Physicochemical Understanding of Self-Aggregation and Microstructure of a Surface-Active Ionic Liquid [C4mim] [C8OSO3] Mixed with a Reverse Pluronic 10R5 (PO8EO22PO8).

Physicochemical studies on aqueous mixtures of ionic liquids (ILs) and reverse pluronics are limited. Self-aggregation dynamics and microstructure of a surface-active IL (SAIL), 1-butyl-3-methylimidazolium octylsulfate [C4mim] [C8OSO3], in the presence of a reverse pluronic, PO8EO22PO8 (known as 10R5), were studied using isothermal titration calorimetry (ITC), high-resolution nuclear magnetic resonance (NMR), and small-angle neutron scattering (SANS) methods. Also, cryo-/freeze-fracture transmission electron microscopy was employed to determine the microstructures of SAIL/10R5 mixtures. The ITC and NMR results revealed facilitation of SAIL aggregation in the presence of 10R5 forming mixed aggregates as well as free SAIL micelles. 2H spin relaxation rate data pointed out the onset of slow dynamics of the aqueous SAIL/10R5 mixture with an increase in either the former or the latter. Globular morphologies of the mixed species as well as their individual components were corroborated from the measurements. The preferential location of interaction of the SAIL with the 10R5 was identified from 13C NMR chemical shift findings to be in the interfacial region of the assembled SAIL. The formed species were mixed interacted aggregates but not mixed micelles that arise from mixed surfactants. The physicochemical information acquired herein would enrich the literature on the 10R5/SAIL mixed microheterogeneous systems having importance in the making of useful green drug carrier systems and templates for the synthesis of nanomaterials.

Introduction

Green technology requires new solvents to replace common organic solvents having inherent toxicity and high volatility, leading to delivering volatile organic compounds to the atmosphere.[1−8] Ionic liquids (ILs) or molten salts commonly consisting of low-symmetry organic cations and inorganic/organic anions are held together via weak electrostatic interactions (reducing the lattice energy of salts and exhibiting low melting points).[1−8] Furthermore, their physical and chemical properties can be fine-tuned by the appropriate selection of constituent cations and anions, making them as candidates for a broad range of applications, viz., solar cells, fuel cells, lithium-ion batteries, and so forth. In general, ILs are recognized as environmental benign alternatives to volatile organic solvents for a wide variety of physical and chemical (biochemical) processes, viz., separation, purification, catalysis, electrochemistry, self-assembly formation, and so forth.[9−11] Surface-active ILs are abbreviated as SAILs; they are known to form aggregates in aqueous, nonaqueous, and even in surface-inactive IL media.

Information on the functionalization of ILs and their compatibility with stimuli-responsive polymers is available in the literature.[12−15] Although uses of IL–polymer combinations forming compatible components are limited, stimuli-responsive synthetic and commercial polymers that may undergo changes in properties in solution to external stimuli in the presence of ILs are now being recognized as a novel platform for creating intelligent soft materials. Combinations of macromolecules and ILs (as solvents and additives) could offer a change (improve) in the solution properties of polymers to enrich the field of soft science. ILs are “designer solvents” having nonvolatility and wide liquid temperature range; they are smart wet materials that can be employed in an open atmosphere over a prolonged time period, permitting wide temperature and pressure conditions. They also offer scope in designing colloidal systems with matching properties such as, refractive index and density, which are important for scattering and imaging studies.

Pluronics are triblock copolymers like PEO/PPO/PEO [PEO, poly(ethylene oxide) and PPO, poly(propylene oxide)];[16,17] in reverse pluronics, the configurations of the blocks are opposite, that is, PPO/PEO/PPO.[18,19] Reverse pluronics have versatile applications, viz., in detergency, drug delivery, dispersion stabilization, lubrication, cosmetic formulation, emulsification, food packaging, rechargeable batteries, tissue engineering, coating and painting, nanoparticle synthesis, and so forth; they are of minimal immune response.[18] Their self-aggregation and microstructure in solution are related to the concentration and temperature that influence their hydrophobic/hydrophilic characters. Because of varied solubility of the building blocks, these polymers may show different self-assembly formations in solution. Below the critical micelle temperature (CMT), both the pluronics are soluble in water in their monomeric forms. At the critical micelle concentration (cmc) as well as at the CMT, they form self-assembled microstructures.[16,17] Self-aggregation properties of reverse pluronics are different from normal pluronics.[18−20] Pluronics in combination with surfactants are known to achieve better colloidal formulations and properties. The self-aggregation behaviors of copolymers in the presence of surfactants, organic additives, and inorganic additives are reported.[21,22]

