Article pubs.acs.org/JPCC
Self-Organization of a Hydrophilic Short-Chain Ionic Liquid Confined within a Hydrophobic Nanopore Ching-Mao Wu,*,† Szu-Yin Lin,† Kuei-Yu Kao,‡ and Hsin-Lung Chen‡ †
Material and Chemical Research Laboratories, Industrial Technology Research Institute, Chutung, Hsin-Chu, 31040, Taiwan, R.O.C. Department of Chemical Engineering, National Tsing Hua University, Hsin-Chu 30013, Taiwan, R.O.C.
‡
ABSTRACT: Ionogel refers to the nanocomposite material composed of ionic liquid (IL) entrapped within the solid matrix. The performance of ionogel was predominantly dependent on whether the ILs within the pore of the matrix formed ordering or not. Thus far, such ordering information was largely provided via simulating the arrangement of ILs confined within nanopores with ordered geometry. However, experimental evidence was still lacking. Herein, we studied the ordering of a short-chain IL (1-butyl-3-methylimidazolium tertrafluoroborate) confined within the two distinct nanopores (radius-various H2 pore and slit-shaped H3 pore) of the methylated silica matrix with a fractal network by small-angle X-ray scattering. The results obviously indicated that these confined IL molecules indeed formed a self-organized ordering in the H3 nanopores, as is directly observed a correlation peak in the scattering patterns. We speculated on the plausible formation mechanism of this ordering in H3 nanopores according to “the effects of broad pore sizes and rough surface topography on the pore wall”.
1. INTRODUCTION Extremely fast developments in investigating ionic liquids (denoted by ILs) are due to their unique properties:1−6 (1) ILs consist of bulky, asymmetrical ionic pairs that are self-associated without the presence of solvent, and thus being categorized into organic salts;1,2,6 (2) ILs definitely contain a melting temperature below 100 °C, and they are further grouped as room temperature ionic liquids (denoted by RTILs) when liquidus lines of ILs are near or below room temperature;1,3 and (3) ILs involve a negligible vapor pressure at room temperature, excellent thermal stability, high ionic conductivity, and wide electrochemical windows. These special properties lead ILs to be widely applicable as green solvents for chemical synthesis, catalysis, and separations,1−6 except that ILs are also used as electrolytes for electrochemistry, photovoltaics, and other fields.5,7−13 Moreover, a novel hybrid material called “ionogel” is thus investigated via immobilizing ILs within a polymeric or inorganic matrix,5,14−38 which was first developed for entrapping a short-chain IL into a silica matrix to prepare a monolithic porous gel.15,16,18−20 Briefly, the ionogel material can be easily manufactured through a typical sol−gel process, which consists of the hydrolysis and condensation of a metal oxide precursor to form a sol in an aqueous solution involving a base or acid catalyst and ionic liquids, and then is followed by gelation of the sol and aging at ambient temperature. Finally, a monolithic gel thus formed is an “ionogel”. We can also use solvent extraction and subsequent drying to remove the entrapped ionic liquids from ionogels to prepare dry porous gels. Owing to these ionogels involving binary functions of the confined ILs and matrices, they have extensive applications © 2014 American Chemical Society
such as making aerogels/xerogels, supercapacitors, fuel cells, drug deliveries, biosensors, catalysis, and miscellaneous applications, as reported by a recent review paper.6 Despite so many studies regarding the ionogels having been reported, what remains unclear is their dependence on gel structure and behaviors of ILs confined in the matrix on macroscopic performance, accordingly attracting many concerns about the studies on ILs confined within the nanopores with different pore geometries.39−46 These studies mainly focus on the interplays between ion pairs of ILs and pore wall, physicochemical properties, the dynamics, relaxation times, etc. In such investigated ionogels, silica ionogels are most extensively explored as a mainstream system based on recent reports.15,16,20,21,24,33,34,39,41−46 Several results have been presented: (1) First, the confined ILs would affect the resultant 3D gel structures.15,16,20,24 Two typical mesoporous silica frameworks have been revealed via small-angle X-ray scattering (SAXS) and electron microscope methods with short-chain ionic liquids. One is a fractal network constructed of constituent silica particles, which was first revealed by Dai et al., who prepared a silica aerogel monolith using an ionic liquid, 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfony] amide, as the solvent and tetramethyl orthosilicate (TMOS) as the sol−gel precursor via a nonhydrolytic formic acid solvolytic method.15 The other is mesoporous silica with an ordered wormlike pore. That was verified by Zhou et al., who Received: May 21, 2014 Revised: July 8, 2014 Published: July 17, 2014 17764
