Influence of N-Alkylpyridinium Halide Based Ionic Liquids on

Nov 7, 2014 - ... and Number of Water Molecules, (nw) Associated per P123 Micelles for 15% ...... Bowers , J.; Butts , C. P.; Martin , P. J.; Vergara-...
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Influence of N‑Alkylpyridinium Halide Based Ionic Liquids on Micellization of P123 in Aqueous Solutions: A SANS, DLS, and NMR Study Rohit 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 in the presence and absence of N-alkylpyridinium halide ionic liquids (ILs) is investigated using SANS, DLS, and 1H NMR studies. The micellar structural parameters are obtained as a function of variation in alkyl chain length, anions, and concentrations of ILs by fitting the SANS scattering data with a model composed of core−shell form factor and a hard sphere structure factor of interaction. Addition of ILs decreases the micellar core, aggregation number, and hard sphere radius of P123 micelles. From quantitative analysis, we determined the amount of solvent (D2O + IL) present inside the core and the core−shell interface along with cationic head groups. This is further supported by monitoring interaction between ILs and polymer 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. possibilities in designing new polymeric materials.23,24 The prospective practical applications of block copolymers and ILs have been identified in the areas of supported catalyst,25 polymer electrolyte membrane,26 metal ion removal,27 and so forth. Out of all these applications, because of the high conductivity of ILs, polymer electrolytes is a promising field in which they can be utilized as an electrolyte in advanced devices like dye sensitized solar cells,28,29 lithium ion batteries,30 and so forth. Very recently, water based polymer gel electrolytes have also received much attention because of environmental and safety issues.31 In view of this, it is necessary to study the effect of ILs, their interactions, and the location of cation/anion fragments in micelles of amphiphilic block copolymers in aqueous solutions. Individually, surface activity and association behavior of Pluronic block copolymer and long chain cationic surfactants are very well documented in the literature.20,21,32 There are reports on self-assembly and micellization behavior of amphiphilic block copolymer in the presence of long chain cationic surfactants. Hecht et al. studied the influence of DTAB (dodecyl trimethylammonium bromide) on the aggregation behavior of Pluronic F127.10,11 Li et al.12,13 and Singh et al.16 reported that some Pluronics form complex unique supramolecular assemblies in the presence of ionic surfactants

1. INTRODUCTION Poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) (PEO−PPO−PEO) block copolymers are an interesting type of amphiphilic block copolymer commonly known by Pluronic or Polaxamer or Symperonics (ICI) and used extensively in many industrial applications.1,2 In recent years, the scope of their application is much broader and covers uses in nanoparticle synthesis,3,4 in gene delivery,5 as polymer gel electrolyte,6 as templating agents,7 and so forth. The majority of these applications are associated with the micelles that are formed in aqueous solution, and therefore, micellization of block copolymer has attracted great attention.8 In many industrial applications, Pluronics are used in the presence of various cosolvents/surfactants, and therefore, the influence of surfactant in general and ionic surfactant in particular on the association behavior of Pluronic block copolymers is very well documented in the literature.9−17 Ionic liquids (ILs) are receiving considerable attention due to their unique physical properties and have been widely used in the area of organic synthesis, catalyst, electrochemistry, polymer electrolyte, and so forth.18,19 Generally, ILs are composed of alkyl imidazolium or pyridinium cation and a variety of anions. However, the alkylpyridinium based ILs have higher biodegradability compared to imidazolium based ILs, and therefore, they are used in various applications including surface active agents and also behave as short chain cationic surfactants when they are dissolved in water.20−22 The combination of amphiphilic block copolymers and ILs is being recognized for many © XXXX American Chemical Society

