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Revealing the pore characteristics and physicochemical properties of silica ionogels based on different sol-gel drying strategies
Journal of Solid State Chemistry 278 (2019) 120877
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Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Revealing the pore characteristics and physicochemical properties of silica ionogels based on different sol-gel drying strategies Selay Sert Çok, Fatos¸ Koç, Firuz Balkan, Nilay Gizli * _ Ege University, Chemical Engineering Department, Izmir, Turkey
A R T I C L E I N F O
A B S T R A C T
Keywords: Silica ionogel Ionic liquid Supercritical drying Freeze drying Ambient pressure drying
In this study, an exhaustive investigation about the effect of drying methods on the physical properties and microstructures of silica ionogels was conducted. Silica ionogels were synthesized by following one-step sol-gel process and dried either by supercritical, ambient pressure or freeze-drying techniques. After drying periods, characterization studies of the ionogels were carried out by performing FTIR, SEM, BET and TGA analyses. Results have shown that, although they all appeared in a monolithic structure, their microstructural pattern were completely different. Supercritically dried silica ionogel resulted in mesoporous structure with superior properties such as low density (ρ ¼ 0.21 kg/m3), high porosity (ϕ ¼ 90%) and high surface area (646 m2/g). On the other hand, silica ionogel dried in ambient conditions comprised both micro-meso pores and the pores are mainly closed form and freeze dried ionogel had non-homogeneous pore distribution with relatively larger pores.
1. Introduction Ionic liquids can simply be described as organic salts that usually have melting points below 370 K. They are composed of a large organic cation group such as ammonium, phosphonium, pyridinium, imidazolium, etc. and inorganic/organic anion group. Usual it is claimed that ionic liquids are classified as green chemicals due to their negligible vapor pressure, chemical stability and non-flammability. Also, most of them have thermal stability at temperatures higher than 570 K and has large electrochemical window and ionic conductivity [1]. Apart from these superior properties, ionic liquid family attracted great attention of tuning their characteristics due to their wide range of anion-cation combinations and applied in diverse applications. Currently, they are successfully used in electrochemical devices such as batteries, fuel cells, super capacitors, electrochemical solar cells or in electrochromic devices. However, problems such as, packaging, leakage and portability arising from their liquid nature restrict their possible range of applications [2]. To overcome these drawbacks, the immobilization of ionic liquids within organic or inorganic matrices and taking advantage of their excellent properties in the solid state has become a focus of attention nowadays [3–7]. One way to accomplish this is to immobilize the ionic liquid in the form of three dimensional porous structure and to have a solid-like behavior [8].This new class of materials are termed as ionogels and has been found
to be suitable for many device and applications such as optical solvents, sensors, biosensors, catalysis, biocatalysts [2]. Sol-gel method is a frequently used efficient way to produce porous ionogels. Traditional sol-gel method involves the removal of liquid phase and remaining the porous solid network after drying stage (xerogel, cryogel, aerogel) as a target material. In the case of ionogels, ionic liquids are also incorporated into porous network due to negligible vapor pressure of ionic liquids and this new material combines the physical properties of the two networks (solid oxides and ionic liquid) intermingled at nanometer scale [1]. In general, sol-gel method is about the formation of nanostructured solid network because of subsequent hydrolysis and condensation reactions of a silica precursor and is mainly comprised of two parts: formation of wet gel by sol-gel reactions and drying of the wet gel with various techniques. Gel is obtained from the colloidal solution named as sol. A sol usually includes a silica precursor and a proper solvent. After a while, the sol reaches a gelation point during the simultaneous hydrolysis and condensation reactions catalyzed by acid or base addition. During this period, firstly, the primary particles are formed in matrix and then they aggregate as secondary particles and lastly, they link together in a pearl necklace morphology [9]. Final porous structure of the material can end up with very different morphology and different pore sizes as illustrated in Fig. 1. According to the pore sizes, the material result either in
_ * Corresponding author., Ege University, Chemical Engineering Department, 35100, Bornova, Izmir, Turkey. E-mail address: [email protected] (N. Gizli). https://doi.org/10.1016/j.jssc.2019.07.038 Received 11 June 2019; Received in revised form 10 July 2019; Accepted 21 July 2019 Available online 8 August 2019 0022-4596/© 2019 Elsevier Inc. All rights reserved.
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Journal of Solid State Chemistry 278 (2019) 120877
In freeze drying technique, the solvent trapped within the pores is firstly frozen and then sublimated under very low pressures. During freeze-drying, the solvent firstly crosses the liquid-solid equilibrium curve and then crosses the solid - gas equilibrium curves. Hence, because of crystallization of solvent within the pores, final products can be obtained either in crack or even in powder form with very large pores [10]. Pajonk et al. (1990) stated that this disadvantage could be eliminated by replacing the solvent by the solvent, with the lower thermal expansion coefficient and higher sublimation pressure [11]. At this point, using ionic liquids in sol-gel reactions can be a powerful alternative as some of these liquids have lower expansion coefficients than classical solvents such as ethanol and water at the same conditions [12]. Ambient pressure drying technique is another way to dry the gels. During this method, solvent within the pores need to cross the liquid-gas equilibrium curve and surface tension at the liquid – vapor interface within pores develops a capillary tension in pores walls and results in pore destruction and gel shrinkage. Ionic liquids, in this case, may also play an important role for controlling the gel shrinkage by satisfying relaxation on the pore walls against any shrunk due to their negligible vapor pressure in ambient temperatures. On the other hand, supercritical drying is the first and the most widely used drying method in sol-gel chemistry. In this method, solvents trapped in the pores of wet gels are removed under supercritical conditions (above critical temperature (Tcr) and pressure (Pcr)). At the supercritical condition, since there is no discriminations between the liquid and vapor phases, the capillary stresses are no longer present. Therefore, the one can have well defined porous structure by applying supercritical drying during the preparation of silica ionogels. For the first time, Dai et al. (2000) has prepared ionogels by introducing ionic liquid into porous silica gels by following sol-gel approach. Afterwards, several studies have conducted for the preparation of silica ionogels by following various drying strategies [13]. Meera et al. prepared ionic liquid embedded silica based aerogel by following freeze-drying method, whereas Gupta et al. (2012) synthesized silica
Fig. 1. General porous structure of a silica network.
micro porous (pore diameter smaller than 2 nm), mesoporous (pore diameter is between 2 nm and 50 nm) or macro porous (pore diameter larger than 50 nm) nature [9]. During the sol-gel method, drying stage occupies the biggest interest and great effort during the sol gel process as it is the most critical and challenging step in the determination of final morphology and physical characteristics of the silica gels. During the drying process, residual solvents trapped within the pores of the material are removed leaving only the three dimensionally linked silica structure. Solvent removal without any damage in the solid skeleton and pore structure has vital importance to get the afore mentioned superior properties of silica ionogels. Great portion of pore destruction is usually generated during the crossing of phase equilibrium boundaries because of severe surface tension formed between the phases. There are three main types of drying used in sol gel processes: such as freeze-drying (FD), ambient pressure drying (APD) and supercritical drying (SCD).