dx.doi.org/10.1021/jp504984x | J. Phys. Chem. C 2014, 118, 17764−17772
The Journal of Physical Chemistry C
Article
prepared a monolithic mesoporous silica using 1-butyl-3methylimidazolium tetrafluoroborate ([BMIM][BF4]) as a template and solvent through a nanocasting hydrolytic sol− gel method.16 The prepared monolithic silica was composed of an ordered, wormlike pore framework, which was further proposed to be formed by a hydrogen bond-co-π−π stack mechanism.16 (2) The versatile phase behaviors of the confined ILs within nanopores with different pore geometries are presented. These results are mostly based on the close interplay between computational simulation (like MD techniques) and experiments of short-chain ionic liquids confined in the various ordered geometry-shape nanopores. The main results suggest that the IL molecules near the pore wall usually have an orientation parallel to the surface of the wall, but the ones located in the center region will be randomly orientated or layered. Further, a higher density also appears near the wall surface than that in the center region.33,34,39,41−46 (3) Confined ILs also possibly form distinct pore morphologies (such as open or closed pores with various radiuses and open slit-shaped capillary pores, etc.) within the hydrophilic/hydrophobic matrices, as evidenced by the N2 sorption method.21 On the basis of the above, the fundamental understanding of confined ILs in the hosted matrix is extremely important to the resultant structure-performance relationship of ionogels or its porous gels after removing ILs. Recent studies also reported the effects of hydrolytic and nonhydrolytic IL-based sol−gel routes on resultant silicas, as studied by Viau et al.21 They synthesized porous silicas with a series of short-chain RTILs with various hydrophobic to hydrophilic anions but with the same cationic part like [BMIM]+ based on the hydrolytic and nonhydrolytic formic acid sol−gel processes. Interestingly, through using N2 sorption, they measured the typical Type IV isotherms with characteristic type H2 and H3 hysteresis loops despite the hydrolytic or nonhydrolytic methods used.21 Based on IUPAC, the H2 loop usually signals the formation of open or closed pores with various radiuses; however, H3 means the formation of open slit-shaped capillary pores (as schematically shown in Scheme 1, we will denote radius-various pore as H2 pore and slit-shaped pore as H3 pore hereafter).47,48 Such isotherms emerge as a scientific interest to explore what kind of H2 or H3 pores are probably formed through using short-chain ILs as solvents. In addition, this information also provides a possible clue to explore the behaviors of the hydrophilic ILs confined within hydrophobic nanopores with distinct pore geometries. In our previous report, we had studied the structure of silica ionogel and aerogel using [BMIM][BF4] as solvent and TEOS as precursor via a HCl-catalyzed sol−gel method. A stable fractal sol−gel structure with a smooth surface of the pore wall was examined by SAXS.24 Likewise, a type-H2 hysteresis loop on a Type IV isotherm was also measured. However, when bare porous silica materials are subject to the atmospheric environment, these silicas containing moisture-sensitive silanol polar groups will deteriorate its performance with time because capillary stress arising from water absorption will cause the collapse of the sol−gel network structure.49,50 Thus, a proper surface modification by using hydrophobic groups to replace the H from Si−OH is necessary to diminish the water absorption.49,50 However, such a surface modification would strongly influence the formed gel network structures and pore morphologies of porous silicas. The specific area would also decrease with increasing the amount of the added modified agents. Although the dependence of the content of modified agents on the resultant gel structure has been extensively
Scheme 1. A Proposed Scheme Illustrating the Arrangement of the Ionic Liquid [BMIM][BF4] Confined within Type H2 and H3 Nanoporesa
a
Both nanopores are entrapped within the fractal silica matrix and there is hydrophobic methylated rough surface on the pore wall. In this sense, the IL molecules near the wall surface would orient along the rough methylated surface subject to the surface topography. Except for the effect of surface topography, the Type H3 pore indeed provides a broader pore size (2−30 nm) to allow the [BMIM][BF4] molecules in the middle region to reorient along the long-axial direction of the pore so that an ordered arrangement is formed, and thus we can observe a correlation peak in the SAXS profile.
