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Venkateswara A Rao - One of the best experts on this subject based on the ideXlab platform.

  • effect of concentration of trimethylchlorosilane tmcs and hexamethyldisilazane hmdz silylating agents on surface free energy of Silica Aerogels
    Journal of Colloid and Interface Science, 2011
    Co-Authors: D B Mahadik, Venkateswara A Rao, Parvathy A Rao, P B Wagh, S V Ingale, Satish C Gupta
    Abstract:

    Abstract 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/m2 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:NH4OH:NH4F 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/m2. As there is not direct method, we have used Neumann’s equation of state to estimate surface energy of Aerogels.

  • rapid reduction in gelation time and impregnation of hydrophobic property in the tetraethoxysilane teos based Silica Aerogels using nh4f catalyzed single step sol gel process
    Journal of Alloys and Compounds, 2009
    Co-Authors: Digambar Y Nadargi, Ravindra R Kalesh, Venkateswara A Rao
    Abstract:

    Abstract The experimental results on the synthesis and physical properties of tetraethoxysilane (TEOS) based superhydrophobic Silica Aerogels, obtained by single step sol–gel process, are reported and discussed. Superhydrophobic Silica Aerogels were prepared by varying the molar ratio of MeOH/TEOS ( M ) from 11 to 275, molar ratio of NH 4 F/TEOS ( A ) from 1.2 to 9.6 and concentration of NH 4 F from 0.001 to 1 M, respectively. All the alcogels were dried under supercritical conditions. The remarkable results were obtained in terms of drastic reduction in the gelation time which is less than 1 min, using single step sol–gel process. Further, the obtained Aerogels are hydrophobic ( θ  = 155°) without use of any hydrophobic reagents. The obtained hydrophobic property of the Aerogels was improved using mono, di and tri-functional methoxysilanes as co-precursor such as, trimethylmethoxysilane (TMMS), dimethyldimethoxysilane (DMMS) and methyltrimethoxysilane (MTMS), respectively. It was observed that as the order of functional groups of methoxysilane decreases, the contact angle of the water droplet on the aerogel surface increases, indicating the aerogel become more hydrophobic. The highest contact angle of ( θ  = 168°) was observed for trimethylmethoxysilane as a co-precursor. The Aerogels were characterized by bulk density, volume shrinkage, porosity, optical transmission and thermal conductivity measurements. The effect of humidity on the aerogel samples was studied using programmable environmental test chamber. The hydrophobicity of the Aerogels was quantified by contact angle measurements and FTIR studies while microstructural studies were done using Transmission Electron Microscopy (TEM).

  • studies on rheological properties of methyltriethoxysilane mtes based flexible superhydrophobic Silica Aerogels
    Microporous and Mesoporous Materials, 2009
    Co-Authors: Digambar Y Nadargi, Hiroshi Hirashima, Sanjay S Latthe, Venkateswara A Rao
    Abstract:

    Abstract The flexible and superhydrophobic properties of Silica Aerogels are extremely important requirements for their long-term applications. Therefore, the present paper deals with the synthesis and characterization of flexible and superhydrophobic Silica Aerogels utilizing a two-step acid–base catalyzed sol–gel process with a novel precursor, methyltriethoxysilane (MTES), followed by supercritical drying. Solvent and catalysts used for the synthesis were methanol (MeOH), oxalic acid (C 2 H 2 O 4 ) and ammonium hydroxide (NH 4 OH), respectively. Silica alcosol was prepared by varying the MeOH/MTES molar ratio (S) from 6.45 to 19.35 by keeping the each of oxalic acid (0.001 M)/MTES and maintaining a constant ammonium hydroxide (10 M)/MTES molar ratio at 4. It was observed that the Young’s modulus ( Y ) decreased from 15.03 × 10 4 to 3.95 × 10 4  N/m 2 with an increase in S value from 6.45 to 19.35. Utilizing constant molar ratio of MeOH/MTES at 19.35, experiments produced monolithic, less shrinkage and flexible Aerogels. The effect of various sol–gel parameters such as acid and base catalysts concentration and gel-aging on the flexibility and hydrophobicity of the Aerogels were investigated. The Aerogels have been characterized by bulk density, percentage of volume shrinkage, Young’s modulus, elastic limit measurements, strain energy, Poisson’s ratio and contact angle measurements. The microstructural studies were carried out using Transmission Electron Microscopy. The hydrophobicity was confirmed by Fourier Transform Infrared (FTIR) spectroscopy.

