Organic modification of TEOS based silica aerogels using hexadecyltrimethoxysilane as a hydrophobic reagent

The experimental results on the synthesis and characterization of tetraethoxysilane (TEOS) based hydrophobic silica aerogels using hexadecyltrimethoxysilane (HDTMS) as a hydrophobic reagent by two step sol-gel process, are described. The molar ratio of tetraethoxysilane (TEOS), methanol (MeOH), acidic water (0.001 M, oxalic acid) and basic water (10 M, NH₄OH) was kept constant at 1:55:3.25:1.25 and the molar ratio of HDTMS/TEOS (M) was varied from 0 to 28.5 × 10⁻². The organic modification was confirmed by infrared spectroscopic studies, and the hydrophobicity of the aerogels was tested by the contact angle measurements. The maximum contact angle of 152° was obtained for M = 22.8 × 10⁻². The aerogels retained the hydrophobicity up to a temperature of 240 °C and above this temperature the aerogels became hydrophilic. The aerogels were characterized by the thermal conductivity, density, contact angle measurements, optical transmission and scanning electron micrographs.     

Transparent Superhydrophobic silica coatings on glass by sol-gel method

Wetting behavior of solid surfaces is a key concern in our daily life as well as in engineering and science. In the present study, we demonstrate a simple dip coating method for the preparation of Thermally stable, transparent superhydrophobic silica films on glass substrates at room temperature by sol-gel process. The coating alcosol was prepared by keeping the molar ratio of methyltriethoxysilane (MTES), trimethylmethoxysilane (TMMS), methanol (MeOH), water (H₂O) constant at 1:0.09:12.71:3.58, respectively with 13 M NH₄OH throughout the experiments and the films were prepared with different deposition time varied from 5 to 25 h. In order to improve the hydrophobicity of as deposited silica films, the films were derivatized with 10% trimethylchlorosilane (TMCS) as a silylating agent in hexane solvent for 24 h. Enhancement in wetting behavior was observed for surface derivatized silica films which showed a maximum static water contact angle (172°) and minimum sliding angle (2°) for 25 h of deposition time. The superhydrophobic silica films retained their superhydrophobicity up to a temperature of 550 °C. The silica films were characterized by field emission scanning electron microscopy (FE-SEM), surface profilometer, Fourier transform infrared (FT-IR) spectroscopy, thermo-gravimetric and differential thermal analysis (TG-DTA), percentage of optical transmission, water contact angle measurements. The imperviousness behavior of the films was tested with various acids.     

Enhancement in hydrophobicity of silica films using metal acetylacetonate and heat treatment

Water is one of the most affecting chemicals that can cause damage to the solid surface. To protect the surface due to the action of water, the surface should be made hydrophobic. In the present study, the improvement in hydrophobicity of silica films using metal acetylacetonate (M-acac) by employing heat treatment to methyltrimethoxy silane (MTMS) based silica coatings is reported as a novel attempt. Instead of following the established trends of the surface derivatization or co-precursor method, iron acetylacetonate Fe(acac)₃, copper acetylacetonate Cu(acac)₂ and heat treatment were used to incorporate hydrophobicity with silica coatings. As M-acac is readily soluble in organic solvents, Fe(acac)₃ and Cu(acac)₂ were dissolved in methanol (MeOH) and their concentration was varied from 0 to 0.025 M. The coating solution was prepared by optimizing molar ratio of MTMS:MeOH:basic H₂O to 1:7.15:6.34, respectively. Gelation time (t_g) for Cu(acac)₂ containing silica sol and that containing Fe(acac)₃ were noted to be 30 and 55 min, respectively. The substrates were taken out after gelation and heat treated at 150 °C for 2 h. The heat treated films showed a dramatic increase in the static water contact angle from 82° to as high as 142°.     

