Micellization Behavior of Surface Active Ionic Liquids Having Aromatic

Article pubs.acs.org/JPCB

Micellization Behavior of Surface Active Ionic Liquids Having Aromatic Counterions in Aqueous Media Gurbir Singh, Gagandeep Singh, and Tejwant Singh Kang* Department of Chemistry, UGC-Centre for Advance Studies II, Guru Nanak Dev University, Amritsar, 143005, India S Supporting Information *

ABSTRACT: Amphiphilic ionic liquids (ILs) based on 3hexadecyl-1-methyl imidazolium cation, [C16mim]+, having aromatic anions, 4-hydroxybenzenesulfonate, [HBS], benzenesulfonate, [BS], and p-toluenesulfonate, [PTS], as counterions have been synthesized and investigated for their micellization behavior in aqueous medium. The surface activity of investigated ILs has been established by surface tension measurements, whereas bulk behavior has been investigated by conductivity and steady-state fluorescence measurements. The investigated ILs exhibited 2−3 fold lower critical micelle concentration (cmc) as compared to analogous ILs or conventional surfactants with nonaromatic counterions. The polarity of the cybotactic region of pyrene decreases along with decrease in extent of water penetration toward palisade layer of micelle with increase in hydrophobicity of counterion. Relatively more hydrophobic anions, i.e., [BS]− and [PTS]−, have been found to form excimer in palisade layer of micelle, whereas [HBS]− remains in close vicinity of imidazolium head groups of micelle as established from inherent fluorescence of aromatic anions. Isothermal titration calorimetry measurements have provided insights into thermodynamics of micelles. The strength of binding and relative position of aromatic anions in micelle has been found to affect the characteristic properties of micelle as deduced from 1H NMR measurements. The micelles with different sizes and shapes such as spherical, partially elongated, or long rod-like micelles have been observed for different ILs depending of nature of aromatic anions as established from dynamic light scattering and transmission electron microscopy measurements.

1. INTRODUCTION The unique physicochemical properties of ionic liquids (ILs), the materials that are composed only of ions and are liquid below 100 °C, have made them a subject of great interest to researchers around the globe.1 The fine-tuning of the physicochemical properties of ILs such as polarity, viscosity, electrochemical window, and thermal stability along with the possibility of functionalization of ILs is of great advantage for their uses in various applications.1−5 One of the interesting properties of many ILs is their inherent amphiphilicity, which places them in the category of ionic surfactants. The fine-tuning of the polarizability of ionic headgroup and the counterion along with variation in size and functionalization of the alkyl chain provides an opportunity for preparation of better surface active ILs as compared to conventional ionic surfactants. In this regard, a variety of amphiphilic ILs based on different cations such as imidazolium,6−13 pyridinium,7,14 pyrolidinium,15,16 morpholinium,17,18 and even amino acid based cations19 have been explored for their surface activity and micellization behavior in aqueous medium. However, the ILs based on imidazolium cation, [Cnmim]+, have been investigated extensively for their behavior. In this regard, Bowers et al. first investigated the aggregation behavior of ILs, [C4mim][BF4], [C8mim][Cl], and [C8mim][I] in their aqueous © XXXX American Chemical Society

solutions using surface tension, conductivity, and small-angle neutron scattering (SANS) techniques.6 An IL, [C4mim][BF4], having a relatively short chain than required for micellization has been shown to exhibit aggregation in aqueous medium.6,7 Even some of the imidazolium based ILs have been established to exhibit better surface active properties over analogous conventional ionic surfactants.8 The group of Wang et al. investigated the aggregation behavior of [Cnmim][Br] (n = 4, 6, 8, 10, or 12) and other ILs having a variety of anions and cations in aqueous media.10,11 The effect of cationic head groups on micellization behavior of ILs has been investigated by comparing the aggregation behavior of the different ILs with the same anion, [C 8 mim][Br], 4m-[C 8 pyr][Br], and [C8mpyrr][Br]. The hydrophobicity and steric hindrance of the cations as well as binding strength of the cations with the anions were suggested to play an important role in the aggregation. On the similar lines, biamphiphilic ILs, where both cation as well as anion of IL is amphiphilic, have been explored for their micellization behavior and have been found to exhibit better surface active properties as compared to other analogous Received: October 4, 2015 Revised: January 15, 2016

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DOI: 10.1021/acs.jpcb.5b09688 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

Scheme 1. (A) Molecular Structure of Ionic Liquids Investigated in This Work; and (B) A Comparison of cmc Values of Investigated ILs with Analogous ILs or Conventional Surfactants

a

Adapted with permission from ref 13. Copyright 2011 Elsevier. bReference 12. cReference 35.

critical micelle concentration (cmc), i.e., γcmc, the effectiveness of surface tension reduction (Πcmc), adsorption efficiency (pC20), saturation adsorption (τmax), and minimum surface area per molecule (Amin) have been obtained from surface tension measurements along with cmc. The conductivity measurements have provided information about cmc, degree of counterion binding (β), and standard free energy of micellization (ΔGom). Isothermal titration calorimetry (ITC) has been employed to an idea about the thermodynamics of micellization exploiting the obtained values of standard free energy (ΔGom), standard enthalpy (ΔHom), and standard entropy (TΔSom) of micellization. The steady-state fluorescence and dynamic light scattering have provided information about cmc, micropolarity, and intermolecular interactions between anions and size of micelles, respectively. 1H NMR measurements have provided insights into the relative position of counterions in micelle with respect to imidazolium cations and provided information about the internal structure of micelle. The size and shape of the micelles have been corroborated from transmission electron microscopy (TEM) measurements. The micellization behavior of investigated ILs has been compared with conventional ionic surfactants and other reported analogous ILs wherever possible. The presence as well as nature of substituent on the aromatic benzenesulfonate ring (BS) in terms of H-bonding prone relatively hydrophilic hydroxyl group (in case of HBS) and hydrophobic and comparatively bulky methyl group (in case of PTS) and therefore overall size, hydrophobicity, and polarity of aromatic counterions is expected to exhibit a dramatic effect on micellization behavior of ILs.