explored in the organic-solvent-based sol−gel route, such relationships do not directly extend to IL-based sol−gel systems and need to be reprobed. In this present paper, the effects of the ionic liquid, [BMIM][BF4], confined in the hydrophobically functionalized silica matrix on resultant structures of ionogels and dry porous gels (after removing ILs from ionogels) are explored. In our studied system, TEOS and methyltrimethoxysilane (MTMS) were used as coprecursors. The HCl-catalyzed sol−gel process was adopted, in which solvent extraction and subsequent vacuum drying were selected to remove the IL. A series of silica gels were prepared by fixing stoichiometric IL-to-TEOS molar ratio (i.e., nIL/nTEOS = 1) and mediating various MTMS-toTEOS molar ratios (denoted by nMTMS/nTEOS). The effects of nMTMS/nTEOS ratios on gel structures and pore morphologies were explored through SAXS and N2 sorption methods. It will be shown that [BMIM][BF4] molecules confined within hydrophobic slit-like nanopores would self-organize into an ordering arrangement, as observed by a correlation peak in the scattering profile. To our best knowledge, this finding is the first direct observation of self-ordering of short-chain ILs confined in the fractal silica framework.
2. EXPERIMENTAL SECTION 2.1. Materials and Preparations of MTMS-Based Ionogels and Porous Gels. The used materials and preparations are briefly described here. Tetraethyl orthosilicate (TEOS) and the ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), were acquired from Aldrich. Methyltrimethoxysilane (MTMS) was purchased from Acros Organics. These materials were directly utilized without further purification. In this work, the stoichiometric IL-to-TEOS molar ratio (i.e., nIL/nTEOS = 1.0) was selected and MTMS-to-TEOS molar ratios were set as nMTMS/nTEOS = 0.1−0.5. For the preparation of ionogel with nMTMS/nTEOS = 0.1, the prescribed 17765
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amount of IL (6.7107 g) was first mixed with 0.01 M HCl (3.525 g) under mild magnetic stirring. Subsequently, the mixed solution of TEOS (6.1224 g) and MTMS (0.4044 g) was added dropwise at room temperature. After the solution was magnetically stirred for hours, a transparent sol was obtained. The sol was left in an open flask at room temperature for at least 7 days for gelation and an additional 7 days for aging. Then, the embedded ionic liquid was removed from the silica ionogel by extracting the sample with methanol in an extraction and stripping apparatus at 65 °C for extra days. Finally, the monolith of silica gel was dried under a high vacuum and the porous silica gel was obtained. The same processes were also used to prepare the other specimens with different nMTMS/nTEOS ratios. The ionogel and porous gels manufactured here were further ground into powders for the subsequent characterizations. 2.2. Characterizations. The gel structure of the synthesized silica was also probed by a Bruker Nanostar SAXS instrument at room temperature. The SAXS apparatus consisted of a Kristalloflex K760 1.5 kW X-ray generator (operated at 40 kV and 35 mA) and cross-coupled Göbel mirrors for Cu Kα radiation (λ = 1.54 Å), which resulted in a parallel beam of approximately 0.05 mm2 in a cross section at the sample position. The scattering intensity was detected by a Siemens multiwire-type area detector with a 1024 × 1024 resolution mode. The ground silica powders were introduced into the sample cells composed of two Kapton windows for SAXS measurements. All the intensity data were corrected for empty beam scattering, background, and the sensitivity of each pixel on the area detector. The SAXS intensity was output as a function of the scattering vector, q = (4π/λ) sin(θ/2) (θ = scattering angle). The surface topography images were