Control on wetting properties of spin-deposited silica films by surface silylation method

Control on the wettability of solid materials by liquid is a classical and key issue in surface engineering. Optically transparent water-repellent silica films have been spin-deposited on glass substrates at room temperature (∼27 °C). The wetting behavior of silica films was controlled by surface silylation method using dimethylchlorosilane (DMCS) as a silylating reagent. A coating sol was prepared by keeping the molar ratio of methyltrimethoxysilane (MTMS) precursor, methanol (MeOH) solvent, water (H₂O) constant at 1:8.8:2.64 respectively, with 4 M NH₄OH as a catalyst throughout the experiments and the amount of DMCS in hexane was varied from 0 to 12 vol.%. It was found that with an increase in vol.% of DMCS, the water contact angle values of the films increased from 78° to 136°. At 12 vol.% of DMCS, the film shows static water contact angle as high as 136° and water sliding angle as low as 18°. The hydrophobic silica films retained their water repellency up to a temperature 295 °C and above this temperature the films show superhydrophilic behavior. These results are compared with our earlier research work done on silylation of silica surface using hexamethyldisilazane (HMDZ) and trimethylchlorosilane (TMCS). The hydrophobic silica films were characterized by taking into consideration the Fourier transform infrared (FT-IR) spectroscopy, thermo gravimetric-differential thermal (TG-DT) analyses, scanning electron microscopy (SEM), atomic force microscopy (AFM), % of optical transmission, thermal and chemical aging tests, humidity tests, static and dynamic water contact angle measurements.     

Effect of different trimethyl silylating agents on the hydrophobic and physical properties of silica aerogels

An essential feature of the ambient pressure dried aerogel manufacturing process is the end-capping of the reactive silanol groups in the silica wet gel. In this report, we have presented the effect of two different trimethyl silylating agents viz. trimethylchlorosilane (TMCS) and hexamethyldisilazane (HMDZ) on the hydrophobic and physical properties of ambient pressure dried silica aerogels. The hydrogels were prepared by sol-gel processing of sodium silicate precursor (Na₂SiO₃) in the presence of acetic acid catalyst followed by vapour passing treatment and different solvent exchanging steps. The silylating agent in hexane was used for end-capping of the silanols present on the silica surface of the gel. To study silylation behavior silylating agent/Na₂SiO₃ molar ratio was varied from 2.4 to 5.6. The aerogels have been characterized by density, % of volume shrinkage, porosity, % of optical transmission, Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Thermo-gravimetric and Differential Thermal Analysis (TG-DTA) and contact angle measurements.     

Hydrophobic and physical properties of the ambient pressure dried silica aerogels with sodium silicate precursor using various surface modification agents

The experimental results on the synthesis and physical properties of the ambient pressure dried hydrophobic silica aerogels in the presence of various surface modification (silylating) agents are presented. The silica aerogels were prepared with 1.12 specific gravity ion exchanged sodium silicate solution, 1N ammonium hydroxide, solvent exchanged with ethanol and hexane, and surface modification with 20% silylating agent in hexane followed by drying the modified gel up to 200 °C. The molar ratio of sodium silicate, water, ammonium hydroxide and silylating agent was kept at 1:45:4.3 × 10⁻²:5, respectively. The physical properties of the aerogels such as density, % of porosity, pore volume, thermal conductivity and contact angle measurements were studied by using various mono, di and tri alkyl or aryl silylating agents (SAs). The tri alkyl silylating agents produced low % of volume shrinkage (2%), low density (0.06 g/cm³), low refractive index (1.011), more pore volume (16.15cm³/g), high percentage of porosity (96.9%) and hydrophobic (contact angle >150°) silica aerogels. It was found from the Fourier transform infrared spectroscopic (FTIR) studies of the aerogels that the intensity of the bands related to the SiC and CH are more and the SiOH and OH are less with the tri than mono and di alkyl SAs. It was found from the TGA-DTA studies of the aerogels with increase in temperature above 325 °C, the % of weight decrease in TGA and exothermic peak in DTA are more with tri than the mono and di alkyl SAs. The SEM studies of the aerogels showed the large pore and particle sizes in the silica network with the tri alkyl SAs. The % of optical transmission of the aerogels is less with the tri alkyl SAs than the mono and di alkyl SAs. It was found from the contact angle and water adsorption studies that the hydrophobicity of the silica aerogel is more with tri alkyl than the di and mono alkyl silylating agents.     

