Conformational States of Ionic Liquid 1-Ethyl-3-methylimidazolium Bis

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Conformational States of Ionic Liquid 1‑Ethyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide in Bulk and Confined Silica Nanopores Probed by Crystallization Kinetics Study Yogendra Lal Verma and Rajendra Kumar Singh* Department of Physics, Banaras Hindu University, Varanasi 221005, India S Supporting Information *

ABSTRACT: The nonaqueous sol−gel process has been used to synthesize the “ionogels” by confining ionic liquid (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide; [EMIM][TFSI]) into silica gel matrices. The present study is concerned mainly with probing the conformational states of the IL ([EMIM][TFSI]) through crystallization kinetics study of the bulk and confined ionic liquid (IL) in nanopores of silica matrix. The crystallization kinetics has been studied by the isothermal method using differential scanning calorimetry (DSC). For bulk IL, DSC result shows three crystallization peaks due to different conformations of IL molecules. DSC results show that one of these crystallization peaks disappears upon confinement due to interaction of IL molecules with the silica pore wall surfaces. The crystallization kinetics of bulk and confined IL is quantified using the Avarami analysis. Confinement of IL results in a decrease of the Avarami exponent, indicating one-dimensional crystal growth. To support the results obtained from crystallization kinetics study, investigate the properties of confined IL, and to study the morphological properties of silica gel matrices, some other characterization techniques, viz. TGA, XPS, FTIR, BET, SEM, and TEM, have been used. The XPS and FTIR results show the change in the binding energy of constituents of IL molecule and vibrational bands related to IL, respectively. BET, SEM, and TEM analyses display the uniform pore structures in IL confined silica matrices.

1. INTRODUCTION The crystallization process is widely applicable to many materials that can crystallize from melt or amorphous state, e.g., oxide glasses,1 polymers,2 ionic liquids,3 etc. The process of crystallization plays an important role in natural phenomena (e.g., rock formation,4 biomineralization,5 etc.) and also in industrial processes such as synthesis and purification of drugs.6 Recently, ionic liquids (ILs) have aroused increasing amount of interest for chemical and material scientists.7 ILs are organic salts (liquid below 100 °C) that are generally composed of bulky and asymmetric organic cations as well as inorganic and organic anions.7,8 They possess attractive properties such as high thermal stability, low vapor pressure, wide liquidus range, and wide electrochemical window, etc. These attractive properties of ILs make them a potential candidate for being used as a catalyst and also in various electrochemical device applications like batteries, fuel cell, solar cell, dye-sensitized solar cell, supercapacitors, etc.7−9 The liquidus nature of ILs limits their applications in device. Therefore, immobilization of ILs in some solid substrates like SiO2,8−12 TiO2,8,12 CNT,13 polymers,14 etc., are indispensable. These types of IL immobilized solid substrates are termed as “ionogels”. In the recent studies, ionogels have been synthesized by confining ILs in nanopores of different matrices having different pore geometries.8 Silica is one of the extensively used materials to immobilize ILs in confined geometry.15−23 These studies on ionogels have focused mainly on physicochemical properties, © 2015 American Chemical Society

dynamics, relaxation times of ILs in nanopores, and structural and morphological properties of confining geometries.15−23 Various properties of the confined ILs like thermal, optical, dynamical, etc., have been found to be different compared to bulk ILs due to the interaction between silica pore wall surface and confined ILs.8,11,16−23 In spite of exhaustive studies on various properties of ionogels, studies related with crystallization kinetics of bulk and confined ILs in inorganic matrix/polymers are still lacking. Only few recent studies by Pas et al.3 (for bulk IL) and by our group have reported crystallization kinetics of confined ILs in silica matrix22 and IL-based polymer electrolytes.24−26 Pas et al.3 have reported for the first time the isothermal and nonisothermal crystallization kinetics of three ILs having different cations and anions. They found that complete IL was crystallized with decreasing nucleation rates, random nucleation growth, and Johnson−Mehl− Avarmi−Kolmogrov (JMAK) equation applicable to them. However, isothermal crystallization kinetics study of bulk and confined IL ([EMIM][BF4]) in silica matrix22 exhibits that crystallization kinetics behavior of IL depends on the isothermal temperatures and amount of IL. In this study, it has also been reported that upon confinement of IL in silica matrix dimensionality Received: July 11, 2015 Revised: October 3, 2015 Published: October 9, 2015 24381

DOI: 10.1021/acs.jpcc.5b06672 J. Phys. Chem. C 2015, 119, 24381−24392

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Details of these experimental techniques are described in the Supporting Information.

