Journal of Alloys and Compounds 487 (2009) 744–750 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Low-density TEOS-based silica aerogels prepared at ambient pressure using isopropanol as the preparative solvent Askwar Hilonga a , Jong-Kil Kim b , Pradip B. Sarawade a , Hee Taik Kim a,∗ a b Department of Chemical Engineering, Hanyang University, 1271 Sa 3-dong, Sangnok-gu, Ansan-si, Gyeonggi-do 426-791, Republic of Korea E&B Nanotech. Co., Ltd, Republic of Korea a r t i c l e i n f o Article history: Received 21 April 2009 Received in revised form 7 August 2009 Accepted 14 August 2009 Available online 22 August 2009 Keywords: Silica aerogels Ambient pressure drying Isopropanol FTIR TEM a b s t r a c t In this paper, we report experimental results on the synthesis of low-density tetraethoxysilane (TEOS)based silica aerogels prepared with different solvents via an ambient pressure drying (APD) route. Tetraethoxysilane was hydrolyzed and condensed in different solvents (methanol, ethanol, butanol, and isopropanol) using oxalic acid and NH4 OH as the catalysts. To minimize shrinkage due to drying, the surfaces of the gels were modified using trimethylchlorosilane (TMCS) before the APD via a one-step solvent exchange/surface modification process. The effects of different solvents on the physical and textural properties of the resulting aerogels was investigated. It was observed that solvents containing longer chains of alkyl groups (–CH2 –CH3 ) formed high silica polymerization in the alcogels which enhanced a distinct “spring-back effect” during APD, and consequently preserved the highly porous silica network by preventing collapse. Silica aerogels with very low bulk densities (0.041 g/cm3 ), extremely high specific surface areas (1150 m2 /g), and large cumulative pore volumes (5.2 cm3 /g) were successfully synthesized using isopropanol as a preparative solvent. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Aerogels are nanoscale mesoporous light materials possessing a number of exceptional physical properties that attract the attention of researchers in various areas of science and technology. They have a large surface area (500–1500 m2 /g), high porosity (80–99%), low bulk density (∼0.03 g/cm3 ), extremely low thermal conductivities (0.005 W/mk), and unique acoustic properties (sound velocities as low as 100 m/s) [1,2]. Because of these properties, aerogels lend themselves to potential applications such as thermal super-insulators in solar energy systems, refrigerators, thermal flasks [3], internal confinement fusion (ICF) targets for thermonuclear fusion reactions [4], efficient catalysts and catalytic supports [5], storage media for liquid for rocket propellants [6], and radio luminescent devices [7]. Monolithic silica aerogels have been used extensively in the area of high energy physics in Cherenkov radiation detectors [8,9]. The extremely low densities and high surface areas of monolithic aerogels provide an opportunity to improve the performance of various metal-oxide-based devices, including gas- and bio-sensors, batteries, heterogeneous catalysis devices, and low dielectric constant materials for integrated circuits (low-k dielectrics) [10–12]. ∗ Corresponding author. Tel.: +82 31 400 5493; fax: +82 31 500 3579. E-mail address: khtaik@yahoo.co.kr (H.T. Kim). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.08.055 Silica aerogels are produced by the removal of entrapped solvent from the wet gel while maintaining the integrity and high porosity of the gel. Kistler used supercritical organic solvents (ethanol, for example) for drying [13,14]. In general, supercritical fluids have a high solvating power and almost zero surface tension. These make them ideal for the removal of solvents from wet gels while avoiding the cracking and shrinkage caused by the surface tension force associated with the removal of the solvent. Nevertheless, supercritical drying requires high pressures and temperatures, is generally risky, relatively expensive, and greatly hampers large-scale production of porous silica because of the required labor and use of sophisticated instruments [13,15]. To enhance the commercial feasibility of aerogels, an important precondition