An economic and environmentally benign approach for the preparation of monolithic silica aerogels

Abstract

Troublesome drying has always limited the scale-up production and wide-spread application of monolithic aerogels because of the high energy consumption and/or organic solvent utilization. In this report, a gel-emulsion-based one step method was developed for the facile, economic and environmentally friendly preparation of various monolithic silica aerogels. The wet porous silica formed in these systems could be dried at ambient-pressure without the need for solvent exchange. It was demonstrated that the stabilities of the gel-emulsions and the resultant wet porous silica gels originate from the utilization of a specially designed stabilizer, which is a derivative of cholesterol (Chol-OH). With the aid of Chol-OH, the chemical functionalities and the densities of the monolithic silica aerogels could be largely tuned via simple variation of the structures of the precursors, organosilanes, and water content in the gel emulsions. As examples, monolithic silica aerogels from super-hydrophobic (water contact angle larger than 150°) to hydrophilic (water contact angle lower than 25°) and those with a density as low as 0.026 g cm⁻³ were obtained. Moreover, the internal structures of the aerogels can also be tuned by adjusting the reaction rate of the sol–gel process. Morphology studies revealed that the as prepared monolithic silica aerogels possessed inter-connected highly porous structures, and the pore walls are composed of highly ordered meso-pore structures with an average size of ∼25 nm. Their thermal conductivities at ambient pressure, humidity, and temperature (30 °C) were as low as 20.6 mW m⁻¹ K⁻¹, which is 18% lower than that of the reference air (24.4 mW m⁻¹ K⁻¹). Considering the ease of the stabilizer preparation, the gel emulsions, and the characteristics of the aerogel preparation process, we believe that the strategy developed in the present work represents substantial progress in the field of aerogel preparation, and the monolithic silica aerogels created have great potential for real-life applications.

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1 Introduction

Aerogels have found a variety of applications in actuator,¹ catalytic supports,^2,3 adsorption,^4,5 separation,⁶ and space dust collection,^7,8 etc. owing to their large pore volumes, high specific surface areas, low bulk densities and low thermal conductivities.⁹ Meanwhile, aerogels also hold promise for emerging applications in batteries,^10,11 and supercapacitors via introduction of conductive materials.^12–14 To realize the above-mentioned applications, aerogels in general need to be prepared both easily and economically, in particular, avoiding the use of laborious and costly drying techniques, such as supercritical drying, freeze-drying and solvent exchange-based ambient pressure drying (APD). This requires that the wet aerogels be prepared to be mechanically strong enough to conquer their shrinkage during drying and fragility during processing.¹⁵ Because of these reasons, significant efforts have been made to develop various methods/strategies for optimizing drying techniques and improving the mechanical performances of the aerogels.^16,17

In comparison to the recently accomplished reinforcement of aerogels, there has not been a large breakthrough in drying techniques.^18–20 This is mainly because the simpler and cheaper alternative drying method, which can avoid serious shrinkage and cracking due to the high capillary force during the evaporation of the liquid in the pores, has not been put forward until now. Previously, supercritical extraction drying and freeze-drying techniques were developed to solve the problems mentioned to maintain the original spatial configuration of wet gels^21–23 and to obtain integrated aerogel monoliths with ultra-low densities.^24,25 However, these drying methods are time-consuming and somewhat expensive, limiting the scale up of the production and widespread application of aerogels. Therefore, the utilization of a much easier and cost-effective APD method in the preparation of aerogels is of great importance for real-life applications of aerogels.^21,26 Though some aerogels can be obtained using the APD drying method,²¹ surface modification and solvent exchange are usually required for the use of a drying technique, which makes the process rather time-consuming, cost-expensive and environmentally unfriendly. Moreover, shrinkage of the aerogels during the APD process cannot be completely eliminated via solvent exchange, and the densities of the aerogels from this APD process are usually greater than 0.1 g cm⁻³.²⁷ Therefore, for producing real monolithic aerogels, it is a challenge to develop methods using APD as a sole drying approach without the need for solvent exchange and surface modification.

Gel-emulsions look like gels and possess foam-like structures, which consist of deformed droplets.²⁸ Within gel-emulsions, the droplets are called internal or dispersed phase, and they are separated from each other by thin films, which are the continuous phase. Moreover, the continuous phase could be either micro-emulsions or a liquid crystalline phase.²⁹ Therefore, porous monoliths with hierarchical structures are generally produced after polymerization of the monomer-based continuous phase of the starting gel-emulsions. Ever since the gel-emulsion template strategy was demonstrated in 1982,³⁰ many examples have shown that the polymerization of the continuous phase leads to interconnected macroporous polymeric monoliths, which are called organic foams.