of the crystallization of IL reduces from three-dimensional (3D) to one-dimensional (1D), and confinement slows down the crystallization rates of IL. Here, we report the synthesis of ionogels by incorporating different amounts of IL (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide; [EMIM][TFSI]) using the nonhydrolytic sol−gel process. In the present work, the phase transition temperature and isothermal crystallization kinetics of bulk and confined IL have been studied using differential scanning calorimetry (DSC). Conformational states of IL have been probed using crystallization kinetics study. Thermal stability, vibrational properties of confined IL, and the morphological properties of silica matrices have been studied using TGA, FTIR, BET, SEM, and TEM measurements. The X-ray photoelectron spectroscopy (XPS) technique has been employed to see the effect of surface interaction of IL with pore wall surface. From DSC study, it is observed that upon confinement of IL in silica matrix phase transition temperatures (glass transition temperature, Tg; crystallization temperature, Tc; and melting temperature, Tm) have changed, and isothermal crystallization study reveals that the rate of crystallization of IL is decreased upon confinement in silica nanopores. The XPS result shows the change in BE of IL molecule constituents due to interaction between silica pore wall surface and IL, and this interaction is responsible for slowing down crystallization kinetics rate of IL in confinement. FTIR results exhibit the change in vibrational bands related to IL cation and anion. N2-sorption study exhibits that higher concentrations of IL containing silica ionogel reveal cylindrical pore geometry. Morphologies of different loadings of IL containing silica gel matrices have also been studied.

3. RESULTS AND DISCUSSION 3.1. Differential Scanning Calorimetry. Differential scanning calorimetry is essential for observing the phase transition temperature of materials. DSC has been used to measure the glass transition temperature (Tg), crystallization temperature (Tc), and melting temperature (Tm) of bulk and confined IL. DSC thermograms of bulk and confined IL (CIL-1, CIL-2, and CIL-3) are shown in Figure 1. It has been observed

Figure 1. DSC thermograms of bulk IL and confined IL samples (CIL-1, CIL-2, and CIL-3).

that the phase transition temperatures of IL change upon confinement into silica matrix of different pore sizes. Phase transition temperatures (Tg, Tc, and Tm) of bulk and confined IL are given in Table 1. From DSC thermograms, it can be seen that

2. EXPERIMENTAL SECTION 2.1. Chemicals. Chemicals used for the synthesis of the ionogels were tetramethyl orthosilicate (TMOS; 98%) as a metallic precursor and formic acid (GR grade; purchased from Merck, Germany) and IL (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [EMIM][TFSI]; >98%, SigmaAldrich). Before use, the IL was heated at a temperature of 100 °C for 12 h followed by vacuum drying at a pressure of ∼10−3 Torr to remove traces of water. To analyze the water content in IL, a Karl Fischer coulometric titrator (Mettler Toledo C20) was used. The water content was less than 50 ppm in bulk IL. 2.2. Synthesis. Ionogels (IL confined silica matrices) were synthesized using the nonhydrolytic sol−gel process. For this, a mixture of IL and formic acid was added to TMOS at a TMOS/ HCOOH/IL molar ratio of 1:8:x (x = 0.3, 0.5, and 0.7 mol) in a reaction vessel at ambient temperature (25 °C). Further, resulting mixture was stirred for a short time and then left for gelation, and it was found that all the samples gelified in approximately 30−35 min. Finally, the stable monolithic ionogels were formed after 2 weeks aging. The synthesized samples are described as CIL-1, CIL-2, and CIL-3 having 0.3, 0.5, and 0.7 mol % of IL, respectively. 2.3. Characterization. Phase transition temperature and crystallization kinetics study of bulk and confined IL were investigated using a Mettler Toledo DSC-1. Thermal stability, binding energy of confined IL system, and vibrational properties were studied using thermogravimetric analysis (Mettler Toledo TGA/DSC 1), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR), respectively, while morphological characteristics were studied using BET (Brunauer, Emmett, and Teller) analysis, scanning electron microscopy (SEM), and transmission electron microscopy (TEM).

