Using High-Pressure Infrared Spectroscopy to Study the Interactions

Apr 25, 2014 - ABSTRACT: In this study, we used high-pressure infrared spectroscopy to probe the local structures formed between the ionic liquid [BMI...
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Using High-Pressure Infrared Spectroscopy to Study the Interactions between Triblock Copolymers and Ionic Liquids Hai-Chou Chang,* Tsung-Ting Tsai, and Meng-Hsiu Kuo Department of Chemistry, National Dong Hwa University, Shoufeng, Hualien 974, Taiwan S Supporting Information *

ABSTRACT: In this study, we used high-pressure infrared spectroscopy to probe the local structures formed between the ionic liquid [BMI][PF6] and the triblock PEO−PPO−PEO copolymer P123. The signals for the imidazolium C−H units of [BMI][PF6] underwent anomalous frequency shifts upon dilution, induced by order-to-order transitions. Appreciable changes in the relative band intensities of the signals for the imidazolium moieties occurred upon compression. Upon increasing the pressure, the ether C−O−C stretching band underwent dramatic changes, with a shoulder peak appearing near 1097 cm−1 with relatively increasing intensity. It appears that high pressures somehow stabilize the hydrogen bonds formed between [BMI] and P123, possibly forcing the P123 molecules to dissociate the clusters of ionic liquid through enhanced hydrogen bonding with the imidazolium C−H units. Analogous to temperature-dependent behavior, we suggest the possibility of pressure-induced association of P123 molecules through elevation of pressure.

I. INTRODUCTION Triblock poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) (PEO−PPO−PEO) copolymers of the Pluronic series are nonionic surfactants that spontaneously self-assemble into ordered microphases or micelles in the presence of a selective solvent for one or two of the blocks.1−7 When these block copolymers are mixed in a solvent, they exhibit rich phase behavior resulting from their amphiphilicity.1−7 Typically, aggregated structures are stabilized by avoiding direct contact between the solvent and the insoluble blocks. The specific properties of block copolymers offer the possibilities of their applications in many industrial fields (e.g., as carriers for drug delivery, as dispersants, and as vectors for gene therapy). The combination of block copolymers with ionic liquids is an interesting emerging area of research. For example, block polymer micelles can transfer from an aqueous phase at room temperature to a hydrophobic ionic liquid at elevated temperature.4,5 Although ionic liquids are organic compounds, they are environmentally green solvents when compared with traditional organic solvents. Their negligible volatilities and liquid states over broad temperature ranges make them attractive alternatives to volatile organic solvents for many applications.8−10 The nonvolatile character of ionic liquids enables effective product separation from reaction mixtures through distillation, while their unusual solubility characteristics provide an attractive means of separating products from expensive transition metal catalysts.9,10 Typically, ionic liquids feature bulky, asymmetric cations that cannot undergo ready packing. The most extensively studied ionic liquids are 1-alkyl-3methylimidazolium salts.11−22 One of the attractive features of 1-alkyl-3-methylimidazolium cations is their inherent amphiphilic characteristics as surfactants. Previous studies © XXXX American Chemical Society

have clearly established the existence of ion pairs and higherorder aggregate species in ionic liquid systems featuring strongly coordinating anions. 23 Both experimental and theoretical investigations have found that ionic liquids are not homogeneous solvents, but rather that they must be considered as nanostructured materials featuring polar and nonpolar regions.23−27 The presence of nanostructural aggregation in ionic liquids presenting long alkyl chains (n > 10) is not surprising because of their surfactant-like characteristics. Nevertheless, Triolo et al. proposed that the mesoscopic organization may exist in asymmetrical imidazolium ions presenting relatively short alkyl chains (n ≥ 4).27 Several researchers have reported that vibrational spectroscopy can distinguish cation−anion interactions from anion−water or cation−water interactions,12,18 and that ionic liquid/water mixtures are not unstructured homogeneous solutions.20,21 Addition of ionic liquids to a low-molecular-weight triblock copolymer is known to induce the formation of ordered microphases and core/corona micelles.1−3 Structural organization in mixtures of ionic liquids and copolymers is of great interest for a wide variety of applications, including the preparation of dye-sensitized solar cells, the formation of polymer electrolytes, and the exploitation of solvation properties.1−7 Nevertheless, the interactions between ionic liquids and copolymers have not been investigated previously in much detail. Accordingly, in this study, to better understand the microscopic structures of the ionic liquid/copolymer systems, we use pressure as a variable to explore the interactions of ionic Received: March 7, 2014 Revised: April 9, 2014