for large-scale application is to avoid supercritical drying. Although several attempts have been made to synthesize silica aerogels by drying the gels at ambient pressures, there are few reports on the synthesis of TEOS-based silica aerogels at ambient pressure. In addition, the synthesis of monolithic silica aerogels via APD has not been achieved. For instance, Wei et al. synthesized monolithic silica aerogels based on a tetraethoxysilane (TEOS) precursor via ambient pressure drying [16]. The silica aerogel they produced, however, possessed a relatively high density (0.069 g/cm3 ) and a relatively high thermal conductivity (0.079 W/mk), a low surface area (777 m2 /g), and a low pore volume (1.13 cm3 /g) compared with aerogels prepared with a TEOS precursor and supercritical drying [17]. In addition, they employed A. Hilonga et al. / Journal of Alloys and Compounds 487 (2009) 744–750 745 Table 1 Synthesis conditions and the physical properties of TEOS-based silica aerogels by one-step solvent exchange surface modification. No. TEOS/MtOH:H2 O(A): H2 O (B) molar ratio (M1 ) EtOH:TMCS molar ratio (M2 ) Perfluorohexane/TMCS volume ratio (V1 ) Volume shrinkage (%) Bulk density Remarks 1 2 3 4 5 6 7 8 9 1:6.9:3.5:2.2 1:6.9:3.5:2.2 1:6.9:3.5:2.2 1:6.9:3.5:2.2 1:6.9:3.5:2.2 1:6.9:3.5:2.2 1:6.9:3.5:2.2 1:6.9:3.5:2.2 1:6.9:3.5:2.2 0.5 1 1.5 2 1 1 1 1 1 – – – – 10 15 20 25 30 7.2 5.6 5.9 6.3 6.7 6.4 5.4 4.3 5.3 0.67 0.59 0.58 0.57 0.65 0.61 0.59 0.53 0.54 Pieces Few cracks Cracks Pieces Pieces Pieces Cracks Few cracks Cracks a multiple surface modification (MSM) process, which is a complicated and lengthy process (requiring about 6 days), to synthesize monolithic silica aerogels at ambient pressure. For the present study, we employed a one-step solvent exchange and surface modification process that significantly reduced the time needed to synthesize monolithic aerogels (at ambient pressure) from 6 to 2 days. Moreover, the effects of different preparative solvents on the physical and textural properties of TEOS-based silica aerogels synthesized at ambient pressure were investigated. The choice of appropriate solvent is a key factor to consider in developing aerogels with desirable properties. For instance, isopropanol (which contains more branching alkyl groups) provides a high degree of polymerization in the two-step acid–base sol–gel process with a TEOS precursor [18,19]. This high degree of polymerization in the silica network enhances a distinct “spring-back effect” during APD, and consequently preserves the highly porous silica network by preventing collapse [20]. In an earlier publication [21] we reported the synthesis of a high specific surface area TEOS-based silica aerogel with a large pore volume using an ethanol solvent and a two-step sol–gel process at ambient pressure. In our continuous program of developing aerogels that are more suitable for industrial application, we have successfully synthesized monolithic aerogels with more desirable properties: low density (0.041 g/cm3 ), high surface area (1150 m2 /g), and large pore volume (5.2 cm3 /g). This was accomplished using an isopropanol solvent and the two-step acid–base sol–gel polymerization of a TEOS precursor. This paper reports, in detail, the results obtained. 2. Materials and methods 2.1. Preparation of silica alcogel using a two-step sol–gel process Silica alcogels were prepared by a two-step acid–base catalyzed sol–gel process followed by a one-step solvent exchange surface modification and ambient pressure drying, as described elsewhere [22]. The chemicals used for the preparation of the alcosols were tetraethoxysilane (TEOS), Si–(OC2 H5 )4 (Aldrich), ammonium hydroxide (NH4 OH, from Duksan Chemical), methanol (MeOH, CH3 OH), ethanol (EtOH, C2 H5 OH), 1-butanol (C4 H9 OH), isopropanol (C3 H7 OH), oxalic acid (dihydrate, C2 H5 OH), and perfluorohexane (C6 H14 , from Duksan chemical). Double-distilled water was used to prepare the desired concentration of hydrochloric acid and NH4 OH catalysts. Silica wet gels were prepared using the following two steps. In the first step, tetraethoxysilane (TEOS) was diluted with a relevant solvent (methanol, ethanol, butanol, or isopropanol), then mixed with oxalic