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

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including CTAB (cetyltrimethylammonium bromide).16 Moreover, they found that the hydrophobic chain of cationic surfactant gets dissolved in the core of block copolymer micelles and the charge head groups reside at the core−shell interface or in a hydrated shell. In the majority of the studies available in the literature, the cationic surfactants used were composed of long hydrocarbon chain (≥C14) and their concentrations were much above their CMC values. However, there are only a few reports in the literature on the effect of short chain (≤C10) cationic surfactants (ILs) on Pluronic micelles.33,34 Zheng et al.33 reported aggregation behavior and interaction of Pluronic P104 in the presence of 1-butyl-3methyl imidazolium bromide (BmimBr) in aqueous solution using FTIR, FFTEM, DLS, and NMR spectroscopy. Recently, Parmar et al.34 studied the interaction between 1-alkyl-3-methyl imidazolium tetrafluoroborate and Pluronic P103 in aqueous solutions using DLS, SANS, and NMR studies. From the selective NOESY NMR spectrum, they indicated that there is an interaction between the butyl chain of IL and the PO group of P103 micelles. In the literature, so far no reports are available on the variation in micelle parameters as a function of concentrations, cationic head groups, and anions of pyridinium based short chain ILs (≤C8) on micelles of Pluronic block copolymer in water. Here, we report the aggregation behavior of Pluronic P123 [(EO)20(PO)70(EO)20] block copolymer in the presence of various pyridinium based ILs in aqueous media. SANS and DLS studies were used to determine the size and shape of P123 micelles under the influence of alkylpyridinium based ILs. From quantitative SANS analysis, a fraction of solvent in PPO and the interaction of alkyl substituted pyridinium cation and anions with micellar core consisting of PPO block were determined. This is further supported by NMR measurements. The effect of alkyl chain length, anions, and concentration of ILs on micellization of P123 in aqueous solution has been discussed.

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

photon correlator. The light source was an argon ion laser operated at 514.5 nm with a maximum output power of 2 W. The apparent equivalent hydrodynamic radii (Rh) of the micelles were calculated using Stokes−Einstein eq 1.32,38

Rh =

kBT 6πηD0

(1)

where kB is the Boltzmann constant, η is the viscosity of solvent (water) at temperature T, and D0 is the diffusion coefficient. The details of theoretical approach for DLS measurement are given in the Supporting Information. 2.4. Nuclear Magnetic Resonance (NMR). All NMR experiments were conducted on a Bruker Avance 400 spectrometer at a larmor frequency of 400.13 MHz for proton equipped with a microprocessor controlled gradient unit and an actively shielded zgradient coil. All samples were prepared in D2O and TMS was used as an internal standard.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Pluronic P123 (EO20PO70EO20) was purchased from the Sigma-Aldrich, India. All ionic liquids were prepared by the procedure reported in the literature18,21,35,36 and characterized by 1H NMR, TGA, and IR methods. All ILs were stored at 60 °C in a vacuum prior to use and the water content was measured by Karl-Fisher analysis, which found less than 0.05% in all ILs. The detailed 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 Millipore grade distilled water. For SANS and NMR measurements samples were prepared in D2O (>99%, Sigma-Aldrich, India). 2.2. Small Angle Neutron Scattering (SANS). SANS measurements were carried out on micellar solutions of P123 triblock copolymer in the presence of various types of ILs. All solutions were prepared in D2O (99.9 atom % D, Sigma-Aldrich, India). The SANS measurements were performed using a fixed geometry SANS instrument with a sample-to-detector distance of 1.8 m at Dhruva reactor, Trombay, India.37 This spectrometer makes use of a BeO filtered beam which provides a mean wavelength of 5.2 Å and has a wavelength resolution of about 15%. The angular distribution of the scattered neutrons is recorded using an indigenously built onedimensional detector. The accessible wave transfer, q, range of this instrument is 0.015−0.35 Å−1. The solutions were held in a 0.5-cmpath-length UV-grade quartz sample holder with tight fitting Teflon stoppers sealed with parafilm. The theoretical approach to SANS measurements is given in Supporting Information. 2.3. Dynamic Light Scattering (DLS). DLS measurements of aqueous solutions of the polymer with and without ILs were performed using a Malvern 4800 autosizer employing a 7132 digital

3. RESULT AND DISCUSSION In the present model, the core and shell are assumed to have uniform scattering length density (SLD), ρc and ρs, respectively. The SLD value of the solvent or the medium, ρM, is a known quantity calculated (using eq 3 of Supporting Information) for all compositions and considered as input parameters, which matched 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) are associated with the micelles of P123. For the isotropic micellar phase, a MATLAB program has been developed to fit the observed data on an absolute scale with the above description. The seven parameters could be either fixed or adjusted, among which four described the form factor, one for the polydispersity of the micelle core radius and two for the structure factor. These parameters are Rc and Rs (the radii of the core and thickness of shell), ρc and ρs, δ (the polydispersity), volume fraction, ϕ, and hard sphere interaction radius, RHS, of micelles. The variations in the intensity are essentially governed by the polydisperse form factor contribution which depends on the five parameters (Rc, Rs, δ, ρc, ρs), and the structure factor term S(q, RHS, ϕ). It was ensured that a pure core model gives poor B