obtained with scanning electron microscopy (SEM), using a Hitachi S-4200 fieldemission instrument with 15 kV. For sample preparation, the ground silica powder was directly mounted on a stub of metal with copper conductive tape, sputter-coated with a thin layer of gold, and then observed in the microscope. Transmission electron microscopy (TEM) was utilized to examine the real-space morphology. The samples were prepared by diluting the ground silica powder with ethanol under an ultrasonic environment. Then, a droplet of the suspension was deposited onto a copper grid and dried for 1 min before use. Subsequently, the ultrathin samples were examined by a JEOL JEM-2100F field-emission TEM operated at 200 kV. The nitrogen sorption of the porous gel was obtained by a gas adsorption analyzer (Micromeritics ASAP 2010 V5.02 H). Prior to degassing, the samples were pretreated by an isothermal annealing at 90 °C for 1 h, followed by heating to 200 °C. Finally, the samples were degassed at 200 °C under 2 μmHg pressure until they were completely free of moisture and adsorbed gases. For the characterization of siloxane architecture of the prepared porous gel, a 29Si magic angle spinning nuclear magnetic resonance measurement was carried out on a Bruker Angle III 400 NMR spectrometer. Thermogravimetric analyses (TGA) of the prepared silica samples were performed with a TA Instruments Q500 highresolution TGA operating at a heating rate of 10 °C/min from room temperature to 700 °C in a high-purity nitrogen atmosphere. Approximately 10−40 mg of the finely ground sample placed in an open platinum crucible was used.
3. RESULTS AND DISCUSSION 3.1. Gel Structure. 3.1.1. Ionogel Structure. Figure 1a shows the SAXS profiles of the obtained silica ionogels. Three
Figure 1. SAXS profiles of the synthesized silica ionogels as a function of MTMS-to-TEOS molar ratios, in which the scattering patterns were acquired with different q ranges of (a) 0.1−3.5 and (b) 1.0−9.0 nm−1, respectively.
distinct profiles were measured with various nMTMS/nTEOS ratios from 0.0 to 0.5. In the first type, the intensity with nMTMS/nTEOS = 0.1 exhibits two distinct power law regions on the respective scale, in which the intensity in the low-q region (i.e., “massfractal region”) depicts an asymptotic scattering power law of I ≈ q−Dm, with Dm being the mass fractal dimension of the network; on the other hand, the one in the high-q region described as the “surface-fractal region” follows an asymptotic scattering power law of I ≈ q−(6−Ds) with Ds being a surface fractal dimension. Such behaviors of power law regions can be reasonably interpreted in terms of the fractal structure, where an open network with a branched appearance is formed by agglomeration of the clusters. These clusters are formed by the aggregation of the primary silica particles as the monomer. Dm 17766
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by SEM. Figure 2 displays the representative SEM images of the dry gels synthesized with nMTMS/nTEOS = 0.2 (a) and 0.4
is a number between 1 and 3, referring to the fractal openness. The smaller value of Dm signifies a more open structure. Ds always lies in the range of 2 ≤ Ds ≤ 3 and can be used to examine the surface roughness of the constitute monomer, in which Ds = 2 signals that the primary silica particles are spherical in shape with a smooth surface, and Ds = 3 means that the primary silica particles are nonspherical in shape with a rough surface.51−57 As described above, we analyzed the SAXS profiles in Figure 1a and gathered its scattering parameters in Table 1. It is found Table 1. Structural Parameters Obtained from SAXS Profiles ionogel
dry porous gel
nMTMS/ nTEOS
Dm
Ds
qc (nm−1)
Rg,c (nm)
0.1 0.2 0.3 0.4 0.5
1.2 1.7 2.0 2.2 2.5
3.0
0.511
1.96
Dm
Ds
qc (nm−1)
Rg,c (nm)
1.1 1.9 2.5
