Adsorption of toxic organic compounds from water with hydrophobic silica aerogels

Silica monolith aerogels with different degrees of hydrophobicity were prepared by incorporating methyltrimethoxysilane (MTMS) or trimethylethoxysilane (TMES) in standard sol-gel synthesis followed by supercritical drying of gels with carbon dioxide (CO(2)) at 40 degrees C and 100 bar. The hydrophobicity of the aerogels was tested by measuring the contact angle (theta). The aerogels were also characterised by FTIR, DSC, and porosity measurements. Adsorption capacity measurements show that such modified hydrophobic silica aerogels are excellent adsorbents for different toxic organic compounds from water. In comparison to granulated active carbon (GAC) they exhibit capacities which are from 15 to 400 times higher for all tested compounds. Adsorption properties of hydrophobic silica aerogel remain stable even after 20 adsorption/desorption cycles.     

Superhydrophobic and superoleophobic nanocellulose aerogel membranes as bioinspired cargo carriers on water and oil

We demonstrate that superhydrophobic and superoleophobic nanocellulose aerogels, consisting of fibrillar networks and aggregates with structures at different length scales, support considerable load on a water surface and also on oils as inspired by floatation of insects on water due to their superhydrophobic legs. The aerogel is capable of supporting a weight nearly 3 orders of magnitude larger than the weight of the aerogel itself. The load support is achieved by surface tension acting at different length scales: at the macroscopic scale along the perimeter of the carrier, and at the microscopic scale along the cellulose nanofibers by preventing soaking of the aerogel thus ensuring buoyancy. Furthermore, we demonstrate high-adhesive pinning of water and oil droplets, gas permeability, light reflection at the plastron in water and oil, and viscous drag reduction of the fluorinated aerogel in contact with oil. We foresee applications including buoyant, gas permeable, dirt-repellent coatings for miniature sensors and other devices floating on generic liquid surfaces.     

Transport of liquids using superhydrophobic aerogels

The experimental results of the studies on the transportation of water droplets on a superhydrophobic silica aerogel-powder-coated surface are reported. The superhydrophobic silica aerogels were prepared using sol-gel processing of methyltrimethoxysilane (MTMS) precursor, methanol (MeOH) solvent, and base (NH4OH)-catalyzed water followed by supercritical drying using methanol solvent. The molar ratio of NH4OH/MTMS, H2O/MTMS, and MeOH/MTMS were varied from 1.7x10(-1) to 3.5x10(-1), 2 to 8, and 1.7 to 14, respectively, to find out the best-quality aerogels in terms of higher hydrophobicity and high droplet velocity. A specially built device was used for the measurement of velocity of water droplet of size 2.8 mm (+/-0.2 mm) on an inclined surface coated with superhydrophobic aerogel powder. Liquid marbles were prepared by rolling water droplets on aerogel powder and the marble(s) velocities on a noncoated inclined surface were compared with that of the water droplets. It was observed that the microstructure of the aerogel affects the droplet as well as marble velocities considerably. For an aerogel with uniform and smaller particles, the water droplet and marble velocities were observed to be maximum, i.e., 144 and 123 cm/s, respectively, whereas for the aerogels with bigger and nonuniform particles, the water droplet and marble velocities were observed to be minimum, i.e., 92 and 82 cm/s, respectively. The results have been discussed by taking into account the contact angles and microstructural observations.     

超临界干燥法制备SiO2气凝胶的研究

通过溶胶－凝胶法和超临界干燥技术制得了ＳｉＯ２气凝胶，研究了溶液的ｐＨ值对凝胶时间及气凝胶密度的影响，应用ＸＲＤ、ＳＡＸＳ、ＴＥＭ和等方面对气凝胶样品的结构进行了初步研究，结果表明，ＳｉＯ２气凝胶是连续网络的非晶态纳米多孔固体，其基本粒子的平均直径约１０ｎｍ．     

Hydrophobicity and drag reduction properties of surfaces coated with silica aerogels and xerogels