Self-aggregation behaviors of SAILs in the presence of triblock copolymers in aqueous solution are also reported.[12−15,23−26] Vekariya et al.[13] investigated the effect of an isotropic micellar phase of a pluronic (P123) in the presence and absence of alkylpyridinum halide-based SAILs. The change in the dimension of the micelle as a function of alkyl chain length, concentration, and halides is also evaluated. Umapathi and Venkatesu[15] recently studied the effect of ILs on the aggregation behavior of pluronic F108. They show that the ability of ILs to affect the CMT of the copolymers results not only from the charge and size of the anions of the ILs but also from the weak ion–ion interactions in the IL. Studies on a mixed system of copolymer P123 and SAIL (1-pentyl-3-methylimidazolium tetrafluoroborate) reveal that the IL penetrates into the hydrophobic core of P123 micelles at lower concentrations and then invades both the core and the corona region with nearly the same efficiency.[27] The same phenomena is observed in the system of copolymer P104 and 1-butyl-3-methylimidazolium bromide.[12] Ge et al.[14] studied the aggregation behavior of pluronic F127 in the presence of a new kind of double-tailed SAIL (1,3-dioctylimidazolium bromide). Nevertheless, investigations pertaining to self-aggregation and microstructure of reverse pluronics in the presence of self-associating molecules (such as SAILs) are still limited.

Imidazolium alkyl sulfate-based room-temperature ILs are recognized for their halogen-free, nontoxic, and hydrolysis stable properties.[2] Their physicochemical properties are well-documented in the literature.[5] The SAIL, 1-butyl-3-methylimidazolium octylsulfate, from the above family was chosen for its superior properties compared to the halide counterparts.[2] Thakkar et al. provided information on the aggregation of 1-alkyl-3-methylimidazole octylsulfates and their interaction with Triton X-100 micelles.[28] In fact, interaction studies of reverse pluronics with ILs using various physicochemical methods are significantly low. In light of the current growing interest in the interaction between SAILs and reverse pluronics, we have studied the pair, 1-butyl-3-methylimidazolium octylsulfate ([C4mim] [C8OSO3]) and the reverse pluronic 10R5 (PO8EO22PO8), by the isothermal titration calorimetry (ITC), nuclear magnetic resonance (NMR), small-angle neutron scattering (SANS), and cryo-/freeze-fracture transmission electron microscopy (Cryo-/FF-TEM) techniques. We have addressed the following points: (i) monitoring the self-aggregation dynamics of the SAIL in the presence of 10R5 and (ii) examining the types of morphologies exhibited by the SAIL/10R5 mixtures at some specified compositions. The nature of such studies is only limitedly found in the literature. A comprehensive separate study using different types of SAILs in combination with different types of reverse pluronics could also be a potential domain for exploration to be taken up in future.

Results and Discussion

ITC of SAIL and Its Mixture with 10R5

The molecular structure of the SAIL is presented in Figure . ITC enthalphograms for 0, 20, 50, and 100 mM 10R5 as a function of aqueous [SAIL] at 303 K are presented in Figure . ITC evidences a break at 54.9 mM for aqueous SAIL alone, and it is considered as the cmc of [C4mim] [C8OSO3], which is consistent with the value obtained from NMR chemical shifts reported elsewhere.[24] The present NMR data (vide infra) have also indicated the cmc break at 40 mM (at 298 K), and the lower value of the cmc is the consequence of lower temperature (298 K) in NMR than that in ITC (aggregation tendency of ionic surfactants may decrease with increasing temperature in the lower range). In the presence of 20 mM 10R5, the SAIL at 303 K exhibits two distinct inflections/breaks at 30.4 and 63.5 mM (Figure ); the former is the critical aggregation concentration (CAC) that is the formation of small micelles induced by the polymer, which get attached to the polymer. On completion of binding, free micelles are formed in solution, which corresponds to the extended critical micelle concentration (cmce). Both CAC and cmce are [10R5] dependent. At 20, 50, and 100 mM of the polymer, the CAC values are 30.4, 37.9, and 53.5 mM of [SAIL], respectively; the corresponding cmce values are 63.5, 71.4, and 85.9 mM, respectively (Table ). The CAC and cmce dependences on [10R5] are fairly linear (not illustrated). The increasing trends herein found are normally observed for surfactant–nonionic polymer systems.[29,30] The increasing presence of the SAIL makes the enthalpy profiles more endothermic. The induced micelles start forming at the CAC progressively bind to the SAIL till the cmce point is reached. The difference in the enthalpy between cmce and CAC, that is, (ΔHcmc – ΔHCAC), is the total enthalpy change for binding (ΔHBind) of small micelles to 10R5. These values for 20, 50, and 100 mM 10R5 are found to be 1.02, 0.75, and 1.10 kJ mol–1, respectively. They are close which is expected for the same system in identical environments; the concentration effect has only a minor say on the enthalpy of the binding process.