Table 1. Phase Transition Temperatures (Glass Transition Temperature, Tg; Crystallization Temperature, Tc; and Melting Temperature, Tm) of Bulk and Confined IL (CIL-1, CIL-2, and CIL-3) samples

Tg (°C)

Tc (°C)

Tm (°C)

bulk IL CIL-1 CIL-2 CIL-3

−91.9 −90.5 −90.2 −90.2

−58.7, −47.2, −29.5

−15.9, −9.0

−52.0, −36.4 −54.5, −29.6

−19.0 −15.7

the CIL-1 shows only the glass transition temperature (Tg) while bulk IL, CIL-2, and CIL-3 exhibit all the three transitions, viz. Tg, Tc, and Tm. For all the samples Tg has been found to increase ∼2 °C upon confinement. Three crystallization and two prominent melting peaks (Table 1) are observed in bulk IL. These multipeaks of crystallization and melting (Figure 1) show the polymorphism of IL, i.e., ordering of different conformational phases of cations and anions due to their intermolecular interactions.27 However, upon confinement of IL, samples CIL-2 and CIL-3 exhibit two crystallization peaks and single melting peak. In the present study it has been found that three crystalline modifications of IL ([EMIM][TFSI]) in bulk state compared to different conformational changes of IL. There are some theoretical and experimental studies on IL ([EMIM][TFSI]) in which different (single, double, triple, etc.) crystalline modifications have been reported. Paulechka et al.27 have found that the IL [EMIM][TFSI] forms four different types of crystalline modifications. The cation has two distinguishable conformers; trans 24382

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the structure of the anion from trans to cis form. Nayeri et al.36 have also reported that the Raman intensity of the modes associated with cis conformer (TFSI anions; at 740 cm−1) are increased with respect to trans conformers upon confinement of IL in silica gel matrix; i.e., TFSI anions rearrange and establish a cisoid form relative to transoid form in ionogels. Martinelli16 has also reported that a mixture of cis and trans conformations is found at room temperature in the TFSI anion and in the liquid state, while upon confinement in silica this equilibrium shifts toward an increased cis population. They have also reported that the TFSI anion preferably adopts the cisoid conformation and experiences a strong ion−ion interaction upon confinement by employing time-resolved Raman spectroscopy.37 In another study, Ori et al.38 have reported the molecular dynamic simulation of confined IL (1-butyl-3-methylimidazolium and anion bis(trifluoromethylsulfonyl)imide) in amorphous silica at ambient temperature and found that the population of trans conformation of anion is ∼90% in the bulk IL. However, upon confinement of IL, the IL anion percentage of trans conformer decreases from 65% to 38%; i.e., cis conformational arrangement allows more efficient packing of the anions at the silica surface which may affect the crystalline structure of IL, as observed in the present study. In the present investigation, one of the melting peak is absent in bulk IL as well as upon confinement (as shown in Figure 1) which is due to the coexistence of cis and trans conformers of TFSI anion. Moschovi et al.39 have also reported that upon melting of IL [HMIm][TFSI] both the cis and trans conformers coexist. 3.2. Crystallization Kinetics. Isothermal crystallization kinetics studies of bulk and confined IL (CIL-3) are described in this section. In the present investigation, due to comparatively high amount of IL in sample CIL-3, crystallization isotherms were observed only for sample CIL-3. DSC thermogram of sample CIL-2 also results in a similar observation with crystallization peaks having small intensity. While in sample CIL-1, no crystallization/melting peaks are observed. Isothermal crystallization curves of bulk IL and confined IL (CIL-3) are given in Figure 2. Figure 2 reveals that the bulk IL and CIL-2 have three and two crystallization peaks, respectively, as discussed in the DSC section. It can be seen from the Figure 2 that peaks related to the bulk IL and CIL-3 are clear, but for a better understanding of the heat involved in the phase transition, deconvolution has been done. Figure S2 shows the deconvoluted crystallization isotherms of three peaks related to pure IL and two peaks of confined IL at different isothermal crystallization temperatures. From Figure S2, it can be seen that peak positions and their intensity are different