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Table 2. Scattering Length Density of Solvents (ρM), Core Radius (Rc), Scattering Length Density of Core (SLC), Thickness of the 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 P123 Micelles for 15% (w/w) P123 Solutions with and without ILs at 30 °C ρM ×106 Å−2

Rc (nm)

P123

6.38

4.9 ± 0.2

P123+100 mM C8PyCl P123+200 mM C8PyCl

6.29 6.21

P123+100 P123+200 P123+100 P123+200 P123+100 P123+200

mM mM mM mM mM mM

C4PyCl C4PyCl C6PyCl C6PyCl C8PyCl C8PyCl

6.32 6.24 6.31 6.20 6.29 6.21

P123+100 P123+200 P123+100 P123+200 P123+100 P123+200

mM mM mM mM mM mM

C8PyCl C8PyCl C8PyBr C8PyBr C8PyI C8PyI

6.29 6.21 6.22 6.00 6.21 5.93

system

SLC ×106 Å−2

Rs (nm)

SLS ×106 Å−2

δ

1.01 2.6 6.34 0.27 Effect of Concentration of C8PyCl 3.9 ± 0.2 1.19 2.2 6.08 0.33 3.5 ± 0.2 1.49 1.6 5.84 0.40 Effect of Alkyl Chain Length of ILs (CnPyCl, n = 4, 6, 8) 4.7 ± 0.2 1.07 2.1 5.86 0.29 4.4 ± 0.2 1.16 1.6 5.55 0.32 4.4 ± 0.2 1.18 2.2 6.04 0.30 4.0 ± 0.2 1.28 1.5 5.65 0.40 3.9 ± 0.2 1.19 2.2 6.08 0.33 3.5 ± 0.2 1.49 1.6 5.84 0.39 Effect of Anions (C8PyX, X = Cl−, Br−, I−) 3.9 ± 0.2 1.19 2.2 6.08 0.33 3.5 ± 0.2 1.49 1.6 5.84 0.39 3.7 ± 0.2 1.33 1.7 5.76 0.40 3.4 ± 0.2 1.61 1.2 5.66 0.39 3.5 ± 0.2 1.53 1.4 5.45 0.41 3.1 ± 0.2 1.82 1.1 5.27 0.42

fits, with significantly larger χ2 values. As the P123 block copolymer contains only 40 EO groups (which is less than a typical long hydrophilic chain with 100 EO groups), the use of the core−shell model is justified, as it has the advantage of introducing a relatively low number of independent parameters during the fittings.39,40 3.1. Effect of Concentration of IL (C8PyCl). The SANS curves measured for 15% (w/w) P123 in D2O as well as in 100 mM and 200 mM aqueous solutions of C8PyCl are shown in Figure 1a. The SANS distribution curves were fitted with the LMA hard sphere core−shell model described in the theoretical section. 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 were considered as free parameters. For guidance, the SLDs for pure PPO, PEO blocks in melts along with D2O were calculated and the 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 values were obtained by assuming that the PEO and PPO materials have a mass density of 1.018 g.cm−3 equal to the mass density of 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 core compared to PPO melt supports that the fraction of D2O may be present inside the PPO core of micelles. The fitted SLD for PEO (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 shows that it is impossible to fit the data to a pure PEO shell with PEO melt density, and therefore, PEO chains were expected to be solvated by water. The obtained fitting parameters are reported in Table 2 with error bars indicating the range in which no significant changes could be detected in the adjustment. In D2O, the obtained core radius, Rc, (4.9 nm) for 15% (w/ w) P123 is slightly higher than the calculated end to end distance (3.7 nm, randomly coiled model) of PPO in the melt state which shows that the core might be hydrated with water. However, the fitted Rc value is in good agreement with that

RHS (nm)

