Influence of N‑Alkylpyridinium Halide Based Ionic Liquids onMicellization of P123 in Aqueous Solutions: A SANS, DLS, and NMRStudyRohit L. Vekariya,† Vinod K. Aswal,‡ Puthusserickal A. Hassan,§ and Saurabh S. Soni*,†

†Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar 388120, Gujarat, India‡Solid State Physics Division and §Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, Maharashtra,India

*S Supporting Information

ABSTRACT: The isotropic micellar state of Pluronic P123 inthe presence and absence of N-alkylpyridinium halide ionicliquids (ILs) is investigated using SANS, DLS, and 1H NMRstudies. The micellar structural parameters are obtained as afunction of variation in alkyl chain length, anions, andconcentrations of ILs by fitting the SANS scattering datawith a model composed of core−shell form factor and a hardsphere structure factor of interaction. Addition of ILs decreasesthe micellar core, aggregation number, and hard sphere radiusof P123 micelles. From quantitative analysis, we determined the amount of solvent (D2O + IL) present inside the core and thecore−shell interface along with cationic head groups. This is further supported by monitoring interaction between ILs andpolymer micelle using 1H NMR spectroscopy. The results are discussed and explained as a function of concentration of C8PyCl,alkyl chain length, and anions of N-alkylpyridinium halides.

1. INTRODUCTION

Poly(ethylene oxide)−poly(propylene oxide)−poly(ethyleneoxide) (PEO−PPO−PEO) block copolymers are an interestingtype of amphiphilic block copolymer commonly known byPluronic or Polaxamer or Symperonics (ICI) and usedextensively in many industrial applications.1,2 In recent years,the scope of their application is much broader and covers usesin nanoparticle synthesis,3,4 in gene delivery,5 as polymer gelelectrolyte,6 as templating agents,7 and so forth. The majority ofthese applications are associated with the micelles that areformed in aqueous solution, and therefore, micellization ofblock copolymer has attracted great attention.8 In manyindustrial applications, Pluronics are used in the presence ofvarious cosolvents/surfactants, and therefore, the influence ofsurfactant in general and ionic surfactant in particular on theassociation behavior of Pluronic block copolymers is very welldocumented in the literature.9−17

Ionic liquids (ILs) are receiving considerable attention due totheir unique physical properties and have been widely used inthe area of organic synthesis, catalyst, electrochemistry, polymerelectrolyte, and so forth.18,19 Generally, ILs are composed ofalkyl imidazolium or pyridinium cation and a variety of anions.However, the alkylpyridinium based ILs have higher biodegrad-ability compared to imidazolium based ILs, and therefore, theyare used in various applications including surface active agentsand also behave as short chain cationic surfactants when theyare dissolved in water.20−22 The combination of amphiphilicblock copolymers and ILs is being recognized for many

possibilities in designing new polymeric materials.23,24 Theprospective practical applications of block copolymers and ILshave been identified in the areas of supported catalyst,25

polymer electrolyte membrane,26 metal ion removal,27 and soforth. Out of all these applications, because of the highconductivity of ILs, polymer electrolytes is a promising field inwhich they can be utilized as an electrolyte in advanced deviceslike dye sensitized solar cells,28,29 lithium ion batteries,30 and soforth. Very recently, water based polymer gel electrolytes havealso received much attention because of environmental andsafety issues.31 In view of this, it is necessary to study the effectof ILs, their interactions, and the location of cation/anionfragments in micelles of amphiphilic block copolymers inaqueous solutions.Individually, surface activity and association behavior of

Pluronic block copolymer and long chain cationic surfactantsare very well documented in the literature.20,21,32 There arereports on self-assembly and micellization behavior ofamphiphilic block copolymer in the presence of long chaincationic surfactants. Hecht et al. studied the influence of DTAB(dodecyl trimethylammonium bromide) on the aggregationbehavior of Pluronic F127.10,11 Li et al.12,13 and Singh et al.16

reported that some Pluronics form complex unique supra-molecular assemblies in the presence of ionic surfactants

Received: July 25, 2014Revised: November 7, 2014

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including CTAB (cetyltrimethylammonium bromide).16 More-over, they found that the hydrophobic chain of cationicsurfactant gets dissolved in the core of block copolymermicelles and the charge head groups reside at the core−shellinterface or in a hydrated shell. In the majority of the studiesavailable in the literature, the cationic surfactants used werecomposed of long hydrocarbon chain (≥C14) and theirconcentrations were much above their CMC values. However,there are only a few reports in the literature on the effect ofshort chain (≤C10) cationic surfactants (ILs) on Pluronicmicelles.33,34 Zheng et al.33 reported aggregation behavior andinteraction of Pluronic P104 in the presence of 1-butyl-3-methyl imidazolium bromide (BmimBr) in aqueous solutionusing FTIR, FFTEM, DLS, and NMR spectroscopy. Recently,Parmar et al.34 studied the interaction between 1-alkyl-3-methylimidazolium tetrafluoroborate and Pluronic P103 in aqueoussolutions using DLS, SANS, and NMR studies. From theselective NOESY NMR spectrum, they indicated that there isan interaction between the butyl chain of IL and the PO groupof P103 micelles. In the literature, so far no reports are availableon the variation in micelle parameters as a function ofconcentrations, cationic head groups, and anions of pyridiniumbased short chain ILs (≤C8) on micelles of Pluronic blockcopolymer in water.Here, we report the aggregation behavior of Pluronic P123