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liquid/copolymer systems by measuring the spectral features of the C−H units of the imidazolium moieties. The application of high pressure is a powerful method for exploring the polymorphic behaviors of chemical compounds. Using pressure as a variable allows one to change intermolecular interactions in a controlled manner without encountering the major perturbations produced by changes in temperature or chemical composition. Previous studies of ionic liquids have indicated that high pressure can lead to phase transitions and, thereby, changes in physical properties.28−35 Russina et al.35 and Su et al.28 reported the structural organization and phase behavior, respectively, of 1-butyl-3methylimidazolium hexafluorophosphate ([BMI][PF6]) under high pressure. Conformational changes of the butyl chains in [BMI][Cl] at pressures of greater than 0.3 GPa have been observed using high-pressure spectroscopy.36 Under ambient pressure, Raman spectra indicate that the more thermodynamically stable structure of the butyl group of the cation of [BMI][Cl] is the anti−anti form; high pressure, however, switches the crystal packing of the butyl group to the gauche− anti form. In this study, we used variable pressure as a window into the structural organization of C−H groups in mixtures of [BMI][PF6] and the PEO−PPO−PEO copolymer P123.

II. EXPERIMENTAL SECTION Samples were prepared using [BMI][PF6], 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([BMI][TFSA], 98.9%, Fluka), Pluronic L64 (Lot WE195341/1, Fluka), and Pluronic P123 (Batch WPOD509B, BASF). Pluronic L64 and P123 have the chemical formulas EO13PO30EO13 and EO20PO70EO20, respectively, where EO and PO denote ethylene oxide and propylene oxide, respectively. The liquid solutions prepared by mixing were then sonicated for an additional hour. A diamond anvil cell (DAC) of Merrill−Bassett design, having a diamond culet size of 0.6 mm, was used to generate pressures of up to approximately 2 GPa. Two type-IIa diamonds were used for mid-infrared (mid-IR) measurements. The sample was contained in a 0.3 mm diameter hole in a 0.25 mm thick inconel gasket mounted on the DAC. To decrease the absorbance of the samples, CaF2 crystals (prepared from a CaF2 optical window) were placed into the holes and compressed to transparency prior to inserting the samples. A droplet of a sample filled the empty space of the entire hole of the gasket in the DAC, which was subsequently sealed when the opposed anvils were pushed toward one another. Infrared spectra of the samples were measured using a PerkinElmer Fourier transform spectrophotometer (model Spectrum RXI) equipped with a lithium tantalite (LITA) mid-IR detector. The IR beam was condensed through a 5X beam condenser onto the sample in the DAC. Typically, spectra were recorded from 1000 scans at a resolution of 4 cm−1 (data point resolution of 2 cm−1). To remove the absorption of the diamond anvils, the absorption spectrum of the DAC was measured first and subtracted from the spectra of the samples. Pressure calibration was performed following Wong’s method.37,38 The spectra of the samples at ambient pressure were recorded after filling the samples into a cell featuring two CaF2 windows but no spacers.

Figure 1. IR spectra of pure [BMI][PF6] (curve a) and mixtures of [BMI][PF6]/P123 featuring 56 (curve b), 30 (curve c), 23 (curve d), and 13 (curve e) wt % of [BMI][PF6], recorded under ambient pressure.