ϕ

Nagg

ϕM

nw

7.8 ± 0.4

0.19

251 ± 5

11.0

4.3

7.2 ± 0.4 6.0 ± 0.3

0.20 0.22

135 ± 4 79 ± 3

14.3 19.5

4.1 3.8

7.4 7.0 7.3 6.0 7.2 6.0

± ± ± ± ± ±

0.4 0.4 0.4 0.4 0.3 0.3

0.19 0.20 0.20 0.23 0.20 0.24

187 129 172 99 135 79

± ± ± ± ± ±

4 3 4 3 3 2

12.2 13.8 12.8 16.0 14.3 19.8

4.2 4.0 4.0 3.9 4.1 3.8

7.2 6.0 6.2 5.3 5.2 4.9

± ± ± ± ± ±

0.4 0.3 0.4 0.4 0.4 0.4

0.20 0.22 0.22 0.24 0.23 0.26

135 79 94 58 70 44

± ± ± ± ± ±

3 2 3 2 3 2

14.3 19.5 16.8 22.3 20.4 26.6

4.1 3.8 3.9 3.3 3.5 3.0

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 the reported values for P123 micelles available in the literature.41 The aggregation number, Nagg, defined as the number of block copolymer molecules per micelle is given by Nagg =

4π (R c + R s)3 3Vp

(2)

where Rc and Rs are the core and shell radius, respectively, and VP (VEO + VPO) is the volume of the P123 molecule. The calculated Nagg of 15% (w/w) P123 in D2O is 251, which matches very well with the value (Nagg = 244) reported in the literature for 10% (w/w) P123 aqueous solution using light scattering technique and by considering hydrated micelles.42 However, this value is almost 2−3-fold higher than the value reported from SANS study by assuming scattering coming from core only.41 The effects of C8PyCl concentration on SANS distribution curves are shown in Figure 1a. Scattering curves depicted are for a 15% (w/w) aqueous solution of P123 micelles with 100 mM and 200 mM C8PyCl at 30 °C. The figure portrays that, as the C8PyCl concentration is increased, the intensity decreases and the correlation peaks appeared and shift toward higher q. This shows that the addition of C8PyCl altered the intermicellar interaction along with the morphology of P123 micelles. The micellar parameters obtained from the fits with the core−shell model are given in Table 2. During fitting, the SLD of medium (D2O + C8PyCl) was fixed to its calculated values obtained from eq 3 (Supporting Information). A perusal of data given in the table shows that the micellar core radius, Rc, shell thickness, Rs, along with RHS progressively decrease with increase in concentration of C8PyCl from 100 mM to 200 mM. Simultaneously, an increase in volume fraction, ϕ, is attributed to the higher number density of micelles, and hence, micellar correlation function, S(q), becomes the dominant component in the scattering function when C8PyCl was added. This is reflected in the values of RHS (Table 2) and the observed shift C

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D2O and aggregation number, respectively, and Rm is the micellar radius (Rm = Rc + Rs). The aggregation number, Nagg, and the number of water molecules associated with each EO unit, nw, are calculated and depicted in last two columns of Table 2. When D2O was replaced by D2O + 100 mM C8PyCl, the Nagg decreases from 251 to 135 and nw from 4.3 to 4.1. This trend continues with increase in C8PyCl concentration. Thus, a decrease in the micellar radius, Rm, Nagg, and nw is attributed to a decrease in the hydrophobicity of the PPO block due to (i) penetration of IL + D2O in micellar core, and (ii) the presence of hydrated ionic species at the PPO/PEO interface. Dehydration of the PEO shell and hydration of the PPO core or the PPO/PEO interface in the presence of C8PyCl decrease its volume. Therefore, an aqueous solution of C8PyCl proved to be a good solvent for both PPO and PEO blocks, which helps to reduce the interfacial tension between PPO blocks and the solvents, and this favors the formation of smaller micelles of P123 with higher 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 considering PPO melt (0.34 × 10−6 Å−2). The values obtained for core SLD are much higher than for the PPO melt and increased further with an increase in the concentration of C8PyCl. This means that either solvent (D2O or D2O + C8PyCl) having higher SLD diffuses inside the core or that the PPO density per block decreases in a PPO melt. The mixing of medium and PPO inside the core is more plausible since C8PyCl is found to be a good phase transfer catalyst,43,44 i.e., known as a good solvent for the PEO and PPO. In the hypothesis that PPO blocks and solvent/medium molecules do not make change in mean volume when mixed, we can infer a fraction of solvent/medium in the core from the fitted SLD values, ρc, by using the following formula:9 ρc = ϕMρM + (1 − ϕM)ρPPO