[(EO)20(PO)70(EO)20] block copolymer in the presence ofvarious pyridinium based ILs in aqueous media. SANS and DLSstudies were used to determine the size and shape of P123micelles under the influence of alkylpyridinium based ILs. Fromquantitative SANS analysis, a fraction of solvent in PPO and theinteraction of alkyl substituted pyridinium cation and anionswith micellar core consisting of PPO block were determined.This is further supported by NMR measurements. The effect ofalkyl chain length, anions, and concentration of ILs onmicellization of P123 in aqueous solution has been discussed.

2. EXPERIMENTAL SECTION2.1. Chemicals and Materials. Pluronic P123 (EO20PO70EO20)

was purchased from the Sigma-Aldrich, India. All ionic liquids wereprepared by the procedure reported in the literature18,21,35,36 andcharacterized by 1H NMR, TGA, and IR methods. All ILs were storedat 60 °C in a vacuum prior to use and the water content was measuredby Karl-Fisher analysis, which found less than 0.05% in all ILs. Thedetailed structure of ILs along with critical micelle concentration(CMC) and neutron scattering length densities are given in Table 1.Aqueous solutions of Pluronic and ILs were prepared in Milliporegrade distilled water. For SANS and NMR measurements sampleswere prepared in D2O (>99%, Sigma-Aldrich, India).2.2. Small Angle Neutron Scattering (SANS). SANS measure-

ments were carried out on micellar solutions of P123 triblockcopolymer in the presence of various types of ILs. All solutions wereprepared in D2O (99.9 atom % D, Sigma-Aldrich, India). The SANSmeasurements were performed using a fixed geometry SANSinstrument with a sample-to-detector distance of 1.8 m at Dhruvareactor, Trombay, India.37 This spectrometer makes use of a BeOfiltered beam which provides a mean wavelength of 5.2 Å and has awavelength resolution of about 15%. The angular distribution of thescattered neutrons is recorded using an indigenously built one-dimensional detector. The accessible wave transfer, q, range of thisinstrument is 0.015−0.35 Å−1. The solutions were held in a 0.5-cm-path-length UV-grade quartz sample holder with tight fitting Teflonstoppers sealed with parafilm. The theoretical approach to SANSmeasurements is given in Supporting Information.2.3. Dynamic Light Scattering (DLS). DLS measurements of

aqueous solutions of the polymer with and without ILs wereperformed using a Malvern 4800 autosizer employing a 7132 digital

photon correlator. The light source was an argon ion laser operated at514.5 nm with a maximum output power of 2 W. The apparentequivalent hydrodynamic radii (Rh) of the micelles were calculatedusing Stokes−Einstein eq 1.32,38

πη=R

k TD6h

B

0 (1)

where kB is the Boltzmann constant, η is the viscosity of solvent(water) at temperature T, and D0 is the diffusion coefficient. Thedetails of theoretical approach for DLS measurement are given in theSupporting Information.

2.4. Nuclear Magnetic Resonance (NMR). All NMR experi-ments were conducted on a Bruker Avance 400 spectrometer at alarmor frequency of 400.13 MHz for proton equipped with amicroprocessor controlled gradient unit and an actively shielded z-gradient coil. All samples were prepared in D2O and TMS was used asan internal standard.

3. RESULT AND DISCUSSIONIn the present model, the core and shell are assumed to haveuniform scattering length density (SLD), ρc and ρs, respectively.The SLD value of the solvent or the medium, ρM, is a knownquantity calculated (using eq 3 of Supporting Information) forall compositions and considered as input parameters, whichmatched very well with the output values obtained after fitting(Table 2). Moreover, as documented previously for CTAB,15

we assume that all IL molecules added (below their CMC) areassociated with the micelles of P123. For the isotropic micellarphase, a MATLAB program has been developed to fit theobserved data on an absolute scale with the above description.The seven parameters could be either fixed or adjusted, amongwhich four described the form factor, one for the polydispersityof the micelle core radius and two for the structure factor.These parameters are Rc and Rs (the radii of the core andthickness of shell), ρc and ρs, δ (the polydispersity), volumefraction, ϕ, and hard sphere interaction radius, RHS, of micelles.The variations in the intensity are essentially governed by thepolydisperse form factor contribution which depends on thefive parameters (Rc, Rs, δ, ρc, ρs), and the structure factor termS(q, RHS, ϕ). It was ensured that a pure core model gives poor

Table 1. Structure, CMC, and Neutron Scattering LengthDensity (SLD) of Ionic Liquids Used in These Studies