  • methyltriethoxysilane new precursor for synthesizing Silica Aerogels
    Journal of Alloys and Compounds, 2009
    Co-Authors: Digambar Y Nadargi, Venkateswara A Rao
    Abstract:

    Abstract Traditionally, Silica Aerogels are synthesized using three silicon alkoxides, namely, tetraethoxysilane (TEOS), tetramethoxysilane (TMOS) and methyltrimethoxysilane (MTMS) for various applications in science as well as in technology. Among all these precursors, only MTMS-based Silica Aerogels are inherently superhydrophobic with so far reported, the highest contact angle of 173°. In the present paper, we reported a new precursor, namely, methyltriethoxysilane (MTES) for synthesizing the Silica Aerogels having the novel properties as that of MTMS-based Aerogels. The Aerogels have been prepared using the MTES by two-stage acid–base catalyzed sol–gel process followed by supercritical drying. The solvent and catalysts used for the synthesis were methanol (MeOH), oxalic acid (C 2 H 2 O 4 ) and ammonium hydroxide (NH 4 OH), respectively. The Aerogels of different densities were obtained by varying the molar ratio of MeOH/MTES ( S ) from 6.45 to 19.35. In order to get good quality Aerogels in terms of low density, high contact angle and less volume shrinkage, the oxalic acid ( A ) and NH 4 OH ( B ) concentrations were varied from 0 to 1 and from 2 to 13.36 M, respectively. Monolithic Aerogels have been obtained for the values of A  = 0.01 M and B  = 13.36 M. Simultaneously, the Aerogels are superhydrophobic with contact angle as high as 163°. Furthermore, the effects of molar ratio of H 2 O/MTES ( W 1), i.e. acidic water and H 2 O/MTES ( W 2), i.e. basic water on the physical properties of the Aerogels have also been studied. The molar ratio of MTES:MeOH:acidic water:basic water was optimized at 1:19.35:3.57:3.57, respectively. The aerogel thermal stability was studied by TGA–DTA while the hydrophobicity was quantified in terms of the contact angle measurements and FTIR studies. The as-prepared Aerogels have been characterized by bulk density, porosity, volume shrinkage, thermal conductivity, contact angle measurements, transmission electron microscopy (TEM) and Fourier transform infrared (FTIR) spectroscopy. The physical properties of the Aerogels have been explained by taking into account of sol–gel reactions and the gel network formation.

  • effect of different trimethyl silylating agents on the hydrophobic and physical properties of Silica Aerogels
    Applied Surface Science, 2008
    Co-Authors: Poonam M Shewale, Venkateswara A Rao, Parvathy A Rao
    Abstract:

    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.

Parvathy A Rao - One of the best experts on this subject based on the ideXlab platform.

  • effect of concentration of trimethylchlorosilane tmcs and hexamethyldisilazane hmdz silylating agents on surface free energy of Silica Aerogels
    Journal of Colloid and Interface Science, 2011
    Co-Authors: D B Mahadik, Venkateswara A Rao, Parvathy A Rao, P B Wagh, S V Ingale, Satish C Gupta
    Abstract:

    Abstract 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/m2 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:NH4OH:NH4F 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/m2. As there is not direct method, we have used Neumann’s equation of state to estimate surface energy of Aerogels.

  • effect of different trimethyl silylating agents on the hydrophobic and physical properties of Silica Aerogels
    Applied Surface Science, 2008
    Co-Authors: Poonam M Shewale, Venkateswara A Rao, Parvathy A Rao
    Abstract:

    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.