ILs and conventional ionic surfactants.20−24 There are investigations pertaining to effect of varying counterions on aggregation behavior of [C8mim]+ with [Cl]− or [Br]−, [NO3]−, [CH3COO]−,[CF3COO]−, [CF3SO3], and [ClO4]− anions.25 The anions have been found to exert drastic effect on micellization behavior of IL. The nature of counterion affects various characteristic properties of micellization such as critical micelle concentration (cmc), size, and shape of micelle of ILs similar to that of conventional ionic surfactants. Similarly, the aromatic anions as counterions or as additive comicellized with conventional ionic surfactants such as tetradecyltriethylammonium bromide (TTAB) and cetyltrimethylammonium bromide (CTAB) have been found to effect the cmc, size, and shape of the micelles.26−32 It is discussed that the hydrophobicity, size, degree of hydration, substitution pattern, and nature of substituent of aromatic counterion governs the characteristic properties of micelles.26−32 To date, ILs with different cationic head groups has been investigated for their micellization behavior with a variety of counterions. However, there exists no report on synthesis and micellization behavior of amphiphilic ILs appended with aromatic counterions. The motive of the present work is to investigate the effect of aromatic counterions on the micellization behavior of imidazolium based amphiphilic cation, [C16mim]+, where unique micellization behavior could be observed. Another question of fundamental interest is how substitution upon the aromatic anion would affect the outcome in terms of characteristic properties and thermodynamics of micellization. A comparison of observed micellization behavior of the ILs investigated in this work with that of other reported homologous amphiphilic ILs or conventional surfactants with different inorganic counterions would be interesting. In this regard, the better surface active properties of the investigated ILs over analogous conventional ionic surfactants or other reported ILs along with other characteristic properties of forming micelles that differ in shape and size by varying the aromatic counterion add to the importance of work. It is anticipated that the present work will open up a new research window for the synthesis and characterization of amphiphilic ILs with aromatic anions. We have synthesized, characterized, and investigated the micellization behavior of 1-hexadecyl-3-methyl imidazolium cation, [C16mim]+, based ILs, [C16mim][X], where [X] = benzenesulfonate (BS), 4-hydroxybenzenesulfonate (HBS), and p-toluenesulfonate (PTS). Surface tension measurements were performed to get insight into the surface activity of investigated ILs. A variety of surface parameters such as surface tension at

2. MATERIALS AND METHODS 2.1. Materials. 1-Methylimidazole (>99%), 1-chlorohexadecane (>99%), sodium benzenesulfonate (>99%), sodium 4hydroxybenzenesulfonate (>99%), and sodium p-toluenesulfonate (>99%) were purchased from Sigma-Aldrich and used without further purification. Methanol, acetone, and diethyl ether (AR grade) were purchased from SD Fine-Chem Ltd., Mumbai, India. Pyrene was purchased from Sigma-Aldrich and used after recrystallization from ethanol. The procedure of synthesis of [C16mim][X], where X = benzenesulfonate (BS), 4-hydroxybenzenesulfonate (HBS), and p-toluenesulfonate (PTS) along with their characterization data (1H NMR and HRMS) are provided in Annexure S1 (Supporting Information). Scheme 1A shows the molecular structure of investigated ILs. 2.2. Methods. Surface tension measurements were performed using a DataPhysics DCAT II automated B


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The Journal of Physical Chemistry B tensiometer employing the Wilhelmy plate method within the accuracy of ±0.1 mN m−1 at 298.15 K. The specific conductance was measured using a digital conductivity meter (Systronics 308) employing a cell of unit cell constant at 298.15 K. Measurements for surface tension and conductivity were performed in triplicate with an uncertainty of less than 0.4% and 0.5%, respectively. The temperature for surface tension and conductivity measurements was controlled by using Julabo water thermostat within ±0.1 K. MicroCal ITC200 microcalorimeter, equipped with an instrument controlled Hamilton syringe having a volume capacity of 40 μL, was employed for isothermal titration calorimetric measurements. Steady-state fluorescence was performed using a PerkinElmer Luminescence spectrometer LS-55 using pyrene as an external fluorescent probe (2 × 10−6 M) at an excitation wavelength of 334 nm at 298.15 K. The temperature was controlled using built-in temperature controller within ±0.1 K by using circulating water bath. The spectra were recorded between 350 and 450 nm using an excitation and emission slit width of 2.5 nm, each. The titration method was used for surface tension, conductivity, and fluorescence measurements, where stock solution of respective IL was added to the fixed volume of water in sample holder followed by stirring for complete solubilization. Dynamic light scattering (DLS) measurements were performed using a light scattering apparatus (Zeta-sizer, nanoseries, nano-ZS) Malvern Instruments, equipped with a built-in temperature controller having an accuracy of ±0.1 K. A quartz cuvette having path length of 1 cm was used for measurement, and an average of 10 measurements was considered as experimental data. Data were analyzed using the standard algorithms and is reported with an uncertainty of less than 8%. 1H NMR measurements have been performed on Bruker Ascend 500 spectrometer (AVANCE III HD console) in 10% D2O−H2O mixtures. Transmission electron microscopy (TEM) measurements were performed on JEM-2100 electron microscope at a working voltage of 200 kV. A drop of freshly prepared micellar solution of ILs was placed on a carbon coated copper grid (300 mesh), and the residual solution was blotted off. The samples were dried in air at room temperature for 24 h before measurements. The DLS and TEM measurements were performed on micellar solutions of ILs at a concentration twice of their critical micelle concentration (cmc) values.

Figure 1. Variation of surface tension (γ) in aqueous solutions of different ILs as a function of their concentration at 298.15 K.

counterions. It is established that with increase in hydrophobicity of the added small chain IL with aromatic anion, the cmc values of IL, 1-dodecyl-3-methylimidaozlium bromide decreases substancially.33 Further, it is speculated that Hbonding ability of the [HBS]− with the water molecules along with greater hydration hinders the binding of [HBS]− with imidazolium ions leading to comparatively weaker screening of electrostatic repulsions between the imidazolium head groups forming micelle. Scheme 1B shows the comparison of cmc values for various analogous ILs or conventional cationic surfactants having different counterions with ILs reported here. It is clear that the ILs having aromatic counterions exhibit 2−3 fold lower cmc as compared to analogous ILs having different counterions. This can be attributed to stronger affinity of aromatic anions toward imidazolium head groups of surfactant ion due to their bulky nature and greater hydrophobic character, which effectively screens the electrostatic repulsions between imidazolium head groups and thereby promotes the micellization. Further, the presence of π−π interactions between aromatic counterions and imidazolium head groups could add to the effective lowering of cmc. The values of surface tension at cmc, γcmc, decreases for ILs with different anions following the order: [HBS]− ≈ [BS]− > [PTS]− suggesting that the IL, [C16mim][PTS] is highly effective surfactant owing to greater hydrophobicity of [PTS]−. As expected, the effectiveness of surface tension reduction (Πcmc) follows the reverse order to that of γcmc. Surprisingly the IL with [HBS]− showed almost identical γcmc and Πcac values to that of relatively more hydrophobic [BS]−. This could be due to H-bonding ability of the [HBS]− with that of imidazolium headgroup, which can compensate for the relatively poor hydrophobic character in modifying the surface properties. The variation of adsorption efficiency (pC20) for different ILs is in line with variation in cmc, where the higher magnitude of pC20 in case of [C16mim][PTS] indicates that more hydrophobic is the counterion, more capable is the IL to reduce the surface tension of water. Further, to get the idea about packing of ILs at air−water interface, two important parameters, i.e., saturation adsorption (τmax) and minimum surface area per molecule (Amin) has been calculated from surface tension data using following relationship:34,35 1 τmax = 1023/NA × A min = − (∂γ /∂ ln C)T (1) nRT