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fits, with significantly larger χ2 values. As the P123 blockcopolymer contains only 40 EO groups (which is less than atypical long hydrophilic chain with 100 EO groups), the use ofthe core−shell model is justified, as it has the advantage ofintroducing a relatively low number of independent parametersduring the fittings.39,40

3.1. Effect of Concentration of IL (C8PyCl). The SANScurves measured for 15% (w/w) P123 in D2O as well as in 100mM and 200 mM aqueous solutions of C8PyCl are shown inFigure 1a. The SANS distribution curves were fitted with theLMA hard sphere core−shell model described in the theoreticalsection. During the fitting, the SLD of medium (D2O or D2O+IL) was kept fixed to the calculated values obtained from eq 3(Supporting Information), while the core and shell SLDs wereconsidered as free parameters. For guidance, the SLDs for purePPO, PEO blocks in melts along with D2O were calculated andthe values are 0.34 × 10−6 Å−2, 0.62 × 10−6 Å−2, and 6.38 ×10−6 Å−2 for PPO, PEO, and D2O, respectively. These valueswere obtained by assuming that the PEO and PPO materialshave a mass density of 1.018 g.cm−3 equal to the mass densityof the P123 block copolymer. For 15% (w/w) P123 in D2O,the fitted core and shell SLDs are 1.01 × 10−6 Å−2 and 6.34 ×10−6 Å−2, respectively. Thus, the higher SLD of PPO corecompared to PPO melt supports that the fraction of D2O maybe present inside the PPO core of micelles. The fitted SLD forPEO (6.34 × 10−6 Å−2) is far different than the calculated value(0.62 × 10−6 Å−2) but closer to the SLD of D2O. This showsthat it is impossible to fit the data to a pure PEO shell withPEO melt density, and therefore, PEO chains were expected tobe solvated by water. The obtained fitting parameters arereported in Table 2 with error bars indicating the range inwhich no significant changes could be detected in theadjustment.In D2O, the obtained core radius, Rc, (4.9 nm) for 15% (w/

w) P123 is slightly higher than the calculated end to enddistance (3.7 nm, randomly coiled model) of PPO in the meltstate which shows that the core might be hydrated with water.However, the fitted Rc value is in good agreement with that

reported by Ganguly et al.41 for 10% (w/w) P123 in D2O (Rc =4.79 nm). Other micellar parameters are also in accord with thereported values for P123 micelles available in the literature.41

The aggregation number, Nagg, defined as the number of blockcopolymer molecules per micelle is given by

π=

+N

R RV

4 ( )3aggc s

3

p (2)

where Rc and Rs are the core and shell radius, respectively, andVP (VEO + VPO) is the volume of the P123 molecule. Thecalculated Nagg of 15% (w/w) P123 in D2O is 251, whichmatches very well with the value (Nagg = 244) reported in theliterature for 10% (w/w) P123 aqueous solution using lightscattering technique and by considering hydrated micelles.42

However, this value is almost 2−3-fold higher than the valuereported from SANS study by assuming scattering coming fromcore only.41

The effects of C8PyCl concentration on SANS distributioncurves are shown in Figure 1a. Scattering curves depicted arefor a 15% (w/w) aqueous solution of P123 micelles with 100mM and 200 mM C8PyCl at 30 °C. The figure portrays that, asthe C8PyCl concentration is increased, the intensity decreasesand the correlation peaks appeared and shift toward higher q.This shows that the addition of C8PyCl altered the intermicellarinteraction along with the morphology of P123 micelles. Themicellar parameters obtained from the fits with the core−shellmodel are given in Table 2. During fitting, the SLD of medium(D2O + C8PyCl) was fixed to its calculated values obtainedfrom eq 3 (Supporting Information). A perusal of data given inthe table shows that the micellar core radius, Rc, shell thickness,Rs, along with RHS progressively decrease with increase inconcentration of C8PyCl from 100 mM to 200 mM.Simultaneously, an increase in volume fraction, ϕ, is attributedto the higher number density of micelles, and hence, micellarcorrelation function, S(q), becomes the dominant componentin the scattering function when C8PyCl was added. This isreflected in the values of RHS (Table 2) and the observed shift

Table 2. Scattering Length Density of Solvents (ρM), Core Radius (Rc), Scattering Length Density of Core (SLC), Thickness ofthe Shell (Rs), Scattering Length Density of Shell (SLS), Polydispersity (δ), Hard Sphere Radius (RHS), Volume Fraction (ϕ),Aggregation Number (Nagg), Fraction of Medium (D2O + IL), ϕM, and Number of Water Molecules, (nw) Associated per P123Micelles for 15% (w/w) P123 Solutions with and without ILs at 30 °C

system ρM ×106 Å−2 Rc (nm) SLC ×106 Å−2 Rs (nm) SLS ×106 Å−2 δ RHS (nm) ϕ Nagg ϕM nw

P123 6.38 4.9 ± 0.2 1.01 2.6 6.34 0.27 7.8 ± 0.4 0.19 251 ± 5 11.0 4.3Effect of Concentration of C8PyCl