Fig. 2. Demonstration of sol preparation steps with related reactions. 2
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Journal of Solid State Chemistry 278 (2019) 120877
Fig. 3. Surface modification reaction of silica ionogels.
Fig. 4. Drying methods.
and dried by using supercritical drying, ambient pressure drying and freeze drying. Differences between the final physical, chemical and morphological properties of the silica ionogels were revealed with the aid of FTIR, SEM, BET and TGA analyses.
ionogels under ambient conditions [14,15]. Karout et al. on the other hand, studied the effect of confinement of ionic liquids on silica aerogels gelation time produced by partial evaporative drying followed by supercritical drying [16]. More recently, Ivanova et al. (2018) used imidazolium based ionic liquids as solvents in sol-gel reactions and prepared silica ionogels by following supercritical CO2 drying and investigated the influence of ionic liquids on gelation time and the solid properties of final ionogels [17]. As far as we know, hardly any study has investigated the effect of different drying methods on the physical and morphological characteristics of a specific silica ionogels and give a brief comparison between drying methods for the same material. In the present study, silica based ionogels are prepared by one-step sol-gel method by using short chain imidazolium based ionic liquid with fluorinated anion group
2. Experimental procedure 2.1. Materials Tetraethylorthosilicate (TEOS, 98%), 3-aminopropyl-triethoxysilane (APTES) were used as sol precursors and purchased from Sigma Aldrich. Ethanol, n-hexane and an imidazolium based ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMIMTF2N) (IL, 98%) 3
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Journal of Solid State Chemistry 278 (2019) 120877
were selected as solvents and were supplied from Sigma Aldrich. HCl was included in sol-gel process as acid catalyst. 3-metacryloxypropyltrimethoxysilane (MEMO) served as sylating agent during surface modification step. Carbon dioxide (CO2) with purity 99.95% used during the SCD step as supercritical fluid and was supplied by HABAS Company. 2.2. Methods The silica ionogels were synthesized by following sol-gel process. Synthesis steps are; sol preparation (Fig. 2), surface modification (Fig. 3), and drying (Fig. 4). Type and amount of the sol components were selected with reference to our previous studies [18,19].
Fig. 5. Physical appearances of the silica ionogels.
Table 1 Physical properties of the samples IG-APD, IG-SCD and IG-FD.
2.2.1. Sol preparation and gel formation During the sol preparation, TEOS and IL were hydrolyzed by using 0.01 M HCl and EtOH for 90 min stirring at 25 C. After stirring, the sol was placed in an ultrasonic medium and APTES was added to the sol to start condensation reaction. The sol component consists of TEOS: APTES: IL: EtOH: HCl with the molar ratios of 1:0.47:0.14:6.3:7.4 105. Hydrolysis and condensation steps were demonstrated in Fig. 2.
2.2.3. Drying The most significant and challenging step to attain the final physical form and morphology of silica ionogels is drying. Each ionogel was dried by applying different methods in this study (Fig. 4). First group of ionogels abbreviated as IG-APD were dried under ambient pressure for three days. Second group of ionogels (IG-SCD) were dried for 3 h by using supercritical CO2 dryer (Leica EM CPD300). During this method, the pressure of the vessel was increased to 76 bar and the temperature was raised up to 32 C so that CO2 transformed into a supercritical fluid. At this state solvent trapped within the pores was replaced with supercritical CO2 in 15 cycles. Then, pressure of the vessel was decreased down to atmospheric conditions at constant temperature by releasing CO2 gas and leaving the pores of the ionogels totally evacuated. Last sample IG-FD was dried under cryogenic conditions by lyophilizator (Christ 1.2 D Alpha Plus). As a first step of the method, the sample was frozen at 20 C overnight and the frozen solvent was sublimated by exposing the vessel to high vacuum for 12 h.
%Porosity ¼
(1)
1 ρb=ρ x100
%Volume shrinkage ¼
(2)
s
1 Va V x100 g
Volume Shrinkage, %
1.4 0.21 0.75
36 90 65
5 18 28
Silica ionogels dried in each type of drying condition appeared in a monolithic structure as shown in Fig. 5. Especially for freeze dried ionogel, it is a very noteworthy outcome since silica based aerogels can be obtained mostly in granular or even in powder form after freeze drying [10,21–23]. In this respect, it should be noticed that the presence of ionic liquid in silica ionogel structure helped the pore walls against collapse during the crystallization of solvent by bringing the pore walls in a more elastic nature. Table 1 shows the physical properties of the samples after drying. Each ionogel exhibited different degree of volumetric shrinkage and hence they ended up with different density values. While supercritical dried ionogel had the lowest volumetric shrinkage and density values, ionogel dried in ambient conditions had the highest volumetric shrinkage and highly dense structure. Identification of chemical structures was analyzed with the aid of FTIR analysis (Fig. 6). According to FTIR results, the absorption peaks around 1080 cm1 can be related with the asymmetric Si–O–Si bonding and peaks around 695 cm1 with the symmetric stretching vibration of Si–O–Si and they are good sign about the successful development of silica network. The terminal Si–CH3 and –CH3 groups observed near 1350 cm1 can affirm the successful replacement of radical groups with hydroxyl groups on silica surfaces. In addition, the absorption peaks around 1750 cm1 can be attributed to aromatic ring of ionic liquid. FTIR
Bulk densities of the silica ionogels obtained by applying different drying periods were measured as mass to volume ratio, then the porosity and volume shrinkage of the samples were determined using the formula below: bulk
Porosity, %
IG-APD IG-SCD IG-FD
3. Results and discussion
2.3. Characterization
Density, g/cm3
where ρb is the bulk density of ionogels and ρs is the skeleton density of the silica ionogels, generally the value of ρs is 2.2 g/cm3 [20]. Va and Vg in Eq. (3) denote the volumes of the samples after drying and before the solvent exchange, respectively. The morphology and the microstructure of silica ionogels were examined by using Scanning Electron Microscope (SEM) (PHILIPS, XL 30S FEG) with magnification rate of 105. In addition, chemical compositions of the ionogels were identified by performing FTIR (PERKIN ELMER, Spectrum 100, ABD) analysis. Surface area, average pore volume and average pore diameters of the silica ionogels were determined by physisorption of N2 at 77 K by using Quanta chrome Corporation, Autosorb-6. BET technique was used to determine surface area of the samples and the average pore size of the samples were obtained with the aid of BJH (Barrett, Joyner, Halenda) technique. Before the measurement, ionogels were subjected to degassing operation at 130 C over night to remove residual moisture and adsorbed gases. To investigate the thermal stability of the samples, thermo-gravimetric analysis (TGA) was carried out by TA Instruments SDT Q600 device that operates at a heating rate of 5 C/min from room temperature to 600 C under a nitrogen environment.