Surface chemical modification of TEOS based silica aerogels synthesized by two step (acid-base) sol-gel process

The present paper describes the comparative studies on the hydrophobic and physical properties of the tetraethoxysilane (TEOS) based silica aerogels prepared by two step sol-gel process followed by supercritical drying. Silica alcogels were prepared by keeping the molar ratio of TEOS:methanol (MeOH):H₂O (acidic):H₂O (basic) constant at 1:33:3.5:3.5 with oxalic acid and ammonium hydroxide concentrations fixed at 0.001 and 1 M, respectively. In all, nine different co-precursors (CP) of the type R_nSiX_4 − n, have been used. The aerogels have been characterized by density, porosity, percentage of volume shrinkage, optical transmission, contact angle and thermal conductivity measurements. The surface chemical modification of silica aerogels was confirmed by the presence of CH and SiC peaks at 2900, 1450 and 840 cm⁻¹, respectively, from the Fourier transform-infrared spectroscopy (FT-IR). The microstructure of the aerogels was studied using transmission electron microscopy (TEM) and scanning electron microscopy (SEM) techniques. In addition to these studies, the stability of the hydrophobic aerogels against an organic impurity (methanol, in the present studies) in water has also been studied.     

Magnetic hydrophobic nanocomposites: Silica aerogel/maghemite

Magnetic hydrophobic aerogels (MHA) in the form of nanocomposites of silica and maghemite (γ-Fe₂O₃) were prepared by one step sol–gel procedure followed by supercritical solvent extraction. Silica alcogels were obtained from TEOS, MTMS, methanol and H₂O, and Fe(III) nitrate as magnetic precursor. The hydrophobic property was achieved using the methytrimethoxysilane (MTMS) as co-precursor for surface modification. The so produced nanocomposite aerogels are monolithic, hydrophobic and magnetic. The interconnected porous structure hosts ∼6 nm size γ-Fe₂O₃ particles, has a mean pore diameter of 5 nm, and a specific surface area (SSA) of 698 m²/g. Medium range structure of MHA is determined by SAXS, which displays the typical fractal power law behavior with primary particle radius of ∼1 nm. Magnetic properties of the nanoparticle ensembles hosted in them are studied by means of dc-magnetometry.     

Optically transparent, superhydrophobic methyltrimethoxysilane based silica coatings without silylating reagent

The superhydrophobic surfaces have drawn lot of interest, in both academic and industries because of optically transparent, adherent and self-cleaning behavior. Surface chemical composition and morphology plays an important role in determining the superhydrophobic nature of coating surface. Such concert of non-wettability can be achieved, using surface modifying reagents or co-precursor method in sol-gel process. Attempts have been made to increase the hydrophobicity and optical transparency of methyltrimethoxysilane (MTMS) based silica coatings using polymethylmethacrylate (PMMA) instead of formal routes like surface modification using silylating reagents. The optically transparent, superhydrophobic uniform coatings were obtained by simple dip coating method. The molar ratio of MTMS:MeOH:H₂O was kept constant at 1:5.63:1.58, respectively with 0.5 M NH₄F as a catalyst and the weight percent of PMMA varied from 1 to 8. The hydrophobicity of silica coatings was analyzed by FTIR and contact angle measurements. These substrates exhibited 91% optical transmittance as compared to glass and water drop contact angle as high as 171 ± 1°. The effect of humidity on hydrophobic nature of coating has been studied by exposing these films at relative humidity of 90% at constant temperature of 30 °C for a period of 45 days. The micro-structural studies carried out by transmission electron microscopy (TEM).     

