A. Venkateswara Rao

Preparation of MTMS based transparent superhydrophobic silica films by sol-gel method.

Superhydrophobic surfaces with water contact angle higher than 150 degrees generated a lot of interest both in academia and in industry because of the self-cleaning properties. Optically transparent superhydrophobic silica films were synthesized at room temperature (27 degrees C) using sol-gel process by a simple dip coating technique. The molar ratio of MTMS:MeOH:H(2)O (5 M NH(4)OH) was kept constant at 1:10.56:4.16, respectively. Emphasis is given to the effect of the surface modifying agents on the hydrophobic behavior of the films. Methyl groups were introduced in the silica film by post-synthesis grafting from two solutions using trimethylchlorosilane (TMCS) and hexamethyldisilazane (HMDZ) silylating agents in hexane solvent, individually. The percentage of silylating agents and silylation period was varied from 2.5 to 7.5% and 1 to 3 h, respectively. The TMCS modified films exhibited a very high water contact angle (166+/-2 degrees) in comparison to the HMDZ (138+/-2 degrees) modified films, indicating the water repellent behavior of the surface. When the TMCS and HMDZ modified films were heated at temperatures higher than 350 degrees C and 335 degrees C, respectively, the films became superhydrophilic; the contact angle for water on the films was smaller than 5 degrees. Further, the humidity study was carried out at a relative humidity of 85% at 30 degrees C temperature over 30 days. The films have been characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FT-IR), % optical transmission, humidity tests and contact angle (CA) measurements.

Effect of methyltrimethoxysilane as a synthesis component on the hydrophobicity and some physical properties of silica aerogels

The effects of adding methyltrimethoxysilane (MTMS) to the synthesis formulation on the hydrophobicity and physical properties of silica aerogels are reported. The molar ratio of the methanol (MeOH) solvent, water (H2O), and the ammonia (NH4OH) catalyst to tetramethoxysilane (TMOS) precursor was fixed at 1TMOS:12MeOH:4H2O:3.6×10−3NH4OH throughout the experiment and the MTMS/TMOS molar ratio M was varied from 0 to 1.55. After gelation, the alcogels were dried supercritically by high-temperature solvent extraction. The hydrophobicity of the resulting aerogels was tested by measuring the water uptake by the aerogel as a function of time, after putting them directly on the surface of water. It was found that for M 90% in the visible range), whereas for M>1.03 the aerogels were more hydrophobic but semi-transparent to opaque. Aerogels that possessed good hydrophobicity and transparency (∼85% in the visible range) were obtained with an M≈0.70. An increase in the MTMS content in the gels shifted the pore size distribution towards larger pore radii with a broad distribution. In order to determine the thermal stability of the hydrophobic nature of the aerogels, they were heat-treated in air in the temperature range between 25 and 350°C. It was found that below 280°C the aerogel samples showed hydrophobic properties, whereas above 280°C the hydrophobicity vanished. This is due to the disappearance of the CH3 groups in the aerogels. The aerogels were characterized by optical transmittance, pore size distribution, BET surface area and infrared spectroscopy measurements.

Comparative studies of the physical and hydrophobic properties of TEOS based silica aerogels using different co-precursors

Abstract In the present paper, the experimental results on the physical and hydrophobic properties of tetraethoxysilane (TEOS) based silica aerogels using six different organosilane co-precursors (C.P) of the type RnSiX4-n as synthesis components, are reported and discussed. The aerogels have been produced by sol-gel processing followed by supercritical drying using methanol solvent extraction. The molar ratio of TEOS, ethanol (EtOH), water (0.001M oxalic acid (H2C2O4) catalyst) was kept constant at 1:5:7, respectively, and the molar ratio of C.P/TEOS (A) was varied from 0.1 to 0.6 and compared the aerogel properties. The hydrophobicity of the aerogels has been tested by the contact angle measurements. The contact angle (θ) has been found to be the highest (θ =136˚) for the trimethylethoxysilane (TMES) co-precursor, while for the other co-precursors it is in between 12˚8 and 13˚8. The surface chemical modification of the hydrophobic aerogels has been studied using Fourier Transform Infrared Spectroscopy (FTIR). As the C.P/TEOS molar ratio increased, the intensity of the C–H and Si–C peaks in the FTIR spectra increased, clearly indicating the organic modification of the aerogels. The aerogels based on mono-alkyl (CH3) trialkoxysilane co-precursor have shown higher optical transmission (≈65%) compared to the phenyl, di or tri alkyl alkoxysilanes (5–50%). The trialkyl modified aerogels showed the lowest bulk density (118.3 kg/m3) and volume shrinkage (<2%). The alkyl alkoxy/chloro–silane modified aerogels have been found to be thermally stable up to a maximum temperature of 573 K, whereas the phenyl trialkoxysilane modified aerogels are stable up to a temperature as high as 823 K. The aerogels have been characterized by scanning electron microscopy, thermogravimetric and differential thermal analyses.

