-
[show abstract]
[hide abstract]
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.
Journal of Sol-Gel Science and Technology 04/2012; 36(3):285-292. · 1.63 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Aerogels are lightweight, highly transparent, thermally insulating materials comprising interconnected nanostructured pores.
Low surface energy aerogels were prepared from ambient pressure drying of sodium silicate-based gels by modifying the pore
surfaces with silylating agents including trimethylchlorosilane (TMCS), hexamethyldisiloxane (HMDSO), and hexamethyldisilazane
(HMDZ), in combination with each other. Hydrophobic properties of the resulted aerogels were studied by contact angle measurements.
Fourier-transform infrared spectroscopy (FTIR) was used to monitor the changes in chemical bonds within the aerogels due to
surface modification. The microstructure was studied by transmission electron microscopy (TEM). Effect of temperature on the
hydrophobicity of the aerogels was studied by thermogravimetric analysis/differential thermal analysis (TGA-DTA). Surface
modification of silica gels with various mixtures of surface-modification agents showed different behaviors. Aerogels made
by HMDZ and HMDSO combination comprised 5nm pores and particles and showed a high surface energy, whereas aerogels prepared
by HMDSO and TMCS combination had lower surface energy with relatively larger particle and pore sizes with a more uniform
distribution of both. The properties of the latter sample were attributed to a greater degree of surface modification and
negligible condensation of OH groups. This preparation produced silica aerogels with a low density (0.042g/cc), low surface
energy (3.39Ncm−1), low thermal conductivity (0.050WK−1m−1), high optical transmission (85% at 700nm) and hydrophobic (154° contact angle) with high hydrophobic thermal stability
(425°C). Moreover, the contact angle for materials prepared by this method decreased negligibly over 12months’ storage in
ambient conditions.
Journal of Materials Science 04/2012; 45(1):51-63. · 2.02 Impact Factor
-
[show abstract]
[hide 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/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 Neumann's equation of state to estimate surface energy of aerogels.
Journal of Colloid and Interface Science 04/2011; 356(1):298-302. · 3.07 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In the present paper, attempts have been made to produce transparent silica aerogels with low density and better hydrophobicity
by controlled sol–gel route and subsequent atmospheric pressure drying. The hydrogels were prepared by hydrolysis and polycondensation
of sodium silicate (Na2SiO3) in the presence of acetic acid catalyzed water followed by several washing steps with water, methanol and hexane, respectively.
The surface modification of the wet gel was carried out using a mixture of hexamethyldisilazane (HMDS) in hexane. Since, the
sol–gel chemistry provides a straightforward method to control the physical and optical properties of the aerogels, the influence
of various sol–gel parameters viz. gel washing time, molar ratios of CH3COOH/Na2SiO3 and HMDS/Na2SiO3 and silylation period on the physical and optical properties of the aerogels have been investigated. The aerogels have been
characterized by bulk density, Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric and Differential Thermal
Analysis (TG-DTA), Atomic Absorption Spectroscopy (AAS), Scanning Electron Microscopy (SEM) studies and Contact angle measurements.
Journal of Sol-Gel Science and Technology 02/2009; 49(3):285-292. · 1.63 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The present paper deals with the synthesis and characterization of low density and hydrophobic silica aerogels dried at ambient
pressure using low cost sodium silicate precursor. The hydrogels were prepared by sol–gel processing of sodium silicate precursor
and acetic acid catalyzed water followed by vapour passing and solvent exchange with methanol. The mixture of MeOH:trimethylchlorosilane
(TMCS):hexane was used for the end capping of the silanols present on the silica surface. The process was optimized by varying
vapour passing time, gel ageing time, molar ratios of H2O/Na2SiO3, CH3COOH/Na2SiO3 and TMCS/Na2SiO3 and silylation period. The aerogels have been characterized by bulk density, % of volume shrinkage, porosity, Fourier transform
infrared spectroscopy, thermogravimetric and differential thermal analysis and contact angle measurements. The best quality
silica aerogels in terms of low density (0.073g/cc), higher porosity (96%) and better hydrophobicity (θ=146°) have been obtained with the molar ratio of Na2SiO3:H2O:CH3COOH:TMCS at 1:166.6:2.25:11.9.