P123+100 mM C8PyCl 6.29 3.9 ± 0.2 1.19 2.2 6.08 0.33 7.2 ± 0.4 0.20 135 ± 4 14.3 4.1P123+200 mM C8PyCl 6.21 3.5 ± 0.2 1.49 1.6 5.84 0.40 6.0 ± 0.3 0.22 79 ± 3 19.5 3.8

Effect of Alkyl Chain Length of ILs (CnPyCl, n = 4, 6, 8)P123+100 mM C4PyCl 6.32 4.7 ± 0.2 1.07 2.1 5.86 0.29 7.4 ± 0.4 0.19 187 ± 4 12.2 4.2P123+200 mM C4PyCl 6.24 4.4 ± 0.2 1.16 1.6 5.55 0.32 7.0 ± 0.4 0.20 129 ± 3 13.8 4.0P123+100 mM C6PyCl 6.31 4.4 ± 0.2 1.18 2.2 6.04 0.30 7.3 ± 0.4 0.20 172 ± 4 12.8 4.0P123+200 mM C6PyCl 6.20 4.0 ± 0.2 1.28 1.5 5.65 0.40 6.0 ± 0.4 0.23 99 ± 3 16.0 3.9P123+100 mM C8PyCl 6.29 3.9 ± 0.2 1.19 2.2 6.08 0.33 7.2 ± 0.3 0.20 135 ± 3 14.3 4.1P123+200 mM C8PyCl 6.21 3.5 ± 0.2 1.49 1.6 5.84 0.39 6.0 ± 0.3 0.24 79 ± 2 19.8 3.8

Effect of Anions (C8PyX, X = Cl−, Br−, I−)P123+100 mM C8PyCl 6.29 3.9 ± 0.2 1.19 2.2 6.08 0.33 7.2 ± 0.4 0.20 135 ± 3 14.3 4.1P123+200 mM C8PyCl 6.21 3.5 ± 0.2 1.49 1.6 5.84 0.39 6.0 ± 0.3 0.22 79 ± 2 19.5 3.8P123+100 mM C8PyBr 6.22 3.7 ± 0.2 1.33 1.7 5.76 0.40 6.2 ± 0.4 0.22 94 ± 3 16.8 3.9P123+200 mM C8PyBr 6.00 3.4 ± 0.2 1.61 1.2 5.66 0.39 5.3 ± 0.4 0.24 58 ± 2 22.3 3.3P123+100 mM C8PyI 6.21 3.5 ± 0.2 1.53 1.4 5.45 0.41 5.2 ± 0.4 0.23 70 ± 3 20.4 3.5P123+200 mM C8PyI 5.93 3.1 ± 0.2 1.82 1.1 5.27 0.42 4.9 ± 0.4 0.26 44 ± 2 26.6 3.0

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in correlation peak position to higher q values (Figure 1a) anddecrease in the mean intermicellar distance.In order to obtain information on the effect of IL on the

hydration of PEO, the number of water molecules, nw, attachedto each EO group of the copolymer molecule in the hydratedshell is calculated as

=−π( )

nR R

V N n

[ ]

2w

43 m

3c

3

D O agg2 (3)

where n = 20 is the number of EO units in each PEO block ofthe copolymer molecule, VD2O and Nagg are the volume of a

D2O and aggregation number, respectively, and Rm is themicellar radius (Rm = Rc + Rs).The aggregation number, Nagg, and the number of water

molecules associated with each EO unit, nw, are calculated anddepicted in last two columns of Table 2. When D2O wasreplaced by D2O + 100 mM C8PyCl, the Nagg decreases from251 to 135 and nw from 4.3 to 4.1. This trend continues withincrease in C8PyCl concentration. Thus, a decrease in themicellar radius, Rm, Nagg, and nw is attributed to a decrease inthe hydrophobicity of the PPO block due to (i) penetration ofIL + D2O in micellar core, and (ii) the presence of hydratedionic species at the PPO/PEO interface. Dehydration of thePEO shell and hydration of the PPO core or the PPO/PEOinterface in the presence of C8PyCl decrease its volume.Therefore, an aqueous solution of C8PyCl proved to be a goodsolvent for both PPO and PEO blocks, which helps to reducethe interfacial tension between PPO blocks and the solvents,and this favors the formation of smaller micelles of P123 withhigher number density and volume fraction.We were unable to fit the scattering curves when the SLD of

the PPO core was fixed to the value calculated by consideringPPO melt (0.34 × 10−6 Å−2). The values obtained for core SLDare much higher than for the PPO melt and increased furtherwith an increase in the concentration of C8PyCl. This meansthat either solvent (D2O or D2O + C8PyCl) having higher SLDdiffuses inside the core or that the PPO density per blockdecreases in a PPO melt. The mixing of medium and PPOinside the core is more plausible since C8PyCl is found to be agood phase transfer catalyst,43,44 i.e., known as a good solventfor the PEO and PPO. In the hypothesis that PPO blocks andsolvent/medium molecules do not make change in meanvolume when mixed, we can infer a fraction of solvent/mediumin the core from the fitted SLD values, ρc, by using thefollowing formula:9

