Effect of solvent exchange on silica aerogel properties via ambient pressure drying

Effect of solvent exchange on silica aerogel properties via ambient pressure drying | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Effect of solvent exchange on silica aerogel properties via ambient pressure drying Xiaodong Hu, Hao Li, Chunyi Tong, Shun Yang, Yuqiong Li, Rilong Zhu, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4137303/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 19 Aug, 2025 Read the published version in Journal of Sol-Gel Science and Technology → Version 1 posted 7 You are reading this latest preprint version Abstract Silica aerogel has been widely used in aerospace, energy construction, petrochemical industry, etc. Its physicochemical properties strongly correlate to the modification methods, such as aging and surface hydrophobicity treatments. Among these methods, solvent exchange plays a key role in synthesizing silica aerogel by ambient pressure drying. In order to facilitate the ambient pressure drying preparation of silica aerogel for industrial application, we designed an efficient automatic solvent exchange device for silica wet gels, and systematically studied the influence of solvent exchange on the physicochemical properties of silica aerogel via ambient pressure drying. After aging and surface hydrophobic modification, the silica wet gels undergo solvent exchange at different temperatures and times. The results show that with the increase of temperature and time, the moisture content of solvent in silica wet gel, as well as the bulk density and thermal conductivity of silica aerogel decrease. Moreover, we studied the effect of solvents with different surface tensions on the physicochemical properties of silica aerogels. The use of solvents with low surface tension can produce silica aerogels with lower bulk density and thermal conductivity, while the specific surface area, pore diameter and pore volume increase. These findings emphasize the importance of solvent exchange to improve the ability of gel particle network skeleton to withstand irreversible pore collapse via ambient pressure drying. Aerogel solvent exchange thermal conductivity FTIR BET Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 1. Introduction Aerogel is generally composed of nano particles or polymers with air as the dispersion medium [ 1 ]. As a three-dimensional nano porous material with ultra-high porosity, it usually has extremely low density (~ 0.003-0.1g/cm 3 ) [ 2 , 3 ], high porosity (~ 80-99.8%), ultra-low thermal conductivity (~ 0.005-0.1W/(m·K)), and high specific surface area (~ 500-1200m 2 /g) [ 4 , 5 ]. Because of its high thermal insulation, it is widely used in aerospace, petrochemical, industrial catalysis and other fields[ 6 , 7 ]. At present, aerogel materials are mainly divided into three categories: inorganic aerogel, organic aerogel and composite aerogel. As a typical representative of aerogel materials, silicon dioxide aerogel is the earliest and relatively more mature aerogel in high-temperature thermal insulation aerogels. Silica aerogels are usually prepared by the sol gel method, by catalytic hydrolysis polycondensation of alkoxides such as tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS) [ 8 ], followed by aging and solvent exchange processes. During the preparation and drying of aerogels, the pore structure of aerogels is often destroyed by the great capillary force of solvent volatilization. Supercritical drying technology(SCD) is a good method to solve the hole collapse, but it has the problems of complicated operation and high cost, so it can not be applied on a large scale[ 9 – 11 ]. In contrast, ambient pressure drying (APD) has significant competitive advantages in terms of cost and safety. However, the difficulty lies in how to make the silica gel resist the capillary tension acting on the framework under ambient pressure drying, and not shrink and collapse due to the pressure difference on the pore wall of the gel, and the porosity of the gel is basically not affected [ 10 , 11 ]. To address these issues in APD, a lot of research work has been carried out through: (1) strengthening and optimizating gel matrix, which improves the resistance to stress generated during drying by enhancing the mechanical strength of gel skeleton structure; (2) reducing the irreversible shrinkage caused by hydroxyl condensation through reducing the force of pore fluid on gel skeleton during drying, so that gel can still maintain its original skeleton structure after ambient pressure drying [ 5 , 12 – 28 ]. Beside these, modification on aging process of wet gel can also effectively enhance the skeleton structure[ 29 – 36 ]. For instance, Einarsrud et al. [ 29 ] put the wet gel in TEOS solution for aging, so that silica can further deposit on the gel skeleton to obtain an enhanced gel network, thus preparing low density silica aerogel under APD for the first time by aging enhancement method. Haereid et al. [ 30 , 31 ] used TEOS/methanol (MeOH) as the aging solution to increase the shear modulus of gel to 1.8 MPa and 7.4 MPa after aging, which proved that the stiffness and strength of gel can be effectively increased by aging means. Although a lot of research work has been carried out on the preparation of silica aerogel by APD, the research on solvent replacement process is rare. In this study, we designed an facile and massive production applicable solvent exchange device for wet gel, and investigated the effect of solvent exchange process on the physicochemical properties of silica aerogel under APD. Unlike previous studies, this work does not involve gel aging and surface modification of wet gel, but focuses on solvent exchange. we designed an efficient automatic solvent exchange device for silica wet gels, and systematically studied the influence of solvent exchange on the physicochemical properties of silica aerogel via APD. With acetone as the exchange solvent, the silica wet gel, after aging and surface hydrophobic modification, underwent solvent exchange at 65 ℃ for 24 h, and the silica aerogel prepared under APD had relatively better performance. The density and thermal conductivity were as low as 0.129 g/cm 3 and 0.027 W/m·K, and the specific surface area was 460.75 m 2 /g. Moreover, we studied the effect of solvents with different surface tensions on the physicochemical properties of silica aerogels. Using the solvent with low surface tension can obtain silica aerogel with excellent performance via APD. The bulk density and thermal conductivity are as low as 0.101 g/cm 3 and 0.016 W/m·K, while the specific surface area, pore diameter and pore volume can reach 670.35 m 2 /g, 14.93 nm and 2.5 ml/g, respectively. In conclusion, solvent exchange has a particularly profound impact on the synthesis of silica aerogels by APD. 2. Experimental section 2.1 Materials Tetramethoxysilane (TMOS, AR), methanol (MeOH, 99.9%), isopropanol (IPA, 99.5%), ammonia aqueous solution (25–28 wt%), acetone (≥ 99.9%) and n-hexane (≥ 99.9%) were provided by Shanghai Titan Scientific Co., Ltd (Shanghai, China). Hexamethyldisilazane (HMDS, 99%) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Decafluoropentane (Vertrel XF, 99.5%) were provided by Chemours Co., Ltd (Shanghai, China). Methoxy-9-fluorobutane (HFE-7100, 99.5%) was obtained from Shanghai Kunbond Chemical Co., Ltd (Shanghai, China). All the chemical reagents were used as-received without further purification. 