Effect of methyltrimethoxysilane as a co-precursor on the optical properties of silica aerogels

Abstract To produce monolithic durable hydrophobic silica aerogels with high direct optical transmittance and low diffusion of light, systematic and detailed experimental investigations were carried out by adding methyltrimethoxysilane (MTMS) in the sol–gel processing of silica alcogels. The alcogels were dried supercritically using the high temperature alcohol method. A series of aerogel samples of different molar ratio combinations (0–1.85) of MTMS/tetramethoxysilane (TMOS) was optically examined in the UV–Visible–NIR range by a spectrophotometer equipped with an integrating sphere. The overall transmittance of the aerogels in the visible range decreased from 93% to 10% with increased MTMS/TMOS molar ratio (A) from 0 to 1.65, respectively. The most relevant parameter being studied was the direct/hemispherical transmittance ratio (τ). The hydrophobicity of the aerogels was measured from the contact angle and it varied from 50° to 140° depending upon the molar ratio combination, the higher being for the higher A value. A good compromise of high direct optical transmittance (τ≈80% at 750 nm for 1 cm thick sample) for durable hydrophobic (θ≈110°) aerogels with low volume shrinkage (

Water repellent porous silica films by sol-gel dip coating method

The wetting of solid surfaces by water droplets is ubiquitous in our daily lives as well as in industrial processes. In the present research work, water repellent porous silica films are prepared on glass substrate at room temperature by sol-gel process. The coating sol was prepared by keeping the molar ratio of methyltriethoxysilane (MTES), methanol (MeOH), water (H(2)O) constant at 1:12.90:4.74, respectively, with 2M NH(4)OH throughout the experiments and the molar ratio (M) of MTES/Ph-TMS was varied from 0 to 0.22. A simple dip coating technique is adopted to coat silica films on the glass substrates. The static water contact angle as high as 164° and water sliding angle as low as 4° was obtained for silica film prepared from M=0.22. The surface morphological studies of the prepared silica film showed the porous structure with pore sizes typically ranging from 200nm to 1.3μm. The superhydrophobic silica films prepared from M=0.22 retained their superhydrophobicity up to a temperature of 285°C and above this temperature the films became superhydrophilic. The porous and water repellent silica films are prepared by proper alteration of the Ph-TMS in the coating solution. The prepared silica films were characterized by surface profilometer, scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier Transform Infrared (FT-IR) spectroscopy, humidity tests, chemical aging tests, static and dynamic water contact angle measurements.

Hydrophobic properties of TMOS/TMES-based silica aerogels

Abstract The hydrophobic properties of tetramethoxysilane (TMOS)-based silica aerogels by incorporating trimethylethoxysilane (TMES) as a synthesis component, are described. The molar ratio of TMES/TMOS ( M ) was varied from 0 to 4.0 by keeping the TMOS, methanol (MeOH), water (H 2 O) and ammonium hydroxide (NH 4 OH), molar ratio constant at 1:14:4:3.7×10 −3 . The hydrophobic properties of the aerogels were studied using contact angle measurements, infrared spectroscopy and thermal analysis. The contact angle, θ increased from 100 to 140° for M =0.5 to 4. While the volume shrinkage of the aerogels increased whereas the bulk density decreased with increased M values. The hydrophobic aerogels are thermally stable up to a temperature of 300°C and above this temperature the aerogels become hydrophilic.