where ϕM, ρM, and ρPPO are the volume fraction of medium in the core, and the SLD of medium and PPO in the core, respectively. For P123 micelles in the absence of IL, we found that 11% of D2O was present inside the core, which is somewhat lower (24% for 2.5% (w/w) P123 at 40 °C) than the value reported by Manet et al.40 This difference might be due to the difference in the concentration of P123 micellar solutions. A further increase in ϕM was noticed with increase in the concentration of C8PyCl (ϕM = 14.3% for 100 mM and 19.5% for 200 mM C8PyCl). This concurred with IL mediated diffusion of D2O inside the core, which is also further supported by the observed decrease in nw. Therefore, due to high hydration of the PPO core, the micellization becomes less favorable since a better environment for both PEO and PPO blocks of copolymer occurred in the presence of C8PyCl. A similar kind of observation was noticed by Singh et al.16 for micelles of F88 and P105 block copolymers in the presence of cetyl tetra ammonium bromide (CTAB). In this report, authors found that at low concentration of CTAB, the hydrophobic chains of surfactant penetrate inside the hydrophobic PPO core of Pluronic micelles and charged head groups resides at the hydrated interface of the core−shell region. As a result of this a reduction in micellar dimension was noticed. Kaur et al.17 also made a similar observation for Pluronic L64 with twin train cationic surfactants, DDAB (didodecyldimethylammonium bromide), DTDAB (ditetradecyldimethylammonium bromide, and DHDAB (dihexadecydimethylammonium bromide).

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

in correlation peak position to higher q values (Figure 1a) and decrease 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, attached to each EO group of the copolymer molecule in the hydrated shell is calculated as nw

4π [R 3 − R c 3] ( 3 ) m =

VD2ONagg 2n

(4)

(3)

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

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3.2. Effect of Alkyl Chain Length of IL. The effect of alkyl chain length of IL, CnPyCl (n = 4, 6, 8) on the structure of P123 micelles has been investigated at 30 °C. Figure 1b shows SANS curves measured for 15% (w/w) P123 micelle in 100 mM CnPyCl (n = 4, 6, 8) aqueous solutions. No significant change in the intensities for 15% (w/w) P123 has been noticed when 100 mM C4PyCl was added, while the presence of C6 and C8PyCl induced a slight decrease in intensity. The correlation peak shifts to higher q for micelles of P123, when D2O was replaced with D2O + CnPyCl (n = 4, 6, 8). This is ascribed to the increase in the number density/micellar volume fraction. The increase in number density of micelles is probably responsible for the observed decrease in Rs, Rc, RHS and the increase in volume fraction ϕ (see Table 2). The decrease in Rs as a function of alkyl chain length reveals a high degree of D2O penetration in PPO core and PPO/PEO interface which ultimately reduces the Rc. This is further supported by the variation in nw with chain length. Due to the hydrophobic nature of the long alkyl chain in C8PyCl, the number of available water molecules per PEO chain becomes lower as compared to C4PyCl. To our surprise, an increasing trend in SLC values was observed when IL with shorter chain (butyl) is replaced by C6 (hexyl) and C8PyCl (octyl), which indicates hydration of the core due to penetration of medium (D2O+IL) inside the PPO core. The calculated ϕM for these systems using eq 4 are 12.2%, 12.8%, and 15.3% for C4PyCl, C6PyCl, and C8PyCl, respectively. This means that the longer the alkyl chain length, the better the penetration of medium inside the hydrophobic PPO blocks. But as these alkyl chains are on positively charged nitrogen, the chances of hydration for the PPO/PEO interface and shrinkage of the core is greater in the case of C8PyCl, which leads to micelles of smaller size. The SANS measurements on 15% (w/w) P123 in the presence of 200 mM of CnPyCl have also been carried out (Figure 2a) and the obtained micellar parameters are given in Table 2. Here, similar trends in all the micellar parameters are found to those observed in the case of variation in concentration of C8PyCl (as discussed earlier). From Figure 1b and Table 2, it is clear that the addition of 100 mM C4PyCl does not affect the scattering intensity as well as the fitted micellar parameters. All the values of Rc, RHS, Rs, ϕ, and σ are almost invariable compared to the micelles of P123 in D2O; hence, addition of C4PyCl does not alter the size and geometry of P123 micelles but is useful to charge the micelles by making a cooperative assembly with them. Behera et al. showed that, for Triton X-100, 1-butyl 3-methyl imidazolium hexafluorophosphate does not have any appreciable effect on CMC, but the structure of the micelle is markedly modified. This was due to the relatively high hydrophobic nature of the PF6− anion compared to halide ions.45 Zheng et al. showed that with the addition of IL, 1-butyl-3-methyl imidazolium bromide affects aggregation of the Pluronic P104 [(PEO)27-(PPO)61(PEO)27], and at high concentration of IL (near its own CMC), very large aggregates with a diameter of ∼500 nm were formed.33 Similarly, Dey et al. reported the solution dynamics of P123 micelles at different concentrations of 1-pentyl-3methyl imidazolium bromide using Coumarin 480 dye as a probe in femtosecond measurements.46 3.3. Effect of Halide Anions of IL. Effects of halide ions of IL on SANS distribution curves are shown in Figure 1c. The SANS scattering curves for 15% (w/w) P123 micelles in 100 mM of C8PyX (X = Cl−, Br−, I−) at 30 °C are depicted in the figure. Unlike the effect of concentration and alkyl chain length