Figure 1

Structure of SAIL 1-butyl-3-methylimidazolium octylsulfate [C4mim] [C8OSO3] used in the present study.

Figure 2

ITC produced enthalpy of dilution of the SAIL ([C4mim] [C8OSO3]) in water and in 10R5 solution at 303 K. Transition points in the curves are indicated in the diagram. In each of 20, 50, and 100 mM 10R5, the first transition point is CAC and the second transition point is cmce as marked in the graphs. For pure SAIL (curve with blue symbol), the transition point is the cmc.

Table 1

Self-Aggregation Pertinent Parameters of IL/10R5/D2O Mixtures at Different Copolymer Concentrations Obtained from ITC (at 303 K) and NMR SD (at 298 K)

	cmc/CAC (mM)		cmce (mM)
[10R5] (mM)	ITC	NMR	ITC	NMR
0	54.9	40
20	30.4	30	63.5	65
50	37.9	35	71.4	70
100	53.5		85.9

We may herein discuss the results of a recent study[23] of cationic (SAIL) alkyl imidazolium chloride of varied chain lengths with 10R5. There three breaks are found instead of two, which occur in conventional surfactant and water-soluble nonionic polymer systems.[30] We have observed two breaks instead of three; the anionic IL used here has a different self-assembling property with 10R5. We may mention here that the method used in the referred study[23] where turbidimetry showed distinct three breaks, the other methods conductometry, calorimetry, and fluorimetry showed two breaks, and the second break in tensiometry was very weak, and the [SAIL] profile in tensiometry was way off from expectation.[23] The above reported deviation from normal warrants confirmation by further studies.

1H Self-Diffusion and Spin-Relaxation Study of IL/10R5/D2O Mixtures

Translational self-diffusion (SD) and spin-relaxation are versatile NMR techniques useful for probing both global and local dynamics, which can infer reliable information of self-aggregation and microstructure of polymer–surfactant systems.[31−33] Earlier studies revealed nonaggregation of 10R5 at 298 K.[19] Self-aggregation may, on the other hand, occur at higher concentrations and temperatures[18,21,34] (cmc = 234 mM at 303 K[34] and 30 mM at 323 K[18]). The cmc values of 10R5 at different temperatures are found elsewhere.[21]1H and 13C chemical shift assignments made for both SAIL[24] in D2O (see Figure S1) and 10R5 based on triblock copolymer systems are reported elsewhere.[35] Interestingly, our NMR spin–lattice relaxation rate (R1), spin–spin relaxation rate (R2), and SD data signature visibly break around 20 mM (Figure S2) at 298 K, which suggests some sort of association of 10R5 molecules [may be a premicelle type of aggregation-like sodium dodecylsulfate (SDS), etc.]. Star-shape flakelike species with low aggregation number may also occur (Naskar et al.[18]); this observation necessitates further study.

1H SD coefficients (DIL) for various protons of SAIL/D2O as a function of [SAIL] are measured (Figure A). In the presence of 10R5, a noticeable diffusional change in anionic α-CH2 relative to cationic α-CH2 of the SAIL is observed (see Table S1). Anionic α-CH2 protons of the SAIL are used in the diffusion analysis (Figure A). DIL is weakly sensitive to [SAIL] till 40 mM; later, it is perceptively responsive to the concentration. Thus, 40 mM is taken as the cmc of the SAIL, and it is consistent with the value obtained from 1H NMR spin-relaxation (Figure S3) and also in an earlier report.[24] The presence of 20 mM 10R5 noticeably affects the DIL of the SAIL. The added polymer decreases the DIL, and this tendency enhances with a further rise in [10R5] (20–50 mM); the CAC and cmce breaks (indicated with arrowheads) are shifted to higher concentrations. Thus, the NMR results support the findings of ITC (Figure ). All of the above diffusion inferences are well-corroborated with that of 1H spin–lattice relaxation rate (R1) data shown in Figure S3 (CAC and cmce are not marked therein). A compilation of self-aggregation pertinent parameters obtained from ITC and NMR is presented in Table . It may be noted that for pure SAIL, the cmc formation is distinctly visible with appreciable slope changes of D. In the presence of 10R5, initially, the CAC forms with a less change in the slope (for the formed induced micelles are small). At cmce, in addition to monomers and small micelles, larger micelles start to form. In overall respect, there a large change in the slope is also not expected; hence, the change is moderate.

Figure 3

(A) Variable-IL SD profiles for IL and 10R5 (20 and 50 mM) system and (B) 13C chemical shifts of IL for IL (200 mM)/10R5/D2O relative to IL (200 mM)/D2O that is Δδ (=δ10R5+IL – δIL) as a function of [10R5] (all NMR measurements at 298 K).