Effect of concentration of trimethylchlorosilane (TMCS) and hexamethyldisilazane (HMDZ) silylating agents on surface free energy of silica aerogels

The surface free energy of a solid determines its surface and interfacial behavior in processes like wetting and adhesion which is crucial for silica aerogels in case of organic liquid absorption and transportation of chemicals at nano-scale for biotechnological applications. Here, we have demonstrated that the surface free energy of aerogels can be tuned in wide range from 5.5892 to 0.3073 mJ/m(2) by modifying their surface using TMCS and HMDZ silylating reagents. The alcogels were prepared by two step acid-base catalyzed process where the molar ratio of precursors Tetraethoxysilane (TEOS):Methanol (MeOH):Oxalic acid:NH(4)OH:NH(4)F was kept at optimal value of 1:2.7:0.18×10(-4):0.02:0.22×10(-3), respectively. To modify gel surfaces, TMCS and HMDZ concentration have been varied from 3% to 12% and such alcogels were dried at ambient pressure. It is observed from FTIR for aerogels that increase in concentration of silylating reagent resulted increase in hydrophobicity. This leads to increase in contact angle for water from 123° to 155° but leads to decrease in surface free energy from 5.5892 to 0.3073 mJ/m(2). As there is not direct method, we have used Neumanns equation of state to estimate surface energy of aerogels.

Crystal growth in gel media

An attempt has been made in this review to cover the existing information on the growth of single crystals in gels. Preparation of various types of gels, in which crystals can be grown, has been briefly described. The growth of single crystals from gels is reviewed using the following classifications: (i) Crystal growth by reaction. (ii) Crystallization by complex dilution method. (iii) Crystal growth by reduction of solubility. A compact list of some important crystals grown in gels is given. Growth mechanism and nucleation problems are discussed. Morphologies of various gel-grown crystals are described. Researches on gel-grown single crystals are also briefly described.

A new route for preparation of sodium-silicate-based hydrophobic silica aerogels via ambient-pressure drying

Abstract An in-depth investigation into the synthesis of hydrophobic silica aerogels prepared by the surface derivatization of wet gels followed by subsequent drying at ambient pressure is reported. The following sol–gel parameters were examined for their effect on the physical properties of the derived aerogels: number of gel washings with water, percentage of hexane or methanol in silylating mixture, molar ratio of tartaric acid: Na2SiO3, gel aging period, weight% of silica, trimethylchlorosilane (TMCS) percentage, and silylation period. These parameters were varied from 1 to 4, 0 to 100%, 0.27 to 1.2, 0 to 4 h, 1.5 to 8 wt.%, 20 to 40% and 6 to 24 h, respectively. The properties of hydrophobic silica aerogels synthesized by this new route were investigated in terms of bulk density, percentage volume shrinkage, percentage porosity, thermal conductivity and contact angle with water, and by Fourier transform infrared spectroscopy (FTIR). The as-prepared hydrophobic silica aerogels exhibited high temperature stability (up to approximately 435 °C) as measured by thermogravimetric/differential thermal analysis (TGA-DTA). The optimal sol-gel parameters were found to be a molar ratio of Na2SiO3:H2O : tartaric acid : TMCS of 1 : 146.67 : 0.86 : 9.46, an aging period of 3 h, four washings with water in 24 h and the use of a 50% hexane- or methanol-based silylating mixture. Aerogels prepared with these optimal parameters were found to exhibit 50% optical transparency in the visible range, 84 kg m−3 density, 0.090 W mK−1 thermal conductivity, 95% porosity and a contact angle of 146° with water.

Synthesis and Characterization of Hydrophobic Silica Aerogels Using Trimethylethoxysilane as a Co-Precursor

The hydrophobic property is one of the most important requirements for the long-term use of silica aerogels for transparent or translucent window insulation and opaque thermal insulating systems. Therefore, the present paper deals with the synthesis and characterization of hydrophobic silica aerogels 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) and ammonia (NH4OH) catalyst constant at 1:12:4:3.7 × 10−3 respectively throughout the experiments and the TMES/TMOS molar ratio (A) was varied from 0 to 2.35. The resulting silica alcogels were dried supercritically by high-temperature alcohol solvent extraction. Hydrophobicity of the aerogels was tested by measuring the percentage of water adsorbed by the aerogels after putting them directly on the surface of water under humid conditions. Alternately, the hydrophobicity was also tested by contact angle measurements. It was found that as the A value increased, the hydrophobicity of the aerogels increased but the optical transmission decreased from 93% to less than 5% in the visible range. The thermal stability of the aerogels was studied in the temperature range from 25 to 400°C. The hydrophobic nature of the aerogels was maintained up to a temperature of 300°C. The aerogels were characterized by infrared spectroscopy, optical transmittance, Scanning electron microscopy (SEM) and contact angle measurements. The results have been discussed by taking into account the hydrolysis and condensation mechanisms.