acid with constant stirring for 30 min. In the second step, after 24 h, the base catalyst (NH4 OH) was added (drop by drop) into the acid catalyzed silica sol. The molar ratio of TEOS: alcohol: H2 O (acidic): H2 O (basic) was kept constant at 1:6.9:3.5:2.2 (Table 1). The oxalic acid and NH4 OH concentrations were kept constant at 0.01 and 0.1 M, respectively. To study the effects of solvents on the properties of the dried silica aerogels, four alcogel samples were prepared with four different solvents: methanol, ethanol, 1-butanol, and isopropanol. The as-prepared alcosols were poured into an air-tight 50 ml beaker and were kept for gelation at room temperature. After gelation of the sols, a small quantity of the relevant alcohol was added into the targeted gel to prevent the evaporation of the pore solvent which would normally lead to shrinkage and cracking of the gel. ± ± ± ± ± ± ± ± ± 0.5 0.3 0.4 0.6 0.2 0.5 0.2 0.5 0.4 2.2. One-step solvent exchange surface modification of silica alcogels and the drying process To minimize drying-related shrinkage, the surfaces of the gels were modified using an alcohol/TMCS/perfluorohexane solution via a one-step solvent exchange surface modification process before the APD. The one-step solvent exchange and surface modification were carried out by immersing the aged alcogels in the alcohol/TMCS/perfluorohexane solution at room temperature for 24 h. To determine the optimal conditions for the modification process, the molar ratio of alcohol/TMCS (M1 ) was varied from 0.5 to 2. Also, the volumetric ratio of perfluorohexane and TMCS (V2 ) was varied from 10 to 30. In the course of the experiments, we observed that the alcohol/TMCS molar ratio had no significant effect on the monolithicity of the dried aerogels in the tested range of ratios. Thus, the molar ratio of alcohol/TMCS was fixed at 1 in additional experiments. The volumetric ratio of perfluorohexane/TMCS was also fixed at 25 since fewer cracks were observed at this ratio, as is reported elsewhere [23]. After the surface modification of the alcogels, they were washed with perfluorohexane to remove the unreacted TMCS. The alcogels were dried at 50 ◦ C for 12 h, and then at 80 ◦ C for 2 h to reduce drying-related shrinkage. For complete evaporation of the pore liquid, the gels were finally dried at 200 ◦ C for 1 h. The aerogel samples were allowed to cool to room temperature and were then characterized by various techniques. 2.3. Characterization methods The specific surface areas and pore size distributions (PSDs) of the aerogels were analyzed by Brunauer, Emmett and Teller (BET) and BJH nitrogen gas adsorption and desorption methods (ASAP 2000, Micromeritics, USA). BET analysis of the amount of N2 gas adsorbed at various partial pressures (five points 0.05 < p/po < 0.3, nitrogen molecular cross-sectional area = 0.162 nm2 ) was used to determine the surface area, and a single condensation point (p/po = 0.99) was used to find the pore size and pore volume. Before N2 adsorption, the sample was degassed at 200 ◦ C. Pore size distributions were calculated from the desorption isotherms [24]. To study the thermal stability of the aerogels in terms of the retention of hydrophobicity, the samples were examined by thermogravimetric and differential thermal analysis (TG-DTA). The hydrophobic nano-porous silica aerogels with a weight of 6 mg were heat-treated in air, from room temperature (25 ◦ C) up to 1000 ◦ C, with a controlled heating rate of 1.5 ◦ C min−1 , using a microprocessor-based Parr temperature controller (Model 4846) connected to a muffle furnace (A.H. JEON Industrial Co. Ltd., Korea). Here, thermal stability refers to the temperature up to which the silica aerogel retains its hydrophobicity [25]. The bulk densities of the aerogels were calculated from their mass to volume ratios. The volume was calculated after measuring the dimensions of the monolithic aerogels using Vernier calipers. An electronic microbalance (Model OHAUS EPG214C, USA) was used to measure the masses of the samples. The percents of volume shrinkage and porosity of the prepared aerogels were determined as follows: % Volume shrinkage = % Porosity =  1− b s   1− Va Vg × 100  × 100 (1) (2) where Va