Journal of Porous Materials 01/2009; 16(1):101-108. · 1.24 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Experimental results on the physico-chemical properties of ambiently dried sodium silicate based aerogels catalyzed with various
acids are reported. The aerogels were prepared by hydrolysis and polycondensation of sodium silicate followed by subsequent
washings, surface chemical modification and ambient pressure drying using 10 various acid catalysts consisting of strong and
weak acids. The strength and concentration of acids have the major effect on the gelation of sol and hence the physico-chemical
properties of the silica aerogels. Strong acids such as HCl, HNO3 and H2SO4 resulted in shrunk (70–95%) aerogels whereas weak acids such as citric and tartaric acids resulted in less shrunk (34–50%)
aerogels. The physical properties of silica aerogels were studied by measuring bulk density, volume shrinkage (%), porosity
(%), pore volume, thermal conductivity, contact angle with water, Transmission Electron Microscopy (TEM), Atomic Absorption
Spectroscopy (AAS), Fourier Transform Infrared Spectroscopy (FTIR), Thermo Gravimetric-Differential Thermal (TG-DT) analyses
and N2 adsorption–desorption BET surface analyzer. The best quality silica aerogels in terms of low density (0.086g/cm3), low volume shrinkage (34%), high porosity (95%), low thermal conductivity (0.09W/mK) and hydrophobic (148°) were obtained
for molar ratio of Na2SiO3:H2O:citric acid:TMCS at 1:146.67:0.72:9.46 with 20min gelation time. The resulting aerogels exhibited the thermal stability
up to around 420°C.
Journal of Sol-Gel Science and Technology 01/2009; 50(1):87-97. · 1.63 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The experimental results on the preparation of low thermal conductivity and transparent ambient pressure dried silica aerogels
with the sodium silicate solution, TMCS silylating agent with methanol, isopropyl alcohol, hexane and xylene solvents, are
reported. This study is focussed on the effect of preparation conditions such as varying the number of preparation steps,
pH of the hydrosol and hydrogel ageing temperature, for the production of the low thermal conductive silica aerogels and the
results are analysed. Density, thermal conductivity, % of optical transmission and contact angle of the aerogels were measured.
The Fourier Transform Infrared Spectroscopy (FTIR) studies revealed the presence of Si–C and C–H along with the Si–O–Si and
OH bonds and their intensities strongly depend on the processing steps, pH of the hydrosol and gel ageing temperature. The
UV–Visible spectra indicated the % of optical transmission of the aerogels decreased with increasing the number of processing
steps, increase in the pH of the hydrosol from 3 to 8 and decreased for ageing temperature up to 50°C. Further increase in
temperature >50°C, the % of optical transmission of the aerogels increased. The TGA-DTA data showed the thermal stability
of the aerogels with respect to hydrophobicity is 325°C. Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy
(SEM) analyses revealed the nanostructure of the aerogels. The porosity of the aerogels was studied using the pore size distribution.
Silica aerogels with low density (0.051g/cc), low thermal conductivity (0.049W/mK), optical transmission (65%), high hydrophobicity
(159°) and resistance to humid atmosphere >1year was obtained in the present studies.
Journal of Sol-Gel Science and Technology 06/2008; 47(1):85-94. · 1.63 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Considering the importance of the highly porous, low density, transparent and nanostructured hydrophobic silica aerogels in
the scientific and industrial applications, the experiments have been carried out to prepare the low density silica aerogels
using the 1.12 specific gravity water glass (sodium silicate, Na2SiO3) precursor, ammonium hydroxide (NH4OH) catalyst, trimethylchlorosilane (TMCS) silylating agent, various first exchanging protic solvents and hexane as a second
exchanging aprotic solvent. The first exchanging solvents used were: methanol, ethanol, propanol, isopropanol, butanol, isobutanol
and hexanol. The molar ratio of the Na2SiO3:H2O:NH4OH:TMCS was kept constant at 1:56:0.02:0.4 respectively. The ambient pressure dried method was used for the preparation of
hydrophobic silica aerogels. The effect of the exchanging protic solvents on the physical properties of the aerogels such
as density, % of volume shrinkage, % of porosity, % of optical transmission, thermal conductivity, thermal stability and contact
angle of the aerogels with water, were studied. FTIR studies were carried out to confirm the silylation of the aerogel samples.
