dynamics decreases
leading to reduction in the average lifeꢀtime.
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In the present context, it is desirable to make comparison between the changes in τavg on
going from [NMP][Hex]ꢀinꢀCy to [Chl][Hex]ꢀinꢀCy RMs at comparable compositions. In
former system, the average lifeꢀtime is 11.70 ns with time constants of 1.29 ns (37%), 3.18
(15%) and 12.82 ns (46%) at R = 0.30. However, at this R value, the average lifeꢀtime
becomes 8.39 ns with time constants 0.96 ns (38%), 2.95 (30%) and 10.79 ns (29%) for the
latter one. Similar trend is also evidenced at R value of 0.70. This shows that the interactions
between the protic [NMP][Hex] with the polar head group of Tweenꢀ80 and with other
[NMP][Hex] molecules are stronger than the case of [Chl][Hex]. Therefore, a reduction in the
mobility of [NMP][Hex] molecules at the proximity of reverse micellar cavity surface is
expected due to strong interaction. Earlier, Shirota et al.62 was observed similar type of
behaviour in the solvation dynamics of protic formamide (FA) and aprotic
dimethylformamide (DMF) in AOT/nꢀheptane RMs.
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FIG. 8. Hydrodynamic diameter of the droplets of ILs/Tweenꢀ80/cyclohexane RMs as a
function of molar ratio of IL to surfactant (R) at different temperatures, where ILs are (A)
[NMP][Hex] and (B) [Chl][Hex]. View in color for best clarity.
In order to scrutinize the effects of the ILs on the thermal stability of formulated ILꢀinꢀoil
RMs more directly, we examined the droplet size and viscosity of formulated systems in the
temperature range of 298ꢀ313 K. However, to our knowledge, the influence of temperature on
IL entrapped RMs scarcely reported.46,48,72 Therefore, it would be very interesting and of
utmost prominence to investigate their microstructural change of RM droplets based on nonꢀ
ionic surfactant with temperature. Figs. 8A and B show the temperatureꢀdependent size
variation of ILꢀinꢀCy RMs through DLS measurement. The corresponding size distribution
plots are provided in Supporting Information (Figs. S6ꢀS7). It is evident from the DLS
measurements that the IL based RMs are monodisperse in nature, where the size of droplets
decreases with increase in temperature in all cases. For example, increasing temperature from
298 to 313 K, the droplet sizes decreases from 135 to 75 nm and 52 to 45 nm for
[NMP][Hex] and [Chl][Hex]ꢀbased RMs at constant R of 0.70, respectively. This shows that
the RMs retain their structural integrity across the temperature range studied. Most notable
observation is that the RMs was not destroyed even after further rise in temperature. Rather
single phase transparent RMs was observed even as the temperature approaches the boiling
point of Cy (347 K). Reports are scarce for such a stable nonꢀionic surfactant supported RMs.
We have also measured the viscosity of [NMP][Hex] or [Chl][Hex]/Tweenꢀ80/Cy RMs at the
similar temperature range used in DLS measurement. The temperature dependence of
viscosity of these systems as a function of R is illustrated in Figs. 9A and B. With gradual
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increasing of temperature, the viscosity of the RMs gradually decreases. Strong electrostatic
interaction between the [NMP]+ or [Chl]+ and POE of Tweenꢀ80 decreases with gradual
increase in temperature and as consequence viscosity decreases.
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FIG. 9. (A) Three dimensional representation of viscosity as a function of R and temperature
(T) for [NMP][Hex]/Tweenꢀ80/cyclohexane RMs. (B) Temperatureꢀdependent viscosity
profile for [NMP][Hex] or [Chl][Hex]/Tweenꢀ80/cyclohexane RMs at different values of R.
View in color for best clarity.
Activation energies (Ea) associated with these processes can be calculated from analysis of
temperature dependency of viscosity. From an Arrhenius plot the Ea, can be calculated by;
η = Aꢞꢟ
(4)
ꢠ
ꢂ
ꢡꢢ
Where, R is the universal gas constant, and A is the preꢀexponential factor. The energy
required to move individual droplets in an environment of surrounding RMs is given by
interactions between individual aggregates, known as the activation energy.73 The values of
Ea were evaluated from the slope of the logarithmic plot of 1/η against 1/T, which illustrated
in Fig. 10 for [NMP][Hex] or [Chl][Hex]/Tweenꢀ80/Cy RMs at different R values. The Ea
obtained for [NMP][Hex]ꢀbased systems are 26.52 and 26.77 kJ/mol at R of 0.30 and 0.70,
respectively. Whereas, these values are 25.96 and 26.19 kJ/mol at R of 0.30 and 0.70,
respectively for [Chl][Hex]ꢀbased systems. Similar values for RMs are reported in literature
but were obtained using different approach.72,74 The Ea value for [NMP][Hex]ꢀbased system
is higher compared to [Chl][Hex]ꢀbased system. Generally, Ea value depends on the structure
of RTILs and it is the energy barrier for the ions to move past each other. Thus the larger Ea
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of [NMP][Hex] systems may be due to the stronger hydrogen bonding environment in
[NMP][Hex]/Tweenꢀ80/Cy system, and as a result its orientational motion is hindered.74
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FIG. 10. Arrhenius plots of variation of viscosity in logarithmic function with inverse of
temperature for [NMP][Hex] or [Chl][Hex]/Tweenꢀ80/cyclohexane RMs at different values
of R. View in color for best clarity.