2.2 Synthesis of the silica aerogels All silica aerogels were prepared according to the process scheme depicted (Fig. 1 ). Firstly, 45 g of TMOS was dispersed in 110 g of MeOH, followed by adding 10 g of deionized water, and stirred at room temperature for 15 minutes after adding 15 g of ammonia water (0.1mol L − 1 ). Gelation occurred approximately 30 min after the static polycondensation reaction at room temperature. Secondly, after aging for 24 h at 60 ℃ isolated from air, the wet gel was added with 1000 g of mixed solution of HMDS and IPA (molar ratio is 1:12) at 60 ℃ for 3 times of surface hydrophobic modification, 2 h each time. Then, solvent exchange begins at different temperature (55, 60, 65 and 70 ℃) and times (0, 8, 16, 24 and 32 h). In addition, in order to study the effects of different solvents such as Acetone, Methanol, n-Hexane, Vertrel XF and HFE-7100 on the properties of silica aerogels, solvent exchange can be carried out under specific temperature and time conditions. Finally, the silica aerogels with different properties were obtained by drying at 80°C under ambient pressure for 12 h. 2.3 Dynamic solvent exchange In order to shorten the preparation period of silica aerogel and more effectively study the influence of solvent exchange on the physico-chemical properties of silica aerogel, a self-made dynamic solvent exchange device was used (Fig. 2 ). Put the stainless steel tray with punching holes containing silica wet gel in a tightly lockable container, fill the container with solvent, so that the silicon wet gel is completely covered, and the container is connected to the peristaltic pump through a hose. Use the pump to circulate the solvent. Place the container in thermostat water bath to ensure that solvent exchange takes place at a constant temperature. An important step before drying is solvent exchange, and the process overview of solvent exchange is shown in Fig. 3 . In the silica wet gel, the pores are filled with water and a few unreacted precipitates, oligomers and solvents used in the synthesis process. Solvent exchange actually involves replacing the liquid in the pores with a solvent. 2.4 Characterizations The moisture content in the solvent during solvent exchange of silica wet gel is measured by Automatic Karl Fischer Moisture Analyzer (V100, Shanghai, China), the measurement range is 0.003-100%, and the maximum possible error is ± 0.3%. The bulk density of silica aerogel is measured by a touch electronic density instrument (AE124J, Shanghai, China) with a test deviation of 0.01%. This instrument is used to test solids whose density is lower than that of water. The test principle is to use the drainage method, and use an inverted glass cup to press the solids to be measured into a beaker filled with water for testing. As silica aerogels were not perfectly regular in shape, the thermal conductivity is measured by crushing a large sample into powder. This measurement is carried out by using the flat plate heat flow meter method at 25 ℃ through the thermal resistance test system (DRL-III, Xiangtan, China). In this process, the polyurethane rigid foam frame with a length and width of 10cm and a height of 1cm (the frame thickness is 0.1cm) shall be prefabricated in advance, and the aerogel powder shall be filled with the frame during the test. Fourier transform infrared (FTIR) spectra in the region from 4000 to 500 cm − 1 were obtained by a Fourier transform infrared spectrometer (FTIR, Thermo Scientific iN10, USA) after the silica aerogels were ground and pressed with KBr powders.The specific surface area, pore size and pore volume of silica aerogels were estimated by measuring nitrogen adsorption-desorption isotherms at 77 K using specific surface & pore size analysis instrument (3H-2000PS2, Beijing, China). The specific surface areas of the aerogels were determined by the Brunauer-Emmette-Teller (BET) method, and the pore size distributions and average pore diameters of the silica aerogels were analyzed by the Barrett-Joyner-Halenda (BJH) method. The morphology and the element composition of the silica aerogel were assessed by means of scanning electron microscopy (SEM, ZEISS Sigma 300, Germany) at a 15-kV accelerating voltage after slices of the aerogel samples were coated with a thin platinum layer. Thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) were performed using a thermogravimetric analyzer (Netzsch TG 209 F1, Germany) in air at 10 ℃ minutes to 1200 ℃. 3. Results and discussion 3.1 bulk density and thermal conductivity of silica aerogels The silica aerogels prepared by APD is strongly affected by the solvent exchange conditions (Table 1 , Table 2 , Fig. 4 , and Fig. 5 ), including moisture content in solvent, solvent exchange temperature and times. Table 1 shows that the influence of solvent exchange temperature and time with Acetone as solvent on the physical properties of silica aerogel, and Table 2 shows that the influence of different solvents on the physical properties of silica aerogel via APD (Solvent exchange at 65 ℃ for 24 h). Figure 4 shows that the moisture content, bulk density and thermal conductivity of silica aerogel decrease with increasing solvent exchange durations and this is an universal trend for all solvent exchange temperatures. Under APD, the lower the moisture content in the solvent, the higher the solvent exchange effect, and the easier to obtain low bulk density and thermal conductivity silicon aerogels. the decrement in moisture content, bulk density, and thermal conductivity, reaches a peak at 8 h, and hardly changed beyond that. In the solvent exchange process of silica wet gel, the potential aging process continues to occur, which can also be called secondary aging, and aging is a dissolution precipitation process like Ostwald ripening [ 37 , 38 ]. In this process, the condensation reaction in the gel network framework and the degree of siloxane cross-linking continue to increase. Meanwhile silica and unreacted oligomers in the solution dissolve and precipitate onto the existing particle network and particle neck, significantly increasing the strength and stiffness of the gel framework [ 37 ]. The increase of solvent exchange temperature strengthens the kinetics of this process, while the increase of solvent exchange time provides more time for these reactions to occur. Together, the two lead to a higher mechanical stiffness of the particle network structure, which makes the gel network skeleton still able to resist the destruction of capillary force during normal pressure drying. However, if the solvent exchange temperature is too high, the gel network skeleton will become thick due to excessive precipitation, which will directly lead to the increase of the density and thermal conductivity of the silicon aerogel (Fig. 4 ). Figure 5 shows that the solvent type is crucial to the physical properties, bulk density and thermal conductivity of silica aerogels. After solvent exchange of wet gel, the solvent volatilization in the pores of gel occurs under APD. The solvent volatilization with lower surface tension will cause less stress on the pore wall, thus reducing the pore collapse to obtain silicon aerogel with better performance. Table 1 Influence of solvent exchange temperature and time with Acetone as solvent on the physical properties of silica aerogel. Solvent exchange temperature (℃) Solvent exchange time (hrs.) MC (wt%) a Bulk density (g/cm 3 ) b Thermal conductivity (W/m·K) c 55 0 15.14 0.792 0.086 8 3.22 0.362 0.051 16 0.94 0.205 0.037 24 0.85 0.131 0.030 32 0.52 0.130 0.030 60 0 15.16 0.801 0.091 8 2.93 0.378 0.048 16 0.88 0.211 0.032 24 0.79 0.129 0.031 32 0.51 0.129 0.029 65 0 15.15 0.799 0.089 8 2.42 0.378 0.041 16 0.71 0.211 0.028 24 0.68 0.129 0.027 32 0.50 0.130 0.029 70 0 15.10 0.805 0.094 8 2.21 0.378 0.035 16 0.65 0.211 0.036 24 0.64 0.212 0.041 32 0.50 0.224 0.043 Estimated uncertainties: a Moisture content in solvent, 0.3% relative. b Bulk density, 0.01% relative. c Thermal conductivity, 3% relative. Table 2 Influence of different solvents on the physical properties of silica aerogel via APD (Solvent exchange at 65 ℃ for 24 h). Solvent Surface tension mN/m (25℃) Bulk density (g/cm 3 ) a S BET (m 2 /g) b Pore diameter (nm) c Pore volume (ml/g) d Thermal conductivity (W/m·K) e Acetone 23.5 0.129 460.75 9.39 1.08 0.027 MeOH 22.6 0.128 473.38 11.29 1.33 0.022 n-Hexane 18.4 0.117 539.65 12.37 2.17 0.020 Vertrel XF 14.1 0.108 577.02 14.28 2.06 0.017 HFE-7100 13.6 0.101 670.35 14.93 2.50 0.016 Estimated uncertainties: a Bulk density, 0.01% relative. b S BET , around 15 m 2 /g. c Pore diameter, around 0.5 nm. d Pore volume, around 0.1 ml/g. e Thermal conductivity, 3% relative. 