Imidazolium Ionic Liquids as Designer Solvents Confined in Silica Nanopores

Composite silica xerogels were prepared via acid catalysed sol–gel route using tetraethoxysilan (TEOS) as silica precursor, and 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4] or 1-butyl-3-methylimidazolium chloride [BMIM][Cl] ionic liquids, used simultaneously as co-solvents, catalysts and pore templates, at various IL-to-silica ratios. Morphology of the xerogels prepared using the different IL templating agents were investigated using scanning electron microscopy (SEM), nitrogen sorption and small angle neutron scattering (SANS). The thermal behavior of the composites was analyzed by thermal gravimetry, whereas the compositions were checked by infrared spectroscopy and EDX. The differences in the morphology and thermal behavior of the composites due to the different IL additives were revealed.


Introduction
Ionic liquids (IL) are well known as designer solvents for the synthesis of new compounds and functional materials. Thanks to various possibilities in tailoring their molecular structure and fine tuning their solvent properties [1], they are recognized as a third group of solvents, following water and organic solvents. In spite of this, they are considerably affected by Lewis acidity or basicity of their cations or anions (Coulombic interactions), by the directionality of interactions between cations and anions, and by the van der Waals interactions between ions [2], they are typically characterized by unique solvent properties. The selection of different anions or usage of different alkyl groups in the heterocyclic rings of the cations, permits the fine-tuning of physicochemical properties of ILs, such as viscosity, solvation, catalytic activity, hydrophobicity and melting point [3]. By choosing various anions, their miscibility with water and with other solvents, and their ability to form hydrogen bonds can be varied. By choosing different cations, their hydrophobicity can be also varied, causing changes in the properties of the composite materials prepared with ILs [4,5]. ILs used as pore templates in polymer matrices permit tailoring of the gel pore size [6]. Using ILs the stability of polymeric gels can be improved. With increased aging time the pore volume diminishes, whereas short aging produces stronger gel network; these processes are controlled by the solvent evaporation speed [7]. It has been shown that the addition of small amount of IL to aqueous mixtures of silica precursors slows down the polycondensation reaction and produces primary silica particles of larger sizes [8]. Composite materials based on silica and prepared by sol-gel route typically have

FT-IR Analysis of the Neat Ionic Liquids, Xerogels and Xerogels after Solvent Extraction
The FT-IR spectra of neat ILs, the xerogel samples and the materials resulted after the IL extraction (named "e"), are presented in Figures 1-3 and the band assignments are given in Tables 1 and 2.

of 1
The Si-OH bond stretching [32] around 950 cm −1 , is better observed in samples pre pared with [BMIM] [Cl]. The Si-O bond rocking [32] vibration around 450 cm −1 , is observ able in all obtained samples.

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The Si-OH bond stretching [32] around 950 cm −1 , is better observed in samples prepared with [BMIM] [Cl]. The Si-O bond rocking [32] vibration around 450 cm −1 , is observable in all obtained samples.