ρ ϕ ρ ϕ ρ= + −(1 )c M M M PPO (4)

where ϕM, ρM, and ρPPO are the volume fraction of medium inthe core, and the SLD of medium and PPO in the core,respectively. For P123 micelles in the absence of IL, we foundthat 11% of D2O was present inside the core, which issomewhat lower (24% for 2.5% (w/w) P123 at 40 °C) than thevalue reported by Manet et al.40 This difference might be due tothe difference in the concentration of P123 micellar solutions. Afurther increase in ϕM was noticed with increase in theconcentration of C8PyCl (ϕM = 14.3% for 100 mM and 19.5%for 200 mM C8PyCl). This concurred with IL mediateddiffusion of D2O inside the core, which is also furthersupported by the observed decrease in nw. Therefore, due tohigh hydration of the PPO core, the micellization becomes lessfavorable since a better environment for both PEO and PPOblocks of copolymer occurred in the presence of C8PyCl. Asimilar kind of observation was noticed by Singh et al.16 formicelles of F88 and P105 block copolymers in the presence ofcetyl tetra ammonium bromide (CTAB). In this report, authorsfound that at low concentration of CTAB, the hydrophobicchains of surfactant penetrate inside the hydrophobic PPO coreof Pluronic micelles and charged head groups resides at thehydrated interface of the core−shell region. As a result of this areduction in micellar dimension was noticed. Kaur et al.17 alsomade a similar observation for Pluronic L64 with twin traincationic surfactants, DDAB (didodecyldimethylammoniumbromide), DTDAB (ditetradecyldimethylammonium bromide,and DHDAB (dihexadecydimethylammonium bromide).

Figure 1. SANS scattering curves for 15% (w/w) P123 in D2O + 100mM IL at 30 °C, (a) various concentrations of C8PyCl, (b) variation inalkyl chain length of ILs, and (c) various anions of ILs.

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3.2. Effect of Alkyl Chain Length of IL. The effect of alkylchain length of IL, CnPyCl (n = 4, 6, 8) on the structure ofP123 micelles has been investigated at 30 °C. Figure 1b showsSANS curves measured for 15% (w/w) P123 micelle in 100mM CnPyCl (n = 4, 6, 8) aqueous solutions. No significantchange in the intensities for 15% (w/w) P123 has been noticedwhen 100 mM C4PyCl was added, while the presence of C6 andC8PyCl induced a slight decrease in intensity. The correlationpeak shifts to higher q for micelles of P123, when D2O wasreplaced with D2O + CnPyCl (n = 4, 6, 8). This is ascribed tothe increase in the number density/micellar volume fraction.The increase in number density of micelles is probablyresponsible for the observed decrease in Rs, Rc, RHS and theincrease in volume fraction ϕ (see Table 2). The decrease in Rsas a function of alkyl chain length reveals a high degree of D2Openetration in PPO core and PPO/PEO interface whichultimately reduces the Rc. This is further supported by thevariation in nw with chain length. Due to the hydrophobicnature of the long alkyl chain in C8PyCl, the number ofavailable water molecules per PEO chain becomes lower ascompared to C4PyCl. To our surprise, an increasing trend inSLC values was observed when IL with shorter chain (butyl) isreplaced by C6 (hexyl) and C8PyCl (octyl), which indicateshydration of the core due to penetration of medium (D2O+IL)inside the PPO core. The calculated ϕM for these systems usingeq 4 are 12.2%, 12.8%, and 15.3% for C4PyCl, C6PyCl, andC8PyCl, respectively. This means that the longer the alkyl chainlength, the better the penetration of medium inside thehydrophobic PPO blocks. But as these alkyl chains are onpositively charged nitrogen, the chances of hydration for thePPO/PEO interface and shrinkage of the core is greater in thecase of C8PyCl, which leads to micelles of smaller size. TheSANS measurements on 15% (w/w) P123 in the presence of200 mM of CnPyCl have also been carried out (Figure 2a) andthe obtained micellar parameters are given in Table 2. Here,similar trends in all the micellar parameters are found to thoseobserved in the case of variation in concentration of C8PyCl (asdiscussed earlier).From Figure 1b and Table 2, it is clear that the addition of

100 mM C4PyCl does not affect the scattering intensity as wellas the fitted micellar parameters. All the values of Rc, RHS, Rs, ϕ,and σ are almost invariable compared to the micelles of P123 inD2O; hence, addition of C4PyCl does not alter the size andgeometry of P123 micelles but is useful to charge the micellesby making a cooperative assembly with them. Behera et al.showed that, for Triton X-100, 1-butyl 3-methyl imidazoliumhexafluorophosphate does not have any appreciable effect onCMC, but the structure of the micelle is markedly modified.This was due to the relatively high hydrophobic nature of thePF6