As for the production of macroporous oxide monoliths by the gel-emulsion template approach, researchers usually encounter two fundamental problems: (1) most of the precursors of the oxides are extremely reactive with water; (2) the large amounts of alcohols produced in the hydrolysis of the precursors, which are usually metal alkoxides, will destroy the emulsions because of their tendency to mix with both oil and water. This ground breaking work was reported by Imhof and Pine in 1997.³¹ In the work, ordered macroporous silica, titania and zirconia monoliths were produced using non-aqueous emulsions as templates combined with sol–gel reactions. However, for silica aerogels, the water-based gel-emulsions may be used because of the moderate and slow reactions of the commonly used silica precursors with water compared to other metal oxide precursors.³² However, the preparation of monolithic silica aerogels via the water-based gel-emulsion template strategy with APD as the sole drying technique has not been reported till now. The key point for achieving this success is innovation in the stabilizer, which is the basis of the gel-emulsions. As previously reported, the stabilizers most commonly found for water-based gel-emulsions are surfactants.^33,34 It is believed that the stabilizer aggregates into films and stays at the interface between the dispersed phase and the continuous phase, in other words, around the dispersed droplets. Hereby, these interface films could resist destruction of ethanol produced during hydrolysis of the oxide precursors^35,36 or organosilanes for silica and maintain the internal structure of the emulsion, laying the foundation for the successful production of the final porous silica monoliths, provided they are sufficiently stable.^37,38 Currently,^39–42 the surfactants used in previous reports mainly include nonionic block copolymers such as Pluronic F127 and tetradecyl-trimethylammonium bromide-like cationic surfactants, etc.

Herein, we, for the first time, report a new stabilizer, from which super-stable gel-emulsions of water in organosilanes were created, and the gel-emulsions produced could be used for template preparation of monolithic silica aerogels. Fortunately, the wet silica aerogels resulting from the systems are strong enough that post drying can be conducted at ambient pressure without the need for solvent exchange and surface modification. This paper reports the details.

2 Experimental section

2.1 Reagents and materials

1,8-Dibromooctane (98%) and sodium hydride (60% suspension in oil) were purchased form Alfa Aesar. Glycidol (96%) was purchased from Sigma-Aldrich. 15-Crown-5 ether (97%) was purchased from Xiya Reagent Co., Ltd. Cholesterol (95%), potassium hydroxide (≥85.0%), THF (≥99.0%), methanol (≥99.5%), hexane (≥97.0%), ethyl acetate (≥99.5%), dichloromethane (≥99.5%), petroleum ether (60–90 °C) and acetonitrile (≥99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Ethenyltriethyloxysilane (≥98.0%), tetraethoxysilane (≥98.0%), chloropropyltriethoxysilane (≥98.0%) and dimethylamine (40% in water) were purchased from Aladdin Reagent Inc. All chemicals were used as received without further purification, and all organic solvents were purified by simple distillation.

2.2 Synthetic procedures

Synthesis of compound 1. Compound 1 was synthesized according to a procedure previously published.⁴³ Typically, a mixture of cholesterol (6.00 g, 15.5 mmol), 1,8-dibromooctane (14.29 mL, 77.5 mmol), KOH (3.48 g, 62 mmol) and dry THF (50 mL) was stirred under a nitrogen atmosphere at 80 °C for 48 h. The resulting solution was concentrated, and the residue was diluted with CH₂Cl₂ (50 mL), washed with brine (50 mL), and dried over MgSO₄. The MgSO₄ was removed by vacuum filtration, and the filtrate was evaporated to dryness. The resulting yellow oil was purified by column chromatography on a silica gel column with petroleum ether, CH₂Cl₂ (v/v, 1 : 1), as the eluent to afford compound 1 as a white solid (5.8 g, 65%). Melting point: 69–71 °C. FTIR (KBr, cm⁻¹): 2931 (CH), 2852 (CH), 1461 (CH), 1381 (CH), 1103 (–C–O–). ¹H NMR (δ (ppm), 400 MHz, CDCl₃): 5.40–5.28 (m, 1H, CH(Chol)), 3.53–3.33 (m, 4H, –CH₂Br, –CH₂O–), 3.20–3.03 (m, 1H, –OCH–), 2.44–0.56 (m, 55H, –CH₂–, Chol).

Synthesis of compound 2. NaH (240 mg, 5.26 mmol, 60% suspension in oil) was added to 4 mL of dry THF to give a suspension that was cooled to 0 °C under nitrogen. To this suspension was added glycidol (0.09 mL, 1.35 mmol) dissolved in 1 mL of THF along with 15C-5 (0.05 mL, 0.26 mmol). The mixture was then allowed to stir at room temperature for 30 min. Then, compound 1 (1.52 g, 2.63 mmol) was added dropwise. After 10 h, the reaction mixture was cooled to 0 °C, quenched with the addition of H₂O, and the aqueous phase was extracted 3 times with CH₂Cl₂. The organic extracts were dried with MgSO₄ and concentrated. The crude mixture was purified with flash chromatography (hexanes/EtOAc 6 : 1) to give compound 2 as a white solid (500 mg, 0.88 mmol, 65%). Melting point: 68–70 °C. Anal. calcd for C₃₈H₆₆O₃: C, 79.94; H, 11.65; found: C, 79.60; H, 11.70%. MS (m/z, [M + Na]⁺): calculated: 593.4910, found: 593.4895. FTIR (KBr, cm⁻¹): 2931 (CH), 2852 (CH), 1461 (CH), 1381 (CH), 1103 (–C–O–). ¹H NMR (δ, ppm, 400 MHz, CDCl₃): 5.34 (d, 1H, CH(Chol)), 3.83–3.62 (m, 1H, –O–CH₂–CH–), 3.53–3.33 (m, 5H, –O–CH₂–CH–), 3.20–3.03 (m, 2H, –OCH–), 2.91–2.72 (m, 1H, –O–CH₂–CH–), 2.61 (d, 1H, –O–CH₂–CH–), 2.42–0.60 (m, 55H, –CH₂–, Chol).