2.2.2. Aging and surface modification After gelation was fully completed, the silica ionogels were subjected to an aging period of 24 h in a polypropylene cylindrical mold for further continuation of the condensation reactions. Afterwards, surface modification was performed by replacing hydroxyl groups exist on the surface of the ionogel with the alkyl groups to reconstruct the surface of the samples and satisfy a hydrophobic structure. Modification step was performed by immersing the ionogels in MEMO and n-hexane mixture with a volumetric ratio of 50% at room temperature for 12h. Surface modification reaction is given as in Fig. 3.
ρb g=cm3 ¼ mbulk=V
Sample ID
(3)
4
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Journal of Solid State Chemistry 278 (2019) 120877
Fig. 6. FTIR Spectrum of a) IG-SCD, b) IG-FD and c) IG-APD.
Fig. 7. SEM images of samples with magnification rates of 100,000.
results showed that each sample had well-developed silica network, also, according to the peaks observed around 1750 cm1, the one may conclude that incorporation of ionic liquid on the silica gel structure is succesfully established for all samples. Morphological properties of the silica ionogels were observed by SEM images given in Fig. 7. Although all the samples had nearly similar physical appearances, their microstructural pattern highly differed from another. SEM images showed that porous network of IG-APD comprised both micro and mesopores. According to IUPAC’s definition, the pores can be mainly categorized either as open or closed pores (Fig. 8). According to the definition, closed pores are described as the pores which are totally isolated from their neighbours, as in region (a) in Fig. 8. On the other hand, pores which have a continuous channel of communication with the external surface of the body, like (b) (c) (d) (e) and (f), are described as open pores. Among the open pores, some may be open only at one end (like b) and (f); they are then called as blind (i.e. deadend, or saccafe) pores. Others may be open at two ends (through pores), like around (e) [24]. According to SEM images, the sample IG-APD seems to contain mainly closed or blind pores (like a and b in Fig. 8). It may be caused by the existence of ionic liquid remaining on the walls of the pore. Since excess ionic liquid seems to cover the silica particles, it ended up with a decrease in the size of the pores. Micropores in the structure can also be aroused from the type of the ionic liquid. The anion group of selected ionic liquid [Tf2N]- may interact with silica gel by the formation of strong hydrogen bonds and hence, it can form less compact ion aggregates with the imidazolium cation and π-π stacking of
Fig. 8. Schematic cross-section of a porous solid [24].
imidazolium rings of EMIMTf2N can block the pores and deteriorates the porous network [17]. On the other hand, SEM image of IG-FD revealed that, even though the existence of IL has milden the solvent expansion during the phase change from solid state to vapor state and protected the three dimensional solid network, a non-homogeneous pore distribution and large open pores existed in the network. It may again be originated from the crystallization process and may be avoided by changing the type of the 5
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Journal of Solid State Chemistry 278 (2019) 120877
Fig. 9. Nitrogen adsorption - desorption isotherms and the BJH pore size distribution of the ionogels synthesized via APD(a-a’), SCD(b-b’) and FD(c-c’).
ionic liquid and increasing the aging time. SEM image of IG-SCD, however, exhibited well-defined open porous structure that contains mesopores with more homogeneous pore distribution. Results of SEM analyses were also proven by the pore analysis. The N2 adsorption-desorption isotherms were displayed in Fig. 9 coupled with the corresponding the pore size distribution (PSD) curves abbreviated as a’, b’ and c’. According to the IUPAC classification, both isotherms in Fig. 9 can be categorized as Type IV, which denotes to mesoporous characteristics of the ionogels. For mesoporous materials, adsorption occurs on micropores at very low pressure and it is followed by adsorption on mesopores with capillary condensation occurring at higher pressure that leads to hysteresis loops [14,25]. An interesting outcome is that the hysteresis loops of the samples in Fig. 9 highly differ from each other, which indicates that the samples has ended up with different pore morphologies after drying periods. Pores may also be classified according to their shape: they may be cylindrical (either open (c) or blind (f)), ink-bottle shaped (b), finnel shaped (d) or slit-shaped [24]. Trends in adsorption-desorption isothems may be help us to understand the pore geometry within the structures of different ionogels. In the adsorption-desorption isotherms, the first inflection point corresponds to maximum monolayer adsorption. For IG-FD, the first inflection point appears that relative pressure of 0.8 and adsorption could not reach saturation. Isotherm of IG-FD had an H3-type hysteresis loop with a sharp upturn in the high relative pressure region, which indicates the limited capillary condensation associated with presence of
Table 2 Pore characteristics of silica ionogels. Sample ID
Surface Area, m2/ g
Pore volume, cm3/ g
Average pore diameter, nm
IG-APD IG-SCD IG-FD
95 646 130
0.14 0.43 0.22
1.43 4.21 10.3
macropores in the structure where the liquid has not condensed [24,25]. PSD curve of IG-FD represents wider than that of IG-SCD and IG-APD that is consistent with its macro-meso pore nature. Specific surface area of IG-FD (130 m2/g) being determined from the BET method is also relatively low in this respect. On the other hand, the sample IG-SCD exhibits hysteresis loop of type H2 showing that mesopores are appeared as inkbottles with narrow necks and adsorption isotherms is governed by delayed condensation. Because of its mesoporous structure, IG-SCD had the largest surface area with the highest average pore volume (Table 2). The adsorption-desorption isotherm of IG-APD accompanied by the type of H4 with less steep and elongated hysteresis loop which indicates that non-uniform distribution of the pores and disordered mesopores and micropores. In Fig. 9-a, the one can see that hysteresis loop has not been completely enclosed due possibly to the formation of very narrow slit pores or bottle shaped pores which causes large amount of micropores in structure and deteriorates the mesoporous nature. As exhibited in PSD 6
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Journal of Solid State Chemistry 278 (2019) 120877
Fig. 10. Results of thermo-gravimetric analysis of silica ionogels.