2.5 2.7 2.8 2.7 2.5
0.516 0.451 0.408 0.340 0.340
1.94 2.22 2.45 2.94 2.94
that the scattering intensity with nMTMS/nTEOS = 0.1 in the highq surface-fractal region (i.e., q > 0.511 nm−1) exhibits a slope of −3.0 (i.e., Ds = 3.0), indicating an extremely rough interface between IL and the silica pore wall. On the other hand, the intensity obtained from the low-q mass-fractal region (i.e., q < 0.511 nm−1) presents the Dm of 1.2, thus attributing to the formation of the open linear fractal structure. The size of the primary particle could further be calculated via the crossover between the intensities of two power law regions, as denoted by the scattering vector qc in the figure, which corresponds to the reciprocal of the radius of gyration Rg,c of the constitute primary particle (qc = 1/Rg,c). In the case with nMTMS/nTEOS = 0.1, the Rg,c is 1.96 nm. The second type of profile was measured from ionogels with 0.2 ≤ nMTMS/nTEOS ≤ 0.4, these three intensities only indicate the single mass-fractal power law regions with Dm from 1.7 to 2.2. This reflects that the openness of the fractal structure is obviously reduced with increasing MTMS amounts. A compact fractal structure would be yielded at a higher MTMS-to-TEOS ratio. Besides, it is worth noting that the ionogel with nMTMS/ nTEOS = 0.5 displays the third type of profile, which still could be ascribed as a fractal structure at low-q range, but some information disclosed at high-q range is not clear. Subject to the limited q-range, we could not do any further analyze here and a wider q range was necessary. Figure 1b shows the remeasured SAXS patterns acquired from a much broader q-range of 1.0− 9.0 nm−1. By contrast, the profile with nMTMS/nTEOS = 0.5 indeed exhibits a correlation peak positioned at qm = 2.80 nm−1, which corresponds to a characteristic distance (=2π/qm) of 2.24 nm. Such a length scale is smaller than the size of the primary silica particle (Rg,c = 1.96 nm), and thus is not assigned to the contribution from the correlation of ordering arrangement among silica walls. The most plausible reason originates from short-ranged inter-IL interactions within the pores. Subject to unfavorable interactions between the hydrophilic IL layer and the hydrophobic MTMS-based silica wall, these IL molecules entrapped in the nanopores of the silica matrix might selforganize into a short-ranged ordered arrangement, thereby giving rise to a correlation peak in the scattering profile. 3.1.2. Dry Porous Gel Structure. The surface morphologies of the prepared dry MTMS-based porous gels were examined
Figure 2. SEM images showing the real-space morphologies of the dry silica gels synthesized with nMTMS/nTEOS = 0.2 (a) and 0.4 (b), respectively. The corresponding TEM micrograph of nMTMS/nTEOS = 0.4 is shown in panel c.
(b), respectively. Both images display the close packing among these island-like nanoparticles, signaling that these MTMSmodified silica gels form the framework with a branched and self-similar appearance, as further verified by the TEM image with nMTMS/nTEOS = 0.4 (Figure 2c). The structural details were characterized by SAXS. Figure 3 shows the SAXS profiles of these dry gels as a function of various nMTMS/nTEOS ratios. In comparison with Figure 1a, all the intensities also present two distinct power law regions on the respective scale. It can be found that all the scattering patterns in the high-q region (i.e., q > 0.340−0.516 nm−1) express a slope between −3.5 and −3.2 (thus Ds = 2.5−2.8), indicating a much rougher surface existing on the surface of the resulting silica walls. On the other hand, with increasing nMTMS/ nTEOS molar ratios, the intensities obtained from q < 0.52 nm−1 were categorized into two regimes. In regime I, the intensities measured from nMTMS/nTEOS ≤ 0.2 present the Dm of