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

of IL, clear shifting of the correlation peak at high q region was noticed upon addition of 100 mM C8PyX. While variation of salts has no perceptible effect on the peak position of scattering curves, a constant value of micellar volume fraction or number density is expected. Such shifts and a slight decrease in the scattering intensities suggest a reduction in the size of the micelle. A close look at the micellar parameters depicted in Table 2 discloses that the values of Rc, RHS, Rs, and Nagg are reduced along with enhancement in values of ϕ. A decrease in micelle core, hard sphere radius, (overall decrease in micellar size) follows the order I− > Br− > Cl−. These trends for halide ions pursue similar behavior compared to the general trends of alkali metal halide salts (Hofmeister effect). The I− has a more dehydrating effect on the micellar shell but increases the hydration of the PPO core, which increases the solubility of polymer more than Cl−, i.e., the effectiveness of the halide ions in enhancing the solubility of the PPO core increases with increased size with the order F− < Cl− < Br− < I−.47 Additionally, the presence of alkyl substituted quaternary ammonium ions is responsible for the decreasing water structure formation power of halide ions. Due to the complex formation between the PEO chains and the cationic headgroup of IL, an increase in the degree of hydration of PPO core for I− compared to Cl− may occur. This is further supported by an observed 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% for Cl−, Br−, and I−, respectively. Among these three halide anions, E

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iodide has smaller hydration radius; therefore, it is close to the quaternary ammonium pyridine cation and hence effectively reduces the electrostatic repulsion between counter cations, which results in more penetration of D2O along with C8PyI into the micellar core. As a consequence, hydration of PPO or the PPO/PEO interface favors the association of smaller micelles. This trend is further continued with the increase in concentration of C8PyX from 100 mM to 200 mM (Figure 2b and 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

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.

Supporting Information Figures S1 and S2. Figure 3a,b shows the representative graphs of correlation function ln g1(τ) vs relaxation time (μs) and average relaxation time (τ) vs concentration of ILs, respectively, for various halide anions. The solid lines in Figure 3a represent the theoretical fit to the experimental data by the method of cumulants with relaxation time τ. Analysis using the constraint regularization method, CONTIN, also revealed a unimodal distribution of relaxation rates, supporting the validity of cumulant results. The initial negative slope in Figure 3a indicates that the micelles of P123 in the presence of I− ions are smaller in size and so they diffuse more rapidly compared to the other anions, Cl− and Br−. The latter ions diffuse slowly compared to the earlier case. The variation of g1(τ) as a function of relaxation time (τ) for other

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

systems and effect of concentration of ILs and alkyl chain length are also depicted in Supporting Information Figures S1 and S2, respectively. Figure 3b portrays the variation of average relaxation time with concentration of ILs having various anions. A systematic decrease in relaxation time with concentration of IL confirms the reduction in micellar size. F