Synthesis of compound Chol-OH. A mixture of compound 2 (0.5 g, 0.88 mmol), 40% dimethylamine (0.44 mL, 2.63 mmol), and dry THF was stirred at 65 °C for 12 h. The solution was concentrated, and the residue was diluted with CH₂Cl₂ (20 mL), washed with brine (50 mL), and dried over MgSO₄. The MgSO₄ was removed by vacuum filtration, and the filtrate was evaporated to dryness. The crude mixture was purified with acetonitrile to give compound Chol-OH as a white solid (270 mg, 0.44 mmol, 50%). Melting point: 49–51 °C. Anal. calcd for C₄₀H₇₃NO₃: C, 77.99; H, 11.94; N, 2.27; found: C, 77.85; H, 11.66%; N, 2.17. MS (m/z, [M + H]⁺): calculated: 616.5669, found: 616.5679. FTIR (KBr, cm⁻¹): 3420 (NH), 2931 (CH), 2852 (CH), 1461 (CH), 1381 (CH), 1103 (–C–O–). ¹H NMR (δ (ppm), 400 MHz, CDCl₃): 5.40–5.28 (m, 1H, CH(Chol)), 3.83–3.62 (m, 1H, –O–CH₂–CH–), 3.53–3.33 (m, 5H, –O–CH₂–CH–), 3.20–3.03 (m, 1H, –OCH–), 2.56–0.51 (m, 61H, –CH₂–, Chol, –N(CH₃)₂).

2.3 Silica aerogels preparation

Gel-emulsions. The starting emulsions were prepared with distilled water used as the aqueous phase, organosilanes used as the oil phase, and Chol-OH as the stabilizer. After the Chol-OH was dissolved in the organosilanes, the resulting solution was mixed with distilled water using a homogenizer (IKA T10) operated at 11 400 rpm for 2 min. The resulting emulsion is referred as the gel-emulsion if the fluidity of the emulsion is lost when the test tube was turned upside down. Unless stated otherwise, the water volume fraction was fixed at 80% as the most stable gel-emulsions for all the organosilanes, and the content (w/v) of stabilizer was kept at 15% based on the oil phase at the optimum dosage.

Silica aerogels. The starting gel-emulsions were put into an ammonia bath at room temperature for 3–5 days. Then, the ambient drying was conducted at room temperature for 3 d, or 50 °C for 1 d, and monolithic silica aerogels were obtained. Collapse and shrinkage of the aerogels were not observed during the sol–gel reaction and drying processes. The dried silica aerogels were finally immersed in purified THF for 2 h and then filtered. The aerogels were further washed with a small amount of THF for another 2 times to make sure they were free of the stabilizer. Regeneration of the stabilizer was realized by drying its THF solution in a rotary evaporator.

2.4 Characterization of the W/O emulsions and the aerogels

Optical micrographs. The measurements were taken on a Zeiss (Axio Observer A1) optical microscope. Before examination, the gel-emulsions, without further dilution, were placed on a slide glass holder and covered with a thin glass slide.

SEM observation. The microstructures of the monolithic silica aerogels were studied by imaging the fracture surfaces using a TM3030 scanning electron microscopy spectrometer at an accelerating voltage of 15 kV. Specimens of the aerogel samples were coated with Au before observation.

FE-SEM observations. The morphological structures of the selected monoliths were observed using a Hitachi, Ltd. SU8020 field emission gun scanning electron microscope (FE-SEM, Zeiss UltraPlus Analytical). The measurements were performed under high vacuum. Prior to measurement, the samples were coated with a thin layer of platinum.

Contact-angle tests. The contact angles of the monoliths were measured using a Dataphysics OCA20 contact-angle system at ambient temperature.

Rheological measurements. The storage stability of the gel-emulsions was evaluated by optical micro-rheology (Rheolaser LAb from Formulaction, French). In the experiment, 4 mL of the gel-emulsion in a flat-bottomed cylindrical glass tube (height 45.0 mm, external diameter 14.0 mm) was prepared. The resulting gel-emulsion was immediately placed in the sample chamber of the system. All the measurements were taken at ambient temperature (25 °C).