3.2 FTIR results of the silica aerogels The wet gel was solvent exchanged with acetone, MeOH, n-hexane, Vertrel XF and HFE-7100 at 65 ℃ for 24 h, under APD. Its infrared (IR) spectrum is shown in Fig. 6 . The C-H tensile vibration peak at 2972 cm − 1 was observed within the range of 2800–3000 cm − 1 [ 39 ], and a characteristic bending vibration peak of Si-CH 3 was observed at 1273 cm − 1 . The infrared absorption peaks of Si-O-C are in the range of 1000–1300 cm − 1 . The two absorption peaks of 1121cm − 1 and 1031cm − 1 in Fig. 6 are mainly caused by the stretching vibration of C-O-C and Si-O-C [ 40 , 41 ], while the absorption peaks at 781 cm − 1 and 1273 cm − 1 belong to the asymmetric stretching vibration of Si-C. 3.3 Macrostructure and nanostructure assessment of the silica aerogels Figure 7 shows the microstructure of silica aerogels obtained under APD by solvent exchange with acetone, MeOH, n-Hexane, Vertrel XF and HFE-7100 respectively. As indicated, the sample conforms to the typical inorganic gel morphology, and shows a concave convex surface composed of a large number of spherical nanoparticles. Using Vertrel XF and HFE-7100 with lower surface tension for solvent exchange can obtain silicon aerogel with better network pore structure. The N 2 adsorption desorption isotherm and pore size distribution of the silica aerogels prepared by solvent exchange with acetone, MeOH, n-Hexane, Vertrel XF and HFE-7100 are shown in Fig. 8 . the isotherm of all samples has a type IV curve, and the isotherm of the five samples shows a type H1 hysteresis loop, which has the mesoporous structure characteristics of columnar pores [ 42 ]. It can be inferred from Fig. 8 and Table 2 that with the decrease of solvent surface tension, the specific surface area of silica aerogel increases in the range of 460.75 to 670.35m 2 /g, while the average pore size, pore volume and thermal conductivity will continuously decrease. When HFE-7100 is used as the exchange solvent, the silica aerogel obtained under APD conditions has the best performance, with a specific surface area of 670.35m 2 /g, an average pore diameter of 14.93 nm, a pore volume of 2.5ml/g, and a thermal conductivity as low as 0.016 W/m·K. 3.4 Thermal stability of the silica aerogels Figure 9 shows the TG-DSC scanning curve of silica aerogel in air atmosphere. There is an obvious exothermic peak in the DSC curve. The exothermic peak starts when the temperature rises to 336.8 ℃, and reaches the peak at 380 ℃. This is because the silicon dioxide aerogel structure -Si (CH 3 ) 3 reacts with oxygen in the air to form Si-OH to release heat [ 43 ]. From the TG curve, there are two obvious weight losses in the heating process of silica aerogel from 25 ℃ to 800 ℃: the first weight loss is 380 ℃, and the weight loss rate is 2.34%, which is mainly caused by the fracture of -Si (CH 3 ) 3 and the volatilization of residual solvent; The second weight loss occurred at 380–700 ℃, with a weight loss rate of 36.22%. During this stage, the DSC curve showed a significant exothermic peak starting at 700 ℃ and reaching its peak at 740 ℃. When the temperature is above 800 ℃, the TG curve decreases slowly, and a condensation reaction occurs between Si-OH to produce Si-O-Si. 4. Conclusion Based on the wet gel solvent exchange in the preparation of silica aerogels, we designed an efficient automatic solvent exchange device for silica wet gels, and systematically studied the influence of solvent exchange on the physicochemical properties of silica aerogel via APD. Using acetone as the exchange solvent, after aging and surface hydrophobic modification of silica wet gel, solvent exchange can be completed in only 24 hours at 65 ℃ through this device. We can obtain silica aerogel with good performance by APD, its density and thermal conductivity are as low as 0.129 g/cm 3 and 0.027 W/m·K, respectively, and its specific surface area is 460.75 m 2 /g. In addition, using this self-designed device, we also studied the influence of solvents with different surface tensions on the physicochemical properties of silica aerogels under the same solvent exchange temperature (65℃) and time (24 h) mentioned above. The results shows that using low surface tension solvent HFE-7100 (25 ℃ surface tension is 13.6 mN/m), we can obtain silica aerogels with excellent performance by APD, and the bulk density and thermal conductivity are as low as 0.101 g/cm 3 and 0.016 W/m·K, respectively, and the specific surface area, pore size, and pore volume can reach 670.35 m 2 /g, 14.93 nm, and 2.5 ml/g, respectively. The solvent exchange of wet gel is accompanied by this potential aging process. The interparticle neck of silica pearl necklace structure is strengthened through the dissolution and precipitation process like Ostwald ripening. This process is driven by minimizing the energy by reducing the total surface area. The drying process magnified the influence of solvent exchange process on the structure and stability of gel skeleton. Due to the strong capillary force and mechanical deformation (spring-back effect) that occurred in the process of drying under ambient pressure but mostly did not exist in the supercritical drying process, the samples with insufficient solvent exchange will have strong irreversible pore collapse. Moreover, the proposed method is highly applicable for industrial applications. Therefore, compared with aging and surface hydrophobic modification, the solvent exchange process is critical for the successful synthesis of low-density silica aerogels through APD. Declarations Author Contribution H. LH. Y wrote the main manuscript text and L prepared figures 1-9. All authors reviewed the manuscript. Acknowledgements This work was supported by the Fundamental Research Funds for the Science & Technology Department of Sichuan Province References Z. Yang, D. Zhu, H. 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Cite Share Download PDF Status: Published Journal Publication published 19 Aug, 2025 Read the published version in Journal of Sol-Gel Science and Technology → Version 1 posted Editorial decision: Revision requested 05 Apr, 2024 Reviews received at journal 29 Mar, 2024 Reviewers agreed at journal 27 Mar, 2024 Reviewers invited by journal 25 Mar, 2024 Editor assigned by journal 21 Mar, 2024 Submission checks completed at journal 20 Mar, 2024 First submitted to journal 20 Mar, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4137303","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":282131168,"identity":"9b999a5d-dd86-4d59-a937-02ea542a16c9","order_by":0,"name":"Xiaodong Hu","email":"","orcid":"","institution":"Hunan University","correspondingAuthor":false,"prefix":"","firstName":"Xiaodong","middleName":"","lastName":"Hu","suffix":""},{"id":282131169,"identity":"f30c0ac4-f2b1-4a0c-b050-9c6a3424bd15","order_by":1,"name":"Hao Li","email":"","orcid":"","institution":"Hunan University","correspondingAuthor":false,"prefix":"","firstName":"Hao","middleName":"","lastName":"Li","suffix":""},{"id":282131170,"identity":"9d74f734-6b08-401e-b043-9ce7d041ac4f","order_by":2,"name":"Chunyi Tong","email":"","orcid":"","institution":"Hunan University","correspondingAuthor":false,"prefix":"","firstName":"Chunyi","middleName":"","lastName":"Tong","suffix":""},{"id":282131172,"identity":"ec0508ef-e838-4112-ab09-9c1d48698912","order_by":3,"name":"Shun Yang","email":"","orcid":"","institution":"Leshan Vocational and Technical College","correspondingAuthor":false,"prefix":"","firstName":"Shun","middleName":"","lastName":"Yang","suffix":""},{"id":282131173,"identity":"40542d17-7cd0-43b9-8690-c2329420ebf6","order_by":4,"name":"Yuqiong Li","email":"","orcid":"","institution":"Leshan Vocational and Technical College","correspondingAuthor":false,"prefix":"","firstName":"Yuqiong","middleName":"","lastName":"Li","suffix":""},{"id":282131176,"identity":"fb70c353-cd41-49cc-b707-0142e0c2560f","order_by":5,"name":"Rilong Zhu","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAvUlEQVRIiWNgGAWjYFAC5gYJBgabBDCbhzgtjCAtaaRrOUyCFvnZBxtvfNxxPs9cIoHxwds2Bnlzgnb0JTZbzjxzu9hyRgKz4dw2BsOdDQS0MPMwtknztt1O3HAjgQ3IYEgwOEBACxtIy9+2cyAt7L+J0sID0sLYdgBsCzNRWiR4GJste9uSEzecedgsOeechOEGQlrke5gP3vjZZpe44XjywQ9vymzkCdqCBBgbQLYSr34UjIJRMApGAW4AAHLqPrqnatm+AAAAAElFTkSuQmCC","orcid":"","institution":"Hunan University","correspondingAuthor":true,"prefix":"","firstName":"Rilong","middleName":"","lastName":"Zhu","suffix":""},{"id":282131177,"identity":"c42ae89c-9743-4eab-b1ec-505d2f59bb6f","order_by":6,"name":"Deliang He","email":"","orcid":"","institution":"Hunan