  • microstructural and physical properties of the ambient pressure dried hydrophobic Silica Aerogels with various solvent mixtures
    Journal of Non-crystalline Solids, 2008
    Co-Authors: Parvathy A Rao, Venkateswara A Rao
    Abstract:

    Abstract The experimental results on the microstructural and physical properties of the ambient pressure dried hydrophobic Silica Aerogels with various solvent mixtures have been reported. The Aerogels were prepared with sodium Silicate precursor, ammonium hydroxide catalyst, trimethylchlorosilane (TMCS) silylating agent, solvent mixture of methanol–isopropanol (MeOH/IPA) and various aprotic solvent mixtures namely, hexane and benzene (HB), hexane and toluene (HT), hexane and xylene (HX), heptane and benzene (HpB), heptane and toluene (HpT), heptane and xylene (HpX). The physical properties of the Aerogels such as % of volume shrinkage, density, % of optical transmission, surface area, % of porosity, pore volume, thermal conductivity and heat capacities of the Aerogels were studied. The hydrophobicity of the Aerogels was studied by contact angle measurements. The HX and HpX Aerogels have been found to be more hydrophobic (contact angle, θ  > 155°) than the other Aerogels. It has been observed that the % of weight increase is highest (1%) for the HT Aerogels and lowest (0.25%) for HpX Aerogels by keeping them at 70% humidity for 350 h. Further, the Aerogels have been characterized by pore size distribution (PSD), Fourier transform infra red spectroscopy (FTIR) and thermogravimetric and differential thermal (TG-DGA) analysis and transmission electron microscopy (TEM) techniques. The results have been discussed by taking into account the surface tension, vapor pressure, molecular weight and chain length of the solvents. Low density (0.051 g/cc), hydrophobic (165°), transparent (85%), low thermal conductive (0.059 W/m K), low heat capacity (180 kJ/m 3  K) and highly porous (97.38%) Silica Aerogels were obtained with HpX solvent mixture.

  • hydrophobic and physical properties of the ambient pressure dried Silica Aerogels with sodium Silicate precursor using various surface modification agents
    Applied Surface Science, 2007
    Co-Authors: Parvathy A Rao, Venkateswara A Rao, G M Pajonk
    Abstract:

    Abstract 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/cm 3 ), low refractive index (1.011), more pore volume (16.15cm 3 /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 Si C and C H are more and the Si OH 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.

  • hydrophobic and physical properties of the two step processed ambient pressure dried Silica Aerogels with various exchanging solvents
    Journal of Sol-Gel Science and Technology, 2005
    Co-Authors: Parvathy A Rao, Venkateswara A Rao, G M Pajonk
    Abstract:

    The experimental results by using various exchanging solvents in the preparation of two step (acid and base) processed ambient pressure dried hydrophobic Silica Aerogels, are reported. Silica alcogels were prepared by hydrolysis with oxalic acid and condensation with NH4OH of ethanol diluted tetraethylorthoSilicate (TEOS) precursor and hexamethyldisilazane(HMDZ) methylating agent. The exchanging solvents used were: hexane, cyclohexane, heptane, benzene, toluene and xylene. The physical properties such as % of volume shrinkage, density, pore volume, % of porosity, thermal conductivity, % of optical transmission, surface area, pore size distribution and contact angle (θ) of the Silica Aerogels with water, were measured as a function of EtOH/TEOS molar ratios (R) for all the exchanging solvents. It was found that the physical and hydrophobic properties of the Silica Aerogels strongly depend on the nature of the solvent and R. Heptane solvent resulted in highly transparent (≈90% optical transmission at 700 nm for 1 cm thick sample), low density (≈0.060 g/cm3), low thermal conductive (≈0.070 W/m·K), high % of porosity (97%), high surface area (750 m2/g), uniform porosity and hydrophobic (θ ≈ 160°) Aerogels compared to other solvents. On the otherhand, xylene resulted in Aerogels with higher hydrophobicity (θ ≈ 172°) among other solvents.

G M Pajonk - One of the best experts on this subject based on the ideXlab platform.

  • hydrophobic and physical properties of the ambient pressure dried Silica Aerogels with sodium Silicate precursor using various surface modification agents
    Applied Surface Science, 2007
    Co-Authors: Parvathy A Rao, Venkateswara A Rao, G M Pajonk
    Abstract:

    Abstract 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/cm 3 ), low refractive index (1.011), more pore volume (16.15cm 3 /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 Si C and C H are more and the Si OH 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.