Superhydrophobic surfaces have application in self-cleaning, anti-fouling and drag reduction. Most superhydrophobic surfaces are constructed using complex fabrication methods. An alternative method is to use sol–gel methods to make hydrophobic aerogel and xerogel surfaces. In this work, hydrophobic silica aerogels and xerogels were made from the silica precursors tetramethoxysilane (TMOS) and methyltrimethoxysilane (MTMS) in volume ratios MTMS/TMOS of 0–75 % using a base-catalyzed recipe. Overall hydrophobicity was assessed using contact angle measurements on surfaces prepared from crushed aerogel and xerogel powders. The surfaces made from aerogels were super-hydrophobic (with contact angles of 167°–170°) for all levels of MTMS (10–75 %). Of the xerogel-coated surfaces, those made with 50 % MTMS were hydrophobic and with 75 % MTMS were superhydrophobic. Chemical hydrophobicity was assessed using Fourier transform infrared spectroscopy, which showed evidence of Si–CH₃ and Si–C bonds in the aerogels and xerogels made with MTMS. Morphological hydrophobicity was assessed using SEM imaging and gas adsorption. The drag characteristics of the aerogel- and xerogel-coated surfaces were measured using a rotational viscometer. Under laminar flow conditions all of the hydrophobic aerogel-coated surfaces (10–75 % MTMS) were capable of capturing an air bubble, thereby reducing the drag on a horizontal rotating surface by 20–30 %. Of the xerogel-coated surfaces, only the one made from 75 % MTMS could capture a bubble, which led to 27 % drag reduction. These results imply that morphological differences between silica aerogels and xerogels, rather than any differences in their chemical hydrophobicity, give rise to the observed differences in hydrophobicity and drag reduction for the sol–gel-coated surfaces.     

3D hierarchical MnO2 aerogels with superhydrophobicity for selective oil–water separation

Inorganic nanowire aerogel with low density, high specific surface area and high porosity has received increasing attention in the field of materials physics and chemistry because of not only the unique structural and physical features of metallic oxide but also low cost, environmental friendliness and earth abundant of precursor materials. In this work, MnO₂ nanowire aerogels (MNA) with ultralow density, and stable 3D hierarchical structures was successfully fabricated by freeze‐drying processes using MnO₂ nanowire as building blocks. The length of MnO₂ nanowires exceeds 100 μm, making it easier to cross‐link and self‐assemble into a 3D network of aerogels, and the acid and alkali resistance of MnO₂ enables it to adapt to extreme environments. Simultaneously, the monodispersed MnO₂ nanowire was prepared by the hydrothermal method, followed by acid treatment. To obtain superhydrophobic properties and achieve selective oil adsorption, the surfaces of nanowire aerogels were grafted the hydrophobic groups with low surface energy via vapor deposition. It is indicated that the obtained 3D hierarchical MNA show both superhydrophobic and super‐lipophilic properties simultaneously with a high‐water contact angle of 156°  ±  2° and an oil contact angle of 0°. And the MNA exhibited a high oil adsorption capacity of 85–140 g/g, thereby indicating its potential applications in oil/water separation. More importantly, the resulting MNA can be recycled ten cycles without loss of oil absorption capacity (more than 120 g/g). The results presented in this work demonstrate that the as‐prepared nanowire aerogel may find applications in chemical separation and environmental remediation for large‐scale absorption of oils from water.     

Organofunctional Silica Aerogels

Organically modified silica aerogels were prepared from mixtures of tetramethoxysilane (TMOS) and organofunctional alkoxysilanes RSi(OMe)₃ with R=mercaptopropyl, diphenylphosphinoethyl and chloropropyl. The base catalyzed hydrolysis and condensation reactions, followed by supercritical drying with liquid carbon dioxide were investigated. Starting from 9:1 mixtures of TMOS and RSi(OMe)₃, incorporation of the functional moieties succeeded quantitatively. Increasing the percentage of RSi(OMe)₃ to 20% or 40% leads to an incomplete condensation of the RSiO_1,5 units to the SiO₂ network. Compared with an unmodified silica aerogel, the microstructure of the resulting hybrid aerogels is nearly uninfluenced for the 9:1 mercapto-and chloro-modified samples, while in the phosphino-modified sample the typical pore radii distribution is disturbed by the bulky organic groups. The organofunctional aerogels decompose between 210 and 650°C.     