University","correspondingAuthor":false,"prefix":"","firstName":"Deliang","middleName":"","lastName":"He","suffix":""},{"id":282131179,"identity":"4984f6d5-a799-4eb1-af8c-70c332109db4","order_by":7,"name":"Baicheng Weng","email":"","orcid":"","institution":"Central South University","correspondingAuthor":false,"prefix":"","firstName":"Baicheng","middleName":"","lastName":"Weng","suffix":""}],"badges":[],"createdAt":"2024-03-20 12:57:23","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4137303/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4137303/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s10971-025-06831-4","type":"published","date":"2025-08-19T16:29:46+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":53263451,"identity":"8d39d8f5-c989-4500-bf4e-7684c46cd4e5","added_by":"auto","created_at":"2024-03-22 15:05:24","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":45113,"visible":true,"origin":"","legend":"\u003cp\u003eSynthetic illustration on silica aerogel preparation\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4137303/v1/f2490a1a9daf81de1da211cf.jpg"},{"id":53263448,"identity":"2057ebba-4593-4f49-8a91-d2ce9858bebd","added_by":"auto","created_at":"2024-03-22 15:05:23","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":62201,"visible":true,"origin":"","legend":"\u003cp\u003eSetup for dynamic solvent exchange: ① thermostat water bath; ② container for new solvent, with inlet for adding new solvent (marked with red circle)); ③ container for solvent exchange of wet gel; ④ tube pump with adjustable speed, used to introduce new solvents into the container for solvent exchange (the direction of circulation of solvent is marked with red arrows); ⑤ tube pump with adjustable speed, used to export the solvent to the container for waste solvent after solvent exchange; ⑥ container for waste solvent; ⑦ silica wet gel; ⑧ stainless steel tray with punching holes is placed inside the solvent exchange container to hold wet gel, which will make the solvent exchange of wet gel more efficient.\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4137303/v1/e1ac3179dfa87ccf925c719d.jpg"},{"id":53263445,"identity":"9d0ee62a-3c6b-4c3d-b8ca-2ecbb86af943","added_by":"auto","created_at":"2024-03-22 15:05:23","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":44291,"visible":true,"origin":"","legend":"\u003cp\u003eScheme of solvent exchange of silica aerogels: a) after the sol-gel, aging and surface hydrophobic modification processes, the pores are filled with water and unreacted chemicals; b) during solvent exchange, water inside of the pores is replaced by solvent; c) Finally, only solvent is present in the pores.\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4137303/v1/fab67a9b56b24f665e227f89.jpg"},{"id":53263447,"identity":"3da3a208-c983-4503-b5e2-dc7b80b47df0","added_by":"auto","created_at":"2024-03-22 15:05:23","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":64447,"visible":true,"origin":"","legend":"\u003cp\u003eInfluence of solvent exchange temperature and time on (a) the moisture content, (b) bulk density and (c) thermal conductivity of silica aerogels.\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4137303/v1/663503ee0f794ee74acb6ba0.jpg"},{"id":53263450,"identity":"3f8c111d-8e73-40a6-9f04-90b5394e2396","added_by":"auto","created_at":"2024-03-22 15:05:24","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":48361,"visible":true,"origin":"","legend":"\u003cp\u003eInfluence of solvent on the thermal conductivity and bulk density of silica aerogels.\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4137303/v1/3ca547e0de9fab3ee8ea02b7.jpg"},{"id":53263449,"identity":"c920c7f0-81a9-4736-8867-3bb38d1573cd","added_by":"auto","created_at":"2024-03-22 15:05:24","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":36901,"visible":true,"origin":"","legend":"\u003cp\u003eFTIR spectra of silica aerogels prepared by solvent exchange of acetone, methanol, n-hexane, Vertrel XF and HFE-7100.\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4137303/v1/e77a71650214547f826f451e.jpg"},{"id":53263446,"identity":"95e9f785-b98d-4b9c-9a72-1eb06a7dee29","added_by":"auto","created_at":"2024-03-22 15:05:23","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":143542,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of silica aerogels prepared by solvent exchange of acetone, methanol, n-hexane, Vertrel XF and HFE-7100.\u003c/p\u003e","description":"","filename":"7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4137303/v1/a998a7d565754f33eeb73556.jpg"},{"id":53263452,"identity":"6360aaa6-7a30-4a64-a1b3-73d69eb06693","added_by":"auto","created_at":"2024-03-22 15:05:24","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":87726,"visible":true,"origin":"","legend":"\u003cp\u003eN\u003csub\u003e2\u003c/sub\u003e adsorption-desorption isotherms and pore size distributions of silica aerogels prepared by solvent exchange of acetone, methanol, n-hexane, Vertrel XF and HFE-7100.\u0026nbsp;\u003c/p\u003e","description":"","filename":"8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4137303/v1/673d24b0f62cc52b95c99f35.jpg"},{"id":53263441,"identity":"c06106af-eb98-440a-bee4-d08f6243dde3","added_by":"auto","created_at":"2024-03-22 15:05:23","extension":"jpg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":46792,"visible":true,"origin":"","legend":"\u003cp\u003eTG and DSC curves of silica aerogels prepared by solvent exchange of HFE-7100.\u003c/p\u003e","description":"","filename":"9.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4137303/v1/e4c8843b3dc5177e8d0b3382.jpg"},{"id":89847826,"identity":"9ba6dbe8-bc7e-4ebc-a861-ebf3d83661b8","added_by":"auto","created_at":"2025-08-25 16:44:27","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1349108,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4137303/v1/0332f9a7-f095-4747-b16a-614ff8b9a1b5.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Effect of solvent exchange on silica aerogel properties via ambient pressure drying","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eAerogel is generally composed of nano particles or polymers with air as the dispersion medium [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. As a three-dimensional nano porous material with ultra-high porosity, it usually has extremely low density (~\u0026thinsp;0.003-0.1g/cm\u003csup\u003e3\u003c/sup\u003e) [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e], high porosity (~\u0026thinsp;80-99.8%), ultra-low thermal conductivity (~\u0026thinsp;0.005-0.1W/(m\u0026middot;K)), and high specific surface area (~\u0026thinsp;500-1200m\u003csup\u003e2\u003c/sup\u003e/g) [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Because of its high thermal insulation, it is widely used in aerospace, petrochemical, industrial catalysis and other fields[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. At present, aerogel materials are mainly divided into three categories: inorganic aerogel, organic aerogel and composite aerogel. As a typical representative of aerogel materials, silicon dioxide aerogel is the earliest and relatively more mature aerogel in high-temperature thermal insulation aerogels.