Vibration Bands of Ils and Ils in Silica Matrix
The C=C stretching from the imidazolium ring bands around 1635 cm −1 [24] and can be well distinguished in the neat [BMIM] [Cl].
The imidazolium ring CH 3 -HCH asymmetric stretching vibration at 3150 cm −1 and the imidazolium (N)CH 2 -HCH symmetric stretching vibration at 3090 cm −1 [25]; as well as aliphatic methylene group vibrations at 2962 and 2873 cm −1 [24,26], are present in the neat ILs and in the composites with high IL concentration. After the ILs extraction these bands disappear. The in-plane C-C and C-N stretching vibrations of the imidazolium ring around 1570 cm −1 [27,28] are visible in the neat ILs and in the xerogels with high IL content. In the extracted samples of the BF 4 series this band is still visible, showing proving that [BMIM][BF 4 ] was not fully removed in these samples.     The ring CH 3 -CN stretching vibration [25], C-C vibration [29,30] and due to asymmetric C-H bond stretching vibration [31] of the alkyl chain bands are around 1463 cm −1 (from literature, found at 1476 cm −1 ) [27]. The band is well visible in the neat ILs and in the xerogels with increased ILs concentration, disappearing in the samples where the ILs has been extracted.
The in-plane C-H deformation vibration of imidazolium ring [27], around 1167-1170 cm −1 , is superposed with the silica matrix bands and can be seen separately only in the xerogels with high [BMIM][Cl] concentration. In the IM-0.5-Cl sample this band is even stronger than the characteristic band of the silica matrix at 1084 cm −1 [32].
One of the characteristics bands of BF 4 group at 1135 cm −1 [27] is superposed with the broad band of the silica matrix at 1084 cm −1 and could be distinguished only in the neat IL at 1038 cm −1 .
At 848 cm −1 is observable the characteristic band due to C-N stretching vibration [24] and B-F symmetric stretching vibration [25] only in the neat [BMIM] [BF 4 ].
Between 758-808 cm −1 are observable in all the samples, the characteristic band due to C-C stretching vibration [32].
Only in the neat [BMIM] [BF 4 ] IL and in the xerogels synthetized with it, the characteristic band due to BF 4 symmetric stretching vibration [25,27] is observable around 750 cm −1 . In the xerogel with the highest IL concentration, the band at 525 cm −1 is from the BF 4 scissoring vibrations [25]. The characteristic band for ring HC-CH asymmetric bending [25] vibration around 695 cm −1 are observable only in the neat ILs. The ones for ring bending [25] vibration around 650 cm −1 and 620 cm −1 are observable not only in the neat ILs, but also in the xerogels containing ILs in higher concentrations (the band around 650 cm −1 , only for the xerogels containing [BMIM][BF 4 ]).  The Si-OH bond stretching [32]

Nitrogen Porosimetry
The xerogel composite samples containing the ionic liquids were essentially nonporous or had closed pores, resulting in non-measurable porosity by nitrogen sorption. The xerogels with after IL extraction were further analysed and their porosity evaluated.
The nitrogen sorption isotherms and the pore size distribution for the samples synthetized with [BMIM] [BF 4 ] are shown in Figure 4. In the case of samples synthetized with [BMIM][Cl], the results indicated that all pores were closed, and the surface area was close to 0.

Nitrogen Porosimetry
The xerogel composite samples containing the ionic liquids were essentially rous or had closed pores, resulting in non-measurable porosity by nitrogen sorp xerogels with after IL extraction were further analysed and their porosity evaluate The nitrogen sorption isotherms and the pore size distribution for the sam  The textural parameters are collected in Table 3. In case of blank sample, IM isotherms are type IVa with H4 hysteresis type, meaning that the sample pres microporosity beside mesoporosity. When [BMIM][BF4] was added, a type IVa  The textural parameters are collected in Table 3. In case of blank sample, IM-0-e, the isotherms are type IVa with H4 hysteresis type, meaning that the sample presents also microporosity beside mesoporosity. When [BMIM] [BF 4 ] was added, a type IVa isotherm with a H2b hysteresis has been obtained in all cases. This type of hysteresis is specific for samples with broad pore size distribution. By analysing the data, it was observed that both, the surface area and rugosity are decreasing with the increase in [BMIM] [BF 4 ] concentration.
In case of blank sample, IM-0-e, microporosity constitutes 70 % (193 m 2 /g) from the total specific surface area of 273 m 2 /g. In the samples synthetised with [BMIM][BF 4 ], mesoporosity only has been observed and the specific surface area decreased with increasing the IL amount). At the same time, the total pore volume was four times higher than that in silica for the intermediate [ 4 ] remained attached to the aerogel network after the attempt to remove it via thermal treatment, thermal analysis demonstrating that around of 6% still remained entrapped into the silica matrix [34,35].