Size measurements. The average sizes of the ‘voids’ (i.e., emulsion droplets) were measured using Nano Measurer software.

Density of the monoliths. Densities of the monoliths were calculated via the following equation, where ρ is density of a monolith (g cm⁻³), m is the weight (g), and v is the volume.

ρ = m/v

Thermal conductivity measurements. The thermal conductivity measurements were performed on XIATECH TC3010 thermal conductivity tester using the instantaneous heat ray method at room temperature. To test the thermal conductivity, we put the hot wire between two of the same samples and placed a weight on top of the stacked samples. The aerogel monolith can make good contact with the hot wire under slight compression. For each sample, three measurements were performed, and the obtained average value was taken as the conductivity of the sample.

3 Results and discussion

3.1 Preparation of the stabilizer and the gel-emulsions

The synthesis route of the stabilizer, Chol-OH, and the preparation procedures of the monolithic silica aerogels are schematically shown in Scheme 1. As shown in the scheme, it is seen that unlike conventional sol–gel processes, the preparation of monolithic silica aerogels in the present method is based on the combination of the gel-emulsion template technique and sol–gel reactions of suitable organosilanes, followed by the direct solvent-exchange free ambient-pressure drying (APD) process. The key point of this approach is the utilization of a specially designed amphiphilic cholesterol derivative, Chol-OH. It is the use of Chol-OH that stabilizes the gel-emulsions of water in the organosilanes, (EtO)₃SiR, where Et stands for an ethyl group, R for vinyl, benzyl, methyl, chloropropyl, and ethoxy. As reported in the literature, the stabilizer of Chol-OH synthesized in the present work possesses a low-molecular-mass gelator (LMMGs)-like structure,^44,45 implying that the compound has a strong tendency to form aggregates in the position the molecules stay. Inspection of the structure of the stabilizer shows that the molecules of the compound will preferentially stay at the oil/water interference and form an interfacial film because of their aggregation, which reduces the interfacial energy and leads to the stabilization of the gel-emulsion. Subsequent introduction of NH₃ into the gel-emulsions induces hydrolysis and condensation of the organosilanes, which consist of the continuous phases of the gel-emulsions, resulting in inter-connected silica skeletons. Finally, monolithic silica aerogels are obtained by simple evaporation of water within the wet silica aerogels. It is to be noted that evaporation of water within the system is not a simple task. This is because, unlike other solvents, the vaporization heat of water is high and shrinkage or even collapse of the pristine silica skeleton may occur because of the high energy-consumption, which may explain why supercritical drying, freeze-drying, or solvent exchange-based APD processes are usually employed for the post-treatment of the pristine porous silica. However, upon utilization of the as synthesized stabilizer, Chol-OH, the post-treatment becomes much simpler, and the treatment can be accomplished using a simple, economic solvent exchange free APD process. To demonstrate the potential of the stabilizer, five different organosilanes were used as sample oil phases for the preparation of the water in oil gel-emulsions, and the corresponding monolithic silica aerogels with different surface structures and properties were obtained. Table 1 depicts the structures of the organosilanes, the compositions of the corresponding gel-emulsions, and the associated monolithic silica aerogels.

Scheme 1 Schematic of the fabrication procedures for monolithic silica aerogels using a gel-emulsion template/sol–gel strategy and our APD process.

Table 1 Organosilanes used as oil phases and precursors, their gel-emulsions and associated final silica aerogel monoliths^a

Organic silanes	Water contents	Test	GE-emulsions	Size^b (μm)	Aerogel monoliths	Shrinkage (%)	Bulk density (g cm^−3)	Size^c (μm)
a [Chol-OH] = 15% (w/v) based on the oil phase; GE = gel-emulsion; E = emulsion.b Size indicates the mean size in diameter of gel-emulsion droplets.c Size indicates the mean size in diameter of macropores of monolith-type aerogels.
	75%	GE	GE-vinyl	—	AM-vinyl	0	0.090	—
	80%	GE		4.0		0	0.075	4.1
	90%	GE		5.4		0	0.050	5.7
	95%	GE		9.4		0	0.026	14.3
	75%	GE	GE-benzyl	—	AM-benzyl	4	0.140	—
	80%	GE		11.1		0	0.080	29.6
	90%	E		—		—	—	—
	75%	GE	GE-methyl	—	AM-methyl	3	0.100	—
	80%	GE		10.1		0	0.074	12
	90%	GE		—		5	0.065	—
	75%	GE	GE-chloro	—	AM-chloro	4	0.130	—
	80%	GE		8.8		0	0.081	14.9
	90%	E		—		—	—	—
	75%	GE	GE-ethoxy	—	AM-ethoxy	7	0.108	—
	80%	GE		7.1		2	0.056	7.1
	90%	GE		—		5	0.045	—

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3.2 Properties of the gel-emulsions