It was found that the exchanging protic solvents have profound effect on the physical and hydrophobic properties of the aerogels.
Low density (0.07g/cm3), high porosity (96.6 %), low thermal conductivity (0.091W/mK), high contact angle (166°) silica aerogels could be prepared
by using the isopropanol first exchanging solvent followed by the hexane as the second exchanging solvent along with the TMCS
silylating agent with sodium silicate precursor.
Journal of Porous Materials 01/2008; 15(5):507-512. · 1.24 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Experimental results obtained on the preparation of hydrophobic silica aerogels by ambient pressure drying method using the
sodium silicate precursor with the variation of solvent exchanging process, are reported. The silica hydrogel was prepared
by passing the 1.12 specific gravity sodium silicate through the Amberlite (TM) 120 Na+ resin and addition of 1M ammonium hydroxide to silicic acid. The gel was kept in an oven for 3h to strengthen the gel.
Solvent exchange was carried out with ethanol and hexane for 36h each followed by 24h silylation using 20% hexamethyldisilazane
(HMDZ) in hexane. Unreacted HMDZ was washed with hexane by keeping the gel in hexane for 24h. Solvent was decanted and the
gel was dried for 24h by keeping the gel at 50°C for 6h, at 150°C for 12h and at 200°C for 6h. The low density (0.06g/cm3), highly porous (96.9%), highly hydrophobic (contact angle of 160°), low thermal conductivity (0.07W/mK) aerogels were
obtained for the process of three times exchange with ethanol and three times exchange with hexane in 36h each, followed
by silylation with 20% HMDZ in hexane and two times washing with hexane in 24h. FTIR studies showed the increase in the intensity
of the Si–H and C–H bands of the aerogels with the increase of solvent exchanging times because of increase in silylation
for more times of solvent exchange processes. It was found from the TG–DTA studies that the hydrophobicity of the aerogels
retained up to the temperature of 325°C. Water absorption studies show that the aerogels were remained hydrophobic up to
4months when the aerogels were placed over the water as well as for up to 60h in a 90% humid atmosphere. SEMs of the aerogels
reveal that the pore sizes of the silica network increased, so the percentage of optical transparency decreased with the increase
in exchange times with ethanol and hexane.
Journal of Materials Science 01/2007; 42(20):8418-8425. · 2.02 Impact Factor
-
[show abstract]
[hide abstract]
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.
Journal of Materials Science 06/2005; 40(13):3481-3489. · 2.02 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The experimental results of the influence of drying control chemical additives (DCCAs)—such as formamide (FA), N-methyl formamide (NMF), N-N-dimethyl formamide (DMF), acetamide (AA), glycerol (GLY), and oxalic acid (OXA)—on the physical and optical properties of nanocrystalline cadmium sulfide (CdS)–doped silica xerogels were reported in this paper. Tetraethylorthosilicate [TEOS, Si(OC2H5)4] was used as a precursor for the three-dimensional silica network in which the CdS nanocrystallites were trapped. Silica alcosol was prepared by taking the mixture of ethanolic (EtOH) TEOS, water (H2O), hydrochloric acid (HCI), and ammonium hydroxide (NH4OH). Ethanolic cadmium acetate [Cd(CH3COO)22H2O] and thiourea [CS(NH2)2] were added to the alcosol for the formation of CdS crystallites. To study the effect of DCCAs on the physical and optical properties of nanocrystalline CdS-doped silica xerogels, the molar ratio of TEOS:EtOH:H2O:HCl:NH4OH:Cd(CH3COO2)2 2H2O:CS(NH2)2 was kept constant at 1:5:7:0.01:0.0027:0.001:0.002, respectively, and the molar ratio of DCCA/TEOS was varied from 0.001 to 1. The addition of DCCAs (e.g., formamide) resulted in decreased gelation time (tg) from 240 to 0.5). It has been found that the percentage of volume shrinkage of the samples, when heated at 300C for 3 hr, was more (>15%) for OXA, GLY, and DMF and the shrinkage was less (< gly="">< dmf="">< aa="">< nmf="">< fa.="" from="" xrd="" studies,="" the="" structure="" of="" the="" cds="" crystallites="" in="" the="" silica="" matrix="" was="" found="" to="" be="" hexagonal="">
Journal of Materials Synthesis and Processing 12/2001; 10(1):7-16.