(planar) and gauche (nonplanar) and anion TFSI has also two conformers cis and trans.28 Therefore, a possibility of packing of the [EMIM] and [TFSI] ions in various ways with different mutual orientations of the cation and anion may be more potential cause of polymorphism. In another study, Paulechka et al.29 have reported the structures of [EMIM] [TFSI] crystalline modification using combined use of X-ray crystallography, IR spectroscopy, and quantum-chemical calculations. They have reported the considerable change in vibrational bands of cations and anions in the wavenumber range 900−800 cm−1 on going from liquid to crystalline state. These results demonstrate the cis and trans conformation of cation and anion in different crystal phase. However, Choudhury et al.30 reported that [EMIM][TFSI] forms single crystal on in situ crystallization. Gometz et al.31 have also studied the thermal properties of IL [EMIM][TFSI] during different cooling, heating, and quenching rates and found that this IL exhibits three crystallization peaks during quenching, and it also gives more than one melting/endothermic peak at a heating rate 2 K/min, which evidenced the presence of polymorphism. Apart from these studies, it has also been reported computationally (DFT calculations) and experimentally (using Raman and IR spectroscopy)28,32−35 that TFSI is flexible molecule which can acquire two different conformations: cis and trans. Both the conformers are usually present in the liquid state of IL and their concentration in solid/liquid phase can change the chemical and physical properties of ILs. However, the trans conformer is more stable than the cis conformer. The trans and cis conformers of TFSI anion reveal different Raman and IR spectra, as confirmed by experimental and computational studies.34,35 Upon confinement of IL in silica matrices, it has been found that CIL-2 and CIL-3 exhibit two crystallization peaks and single melting peak; i.e., one of the crystalline peaks related to bulk IL is absent. The absence of one of the three crystallization peaks is due to the interaction of IL with the silica pore wall surface which affected the structure of anion from trans to cis conformer as observed in quantum chemical calculations using Gaussian 03 program (discussed in section 3.9). The schematic representation of IL molecules before confinement and after confinement is given in Figure S1 of the Supporting Information. From Figure S1, it can be seen that two anion conformers (cis and trans) are present in bulk IL state. They form more than three possible combinations of IL molecules by combining with cation at different positions. However, upon confinement of IL molecules in silica nanopores, a large number of trans anion convert into cis anion conformers with two possible attachment with cations. As discussed in section 3.9, the oxygen of the silica is interacted with hydrogen of the cation ring which also hinders

Figure 2. Isothermal crystallization curves for (a) bulk IL and (b) confined IL (CIL-3) at different isothermal crystallization temperatures. 24383

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Figure 3. Heat flow vs time plots (a−c), Xt vs time plots (d−f), and Avrami plots (g−i) during isothermal crystallization for three crystal phases of bulk IL at different crystallization temperatures (viz., Tc = −63, −64, −65, and −67 °C).

at different isothermal temperatures (−63 to −67 °C for pure IL and −61 to −67 °C for confined IL). The amount of heat evolved during the formation of crystal phase has been analyzed by evaluating relative degree of crystallinity (Xt):40 t

dH dt ( dt ) Xt = = ∞ Q∞ ∫0 ( ddHt ) dt

Qt

∫0

increasing with decreasing isothermal temperature (−63 to −67 °C), while for second and third Tc peaks, t1/2 decreases with decreasing isothermal crystallization temperature. In the case of confinement also, t1/2 is increasing with decreasing isothermal crystallization temperature for the first peak while for the second peak no regular trend is obtained. Overall, it has been found that the crystallization half-time (t1/2) is increased upon confinement of IL in silica nanopores. This may be due to the interaction of IL cation with silica pore wall surface which may result the higher packing efficiency, or density, for the IL at the silica surface.16 The crystallization kinetics of pure IL and confined IL (CIL-3) under isothermal conditions has been investigated using the Avrami method.41,42 According to this method, the relative degree of crystallinity, Xt, is related to crystallization time t, i.e.

(1)

where Qt and Q∞ are the heat generated at time t and infinity time, respectively, and dH/dt is the rate of heat evolution. The relative crystallinity vs time plots of pure IL and confined IL (CIL-3) at different temperatures are shown in Figures 3a−i and 4a−f, respectively. The crystallization half-time (t1/2) (i.e., time taken for 50% crystallization of samples) is an important parameter for studying crystallization kinetics. The crystallization half-time (t1/2) is directly estimated from the plots of relative crystallinity vs time. From Figures 3 and 4, it can be seen that all the curves (relative crystallinity vs time) related to three crystallization peaks of bulk IL and two peaks of CIL-3 show sigmoid nature. The crystallization half-times (t1/2) are given in Table 2. From the Table 2, it can be seen that the crystallization half-time (t1/2) for first crystallization (Tc) peak of bulk IL is

X t = 1 − exp( −Kt n)

(2)

where K is the crystallization rate constant [(time)−n] which depends on crystallization temperature and n is the Avrami exponent whose value depends on the characteristics of nucleation and growth mechanism of the crystal. This equation is usually written in the form of the double logarithmic log[− ln(1 − X t )] = log K + n log t 24384

(3)

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Figure 4. Heat flow vs time plots (a, b), Xt vs time plots (c, d), and Avrami plots (e, f) during isothermal crystallization for two crystal phases of confined IL (CIL-3) at different crystallization temperatures (viz., Tc = −61, −63, −65, and −67 °C).