In order to evaluate the free fraction (pf) of IL in the IL/10R5/D2O mixture, a prior knowledge of both Df and Db is essential (see eqs −1a). In this regard, a low concentration (5 mM) of IL, well below its cmc (40 mM), was used to measure D noted to be 6.67 (±0.01) × 10–10 m2/s, which was considered as Df, whereas Db was observed using tetramethylsilane (TMS) an external probe in the ternary mixtures, I–III, as it solubilizes in the hydrophobic portions of the mixed aggregates/micelles.[36] In the present study, we assume that the intermolecular interactions are weak as the concentrations of both IL (200 mM) and 10R5 (200 mM) used were much lower (not that high to make strong interaction of the formed aggregated species). In this regard, the influence of intermicellar interactions to diffusion[37] is not accounted. The obtained diffusion coefficients for mixtures I–III are 1.21 (±0.01), 0.89 (±0.01), and 0.97 (±0.01) × 10–10 m2/s, respectively. The value of pf is found from the knowledge of Df, Db, and D (see eq ), and such a procedure is reported elsewhere.[31,38] The results are displayed in Figure S4. From the above diffusion data [using the viscosity (η) of D2O at 298 K[39] = 1.1 mPa s], the corresponding RH (using eq ) values for mixtures I–III are 16.5, 22.4, and 20.5 Å. The observed value of RH for mixture I fairly matches with the reported values of 14.1 Å by DLS[40] and 13.5 Å by SANS.[28] From the RH values, it may be concluded that the sizes of 10R5/SAIL mixed aggregates are not significantly different from the SAIL micelles; the microstructure of free IL micelle is essentially comparable with the 10R5/IL mixed aggregates.

2H NMR Spin-Relaxation of IL/10R5/H2O Mixtures

NMR spin-relaxation, in particular, 1H R1 and R2 data are sensitive to both inter- and intramolecular dipolar interactions, which complicates 1H spin-relaxation analysis.[31,41] In this regard, 2H spin-relaxation rates (R2 = 1/T2) and (R1 = 1/T1) and their difference ΔR (=R2 – R1) plotted against both the concentration and temperature yield reliable information on the self-aggregation of surfactants in the presence of polymers, as 2H senses intramolecular interactions.[32,33,41] In the present study, ternary mixtures of SAIL-α-d2/10R5/H2O with specific compositions have been investigated using 2H NMR as a probe and the data are presented in Table . A close inspection of 2H data (Table ) reveals the following: (i) unimeric IL (20 mM) when combines with relatively lower [10R5] (∼50 mM) exhibits ΔR ≈ 0, pointing out disfavor of aggregation, whereas at higher [10R5], a nonzero value of ΔR (∼4) suggests the presence of random/local aggregates, (ii) upon raising [SAIL] from 20 to 50 mM does not favor aggregation at lower [10R5] (∼20 mM), but an increase in [10R5] by an order of magnitude (∼200 mM) results a finite ΔR (∼5) evidencing signature of local aggregates, and (iii) the combination of 10R5 with micellized SAIL (>cmc) results finite ΔR (∼6) also indicating random/local aggregates. In general, notable changes in ΔR are indications of the formation of nonspherical aggregates such as ellipsoids and rodlike. However, the weak changes in ΔR indicate spherical/globular shapes supplemented by Cryo-/FF-TEM (vide-infra). It is noted that the presence of random aggregates arises from the interaction between the hydrophobic regions of PPO and the IL anion.

Table 2

2H NMR Spin-Relaxation Data at 298 K for the Ternary Mixtures Composed of SAIL-α-d2/10R5/H2O

composition (mM) [IL]/[10R5]	R2 (s–1)	R1 (s–1)	ΔRa (s–1)
20–0	5.17 (0.1)	5.13 (0.06)	0.04 (0.11)
20–20	6.36 (0.06)	6.36 (0.07)	0 (0.09)
20–50	8.32 (0.38)	8.07 (0.1)	0.25 (0.39)
20–200	23.6 (0.21)	19.81 (0.54)	3.79 (0.58)
50–0	7.4 (0.02)	6.72 (0.04)	0.68 (0.04)
50–20	9.15 (0.04)	8.06 (0.07)	1.09 (0.08)
50–200	25.1 (0.11)	20.28 (0.26)	4.82 (0.29)
200–0	16.78 (0.03)	12.69 (0.09)	4.09 (0.09)
200–20	22.45 (0.03)	15.11 (0.08)	7.34 (0.08)
200–200	26.78 (0.1)	20.17 (0.25)	6.61 (0.27)

The error in ΔR corresponds to the propagated error resulting from the errors in R2 and R1, displayed in parenthesis.