\u003c/p\u003e \u003cp\u003eSilica aerogels are usually prepared by the sol gel method, by catalytic hydrolysis polycondensation of alkoxides such as tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS) [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e], followed by aging and solvent exchange processes. During the preparation and drying of aerogels, the pore structure of aerogels is often destroyed by the great capillary force of solvent volatilization. Supercritical drying technology(SCD) is a good method to solve the hole collapse, but it has the problems of complicated operation and high cost, so it can not be applied on a large scale[\u003cspan additionalcitationids=\"CR10\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. In contrast, ambient pressure drying (APD) has significant competitive advantages in terms of cost and safety. However, the difficulty lies in how to make the silica gel resist the capillary tension acting on the framework under ambient pressure drying, and not shrink and collapse due to the pressure difference on the pore wall of the gel, and the porosity of the gel is basically not affected [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. To address these issues in APD, a lot of research work has been carried out through: (1) strengthening and optimizating gel matrix, which improves the resistance to stress generated during drying by enhancing the mechanical strength of gel skeleton structure; (2) reducing the irreversible shrinkage caused by hydroxyl condensation through reducing the force of pore fluid on gel skeleton during drying, so that gel can still maintain its original skeleton structure after ambient pressure drying [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan additionalcitationids=\"CR13 CR14 CR15 CR16 CR17 CR18 CR19 CR20 CR21 CR22 CR23 CR24 CR25 CR26 CR27\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eBeside these, modification on aging process of wet gel can also effectively enhance the skeleton structure[\u003cspan additionalcitationids=\"CR30 CR31 CR32 CR33 CR34 CR35\" citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. For instance, Einarsrud et al. [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e] put the wet gel in TEOS solution for aging, so that silica can further deposit on the gel skeleton to obtain an enhanced gel network, thus preparing low density silica aerogel under APD for the first time by aging enhancement method. Haereid et al. [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e] used TEOS/methanol (MeOH) as the aging solution to increase the shear modulus of gel to 1.8 MPa and 7.4 MPa after aging, which proved that the stiffness and strength of gel can be effectively increased by aging means.\u003c/p\u003e \u003cp\u003eAlthough a lot of research work has been carried out on the preparation of silica aerogel by APD, the research on solvent replacement process is rare. In this study, we designed an facile and massive production applicable solvent exchange device for wet gel, and investigated the effect of solvent exchange process on the physicochemical properties of silica aerogel under APD. Unlike previous studies, this work does not involve gel aging and surface modification of wet gel, but focuses on solvent exchange.\u003c/p\u003e \u003cp\u003ewe designed an efficient automatic solvent exchange device for silica wet gels, and systematically studied the influence of solvent exchange on the physicochemical properties of silica aerogel via APD. With acetone as the exchange solvent, the silica wet gel, after aging and surface hydrophobic modification, underwent solvent exchange at 65 ℃ for 24 h, and the silica aerogel prepared under APD had relatively better performance. The density and thermal conductivity were as low as 0.129 g/cm\u003csup\u003e3\u003c/sup\u003e and 0.027 W/m\u0026middot;K, and the specific surface area was 460.75 m\u003csup\u003e2\u003c/sup\u003e/g. Moreover, we studied the effect of solvents with different surface tensions on the physicochemical properties of silica aerogels. Using the solvent with low surface tension can obtain silica aerogel with excellent performance via APD. The bulk density and thermal conductivity are as low as 0.101 g/cm\u003csup\u003e3\u003c/sup\u003e and 0.016 W/m\u0026middot;K, while the specific surface area, pore diameter and pore volume can reach 670.35 m\u003csup\u003e2\u003c/sup\u003e/g, 14.93 nm and 2.5 ml/g, respectively. In conclusion, solvent exchange has a particularly profound impact on the synthesis of silica aerogels by APD.\u003c/p\u003e"},{"header":"2. Experimental section","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Materials\u003c/h2\u003e \u003cp\u003eTetramethoxysilane (TMOS, AR), methanol (MeOH, 99.9%), isopropanol (IPA, 99.5%), ammonia aqueous solution (25\u0026ndash;28 wt%), acetone (\u0026ge;\u0026thinsp;99.9%) and n-hexane (\u0026ge;\u0026thinsp;99.9%) were provided by Shanghai Titan Scientific Co., Ltd (Shanghai, China). Hexamethyldisilazane (HMDS, 99%) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Decafluoropentane (Vertrel XF, 99.5%) were provided by Chemours Co., Ltd (Shanghai, China). Methoxy-9-fluorobutane (HFE-7100, 99.5%) was obtained from Shanghai Kunbond Chemical Co., Ltd (Shanghai, China). All the chemical reagents were used as-received without further purification.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Synthesis of the silica aerogels\u003c/h2\u003e \u003cp\u003eAll silica aerogels were prepared according to the process scheme depicted (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Firstly, 45 g of TMOS was dispersed in 110 g of MeOH, followed by adding 10 g of deionized water, and stirred at room temperature for 15 minutes after adding 15 g of ammonia water (0.1mol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). Gelation occurred approximately 30 min after the static polycondensation reaction at room temperature. Secondly, after aging for 24 h at 60 ℃ isolated from air, the wet gel was added with 1000 g of mixed solution of HMDS and IPA (molar ratio is 1:12) at 60 ℃ for 3 times of surface hydrophobic modification, 2 h each time. Then, solvent exchange begins at different temperature (55, 60, 65 and 70 ℃) and times (0, 8, 16, 24 and 32 h). In addition, in order to study the effects of different solvents such as Acetone, Methanol, n-Hexane, Vertrel XF and HFE-7100 on the properties of silica aerogels, solvent exchange can be carried out under specific temperature and time conditions. Finally, the silica aerogels with different properties were obtained by drying at 80\u0026deg;C under ambient pressure for 12 h.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Dynamic solvent exchange\u003c/h2\u003e \u003cp\u003eIn order to shorten the preparation period of silica aerogel and more effectively study the influence of solvent exchange on the physico-chemical properties of silica aerogel, a self-made dynamic solvent exchange device was used (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Put the stainless steel tray with punching holes containing silica wet gel in a tightly lockable container, fill the container with solvent, so that the silicon wet gel is completely covered, and the container is connected to the peristaltic pump through a hose. Use the pump to circulate the solvent. Place the container in thermostat water bath to ensure that solvent exchange takes place at a constant temperature. An important step before drying is solvent exchange, and the process overview of solvent exchange is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. In the silica wet gel, the pores are filled with water and a few unreacted precipitates, oligomers and solvents used in the synthesis process. Solvent exchange actually involves replacing the liquid in the pores with a solvent.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Characterizations\u003c/h2\u003e \u003cp\u003eThe moisture content in the solvent during solvent exchange of silica wet gel is measured by Automatic Karl Fischer Moisture Analyzer (V100, Shanghai, China), the measurement range is 0.003-100%, and the maximum possible error is \u0026plusmn;\u0026thinsp;0.3%. The bulk density of silica aerogel is measured by a touch electronic density instrument (AE124J, Shanghai, China) with a test deviation of 0.01%. This instrument is used to test solids whose density is lower than that of water. The test principle is to use the drainage method, and use an inverted glass cup to press the solids to be measured into a beaker filled with water for testing. As silica aerogels were not perfectly regular in shape, the thermal conductivity is measured by crushing a large sample into powder. This measurement is carried out by using the flat plate heat flow meter method at 25 ℃ through the thermal resistance test system (DRL-III, Xiangtan, China). In this process, the polyurethane rigid foam frame with a length and width of 10cm and a height of 1cm (the frame thickness is 0.1cm) shall be prefabricated in advance, and the aerogel powder shall be filled with the frame during the test. Fourier transform infrared (FTIR) spectra in the region from 4000 to 500 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e were obtained by a Fourier transform infrared spectrometer (FTIR, Thermo Scientific iN10, USA) after the silica aerogels were ground and pressed with KBr powders.The specific surface area, pore size and pore volume of silica aerogels were estimated by measuring nitrogen adsorption-desorption isotherms at 77 K using specific surface \u0026amp; pore size analysis instrument (3H-2000PS2, Beijing, China). The specific surface areas of the aerogels were determined by the Brunauer-Emmette-Teller (BET) method, and the pore size distributions and average pore diameters of the silica aerogels were analyzed by the Barrett-Joyner-Halenda (BJH) method. The morphology and the element composition of the silica aerogel were assessed by means of scanning electron microscopy (SEM, ZEISS Sigma 300, Germany) at a 15-kV accelerating voltage after slices of the aerogel samples were coated with a thin platinum layer. Thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) were performed using a thermogravimetric analyzer (Netzsch TG 209 F1, Germany) in air at 10 ℃ minutes to 1200 ℃.