3. RESULTS AND DISCUSSION 3.1. Surface Active Behavior of Ionic Liquids. The surface activity of synthesized ILs has been established using surface tension measurements. Figure 1 shows the variation of surface tension, γ, in aqueous solutions of investigated ILs as a function of their concentration. The break point corresponding to onset of plateau region in surface tension profiles indicates the critical micelle concentration (cmc), the values of which are given in Table 1. The variation in molecular structure of counterion has a remarkable effect on cmc values as can be seen from Table 1. The cmc values for investigated ILs for different anions follow the order: [HBS]− > [BS]− > [PTS]−. Both [HBS]− and [PTS]− are substituted derivatives of [BS]−; however, the difference lies in their relative hydrophobicity or bulkiness of substituent. The presence of a p-hydroxyl group in [HBS]− makes it comparatively more hydrophilic as compared to [PTS]− appended with relatively bulky hydrophobic pmethyl group, where [BS]− exhibit an intermediate character. Therefore, the variation in cmc can be correlated with relative hydrophobicity and extent of hydration of the aromatic C


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Table 1. Critical Micelle Concentration (cmc) Determined Using Surface Tension (ST), Conductivity (Cond.), Steady-State Fluorescence Using Pyrene as Polarity Probe (Flr), Steady-State Fluorescence Exploiting Inherent Fluorescence of Aromatic Anions of ILs (Flr.IL), and Isothermal Titration Calorimetry (ITC) and Various Parameters Obtained from Surface Tension (ST) Measurements at 298.15 K for Various ILs cmc/mM

ST

IL

ST

Cond.

Flr.

Flr.IL

ITC

Ave.

γcmc

Πcmc

pC20

106Γ

Amin

[C16mim][HBS] [C16mim][BS] [C16mim][PTS]

0.38 0.33 0.20

0.36 0.32 0.21

0.41 0.33 0.29

0.31 0.29 0.25

0.34 0.32 0.21

0.36 ± 0.03 0.32 ± 0.03 0.23 ± 0.02

33.5 33.7 31.0

37.7 37.5 40.2

−4.05 −4.55 −4.62

2.62 2.26 2.13

63 73 78

γcmc and Πcmc are in mN m−1, 106Γ is in μmol·m−2, Amin is in Å2, and ΔGoad is in kJ mol−1. Ave. represents the average value of cmc obtained from different techniques. a

Figure 2. Variation of (■) specific conductance; and (□) molar conductance in aqueous solutions of different ILs: (A) [C16mim][HBS]; (B) [C16mim][BS]; and (C) [C16mim][PTS] as a function of concentration at 298.15 K. The variation of molar conductance is plotted with respect to square root of concentration.

investigated ILs is fit well into two straight lines with different slopes and the point of intersection of two straight lines marks the critical micelle concentration (cmc). The observed values of cmc are found in good agreement with those observed from tensiometry and are provided in Table 1. The degree of counterion binding (β) have been calculated as β = 1 − S2/S1 where S2 and S1 are slopes of conductivity before and after the cmc, respectively.36 Obtained values of β are provided in Table 2. Very surprisingly, for the investigated ILs having variations in

where (∂γ/∂ ln C) indicates the premicellar slope in the surface tension versus concentration plot and n is a prefactor that denotes the number of species formed in solution by the dissociation of the amphiphile (n = 2) and other terms have usual meanings. The obtained values of τmax and Amin for different ILs are provided in Table 1. The variation of τmax for ILs with different anions follows the order: [C16mim][HBS]− > [C16mim][BS]− > [C16mim][PTS]−, similar to that observed for pC20 values. As expected, the reverse order is followed by variation in Amin, which indicates that the packing density of surfactant ions at air−water interface follows the order: [C16mim][HBS] > [C16mim][BS] > [C16mim][PTS]. The investigated aromatic anions are hydrotropes and are expected to occupy the air−solution interface along with the surfactant ions. It is speculated that [HBS]− forms H-bonding with the imidazolium cation at the air−water interface along with surrounding water molecules. Here [HBS]− could intercalate between the adsorbed imidazolium ions partially beneath the air−water interface leading to a closer packing of surfactant ions at air−water interface takes place. In case of [BS]− and [PTS]−, the position of adsorption at air−solution interface seems to be relatively closer to the air−water interface with a nonplanner orientation between imidazolium head groups leading to lose arrangement of surfactant ions at air−solution interface. The orientation of the counterions at air−water interface is a matter of detailed investigation and will be taken up in future communications. 3.2. Micellization of Ionic Liquids in Bulk. 3.2.1. Conductometry. The micelle formation of investigated ILs in bulk has been explored first by conductivity measurements. The variation of specific conductance in aqueous solution of different ILs as a function of concentration of ILs is shown in Figure 2A−C. The observed conductivity profiles for all the

Table 2. Degree of Counterion Binding (β), Standard Free Energy (ΔGom), Enthalpy (ΔHom), and Entropy (TΔSom)b Change of Micellization, and Polarity Indicator (I1/I3) for Cybotactic Region of Pyrene in Micelles at 298.15 K IL

βa

ΔGoma

ΔGomb

ΔHomb

TΔSomb

I1/I3c

[C16mim][HBS] [C16mim][BS] [C16mim][PTS]

0.36 0.31 0.30

−38.7 −40.0 −41.0

−39.1 −40.7 −40.9

−15.6 −9.5 −11.5

23.6 30.9 26.9

1.26 1.20 1.11

Conductivity. bIsothermal titration calorimetry. cSteady-state fluorescence. Units of ΔGom, ΔHom, and TΔSom are kJ mol−1.

a

the nature of counterion, a decrease in β with increase in hydrophobicity of the counterion is observed which is contrary to general observations.11,25 It is also important to mention that β decreases along with the decrease in cmc values, which seems quite interesting. In general, more hydrophobic counterions decrease cmc favorably via increased counterion binding, however, for the investigated ILs, β decreases with increased hydrophobicity of the counterions. The reason for such behavior can be described on the basis of hydrotropic nature. The investigated organic anions are speculated to form ion-pair complexes with the [C16mim]+ similar to that observed in the D


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Figure 3. (A) A representative emission spectra of pyrene in aqueous solutions of [C16mim][HBS] at different concentrations of IL below cmc and (B) variation of I1/I3 in aqueous solutions of different ILs as a function of concentration at 298.15 K.