As previously mentioned, the formation of the gel-emulsions used in the present work is simple. Direct mixing and agitation of a suitable amount of deionized water with the Chol-OH solution of one of the organosilanes listed in Table 1 could result in a stable gel-emulsion. Optimization of the concentration of the stabilizer in the organic phase and the volume fraction of water in the system was systematically conducted. It was revealed that systems with more than 10 wt% of Chol-OH in the organosilane phase and 74% or more volume fraction of water exist as stable gel emulsions; see Fig. 1a as an example. In contrast, systems with less than 74% of water appear as conventional emulsions (Fig. 3a). Furthermore, gel-emulsions were not observed when the volume fraction of water exceeded 97% (Fig. 3b–e). These results suggest: (1) the gel emulsions belong to the conventional liquid/liquid type⁴⁶ rather than the LMMGs-based liquid/gel type,^47,48 and (2) water is dispersed in the systems as an internal phase, implying that hydrolysis and condensation of the oil phase may result in interconnected porous silica skeletons from which monolithic silica aerogels may be obtained after a proper post treatment. To confirm if water is the real internal phase, a fluorescence microscopy measurement was further conducted by taking the system of GE-vinyl as an example sample. The result is shown in Fig. S4.† As expected, the hydrophobic and hydrophilic fluorescent probes exist separately in the continuous phase and the dispersed phase, confirming the W/O nature of the resultant gel-emulsion.

Fig. 1 An example water in organosilane gel-emulsion with Chol-OH as the stabilizer (15%, w/v of the oil phase; water volume fraction 80%, v/v). (a) A picture of one of the gel emulsions; (b–f) optical microscopic images of the five gel-emulsions with GE-vinyl, GE-benzyl, GE-methyl, GE-chloro, and (f) GE-ethoxy as the oil phases, respectively, indicating similar foam-like structures and different droplet sizes.

On the basis of the above studies, the gel-emulsions with 15 wt% of stabilizer and 80% (v/v) water were investigated. First, the structures of the five gel-emulsions were examined by optical microscopy, and the results are shown in Fig. 1. Reference to the images reveals that all the gel-emulsions display typical foam-like structures despite the different organosilanes that were used as the oil phases. Meanwhile, further observation finds a large distinction in the droplet sizes and continuous phase structures among the gel-emulsions under study. Clearly, the mean droplet size of the GE-benzyl-based system is the largest and shows a wider size distribution; while the GE-vinyl system displays the smallest droplet sizes and a much narrower size distribution, as summarized in Table 1. Actually, the difference in the droplet sizes explains why the final monolithic silica aerogels possess different porous structures, which will be discussed later.

The storage stability of a gel-emulsion is crucial for its application as a template. This is because the hierarchical structures and high porosity of the resulting monolithic aerogels could only be formed after a long time sol–gel reaction, which generally lasts from 2 to 7 d depending on the reaction conditions. For this reason, optical micro-rheology measurements were conducted to examine the mean square displacement (MSD) of probing particles against the decorrelation time of the gel-emulsions by taking the GE-vinyl and GE-ethoxy systems as examples. The decorrelation time is the time scale of the observation. The measurements were conducted just after sample preparation and continued for 100 h. Fig. 2 shows the MSD curves with respect to decorrelation time at selected aging times. As expected, the two samples show very different characteristic patterns by which the MSD values evolve with the decorrelation times, and the plots for each system vary with the aging times in different ways. As for GE-vinyl, the nonlinear variation of the MSD with time is markedly observed during the first 36 h, indicating the typical viscoelastic behavior of the gel-emulsion.^49,50 Then, despite the instability of the gel-emulsion, the curves gradually tend to shift upwards in the following time intervals from 40 to 80 h, showing that the oil–water interface is likely destroyed, possibly due to the slow reaction of the organosilane with water. This leads to the fluidity of the gel-emulsion, as shown in the inset of the Fig. 2. The viscoelastic property of the gel-emulsion recovers 80 h later, revealing that the stable oil–water interface is re-built, which may be a result of the generation of a silica skeleton via the sol–gel reaction. Comparatively speaking, the MSD of the GE-ethoxy system is obviously lower than that of GE-vinyl, indicating the higher elasticity and better stability of this system than that of GE-vinyl. More importantly, during the testing time scale, the MSD curves of this system with the ageing time always maintained a character of nonlinear variation and slowly shifted downwards, indicating the gradual reinforcement of the oil–water interface possibly due to the formation of a silica skeleton, a result of the interfacial sol–gel reaction. This finding also reveals that the influence of the reaction of the organosilane with water on the stability of the interface is negligible because of the instantaneous regeneration of a stronger interface due to the fast reaction of GE-ethoxy with water. In brief, the Chol-OH-based gel-emulsions under study are stable enough for further use as templates in the preparation of monolithic silica aerogels.

Fig. 2 Selected plots of MSD with respect to the decorrelation time at selected aging time, t_w, for gel-emulsions with a Chol-OH mass fraction of 15% based on the oil phase and water volume fraction of 80%. (a) GE-vinyl, and (b) GE-ethoxy. The inset shows the appearance of gel-emulsions at different interval times, exhibiting an evaluation of rheological behaviors over time.