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.1 bulk density and thermal conductivity of silica aerogels\u003c/h2\u003e \u003cp\u003eThe silica aerogels prepared by APD is strongly affected by the solvent exchange conditions (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, and Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), including moisture content in solvent, solvent exchange temperature and times. Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows that the influence of solvent exchange temperature and time with Acetone as solvent on the physical properties of silica aerogel, and Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows that the influence of different solvents on the physical properties of silica aerogel via APD (Solvent exchange at 65 ℃ for 24 h).\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows that the moisture content, bulk density and thermal conductivity of silica aerogel decrease with increasing solvent exchange durations and this is an universal trend for all solvent exchange temperatures. Under APD, the lower the moisture content in the solvent, the higher the solvent exchange effect, and the easier to obtain low bulk density and thermal conductivity silicon aerogels. the decrement in moisture content, bulk density, and thermal conductivity, reaches a peak at 8 h, and hardly changed beyond that. In the solvent exchange process of silica wet gel, the potential aging process continues to occur, which can also be called secondary aging, and aging is a dissolution precipitation process like Ostwald ripening [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. In this process, the condensation reaction in the gel network framework and the degree of siloxane cross-linking continue to increase. Meanwhile silica and unreacted oligomers in the solution dissolve and precipitate onto the existing particle network and particle neck, significantly increasing the strength and stiffness of the gel framework [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. The increase of solvent exchange temperature strengthens the kinetics of this process, while the increase of solvent exchange time provides more time for these reactions to occur. Together, the two lead to a higher mechanical stiffness of the particle network structure, which makes the gel network skeleton still able to resist the destruction of capillary force during normal pressure drying. However, if the solvent exchange temperature is too high, the gel network skeleton will become thick due to excessive precipitation, which will directly lead to the increase of the density and thermal conductivity of the silicon aerogel (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows that the solvent type is crucial to the physical properties, bulk density and thermal conductivity of silica aerogels. After solvent exchange of wet gel, the solvent volatilization in the pores of gel occurs under APD. The solvent volatilization with lower surface tension will cause less stress on the pore wall, thus reducing the pore collapse to obtain silicon aerogel with better performance.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eInfluence of solvent exchange temperature and time with Acetone as solvent on the physical properties of silica aerogel.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSolvent exchange temperature (℃)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSolvent exchange time (hrs.)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMC (wt%)\u003csup\u003ea\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eBulk density (g/cm\u003csup\u003e3\u003c/sup\u003e)\u003csup\u003eb\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eThermal conductivity (W/m\u0026middot;K)\u003csup\u003ec\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"4\" rowspan=\"5\"\u003e \u003cp\u003e55\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e15.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.792\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.086\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3.22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.362\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.051\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.94\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.205\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.037\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.85\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.131\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.030\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e32\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.52\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.130\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.030\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"4\" rowspan=\"5\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e15.16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.801\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.091\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.93\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.378\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.048\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.88\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.211\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.032\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.79\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.129\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.031\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e32\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.51\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.129\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.029\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"4\" rowspan=\"5\"\u003e \u003cp\u003e65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e15.15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.799\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.089\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.42\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.378\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.041\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.71\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.211\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.028\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.68\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.129\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.027\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e32\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.130\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.029\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"4\" rowspan=\"5\"\u003e \u003cp\u003e70\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e15.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.805\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.094\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.378\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.035\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.211\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.036\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.64\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.212\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.041\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e32\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.224\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.043\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"5\"\u003eEstimated uncertainties:\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003csup\u003ea\u003c/sup\u003e Moisture content in solvent, 0.3% relative.