considered as the region around solute molecule where ordering of solvent molecules is modified by the presence of solute. On the other hand, the intrinsic fluorescence of aromatic counterions, i.e., [HBS]−, [BS]−, and [PTS]− has been exploited to get information about the nature and composition of Stern and Palisade layer of micelle, where the counterions are expected to reside upon micellization. It is important to define both Stern and Palisade layer for better understanding. The Stern layer of micelle is generally considered as the layer around the core of an ionic micelle which consists of ionic head groups and the tightly bound oppositely charged counterions. The term Palisade layer is normally used in the case of nonionic micelles and is used in this study to differentiate between the areas of adsorption of counterions. The Palisade layer of micelle is considered as the area between oil like hydrophobic core of micelle and stern layer comprised of a few carbon atoms originating from the ionic headgroup. The fluorescent nature of [PTS]− as a counterion in short chain IL 1-ethyl-3methylimidazolium p-toluenesulfonate, [C2mim][PTS] is already established, whereas other investigated counterions are found to be fluorescent, weak but within good detection limit.38 The observed fluorescence behavior using pyrene as a fluorescent probe and using fluorescent anions as intrinsic probe is discussed below. 3.2.2.1. Fluorescence Using Pyrene as Fluorescent Probe. The ratio (I1/I3) of intensity of first (I1) to third (I3) vibration bands of pyrene fluorescence spectrum is very much sensitive to polarity of cybotactic region.39 In aqueous solutions of conventional surfactants or ILs, a drop in value of I1/I3 indicates the onset of micellization as the concentration of surfactant is increased and, therefore, has been successfully utilized to get insights into micellization behavior of conventional surfactants as well as amphiphilic ILs.10,17,19,39 Figure 3A and B shows the representative fluorescence spectrum of pyrene in aqueous solutions of [C16mim][HBS] at different concentrations of IL below cmc and the variation of I1/I3 in aqueous solutions as a function of concentration of different ILs, respectively. For all the investigated ILs, I1/I3 remains almost constant in premicellar region followed by a decline before reaching a plateau region. The solubilization of pyrene in hydrophobic regions of micelle as a consequence of micellization leads to decrease in I1/I3. The concentration corresponding to middle point of transition is considered as cmc and is provided in Table 1. The obtained values of cmc are

case of aqueous surfactant solutions having added hydrotropes and the tendency of ion-pair complex formation increases with an increase in hydrophobicity of the organic anions.37 This suggests that even before the micellization, the aromatic anions remain partially associated with [C16mim]+ and the degree of counterion binding obtained from conductivity measurements can only be analyzed quantitatively in the case of investigated ILs and comparison with other reported ILs or conventional ionic surfactants would be problematic. Such ion-pair formation prior to micellization could also exert the influence on the surface behavior of ILs where a greater reduction in surface area has been observed with the IL having the most hydrophobic counterion. There are some exceptions where it has been established that the cmc and β values are not necessarily interrelated.30 It is argued that although β, which reflects the fraction of counterions bound with the surfactant ion, is comparable for investigated ILs. However, there could be a difference in strength or locus of binding of counterions. This could lead to varying extents of shielding of electrostatic repulsions between the head groups and hence, different cmc values have been observed. The pseudophase model of micellization has been applied to get the value of standard free energy of micellization (ΔGom) using the following equation:34 ΔGmo = (1 + β)RT ln Xcmc

(2)

where R is the gas constant, T is temperature in K, Xcmc is cmc in mole fraction, and β is the degree of counterion binding. The calculated values of ΔGom are given in Table 2. The increasing magnitude of negative values of ΔGom for the analogous investigated ILs with increasing hydrophobicity of the counterion indicates that the micellization of ILs is hydrophobically driven. The findings suggest that the more hydrophobic counterion being less hydrated interacts strongly with the forming micelles and effectively screens the electrostatic repulsions between cationic head groups. This results in decrease in cmc and increase in magnitude of ΔGom for investigated ILs while going from [C16mim][HBS]− to [C16mim][PTS]−. 3.2.2. Fluorescence Measurements. The steady-state fluorescence measurements using pyrene as a polarity sensitive probe has been carried out to get idea about cmc and polarity of cybotactic region of pyrene in micelles. Cybotactic region is E


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Figure 4. Normalized emission spectra of (A) [C16mim][HBS]; (B) [C16mim][BS]; and (C) [C16mim][PTS] in aqueous solutions at different concentrations ranging from below cmc to above cmc. The inset of A−C shows the variation of emission intensity as a function of concentration of respective ILs. An encircled area in B and C indicates the shape change in emission spectra as a consequence of formation of excimer. Part D shows the variation of ratio of excimer emission (IE) to emission from monomers (IM) in aqueous solutions of different ILs as a function of concentration.

[C16mim][PTS], the values resembles with toluene (I1/I3 = 1.1) or ethanol (I1/I3 = 1.0).39 3.2.2.2. Intrinsic Fluorescence of ILs. The fluorescence nature of investigated ILs has been established by measuring the fluorescence spectra at different excitation wavelengths as shown in Figure S1. The invariance of emission maxima (λmax) at different excitation wavelengths indicates the fluorescence nature of ILs owing to presence of aromatic anions. It is important to mention that the imidazolium cation absorbs in the same wavelength region as that of counterions; however, both the absorbance and the emission from imidazolium cation is much weaker as compared to that of the organic anions at the employed excitation wavelength. Figure 4A−C shows the normalized emission spectra for different ILs at varying concentration of ILs and the variation in emission intensity is shown in insets of respective figures. As can be seen from the insets of Figure 4A−C, the emission intensity (Iem), at λmax increases linearly with a relatively higher slope at IL concentrations below cmc to that observed at higher concentrations after micellization. The breakpoint in the profiles of Iem is due to change in environment of counterions as a consequence of their transfer from aqueous solutions to relatively nonpolar environment in micelle and corresponds to cmc (Table 1). The variation in slope of Iem, before and after cmc, is maximum in case of [C16mim][HBS], whereas [C16mim][BS] and [C16mim][PTS] shows similar behavior. This could be due to insertion of [HBS]− between the

in good agreement with those obtained from other methods within the limits of technique used. The I 1/I3 upon micellization for different ILs follows the order: [C16mim][HBS] (1.25) > [C16mim][BS] (1.18) > [C16mim][PTS] (1.10), which indicates the decreasing polarity of cybotactic region of pyrene. The variation can be analyzed in terms of increased extent of packing of cationic head groups in Stern layer of micelle while going from [C16mim][HBS] to [C16mim][PTS]. The observations are in line with the variation of β for different ILs to some extent as β values are almost identical in case of [C16mim][BS] and [C16mim][PTS]. Therefore, as discussed in earlier section, the strength of binding and relative position of counterions in micelle are thought to be other factors governing the compactness of micelle by varying extent of shielding of electrostatic repulsions between ionic head groups. On the other hand, it has been reported that the water can penetrate into the micelle up to 4 carbon atoms from headgroup.40 Therefore, it is inferred from the values of I1/I3 that the extent of water penetration is least in case of [C16mim][PTS], which is in corroboration with the increased hydrophobic character of counterion while going from [C16mim][HBS] to [C16mim][PTS]. The polarity of cybotactic region of micelle (I1/I3 = 1.10−1.25) in [C16mim][HBS] is close to that of methanol (I1/I3 = 1.33), in [C16mim][BS], it is close to that of that of tetrahydrofuran (I1/I3 = 1.20) or trichloromethane (I1/I3 = 1.20) and in case of F