and Vg are the volumes of the aerogel and alcogel, respectively, and s and b are the skeletal and bulk densities (of silica aerogel), respectively. Surface modification was confirmed using infrared (IR) spectroscopy (PerkinElmer, Model No. 783). For this purpose, the as-prepared silica aerogels were ground into powders and pressed to from a sample pellet for FTIR measurements. The contact angle () measurements were performed using a contact angle meter (CAM 200 Automates contact angle analyzer, Finland) to quantify the degree of hydrophobicity. A 12 ␮l (2 mm) water droplet was placed at three different places on the surface of the hydrophobic aerogels, and the average measured value was then taken to be the contact angle (). The optical transmittances of the aerogels (sample 10 15 40 70 Gelation time (min) thickness: 10−2 m) were measured at a wavelength of 750 nm (Systronics 119 spectrophotometer). Microstructure studies of the aerogel samples were carried out by transmission electron microscopy (TEM, JSM 6700 F, JEOL). 3. Results and discussion Silica alcogels were prepared by two-step hydrolysis and condensation of diluted tetraethoxysilane (TEOS) in the presence of acid (oxalic acid)–base (ammonium hydroxide) catalysts and selected solvents, as shown in the following reactions: Hydrolysis: Si(O–X)4 + 4H2 O → Si(OH)4 + 4X–OH (3) n(Si(OH)4 + (OH)4 Si) → n((OH)3 Si–O–O(OH)3 ) + nH2 O 85 86 89 91 157 159 162 168 0.7 0.4 0.2 0.5 ± ± ± ± 87 91 94 96 9 10 11 14 3.1 3.7 4.5 5.2 0.056 0.047 0.044 0.041 Mean pore size (nm) Density (g/cm3 ) (4) 3.1. Mechanism of surface modification and hydrophobic properties of TEOS-based silica aerogels Pore volume (cm3 /g) Porosity (%) Contact angle ( ◦ ) Optical transmittance (%) Condensation: The silica network formed due to the polymerization of the TEOS precursor was weaker than those obtained with the polymerization of colloidal silica. During the drying process, liquid and vapor coexisted within the pores of the gel. As the liquid began to evaporate, a meniscus was formed at the liquid–vapor interface. The formation of a liquid–vapor interface within the gel resulted in surface tension and created concave menisci in the pores of the gel. As evaporation progressed, the menisci receded into the gel body and built up a compressive force that acted on the walls of the pores. This caused considerable shrinkage due to the partial collapse of the gel network [26]. In addition, the terminal silanol groups (Si–OH) present on the silica surface underwent a condensation reaction, forming new siloxane bonds which ultimately resulted in an irreversible shrinkage of the gel [27]. This shrinkage can be reduced by attaching non-polar alkyl groups to the silica surface by replacing H by OH. Gel collapse also ceases when the gel structure is strong enough to withstand the tensile strength of the liquid. Therefore, to reduce the irreversible shrinkage of the alcogel during the drying process, the surface of the alcogel was organically modified by tri-methyl groups (found in TMCS). This was done via a one-step solvent exchange and surface modification processes using an alcohol/TMCS/perfluorohexane solution. The chemical reactions in the one-step solvent exchange and surface modification are as follows: CH3 OH + (CH3 )3 –Si–Cl → (CH3 )3 –Si–O–CH3 + HCl (5) C2 H5 OH + (CH3 )3 –Si–Cl → (CH3 )3 –Si–O–CH2 CH3 + HCl (6) C3 H7 OH + (CH3 )3 –Si–Cl → (CH3 )3 –Si–O–CH(CH3 )2 + HCl (7) C4 H9 OH + (CH3 )3 –Si–Cl → (CH3 )3 –Si–O–(CH2 )3 CH3 + HCl (8) (CH3 )3 –Si–Cl + (9) Si–OH → Si–O–Si(CH3 )3 + HCl Surface area (m2 /g) 978 1044 1080 1150 Methanol (MeOH) Ethanol (EtOH) Butanol (BuOH) Isopropanol (IsoPrOH) R–OH + (CH3 )3 –SiCl → (CH3 )3 Si–O–R + HCl Solvent Table 2 Comparison in physical and textural properties of the TEOS-based silica aerogels prepared with various solvents by one-step solvent exchange surface modification followed by ambient pressure drying. Pieces Pieces Multiple cracks Monolithic A. Hilonga et al. / Journal of Alloys and Compounds 487 (2009) 744–750 Remarks 746 (10) Thus, alcohols reacted with TMCS to form alkoxytrimethylsilane [(CH3 )3 Si–O–R] (Eq. (10)), and consequently, the hydrophilic surface of the silica network became hydrophobic. As