Fabrication of superhydrophobic cotton fabrics by silica hydrosol and hydrophobization

Superhydrophobic cotton fabrics were prepared by the incorporation of silica nanoparticles and subsequent hydrophobization with hexadecyltrimethoxysilane (HDTMS). The silica nanoparticles were synthesized via sol-gel reaction with methyl trimethoxy silane (MTMS) as the precursor in the presence of the base catalyst and surfactant in aqueous solution. As for the resulting products, characterization by particle size analyzer, scanning electron microscopy (SEM), scanning probe microscopy (SPM), X-ray photoelectron spectroscopy (XPS), and thermal gravimetric analysis (TGA) were performed respectively. The size of SiO₂ nanoparticles can be controlled by adjusting the catalyst and surfactant concentrations. The wettability of cotton textiles was evaluated by the water contact angle (WCA) and water shedding angle (WSA) measurements. The results showed that the treated cotton sample displayed remarkable water repellency with a WCA of 151.9° for a 5 μL water droplet and a WSA of 13° for a 15 μL water droplet.     

Transparent water repellent silica films by sol-gel process

Non-wettable surfaces with high contact angles and facile sliding angle of water droplets have received tremendous attention in recent years. The present paper describes the room temperature (∼27 °C) synthesis of dip coated water repellent silica coatings on glass substrates using iso-butyltrimethoxysilane (iso-BTMS) as a co-precursor. Emphasis is given to the influence of the hydrophobic reagent (iso-BTMS) on the water repellent properties of the silica films. Silica sol was prepared by keeping the molar ratio of tetraethoxysilane (TEOS) precursor, methanol (MeOH) solvent, water (H₂O) constant at 1:16.53:8.26 respectively, with 0.01 M NH₄F throughout the experiment and the molar ratio of iso-BTMS/TEOS (M) was varied from 0 to 0.965. The effect of M on the surface structure and hydrophobicity has been researched. The static water contact angle values of the silica films increased from 65° to 140° and water sliding angle values decreased from 42° to 16° with an increase in the M value from 0 to 0.965. The water repellent silica films are thermally stable up to a temperature of 280 °C and above this temperature the film shows hydrophilic behavior. The water repellent silica films were characterized by the Fourier Transform Infrared (FT-IR) Spectroscopy, Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), % of optical transmission, thermal and chemical aging tests, humidity tests, static and dynamic water contact angle measurements.     

Fabrication of hydrophobic alumina aerogel monoliths by surface modification and ambient pressure drying

Hydrophobic crack-free alumina aerogel monoliths were fabricated by -Si(CH₃)₃ (trimethylsilyl substituent) modification of alcogels followed by an ambient pressure drying procedure. One-step solvent exchange and surface modification were simultaneously progressed by immersing alumina alcogels in trimethylmethoxysilane (TMMOS)/hexane solution. It is found that the hydrophobic property of alumina aerogels is affected by the contents of TMMOS from the measurements of contact angle and Fourier transform infrared spectrometry. Thermogravimetry/differential scanning calorimetry analyses reveal that the modified aerogels maintain their hydrophobic behavior up to a temperature of 260 °C. The structure and morphology of the obtained hydrophobic alumina aerogels were characterized by the measurements of N₂ physical adsorption and scanning electron microscopy, which showed that they were highly porous materials with narrow slit-like pore geometry and a high degree of pore size uniformity.     

浸渍过程对纳米纤维素/二氧化硅复合气凝胶结构与性能研究

纳米纤维素(CNF)气凝胶兼具传统气凝胶的优异特性和自身优良的生物相容性和可降解性,在很多领域应用前景广阔。然而纤维素自身的超亲水性严重限制了其更广泛的应用,为改善纤维素气凝胶的亲水性能,提高其综合应用性能,采用简单浸渍法在纤维素气凝胶基体中引入二氧化硅(SiO₂)颗粒制备纳米纤维素/二氧化硅复合气凝胶,利用傅里叶红外光谱仪(FTIR)分析纤维素气凝胶和复合气凝胶的化学结构;用扫描电镜(SEM)观察气凝胶的微观结构;测定气凝胶的物理、力学性能和接触角。结果表明,复合气凝胶在3 340 cm-1处-OH吸收峰较纤维素气凝胶均有所减弱,表明SiO₂的引入促使Si-OH形成,也降低了气凝胶的亲水性,同时有Si-CH₃和Si-O-Si吸收振动峰出现,表明三甲基氯硅烷(TMCS)的改性作用以及纤维素与SiO₂颗粒之间形成稳定化学键连接。浸渍时间影响硅含量,进而影响气凝胶的密度、比表面积和孔隙率。当浸渍时间为10 min时制备的复合气凝胶性能较好,其微观结构分布均匀,具有疏水性能,接触角可达152°,同时气凝胶仍具有较好的力学性能和较低密度,其压缩模量和压缩性能分别为5.91和1.38 MPa,密度为0.1 g·cm-3。     