adding P123, red-shifting to 3121 and 3165 cm−1, respectively, in Figure 1b. Parts b−e of Figure 1 also reveal the appearance of a shoulder peak near 3081 cm−1. In parts d and e of Figure 1, we observe significant increases in the ratios of the intensities of the imidazolium C−H bands (I3115/I3158) for the [BMI][PF6]/ P123 mixtures containing concentrations of the low ionic liquid (wt % < 25). The increases in the relative intensities of the bands at 3115 and 3081 cm−1 in Figures 1d and 1e and the appearance of the shoulder peak near 3081 cm−1 suggest that the imidazolium C−H vibrations were red-shifted as a result of hydrogen bonding interactions between [BMI] and P123.3,32 In other words, P123 appeared to be capable of breaking the cation−anion interactions as a result of the formation of stable cation−P123 and anion−P123 interactions. Figures 2 and S1 (Supporting Information) plot the frequencies of the two imidazolium C−H bands with respect to the weight percentages of [BMI][PF6] and [BMI][TFSA], respectively. We observe that the imidazolium C−H bands of [BMI][PF6] and [BMI][TFSA] underwent anomalous frequency shifts upon dilution. The imidazolium C−H peaks of [BMI][PF6] experienced mild red-shifts in frequency when diluted to high concentrations of [BMI][PF6] (wt % > 30), but red-shift strongly at lower concentrations (wt % < 30). The

III. RESULTS AND DISCUSSION Figure 1 displays IR spectra recorded at ambient pressure of pure [BMI][PF6] (Figure 1a) and [BMI][PF6]/P123 mixtures having [BMI][PF6] contents of 56 wt % (Figure 1b), 30 wt % (Figure 1c), 23 wt % (Figure 1d), and 13 wt % (Figure 1e). The absorption spectrum of pure [BMI][PF6] exhibits two discernible peaks at 3127 and 3170 cm−1, corresponding to coupled imidazolium C−H stretching vibrations.29−34 These imidazolium C−H stretching modes underwent changes upon B

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Among the many interactions occurring within ionic liquids, Coulombic forces between cations and anions are predominant.9,10 A bulky anion, such as TFSA−, exhibits weaker cation−anion interactions. The effect of ionic liquids on the properties of block copolymers has been studied extensively. For example, the formation of ordered morphologies in mixtures of block copolymers and various ionic liquids has been reported.39,40 Wanakule et al. noted that changing the anion of the ionic liquid results in a dramatic change in phase behavior.40 Figure 3 displays IR spectra of a [BMI][PF6]/P123 mixture containing 21 wt % [BMI][PF6] recorded at ambient pressure

Figure 2. Concentration-dependence of the stretching frequencies of the imidazolium C−H bonds [the bands at (A) 3170 and (B) 3127 cm−1] of [BMI][PF6]/P123 (diamonds), [BMI][PF6]/L64 (squares), [BMI][TFSA]/P123 (triangles), and [BMI][TFSA]/L64 (crosses).

anomalous frequency shifts suggest that order-to-order transitions may have occurred, with [BMI][PF6]/P123 and [BMI][PF6]/L64 mixtures tending to form ordered morphologies as the [BMI][PF6] concentration is decreased.1−3 A previous study has revealed similar phase transition behavior in [BMI][PF6]/P123 mixtures, with a lyotropic liquid crystal phase forming at high [BMI][PF6] concentrations; at lower [BMI][PF6] concentrations (wt % < 30), P123 molecules further break the liquid crystalline phase to form the lamellar phase in a bilayer structure.2 As shown in Figures 1 and 2, hydrogen bonding between P123 and [BMI][PF6] leads to changes in the relative intensities of the bands at 3115 and 3081 cm−1 (Figures.1d−e) and to red-shifts in the frequencies of the imidazolium C−H bands (Figure 2) in the region of low concentrations of [BMI][PF6] (wt % < 30). Thus, the concentration-dependent IR absorptions of the imidazolium C−H vibrations can be employed to monitor aggregation and local structural changes in ionic liquid/copolymer mixtures. Figure 2 reveals decreases in the frequencies of the signals for the imidazolium C−H units when [BMI][TFSA] was present at high concentrations (wt % > 30). In contrast, at low concentrations of [BMI][TFSA] (i.e., wt % < 30), we observe almost no changes in the imidazolium C−H frequencies of [BMI][TFSA]. These anomalous shifts in frequencies are in agreement with our previous finding of certain solvation structures around the cations in copolymer-rich regions (wt % < 30) of [EMI][TFSA]/L64 systems.32 Inspection of the concentration-dependence revealed that the L64 (EO13PO30EO13) and P123 (EO20PO70EO20) mixtures induced almost identical frequency shifts; thus, the phase behavior of these ionic liquid/copolymer systems is sensitive to the nature of the anion, rather that the chain length of the copolymer.