Wettability study of surface modified silica aerogels with different silylating agents

In wettability study, surface free energy interactions are of crucial importance for silica aerogels in which absorption of organic liquids and transportation of chemicals carried out for chemical and biotechnological applications. In present study, we have used Lifshitz–van der Waals/acid–base approach for calculation of surface free energy of aerogel sample. We have investigated that the surface free energy values of aerogels are 45.95, 51.42 and 45.69 mJ/m² by modifying their surfaces using 7 % chlorotrimethylsilane (TMCS), dimethyldichlorosilane (DMDCS) and hexamethyldisilazane (HMDZ) silylating reagents with solvent, respectively. The alcogels were prepared by two step acid–base catalyzed process where the molar ratio of precursors tetraethoxysilane:methanol:oxalic acid:NH₄OH:NH₄F was kept at optimal value of 1:16.5:0.71:0.58:0.60:0.98, respectively. To modify gel surfaces, TMCS, DMDCS and HMDZ concentration have been varied from 5 to 12 % and such alcogels were dried at ambient pressure. The aerogels have been characterized by fourier transform infrared spectroscopy, scanning electron microscopy, thermo-gravimetric and differential thermal analysis and Wetting properties of silica aerogel surfaces was studied by contact angle measurements. The surface chemical composition of DMDCS modified silica aerogels was studied by using X-ray photoelectron spectroscopy. As there is not any direct method, we have used Lifshitz–van der Waals/acid–base approach which gives, polar and non-polar components of aerogels surface free energy.     

SANS study of hybrid silica aerogels under “in situ” uniaxial compression

We have modified the inorganic silica network of aerogels with polydimethylsiloxane (PDMS), a hydroxyl-terminated polymer, to obtain an organic modified silicate (ORMOSIL). Reactions were assisted by high-power ultrasounds. The resulting gels were dried under supercritical conditions of the solvent to obtain a monolithic sono-aerogel. The mechanical behaviour of these aerogels can be tuned from brittle to rubbery as a function of the organic polymer content. In order to determine the links between the mechanical behaviour and modifications made to the microstructure, SANS (small-angle neutron scattering) experiments were carried out. To measure the intensities under “in situ” uniaxial compression of the aerogel, a specific sample-holder was built. Under uniaxial compression the 2D-diagrams were significantly anisotropic (butterfly pattern), indicating the rearrangement of the polymer. The form factor of these aerogels is described well by two correlation lengths, small microporous silica clusters surrounded by entangled polymer chains of 6 nm average size (blobs), which form a larger secondary level of agglomerates governed by the “frozen-in” elastic constraints.     

Optically transparent silica aerogels based on sodium silicate by a two step sol–gel process and ambient pressure drying

Interest in improving the optical transmission of sodium silicate-based aerogels by ambient pressure drying led to the synthesis of aerogels using a two-step sol–gel process. To produce optically transparent silica aerogel granules, NH₄F (1 M) and HCl (4 M) were used as hydrolyzing and condensation catalysts, respectively. The silica aerogels were characterized by their bulk density, porosity (%), contact angle and thermal conductivity. Optical transmission of as synthesized aerogels was studied by comparing the photos of aerogel granules. Scanning electron microscopic study showed the presence of fractal structures in these aerogels. The degree of transparency in two step sol–gel process-based aerogels is higher than the conventional single step aerogels. The N₂ adsorption–desorption analysis depicts that the two step sol–gel based aerogels have large surface areas. Optically transparent silica aerogels with a low density of ∼0.125 g/cc, low thermal conductivity of ∼0.128 W/mK and higher Brunauer, Emmett, and Teller surface area of ∼425 m²/g were obtained by using NH₄F (1 M), HCl (4 M), and a molar ratio of Na₂SiO₃::H₂O::trimethylchlorosilane of 1::146.67::9.46. The aerogels retained their hydrophobicity up to 500 °C.     

勃姆石纤维增强二氧化硅气凝胶的制备及性能

以六水合氯化铝为铝源, 通过水热法制备勃姆石纤维; 以甲基三甲氧基硅烷和正硅酸乙酯为硅源共前驱体, 采用溶胶-凝胶法进而常压干燥制备了勃姆石纤维掺杂的二氧化硅复合气凝胶; 探究了勃姆石纤维的掺杂量对复合气凝胶性能的影响. 当勃姆石纤维的掺杂量(质量分数)为1%时, 气凝胶的机械性能最好, 能够承受17.1%的压缩应变, 最大压缩强度为1.12 MPa, 压缩模量高达2.57 MPa, 复合气凝胶在150 ℃仍然具有较低的导热系数(0.0670 W·m^?1·K^?1). 勃姆石纤维能够一定程度地抑制二氧化硅颗粒在高温下的烧结和相转变, 对二氧化硅气凝胶的耐高温性能有显著的提升作用, 复合气凝胶在1100 ℃高温热处理后, 仍能保持良好的隔热性能和较高的机械强度.     