Thermal Analysis
Thermal gravimetric curves are shown in Figure 5. The first weight loss step below 150 • C shows the evaporation of the water. The water content in the pure silica material IM-0 is much higher than in the more hydrophobic silica-IL composites. The water evaporation is much slower in the composites, and it continues until approximately 250-350 • C, for the different samples. Above 250 • C, a different behavior is observed between the samples of the two series. For all three samples in the IM-BF4 series, a sharp onset of a large mass loss is seen at 300 • C, followed by a second smaller mass loss starting at 500 • C. For the IM-Cl series, only one mass loss process is observed, which is broader than that in the IM-BF4 series, and starts at lowest temperature 230 • C for IM-Cl-0.1, and at higher temperature of 330 • C for IM-Cl-0.5. The organic groups of the Ils degrade and burn out in this step, furthermore no weight change occurs above 400 • C.
Comparing the observed behaviors, it can be said that the BF 4 -containing samples contain less water, that evaporate below 150 • C, and they are stable until higher temperature than the other series. Further on, the degradation and elimination of the BF 4 group is responsible for the second mass loss region between 500 and 650 • C.
Concerning the thermal stability of the composite xerogels, by increasing the content of the more hydrophilic

SANS Analysis
Small-angle neutron scattering curves of the nanocomposites are shown in Figure 6. The scattering data show the characteristic features of dry xerogels, which usually display a heterogeneous structure in the size range 1-50 nm, resulting from the non-uniform arrangement of the silica phase and the interparticle voids [36][37][38]. The shapes of the scattering curves for the two series of IL-silica composites are markedly different: samples containing [BMIM][BF4] show a high intensity plateau in the momentum transfer (Q) range 0.02-0.1 Å −1 indicating strongly heterogeneous structure with characteristic dimensions of 5-10 nm. The scattering intensities for the blank xerogel and samples prepared with [BMIM] [Cl] in this Q range are by 1-2 orders of magnitude weaker, and only at the lowest Q the upturn of the intensity curve is seen, corresponding aggregates with size larger than 30 nm. The measured data have been analyzed by the empirical model function proposed by Beaucage [39], which provides the domain size, characterized by radius of gyration Rg, and the power law exponent P that characterizes the interface roughness between the constituent phases or particles: Parameter values obtained by least squares fitting are collected in Table 4. The power law exponents for the two lower IL content materials are close to −4, indicating sharp interfaces according to Porod law, between the silica and the IL or water phases, whereas in the highest IL content sample IM-BF4-0.5 this value is reduced to −3.1, corresponding to a rougher interface. The similar smooth character of the interface in dry [BMIM][BF4]-silica composite aerogels was also found by small-angle X-ray scattering by Wu and Lin [38]. The strong difference in the nanostruture of the composite gels prepared by using the two