did. Silica ionogels dried with this method can appear in monolithical structure with very low percentage volumetric shrinkage (5%) and they also showed good thermal stability up to 410 C. For the other drying methods, selecting an alternative ionic liquid with different alkyl group (longer alkyl chain) can be a powerful approach for the further studies.
curve of IG-APD pores are majorly located in the range of 1.5–2.5 nm. IGAPD has also the lowest specific surface area (95 m2/g) as the presence of ionic liquid on the pore walls as thin films drastically restricted achievable the surface area and yielding a more closed-porous structure. To investigate the thermal stability of the prepared silica ionogels thermo-gravimetric analysis (TGA) was performed to understand the effect of ionic liquid confinement in the silica matrix and to observe the thermal degradation of the silica ionogels with temperature. Each sample exhibited nearly the same thermal behavior. According to TGA results, small weight loss up to 80 C for three IGs can be attributed to the residual solvent evaporation, which corresponds to about 5%wt. Moreover, weight loss occurring between 100 C and 400 C can be related with the oxidation of radical groups that formed after modification steps. Finally, weight loss in both samples after 395 C, 400 C and 410 C can be attributed to the degradation of ionic liquid confined within the IG-APD, IG-FD, IG-SCD network, respectively (Fig. 10).
Acknowledgements This study was financially supported by Ege University Scientific Research Fund under the contract number of 18MUH019. References [1] A. Vioux, L. Viau, S. Volland, J. Le, Use of ionic liquids in sol-gel ; ionogels and applications 13 (2010) 242–255, https://doi.org/10.1016/j.crci.2009.07.002. [2] M.P. Singh, R.K. Singh, S. Chandra, Studies on imidazolium-based ionic liquids having a large anion confined in a nanoporous silica gel matrix, J. Phys. Chem. B 115 (2011) 7505–7514, https://doi.org/10.1021/jp2003358. [3] M. Ivanova, S. Kareth, E.T. Spielberg, A.V. Mudring, M. Petermann, Silica ionogels synthesized with imidazolium based ionic liquids in presence of supercritical CO2, J. Supercrit. Fluids 105 (2015) 60–65, https://doi.org/10.1016/ j.supflu.2015.01.014. [4] L. Viau, M.A. Neouze, C. Biolley, S. Volland, D. Brevet, P. Gaveau, P. Dieudonne, A. Galarneau, A. Vioux, Ionic liquid mediated sol-gel synthesis in the presence of water or formic acid: which synthesis for which material? Chem. Mater. 24 (2012) 3128–3134, https://doi.org/10.1021/cm301083r. [5] M.P. Singh, R.K. Singh, S. Chandra, Ionic liquids confined in porous matrices: physicochemical properties and applications, Prog. Mater. Sci. 64 (2014) 73–120, https://doi.org/10.1016/j.pmatsci.2014.03.001. [6] C.M. Wu, S.Y. Lin, H.L. Chen, Structure of a monolithic silica aerogel prepared from a short-chain ionic liquid, Microporous Mesoporous Mater. 156 (2012) 189–195, https://doi.org/10.1016/j.micromeso.2012.02.039. [7] M.A. Neouze, J. Le Bideau, P. Gaveau, S. Bellayer, A. Vioux, Ionogels, new materials arising from the confinement of ionic liquids within silica-derived networks, Chem. Mater. 18 (2006) 3931–3936, https://doi.org/10.1021/cm060656c. [8] J. Le Bideau, L. Viau, A. Vioux, Ionogels, Ionic liquid based hybrid materials, Chem. Soc. Rev. 40 (2011) 907–925, https://doi.org/10.1039/c0cs00059k. [9] J. Zhang, Y. Ma, F. Shi, L. Liu, Y. Deng, Microporous and mesoporous materials room temperature ionic liquids as templates in the synthesis of mesoporous silica via a sol – gel method 119 (2009) 97–103, https://doi.org/10.1016/ j.micromeso.2008.10.003. [10] H. Maleki, L. Dur~aes, A. Portugal, An overview on silica aerogels synthesis and different mechanical reinforcing strategies, J. Non-Cryst. Solids 385 (2014) 55–74, https://doi.org/10.1016/j.jnoncrysol.2013.10.017. [11] G.M. Pajonk, M. Repellin-Lacroix, S. Abouarnadasse, J. Chaouki, D. Klavana, From sol-gel to aerogels and cryogels, J. Non-Cryst. Solids 121 (1990) 66–67, https:// doi.org/10.1016/0022-3093(90)90106-V. [12] A. Pons, L. Casas, E. Estop, E. Molins, K.D.M. Harris, M. Xu, A new route to aerogels : Monolithic silica cryogels 358 (2012) 461–469, https://doi.org/10.1016/ j.jnoncrysol.2011.10.031.