  • synthesis of flexible Silica Aerogels using methyltrimethoxysilane mtms precursor
    Journal of Colloid and Interface Science, 2006
    Co-Authors: Venkateswara A Rao, Hiroshi Hirashima, Sharad D Bhagat, G M Pajonk
    Abstract:

    Abstract The experimental results on the synthesis of flexible and superhydrophobic Silica Aerogels using methyltrimethoxysilane (MTMS) precursor by a two-step (acid–base) sol–gel process followed by the supercritical drying, are reported. The effects of various sol–gel parameters on the flexibility of the Aerogels have been investigated. The Aerogels of different densities were obtained by varying the molar ratio of MeOH/MTMS ( S ) from 14 to 35, with lower densities for larger S values. It has been observed that the Young's modulus ( Y ) decreased from 14.11 × 10 4 to 3.43 × 10 4 N / m 2 with the decrease in the density of the Aerogels from 100 to 40 kg/m3. Simultaneously, the Aerogels are superhydrophobic with a contact angle as high as 164°. The superhydrophobic Aerogels are thermally stable up to a temperature of 530 K, above which they become hydrophilic. The Aerogels have been characterized by bulk density, percentage volume shrinkage, and porosity measurements. The microstructures of the Aerogels have been studied using the transmission electron microscopy (TEM). The Young's modulus of the Aerogels has been determined by an uniaxial compression test. The variation of physical properties of the Aerogels has been explained by taking into consideration the hydrolysis, condensation reactions, the resulting colloidal clusters and their network formation.

  • hydrophobic and physical properties of the two step processed ambient pressure dried Silica Aerogels with various exchanging solvents
    Journal of Sol-Gel Science and Technology, 2005
    Co-Authors: Parvathy A Rao, Venkateswara A Rao, G M Pajonk
    Abstract:

    The experimental results by using various exchanging solvents in the preparation of two step (acid and base) processed ambient pressure dried hydrophobic Silica Aerogels, are reported. Silica alcogels were prepared by hydrolysis with oxalic acid and condensation with NH4OH of ethanol diluted tetraethylorthoSilicate (TEOS) precursor and hexamethyldisilazane(HMDZ) methylating agent. The exchanging solvents used were: hexane, cyclohexane, heptane, benzene, toluene and xylene. The physical properties such as % of volume shrinkage, density, pore volume, % of porosity, thermal conductivity, % of optical transmission, surface area, pore size distribution and contact angle (θ) of the Silica Aerogels with water, were measured as a function of EtOH/TEOS molar ratios (R) for all the exchanging solvents. It was found that the physical and hydrophobic properties of the Silica Aerogels strongly depend on the nature of the solvent and R. Heptane solvent resulted in highly transparent (≈90% optical transmission at 700 nm for 1 cm thick sample), low density (≈0.060 g/cm3), low thermal conductive (≈0.070 W/m·K), high % of porosity (97%), high surface area (750 m2/g), uniform porosity and hydrophobic (θ ≈ 160°) Aerogels compared to other solvents. On the otherhand, xylene resulted in Aerogels with higher hydrophobicity (θ ≈ 172°) among other solvents.

  • effect of preparation conditions on the physical and hydrophobic properties of two step processed ambient pressure dried Silica Aerogels
    Journal of Materials Science, 2005
    Co-Authors: Parvathy A Rao, G M Pajonk, A V Rao
    Abstract:

    The experimental results on the physical and hydrophobic properties of the ambient pressure dried Silica Aerogels as a function of sol-gel and drying conditions, are reported.The Aerogels have been produced by a two stage (acidic and basic) catalytic sol-gel process using tetraethylorthoSilicate (TEOS) precursor, oxalic acid (OXA) and ammonium hydroxide (NH4OH) catalysts, ethanol (EtOH) solvent and hexamethyldisilazane (HMDZ) silylating agent at 200∘C.The molar ratios of HMDZ/TEOS (M), OXA/TEOS (A) NH4OH/TEOS (B), acidic H2O/TEOS (Wa) basic H2O/TEOS (Wb), EtOH/TEOS (S) were varied from 0.09 to 0.9, 3.115 × 10− 5 to 3.115 × 10−3, 4 × 10− 3 to 8 × 10− 2, 2 to 9, 1.25 to 5 and 1 to 16 respectively. The physical properties such as the percentage (%) of volume shrinkage, density, thermal conductivity, percentage of porosity, the percentage of optical transmission and contact angle have been found to be strongly dependent on the sol-gel parameters. It was found from the FTIR spectra of the Aerogels that with the increase of M, the bands at 3500 and 1600 cm− 1 corresponding to H-OH and Si-OH respectively decreased and the bands at 840 and 1250 cm− 1 due to Si-C and 2900 and 1450 cm−1 due to C-H increased. The best quality Silica Aerogels in terms of low density, low volume shrinkage, low thermal conductivity, high hydrophobicity and high optical transmission have been obtained with the molar ratio of TEOS:EtOH:acidicH2O:basicH2O:OXA:NH4OH:HMDZ at 1:8:3.75:2.25:6.23 × 10− 5: 4 × 10− 2:0.36 respectively, by ambient pressure dried method.

  • effect of methyltrimethoxysilane as a co precursor on the optical properties of Silica Aerogels
    Journal of Non-crystalline Solids, 2001
    Co-Authors: Venkateswara A Rao, G M Pajonk
    Abstract:

    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 (

Irina Smirnova - One of the best experts on this subject based on the ideXlab platform.

  • adsorption of co2 moisture and ethanol at low partial pressure using aminofunctionalised Silica Aerogels
    Chemical Engineering Journal, 2013
    Co-Authors: K. Wörmeyer, Irina Smirnova
    Abstract:

    Abstract Common applications for CO2 adsorption require the removal of CO2 at high partial pressures. Hence, a number of adsorbents have already been optimised for this task. Nonetheless, new fields for CO2 adsorption at low partial pressure are emerging, like the removal of CO2 from atmosphere or enclosed inhabited environments in submarines, space crafts or aircraft. One way to increase the energetic efficiency of the latter application would be to use tailor made adsorbents in the closed loop environmental control systems able to remove all desired species in one adsorption step. First off all these adsorbents should feature a high adsorption capacity at low CO2 partial pressures. Furthermore, the adsorbents should allow regeneration at low temperatures, have the ability to work in moist conditions and adsorb small amounts of VOCs like ethanol. In this work, the ability of Silica Aerogels functionalised with a mono- and a tri-aminosilane to fulfil the desired requirements was investigated. Both adsorption of pure components and adsorption from gas mixtures were used to characterise the adsorbents. The Aerogels showed a high CO2 adsorption capacity at low partial pressure (250 Pa) despite a decreasing specific surface area due to functionalisation. In presence of humidity an increase in the CO2 adsorption capacity takes place. Both types of amino-functionalised Aerogels are able to adsorb water, ethanol and CO2 simultaneously. The Aerogels could be regenerated in an air flow at 70 °C and a mild vacuum (25,000 Pa) with negligible losses in adsorption capacity. Thus, functionalised Silica aerogel possesses properties for an application as adsorbents in a closed loop environmental control system.

  • adsorption and thermal release of highly volatile compounds in Silica Aerogels
    Journal of Supercritical Fluids, 2009
    Co-Authors: Babu S K Gorle, Irina Smirnova, Mark A Mchugh
    Abstract:

    Abstract This work reports the adsorption/desorption behaviour of two highly volatile compounds (1-menthol and 2-methoxy pyrazine) in hydrophobic and hydrophilic Silica Aerogels where CO2 is used as an effective solvent to deliver these compounds to the inner surfaces of the Aerogels. The thermal desorption behaviour of both substances depends intimately on the strength of the interactions of the absorbed substance with the type and availability of hydrophobic and hydrophilic surface groups in each aerogel. For example, as much as ∼50 wt% of either compound remains absorbed on the hydrophilic aerogel surface at temperatures of ∼250–300 °C, whereas these same compounds are released from the hydrophobic aerogel at 100–150 °C lower temperatures. The results reported here provide insight into the application of Silica gels as delivery/storage devices with potential applications in the food, drug, flavors, and other allied industries.