13C NMR Chemical Shift of IL/10R5/D2O Mixtures

13C chemical shifts (Δδ′s) of proton–carbon of the IL in the mixtures, 200 mM SAIL/10R5/D2O relative to 200 mM SAIL/D2O, are monitored as a function of [10R5] and displayed in Figure B. The SAIL carbons C1a, C2a, C3a, C8a, C2c, C4c, C5c, C6c, C7c, C8c, and C10c are used in the analysis (see Figure B), and other carbons are not considered because of their tentative assignments. Interestingly, the C1a carbon encounters notable upfield chemical shifts, whereas other SAIL carbons (considered) show downfield shifts with increasing [10R5]. The notable increasing upfield shifts of C1a (increased Δδ) signature localization of 10R5 on the SAIL aggregate near the palisade surface region (at 200 mM ≈ 5 cmc). Other carbons show weak downfield shifts and their moderate increase with increasing [10R5] for gauche-trans conformational changes in the hydrocarbon chains corroborating with ionic surfactants such as SDS, mixtures of tetradecyl- and cetyl-trimethylammonium bromide, and so forth.[42] Fair downfield changes observed for carbons, in particular, C4c, C5c, C6c, C2a, C3a, and C10c, indicate penetration of 10R5 up to these carbons in the SAIL micelle. The present observation is in line with 13C chemical shifts found in the SDS–triblock copolymer combination.[42,43] Significant downfield shifts of the anionic methyl carbon (C8a) of the SAIL are taken as disparity in the chain length between hydrophobic PPO of 10R5 and the SAIL alkyl chain based on the report in the literature.[42,43] The downfield shifts start rising sharply from 10R5 = 60 mM (>cmc) and maximize at 200 mM. The interaction between SAIL micelles and starlike tiny premicellar flakes of 10R5 has a say on the shift. The preferential localization of 10R5 flakes on the SAIL micelle surface enhances with increasing [10R5].

Microstructures of Formed Aggregates Assessed by FF-TEM, Cryo-TEM, NMR, and SANS

In this endeavor, we address the assessment of microstructures of the formed aggregates of IL/10R5 combinations in aqueous solution by physical methods, such as TEM, NMR, and SANS with reference to four ternary mixtures of 10R5/SAIL/D2O: (A) SAIL (20 mM)/10R5 (20 mM)/D2O; (B) SAIL (50 mM)/10R5 (20 mM)/D2O; (C) SAIL (20 mM)/10R5 (200 mM)/D2O; and (D) SAIL (50 mM)/10R5 (200 mM)/D2O. TEM results are presented in Table .

Table 3

Size of Aggregates Obtained for Mixtures A–D from Cryo-/FF-TEM, NMR, and SANSb

				SANSq
mixture	Cryo-TEM (Å)	FF-TEM (Å)	NMRa (Å)	a (Å)	b = c (Å)
A	15–18	40–45	4.71	32.2	11.2
B	13–18	13–17	4.35	25.7	11.5
C	10–15	40–50	48.5	Rg = 7 Å
D	13–15	15–22	26.2

Reported from the NMR SD data by utilizing TMS as a probe and corresponding aggregate sizes evaluated using Stokes–Einstein eq . The values of mixtures A and B are off the trends found from other methods.

q: aggregation numbers for mixture A = 16 (IL 8, 10R5 8) and mixture B = 17 (IL 12, 10R5 5) using eqs –5. A: IL (20 mM)/10R5 (20 mM)/D2O; B: IL (50 mM)/10R5 (20 mM)/D2O; C: IL (20 mM)/10R5 (200 mM)/D2O; and D: IL (50 mM)/10R5 (200 mM)/D2O.

Mixed aggregate sizes obtained from NMR SD coefficients using TMS as a solubilizate are also included in Table . For the mixture A, nearly spherical aggregates of radii 40–45 Å are found from FF-TEM measurements (Figure A(a)). The morphological features of mixture B (Figure B(a)) are similar to that of mixture A, but quantitatively their sizes (13–15 Å) are lower than that of mixture A. Larger spherical aggregates (40–50 Å) are observed in mixture C (Figure C(a)); aggregates of sizes 15–22 Å are witnessed in mixture D presented in Figure D(a). Relatively densely arranged particles are found, which is in line with the higher viscosity of mixture D. In essence, FF-TEM findings give direct evidence of the overall formation of spherical aggregates. Cryo-TEM produces comparable results for all of the mixtures presented in Figure A(b)–D(b); the values are presented in Table . The NMR data for mixtures A and B are of much lower sizes (∼5 Å) and are presented in Table with reservations. At this juncture, we cannot rationalize the observation of much lower sizes encountered in both mixtures A and B until more experiments with close compositions are at hand. Mixture C exhibited mixed aggregates of large size about 50 Å, and a size dimension of about 26 Å was found for mixture D (Table ). The data of mixtures C and D reasonably agreed with FF-TEM. NMR data for mixtures C and D reasonably agreed with FF-TEM.