Table 2. Different Crystallization Parameters of Samples Obtained by Avrami Plots in the Isothermal Crystallization Method crystallization rate constant [K (s−n)]

Avrami exponent (n) samples bulk IL

confined IL

crystallization half-time [t1/2 (s)]

(Tc)iso (°C)

n1

n2

n3

K1

K2

K3

t1/2(1)

t1/2(2)

t1/2(3)

−63 −64 −65 −67 −61 −63 −65 −67

1.8 1.9 1.80 1.84 1.07 1.02 1.13 1.16

2.78 2.92 3.36 2.98

4.0 3.92 3.82 4.1 1.30 1.30 1.11 1.11

1.76 × 10−4 9.77 × 10−5 1.14 × 10−4 6.16 × 10−6 3.44 × 10−3 1.50 × 10−3 7.14 × 10−4 5.54 × 10−4

8.071 × 10−8 5.53 × 10−8 7.58 × 10−9 5.01 × 10−8

1.86 × 10−11 6.02 × 10−11 2.13 × 10−10 1.62 × 10−10 9.77 × 10−9 2.29 × 10−8 8.35 × 10−6 3.45 × 10−4

42 60 79 90 45 70 108 140

224 200 189 182

347 292 240 176 223 312 302 306

Here, the Avrami exponent n and crystallization rate constant K can be obtained from the slope and intercept in the plot of log[−ln(1 − Xt)] vs log t. The Avrami plots related to various crystallization peaks of bulk IL and confined IL at different isothermal temperatures are given in Figures 3 and 4, respectively. The Avrami exponent n and crystallization rate constant K are determined by fitting the curve of log[−ln(1 − Xt)] versus log t,

and values are given in Table 2. From the Table 2, it can be seen that the values of Avrami exponent n1 for the first crystal phase of bulk IL lies in the range of 1.79−1.86 for different isothermal crystallization temperature, while for the second and third crystal phase of bulk IL, Avrami exponents n2 and n3 vary in the range of 2.78−3.36 and 3.8−4.1, respectively. This suggests that bulk IL shows three-dimensional (3D) crystal growth. The value of 24385

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absorbed water.12 The inset of Figure S3 also shows that weight loss occurred (in temperature range 40−150 °C) in confined IL samples increases as the amount of silica content increased. It shows that higher amount of silica containing samples have more unhydrolyzed silanol (Si−OH) groups, which causes more weight loss in these samples (CIL-1, CIL-2, and CIL-3). Further, all confined IL samples (CIL-1, CIL-2, and CIL-3) show single-step decomposition approximately at the same temperature (Tstart ∼ 350 °C). Thus, TGA thermograms exhibit that the decomposition temperature of IL is not found to change upon confinement. TGA themograms also reveal that after complete decomposition of confined IL, the wt % of the remaining samples is nearly equal to the wt % of silica present in the samples (CIL-1, CIL-2, and CIL-3). 3.4. XPS Study. XPS study of the confined IL (CIL-3) in silica matrix has been carried out to investigate the interactions of IL with the silica pore wall surface. The survey scan spectra of confined IL (CIL-3) are given in Figure 5a, which shows the

Avrami exponent is nearly equal to 1 for both the crystallization peaks observed in confined IL (CIL-3) which indicates onedimensional crystal growth. Thus, dimensionality of crystal growth gets reduced in confined geometry of nanopores. 3.3. Thermogravimetric Analysis. Thermogravimetric analysis of bulk IL and confined IL (CIL-1, CIL-2, and CIL-3) is shown in Figure S3. Before the TGA measurements, bulk IL and confined IL samples were heated at 100 °C for 12 h under vacuum (10−3 Torr) to remove the traces of water, organic solvents, and other volatile impurities. From the TGA thermograms, it can be seen that the bulk IL shows onset decomposition temperature (Tstart) at 350 °C followed by single-step decomposition. From the inset of Figure S3, it can be seen that a very small weight loss occurred in the bulk IL (0.1 wt %) and confined IL samples in the temperature range 40−150 °C. It can be attributed to the evaporation of OH groups (due to some unintentional unhydrolyzed silanol (Si−OH) groups), physically and chemically

Figure 5. XPS spectra of confined IL ([EMIM][TFSI]): (a) survey spectra and high resolution spectra for (b) Si 2p, (c) S 2p Si 2s, (d) C 1s, (e) O 1s, and (f) F 1s. 24386

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Figure 6. FTIR spectra of (a, b) bulk IL and confined IL samples (CIL-1 CIL-2, and CIL-3) in the regions 3250−2900 and 1700−400 cm−1; (c−f) deconvoluted spectra of bulk IL, CIL-3, CIL-2, and CIL-1, respectively, in the region 3250−3050 cm−1.