  • hydrophilic Silica Aerogels as dermal drug delivery systems dithranol as a model drug
    European Journal of Pharmaceutics and Biopharmaceutics, 2008
    Co-Authors: U Guenther, Irina Smirnova, Reinhard H.h. Neubert
    Abstract:

    A special class of porous Silica materials, Silica Aerogels, was recently shown to be a potential candidate for oral drug delivery systems. It was demonstrated, that stability of drugs and their dissolution rate can essentially be improved through the adsorption on to these materials. In this work, drug loaded Silica Aerogels are firstly applied as dermal drug delivery systems. Dithranol is used as a representative drug since there is a need to enhance its dermal availability. The unstable and nearly water-insoluble drug exhibits a poor penetration. Release of dithranol from Aerogels into various semi-solid formulations and its dissolution as well as the release and penetration into artificial membranes were investigated by Fourier-transform infrared attenuated total reflection (FTIR-ATR) spectroscopy. Two model membranes (one hydrophilic and one lipophilic) were applied. Several formulations were tested and the most promising one was used in order to study the penetration of dithranol into human stratum corneum (SC). Dithranol adsorbed on hydrophilic Silica Aerogels exhibited superior penetration behaviour compared to that of the standard ointment (dithranol in white soft paraffin).

  • dissolution rate enhancement by adsorption of poorly soluble drugs on hydrophilic Silica Aerogels
    Pharmaceutical Development and Technology, 2005
    Co-Authors: Irina Smirnova, Matthias Seiler, Supakij Suttiruengwong, Wolfgang Arlt
    Abstract:

    The aim of this work is to evaluate the feasibility of hydrophilic Silica Aerogels as drug carriers and to investigate the influence of the Aerogels properties on the release rate of poorly water‐soluble drugs. Hydrophilic Silica Aerogels of different densities were loaded with two model drugs, ketoprofen and griseofulvin, by adsorption from their solution in supercritical CO2. It is demonstrated that up to 30 wt% of ketoprofen and 5.4 wt% of griseofulvin can be deposited on hydrophilic Aerogels through physical adsorption. The obtained drug‐aerogel formulations were characterized by IR‐ and UV‐spectroscopy, X‐ray diffraction and scanning electron microscopy. Release kinetics of both drugs were studied in vitro. The release rate of ketoprofen from the drug‐aerogel formulation is much faster than that of the corresponding crystalline drugs. The release rate of ketoprofen increases in 500% and that of griseofulvin in 450%, respectively. The reasons for the release enhancement are the enlarged specific surfa...

  • dissolution rate enhancement by adsorption of poorly soluble drugs on hydrophilic Silica Aerogels
    Pharmaceutical Development and Technology, 2005
    Co-Authors: Irina Smirnova, Matthias Seiler, Supakij Suttiruengwong, Wolfgang Arlt
    Abstract:

    The aim of this work is to evaluate the feasibility of hydrophilic Silica Aerogels as drug carriers and to investigate the influence of the Aerogels properties on the release rate of poorly water-soluble drugs. Hydrophilic Silica Aerogels of different densities were loaded with two model drugs, ketoprofen and griseofulvin, by adsorption from their solution in supercritical CO2. It is demonstrated that up to 30 wt% of ketoprofen and 5.4 wt% of griseofulvin can be deposited on hydrophilic Aerogels through physical adsorption. The obtained drug-aerogel formulations were characterized by IR- and UV-spectroscopy, X-ray diffraction and scanning electron microscopy. Release kinetics of both drugs were studied in vitro. The release rate of ketoprofen from the drug-aerogel formulation is much faster than that of the corresponding crystalline drugs. The release rate of ketoprofen increases in 500% and that of griseofulvin in 450%, respectively. The reasons for the release enhancement are the enlarged specific surface area of drugs by adsorption on Aerogels compared to their crystalline form and the immediate collapse of aerogel network in aqueous media. The dissolution rate of poorly water soluble drugs can be significantly enhanced by adsorption on highly porous hydrophilic Silica Aerogels.