− anion compared to halide ions.45 Zheng et al. showed thatwith the addition of IL, 1-butyl-3-methyl imidazolium bromideaffects aggregation of the Pluronic P104 [(PEO)27-(PPO)61-(PEO)27], and at high concentration of IL (near its ownCMC), very large aggregates with a diameter of ∼500 nm wereformed.33 Similarly, Dey et al. reported the solution dynamicsof P123 micelles at different concentrations of 1-pentyl-3-methyl imidazolium bromide using Coumarin 480 dye as aprobe in femtosecond measurements.46

3.3. Effect of Halide Anions of IL. Effects of halide ions ofIL on SANS distribution curves are shown in Figure 1c. TheSANS scattering curves for 15% (w/w) P123 micelles in 100mM of C8PyX (X = Cl−, Br−, I−) at 30 °C are depicted in thefigure. Unlike the effect of concentration and alkyl chain length

of IL, clear shifting of the correlation peak at high q region wasnoticed upon addition of 100 mM C8PyX. While variation ofsalts has no perceptible effect on the peak position of scatteringcurves, a constant value of micellar volume fraction or numberdensity is expected. Such shifts and a slight decrease in thescattering intensities suggest a reduction in the size of themicelle. A close look at the micellar parameters depicted inTable 2 discloses that the values of Rc, RHS, Rs, and Nagg arereduced along with enhancement in values of ϕ. A decrease inmicelle core, hard sphere radius, (overall decrease in micellarsize) follows the order I− > Br− > Cl−. These trends for halideions pursue similar behavior compared to the general trends ofalkali metal halide salts (Hofmeister effect). The I− has a moredehydrating effect on the micellar shell but increases thehydration of the PPO core, which increases the solubility ofpolymer more than Cl−, i.e., the effectiveness of the halide ionsin enhancing the solubility of the PPO core increases withincreased size with the order F− < Cl− < Br− < I−.47

Additionally, the presence of alkyl substituted quaternaryammonium ions is responsible for the decreasing waterstructure formation power of halide ions. Due to the complexformation between the PEO chains and the cationic headgroupof IL, an increase in the degree of hydration of PPO core for I−

compared to Cl− may occur. This is further supported by anobserved increase in volume fraction of medium inside the core,ϕM, as well as a decrease in nw with the order I− > Br− > Cl−.The obtained values of ϕM are 14.3%, 16.8%, and 26.6% forCl−, Br−, and I−, respectively. Among these three halide anions,

Figure 2. SANS scattering curves for 15% (w/w) P123 in D2O + 200mM IL at 30 °C: (a) variation in chain length of ILs and (b) variousanions of ILs.

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iodide has smaller hydration radius; therefore, it is close to thequaternary ammonium pyridine cation and hence effectivelyreduces the electrostatic repulsion between counter cations,which results in more penetration of D2O along with C8PyIinto the micellar core. As a consequence, hydration of PPO orthe PPO/PEO interface favors the association of smallermicelles. This trend is further continued with the increase inconcentration of C8PyX from 100 mM to 200 mM (Figure 2band Table 2).3.4. DLS Measurement. DLS studies on the effect of

various types of N-alkylpyridinium halides on the 5% (w/w)P123 copolymer solution are shown in Figures 3−5 and

Supporting Information Figures S1 and S2. Figure 3a,b showsthe representative graphs of correlation function ln g1(τ) vsrelaxation time (μs) and average relaxation time (τ) vsconcentration of ILs, respectively, for various halide anions.The solid lines in Figure 3a represent the theoretical fit to theexperimental data by the method of cumulants with relaxationtime τ. Analysis using the constraint regularization method,CONTIN, also revealed a unimodal distribution of relaxationrates, supporting the validity of cumulant results. The initialnegative slope in Figure 3a indicates that the micelles of P123in the presence of I− ions are smaller in size and so they diffusemore rapidly compared to the other anions, Cl− and Br−. Thelatter ions diffuse slowly compared to the earlier case. Thevariation of g1(τ) as a function of relaxation time (τ) for other

systems and effect of concentration of ILs and alkyl chainlength are also depicted in Supporting Information Figures S1and S2, respectively. Figure 3b portrays the variation of averagerelaxation time with concentration of ILs having various anions.A systematic decrease in relaxation time with concentration ofIL confirms the reduction in micellar size.

Figure 3. Variation of (a) correlation function vs relaxation time, and(b) average relaxation time vs concentration of ILs (C8PyX, X = Cl−,Br−, I−) for micelles of 5% (w/w) P123 in aqueous solution at 30 °C.

Figure 4. Size distribution plot of 5% (w/w) P123 with varyingconditions: (a) effect of anions, (b) effect of concentration of C8PyCl,and (c) effect of alkyl chain length of ILs at 30 °C. (For better claritythe data in graph is shifted with the multiplication sequence of 2, 4,and 6.)