SANS Analysis
Small-angle neutron scattering curves of the nanocomposites are shown in Figure 6. The scattering data show the characteristic features of dry xerogels, which usually display a heterogeneous structure in the size range 1-50 nm, resulting from the non-uniform arrangement of the silica phase and the interparticle voids [36][37][38]. The shapes of the scattering curves for the two series of IL-silica composites are markedly different: samples containing [BMIM][BF 4 ] show a high intensity plateau in the momentum transfer (Q) range 0.02-0.1 Å −1 indicating strongly heterogeneous structure with characteristic dimensions of 5-10 nm. The scattering intensities for the blank xerogel and samples prepared with [BMIM][Cl] in this Q range are by 1-2 orders of magnitude weaker, and only at the lowest Q the upturn of the intensity curve is seen, corresponding aggregates with size larger than 30 nm. The measured data have been analyzed by the empirical model function proposed by Beaucage [39], which provides the domain size, characterized by radius of gyration Rg, and the power law exponent P that characterizes the interface roughness between the constituent phases or particles: Parameter values obtained by least squares fitting are collected in Table 4. The power law exponents for the two lower IL content materials are close to −4, indicating sharp interfaces according to Porod law, between the silica and the IL or water phases, whereas in the highest IL content sample IM-BF4-0.5 this value is reduced to −3.1, corresponding to a rougher interface. The similar smooth character of the interface in dry [BMIM][BF 4 ]-silica composite aerogels was also found by small-angle X-ray scattering by Wu and Lin [38]. The strong difference in the nanostruture of the composite gels prepared by using the two ILs can be understood considering the self-associtation features of the various ILs in their aqueous solutions. The hydrophilic Cl − anion in [BMIM][Cl] favours a homogeneous distribution of the cation and anion in their aqueous solutions [40], whereas the less hydrophilic, and moderately hydrophobic BF 4 − anion [41] [14,42,43]. This type of microheterogeneity is observable by small-angle neutron scattering, and is characterized by domain sizes around 1-2 nm at room temperature. The heterogeneity in aqueous IL solutions is strongly temperature dependent [43], and can also be affected by the presence of cosolvents, including TEOS and its hydrolysis products. Thus, addition of TEOS to solutions of tri-block copolymers results in change of the micelle shape from spherical to elongated [44]. In the sol-gel synthesis mixtures the heterogeneous nature of the mixed solvent leads to different distribution of the silica precursors in the water-rich and IL-rich domains, causing therefore different dynamics of the reactions and resulting, in turn, in different nanostructures of the composites made with the two kinds ILs. neutron scattering, and is characterized by domain sizes around 1-2 nm at room temperature. The heterogeneity in aqueous IL solutions is strongly temperature dependent [43] and can also be affected by the presence of cosolvents, including TEOS and its hydrolysis products. Thus, addition of TEOS to solutions of tri-block copolymers results in change of the micelle shape from spherical to elongated [44]. In the sol-gel synthesis mixtures the heterogeneous nature of the mixed solvent leads to different distribution of the silica precursors in the water-rich and IL-rich domains, causing therefore different dynamics of the reactions and resulting, in turn, in different nanostructures of the composites made with the two kinds ILs.

Scanning Electron Microscopy and EDS Analysis
Morphology of all prepared materials has been characterized by scanning electron microscopy. Images of samples from both series with the highest IL content are shown in Figure 7. Materials appeared as solid non-porous monolith particles with smooth surfaces with sizes mainly between 10 and 100 µm. No discernible difference could be seen be-

Scanning Electron Microscopy and EDS Analysis
Morphology of all prepared materials has been characterized by scanning electron microscopy. Images of samples from both series with the highest IL content are shown in Figure 7. Materials appeared as solid non-porous monolith particles with smooth surfaces with sizes mainly between 10 and 100 µm.

Conclusions
Based on their unique characteristics with a high flexibility in varying the cations and the anions, the ILs have been generally named as designer solvents. In the present study Ils with imidazolium cation and different anions, tetrafluoroborate chloride, were used for preparation of composite xerogels. The FT-IR and EDX analyses proved the entrapment of the two Ils into the silica matrix, the majority of the vibration bands being stronger with the increase in ILs concentration. The xerogel synthetised without IL present mesoporosity and microporosity as well. Different characteristic dimensions of the mesoscopic structure have been obtained for the two types of Ils, and attributed to the different character of structuring in the aqueous solutions of the two IL, which remains also in the mixtures with the silica precursor.