Iron oxide nanoparticles hosted in silica aerogels

The investigated hybrid materials consist of non-agglomerated iron oxide particles hosted in silica aerogels. The composite material can be produced as a monolith, in any shape, and with different dilutions of the iron oxide phase. Two sol–gel chemistry routes have been followed: a solution of Fe(NO₃)₃·9H₂O has been added either to the silica gel or to the initial sol; in the latter, the iron salt provided the water required for the gel polymerisation. To obtain monolithic aerogels, the gels were dried by hypercritical solvent evacuation. On the other hand, some gels were dried by slow and controlled evaporation of the solvent, resulting in xerogels. Several heat treatments have been performed and the iron oxide particle phase, growth mechanism and crystallinity have been analysed. The composite materials were characterised by means of X-ray diffraction, M?ssbauer spectrometry and superconducting quantum interference device magnetometry. It was observed that the particle sizes (in the nanometre range) and the thermal stability of the iron oxide phases strongly depend on the preparation method that determines the microstructure of the host material. Consequently, the magnetic properties of the nanoparticles can be controlled via synthesis conditions, matrix properties and thermal treatments. Received: 5 March 2001 / Accepted: 16 June 2001 / Published online: 30 August 2001     

Diffusivity control in nanoporous membrane through organic-inorganic hybridization

Nanoporous inorganic materials have attracted great interest due to their potential application as nanofilters, drug delivery carriers and adsorbents. In order to control the molecular passage through nanopores, we have modified the pore channel of inorganic materials with organic moieties and investigated the diffusion pattern of small molecules. The surface was modified by octyltriethoxysilane (OTS) by refluxing in toluene for 12 h. The water contact angle of OTS modified zirconia membrane was observed ∼110° showing hydrophobic surface. Contact angles to various solvents were also examined to verify the self-assembled monolayer of octyl chains on the inorganic membrane. The molecular passage patterns of both pristine and modified nanoporous membrane were evaluated by means of the diffusivity of small dye molecule, azobenzene. The diffusion coefficients of azobenzene on both membranes were measured in various solvents on the basis of Fick’s diffusion law. The diffusivities in various solvents for pristine and its modified zirconia membrane were determined. The diffusivity was observed to be influenced by surface energy of both membrane and solvent as well as the polarity of solvents.     

TMOS based water repellent silica thin films by co-precursor method using TMES as a hydrophobic agent

The present paper describes the room temperature synthesis of dip coated water repellent silica coatings on glass substrates using trimethylethoxysilane (TMES) as a co-precursor. Silica sol was prepared by keeping the molar ratio of tetramethoxysilane (TMOS) precursor, methanol (MeOH) solvent, water (H₂O) constant at 1:29.27:2.09 respectively, with 0.5 M NH₄OH throughout the experiments and the TMES/TMOS molar ratio (M) was varied from 0 to 3.8. It was found that with an increase in M value, the roughness and hydrophobicity of the films increased, however the optical transmission decreased from 93% to 57% in the visible range. The hydrophobic silica films retained their hydrophobicity up to a temperature of 250 °C and above this temperature the films became hydrophilic. The hydrophobic silica thin films were characterized by taking into consideration the surface roughness studies, Fourier transform infrared (FT-IR) spectroscopy, percentage of optical transmission, scanning electron microscopy (SEM) and contact angle measurements.     