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Scheme 2. A Schematic Representation Showing the Relative Position of Aromatic Anions and Water Molecules with Respect to [C16mim] Cation in Micelles of Different ILs with Varying Shapes and Sizes in Aqueous Medium

that of monomer. The quantitative variation in formation of excimer can be seen from Figure 4D displaying the excimer to monomer emission fluorescence ratio (IM/IE) normalized to the intensity maxima in each case at highest investigated concentration for better clarity. In case of [C16mim][HBS], there exist only a weak signature of excimer formation starting at very low concentrations, which constraints at higher concentrations close to cmc as can be seen from inset of Figure 4D. This indicates that upon micellization, the interactions between [HBS]−, although weaker, diminishes after micellization. This may be due to parallel orientation of [HBS]− anion to that of imidazolium head groups, where there exist π−π interactions between them rendering the formation of excimer. Therefore, the [HBS]− due to its relatively hydrophilic nature to that of other investigated anions, is expected to remain in Stern layer of micelle in contact with water molecules. The relatively larger value of I1/I3 of pyrene in micellar solutions of [C16mim][PTS] as discussed earlier also points toward the penetration of water inside the micelle in vicinity of ionic head groups. Here, the sulfate group screens the electrostatic repulsions between cationic head groups supported by π−π interactions between [HBS] − and imidazolium cation, where hydroxyl group orients toward the palisade layer. As can be seen from Figure 4D, contrary to that observed for [HBS]−, for both [BS]− and [PTS]−, the excimer formation only starts close to micellization indicating that these aromatic anions in micelle are close enough to exhibit π−π* interactions via overlap of π−π* orbital. This overlap at the micelle-water interface is ruled out due to following reasons: (i) these aromatic anions are expected to reside in the palisade layer of micelle considering their hydrophobic nature; (ii) such overlap would result into non- or less efficient screening of electrostatic repulsions between imidazolium head groups which would destabilizes the micelle. Therefore, it is natural to assume that due to greater hydrophobicity of the concerned anions, they got buried into the palisade layer of micelle, where sulfate group interacts with positively charged imidazolium

imidazolium head groups in Stern layer of micelle, where the π−π interactions between oppositely charged ions leads to effective quenching of fluorescence. The behavior is quite similar to that observed for variation in β obtained from conductivity measurements. Considering the greater hydrophobicity of [BS]− and [PTS]−, these anions are expected to remain in the palisade layer of micelle in vicinity of imidazolium head groups without experiencing much influence from πelectron cloud of imidazolium ring as corroborated by 1H NMR measurements discussed later. The emission spectra observed has been analyzed in terms of excimer formation by fluorescent anions in micellar solutions of ILs, which could provide information about the relative position of aromatic anions with respect to imidazolium cation in micelle. The formation of dimers/aggregates of anions or interactions of anions with imidazolium cation in ground state is ruled out based on constancy in shape of the absorption spectra (Figure S2, Supporting Information) of investigated ILs in the investigated concentration range with the exception of [C16mim][HBS]. A noticeable change in shape of UV−vis absorption spectra of [C16mim][HBS] with varying concentration is assigned to possible π−π interactions between [HBS]− and imidazolium cation in Stern layer of micelle. Significant differences in shape of emission spectra of fluorophores as a function of concentration have been observed. Both [BS]− and [PTS]− exhibit the excimer band as obtained by subtraction of the spectrum of most diluted solution of ILs from the obtained spectrum at higher concentration of IL normalized to the same intensity,38 whereas a very weak excimer band is observed in the case of [HBS]− around 307 nm. The excimer formation by [BS]− as well as [PTS]− is expected to occur in deeper Stern layer or upper palisade layer away from water in relatively nonpolar environment. The excimer band in case of IL with [BS]− and [PTS]− is centered around 319 nm, which is in close proximity to monomer band centered around 285−290 nm and indicates that the excited state of excimer is weakly stable as compared to G


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Figure 5. (A) A representative differential power profile obtained for [C16mim][HBS] and (B) variation of enthalpy changes in aqueous solutions of different ILs as a function of concentration at 298.15 K.

used to obtain ΔHom and cmc values is described in inset of Figure 5B. Various thermodynamic parameters such as cmc, ΔGmo , ΔHmo , and TΔSmo have been deduced from ITC measurements using the method described elsewhere and are reported in Table 2.17,42 In brief, the value of 1 − p/n (p = effective charge and n = aggregation number of micelle) essentially used in ITC measurements,42 which is equal to the degree of counterion binding, β, has been deduced from conductivity measurements to evaluate the value of ΔGom using the pseudophase model of micellization as suggested by other researchers.42 Using the Gibbs−Helmholtz equation, the related entropy change has been calculated. The investigated ILs exhibit exothermic enthalpy changes upon micellization following the order: [C16mim][HBS] > [C16mim][PTS] > [C16mim][BS]. For the ILs with same alkyl chain length, the extent of hydrophobic interactions due to alkyl chain giving rise to exothermic enthalpy change is assumed to be almost same in magnitude. Therefore, the variation in ΔHom seems to be controlled by nature of the counterion in terms of its extent of binding with micelle and hydrophobicity eventually effecting the site of binding and aggregation number of micelle. As discussed earlier, the variation of β follows the reverse order to that of enthalpy change for investigated ILs suggesting that the screening of electrostatic repulsions between ionic head groups which give rise to exothermic changes are not controlling the enthalpy changes. It seems that the greater dehydration of more hydrophilic [HBS]− upon micellization gives rise to large exothermic changes. The other reason behind such observations could be the π−π interactions arising from the aromatic anions with imidazolium cations as in the case of [HBS]− in the Stern layer of micelle.9 The presence of such π−π interactions between [HBS]− and imidazolium cation are also speculated to be present from fluorescence measurements whereas such interactions are negligible in case of [BS]− or [PTS]−. The role of H-bonding interactions between [HBS] − and water molecules cannot be completely ruled out as H-bonding interactions could give rise to large exothermic changes. On the other hand, the excimer formation, where self-repulsion between aromatic anions and weaker π−π* interactions could lead to exothermic enthalpy changes. [HBS]− have been found not to form excimer ruling out the possibility of electrostatic repulsions between anions at micelle-water interface. Thus, the greater extent of dehydration of [HBS]− upon micellization