3.3 Preparation and properties of monolithic silica aerogels

The sol–gel reaction of the organosilanes in the gel-emulsions was initiated by putting the system under an ammonia atmosphere. The gel-emulsion was kept in the atmosphere for 72 h to ensure completion of the reaction, resulting in a complete porous silica skeleton, which lays the foundation for further obtaining the expected monolithic silica aerogels. After completion of the sol–gel reaction, monolithic silica aerogels with different surface structures were obtained by direct drying of the wet samples at a suitable temperature and ambient pressure. The well encountered destabilization of gel-emulsions induced by alcohol released during hydrolysis of the silica precursors was not observed in all of the systems under study, which was an indication of the excellent stability of the as-prepared gel-emulsions. This extraordinary stability may be attributed to the special structure of the stabilizer, Chol-OH, created in the present work. As previously mentioned, Chol-OH possesses a typical gelator-like structure (Scheme 1) in spite of no observation of gelation in the organosilanes. It is known that cholesteryl derivatives stand for a typical class of LMMGs because the cholesteryl moiety has a strong tendency to form ordered aggregates via inter-molecular van der Waals forces.^51,52 Therefore, it is believed that unlike conventional surfactants, the molecules of Chol-OH may stay at the oil–water interface region because of their surfactant-like structure and property and self-assemble into a super-stable interface film there, which stabilizes the droplets and inhibits their coalescence. To the best of our knowledge, this is the first report that monolithic silica aerogels can be prepared with only organosilanes as the oil phase, which reduces the environmental impact of the monolithic aerogel production based on the combined utilization of the well-known gel-emulsion template and sol–gel reaction techniques. In fact, there was no sign of collapse of the gel-emulsion during the sol–gel reaction or shrinkage of the porous silica skeleton during the solvent exchange free APD process.

Fig. 3 shows the internal structures of the as-prepared monolithic silica aerogels. It is seen that whatever the emulsion oil phase is, the first observation emerging from the SEM images is the typical foam-like structure, which are well correlated with the structures of the corresponding gel-emulsions shown in Fig. 1, which is a well-established phenomenon observed in gel-emulsion template strategy.^53,54 Specifically, the sizes of the macroscopic voids are different from each other among the monolithic silica aerogels obtained from different gel emulsions, and the mean diameters of the voids range from about 4 μm to 30 μm (Table 1). The differences in the void sizes are in line with the distinction in the droplet sizes of the starting gel-emulsions. A more careful comparison of the pictures shown in Fig. 3 and those in Fig. 1 shows that for the AM-vinyl and AM-ethoxy aerogels, the average diameters of their voids are almost equal to the droplet sizes of the corresponding gel-emulsions, suggesting no significant coalescence between the droplets during the sol–gel reactions and confirming the unusual stability of the two starting gel-emulsions. For the other three gel-emulsions, however, coalescence should have occurred because the sizes of the macropores are significantly larger than those of the droplets of the corresponding starting gel-emulsions, as summarized in Table 1.

Fig. 3 SEM images of the as-prepared monolithic silica aerogels: (a) AM-vinyl, (b) AM-benzyl, (c) AM-methyl, (d) AM-chloro, and (e) AM-ethoxy. (f) Representative photographs of the monolithic silica aerogel of AM-vinyl before and after cutting.

Further inspection of the structures of the monolithic silica aerogels reveals that the macropores are usually deformed, which must be a result of exceeding 74% of the volume fraction of the dispersed phase, a critical geometrical value for filling the reaction vessel with regular balls. Another important finding is that the as prepared monolithic aerogels can be cut into almost any shapes, as shown in Fig. 3f, which is convenient for applications. In addition, the GE-vinyl system was found to be the most stable one among the five gel-emulsions under study as the aerogels from this system show no shrinkage during the drying process. The GE-vinyl system was taken as an example for further investigations of the effects of the components of the gel-emulsions and the reaction conditions on the porous structures of the resultant monolithic aerogels.

The pictures also demonstrated that the macroscopic pores in the monolithic silica aerogels are inter-connected via a few throats. In other words, they have open-cell internal structures. For specific applications, the presence of rich throats in monolithic aerogels is highly desired because of its importance for mass transfer. To this end, the sol–gel reaction rate was adjusted by alternating the concentration of ammonia in the surrounding atmosphere. It was found that the reaction rate of the sol–gel process can be dramatically altered via variation of the concentration of ammonia, allowing for great adjustments in the internal structures of the final monolithic aerogels. In this way, a complete open-cell structure was obtained as shown in Fig. 4a, where the AM-vinyl was taken as an example aerogel. The microstructures of this material were further examined using an FE-SEM technique, and the results are depicted in Fig. 4b and c, respectively. Interestingly, as shown in the pictures, the aerogel shows highly ordered mesoporous structures with an average diameter of ∼25 nm. The formation of the mesoporous structure is generally believed to originate from the micro-emulsion structures in the continuous phase of the gel-emulsion, which must be a result of the aggregation of the stabilizer in the continuous phase.⁵⁵

Fig. 4 FE-SEM images of the AM-vinyl aerogels at different magnifications. The very low sol–gel reaction rate produces monolithic silica aerogels possessing open-cell and well-ordered mesoporous structures.