Conclusions
Based on their unique characteristics with a high flexibility in varying the cations and the anions, the ILs have been generally named as designer solvents. In the present study, Ils with imidazolium cation and different anions, tetrafluoroborate chloride, were used for preparation of composite xerogels. The FT-IR and EDX analyses proved the entrapment of the two Ils into the silica matrix, the majority of the vibration bands being stronger with the increase in ILs concentration. The xerogel synthetised without IL present mesoporosity and microporosity as well. Different characteristic dimensions of the mesoscopic structure have been obtained for the two types of Ils, and attributed to the different character of structuring in the aqueous solutions of the two IL, which remains also in the mixtures with the silica precursor.  4 ], containing samples were gelled from 3 h up to the next day, therefore in less than 24 h; the blank sample and the [BMIM][Cl] containing samples, gelled in four days. The xerogel samples were obtained after drying (12 h) at 100 • C (for the IM-Cl-0.5 sample another 6 h of drying were necessary). For comparison purposes, blank sample was prepared using the same procedure, but without ionic liquid (sample IM-0); instead, an equivalent quantity of water was added (6.25 mL H 2 O). The imidazolium ionic liquids were extracted from the xerogels samples by using the method of extraction with a solvent. For 1 g of xerogel material, around 25 mL ethanol was added. The extraction was realized with 5 h of stirring, and then the solutions were left overnight in ethanol. In the next day, the operation was repeated with a new ethanol washing solution, therefore, another 5 h of stirring and then, left overnight for complete filtration on the filter paper and drying at room temperature. In the third day, the samples were dried at 40 • C for one hour and at 100 • C, for 2 h. The efficiency of the extraction method was checked by IR analysis, taken before and after the extraction.

Characterization Methods
FT-IR spectra were recorded in KBr pellets using a JASCO FT/IR-4200 apparatus. Samples for transmittance measurements were prepared typically by mixing 3 mg of sample with 600 mg of KCl. The powdered samples were pelletized by applying 7.5 tons for several minutes under vacuum of several mm Hg.
SANS measurements were performed on the Yellow Submarine instrument at BNC in Budapest (Hungary) [45]. Collimation distance of 5 m and circular beam apertures of 25 and 7 mm in diameter defined the incoming beam divergence, whereas the sample for detector distances of 1.2 and 5.5 m and mean neutron wavelength of 4.1 Å defined the momentum transfer range of 0.015-0.42 Å −1 . The scattered neutrons were detected by a two-dimensional position-sensitive BF 3 gas detector (64 × 64 cells of 1 cm × 1 cm area). The raw data were corrected for sample transmissions and scattering of the empty cell, and converted to absolute units by comparison with the incoherent scattering of a 1 mm thick water sample. The powdered samples were filled into Hellma quartz cells of 2 mm flight path, and the measurements were conducted at room temperature. Nitrogen sorption was measured at 77K with a QuantaChrome Nova 2000-e instrument (Boynton Beach, Florida, United States). Before measurements, each sample was outgassed at room temperature for 6 h. The NovaWin software was used to evaluate the isotherms. The specific surface area was determined by the Brunauer-Emmett-Teller (BET) method in the relative pressure (P/P 0 ) range 0.01-0.25. The surface area of micropores was determined in the 0.15-0.50 P/P 0 interval using V-t method. The pore size distribution was obtained using the BJH (Barrett-Joyner-Halenda) method and density functional theory method (DFT). The total pore volumes were determined using the point closest to 1 for the relative pressure P/P 0 . Surface fractal dimension was determined by FHH (Frenkel-Halsey-Hill) method from the adsorption branch.
Thermal measurements were performed on a Setaram LabsysEvo (Lyon, France) TG-DSC system, in flowing (90 mL/min) high purity (99.999%) synthetic air (20% O 2 in N 2 ) atmosphere. Samples were weighed into 100 µL Al 2 O 3 crucibles (the reference cell was empty) and were heated from 25 • C to 800 • C with a heating rate of 10 • C/min. The obtained data was blank corrected and further processed with the thermoanalyzer's processing software (Calisto Processing, ver. 2.08, Wheaton, IL, USA).
The morphology and surface of the prepared materials have been analyzed by scanning electron microscopy using a LEO 1540XB system (Carl Zeiss, Jena, Germany) equipped with an Ultimax 40 energy dispersive spectroscopy (EDS) Si drift detector (Oxford Instruments, Abingdon, UK). SEM images were recorded at different magnifications up to 20,000× using 2.0 kV accelerating voltage. During EDS analysis an accelerating voltage of 5 kV was used.