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The average τ value obtained from cumulative analysis wasused to calculate the diffusion coefficient as well as hydro-dynamic radius, Rh, using Stokes-Einstein equation. Figures 4and 5 indicate that the change in halide ion of C8PyX (X = Cl−,Br−, I−) induces an increase in polydispersity along with themicellar size. The order for extent of increase in average Rh isCl− < Br− < I− which is in agreement with the results obtainedfrom SANS analysis. The distribution of the apparent radius ofthe P123 micelles with various ILs as obtained by CONTINanalysis of the DLS data is summarized in Figures 4 and 5. A5% (w/w) P123 aqueous solution in the absence of IL has Rhvalue 9.5 nm at 30 °C, which is in good agreement with theliterature value 9.4 nm.48 Upon addition of 25 mM C8PyCl, noappreciable change in Rh was observed. As the concentration ofC8PyCl increases from 25 mM to 100 mM, the extent ofdecrease in the micelle dimension was observed, whichconfirms shift to shorter radius and narrowing of the scatteringintensity (Figure 4b and Supporting Information Table S1). Inthe case of 100 mM and 200 mM CnPyCl (n = 4, 6, 8), thereduction in Rh of the P123 micelle is observed (Figures 4c and5b). For the shorter chain length, C4PyCl and C6PyCl, theaverage hydrodynamic radius is slightly reduced, the sizedistribution becomes asymmetric (right-skewed), and the width

of the distribution increases. This suggests that the poly-dispersity of the micelles increases with increasing alkyl chainlength of ILs. Additionally, the micelle undergoes a reduction insize, keeping spherical geometry, which is also observed fromthe SANS analysis using a hard sphere core−shell model.

3.5. NMR Measurements. To obtain information con-cerning the detailed interaction sites between ILs and differentmoieties of the block copolymer species and to obtain a clearmolecular level mechanism of the effect of ILs on themicellization of PEO−PPO−PEO block copolymers, 1HNMR measurements were carried out. 1H NMR spectra of15% (w/w) P123 in D2O along with pure ILs and their mixturewere taken at 25 °C (Supporting Information Figures S3−S5).According to the spectra, the singlet at ∼1.10 ppm is attributedto protons of PO −CH3 groups and the additional broad peaksat 3.34−3.48 ppm and sharp peak at ∼3.64 ppm belong to theprotons of PO−CH2− and EO −CH2− groups, respectively.The signal ∼4.80 ppm is the residual signal of HDO, and allthese assignments are in good agreement with reportedvalues.49,50 It is well-known that the chemical shift is sensitiveto the chemical nature of the related protons and a penetrationof protons into nonpolar or polar media would possiblyinduced an upfield or downfield shift due to the change inmagnetic susceptibility of the protons.51,52

In view of the above and in order to understand themechanism, interaction, and location of IL in micelles of blockcopolymers, the change in chemical shift for PO−CH3, PO−CH2−, PO−CH, EO−CH2, and terminal −CH3 of the alkylchain of ILs has been monitored. The observed change in thechemical shift for the above-mentioned protons is depicted inTable 3 (the complete NMR spectra are available in theSupporting Information). When 100 mM C8PyCl was added toP123 micelles, the chemical shift of PO−CH2−, PO−CH3, andEO−CH2− protons experience a slight downfield shift, whilePO−CH− protons undergo a sudden ∼0.11 ppm upfield shift.A slight (∼0.01 ppm) downfield shift, for EO−CH2−, PO−CH2, and PO−CH3, indicates that the local environment ofPEO and PPO blocks remains almost unchanged. However, adrastic upfield shift for the PO−CH− group indicates theexperience of dehydration, which may occur at the center partof the PPO core due to the possible presence of a long alkylchain of IL. Thus, from 1H NMR results, it seems that onlyPPO at the PPO/PEO interface interacts directly with IL, whilethe PPO located at the center of core could not interact withhydrated IL.It is expected that this trend will continue if the

concentration of IL increases from 100 mM to 200 mM, butsurprisingly all the protons of the PPO and PEO domainundergo strong upfield shift (near the values obtained for pureP123 micelles in D2O). This indicates that hydrated PPOblocks apparently form a little hydrophilic environment, whiledehydrated EO−CH2− experiences a slightly hydrophobicenvironment because of the lower amount of water available inthe shell, due to penetration of alkyl head groups (IL) + D2O atthe PPO domain and the PPO/PEO interface. This supportsour conclusion drawn from SANS study that penetration ofmedium (D2O + IL) into the PPO domain is more in the caseof 200 mM C8PyCl compared to 100 mM C8PyCl. Further, thiscan also be supported by monitoring the chemical shift of EO−CH2− as a function of the alkyl chain length of ILs. IL withshorter chain length (C4, -butyl) undergoes maximum down-field shift compared to C6 and C8PyCl. This strong effect isinferred from the strong interaction between EO−CH2−

Figure 5. Size distribution plot of 5% (w/w) P123 with varyingconditions: (a) effect of anions of 200 mM C8PyX (X = Cl−, Br−, I−)and (b) effect of alkyl chain length of 200 mM CnPyCl (n = 4, 6, 8) at30 °C. (For better clarity the data in graph is shifted with themultiplication sequence of 2, 4, and 6.)