4. Conclusion In this study, the effects of applied drying method on the physical, chemical and morphological characteristics of silica based ionogels were investigated. Silica gels containing imidazolium based ionic liquid were prepared by using one-step sol-gel process and then dried either with supercritical, ambient pressure or freeze-drying methods. After drying periods, it was observed that each ionogel exhibited a monolithic appearance. However, the SEM results have shown that the type of drying methods has remarkable impacts on the final morphology of the silica ionogels. While the ionogel dried in ambient conditions comprised mostly of micro-mesopores with non-homogenous pore distribution with the lowest surface area of 95 m2/g, ionogel dried in cryogenic conditions has large number of macropores in the network with relatively high surface area of 130 m2/g and supercritical dried silica ionogel had mesoporous structure with highly developed silica network with the highest surface area of 646 m2/g. Microporous nature of the IG-APD probably generated from the excess ionic liquid remained on the silica clusters after drying and anion group of selected ionic liquid. For the freeze dried ionogel, however, the existence of ionic liquid helped the material to conserve its monolithical form after drying but yet it caused larger pores with insufficient porous structure. Therefore, it can be concluded that the supercritical drying method leads to form silica ionogels with more superior physical and morphological characteristics than other alternatives 7
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Contents lists available at ScienceDirect
Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Revealing the pore characteristics and physicochemical properties of silica ionogels based on different sol-gel drying strategies Selay Sert Çok, Fatos¸ Koç, Firuz Balkan, Nilay Gizli * _ Ege University, Chemical Engineering Department, Izmir, Turkey
A R T I C L E I N F O
A B S T R A C T
Keywords: Silica ionogel Ionic liquid Supercritical drying Freeze drying Ambient pressure drying
In this study, an exhaustive investigation about the effect of drying methods on the physical properties and microstructures of silica ionogels was conducted. Silica ionogels were synthesized by following one-step sol-gel process and dried either by supercritical, ambient pressure or freeze-drying techniques. After drying periods, characterization studies of the ionogels were carried out by performing FTIR, SEM, BET and TGA analyses. Results have shown that, although they all appeared in a monolithic structure, their microstructural pattern were completely different. Supercritically dried silica ionogel resulted in mesoporous structure with superior properties such as low density (ρ ¼ 0.21 kg/m3), high porosity (ϕ ¼ 90%) and high surface area (646 m2/g). On the other hand, silica ionogel dried in ambient conditions comprised both micro-meso pores and the pores are mainly closed form and freeze dried ionogel had non-homogeneous pore distribution with relatively larger pores.
1. Introduction Ionic liquids can simply be described as organic salts that usually have melting points below 370 K. They are composed of a large organic cation group such as ammonium, phosphonium, pyridinium, imidazolium, etc. and inorganic/organic anion group. Usual it is claimed that ionic liquids are classified as green chemicals due to their negligible vapor pressure, chemical stability and non-flammability. Also, most of them have thermal stability at temperatures higher than 570 K and has large electrochemical window and ionic conductivity [1]. Apart from these superior properties, ionic liquid family attracted great attention of tuning their characteristics due to their wide range of anion-cation combinations and applied in diverse applications. Currently, they are successfully used in electrochemical devices such as batteries, fuel cells, super capacitors, electrochemical solar cells or in electrochromic devices. However, problems such as, packaging, leakage and portability arising from their liquid nature restrict their possible range of applications [2]. To overcome these drawbacks, the immobilization of ionic liquids within organic or inorganic matrices and taking advantage of their excellent properties in the solid state has become a focus of attention nowadays [3–7]. One way to accomplish this is to immobilize the ionic liquid in the form of three dimensional porous structure and to have a solid-like behavior [8].This new class of materials are termed as ionogels and has been found
to be suitable for many device and applications such as optical solvents, sensors, biosensors, catalysis, biocatalysts [2]. Sol-gel method is a frequently used efficient way to produce porous ionogels. Traditional sol-gel method involves the removal of liquid phase and remaining the porous solid network after drying stage (xerogel, cryogel, aerogel) as a target material. In the case of ionogels, ionic liquids are also incorporated into porous network due to negligible vapor pressure of ionic liquids and this new material combines the physical properties of the two networks (solid oxides and ionic liquid) intermingled at nanometer scale [1]. In general, sol-gel method is about the formation of nanostructured solid network because of subsequent hydrolysis and condensation reactions of a silica precursor and is mainly comprised of two parts: formation of wet gel by sol-gel reactions and drying of the wet gel with various techniques. Gel is obtained from the colloidal solution named as sol. A sol usually includes a silica precursor and a proper solvent. After a while, the sol reaches a gelation point during the simultaneous hydrolysis and condensation reactions catalyzed by acid or base addition. During this period, firstly, the primary particles are formed in matrix and then they aggregate as secondary particles and lastly, they link together in a pearl necklace morphology [9]. Final porous structure of the material can end up with very different morphology and different pore sizes as illustrated in Fig. 1. According to the pore sizes, the material result either in
_ * Corresponding author., Ege University, Chemical Engineering Department, 35100, Bornova, Izmir, Turkey. E-mail address: [email protected] (N. Gizli). https://doi.org/10.1016/j.jssc.2019.07.038 Received 11 June 2019; Received in revised form 10 July 2019; Accepted 21 July 2019 Available online 8 August 2019 0022-4596/© 2019 Elsevier Inc. All rights reserved.
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In freeze drying technique, the solvent trapped within the pores is firstly frozen and then sublimated under very low pressures. During freeze-drying, the solvent firstly crosses the liquid-solid equilibrium curve and then crosses the solid - gas equilibrium curves. Hence, because of crystallization of solvent within the pores, final products can be obtained either in crack or even in powder form with very large pores [10]. Pajonk et al. (1990) stated that this disadvantage could be eliminated by replacing the solvent by the solvent, with the lower thermal expansion coefficient and higher sublimation pressure [11]. At this point, using ionic liquids in sol-gel reactions can be a powerful alternative as some of these liquids have lower expansion coefficients than classical solvents such as ethanol and water at the same conditions [12]. Ambient pressure drying technique is another way to dry the gels. During this method, solvent within the pores need to cross the liquid-gas equilibrium curve and surface tension at the liquid – vapor interface within pores develops a capillary tension in pores walls and results in pore destruction and gel shrinkage. Ionic liquids, in this case, may also play an important role for controlling the gel shrinkage by satisfying relaxation on the pore walls against any shrunk due to their negligible vapor pressure in ambient temperatures. On the other hand, supercritical drying is the first and the most widely used drying method in sol-gel chemistry. In this method, solvents trapped in the pores of wet gels are removed under supercritical conditions (above critical temperature (Tcr) and pressure (Pcr)). At the supercritical condition, since there is no discriminations between the liquid and vapor phases, the capillary stresses are no longer present. Therefore, the one can have well defined porous structure by applying supercritical drying during the preparation of silica ionogels. For the first time, Dai et al. (2000) has prepared ionogels by introducing ionic liquid into porous silica gels by following sol-gel approach. Afterwards, several studies have conducted for the preparation of silica ionogels by following various drying strategies [13]. Meera et al. prepared ionic liquid embedded silica based aerogel by following freeze-drying method, whereas Gupta et al. (2012) synthesized silica
Fig. 1. General porous structure of a silica network.
micro porous (pore diameter smaller than 2 nm), mesoporous (pore diameter is between 2 nm and 50 nm) or macro porous (pore diameter larger than 50 nm) nature [9]. During the sol-gel method, drying stage occupies the biggest interest and great effort during the sol gel process as it is the most critical and challenging step in the determination of final morphology and physical characteristics of the silica gels. During the drying process, residual solvents trapped within the pores of the material are removed leaving only the three dimensionally linked silica structure. Solvent removal without any damage in the solid skeleton and pore structure has vital importance to get the afore mentioned superior properties of silica ionogels. Great portion of pore destruction is usually generated during the crossing of phase equilibrium boundaries because of severe surface tension formed between the phases. There are three main types of drying used in sol gel processes: such as freeze-drying (FD), ambient pressure drying (APD) and supercritical drying (SCD).