Figure 4

(A) (a) FF-TEM and (b) Cryo-TEM images of IL (20 mM)/10R5 (20 mM)/H2O (mixture A). (B) (a) FF-TEM and (b) Cryo-TEM images of IL (50 mM)/10R5 (20 mM)/H2O (mixture B). (C) (a) FF-TEM and (b) Cryo-TEM images of IL (20 mM)/10R5 (200 mM)/H2O (mixture C). (D) (a) FF-TEM and (b) Cryo-TEM images of IL (50 mM)/10R5 (200 mM)/H2O (mixture D). Arrows indicate particles’ locations.

Microstructures (shapes and geometries) of the surfactant–polymer assemblies formed by the interaction in solution are also studied by SANS. The experimental SANS profiles of the SAIL/10R5 mixtures A–D are presented in Figure , and the results found are shown in Table . For mixture A, SANS data lead to the ellipsoidal shape of the mixed assemblies [axial ratio (a/b) = 2.8] (Table ). SAIL and 10R5 equally contribute to the aggregation number (16, comprising 8 molecules from each). Increased [SAIL] from 20 to 50 mM (mixture B) produces mixed aggregates near globular with a reduced axial ratio of 2.2 with aggregation number 17, where 12 molecules of IL combine with 5 molecules of 10R5. Increased [SAIL] enhances its increased population in the aggregates and lowers the axial ratio. It corroborates with the reported study of the SDS/10R5 system.[22] Increased [10R5] to 200 mM in C and D produces SANS profiles that do not correspond to the aggregate formation, but to some kind of monomers with Rg (radius of gyration) ≈ 7 Å as if mixtures C and D do not form self-assemblies. In TEM results (presented above), aggregate formations are found in all mixtures A to D. This anomaly arises because the radiation in SANS does not find a contrast with respect to the solvent. According to reports, PEO alone, as a free chain, and a surfactant micelle-decorated chain become practically transparent to the neutron beam.[44,45] We find that the 10R5/SAIL ratios in mixtures A and B are 1 and 0.4, respectively, which in C and D are 10 and 4, respectively. Lower proportions of 10R5 in A and B make SANS detection effective which for C and D at higher proportions of the polymer is ineffective from the standpoint of transparency of the neutron beam referred to above.[44,45] It is obvious that the radiation responses of both PEO and PPO groups in the mixed conditions with the SAIL are close.

Figure 5

(A) SANS profiles for 20 mM 10R5/IL/D2O as a function of (red ○) 20 mM and (blue □) 50 mM IL (at 303 K) and (B) SANS profiles for 200 mM 10R5/IL/D2O as a function of (red ○) 20 mM and (blue □) 50 mM IL (at 303 K).

The microstructures of the aggregates by different methods are on the whole not way off. The NMR method has shown limitations in the size evaluation of mixtures A and B. TEM results are direct and convincing. The presence of distinct aggregates in the FF-TEM as well as Cryo-TEM measurements for mixtures C and D supports the limitations of the SANS method of evaluation discussed above (there, the ellipsoidal morphology produced by SANS in mixture A is an exception). The above variations warrant further attention. The proposed microstructures of the SAIL/10R5 aqueous mixtures are presented in Scheme .

Scheme 1

Schematic Illustration of Microstructures of IL-10R5 Aqueous Mixtures Based on NMR, SANS, and Cryo-/FF-TEM Studies

Conclusions

Interactions between ILs and pluronics are being recently studied for their prospects in the field of materials science. By proper choice, the produced systems may be made environment-friendly. In the present study, different physicochemical techniques such as ITC, NMR, SANS, and Cryo- and FF-TEM are used to probe the aggregation and microstructure formation of the SAIL (1-butyl-3-methyl imidazolium octylsulfate, [C4mim] [C8OSO3]) interacted with the reverse pluronic 10R5 (PO8EO22PO8). ITC, NMR SD, and spin-relaxation point out that the addition of low concentration, such as 20 mM of 10R5 to 20 mM of SAIL, results in a synergistic effect forming mixed globular aggregates. Increased [10R5] from 20 to 50 mM also causes the formation of similar mixed aggregates. Such aggregate formation arises from preferential localization of 10R5 in the interfacial region of the SAIL assembly, as found from the 13C chemical shift analysis. At low [10R5] conditions, mixed ellipsoidal and globular assemblies are found for mixture A and mixture B, respectively, with nearly same composition number, but of different component proportions and sizes as envisaged from the SANS study. The changes in 2H spin-relaxation difference (ΔR) suggest nearly spherical, that is, globular microstructures of the mixed aggregates. Mixtures C and D evidence nearly spherical aggregates by TEM and NMR methods, whereas aggregate formation is not found from SANS measurements; the species comprising the systems are similar to monomers essentially of the larger component 10R5. This is considered as a limitation of the SANS method in the evaluation of the studied systems. It is found that a proper choice of 10R5 and SAIL combinations may yield mixed-aggregate morphologies of desired size and shape. We may herein emphasize that the SAIL behaves as a surfactant and the pluronic acts like a free or assembled species. Their interaction leads to the induced small SAIL assembly formation (as in surfactant–polymer systems) that gets bound with the 10R5 resulting aggregates which are herein attempted to characterize and report. They are mixed aggregates not mixed micelles as found in simple mixtures of surfactants. The products have prospects in the preparation and designing chemically stable nanoobjects, which may be useful in the formation of fuel cells and nanostructure-based applications in the fields of material and biomedical sciences. It is to be noted that the components and, hence, the products of the herein studied systems fall in the category of green materials.