headgroup and aromatic part remains in upper palisade layer near the hydrocarbon chains as shown in Scheme 2. This provides the opportunity for aromatic anions to come closer in the formed micelle to form excimer. The excimer formation is more pronounced in case of [BS]− as compared to [PTS]− upon micellization as can be judged from greater IM/IE ratio for the former at similar concentrations. It is expected that the position of adsorption of counterions in the micelle would affect the thermodynamics of micellization as well as size and shape of micelle via varying degree of screening of electrostatic repulsions between cationic head groups. 3.2.3. Thermodynamic Properties of Micellization. Figure 5A and B shows the variation of differential power for [C16mim][HBS] as a representative and differential enthalpy for investigated ILs, respectively. The plots of differential power for [C16mim][BS] and [C16mim][PTS] are provided in Supporting Information (Figure S3A and B). For all the investigated ILs (Figure 5A and S3 of Supporting Information), endothermic and marginal exothermic changes have been observed below and above cmc, respectively. The enthalpy changes in the aqueous surfactant solution depends on various factors such as apparent molar enthalpy of surfactant ion and counterion, and the micelles, aggregation number (Nagg), degree of counterion binding (β), and the dilution of micelles on addition of stock solution of surfactants.41 The major contribution toward total enthalpy change after cmc is from the dilution of added micelles as all other factors affecting micellization remains almost constant. Therefore, it is assumed that the enthalpy changes taking place after cmc are mainly dominated by dilution of added micelle or electrostatic repulsion between the forming micelles leading to exothermic changes. In general, three different types of enthalpograms, i.e., A, B, and C, have been described as commonly observed for surfactants.41 Similar to other reported ILs or conventional ionic surfactants, all of the investigated ILs exhibit type A enthalpogram as shown in Figure 5B, where a constant value of enthalpy change is observed at concentrations much below the cmc followed by a sharp decline in magnitude of enthalpy change until a plateau is reached after cmc, giving a sigmoidal shape to the curve. For such curves displaying sharp transitions in calorimetric transitions upon micellization, enthalpy of micellization, ΔHmo, can be easily obtained by graphical extrapolation method.41,42 The graphical extrapolation method H


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Figure 6. Expanded 1H NMR spectra of various protons of imidazolium cation and corresponding aromatic counterions in aqueous solutions of investigated ILs as (A) [C16mim][HBS]; (B) [C16mim][BS]; and (C) [C16mim][PTS]. The lower spectrum in each panel (blue) is at concentrations much below cmc, and upper spectra in each panel (red) corresponds to spectra at a concentration above twice the cmc at 298.15 K.

along with its engagement in π−π interactions with imidazolium cations as established by fluorescence measurements leads to larger exothermic changes. However, both [BS]− and [PTS]− forms excimer having different extent of π−π* interactions, which could lead to their different behavior toward cationic head groups in micelle. Further, the factor of dehydration can also play its role in case of [PTS]− owing to hydrophobic hydration of the substituent methyl group. In summary, the dehydration of counterions during micelle formation along with possible π−π interactions between

aromatic anions or aromatic anions and cationic headgroup governs the total enthalpy change. The values of ΔGom varies in accordance with that observed from conductivity measurements, where major contribution toward ΔGom comes from the entropy change. Therefore, it is inferred that the micellization is entropy driven. 3.2.4. Structure and Position of Anions in Micelle. To obtain detailed insight into the position of anions in the micelles, 1H nuclear magnetic resonance (NMR) spectroscopy has been employed. NMR, which utilizes native molecular I


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supports that shielding effect by the anisotropic anion [PTS]−, is prominent. Further the increased broadness of resonance peak for different ILs follows the order: [C16mim][HBS] < [C16mim][BS] < [C16mim][PTS] and is due to decreased relaxation of protons Hb near to the imidazolium headgroup as a consequence of tight packing of alkyl chain. This led to growth of micelles into elongated and rod-like micelles in case of [C16mim][BS] and [C16mim][PTS], respectively (discussed in next section). Further the peaks corresponding to Hc and Hd becomes broadened without any characteristic splitting pattern after cmc in case of all the investigated ILs, which could be accounted for restricted movement of these protons due to packing of monomers in micelle. The behavior of variation of Δδppm for Hc and Hd is similar to that observed for other protons. 3.2.4.2. 1H NMR of Imidazolium Ring and Counterions of ILs. The protons He, Hf, and Hg present on the imidazolium ring have been exploited as an important indicator of micelle formation and its structure, in past.7,43 Being present in the Stern layer of micelle, it is expected that the protons of imidazolium ring could interact with water, anisotropic counterion, and negatively charged sulfate group, and such interactions are expected to affect the observed chemical shift along with π−π interactions between imidazolium cations. All of the protons of imidazolium ring shifts upfield upon micellization in investigated ILs, with the exception of He, which shifts downfield in case of [C 16 mim][BS] and [C16mim][PTS]. The upfield shift in the protons of ionic headgroup have been attributed to the magnetic anisotropy effects of aromatic anions interacting with ionic head groups and have been used to get the average location of counterion in the micelle.31,44,45 Further, the magnitude of upfield shift in ring protons (Hf and Hg) for different ILs upon micellization decreases with increase in hydrophobicity of the counterion as can be seen from Table S1 (Supporting Information). An upfield shift in ring protons can be accounted for the following reasons: (i) a shielding effect produced by the presence of aromatic anions in vicinity of imidazolium headgroup in micelle; (ii) the presence of π−π interactions between the imidazolium rings in micelle; (iii) a weaker interaction between aromatic anion and imidazolium headgroup at the cost of stronger interactions between counterion and water as well as ring protons and water.7,30,43 Therefore, combining the results from other techniques with that obtained from NMR investigations, it is inferred that the aromatic anion, [PTS]− shields the imidazolium protons Hf and Hg to the least extent, which can be due to its presence in the relatively deeper part of Stern layer, whereas [HBS]− owing to its least hydrophobic nature remains in close vicinity or between the imidazolium head groups shielding the ring protons to a greater extent. This fact is also supported by changes in nature and magnitude of chemical shift for alkyl chain protons near to imidazolium headgroup along with results obtained from fluorescence measurements. The presence of relatively stronger π−π interactions between the imidazolium rings is speculated in case of [C16mim][PTS] as compared to [C16mim][BS] leading to effective screening of electrostatic repulsions. This provides stability to micelle resulting in growth of micelle into partially elongated and long rod-like micelles, in case of [C16mim][BS] and [C16mim][PTS], respectively as observed from TEM measurements. For the proton He, as expected, an upfield shift is observed in case of [C16mim][HBS]. This could be due to shielding by anion as well as its weaker interactions with water