Figure 3. IR spectra of the [BMI][PF6]/P123 mixture featuring 21 wt % of [BMI][PF6], recorded under ambient pressure (curve a) and at pressures of 0.4 (curve b), 0.7 (curve c), 1.1 (curve d), 1.5 (curve e), 1.8 (curve f), and 2.5 (curve g) GPa.

(curve a) and at 0.4 (curve b), 0.7 (curve c), 1.1 (curve d), 1.5 (curve e), 1.8 (curve f), and 2.5 (curve g) GPa. As the pressure was elevated to 0.4 GPa (Figure 3b), an appreciable change occurred in the relative band intensities of the imidazolium moieties (I3115/I3158); compression also led to an increase in the intensities of the bands for the hydrogen-bonded C−H units near 3115 and 3081 cm−1. Notably, we observed no significant changes in the relative band intensities upon compression of pure [BMI][PF6]41 or the [BMI][PF6]/P123 mixture containing 90 wt % [BMI][PF6] (Figure S2, Supporting Information). We suspect that elevated pressure somehow stabilizes the conformations of the hydrogen-bonded [BMI]/P123 species, C

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than 0.4 GPa. The imidazolium C−H peaks of the 13 wt % mixture underwent almost no (or mild) shifts in frequency upon increasing the pressure to 1 GPa, then underwent blueshifts upon increasing the pressure beyond 1 GPa. The absence of any drastic changes in frequency at pressures of less than 1 GPa suggests that the crystalline-like transformation did not occur in the 13 wt % [BMI][PF6]/P123 mixture. As revealed in Figure 4, the discontinuous jump occurring around 0.4 GPa becomes less obvious for blends with 41 and 13 wt %. This observation suggest that the cluster structures of [BMI][PF6] may be disturbed by P123 and the [BMI] cations should be surrounded by P123 for low concentration blends. The blueshifts observed at pressures greater than 1 GPa may have originated from the overlap repulsion effect enhanced by hydrostatic pressure. Pressure might also have changed the anharmonic nature of the potential well for C−H stretching in such a way as to increase the observed frequency. We obtained a complementary insight into the behavior of the imidazolium C−H bands by measuring the concentrationdependent variation in the mid-IR spectra under high pressures. Figure 5 presents the IR spectra of pure [BMI][PF6] (curve a) and [BMI][PF6]/P123 mixtures containing contents of [BMI][PF6] of 55 (curve b), 41 (curve c), 21 (curve d), and 13 (curve

with the P123 molecules being forced to dissolute into aggregates upon breaking the cation−anion interactions. The favorable approach of a P123 copolymer strand toward an imidazolium cation occurs through the formation of imidazolium C−H···O hydrogen bonds; such interactions are presumably enhanced at elevated pressure. Our results suggest potential uses of the application of high pressure in separation and dispersion processes. As revealed in Figures 3b−g, the C− H stretching modes exhibited monotonic blue-shifts in frequency upon further compression. Thus, high pressure can be applied to tune the relative weights of the ionic liquid aggregation states in [BMI][PF6]/P123 mixtures. As revealed in Figure S3 (see Supporting Information), we show the results of a control experiment on effect of high pressure on pure P123. The infrared spectra of pure P123 in Figure S3 exhibit absorption bands corresponding to alkyl C−H stretching modes. We note that pure P123 does not have absorption bands near 3100 cm−1. Figure 4 presents plots of the pressure-dependence of the frequencies of the imidazolium C−H bands of pure [BMI]-

Figure 4. Pressure-dependence of the imidazolium C−H stretching frequencies [the bands at (A) 3170 and (B) 3127 cm−1] of [BMI][PF6]/P123 featuring [BMI][PF6] contents of 100 (diamonds), 91 (squares), 55 (triangles), 41 (crosses), and 13 (circles) wt %.