the reaction proceeded, transferable liquids (HCl/residual alcohol) spontaneously come out of the wet gel [28]. During the one-step solvent exchange/surface modification process, the reaction between alcohols and TMCS helped to slow down the rate of reaction of TMCS with the OH groups on the silica surface. This process spontaneously replaced the pore alcohol with perfluorohexane. The chemical surface modification of the alcogels by non-polar alkyl/aryl groups is an indispensable step before the APD as it prohibits the formation of new siloxane bonds between the adjacent silica cluster, and thereby prevents shrinkage of the gel A. Hilonga et al. / Journal of Alloys and Compounds 487 (2009) 744–750 747 Fig. 3. TG/DTA curves obtained in an air atmosphere for the TEOS-isopropanolbased silica aerogel. Fig. 1. A water droplet (2 mm) placed on TEOS-isopropanol-based superhydrophobic silica aerogel surface. [29,30]. In the present work, the surfaces of the TEOS-based alcogels were organically modified by tri-methyl groups present in the trimethylchlorosilane using the one-step solvent exchange and surface modification process explained above. The surface modification of the gels was studied by measuring the contact angle of water droplets on the silica aerogel surface, and the results are tabulated in Table 2. The observed hydrophobicity was ascribed to the attachment of hydrolytically stable –Si–(CH3 )3 groups after the replacement of H by the –OH groups on the silica surface. There was a slight increase in the contact angle of the aerogels with the use of various solvents. The size of the increase occurred in the following order: methanol < ethanol < butanol < isopropanol. The contact angle was found to be highest (168◦ ) for aerogels prepared with the isopropanol solvent (Fig. 1). The surface modification of the gels was also confirmed using Fourier transform infrared spectroscopy (FTIR) as shown in Fig. 2. In the figure, it is evident that apart from the; Si–O–Si ( absorption peak at 1060 cm−1 , all aerogel samples showed Si–CH3 peaks at 850 and 1260 cm−1 [31–33]. The presence of Si–CH3 peaks in the FTIR spectra of all samples confirms the attachment of –Si–CH3 groups from the TMCS to the silica aerogel surface, and hence the surface modification of the aerogels was successful. The degree of intensity of the Si–CH3 peaks is dependent on the solvent used, and occurs in the following order: methanol < ethanol < butanol < isopropanol. Thus, the most surface modification occurred in the alcogel prepared with the isopropanol solvent. The aerogels were heated in a furnace at a range of temperatures to investigate the thermal stability of their hydrophobic natures. It has been observed that aerogels retain their hydrophobic behavior up to a maximum temperature of 450 ◦ C, above which they become hydrophilic. This is due to the fact that at this temperature the surface –CH3 groups, which are the reason for the hydrophobicity, are oxidized. Thermogravimetric (TG) and differential thermal (DT) analysis of the TMCS-modified silica aerogels were employed to confirm the thermal stability and oxidation temperature of the –CH3 groups, respectively. Fig. 3 illustrates the TG/DTA curves obtained for the isopropanol-based TMCS-modified silica aerogel in the temperature range of 25–1000 ◦ C, in an air atmosphere. The weight loss observed up to 200 ◦ C was due to the evaporation of the residual solvent and water molecules from the aerogel. The sharp decrease in weight at the temperature of 450 ◦ C can be attributed to the thermal decomposition of the organic groups (CH3 ) present in the aerogel. By considering the two sharp exothermic peaks in the DTA curve for temperatures above 450 ◦ C, the oxidation of surface methyl groups can be confirmed. The results are consistent with earlier work wherein the thermal stability of TEOS-based silica aerogels was observed up to 380 ◦ C [21]. 