(0.8 eV) in comparison to bulk IL (at 532.7 eV).44 As reported in the bulk IL, BE peaks related to S 2p and F 1s have single peak at 169.1 and 688.8 eV, respectively.44 However, we have found more than one peak for S 2p at 169.6, 168.6, and 160.3 eV (Figure 5c) and for F 1s at 689.2 and 688.6 eV (Figure 5f). Overall, the BE related to the anion constituents has been found to change as compared to the bulk IL, which reveals that upon confinement of IL in silica nanopores, BE of the anion is being modified due to the interaction of the silica pore wall surface. Further, it has found two BE peaks at 103.4 and 104.7 eV attributed to Si 2p (Figure 5b). However, in pure SiO2, single BE peak is reported at 102.9 eV;22 i.e., the former peak is due to Si element of SiO2, and latter one is due to the interaction of IL molecules with the silica pore wall surface. Thus, the observed changes in the BE positions of these elements, viz. C 1s, Si 2p, and O 1s, upon confinement of IL in silica matrix could explain the cause of delayed crystallization process as observed in our isothermal crystallization kinetics studies using DSC. 3.5. FTIR Study. In order to investigate the interaction of IL with silica pore wall surface, FTIR study has been performed. FTIR spectra of bulk IL and confined IL samples (CIL-1, CIL-2, and CIL-3) are given in Figure 6a,b. Figure 6a shows the extended FTIR spectra in the region 3250−2900 cm−1, assigned to C−H vibrational bands related to ring and alkyl chain of IL cation. However, to see the clear spectra in region 3250− 3050 cm−1, the FTIR spectra have been deconvoluted in this region using software PeakFit 4.12. The deconvoluted FTIR

different peaks related to Si 2s, Si 2p, S 2p, C 1s, N 1s, O 1s, and F 1s at their respective binding energy (BE) positions. However, the detailed scan spectra of Si 2p, C 1s, S 2p−Si 2s, O 1s, and F 1s with curve fitting are given in Figure 5b−f. Here, the XPS spectrum for C 1s core level has been fitted with six curves (Figure 5d). From the Figure 5d, it can be seen that the BE of carbon peaks related to the CF3 of anion reveals two peaks at 294.7 and 292.8 eV as compared to the literature value of BE of CF3 at 293.0 eV.43 The peak at 292.8 eV has nearly the same value as near the bulk IL,44 while the BE peak of carbon at higher energy (294.7 eV) has small area, low intensity, and shift observed is ∼1.7 eV as compared to the bulk IL peak (293.0 eV). The BE peak of carbon related to CF3 may split into two peaks: the first one at 294.7 eV due to TFSI anions near the pore wall surface which is interacting with O of the silica pore wall surface and the second one at 292.8 eV due to TFSI anions away from the pore wall surface that shows the BE equal to the bulk IL. Similarly, the BE peaks at 285.0 and 286.2 eV associated with end of ethyl group carbon (C 1s aliphatic chain) and carbon direct bonded to N (C−N) of IL cation, respectively, are found to be shifted in comparison to bulk IL reported at 285.6 and 286.7 eV, respectively.44 Further, we have also found the two peaks of oxygen: one related to the oxygen of SiO2 at 532.7 eV and the second one due to the oxygen present in anion at 533.5 eV (Figure 5e). The BE peak related to O 1s of SiO2 has not been found to change as reported at 532.6 eV,22 while the BE peak of O 1s related to anion has been found to shift upon confinement 24387

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Figure 7. (a) N2-sorption isotherms of samples WCL-1, WCL-2, and WCL-3 and (b) show their corresponding BJH desorption pore size distribution curves. (c) A typical SEM image of WCIL-3 and inset shows the SEM image of CIL-3. (d) TEM image of CIL-3.