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  • methyltrimethoxysilane based monolithic Silica Aerogels via ambient pressure drying
    Microporous and Mesoporous Materials, 2007
    Co-Authors: Sharad D Bhagat, Changsup Oh
    Abstract:

    Abstract We have developed a novel route to monolithic Silica Aerogels via ambient pressure drying by the acid–base sol–gel polymerization of methyltrimethoxysilane (MTMS) precursor. An extent of Silica polymerization in the alcogels plays a crucial role in obtaining the monolithic Aerogels which could be optimized by a proper control over the MeOH/MTMS molar ratio ( S ) during the sol–gel synthesis. The alcogel undergoes the distinct “spring-back effect” at the critical stage of the drying and thereby preserving the highly porous Silica network without collapse. The process yields Silica Aerogels exhibiting very low bulk density and high specific surface area of 0.062 g/cm 3 and 520 m 2 /g, respectively. The average pore diameter and the cumulative pore volume varied from 4.5 to 12.1 nm and 0.58 to 1.58 cc/g, respectively. In addition, the Aerogels are superhydrophobic with contact angle as high as 152°. We anticipate that the new route of the monolithic Silica aerogel production will greatly expand the commercial exploitation of these materials.

  • synthesis of flexible Silica Aerogels using methyltrimethoxysilane mtms precursor
    Journal of Colloid and Interface Science, 2006
    Co-Authors: Venkateswara A Rao, Hiroshi Hirashima, Sharad D Bhagat, G M Pajonk
    Abstract:

    Abstract The experimental results on the synthesis of flexible and superhydrophobic Silica Aerogels using methyltrimethoxysilane (MTMS) precursor by a two-step (acid–base) sol–gel process followed by the supercritical drying, are reported. The effects of various sol–gel parameters on the flexibility of the Aerogels have been investigated. The Aerogels of different densities were obtained by varying the molar ratio of MeOH/MTMS ( S ) from 14 to 35, with lower densities for larger S values. It has been observed that the Young's modulus ( Y ) decreased from 14.11 × 10 4 to 3.43 × 10 4 N / m 2 with the decrease in the density of the Aerogels from 100 to 40 kg/m3. Simultaneously, the Aerogels are superhydrophobic with a contact angle as high as 164°. The superhydrophobic Aerogels are thermally stable up to a temperature of 530 K, above which they become hydrophilic. The Aerogels have been characterized by bulk density, percentage volume shrinkage, and porosity measurements. The microstructures of the Aerogels have been studied using the transmission electron microscopy (TEM). The Young's modulus of the Aerogels has been determined by an uniaxial compression test. The variation of physical properties of the Aerogels has been explained by taking into consideration the hydrolysis, condensation reactions, the resulting colloidal clusters and their network formation.

  • surface chemical modification of teos based Silica Aerogels synthesized by two step acid base sol gel process
    Applied Surface Science, 2006
    Co-Authors: Sharad D Bhagat, Venkateswara A Rao
    Abstract:

    Abstract 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):H 2 O (acidic):H 2 O (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 n SiX 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 C H and Si C 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.

  • synthesis and physical properties of teos based Silica Aerogels prepared by two step acid base sol gel process
    Solid State Sciences, 2004
    Co-Authors: Sharad D Bhagat
    Abstract:

    Abstract The experimental results on the synthesis and physical properties of tetraethoxysilane- (TEOS) based Silica Aerogels produced by two step (acid–base) sol–gel process, are reported. The oxalic acid (A) and NH4OH (B) concentrations were varied from 0 to 0.1 M and from 0.4 to 3 M, respectively. Monolithic and transparent Aerogels have been obtained for the values of A=0.001 M and B=1 M. The effect of time interval (t) before the base catalyst (NH4OH) addition to the acidic sol was studied from 0 to 72 h. The time interval at t=24 h of NH4OH addition was found to be the best, in terms of low volume shrinkage, high optical transmission and monolithicity. The molar ratio of EtOH/TEOS (S) was varied from 3 to 7.5. Monolithic and transparent Aerogels were obtained for an S value of 6.9. Also, the effects of molar ratio of acidic water, i.e., H2O/TEOS (W1) and basic water, i.e., H2O/TEOS (W2) on the physical properties of the Aerogels have been studied. Highly transparent (∼90%) and monolithic Aerogels with lower volume shrinkage (