Fig. 2. Demonstration of sol preparation steps with related reactions. 2
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Fig. 3. Surface modification reaction of silica ionogels.
Fig. 4. Drying methods.
and dried by using supercritical drying, ambient pressure drying and freeze drying. Differences between the final physical, chemical and morphological properties of the silica ionogels were revealed with the aid of FTIR, SEM, BET and TGA analyses.
ionogels under ambient conditions [14,15]. Karout et al. on the other hand, studied the effect of confinement of ionic liquids on silica aerogels gelation time produced by partial evaporative drying followed by supercritical drying [16]. More recently, Ivanova et al. (2018) used imidazolium based ionic liquids as solvents in sol-gel reactions and prepared silica ionogels by following supercritical CO2 drying and investigated the influence of ionic liquids on gelation time and the solid properties of final ionogels [17]. As far as we know, hardly any study has investigated the effect of different drying methods on the physical and morphological characteristics of a specific silica ionogels and give a brief comparison between drying methods for the same material. In the present study, silica based ionogels are prepared by one-step sol-gel method by using short chain imidazolium based ionic liquid with fluorinated anion group
2. Experimental procedure 2.1. Materials Tetraethylorthosilicate (TEOS, 98%), 3-aminopropyl-triethoxysilane (APTES) were used as sol precursors and purchased from Sigma Aldrich. Ethanol, n-hexane and an imidazolium based ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMIMTF2N) (IL, 98%) 3
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were selected as solvents and were supplied from Sigma Aldrich. HCl was included in sol-gel process as acid catalyst. 3-metacryloxypropyltrimethoxysilane (MEMO) served as sylating agent during surface modification step. Carbon dioxide (CO2) with purity 99.95% used during the SCD step as supercritical fluid and was supplied by HABAS Company. 2.2. Methods The silica ionogels were synthesized by following sol-gel process. Synthesis steps are; sol preparation (Fig. 2), surface modification (Fig. 3), and drying (Fig. 4). Type and amount of the sol components were selected with reference to our previous studies [18,19].
Fig. 5. Physical appearances of the silica ionogels.
Table 1 Physical properties of the samples IG-APD, IG-SCD and IG-FD.
2.2.1. Sol preparation and gel formation During the sol preparation, TEOS and IL were hydrolyzed by using 0.01 M HCl and EtOH for 90 min stirring at 25 C. After stirring, the sol was placed in an ultrasonic medium and APTES was added to the sol to start condensation reaction. The sol component consists of TEOS: APTES: IL: EtOH: HCl with the molar ratios of 1:0.47:0.14:6.3:7.4 105. Hydrolysis and condensation steps were demonstrated in Fig. 2.
2.2.3. Drying The most significant and challenging step to attain the final physical form and morphology of silica ionogels is drying. Each ionogel was dried by applying different methods in this study (Fig. 4). First group of ionogels abbreviated as IG-APD were dried under ambient pressure for three days. Second group of ionogels (IG-SCD) were dried for 3 h by using supercritical CO2 dryer (Leica EM CPD300). During this method, the pressure of the vessel was increased to 76 bar and the temperature was raised up to 32 C so that CO2 transformed into a supercritical fluid. At this state solvent trapped within the pores was replaced with supercritical CO2 in 15 cycles. Then, pressure of the vessel was decreased down to atmospheric conditions at constant temperature by releasing CO2 gas and leaving the pores of the ionogels totally evacuated. Last sample IG-FD was dried under cryogenic conditions by lyophilizator (Christ 1.2 D Alpha Plus). As a first step of the method, the sample was frozen at 20 C overnight and the frozen solvent was sublimated by exposing the vessel to high vacuum for 12 h.
%Porosity ¼
(1)
1 ρb=ρ x100
%Volume shrinkage ¼
(2)
s
1 Va V x100 g
Volume Shrinkage, %
1.4 0.21 0.75
36 90 65
5 18 28
Silica ionogels dried in each type of drying condition appeared in a monolithic structure as shown in Fig. 5. Especially for freeze dried ionogel, it is a very noteworthy outcome since silica based aerogels can be obtained mostly in granular or even in powder form after freeze drying [10,21–23]. In this respect, it should be noticed that the presence of ionic liquid in silica ionogel structure helped the pore walls against collapse during the crystallization of solvent by bringing the pore walls in a more elastic nature. Table 1 shows the physical properties of the samples after drying. Each ionogel exhibited different degree of volumetric shrinkage and hence they ended up with different density values. While supercritical dried ionogel had the lowest volumetric shrinkage and density values, ionogel dried in ambient conditions had the highest volumetric shrinkage and highly dense structure. Identification of chemical structures was analyzed with the aid of FTIR analysis (Fig. 6). According to FTIR results, the absorption peaks around 1080 cm1 can be related with the asymmetric Si–O–Si bonding and peaks around 695 cm1 with the symmetric stretching vibration of Si–O–Si and they are good sign about the successful development of silica network. The terminal Si–CH3 and –CH3 groups observed near 1350 cm1 can affirm the successful replacement of radical groups with hydroxyl groups on silica surfaces. In addition, the absorption peaks around 1750 cm1 can be attributed to aromatic ring of ionic liquid. FTIR
Bulk densities of the silica ionogels obtained by applying different drying periods were measured as mass to volume ratio, then the porosity and volume shrinkage of the samples were determined using the formula below: bulk
Porosity, %
IG-APD IG-SCD IG-FD
3. Results and discussion
2.3. Characterization
Density, g/cm3
where ρb is the bulk density of ionogels and ρs is the skeleton density of the silica ionogels, generally the value of ρs is 2.2 g/cm3 [20]. Va and Vg in Eq. (3) denote the volumes of the samples after drying and before the solvent exchange, respectively. The morphology and the microstructure of silica ionogels were examined by using Scanning Electron Microscope (SEM) (PHILIPS, XL 30S FEG) with magnification rate of 105. In addition, chemical compositions of the ionogels were identified by performing FTIR (PERKIN ELMER, Spectrum 100, ABD) analysis. Surface area, average pore volume and average pore diameters of the silica ionogels were determined by physisorption of N2 at 77 K by using Quanta chrome Corporation, Autosorb-6. BET technique was used to determine surface area of the samples and the average pore size of the samples were obtained with the aid of BJH (Barrett, Joyner, Halenda) technique. Before the measurement, ionogels were subjected to degassing operation at 130 C over night to remove residual moisture and adsorbed gases. To investigate the thermal stability of the samples, thermo-gravimetric analysis (TGA) was carried out by TA Instruments SDT Q600 device that operates at a heating rate of 5 C/min from room temperature to 600 C under a nitrogen environment.