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of the distribution increases. This suggests that the polydispersity of the micelles increases with increasing alkyl chain length of ILs. Additionally, the micelle undergoes a reduction in size, keeping spherical geometry, which is also observed from the SANS analysis using a hard sphere core−shell model. 3.5. NMR Measurements. To obtain information concerning the detailed interaction sites between ILs and different moieties of the block copolymer species and to obtain a clear molecular level mechanism of the effect of ILs on the micellization of PEO−PPO−PEO block copolymers, 1H NMR measurements were carried out. 1H NMR spectra of 15% (w/w) P123 in D2O along with pure ILs and their mixture were taken at 25 °C (Supporting Information Figures S3−S5). According to the spectra, the singlet at ∼1.10 ppm is attributed to protons of PO −CH3 groups and the additional broad peaks at 3.34−3.48 ppm and sharp peak at ∼3.64 ppm belong to the protons of PO−CH2− and EO −CH2− groups, respectively. The signal ∼4.80 ppm is the residual signal of HDO, and all these assignments are in good agreement with reported values.49,50 It is well-known that the chemical shift is sensitive to the chemical nature of the related protons and a penetration of protons into nonpolar or polar media would possibly induced an upfield or downfield shift due to the change in magnetic susceptibility of the protons.51,52 In view of the above and in order to understand the mechanism, interaction, and location of IL in micelles of block copolymers, the change in chemical shift for PO−CH3, PO− CH2−, PO−CH, EO−CH2, and terminal −CH3 of the alkyl chain of ILs has been monitored. The observed change in the chemical shift for the above-mentioned protons is depicted in Table 3 (the complete NMR spectra are available in the Supporting Information). When 100 mM C8PyCl was added to P123 micelles, the chemical shift of PO−CH2−, PO−CH3, and EO−CH2− protons experience a slight downfield shift, while PO−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 of PEO and PPO blocks remains almost unchanged. However, a drastic upfield shift for the PO−CH− group indicates the experience of dehydration, which may occur at the center part of the PPO core due to the possible presence of a long alkyl chain of IL. Thus, from 1H NMR results, it seems that only PPO at the PPO/PEO interface interacts directly with IL, while the PPO located at the center of core could not interact with hydrated IL. It is expected that this trend will continue if the concentration of IL increases from 100 mM to 200 mM, but surprisingly all the protons of the PPO and PEO domain undergo strong upfield shift (near the values obtained for pure P123 micelles in D2O). This indicates that hydrated PPO blocks apparently form a little hydrophilic environment, while dehydrated EO−CH2− experiences a slightly hydrophobic environment because of the lower amount of water available in the shell, due to penetration of alkyl head groups (IL) + D2O at the PPO domain and the PPO/PEO interface. This supports our conclusion drawn from SANS study that penetration of medium (D2O + IL) into the PPO domain is more in the case of 200 mM C8PyCl compared to 100 mM C8PyCl. Further, this can also be supported by monitoring the chemical shift of EO− CH2− as a function of the alkyl chain length of ILs. IL with shorter chain length (C4, -butyl) undergoes maximum downfield shift compared to C6 and C8PyCl. This strong effect is inferred from the strong interaction between EO−CH2−

Figure 5. Size distribution plot of 5% (w/w) P123 with varying conditions: (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) at 30 °C. (For better clarity the data in graph is shifted with the multiplication sequence of 2, 4, and 6.)

The average τ value obtained from cumulative analysis was used to calculate the diffusion coefficient as well as hydrodynamic radius, Rh, using Stokes-Einstein equation. Figures 4 and 5 indicate that the change in halide ion of C8PyX (X = Cl−, Br−, I−) induces an increase in polydispersity along with the micellar size. The order for extent of increase in average Rh is Cl− < Br− < I− which is in agreement with the results obtained from SANS analysis. The distribution of the apparent radius of the P123 micelles with various ILs as obtained by CONTIN analysis of the DLS data is summarized in Figures 4 and 5. A 5% (w/w) P123 aqueous solution in the absence of IL has Rh value 9.5 nm at 30 °C, which is in good agreement with the literature value 9.4 nm.48 Upon addition of 25 mM C8PyCl, no appreciable change in Rh was observed. As the concentration of C8PyCl increases from 25 mM to 100 mM, the extent of decrease in the micelle dimension was observed, which confirms shift to shorter radius and narrowing of the scattering intensity (Figure 4b and Supporting Information Table S1). In the case of 100 mM and 200 mM CnPyCl (n = 4, 6, 8), the reduction in Rh of the P123 micelle is observed (Figures 4c and 5b). For the shorter chain length, C4PyCl and C6PyCl, the average hydrodynamic radius is slightly reduced, the size distribution becomes asymmetric (right-skewed), and the width G