Fabrication of gradient density SiO<Subscript>2</Subscript> aerogel

With tetraethoxysilane as the organic precursor, gradient density aerogels were fabricated by three different methods: layer-by-layer gelation, sol-co-gelation and continuous formation technics. Through layer-by-layer method, a 5-layer graded density silica aerogel whose density ranges from 50 to 200 mg/cm³ was obtained, but it existed a dense skin between adjacent layers which could result in density mutation in the interface. In order to optimize its interface character, sol-co-gelation technique was created to improve the interdiffusion and smooth out the density mutation via a self-built device. Finally, on the base of the device and sol-co-gelation technics, a continuous formation process was developed to fabricate the completely gradient density silica aerogel. Optical microscope and X-ray phase contrast method were used to characterize the samples prepared by three different technics and comparatively research their interface feature.     

Polyurea based aerogel for a high performance thermal insulation material

Less fragile lightweight nanostructured polyurea based organic aerogels were prepared via a simple sol–gel processing and supercritical drying method. The uniform polyurea wet gels were first prepared at room temperature and atmospheric pressure by reacting different isocyanates with polyamines using a tertiary amine (triethylamine) catalyst. Gelation kinetics, uniformity of wet gel, and properties of aerogel products were significantly affected by both target density (i.e., solid content) and equivalent weight (EW) ratio of the isocyanate resin and polyamine hardener. A supercritical carbon dioxide (CO₂) drying method was used to extract solvent from wet polyurea gels to afford nanoporous aerogels. The thermal conductivity values of polyurea based aerogel were measured at pressures from ambient to 0.075 torr and at temperatures from room temperature to −120 °C under a pressure of 8 torr. The polyurea based aerogel samples demonstrated high porosities, low thermal conductivity values, hydrophobicity properties, relatively high thermal decomposition temperature (~270 °C) and low degassing property and were less dusty than silica aerogels. We found that the low thermal conductivities of polyurea based aerogels were associated with their small pore sizes. These polyurea based aerogels are very promising candidates for cryogenic insulation applications and as a thermal insulation component of spacesuits.     

Calorimetric study of heptyloxybenzylidene butylaniline in silica aerogels

A high resolution a.c. and relaxation calorimetric study has been carried out on heptyloxybenzylidene butylaniline (7O.4) in two silica aerogels with mass densities rho 0.08 and 0.17g cm -3 . Bulk 7O.4 exhibits strongly first order N-I, N-SA and SC-CrG transitions as well as a mean-field second order SA-SC transition. The 7O.4/aerogel samples exhibit three first order transitions (N-I, N-SA, SA-crystal) that are appreciably shifted and broadened relative to bulk 7O.4. The SA-SC transition is not observed in either of the aerogel samples.     

Fabrication of controllable graphene aerogel with superior adsorption capacity for organic solvents

Controllable graphene aerogel with relative homopores was fabricated with the assistance of block copolymer (P123) and vacuum drying. Thermogravimetric analysis was conducted to obtain the optimum temperature for template removal. The Brunauer–Emmett–Teller method, scanning electron microscopy, Raman spectroscopy, Fourier transform infrared spectroscopy and X-ray diffraction were performed to characterise the obtained aerogel. Results showed that P123 was successfully removed on the surface, and a controlled multi-level porous structure was generated. The optimum ratio of GO and P123 was 1:4 (ca. 2.5 mg/ml), and the adsorption capacities of the produced aerogels for methylene blue, oils and organic solvents were excellent and superior to those aerogels fabricated by traditional approaches. Adsorption kinetics were also further studied.     