probes, has been used in the past for structure elucidation of micelles of surfactants as well as to obtain insights into the relative positions of counterions with respect to surfactant ions in micelle, an important parameter for growth of micelle into different shapes and sizes.7,17,28−30 The conformation and medium effects are dominant factors over deciding the changes in chemical shift along with contribution of other shielding and deshielding effects such as intermolecular aromatic ring current effect. The magnitude and nature (upfield/downfield) change in chemical shift (Δδppm = δmic − δmon) upon micellization relative to that of monomers is an quantitative and qualitative indicator of above-discussed effects.7 Below cmc and after the micellization, the chemical shift for all protons of interest remains constant and is termed as δmon and δmic, respectively. The discussion in this regard is divided into two sections: (i) 1 H NMR of alkyl chain of ILs; (ii) 1H NMR of imidazolium headgroup and respective counterions. 3.2.4.1. 1H NMR of Alkyl Chain of ILs. The molecular structure of [C16mim]+ and different anions having marking of different protons is shown in Figure 6A. Figure 6B−D shows the expanded 1H NMR spectra of investigated ILs below and above cmc. The protons of terminal methyl group (Ha) shift downfield for all investigated ILs upon micellization following the order: [C16mim][HBS] < [C16mim][BS] < [C16mim][PTS], and corresponding Δδppm = δmic − δmon values are provided in Table S1 (Supporting Information). The downfield shift observed for Ha is due to decrease in hydration as a consequence of transfer of alkyl chain from water to the most hydrophobic micellar core upon micellization. The Δδppm for Ha (Table S1, Supporting Information) in different ILs indicates that the core of micelle formed by [C16mim][PTS] is relatively tight and, hence, most hydrophobic in nature as also indicated by fluorescence measurements. Below cmc, the resonance peak corresponding to protons at Hb is marginally asymmetric singlet peak in all the investigated systems. Very surprisingly, it splits into a broadened doublet in case of [C16mim][HBS], and a broadened triplet in case [C16mim][BS] and [C16mim][PTS], after cmc. Here the main peak for Hb shifts downfield, whereas the newly originated peak shifts upfield. The downfield shift for Ha and Hb can be correlated with the increased content of trans conformation in micelle as compared to that in bulk water.7 Further, the broadness of newly originated peak increases while going from [C16mim][HBS] to [C16mim][PTS] and the corresponding Δδppm (Table S1, Supporting Information) for observed downfield and upfield shift for main peak and newly originated peak, respectively follows the same order of variation in Δδppm as followed for Ha. The splitting of main resonance peak into broadened peaks can be accounted for the different electronic environment of Hb present near to the ionic headgroup as compared to that present away from headgroup toward micellar core provided by the adsorbed anisotropic counterions. Earlier, it has been shown by the self-fluorescence of the counterions of different ILs that the more hydrophobic and bulky [PTS]− penetrates deep into the palisade layer of micelle followed by [BS]−, where these anions from excimer via π−π* interactions as suggested from fluorescence measurements. The least splitting of resonance peak corresponding to Hb in case of [C16mim][HBS] supports the observation made by fluorescence measurements that the [HBS]− remains in the Stern layer of micelle near to imidazolium head groups, thereby, affects the alkyl chain protons to least extent. Further, the increasing magnitude of Δδppm in case of [C16mim][PTS] J


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Figure 7. (A) Dynamic light scattering profiles represented as intensity % for micellar solutions of different ILs showing different hydrodynamic diameters (Dh) at 298.15 K; (B−D) transmission electron microscopy (TEM) images of aqueous micellar solutions of different ILs as (B) [C16mim][HBS]; (C) [C16mim][BS]; and (D) [C16mim][PTS] showing micelles with varying shape and size. Inset of images shows an enlarged view of the micelles. The partially elongated micelles in the case of [C16mim][BS] seems to be flexible. The presence of few spherical micelles (marked) in case of [C16mim][PTS] indicates the growth of spherical to rod-like micelles.

0.06−0.08) upon micellization is observed for investigated ILs. There are other factors such as the presence of water in upper palisade layer near imidazolium headgroup, effect of ring current by the imidazolium ring, or the effect of ring current by aromatic anions on the intermolecular protons of aromatic ring.46 As indicated from fluorescence measurements and 1H NMR measurements, it is inferred that both the penetration of water along with ring currents seems to dominate the change in chemical shift in case of [C16mim][HBS], where [HBS]− is present in between the imidazolium groups. The effect of imidazolium ring currents however is assumed to negligible in case of [C16mim][BS] and [C16mim][PTS]. This also suggests that the protons H2 and H6 reside in the Stern layer of micelle; however, the depth of penetration could be different. As expected, the other two equivalent aromatic protons, H3 and H5, experience larger shifts toward the strong field upon aggregation as compared to that of H2 and H6 confirming their presence in relatively more hydrophobic environment. Interestingly, the amount of the observed shift (Δδppm) is different for different anions and follows the order: [HBS]− < [BS]− < [PTS]−. This order can be explained on the basis of the relative positioning and orientation of these anions with respect to the micelle forming cation. The least upfield shift with Δδppm ≈ 0.11 ppm, which is comparable to that observed for H2 the H6 (Δδppm ≈ 0.08 ppm) in the case of [HBS]− point toward the presence these protons in a similar environment between imidazolium head groups, positioning toward the palisade layer. The observation is further corroborated by an

at the cost of enhanced interactions between water and negatively changed sulfate group, which remains above the surface of micelle due to position of [HBS]−. In case of [C16mim][BS] and [C16mim][PTS], a downfield shift via almost equal magnitude (Table S1, Supporting Information) in most acidic proton of the imidazolium ring (Hg) is assumed to be due to its interactions with water present in Stern layer as deshielding via ring current of aromatic anion is ruled out based on the observations made from other techniques. Further, the protons of methyl group directly attached to the imidazolium ring (Hh) shifts upfield for all investigated ILs upon micellization; however, the shift is relatively more in case of [C16mim][HBS], whereas Hh shifts upfield by almost equal magnitude in case of [C16mim][BS] and [C16mim][PTS]. This supports the assumption that [HBS]− remains in close proximity of imidazolium ring, thereby exerting maximum effect on the Hh; however, other factors such as solvation and effect of ring current by imidazolium cation cannot be ruled out. An upfield shift is observed for all the aromatic protons, where the varying magnitude of the shift for different protons upon micellization suggests an environment differing in polarity for various protons. An upfield shift for aromatic protons in line with that observed for aromatic anions with conventional surfactants is due to the transfer of the aromatic protons from the more polar hydrophilic region to a relatively, nonpolar hydrocarbon like environment in micelle.7,44 In case of aromatic protons H2 and H6, a small upfield shift (Δδppm = K