2.2.2. Aging and surface modification After gelation was fully completed, the silica ionogels were subjected to an aging period of 24 h in a polypropylene cylindrical mold for further continuation of the condensation reactions. Afterwards, surface modification was performed by replacing hydroxyl groups exist on the surface of the ionogel with the alkyl groups to reconstruct the surface of the samples and satisfy a hydrophobic structure. Modification step was performed by immersing the ionogels in MEMO and n-hexane mixture with a volumetric ratio of 50% at room temperature for 12h. Surface modification reaction is given as in Fig. 3.
ρb g=cm3 ¼ mbulk=V
Sample ID
(3)
4
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Fig. 6. FTIR Spectrum of a) IG-SCD, b) IG-FD and c) IG-APD.
Fig. 7. SEM images of samples with magnification rates of 100,000.
results showed that each sample had well-developed silica network, also, according to the peaks observed around 1750 cm1, the one may conclude that incorporation of ionic liquid on the silica gel structure is succesfully established for all samples. Morphological properties of the silica ionogels were observed by SEM images given in Fig. 7. Although all the samples had nearly similar physical appearances, their microstructural pattern highly differed from another. SEM images showed that porous network of IG-APD comprised both micro and mesopores. According to IUPAC’s definition, the pores can be mainly categorized either as open or closed pores (Fig. 8). According to the definition, closed pores are described as the pores which are totally isolated from their neighbours, as in region (a) in Fig. 8. On the other hand, pores which have a continuous channel of communication with the external surface of the body, like (b) (c) (d) (e) and (f), are described as open pores. Among the open pores, some may be open only at one end (like b) and (f); they are then called as blind (i.e. deadend, or saccafe) pores. Others may be open at two ends (through pores), like around (e) [24]. According to SEM images, the sample IG-APD seems to contain mainly closed or blind pores (like a and b in Fig. 8). It may be caused by the existence of ionic liquid remaining on the walls of the pore. Since excess ionic liquid seems to cover the silica particles, it ended up with a decrease in the size of the pores. Micropores in the structure can also be aroused from the type of the ionic liquid. The anion group of selected ionic liquid [Tf2N]- may interact with silica gel by the formation of strong hydrogen bonds and hence, it can form less compact ion aggregates with the imidazolium cation and π-π stacking of
Fig. 8. Schematic cross-section of a porous solid [24].
imidazolium rings of EMIMTf2N can block the pores and deteriorates the porous network [17]. On the other hand, SEM image of IG-FD revealed that, even though the existence of IL has milden the solvent expansion during the phase change from solid state to vapor state and protected the three dimensional solid network, a non-homogeneous pore distribution and large open pores existed in the network. It may again be originated from the crystallization process and may be avoided by changing the type of the 5
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Fig. 9. Nitrogen adsorption - desorption isotherms and the BJH pore size distribution of the ionogels synthesized via APD(a-a’), SCD(b-b’) and FD(c-c’).
ionic liquid and increasing the aging time. SEM image of IG-SCD, however, exhibited well-defined open porous structure that contains mesopores with more homogeneous pore distribution. Results of SEM analyses were also proven by the pore analysis. The N2 adsorption-desorption isotherms were displayed in Fig. 9 coupled with the corresponding the pore size distribution (PSD) curves abbreviated as a’, b’ and c’. According to the IUPAC classification, both isotherms in Fig. 9 can be categorized as Type IV, which denotes to mesoporous characteristics of the ionogels. For mesoporous materials, adsorption occurs on micropores at very low pressure and it is followed by adsorption on mesopores with capillary condensation occurring at higher pressure that leads to hysteresis loops [14,25]. An interesting outcome is that the hysteresis loops of the samples in Fig. 9 highly differ from each other, which indicates that the samples has ended up with different pore morphologies after drying periods. Pores may also be classified according to their shape: they may be cylindrical (either open (c) or blind (f)), ink-bottle shaped (b), finnel shaped (d) or slit-shaped [24]. Trends in adsorption-desorption isothems may be help us to understand the pore geometry within the structures of different ionogels. In the adsorption-desorption isotherms, the first inflection point corresponds to maximum monolayer adsorption. For IG-FD, the first inflection point appears that relative pressure of 0.8 and adsorption could not reach saturation. Isotherm of IG-FD had an H3-type hysteresis loop with a sharp upturn in the high relative pressure region, which indicates the limited capillary condensation associated with presence of
Table 2 Pore characteristics of silica ionogels. Sample ID
Surface Area, m2/ g
Pore volume, cm3/ g
Average pore diameter, nm
IG-APD IG-SCD IG-FD
95 646 130
0.14 0.43 0.22
1.43 4.21 10.3
macropores in the structure where the liquid has not condensed [24,25]. PSD curve of IG-FD represents wider than that of IG-SCD and IG-APD that is consistent with its macro-meso pore nature. Specific surface area of IG-FD (130 m2/g) being determined from the BET method is also relatively low in this respect. On the other hand, the sample IG-SCD exhibits hysteresis loop of type H2 showing that mesopores are appeared as inkbottles with narrow necks and adsorption isotherms is governed by delayed condensation. Because of its mesoporous structure, IG-SCD had the largest surface area with the highest average pore volume (Table 2). The adsorption-desorption isotherm of IG-APD accompanied by the type of H4 with less steep and elongated hysteresis loop which indicates that non-uniform distribution of the pores and disordered mesopores and micropores. In Fig. 9-a, the one can see that hysteresis loop has not been completely enclosed due possibly to the formation of very narrow slit pores or bottle shaped pores which causes large amount of micropores in structure and deteriorates the mesoporous nature. As exhibited in PSD 6
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Fig. 10. Results of thermo-gravimetric analysis of silica ionogels.