The porous structures of the resulting aerogels can be further adjusted by varying the water content in the starting gel emulsions. As shown in Fig. 5a and b, the porosity of the GE-vinyl sample is increased with increasing water content. For the system of 95% of water, the apparent density of the monolithic silica aerogel as produced is reduced to only 0.026 g cm⁻³ (Fig. 5c), the lowest value for the silica aerogels produced by employing APD, including those combined with solvent exchange as a drying technique, reported until this paper. Importantly, in this case, APD was still conducted free of solvent exchange. Undoubtedly, this is a great advance in the preparation of the aerogels, which lays the foundation for facile, economic, and environment-friendly large scale production of the monolithic silica aerogels with adjustable internal structures.

Fig. 5 SEM images of (a) AM-vinyl-90% and (b) AM-vinyl-95% aerogels, (c) photograph of a monolithic AM-vinyl-95% aerogel, showing a density of 0.026 g cm⁻³.

For the real-life using of aerogels, their surface property, especially hydrophobicity or hydrophilicity, is crucial, and the contact angles of the as prepared monolithic aerogels were measured. The contact angle of a sample greatly depends upon its surface morphology, roughness and chemical composition and is a direct indication of the hydrophobic–hydrophilic property of the material. Fig. 6 shows the air–water contact angles of the cross-sections of the as-prepared silica aerogels. With reference to the images and data shown in Fig. 6, it can be seen that the contact angles of the samples from AM-vinyl to AM-ethoxy are 151.4°, 142.0°, 132.4°, 124.0°, and 24.0°, respectively, demonstrating the high hydrophobicity of the first four aerogels and the hydrophilicity of the last sample, which is not an unexpected result if the chemical nature of the starting organosilane is considered (cf. the simplified surface chemical structures in Fig. 6). To our great surprise, the very hydrophilic silica aerogel was also obtained using the solvent exchange free APD technique, which was never reported before solvent exchange was used. This is extremely difficult because the interfacial energy of this system is significantly larger than those of the other four systems because of the high affinity of water for the hydrophilic surface of the monolith, which may result in shrinkage or even collapse of the silica skeleton due to evaporation of the dispersed phase with high evaporation heat. The finding of the successful drying of the hydrophilic silica aerogels at ambient pressure without the need for solvent exchange and surface modification can be only explained by the afore-mentioned super-stability of the stabilizer film existing between the dispersed and continuous phases.

Fig. 6 Images of the water contact angles for the five silica aerogel monoliths, where (a), (b), (c), (d) and (e) are the aerogels of AM-vinyl, AM-benzyl, AM-chloro, AM-methyl and AM-ethoxy, respectively. Photographs of water and oil droplets on the surface and a schematic illustrating the silica aerogel surface chemistry of AM-vinyl (f) and AM-ethoxy (g).

The hydrophilicity and hydrophobicity of the monoliths were further evaluated by taking AM-vinyl and AM-ethoxy as examples. As shown in Fig. 6f and g, for the sample of AM-vinyl, the water drop keeps its spherical shape over the whole test time scale, but an oil drop of the same volume was absorbed completely upon addition onto the sample surface, indicating its super-hydrophobicity. On the contrary, for the sample of AM-ethoxy, water was absorbed instantly and completely, but the oil just spread on the surface rather than being absorbed, which is a result of the hydrophobicity of air and the specific surface structure of the sample. As mentioned already, the fact that each silicon atom of the AM-vinyl sample contains one hydrophobic –CH₂=CH₂ group may explain why the sample is hydrophobic, and similarly, for the sample of AM-ethoxy, each silicon atom contains, at least in theory, one –OH group, which explains the hydrophilicity of the sample (cf. the FT-IR spectra in Fig. S5†). Likewise, the hydrophobic nature of other monolithic silica aerogels can also be explained by considering the hydrophobicity of the benzyl, chloropropyl, and methyl substituents on them. Of course, the difference in the micro-structures of the surfaces of the samples also contribute to the tested hydrophobic and hydrophilic properties.

The difference in the hydrophilic properties of the AM-vinyl and AM-ethoxy aerogels were further demonstrated by an additional test. As shown in Fig. 7a, when an AM-vinyl monolith was partly or totally immersed in water with an external force, the surface of the sample was encompassed by a lot of air bubbles, which appear as a silver mirror-like surface because of the formation of a continuous air layer between the super-hydrophobic surface and water, a typical Cassie Baxter non-wetting behavior.^56,57 With a retreating external force, the monolith immediately floated on the water surface without uptake of water, showing a strong repellency against water, suggesting the potential application for purification of oil and/or hydrophobic organic solvents from contaminated water. In sharp contrast, the AM-ethoxy monolith was totally immersed in water without the need for an external force, and it continually absorbed water, exhibiting a strong hydrophilicity (Fig. 7b).