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interactions with imidazolium ring in contact with hydrating water. Although the counterion binding is maximum in case of [HBS]− indicating the effective screening of electrostatic repulsions between the ionic head groups, however, its presence in the Stern layer in contact with water does not affect the curvature of forming micelles leading to formation of spherical micelles. On the other hand, [BS]−, as judged from fluorescence and 1H NMR measurements, owing to its relatively more hydrophobic nature is expected to stay somewhere in upper palisade layer in contact with the imidazolium head groups, where negatively charged sulfate group effectively screens the electrostatic repulsion despite having low counterion binding. This would also affect the curvature of micelle leading to partial growth of micelle as observed from TEM measurements. On the other hand, the [PTS]− owing to its most hydrophobic nature and presence of bulky methyl group gets penetrated deeper into the palisade later of micelle as confirmed by 1H NMR measurements. Further, the tight packing of micelle as suggested by variation in chemical shift for the protons of alkyl chain near to imidazolium headgroup reflects the effective screening of electrostatic repulsions, where the presence of π−π* interactions in excimer of [PTS]− leads to stabilization of micelle. The steric hindrance provided by the [PTS]− along with relatively deeper penetration leading to tight packing of head groups at micelle-water interface results in lowering of curvature of micelle eventually leads to growth of micelles into rod-like structures.

upfield shift of the proton (He) present between the two N atoms in the imidazolium ring, which is encountered only in the case of [C16mim][HBS], and probably due to the ring currents or because of the electron density of the aromatic ring. This confirms the close proximity and the tilted orientation of the aromatic ring with respect to the imidazolium ring. In case of [BS]− and [PTS]−, relatively large upfield shift with Δδppm ≈ 0.14 and ≈ 0.18 ppm, is observed, respectively. This indicates the parallel orientation of these anions with respect to cation, where [PTS]− is more deeply buried into the palisade layer as compared to the [BS]−. In line with this, an upfield shift of comparatively lower magnitude (Δδppm ≈ 0.12) for methyl protons (H4) present at para-position of [PTS] than that for protons H3 and H5 (Δδppm ≈ 0.17) suggests the presence of methyl group away from imidazolium ring currents extended relatively deeper into the palisade layer. The proton H4 present on the aromatic ring in case of [BS]− is also shifted upfield showing Δδppm ≈ 0.17, which confirms straight up to down parallel orientation of anion with respect to the cation from Stern to Palisade layer rather than in any tilted conformation. 3.2.5. Size and Shape of Micelles. Dynamic light scattering (DLS) and Transmission electron microscopy (TEM) have been employed to gain insight into the size and shape of the formed micelles. Figure 7A shows the light scattering profiles for different ILs at a concentration twice that of their respective cmc values. As can be seen from Figure 7A, the formed micelles exhibit hydrodynamic diameter (Dh) ≈ 8 nm in case of [C16mim][HBS] along with a scattering peak ≈40 nm. Considering the extended length of the [C16mim]+ ≈ 4 nm, it is natural to assume that the scattering peak ≈8 nm corresponds to the presence of spherical micelles, whereas a peak ≈40 nm is accounted for the agglomeration of formed micelles. The assumption is supported by the TEM measurements (Figure 7B), where smaller spherical micelles of size ≈7−9 nm along with larger spherical micelles of size ≈30−50 nm are clearly seen. The light scattering profile in aqueous solutions of [C16mim][BS] exhibit a single peak centered at Dh ≈ 10 nm. Considering the greater hydrophobicity of [BS]− as compared to [HBS]− along with varying locus of adsorption of these counterions in micelle as suggested by fluorescence and 1 H NMR measurements, some variation in nature of micelles was expected. TEM measurements (Figure 7C) have established that the marginally elongated micelles forms in case of [C16mim][BS] with dimensions ≈6 nm in width and ≈12 nm in length. Further, these micelles seem to be flexible in nature as can be judged from their shape which is not so regular. In case of [C16mim][PTS], three scattering peaks at Dh ≈ 7 nm, ≈ 35, and ≈ 150 nm have been observed. From analysis of TEM micrographs (Figure 7D), very surprisingly, the presence of elongated rod-like micelles with dimensions ≈7 nm in width and ≈30−150 nm in length have been observed. This suggests that the scattering peaks at lower (≈ 7 nm) and higher Dh (≈ 35−150 nm) ends correspond to scattering from smaller and longer axis of rod-like micelle, respectively. The presence of spherical micelles to rod-like micelles sized ≈7 nm along with relatively long rod-like micelles indicates the growth of spherical micelles to rod-like micelles at such low concentration of ILs above cmc. It has been established that the nature of counterion affects the size and shape of micelle along with other characteristic properties of micelle.26−32 As indicated from fluorescence measurements and confirmed from 1 H NMR measurements, it is clear that the least hydrophobic [HBS]− remains near the Stern layer of micelle exhibiting π−π

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CONCLUSIONS

Ionic liquids (ILs) based on 1-hexadecyl-3-methylimidaozlium cation, [C16mim]+ with aromatic anions, 4-hydroxybenzesulfonate, [HBS]−, benzenesulfonate, [BS]−, and p-toluenesulfonate [PTS]−, have been synthesized and investigated for their micellization behavior in aqueous medium for the first time. Investigated ILs exhibited 2−3 fold lower cmc values as compared to that of conventional surfactants or ILs having inorganic anions as counterions. The degree of counterion binding is not found to affect the cmc values; instead, strength of binding leading to formation of tight ion-pairs and relative position of aromatic anions in micelle driven by hydrophobicity of anions governs the cmc and other micellar parameters. The micellization process is found to be hydrophobically driven. The most hydrophobic and relatively bulky anion, [PTS]−, is found to reside in the upper palisade layer of micelle without any contact with water molecules from fluorescence and 1H NMR spectroscopy. The formation of excimer by [BS]− and [PTS]−, which helps in formation of compact packing of imidazolium cations in micelle. This deep insertion of [PTS]− and tight packing of micelle lowers the micellar curvature leading to formation of rod-like micelle at a concentration just twice to that of cmc, as established from DLS and TEM measurements. On the other hand, relatively hydrophilic [HBS]− resides in the Stern layer of micelle in close contact with imidazolium cations with sulfate group in contact with water, where it effectively shields electrostatic repulsions between head groups but does not affect the curvature of micelle leading to formation of spherical micelles. The position of [BS]− in micelle is somewhere in between to that of [PTS]− and [HBS]−, where a marginal growth of micelle is observed. L


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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b09688. Detailed procedure for synthesis of investigated ILs along with 1H NMR and HRMS data is provided in Annexure S1, emission spectra at different excitation wavelengths, normalized UV−vis absorption spectra, and differential enthalpograms of different ILs are provided in Figures S1, S2, and S3, respectively. Magnitudes of changes in chemical shifts are provided in Table S1 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Tel.: +91-183-2258802, ext. 3207. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to DST, Government of India, for financial assistance to carry out this work with Project Scheme No. SB/FT/CS-057/2013. We are also thankful to the UGC, India, for their UGC-CAS (Centre for Advanced Studies) program and UPE program for creating infrastructure and research facilities at Guru Nanak Dev University, Amritsar. Authors are thankful to Mr. Ravinder Singh for TEM measurements.

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REFERENCES

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Micellization Behavior of Surface Active Ionic Liquids Having Aromatic

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