Electrical and infrared dielectrical properties of silica aerogels and of silica-aerogel-based composites

In this contribution we have studied the key electrical parameters of silica aerogels and of silica-aerogel-based composites, namely the dielectric constants , the dielectric losses tan (at 1 kHz), and the breakdown fields E _b (at 50 Hz). For low-density bulk silica aerogels we find =1.25 and tan =0.0005. E _b is about 500 kV/cm in quasi-homogeneous fields, and of the order of MV/cm in strongly inhomogeneous fields. The dielectric constants of partially densified aerogels increase linearly with density; their dielectric losses are relatively large and their breakdown fields are comparativiely low. The same results are found for aerogels in the form of settled materials, i.e. aerogel granules and powders in air. Acrylate-based aerogel composites with volume fractions larger than 70% have low dielectric constants but their losses are at least 10 times higher than those of low-density aerogels. These materials sustain high local fields in the MV/cm region, while in quasihomogeneous fields, breakdown occurs at about 100 kV/cm. Based on the present results and the interplay with other physical properties (low mechanical resistance, low thermal conductivity, adsorption of water, etc.), silica aerogels and silica aerogel-acrylate-based composites are predicted to have a low potential for electrical insulation.     

水稻中的纳米硅及其紫外吸收

为了了解水稻中硅的亚显微结构及其紫外吸收特性,根据强酸不会腐蚀SiO₂玻璃的事实,选择湿消化方法分离水稻中的硅体。以浓硫酸和硝酸混合液分别处理水稻叶片和稻壳,经多级沉降分离出其中的硅体。X射线光电子能谱结果表明硅体距离表面10 nm以内碳相对质量为35.05%,远高于硅体表面(5.88%),说明硅质壁能够阻止强酸进入硅体内部,避免硅体内氧化,保持硅体结构完整和相对独立的理化性质。电镜显示硅体SiO₂结构致密,由1～2 nm的SiO₂凝胶粒子粘聚而成,纳米颗粒相互融合组成排列方向一致的纳米棒,内部还有微米尺度(≤1μm)和纳米尺度(≤1～2 nm)的隙孔。颖壳硅体最大吸收位于285 nm;叶片纳米硅对紫外辐射的吸收极其有限,表明水稻颖壳和叶片硅体对紫外辐射具有不同的抵抗机制。     

Rapid synthesis of water-glass based aerogels by in situ surface modification of the hydrogels

The objective of the present research was to reduce the processing time of water-glass based aerogels synthesized via an ambient pressure drying. For this purpose we employed a co-precursor method for the surface modification in hydrogels using trimethylchlorosilane (TMCS) and hexamethyldisilazane (HMDS). The surface modification resulted in the displacement of pore water from the hydrogels and thereby absolutely avoiding the time-consuming solvent exchange step. The attachment of trymethylsilyl (Si(CH₃)₃) groups to the silica surface was confirmed by the presence of SiCH₃ peaks at 2900, 1400, 1255 and 845 cm⁻¹ in the Fourier Transform Infrared (FTIR) spectra. The differential thermal analysis (DTA) revealed that the aerogels maintain their hydrophobic behavior up to a maximum temperature of 500 °C above which they become hydrophilic. The physical and textural properties of the silica aerogels have been reported and the results have been discussed by taking into account the surface modification and the amounts of the pore water displaced out from the hydrogels.     

Mechanism of particle thermophoresis in pure solvents

The thermophoresis of particles suspended in a pure solvent is theoretically examined. Thermophoresis is related to the temperature-induced pressure gradient in the solvent surrounding the particle and the resulting relative motion of the particle and the surrounding liquid. The excess pressure is produced by the particle-solvent interaction. As the interaction potential, London-van der Waals forces are considered. Using the known dependence of the interaction potential on the distance between the particle and the solvent molecule, an expression for the thermophoretic mobility (TM) (the particle velocity in a unit temperature gradient) is obtained. The resulting expressions are used to calculate the TM values for silica particles in several organic solvents and water. The calculated TM values for silica particles are of the same order as those reported in the literature. The model is consistent with laboratory measurements of particle thermophoresis, which is weak in water compared with organic solvents. This can be explained by the very low cubic thermal expansion coefficient for water. The calculated retention values for silica particles in thermal field-flow fractionation experiments performed in three organic solvents also follow the order known from literature.