Materials and Methods

Materials

The SAIL (1-butyl-3-methylimidazolium octylsulfate [C4mim] [C8OSO3]) and the reverse pluronic 10R5 that is PO8EO22PO8 (MW ∼2000) were purchased from Aldrich and used as received. Site-specific 2H-labeled SAIL (SAIL-α-d2) was purchased from CortecNet (France). The chemicals 2,2-dimethyl-2-silapentane-5-sulfonate sodium salt (Z97%), TMS, and deuterium oxide (D2O) (Z99.9 atom % D) were also obtained from Aldrich and used as NMR reference standards.

Experimental Methods

Isothermal Titration Calorimetry

For thermometric measurements, an OMEGA ITC microcalorimeter (MicroCal, USA) was used. A concentrated solution of the SAIL (325 μL in a microsyringe) was injected into 1.325 mL of the solvent in the calorimetric cell at equal time intervals (210 s) in multiple steps (32–50 additions) under constant stirring (350 rpm) condition. All measurements were taken at a constant temperature condition maintained by a Neslab RTE 100 circulating water bath. The heat released or absorbed at each step of dilution of surfactant solution in either water or polymer solution was recorded, and the enthalpy change per mole of injectant was calculated by ITC MicroCal Origin 2.9 software. The reproducibility was checked from repeated experimentations. The procedures for the evaluation of the cmc and enthalpy of micellization (ΔHm°) were reported earlier.[18,29] Enthalpy profiles of the SAIL at [10R5] = 0, 20, 50, and 100 mM were determined and presented for analysis. The measurement temperature was 303 ± 0.1 K.

Nuclear Magnetic Resonance

NMR measurements were taken at 298 K using a JEOL FT-NMR (ECA-500) spectrometer operating at 500 MHz. 1H and 13C chemical shifts (δ), 1H spin–lattice relaxation rates (R1), and 1H SD coefficients (D) were measured for SAIL/10R5/D2O and SAIL/D2O mixtures as a function of [SAIL]. For 1H NMR-based experiments (R1 and D), the concentrations of 10R5 were fixed, whereas IL was varied. For 13C chemical shift measurements, [IL] was chosen as 200 mM in view of micellized IL (five times cmc) as well as for sensitivity considerations. For 2H NMR R1 and R2, [IL] and [10R5] used were commensurate with that of R1 and D. TMS was used as a probe to infer intradiffusion coefficients[36] for the specific compositions of SAIL and 10R5, namely, SAIL (200 mM)/D2O (mixture I), SAIL (200 mM)/10R5 (20 mM)/D2O (mixture II), and SAIL (200 mM)/10R5 (50 mM)/D2O (mixture III). 2H spin–lattice relaxation rates (R1) and 2H spin–spin relaxation times (R2) were also taken for the ternary mixtures of SAIL-α-d2/10R5/H2O as a function of both [IL] and [10R5] for limited compositions. The anionic α-CH2/α-CD2 signal of SAIL/SAIL-d2 was used in the diffusion and spin-relaxation analysis.

1H and 13C NMR Chemical Shifts

The 1H and 13C chemical shift data for SAIL (200 mM)/10R5/D2O as a function of [10R5] were recorded (at 298 K) with a view to addressing the preferential location of 10R5 in the SAIL micelle. 1H and 13C chemical shifts referencing procedures were provided in earlier reports.[43,46]

1H Translational SD

For 1H translational SD measurements, the bipolar pulse pair longitudinal encode–decode (BPPLED) sequence[47] was employed and the experimental and processing details were as described elsewhere.[48] The diffusion coefficients were obtained by linear fitting of the experimental data to the Stejskal–Tanner[49] equation, I = Iο e–, where I and Iο are the peak intensities in the presence and absence of gradient pulses; the parameter k = (γΩg)2(Δ – Ω/3), γ is the magnetogyric ratio, g is the gradient amplitude, whereas gradient duration (Ω) and diffusion time (Δ) are 6 and 200 ms, respectively, and D is the translational SD coefficient.[49] A single exponential nature of the magnetization recovery was observed from the plots of intensity versus g2. The estimated error in D was <±2%.