3.2. Effects of alcohol/TMCS molar ratio and perfluorohexane/TMCS volume ratio on the physical properties of TEOS-alcohol-based silica aerogels Fig. 2. Fourier transform infrared spectroscopy spectra of the TEOS-based silica aerogels prepared with various solvent (methanol, ethanol, butanol and isopropanol) and surface modified by alcohol/TMCS/perfluorohexane by one-step solvent exchange and surface modification process. Table 1 shows the physical properties of aerogels synthesized using different alcohols/TMCS molar ratios and perfluorohexane/TMCS volume ratios. The molar ratio of alcohol/TMCS was varied from 0.5 to 2, and the volumetric ratio of cyclo-hexane and TMCS was varied from 10 to 30. It was observed that the alcohol/TMCS ratio had no significant effect on the monolithicity of the dried aerogels at the tested molar ratios. Thus, in further experiments the alcohol/TMCS molar ratio was kept constant at 1. However, the volume ratio of perfluorohexane/TMCS strongly influenced the physical and textural properties of the resulting aerogels. Fewer cracks were observed when the volumetric ratio of perfluorohexane/TMCS was 25, as observed in related studies 748 A. Hilonga et al. / Journal of Alloys and Compounds 487 (2009) 744–750 Fig. 4. Photographs of TEOS-based silica aerogels prepared with (a) methanol and (b) isopropanol solvent. [23,32]. Therefore, in further experiments, the alcohol/TMCS volumetric ratio was fixed at 25. 3.3. Physical properties of TEOS-based silica aerogels To investigate the effects of different solvents on the physical properties of the silica aerogels, the molar ratio of alcohol/TMCS was kept constant at 1, and the volume ratio of perfluorohexane/TMCS was maintained at 25. It was found that the gelation time was strongly dependant on the type of solvent used. A smaller gelation time was observed with the methanol solvent, while the 1-butanol solvent induced a higher gelation time. This may be associated with the lengths of the alkyl groups which normally affect the rates of hydrolysis and condensation. Thus, 1-butanol contains alkyl groups with longer chains that slow down the hydrolysis and condensation reactions, and extend polymerization, thereby reducing gelation time (Table 2). Fig. 4 shows the effect of solvent-type on the monolithicity of the silica aerogel prepared by the two-step sol–gel process with a TEOS precursor via the one-step solvent exchange surface modification process. As shown in Fig. 4(a), the cracking probability during the APD of the alcogels was higher for samples prepared with the solvents with shorter alkyl chains. On the other hand, a monolithic silica aerogel was obtained when a solvent with alkyl groups having longer chains (isopropanol) was used, as shown in Fig. 4(b). This is due to the fact that the increase in chain-length in the alkyl group of a solvent extends the degree of polymerization in the two-step acid–base sol–gel process, consequently decreasing the cracking probability in the alcogel during APD. Hence, the volume shrinkage seen in the aerogels prepared with short-chain alkyl group solvents was higher than that seen in alkyl groups with longer chains (Table 2). The nanostructures of all aerogels investigated by TEM are presented in Fig. 5. The isopropanol solvent strongly promoted the formation of a highly branched polymeric structure in the alcogels. The fine polymeric structure is clearly evident in the TEM images from the aerogels prepared with the isopropanol solvent, Fig. 5. Transmission electron micrographs of TEOS-based silica aerogels prepared with various solvent (a) methanol (b) ethanol (c) butanol and (d) isopropanol. A. Hilonga et al. / Journal of Alloys and Compounds 487 (2009) 744–750 Fig. 6. N2 adsorption–desorption isotherms obtained for the TEOS-based silica aerogels prepared with various solvent and drying at ambient pressure. while the other samples have less polymeric structures. In addition, the aerogels synthesized with the isopropanol solvent showed a well-developed nano-porous structure consisting of aggregations of primary particles of about 15 nm in size, as shown in Fig. 5(d). As shown in Table 2, the bulk density of the aerogels decreased as the length of alkyl chains in the solvent increased. This is because the length of alkyl chain in the solvent tends to increase the degree of polymerization of the TEOS precursor and provide a flexible silica network. The flexible network can withstand the drying stress, so the drying shrinkage of the gel and, consequently, the