4. OVERALL COMPREHENSION
A series of protic and aprotic ILs such as, [NMP][For], [NMP][Hex] and [Chl][Hex] were
synthesized and subsequently entrapped within Tweenꢀ80/Cy RM nanocage in order to
understand the basic difference between the nature of interionic as well as ILꢀsurfactant
interactions. The phase behaviour of the ternary ILs/Tweenꢀ80/Cy mixture showed large
monophasic transparent region, which varied with the nature of cation (protic [NMP]+ or
aprotic [Chl]+) and alkyl chain length of anion ([For]ꢀ and [Hex]ꢀ) of ILs. Swelling behaviour
of these ILs confirmed the formation of IL/O RM droplets, which found to be larger for
[NMP][Hex]ꢀbased systems compared to [Chl][Hex]. The higher shear viscosity for
[NMP][Hex]ꢀbased systems than [Chl][Hex] correlated with the fact that a stronger network
structure developed for the former system, due to increase in dropletꢀdroplet interactions,
which causes the more closely packed RM droplets at comparable IL content (R) compared to
latter system. All these microstructural information actually offers a clue that the
incorporation of [NMP][Hex] in Tweenꢀ80/Cy RMs creates stronger electrostatic or Hꢀ
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bonding interaction with POE chains of Tweenꢀ80. Also, this primary hypothesis supports the
gas phase theoretical calculations which shows weaker Hꢀbonding interaction in ionꢀpair of
[NMP][Hex] compared to [Chl][Hex]. Thus, [NMP]+ is more prone to interact with Tweenꢀ
80 when entrapped in RM confinement. The basic difference between two types of ILs used
herein ([NMP][Hex] and [Chl][Hex]) is the presence of exchangeable proton in [NMP][Hex],
which can form extended Hꢀbonding network with Tweenꢀ80. In order to strengthen the
understanding of formation mechanism of nonꢀaqueous RMs with these two different classes
of ILs, we performed both FTIR and dynamic fluorescence studies. Both of these
measurements revealed stronger binding of the electronegative oxygen atoms of the POE
units with electropositive [NMP]+ ring by weakening the hydrogenꢀbond interaction between
POE groups of Tween. This results in more blue shifting of OꢀH stretching modes of Tweenꢀ
80 and higher average lifeꢀtime of fluorophore inside [NMP][Hex]ꢀbased RMs compared to
[Chl][Hex]ꢀbased systems. Finally, the temperatureꢀdependent DLS and viscosity
measurements confirmed the decrease in droplet size and viscosity due to weakening of the
electrostatic or Hꢀbonding interactions between the ILs and surfactant without any phase
separation. Thus, these systems retain their structural integrity across the whole temperature
range studied. The thermal stability of two different IL RMs based on nonꢀionic Tweenꢀ80
opens up the possibility of application of nonꢀaqueous RMs at high temperature.
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5. SUMMARY
In this report, we have investigated the significant difference observed in flexibility or
internal rigidity of RMs, dictated by the nature of the polar ILsꢀ [NMP][Hex] and [Chl][Hex],
and analyzed by independent methods such as: phase diagram study, DLS, FTIR, rheology,
and fluorescence measurements. The results show that by simply changing the cationic part
of the IL, remarkable changes in the oil/IL interface and microstructures of RMs, such as
modification of the hydrogen bonding ability of the ILs with surfactant molecules can be
achieved. This findings are very important from the viewpoint of formulating the shapeꢀ and
sizeꢀcontrolled reverse micelle systems with critical applications, for examples, in nanoꢀbio
architectonics, where strategic design of nanostructure materials is crucial.68 It is interesting
that the use of ILs in RM systems can widen their field of application by eliminating the
problems of using pure ILs: viz. low solubility of some chemical compounds and high
viscosity. Furthermore, compared to conventional microemulsion systems, the studied
systems are much less expensive due to their low IL content, and also easier to synthesize.
We strongly believe that the current findings will facilitate in better tuning the microemulsion
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properties by varying the type and amount of additives; and will also escalate research on
similar filed.
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Supporting Information
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9
The Supporting Information is available free of charge on the ACS Publications website.
1
Plots of H NMR and 13C NMR spectra of synthesized ILs; Size distribution of [Chl][Hex]
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and [NMP][Hex] ; Variation of the storage modulus (G′), loss modulus (G″)and complex
viscosity (|η*|) relative to the angular frequency (ω) of microemulsion systems; Emission
spectra of ANS in cyclohexane (Cy); Emission spectra of ANS in neat ionic liquids (ILs);
Size distribution plots [NMP][Hex]ꢀbased RMs as a function of temperature; Size distribution
plots [Chl][Hex]ꢀbased RMs as a function of temperature (PDF).
Acknowledgments
The authors are grateful to IIT Madras for financial support, through Institute Research and
Development Award (IRDA): CHY/15ꢀ16/833/RFIR/RAME. We would like to thank Mr.
Prashant Kumar and Sophisticated Analytical Instruments Facility (SAIF), IIT Madras, for
the fluorescence lifeꢀtime measurement and the computer resource of Computer Centre, IIT
Madras is gratefully acknowledged.
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