did. Silica ionogels dried with this method can appear in monolithical structure with very low percentage volumetric shrinkage (5%) and they also showed good thermal stability up to 410 C. For the other drying methods, selecting an alternative ionic liquid with different alkyl group (longer alkyl chain) can be a powerful approach for the further studies.
curve of IG-APD pores are majorly located in the range of 1.5–2.5 nm. IGAPD has also the lowest specific surface area (95 m2/g) as the presence of ionic liquid on the pore walls as thin films drastically restricted achievable the surface area and yielding a more closed-porous structure. To investigate the thermal stability of the prepared silica ionogels thermo-gravimetric analysis (TGA) was performed to understand the effect of ionic liquid confinement in the silica matrix and to observe the thermal degradation of the silica ionogels with temperature. Each sample exhibited nearly the same thermal behavior. According to TGA results, small weight loss up to 80 C for three IGs can be attributed to the residual solvent evaporation, which corresponds to about 5%wt. Moreover, weight loss occurring between 100 C and 400 C can be related with the oxidation of radical groups that formed after modification steps. Finally, weight loss in both samples after 395 C, 400 C and 410 C can be attributed to the degradation of ionic liquid confined within the IG-APD, IG-FD, IG-SCD network, respectively (Fig. 10).
Acknowledgements This study was financially supported by Ege University Scientific Research Fund under the contract number of 18MUH019. References [1] A. Vioux, L. Viau, S. Volland, J. Le, Use of ionic liquids in sol-gel ; ionogels and applications 13 (2010) 242–255, https://doi.org/10.1016/j.crci.2009.07.002. [2] M.P. Singh, R.K. Singh, S. Chandra, Studies on imidazolium-based ionic liquids having a large anion confined in a nanoporous silica gel matrix, J. Phys. Chem. B 115 (2011) 7505–7514, https://doi.org/10.1021/jp2003358. [3] M. Ivanova, S. Kareth, E.T. Spielberg, A.V. Mudring, M. Petermann, Silica ionogels synthesized with imidazolium based ionic liquids in presence of supercritical CO2, J. Supercrit. Fluids 105 (2015) 60–65, https://doi.org/10.1016/ j.supflu.2015.01.014. [4] L. Viau, M.A. Neouze, C. Biolley, S. Volland, D. Brevet, P. Gaveau, P. Dieudonne, A. Galarneau, A. Vioux, Ionic liquid mediated sol-gel synthesis in the presence of water or formic acid: which synthesis for which material? Chem. Mater. 24 (2012) 3128–3134, https://doi.org/10.1021/cm301083r. [5] M.P. Singh, R.K. Singh, S. Chandra, Ionic liquids confined in porous matrices: physicochemical properties and applications, Prog. Mater. Sci. 64 (2014) 73–120, https://doi.org/10.1016/j.pmatsci.2014.03.001. [6] C.M. Wu, S.Y. Lin, H.L. Chen, Structure of a monolithic silica aerogel prepared from a short-chain ionic liquid, Microporous Mesoporous Mater. 156 (2012) 189–195, https://doi.org/10.1016/j.micromeso.2012.02.039. [7] M.A. Neouze, J. Le Bideau, P. Gaveau, S. Bellayer, A. Vioux, Ionogels, new materials arising from the confinement of ionic liquids within silica-derived networks, Chem. Mater. 18 (2006) 3931–3936, https://doi.org/10.1021/cm060656c. [8] J. Le Bideau, L. Viau, A. Vioux, Ionogels, Ionic liquid based hybrid materials, Chem. Soc. Rev. 40 (2011) 907–925, https://doi.org/10.1039/c0cs00059k. [9] J. Zhang, Y. Ma, F. Shi, L. Liu, Y. Deng, Microporous and mesoporous materials room temperature ionic liquids as templates in the synthesis of mesoporous silica via a sol – gel method 119 (2009) 97–103, https://doi.org/10.1016/ j.micromeso.2008.10.003. [10] H. Maleki, L. Dur~aes, A. Portugal, An overview on silica aerogels synthesis and different mechanical reinforcing strategies, J. Non-Cryst. Solids 385 (2014) 55–74, https://doi.org/10.1016/j.jnoncrysol.2013.10.017. [11] G.M. Pajonk, M. Repellin-Lacroix, S. Abouarnadasse, J. Chaouki, D. Klavana, From sol-gel to aerogels and cryogels, J. Non-Cryst. Solids 121 (1990) 66–67, https:// doi.org/10.1016/0022-3093(90)90106-V. [12] A. Pons, L. Casas, E. Estop, E. Molins, K.D.M. Harris, M. Xu, A new route to aerogels : Monolithic silica cryogels 358 (2012) 461–469, https://doi.org/10.1016/ j.jnoncrysol.2011.10.031.
4. Conclusion In this study, the effects of applied drying method on the physical, chemical and morphological characteristics of silica based ionogels were investigated. Silica gels containing imidazolium based ionic liquid were prepared by using one-step sol-gel process and then dried either with supercritical, ambient pressure or freeze-drying methods. After drying periods, it was observed that each ionogel exhibited a monolithic appearance. However, the SEM results have shown that the type of drying methods has remarkable impacts on the final morphology of the silica ionogels. While the ionogel dried in ambient conditions comprised mostly of micro-mesopores with non-homogenous pore distribution with the lowest surface area of 95 m2/g, ionogel dried in cryogenic conditions has large number of macropores in the network with relatively high surface area of 130 m2/g and supercritical dried silica ionogel had mesoporous structure with highly developed silica network with the highest surface area of 646 m2/g. Microporous nature of the IG-APD probably generated from the excess ionic liquid remained on the silica clusters after drying and anion group of selected ionic liquid. For the freeze dried ionogel, however, the existence of ionic liquid helped the material to conserve its monolithical form after drying but yet it caused larger pores with insufficient porous structure. Therefore, it can be concluded that the supercritical drying method leads to form silica ionogels with more superior physical and morphological characteristics than other alternatives 7
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