[PF6] and [BMI][PF6]/P123 mixtures. The imidazolium C−H peaks of pure [BMI][PF6] initially underwent blue-shifts in frequency upon increasing the pressure to 0.4 GPa and then underwent no changes upon increasing the pressure from 0.4 to 1.1 GPa, and finally underwent blue-shifts again upon increasing the pressure beyond 1.5 GPa. This discontinuity in frequency is identical to the trend revealed in our previous reports for pure [BMI][PF6].41 Such behavior may indicate pressure-induced solidification (or crystallization) or some other type of structural transformation at pressures of greater

Figure 5. IR spectra of pure [BMI][PF6] (curve a) and mixtures of [BMI][PF6]/P123 featuring [BMI][PF6] contents of 55 (curve b), 41 (curve c), 21 (curve d), and 13 (curve e) wt %, recorded under a pressure of 2.5 GPa. D

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example, when a molecule that is capable of forming blueshifting hydrogen bonds binds to a site featuring a sufficiently strong electrostatic field to dominate over the overlap effect, that molecule is predicted to display a red-shifting hydrogen bond.43 Our present study suggests the use of high pressure as a means of examining such issues. In light of this finding, the redshifts may be related to the strengthening of the imidazolium C−H---O contact upon compression, i.e., a switch to imidazolium C−H---O hydrogen bond-like. The red-shift of the imidazolium C−H stretch is characteristic of pressureenhanced hydrogen bonds.36,41 For example, pressure-induced red-shifts were observed for 1,3-dimethylimidazolium methyl sulfate.41 Figure 7 presents IR spectra of the [BMI][PF6]/P123 mixture featuring 90 wt % [BMI][PF6] recorded under ambient

e) wt %, recorded under a pressure of 2.5 GPa. In comparison with the spectra of [BMI][PF6]/P123 obtained under ambient pressure (Figure 1), here we observe significant changes in the imidazolium C−H bands in the presence of P123 under high pressure. The dilution of [BMI][PF6] not only red-shifted the position of the imidazolium peaks but also affected the ratio of the intensities of the bands of the imidazolium C−H units (I3127/I3170). The evolution of the spectral features in Figure 5 may have arisen from changes in the local structures of the imidazolium C−H units, with their hydrogen-bonded networks likely perturbed by the high pressure. Figure 6 reveals the concentration dependence of the band frequencies of the two dominant imidazolium C−H stretching

Figure 6. Concentration dependence of the imidazolium C−H stretching frequencies [the bands at (A) 3170 and (B) 3127 cm−1] of [BMI][PF6]/P123 under ambient conditions (diamonds) and under a pressure of 2.5 GPa (crosses).

absorption near 3127 and 3170 cm−1 under ambient pressure and at 2.5 GPa. The imidazolium C−H stretching modes underwent red-shifts in frequency upon adding P123, with strong decreases in frequency under a pressure of 2.5 GPa and relatively weak decreases under ambient pressure. Because the interactions between the ionic liquid and P123 were stabilized upon compression, the concentration-dependence was stronger under the higher pressure (i.e., 2.5 GPa). This observation supports our argument that elevated pressures forced the P123 molecules to dissociate the clusters of ionic liquid through enhanced hydrogen bonding with the imidazolium C−H units. Fundamental studies of weak hydrogen bonds, such as C−H--O interactions, continue to reveal important new finding; one such intriguing aspect is that the C−H covalent bond tends to shorten as a result of formation of a hydrogen bond.42,43 The Scheiner42 and Dannenberg43 groups view red- and blueshifting hydrogen bonds as being very similar in nature. For

Figure 7. IR spectra of the [BMI][PF6]/P123 mixture featuring 90 wt % of [BMI][PF6], recorded under ambient pressure (curve a) and at pressures of 0.4 (curve b), 0.7 (curve c), 1.1 (curve d), 1.5 (curve e), 1.8 (curve f), and 2.5 (curve g) GPa.