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

PO−CH2− signal (δppm)

P123 P123 + 100 mM C8PyCl P123 + 200 mM C8PyCl

3.641 3.646

3.338 3.348

3.624

3.329

P123 P123 + 100 mM C4PyCl P123 + 100 mM C6PyCl P123 + 100 mM C8PyCl

3.641 3.655

P123 P123 + 100 mM C8PyCl P123 + 100 mM C8PyBr P123 + 100 mM C8PyI

system

a

PO−CH- signal (δppm)

PO−CH3 signal (δppm)

Effect of Concentration of C8PyCl 3.590 3.489 3.468

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

1.077 1.085

0.750

1.059

0.747

Effect of Alkyl Chain Length of ILs (CnPyCl, n = 4, 6, 8) 3.338 3.590 1.077 3.357 3.502 1.097

0.858

3.648

3.350

3.495

1.089

0.755

3.646

3.348

3.489

1.085

0.750

3.641 3.646

Effect of Anions (C8PyX, X = Cl−, Br−, I−) 3.338 3.590 1.077 3.348 3.489 1.085

0.750

3.651

3.350

3.491

1.085

0.748

3.657

3.353

3.494

1.087

0.750

Concentration of P123 is 15% (w/w) and all samples were prepared in D2O.

groups and water, which ultimately deshields the −CH2− protons of ethylene oxide moieties. However, as the chain length of the alkyl group increases, due to the hydrophobic nature it reduces the interaction between water and EO segments. As explained earlier for 100 mM C8PyCl, the upfield shift of PO−CH− protons can be justified. Thus, C4PyCl has a lower effect on the micellar dimension compared to the C6PyCl and C8PyCl. If the Cl− ion of C8PyCl is replaced by Br− and I−, due to lower electronegativity and larger atomic size, I− is less hydrated, and as a result more water is available to deshield EO−CH2− and induce an upfield shift compared to Cl−. An increase in hydration of PEO induces an enhancement for the hydrogen bonding structure in water as well as hydration of the hydrophobic groups, PPO. The increasing hydration of PPO with the addition of C8PyX (X = Cl−, Br−, I−) will eventually prevent the occurrence of micellization or favor smaller micelles. From the above NMR results, we found that an interaction between EO segments and IL dominated over the hydrophobic group−IL interaction, because in the micellar state, all of the hydrophobic groups (PPO of P123 block copolymer) are oriented inside the micellar core.

present inside the core of micelles and is concomitant with the increase in volume fraction. The change in the dimension of the micelle as a function of alkyl chain length, concentration, and halides 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 NMR method. The NMR results provide valuable information on the interaction sites of IL molecules with the triblock copolymer species, particularly PEO and PPO. It was shown that the C8PyCl molecules interact directly with the PEO moieties of the triblock copolymer and an indirect interaction of PPO and IL has been observed. However, for the case of the effect of alkyl chain length and halide ions, the downfield shift of the PPO blocks is possible as a result of an increase in hydration of PPO blocks, which eventually prevents the occurrence of micellization and is also responsible for the reduction in size. The results obtained from 1H NMR validate the mechanism or conclusion drawn from quantitative SANS analysis made by assuming the hard sphere core−shell model.

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ASSOCIATED CONTENT

S Supporting Information *

The detailed theory relating SANS and DLS analysis, and DLS, 1 H NMR spectra of block copolymer in the presence of ILs. This material is available free of charge via the Internet at http://pubs.acs.org.

4. CONCLUSIONS By combining SANS, DLS, and 1H NMR data, the detailed quantitative determination of various N-alkylpyridinium halides in the micelles of P123 has been successfully obtained. The isotropic micellar phase of 15% (w/w) P123 with and without IL is well described by an interacting spherical micelle modeled within the framework of the Percus−Yevick approximation using SANS. The fit to the data provided information about the evolution 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 as the ILs were added. This effect is due to the reduction of aggregation number caused by the fraction of solvent/medium

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91 2692 226857 ext. 216. E-mail address: soni_b21@ yahoo.co.in. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS R.L.V. and S.S.S. are thankful to the Chemistry Division of BARC for providing the DLS measurement facility. The H

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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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