bulk density of the resulting aerogels is reduced. Moreover, extensive polymerization leads to dense cross-linking which restricts the relative movement of the polymer chains under the action of an external force, resulting in the formation of a rigid polymer. A low degree of polymerization results in a polymer with a low Young’s modulus which can easily be deformed [34]. The optical transmittance obtained (at 750 nm) was found to be higher (91%) for the aerogels prepared with the isopropanol solvent than the aerogels prepared using solvents with shorter alkyl chains (Fig. 4). As shown in Fig. 4 (a and b), multiple cracks existed inside the aerogels prepared with the methanol solvent. These cracks scattered the incident light, resulting in a reduction in optical transmittance compared to aerogels prepared using the isopropanol solvent. In addition, optical transmission decreased with a decrease in the incident wavelength. This is because the Rayleigh scattering becomes more effective at shorter wavelengths and thereby reduces the optical transmission [35]. 3.4. Textural properties of the TEOS-alcohol-based silica aerogels The specific surface areas of the aerogels, as determined by the standard BET method, are summarized in Table 2. All aerogels exhibited high specific surface areas in the range of 900–1200 m2 /g. The specific surface area was found to be higher for the aerogels prepared using the isopropanol solvent than the aerogels prepared using the solvents with shorter alkyl chains. This is because efficient surface modification by the tri-methyl groups took place in the alcogels prepared using the isopropanol solvent via the onestep solvent exchange surface modification process. The average pore diameters (pd) of the TEOS-based aerogels prepared using different solvents were in the range of 11–14 nm. The increase in pore diameter (particularly observed in isopropanol-based aerogels) can be attributed to the fact that more surface modification and polymerization took place. Isopropanol also enhanced the spring-back effect, resulting in less volume shrinkage and a high pore diameter. The nitrogen adsorption–desorption isotherms obtained at 77 K are shown in Fig. 6. The nitrogen physisorption isotherms obtained 749 Fig. 7. BET pore size distribution profiles of the ambient pressure dried silica aerogels prepared with various solvents. for all samples exhibited Type IV hysteresis loops, which are generally observed in mesoporous materials [36]. The desorption cycles of the isotherms showed hysteresis loops in all of the aerogels, which were attributed to capillary condensation occurring in the mesopores [37]. Fig. 7 shows the pore size distributions (PSDs) of the aerogels synthesized using four different solvents: methanol, ethanol, isopropanol, and butanol. The aerogels prepared with the methanol solvent showed a relatively narrow PSD with a pronounced peak that corresponds to pores with diameters of 8–10 nm. The aerogels prepared with ethanol, isopropanol, and butanol exhibited broad PSDs. The peak pore diameter shifted to a higher value in the aerogels prepared by ethanol, isopropanol, and butanol solvents. All of the aerogels showed a pronounced peak in the mesopore region even after drying at ambient pressure. 4. Conclusions Monolithic silica aerogels with an extremely low density (0.041 g/cm3 ) and large surface area (1150 m2 /g) were synthesized using a two-step acid–base sol–gel polymerization of tetraethoxysilane (TEOS) with isopropanol solvent via a one-step solvent exchange surface modification process and ambient pressure drying (APD). The resulting silica aerogels possessed a low bulk density and high porosity, similar to those produced by conventional supercritical drying. The success of the approach lies in the fact that the two-step sol–gel polymerization of TEOS using the isopropanol solvent promoted formation of a highly branched polymeric structure in the alcogel that withstood drying stress during APD, resulting in a monolithic aerogel. Sufficient chemical modification can be achieved by the one-step solvent exchange surface modification treatment via solvent/TMCS/perfluorohexane. 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