\u003c/p\u003e \u003cp\u003e \u003csup\u003eb\u003c/sup\u003e Bulk density, 0.01% relative.\u003c/p\u003e \u003cp\u003e \u003csup\u003ec\u003c/sup\u003e Thermal conductivity, 3% relative.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eInfluence of different solvents on the physical properties of silica aerogel via APD (Solvent exchange at 65 ℃ for 24 h).\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSolvent\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSurface\u003c/p\u003e \u003cp\u003etension\u003c/p\u003e \u003cp\u003emN/m (25℃)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eBulk density (g/cm\u003csup\u003e3\u003c/sup\u003e) \u003csup\u003ea\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eS\u003csub\u003eBET\u003c/sub\u003e\u003c/p\u003e \u003cp\u003e(m\u003csup\u003e2\u003c/sup\u003e/g)\u003csup\u003eb\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003ePore diameter (nm) \u003csup\u003ec\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003ePore volume (ml/g)\u003csup\u003ed\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eThermal conductivity (W/m\u0026middot;K)\u003csup\u003ee\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAcetone\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e23.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.129\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e460.75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e9.39\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e1.08\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.027\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMeOH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e22.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.128\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e473.38\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e11.29\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e1.33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.022\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003en-Hexane\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e18.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.117\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e539.65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e12.37\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.020\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eVertrel XF\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e14.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.108\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e577.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e14.28\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.017\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHFE-7100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e13.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.101\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e670.35\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e14.93\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.016\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"7\"\u003eEstimated uncertainties:\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003csup\u003ea\u003c/sup\u003e Bulk density, 0.01% relative.\u003c/p\u003e \u003cp\u003e \u003csup\u003eb\u003c/sup\u003e S\u003csub\u003eBET\u003c/sub\u003e, around 15 m\u003csup\u003e2\u003c/sup\u003e/g.\u003c/p\u003e \u003cp\u003e \u003csup\u003ec\u003c/sup\u003e Pore diameter, around 0.5 nm.\u003c/p\u003e \u003cp\u003e \u003csup\u003ed\u003c/sup\u003e Pore volume, around 0.1 ml/g.\u003c/p\u003e \u003cp\u003e \u003csup\u003ee\u003c/sup\u003e Thermal conductivity, 3% relative.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.2 FTIR results of the silica aerogels\u003c/h2\u003e \u003cp\u003eThe wet gel was solvent exchanged with acetone, MeOH, n-hexane, Vertrel XF and HFE-7100 at 65 ℃ for 24 h, under APD. Its infrared (IR) spectrum is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. The C-H tensile vibration peak at 2972 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was observed within the range of 2800\u0026ndash;3000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e[\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e], and a characteristic bending vibration peak of Si-CH\u003csub\u003e3\u003c/sub\u003e was observed at 1273 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The infrared absorption peaks of Si-O-C are in the range of 1000\u0026ndash;1300 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The two absorption peaks of 1121cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1031cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e are mainly caused by the stretching vibration of C-O-C and Si-O-C [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e, \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e], while the absorption peaks at 781 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1273 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e belong to the asymmetric stretching vibration of Si-C.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Macrostructure and nanostructure assessment of the silica aerogels\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e shows the microstructure of silica aerogels obtained under APD by solvent exchange with acetone, MeOH, n-Hexane, Vertrel XF and HFE-7100 respectively. As indicated, the sample conforms to the typical inorganic gel morphology, and shows a concave convex surface composed of a large number of spherical nanoparticles. Using Vertrel XF and HFE-7100 with lower surface tension for solvent exchange can obtain silicon aerogel with better network pore structure.\u003c/p\u003e \u003cp\u003eThe N\u003csub\u003e2\u003c/sub\u003e adsorption desorption isotherm and pore size distribution of the silica aerogels prepared by solvent exchange with acetone, MeOH, n-Hexane, Vertrel XF and HFE-7100 are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e. the isotherm of all samples has a type IV curve, and the isotherm of the five samples shows a type H1 hysteresis loop, which has the mesoporous structure characteristics of columnar pores [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. It can be inferred from Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e and Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e that with the decrease of solvent surface tension, the specific surface area of silica aerogel increases in the range of 460.75 to 670.35m\u003csup\u003e2\u003c/sup\u003e/g, while the average pore size, pore volume and thermal conductivity will continuously decrease. When HFE-7100 is used as the exchange solvent, the silica aerogel obtained under APD conditions has the best performance, with a specific surface area of 670.35m\u003csup\u003e2\u003c/sup\u003e/g, an average pore diameter of 14.93 nm, a pore volume of 2.5ml/g, and a thermal conductivity as low as 0.016 W/m\u0026middot;K.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Thermal stability of the silica aerogels\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e shows the TG-DSC scanning curve of silica aerogel in air atmosphere. There is an obvious exothermic peak in the DSC curve. The exothermic peak starts when the temperature rises to 336.8 ℃, and reaches the peak at 380 ℃. This is because the silicon dioxide aerogel structure -Si (CH\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e reacts with oxygen in the air to form Si-OH to release heat [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. From the TG curve, there are two obvious weight losses in the heating process of silica aerogel from 25 ℃ to 800 ℃: the first weight loss is 380 ℃, and the weight loss rate is 2.34%, which is mainly caused by the fracture of -Si (CH\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e and the volatilization of residual solvent; The second weight loss occurred at 380\u0026ndash;700 ℃, with a weight loss rate of 36.22%. During this stage, the DSC curve showed a significant exothermic peak starting at 700 ℃ and reaching its peak at 740 ℃. When the temperature is above 800 ℃, the TG curve decreases slowly, and a condensation reaction occurs between Si-OH to produce Si-O-Si.