Calorimetric study of phase transitions for octylphenylthiolpentyloxybenzoate in silica aerogels

An a.c. calorimetric study has been carried out on octylphenylthiolpentyloxybenzoate (8S5) in three silica aerogels with mass densities rho 0.08, 0.17 and 0.36g cm . Results for the least porous aerogel (rho 0.36) are completely consistent with those reported previously for 8CB in the same aerogel. Freezing of 8S5 damaged the more porous rho 0.08 and 0.17 aerogels by creating internal voids and cracks that are large compared with the intrinsic pore size. As a result, overlapping sharp bulk-like Cp features and rounded features due to 8S5 in residual aerogel pores were observed, and no quantitative separation of these features could be achieved. 3     

Application of hydrophobic silica based aerogels and xerogels for removal of toxic organic compounds from aqueous solutions

This work is devoted to the application of hydrophobic silica based aerogels and xerogels for the removal of three toxic organic compounds from aqueous solutions. These materials were tested and characterized regarding their morphology, particle size distribution, surface area and porous structure. The equilibrium tests were carried out at different adsorbate concentrations and the experimental data were correlated by means of Langmuir and Freundlich isotherms. The equilibrium data were well described by Langmuir and Freundlich in most cases. The maximum adsorption capacity by Langmuir model was observed for the adsorption of benzene onto aerogel (192.31 mg/g), though the most promising results were obtained for toluene adsorption due to the greater adsorption energy involved. Comparing these results with other reported results, the hydrophobic silica based aerogels/xerogels were found to exhibit a remarkable performance for the removal of benzene and toluene. In addition, the regeneration of previously saturated aerogel/toluene was also investigated by using an ozonation process. The adsorption/regeneration tests with ozone oxidation showed that the aerogel might be regenerated, nevertheless the materials lost their hydrophobicity and thus different methods should be evaluated in forthcoming investigations.     

“Grafting through” approach for synthesis of polystyrene/silica aerogel nanocomposites by in situ reversible addition-fragmentation chain transfer polymerization

Aerogel/polystyrene nanocomposites with mixed free and aerogel-attached polystyrene chains were synthesized using reversible addition-fragmentation chain transfer (RAFT) polymerization. 3-methacryloxypropyldimethylchlorosilane containing a double bond, which could be incorporated into polystyrene chains by a “grafting through” approach, was used as an aerogel modifier. Kinetics of RAFT polymerization of styrene in the presence of modified silica aerogel was studied by monitoring conversion and molar mass values. To further study, attached polymers were detached and their molecular characteristics were compared to free chains. According to results, the presence of silica aerogel particles has a sensible influence on polymerization kinetic and more aerogels result in decreased polymerization rate and conversion. The dispersity (Ð) of polymer chains increased by the addition of silica aerogel. In the case of aerogel-attached polystyrene chains, number-averaged molar mass values were slightly lower than that of free chains. Also, thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) techniques were used to observe the effect of loading on thermal properties of synthesized nanocomposites.     

Preparation and thermal properties of chemically prepared nanoporous silica aerogels

This work focuses on the dependence preparation conditions—structure—physical properties of hydrophobic silica aerogels, all of them prepared under subcritical drying conditions (70 °C and 0.4 atm.), thus aiming at potential application as case insulation filling in heat pumps. The so prepared, millimeter scaled nano-porous hydrophobic silica aerogel granules were analyzed with standard electron microscope and atomic force microscopy, IR spectroscopy, UV/Vis spectroscopy, differential scanning calorimetry and thermal conductivity measurements. The physical properties of the aerogels were compared with commercial aerogel granules. A method for contact angle measurement of micro-droplets situated on the silica granules was proposed to quantify the level of their hydrophobicity.     

Partial transesterification of sunflower oil with ethanol by a silica fiber reinforced aerogel encapsulated lipase

A commercial lyophilized lipase of Burkholderia cepacia, from Amano, was encapsulated in silica aerogels reinforced with silica quartz fiber felt. This biocatalyst was applied in the direct transesterification of sunflower seed oil with ethanol, without any other solvent. When the molar ratio of ethanol to oil was two or three, the oil transformation kinetics was found to be very slow after the formation of 1 mole of fatty ethyl ester per mole of initial triglyceride. For a molar proportion of ethanol to oil ≈1, the recycling activity also decreased gradually in successive tests to reach an activity ≈7% of the initial activity, during the 5th test. Textural and structural analysis of the aerogels before and after catalytic tests showed that this deterioration was associated with a modification of the aerogel, by preferential adsorption of glycerol or possibly other transesterification products such as diglycerides. Besides, it is proposed that one of the cause for the aerogel loss of activity at an initial molar ratio of ethanol:oil of 3:1 was due to a progressive inhibition of the enzyme by excess adsorbed ethanol. The aerogel samples were also compared to a commercial product of lipase immobilized on polymer beads, from Fluka. The silica aerogels somewhat improved, to a limited extent, the activity during recycling.