pressure (curve a) and at pressures of 0.4 (curve b), 0.7 (curve c), 1.1 (curve d), 1.5 (curve e), 1.8 (curve f), and 2.5 (curve g) GPa. The absorption spectrum in Figure 7a exhibits two major peaks at 1169 and 1111 cm−1, corresponding to imidazolium ring-stretching (ionic liquid) and ether stretching (P123), respectively. Consistent with previous studies,3 we assign the absorption signal at 1169 cm−1 to a mixed band of CH3N and CH2N stretching and in-plane ring asymmetric stretching. We E

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also observe a weak shoulder near 1097 cm−1 in Figure 7a. The nearly degenerate peaks at 1111 and 1097 cm−1 indicates that the ether C−O−C stretching band of P123 exists in two different forms. The appearance of the red-shifted shoulder peak at 1097 cm−1 suggests the existence of hydrogen bonds between the ionic liquid and the C−O−C groups of P123.3 Upon compression of the samplethat is, increasing the pressure from ambient (Figure 7a) to high pressures (Figure 7b−g)the ether C−O−C stretching band underwent a dramatic change and the shoulder peak near 1097 cm−1 increased in relative intensity. It is likely that the pressureenhanced interactions between the ionic liquid and P123 induced a transformation of the local C−O−C structures from non−hydrogen-bonded species (signal near 1111 cm−1) to hydrogen bonded configurations (signal near 1097 cm−1). A possible explanation for this effect is the favored interaction of the oxygen atoms of the ether C−O−C groups with the imidazolium cations via C−H···O interactions under higher pressures. In other words, pressure can be used to tune the relative contributions of the non−hydrogen-bonded and hydrogen-bonded components. These observations suggest the existence of microheterogeneity and hydrogen-bonded structures in the [BMI][PF6]/P123 system. The self-organization of block copolymers in solution has been attracting attention recently for their applications in drug delivery and the transporting of small molecules.4−7 For example, the shuttling of micellar structures between a hydrophobic ionic liquid phase and an aqueous phase has been observed in a temperature-dependent process.4,5 Analogous to such temperature-dependent processing, our results from this present study suggest the potential for pressureinduced association of P123 into ionic liquid clusters through changes in pressure.

pure P123 obtained under ambient pressure (curve a), and at 0.4 (curve b), 0.7 (curve c), 1.1 (curve d), 1.5 (curve e), 1.8 (curve f), and 2.5 GPa (curve g) (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.

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*(H.-C.C.) E-mail: [email protected]. Fax: +886-38633570. Telephone: +886-3-8633585. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank National Dong Hwa University and Ministry of Science and Technology (Contract No. NSC 101-2113-M-259006-MY3) for financial support, and Shih-Chun Wei and KaiWen Chang for their assistance.

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REFERENCES

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IV. CONCLUSION We have used IR techniques to examine the interactions between [BMI][PF6] and P123 at pressures as high as approximately 2 GPa. The concentration-dependence of the IR spectral features indicated that the imidazolium C−H bands of [BMI][PF6] display anomalous frequency shifts upon dilution with P123. It appears that order-to-order transitions may occur in such [BMI][PF6]/P123 mixtures. Upon elevation of the pressure, we observed appreciable changes in the relative band intensities. It appears that pressure somehow stabilizes the conformations of the hydrogen-bonded [BMI] and P123 species. A possible explanation for this effect is that the oxygen atoms of the ether C−O−C groups interact favorably with the imidazolium cations through C−H···O interactions under high pressure. Our results suggest that high pressure can be applied to tune the relative weight of the ionic liquid aggregation state in [BMI][PF6]/P123 systems. Elevated pressure appears to force the P123 molecules to disrupt the ionic liquid clusters as a result of enhanced hydrogen bonding with the imidazolium C− H units.

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ASSOCIATED CONTENT

S Supporting Information *

Concentration dependence of the imidazolium C−H stretching frequencies of [BMI][PF6]/P123 (Figure S1), IR spectra of the [BMI][PF6]/P123 mixture featuring 90 wt % [BMI][PF6] recorded under ambient pressure (curve a) and at pressures of 0.4 (curve b), 0.7 (curve c), 1.1 (curve d), 1.5 (curve e), 1.8 (curve f), and 2.5 (curve g) GPa (Figure S2), and IR spectra of F

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