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groups and water, which ultimately deshields the −CH2−protons of ethylene oxide moieties. However, as the chainlength of the alkyl group increases, due to the hydrophobicnature it reduces the interaction between water and EOsegments. As explained earlier for 100 mM C8PyCl, the upfieldshift of PO−CH− protons can be justified. Thus, C4PyCl has alower effect on the micellar dimension compared to the C6PyCland C8PyCl.If the Cl− ion of C8PyCl is replaced by Br− and I−, due to

lower electronegativity and larger atomic size, I− is lesshydrated, and as a result more water is available to deshieldEO−CH2− and induce an upfield shift compared to Cl−. Anincrease in hydration of PEO induces an enhancement for thehydrogen bonding structure in water as well as hydration of thehydrophobic groups, PPO. The increasing hydration of PPOwith the addition of C8PyX (X = Cl−, Br−, I−) will eventuallyprevent the occurrence of micellization or favor smallermicelles. From the above NMR results, we found that aninteraction between EO segments and IL dominated over thehydrophobic group−IL interaction, because in the micellarstate, all of the hydrophobic groups (PPO of P123 blockcopolymer) are oriented inside the micellar core.

4. CONCLUSIONS

By combining SANS, DLS, and 1H NMR data, the detailedquantitative determination of various N-alkylpyridinium halidesin the micelles of P123 has been successfully obtained. Theisotropic micellar phase of 15% (w/w) P123 with and withoutIL is well described by an interacting spherical micelle modeledwithin the framework of the Percus−Yevick approximationusing SANS. The fit to the data provided information about theevolution of the micelle by changing the alkyl chain length,concentration, and halide ions of IL when added to the mixture(P123 + D2O). In particular, by fitting the parameters like SLD,Rc, Rs, and ϕ, we observed that the micelles become smaller asthe ILs were added. This effect is due to the reduction ofaggregation number caused by the fraction of solvent/medium

present inside the core of micelles and is concomitant with theincrease in volume fraction. The change in the dimension of themicelle as a function of alkyl chain length, concentration, andhalides was also evaluated by using the DLS method.The investigation of the interaction of ILs with the P123

block copolymer has been carried out by using the 1H NMRmethod. The NMR results provide valuable information on theinteraction sites of IL molecules with the triblock copolymerspecies, particularly PEO and PPO. It was shown that theC8PyCl molecules interact directly with the PEO moieties ofthe triblock copolymer and an indirect interaction of PPO andIL has been observed. However, for the case of the effect ofalkyl chain length and halide ions, the downfield shift of thePPO blocks is possible as a result of an increase in hydration ofPPO blocks, which eventually prevents the occurrence ofmicellization and is also responsible for the reduction in size.The results obtained from 1H NMR validate the mechanism orconclusion drawn from quantitative SANS analysis made byassuming the hard sphere core−shell model.

■ ASSOCIATED CONTENT*S Supporting InformationThe detailed theory relating SANS and DLS analysis, and DLS,1H NMR spectra of block copolymer in the presence of ILs.This material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Tel.: +91 2692 226857 ext. 216. E-mail address: soni_b21@yahoo.co.in.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSR.L.V. and S.S.S. are thankful to the Chemistry Division ofBARC for providing the DLS measurement facility. The

Table 3. 1H NMR Chemical Shift (δppm) Values of EO−CH2−, PO−CH2−, PO−CH−, and PO−CH3 of P123 in the Presenceand Absence of a Variety of ILsa

systemEO−CH2− signal

(δppm)PO−CH2− signal

(δppm)PO−CH- signal

(δppm)PO−CH3 signal

(δppm)terminal −CH3 of alkyl chain on cation of ILs

(δppm)

Effect of Concentration of C8PyClP123 3.641 3.338 3.590 1.077 -P123 + 100 mMC8PyCl

3.646 3.348 3.489 1.085 0.750

P123 + 200 mMC8PyCl

3.624 3.329 3.468 1.059 0.747

Effect of Alkyl Chain Length of ILs (CnPyCl, n = 4, 6, 8)P123 3.641 3.338 3.590 1.077 -P123 + 100 mMC4PyCl

3.655 3.357 3.502 1.097 0.858

P123 + 100 mMC6PyCl

3.648 3.350 3.495 1.089 0.755

P123 + 100 mMC8PyCl

3.646 3.348 3.489 1.085 0.750

Effect of Anions (C8PyX, X = Cl−, Br−, I−)P123 3.641 3.338 3.590 1.077 -P123 + 100 mMC8PyCl

3.646 3.348 3.489 1.085 0.750

P123 + 100 mMC8PyBr

3.651 3.350 3.491 1.085 0.748

P123 + 100 mM C8PyI 3.657 3.353 3.494 1.087 0.750aConcentration of P123 is 15% (w/w) and all samples were prepared in D2O.

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authors thank UGC-DAE, Mumbai centre, for financial support(CSR-M-172). Authors are also thankful to the Head,Chemistry Department for providing necessary facilities.

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