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eBased on the wet gel solvent exchange in the preparation of silica aerogels, we designed an efficient automatic solvent exchange device for silica wet gels, and systematically studied the influence of solvent exchange on the physicochemical properties of silica aerogel via APD. Using acetone as the exchange solvent, after aging and surface hydrophobic modification of silica wet gel, solvent exchange can be completed in only 24 hours at 65 ℃ through this device. We can obtain silica aerogel with good performance by APD, its density and thermal conductivity are as low as 0.129 g/cm\u003csup\u003e3\u003c/sup\u003e and 0.027 W/m\u0026middot;K, respectively, and its specific surface area is 460.75 m\u003csup\u003e2\u003c/sup\u003e/g. In addition, using this self-designed device, we also studied the influence of solvents with different surface tensions on the physicochemical properties of silica aerogels under the same solvent exchange temperature (65℃) and time (24 h) mentioned above. The results shows that using low surface tension solvent HFE-7100 (25 ℃ surface tension is 13.6 mN/m), we can obtain silica aerogels with excellent performance by APD, and the bulk density and thermal conductivity are as low as 0.101 g/cm\u003csup\u003e3\u003c/sup\u003e and 0.016 W/m\u0026middot;K, respectively, and the specific surface area, pore size, and pore volume can reach 670.35 m\u003csup\u003e2\u003c/sup\u003e/g, 14.93 nm, and 2.5 ml/g, respectively.\u003c/p\u003e \u003cp\u003eThe solvent exchange of wet gel is accompanied by this potential aging process. The interparticle neck of silica pearl necklace structure is strengthened through the dissolution and precipitation process like Ostwald ripening. This process is driven by minimizing the energy by reducing the total surface area. The drying process magnified the influence of solvent exchange process on the structure and stability of gel skeleton. Due to the strong capillary force and mechanical deformation (spring-back effect) that occurred in the process of drying under ambient pressure but mostly did not exist in the supercritical drying process, the samples with insufficient solvent exchange will have strong irreversible pore collapse. Moreover, the proposed method is highly applicable for industrial applications. Therefore, compared with aging and surface hydrophobic modification, the solvent exchange process is critical for the successful synthesis of low-density silica aerogels through APD.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eH. LH. Y wrote the main manuscript text and L prepared figures 1-9. All authors reviewed the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThis work was supported by the Fundamental Research Funds for the Science \u0026amp; Technology Department of Sichuan Province\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eZ. Yang, D. Zhu, H. Li, A chitosan-assisted co-assembly synthetic route to low-shrinkage Al2O3\u0026ndash;SiO2 aerogel via ambient pressure drying, Microporous Mesoporous Mater. 293 (2020) 109781.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eS.S. Prakash, C.J. Brinker, A.J. Hurd, S.M. Rao, Erratum: Silica aerogel films prepared at ambient pressure by using surface derivatization to induce reversible drying shrinkage, Nature. 374 (1995) 439\u0026ndash;443.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eH. Maleki, Recent advances in aerogels for environmental remediation applications: A review, Chem. Eng. 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Colloid Interface Sci, 206 (1998) 205\u0026ndash;211.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"journal-of-sol-gel-science-and-technology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jsst","sideBox":"Learn more about [Journal of Sol-Gel Science and Technology](https://www.springer.com/journal/10971)","snPcode":"10971","submissionUrl":"https://submission.springernature.com/new-submission/10971/3","title":"Journal of Sol-Gel Science and Technology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Aerogel, solvent exchange, thermal conductivity, FTIR, BET","lastPublishedDoi":"10.21203/rs.3.rs-4137303/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4137303/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSilica aerogel has been widely used in aerospace, energy construction, petrochemical industry, etc. Its physicochemical properties strongly correlate to the modification methods, such as aging and surface hydrophobicity treatments. Among these methods, solvent exchange plays a key role in synthesizing silica aerogel by ambient pressure drying. In order to facilitate the ambient pressure drying preparation of silica aerogel for industrial application, we designed an efficient automatic solvent exchange device for silica wet gels, and systematically studied the influence of solvent exchange on the physicochemical properties of silica aerogel via ambient pressure drying. After aging and surface hydrophobic modification, the silica wet gels undergo solvent exchange at different temperatures and times. The results show that with the increase of temperature and time, the moisture content of solvent in silica wet gel, as well as the bulk density and thermal conductivity of silica aerogel decrease. Moreover, we studied the effect of solvents with different surface tensions on the physicochemical properties of silica aerogels. The use of solvents with low surface tension can produce silica aerogels with lower bulk density and thermal conductivity, while the specific surface area, pore diameter and pore volume increase. These findings emphasize the importance of solvent exchange to improve the ability of gel particle network skeleton to withstand irreversible pore collapse via ambient pressure drying.\u003c/p\u003e","manuscriptTitle":"Effect of solvent exchange on silica aerogel properties via ambient pressure drying","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-03-22 15:05:17","doi":"10.21203/rs.3.rs-4137303/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-04-05T11:33:43+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-03-29T09:25:39+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"f93273f6-c8d3-400e-9ee7-9a2669ca77bf","date":"2024-03-27T05:57:53+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-03-25T13:17:25+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-03-21T07:28:22+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-03-20T14:33:00+00:00","index":"","fulltext":""},{"type":"submitted","content":"Journal of Sol-Gel Science and Technology","date":"2024-03-20T12:56:03+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"journal-of-sol-gel-science-and-technology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jsst","sideBox":"Learn more about [Journal of Sol-Gel Science and Technology](https://www.springer.com/journal/10971)","snPcode":"10971","submissionUrl":"https://submission.springernature.com/new-submission/10971/3","title":"Journal of Sol-Gel Science and Technology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"3d9ee4a1-5a96-47b9-b241-133fa15e41ad","owner":[],"postedDate":"March 22nd, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-08-25T16:40:46+00:00","versionOfRecord":{"articleIdentity":"rs-4137303","link":"https://doi.org/10.1007/s10971-025-06831-4","journal":{"identity":"journal-of-sol-gel-science-and-technology","isVorOnly":false,"title":"Journal of Sol-Gel Science and Technology"},"publishedOn":"2025-08-19 16:29:46","publishedOnDateReadable":"August 19th, 2025"},"versionCreatedAt":"2024-03-22 15:05:17","video":"","vorDoi":"10.1007/s10971-025-06831-4","vorDoiUrl":"https://doi.org/10.1007/s10971-025-06831-4","workflowStages":[]},"version":"v1","identity":"rs-4137303","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4137303","identity":"rs-4137303","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}