Because these functional groups are directly involved in trnas to cis conformational changes, these interactions play a crucial role in changing the phase transition temperature as well as slowing down the crystallization of IL in silica nanopores. 3.6. Pore Parameter Analysis. Figure 7a shows the N2sorption isotherms of porous silica gel matrices obtained after the removal of confined IL without destroying the skeleton of silica gel matrices. Isotherms of samples WCIL-1, WCIL-2, and WCIL-3 (samples corresponding to CIL-1, CIL-2, and CIL-3, respectively, with IL removed) show type IV characteristics of mesoporous materials.45 From Figure 7a, it can be seen that the sample WCIL-1 exhibits the asymmetric shape of the hysteresis loop of type H2, i.e., disorder and highly interconnected porous materials, while samples WCIL-2 and WCIL-3 exhibit H1 like hysteresis loop associated with a regular cylindrical like pores. It has found that upon increasing the content of IL pore structure of silica gel matrices has changed. The pore parameters (surface area, pore volume, average pore size) of samples WCIL-1, WCIL2, and WCIL-3 are given in Table S2. From Table S2, it can be seen that on increasing the loading of IL surface area gets decreased along with the increase in pore volume and average pore size. Figure 7b shows the pore size distribution curve of samples WCIL-1, WCIL-2, and WCIL-3, calculated by the BJH (Barrett−Joyner−Halenda) method from the desorption branch. The average pore diameter of samples WCIL-1, WCIL-2, and WCIL-3 are 5.2, 8.9, and 12.1 nm, respectively, as given in Table S2. 3.7. SEM. Typical SEM images of sample CIL-3 (in inset) and WCIL-3 are given in Figure 7c. The SEM micrograph (WCIL-3) reveals the uniform porous structure of silica matrix after extraction of IL from ionogel. However, the SEM micrograph of ionogels (CIL-3) shown in inset of Figure 7c reveals the aggregates of IL containing silica particles.

spectra are shown in Figure 6c−f. From Figure 6c−f, it can be seen that upon confinement of IL in silica nanopores a large shift is observed in vibrational bands of bulk IL related to C−H of the ring at 3185, 3160, and 3103 cm−1. The observed shift in the vibrational bands and their assignment of IL cation and anion are given in Table S1. Along with the vibrational bands of IL ring, shift in bands of alkyl chain of cation and anion are also found. From the Table S1, it can be seen that the C−H vibrational band of alkyl chain of cation at 2973 cm−1 is found to shift at 2971, 2967, and 2968 cm−1 respectively for the CIL-1, CIL-2, and CIL-3. Apart from these changes in vibrational bands of IL cation, shift in vibrational bands of IL anion, viz. SO2 and CF3 group of TFSI anion, have also been observed. These changes observed in vibrational bands of IL upon confinement in silica nanopores clearly indicate that confinement hinders the motion of IL due to interaction of silica pore wall surface and IL molecules. As FTIR spectra reveal that a large change in C−H vibration of the ring and alkyl chain of cation and also the change in SO2 vibration of anion, in the same way, quantum chemical calculation exhibit that oxygen of silica molecule interacts with the C−H of the ring and Si of the silica interacts with oxygen of the anion (more details are given in section 3.9). Both XPS and FTIR study exhibit that upon confinement of IL in silica nanopores, BE and vibrational bands of similar moieties are changed as compared to bulk IL. For example, XPS study reveals the change in BE of elements C, S, O, and F related to moiety CF3, C−H, SO2, etc., and FTIR study also shows the change in vibrational bands related to CF3, C−H, SO2, etc. These changes in BE and vibrational bands are observed due to the interaction of IL molecules to the silica pore wall surface. The change in BE (elements C, S, O, and F related to moiety CF3, SO2) and vibrational bands of CF3 and SO2 group upon confinement clearly indicates the change in conformational states. 24388

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The Journal of Physical Chemistry C

Figure 8. (a−e) Geometries of IL ([EMIM][TFSI]) optimized on the basis of the B3LYP/6-31++g(d,p) level of theory and (g, i) optimized structures of [EMIM][TFSI] in the presence silica obtained using the B3LYP/6-31++g(d,p) level of theory by giving trial inputs (f, h), respectively.

3.8. TEM. Figure 7d shows the typical TEM micrograph of sample CIL-3. From the TEM micrograph, it can be seen that sample shows the wormhole-like porous texture and pores are nearly uniform. Pore size observed from TEM micrograph is ∼2.5 to 13 nm, which is comparable to the pore size obtained from the BJH analysis (average pore diameter is 12.1 nm). 3.9. Quantum Chemical Calculation. Hydrogen bonding plays a crucial role in ion-pair formation.46 Therefore, due to

H-bonding in [EMIM][TFSI], several similar energetically ion pair configurations have been reported in both gas phase and solution phase.47−50 Tsuzaki et al.47 have found 24 ion-pair configurations of [EMIM][TFSI] by Møller−Plesset (MP2) level of geometry optimization, in which three configurations are within 0.5 kcal/mol. In another study, 23 possible configurations of [EMIM][TFSI] have been obtained.48 Furthermore, Kiefer and co-workers49 have reported molecular interactions 24389