In micellar systems, experimental NMR parameters such as chemical shift (δ), spin–spin lattice relaxation rate (R2), spin–lattice relaxation rate (R1), and translational SD coefficient (D) are considered as weighted averages over the NMR time scales because of the fast exchange of surfactant molecules between monomer and micelles.[31] Hence, the observed D is given by[31]where Df and Db represent free (monomer) and bound (micelle) diffusion coefficients, respectively. Here, pf and pb are the free and bound fraction of surfactant, respectively, so that pf + pb = 1.

We then get

The measured Db enables to calculate the hydrodynamic radius (RH) of micelle/aggregate with the aid of Stokes–Einstein relation[31]where all the other symbols have their usual meaning.

2H Spin-Relaxation (R1 and R2)

2H R1 and R2 measurements were made using inversion recovery and Carr–Purcell–Meiboom–Gill pulse sequences, respectively.[50] Magnetization recovery profiles for both R1 and R2 data manifest exponential decays; the corresponding data analyses corroborate three and two parameter fits, respectively. The estimated errors in both R1 and R2 data were <±2%.

Small-Angle Neutron Scattering

SANS measurements were taken using neutron scattering facility at DHRUVA reactor, Trombay, India. SANS profiles were monitored for SAIL/10R5/D2O mixtures as a function of concentration. D2O was used as a solvent instead of H2O as the former is devoid of incoherent background and subsequently improved the contrast in SANS experiments. The wavelength of the incident SANS beam utilized was 5.2 Å with Δλ/λ = 15%. The scattering intensity profiles were monitored in the scattering vector (Q) range of 0.015–0.35 Å–1. The standard protocol details pertaining to the background corrections, empty cell contributions, transmission, and so forth were mentioned in our earlier reports.[28,51]

The SANS experiment probes the differential scattering cross section (dΣ/dΩ) per unit volume against Q and is presented as[52]where n signifies the number density of the micelles, ρm and ρs are the scattering length densities of the micelle and the solvent, respectively, and Vm is the volume of the micelle. P(Q) and S(Q) are the intraparticle and interparticle structure factors, sensitive to the shape and size of micelles and micelle–micelle interactions, respectively. The constant B reflects the background effects in a typical SANS experiment. In the present SANS analysis, P(Q) has been calculated using ellipsoidal micelles[51] and S(Q) by the method of Hayter and Penfold for the screened Coulomb interaction between the charged micelles.[28,53] The composition of mixed micelle (volume and scattering length density) having x1 and x2 mole fractions of surfactant and polymer considering the ideal mixing of the components, respectively, is taken as[53]where N1 and N2 characterize aggregation numbers, ν1 and ν2 are the monomer volumes, and ρ1 and ρ2 indicate the scattering length densities of surfactant and polymer, respectively. The values of ν1 for the surfactant and ν2 for PPO tails of the polymer are taken to be 578.6 and 1544.0 Å3, respectively. The scattering from the PEO part of the polymer is neglected because of its poor contrast. The corresponding ρ1 and ρ2 values used are 0.48 × 1010 and 0.34 × 1010 cm–2, respectively. The scattering length density for D2O is 6.38 × 1010 cm–2. The total aggregation number of the mixed micelle is represented by N (=N1 + N2) and calculated by dividing the volume of mixed micelle by Vm, whereas N1 and N2 are then determined as Nx1 and Nx2, respectively.

The findings were subjected to the nonlinear least-squares fitting of the model scattering and the experimental data. Background corrections were also applied for instrumental smearing during the data analysis. The fitted data were revealed by the solid lines to the experimental data points. The scattering contribution from the hydrophobic core region of the micelle was considered in the SANS data analysis, whereas scattering from corona (hydrated shell) was not taken into account because most of the scattering originated from the core.

Cryo-/Freeze-Fracture Transmission Electron Microscopy

TEM observations of the sample were recorded using the freeze-replica method. The sample was rapidly frozen in liquid propane with a Cryo-preparation system (LEICA EM CPC, LEICA microsystems), and the frozen sample was transferred into a freeze-replica preparing apparatus (FR-7000A, Hitachi Science Systems) and fractured with a glass knife at 153 K. A replica film was prepared by evaporating platinum carbon at 318 K and then carbon at 363 K on the fractured sample.[54] The replica film prepared was washed several times with acetone and distilled water after being taken out of the freeze-replica preparing apparatus and transferred onto a 300 mesh copper grid. The replica thus prepared was examined with a transmission electron microscope (JEM-1200EX, JEOL).