Fig. 7 Digital images of a monolith partially and totally immersed in water by an external force where (a) is a sample of AM-vinyl and (b) a sample of AM-ethoxy.

Silica aerogel is generally recognized as a good heat insulating material because of its low thermal conductivity.⁵⁸ Therefore, the thermal conductivities of the as prepared silica aerogels were measured. The tests were conducted at 32 °C under ambient pressure and humidity using the hot-wire method, which is a well adopted absolute thermal conductivity technique for measuring solids with very low uncertainty.^59–61 The results are shown in Fig. 8. As can be seen from Fig. 8, the thermal conductivities of the as-prepared AM-vinyl, AM-benzyl, AM-chloro, AM-methyl and AM-ethoxy aerogels are 26.1, 24.8, 33.7, 27.2 and 32.1 mW m⁻¹ K⁻¹, respectively, which are all much lower than that of commercially available silica aerogels (100–200 mesh, 98 mW m⁻¹ K⁻¹) and that of glass fibers (80 mW m⁻¹ K⁻¹), which are all commonly used as thermal insulating materials.⁴⁹ However, the values are in good agreement with those of other silica aerogels (17–41 mW m⁻¹ K⁻¹), which were prepared by a classical sol–gel method combined with a super-critical drying technique,¹ indicating their potential for use in special circumstances as energy saving materials.

Fig. 8 The measured thermal conductivities of the reference air and the monolithic silica aerogels with different chemical natures and surface morphologies. The inset shows the variation of the thermal conductivity with the bulk density of AM-vinyl.

The thermal conductivities of the monolithic silica aerogels are well correlated with their densities. Taking the AM-vinyl aerogel as an example, the effect of the density of the aerogels on their thermal conductivities was investigated by varying the water content of the starting gel-emulsions. It was found that the thermal conductivity of the resultant AM-vinyl aerogel decreased from 26.1 to 23.0 and then to 20.6 mW m⁻¹ K⁻¹ with increasing water content in the gel emulsions from 80% to 90% and then to 95% (Table 1). The respective thermal conductivity of the silica aerogels is about 3 mW m⁻¹ K⁻¹ lower than that of the aerogel with a bulk density of 0.025 g cm⁻³ higher, as shown in the inset of Fig. 8. The decrease of the density of the resultant aerogels implies a decrease in the solid mass and the pore wall thickness, causing more air to be trapped within the monoliths, reducing the decreased thermal conductivity of the materials. Moreover, the reduced transport of the air trapped within the meso- and micro-pores smaller than the free mean path of air (∼70 nm) at ambient conditions explains why the thermal conductivities of the samples of AM-vinyl (90%) and AM-vinyl (95%) are lower than that of the reference air.

4 Conclusions

In summary, a series of monolithic silica aerogels were prepared via a one-step sol–gel reaction in the continuous phase of specially designed stabilizer (Chol-OH)-based W/O gel emulsions, in which the oil phase is one of the silica precursors, an organosilane, followed by a solvent exchange free APD process. It has been demonstrated that triethoxyvinylsilane, phenyltriethoxysilane, methyltriethoxysilane, 3-chloro-propyltriethoxysilane, and tetraethoxysilane can be used as example precursors. The prepared gel emulsions showed remarkable stability, and the hydrolysis and condensation of the organosilanes occur without damage to the template, which is a basis for obtaining the final porous silica monoliths. Moreover, it is the stability that allows the drying of the wet aerogels to be performed in a very convenient, economic and environmentally benign way, i.e., drying at ambient pressure without the need for solvent exchange and surface modification. The chemical nature and the densities of the monolithic silica aerogels could be largely tuned via simple variation of the precursor and the water content in the gel emulsions. In this way, monolithic silica aerogels from super-hydrophobic (water contact angle larger than 150°) to extreme hydrophilic (water contact angle lower than 25°) with a density as low as 0.026 g cm⁻³ were prepared. In addition, the internal structures of the aerogels can also be tuned by alternating the reaction rate of the sol–gel process. As confirmed by morphological studies, the as prepared monolithic silica aerogels possess highly porous inter-connected structures, and the macro-pore walls of the materials are composed of highly ordered mesopore structures. The aerogels are mechanically stable and could be cut into any shape required. Their thermal conductivities at ambient pressure, humidity, and temperature (32 °C) could be 18% lower than that of the reference air (24.4 mW m⁻¹ K⁻¹). Considering the ease of the preparation of the stabilizer, the gel emulsions, and the characteristics of the aerogel preparation and drying processes, we believe that the strategy developed represents substantial progress in the preparation of silica aerogels, and the monolithic silica aerogels obtained in the present work have a great potential for real-life applications.

Acknowledgements

The authors acknowledge the financial support from the National Natural Science Foundation of China (21206089) and the Fundamental Research Funds for the Central Universities (GK201402048, GK201603131).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21050c

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