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The Journal of Physical Chemistry C and conformational states of [EMIM][TFSI] via the ab initio method at the DFT and MP2 level of theory. They have reported the different conformers on the basis of various interactions of the [EMIM][TFSI] ion pair in both the phases gas and solution phase, in which the three conformers have lowest energy states having trans conformations of anion. Recently, Vayas et al.50 located 23 minima for the ion pair of [EMIM][TFSI] by the B3LYP/6-31+G(d,p) level of theory in the gas phase. However, in the solution phase, three minima have been located, and also it has been found that [TFSI] anion exists in between cisoid and the transoid rotamers due to the interaction with [EMIM] cation. Similar to these studies,47−50 in the present work, the preferred geometry optimizations of bulk IL ([EMIM][TFSI]) and IL in the presence of SiO2 were carried out on the basis of density functional theory (DFT) with the basis set B3LYP/6-31+g(d,p). DFT calculations were carried out using Gaussian 03 program package.51 We have discussed our DSC results by optimizing the structure of IL with DFT calculation. The bulk IL shows three crystallization peaks due to the existence of various conformations of IL molecule. However, confined IL into the silica nanopores shows only two crystallization peaks because of the absence of any one of the conformations of IL due to the interaction of IL molecules with silica pore wall. To support this argument, DFT calculations have been performed for qualitative description using optimized structures obtained from IL with and without silica molecule. In the case of bulk IL molecule, five conformations have been found (Figure 8a−e) as reported by Kiefer and co-workers.49 They found that three conformers have smaller energy than others, i.e., three conformations of IL are most stable. In similar way, we have also found the three conformations of IL have smaller energies. Therefore, due to these three most stable conformations of IL ion pair, three crystallization peaks are observed by DSC. However, in the optimization of IL molecule in the presence of silica, two trial inputs were given (Figure 8f,h). In which anion is in trans conformation, but after complete optimization, anion formed cis conformation as given in Figure 8g,i. Thus, the absence of one of the crystallization peaks (as observed in DSC thermogram and isothermal crystallization kinetics of confined IL sample; CIL-3) upon confinement of IL into the nanopores of silica matrix is due to the conversion of anion from trans conformers to cis conformers (Figure 8g,i).

also reveals that confinement reduces the dimensionality of IL crystallization from 3D to 1D and also slows down the crystallization rate constant. XPS and FTIR results clearly indicated the interaction between silica pore wall surface to IL molecules which affects the phase transition temperature and crystallization kinetics of IL in nanopores of silica matrix. The quantum chemical calculation also gives the qualitative evidence of interactions due to which change in phase transition temperatures of IL in silica nanopores occur, and these calculations also support the existence of different conformational states in bulk and confined IL.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b06672. Details of characterization techniques, tables, and figures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail rksingh_17@rediffmail.com, rajendrasingh.bhu@gmail. com; Tel (+91) 542 6701541; Fax (+91) 542 2368390 (R.K.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.K.S. gratefully acknowledges the financial support received from the Department of Science & Technology, New Delhi, and Board of Research in Nuclear Sciences BRNS-DAE, India, for carrying out this work. Y.L.V. is thankful to Council of Scientific and Industrial Research, New Delhi, India, for award of Senior Research Fellowship (SRF). We are also thankful to Prof. T. Shripathi (I U C Indore, India) for XPS measurement and Prof. Il-Kwon Oh, KAIST, Korea, for providing SEM and TEM images of our samples.

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REFERENCES

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4. CONCLUSIONS In the present study, we report results on IL ([EMIM][TFSI]) confined silica gel matrices (ionogels). In particular, isothermal crystallization kinetics study of bulk and confined IL has been investigated using differential scanning calorimetry by the Avrami method to probe various conformational states of the IL. Together with the crystallization kinetics study, ionogels have also been characterized by TGA, XPS, FTIR, BET (after removal of IL from silica ionogels), SEM, and TEM. The DSC result exhibits that bulk IL shows three crystallization peaks and two melting peaks. However, upon confinement of IL, two crystallization and single melting peaks with shift in their respective positions as compared to bulk IL are observed for higher amount of IL containing samples (CIL-2 and CIL-3) due to the interaction of IL molecules with the silica pore wall surface. The crystallization kinetics results show that the crystallization parameters such as crystallization half-time (t1/2), relative crystallinity (Xt), crystallization rate constant (K), and Avrami exponent (n) of bulk IL are found different upon confinement of IL in the silica matrix. Moreover, crystallization kinetics study 24390

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