-
[show abstract]
[hide abstract]
ABSTRACT: CdS semiconductor nanocrystals are prepared in SiO2 matrix xerogels by a sol–gel method using ethanolic tetraethylorthosilicate [TEOS, Si(OC2H5)4], catalyst (HCl or NH4OH), cadmium acetate [Cd (CH3COO)2 2 H2O], and thiourea [SC (NH2)2] and drying the alcogels at room temperature and heating up to 250C. The effect of alcosol pH on the CdS crystallite size in silica matrix, color, monolithicity, and transparency of the xerogels is studied by varying the pH from 1 to 10, and maintaining the CdS/SiO2 molar ratio constant at 0.0025. It was found that the color of samples varied from light yellow to orange and transparency decreased from 85 to 15% with the variation of alcosol pH from 1 to 10, respectively. The cracked sample were found for pH < 3,="" opaque,="" powdery="" samples="" for="" the="" ph=""> 8, and transparent monolithic yellow-colored samples in the pH range 3–7. The quantum size effect was observed in the optical absorption spectra of the samples. The threshold peak decreased from 520 to 250 nm, energy increased from 2.4 to 4.9 eV, and the CdS crystallite size decreased from 6 to 1 nm with the variation of pH of the alcosol from 10 to 1, respectively. In the case of the effect of aging, a minimum of 5 days aging is required for monolithic samples and the CdS crystallite to increase in size with and remain constant, even when the aging period is increased for 10 days. Monolithic samples were obtained with drying and heating rates of 0.4 wt.% loss/h and >50C/h drying and heating rates, respectively. The density of the CdS crystallites doped-silica xerogels decreased from 1.57 to 1.30 gm/cm3 with an increase in pH from 1 to 10, respectively. From XRD spectra, the CdS crystallite structure was found to be a hexagonal wurtzite structure. The intensity of the peaks increased and the breadth decreased with the variation of pH of the alcosol from 1 to 10 because of an increase in CdS crystallite size.
Journal of Materials Synthesis and Processing 12/2000; 9(1):11-18.
-
[show abstract]
[hide abstract]
ABSTRACT: Sol–gel process was used for the preparation of Rhodamine 6G (R6G) doped silica xerogels, using tetraethylorthosilicate [TEOS, Si(OC2H5)4] as the precursor for the silica network. Silica alcosol was prepared by hydrolysis and polycondensation of ethanol (EtOH) diluted TEOS in the presence of citric acid (CTA) catalyst. The ethanolic R6G was added to the alcosol to trap R6G molecules inside the SiO2 gel network during the gelation of the TEOS alcosol. The effect of CTA/TEOS molar ratio on the gelation time of the R6G doped TEOS alcosol, transparency and monolithicity of the R6G doped silica xerogel was studied by varying the CTA/TEOS molar ratio from 1.2×10−4 to 180×10−4 by keeping the molar ratios of TEOS:EtOH:H2O:R6G constant at 1:5:7:9.2×10−6, respectively. It was found that the minimum (<70 h) gelation time was observed at higher and lower CTA/TEOS molar ratios of 72×10−4 where as maximum (>180 h) gelation time was observed for CTA/TEOS molar ratio of 72×10−4. While opaque and monolithic R6G doped SiO2 xerogels were obtained for <4.8×10−4 CTA/TEOS molar ratios, whereas cracked and transparent xerogels were obtained for >120×10−4 molar ratios of CTA/TEOS. Transparent, homogeneous and monolithic samples were obtained between 4.8×10−4 and 120×10−4 of CTA/TEOS molar ratios. Leaching out property was studied by using water, methanol and ethanol solvents for the R6G doped SiO2 xerogels of 9.2×10−6 and 12×10−4 of R6G/TEOS and CTA/TEOS molar ratios, respectively, and found that R6G molecules were trapped in the pores of the SiO2 network.Bleaching out phenomena of the R6G doped SiO2 xerogels was studied by focusing the high intensity light on some part of the samples for a period of 1 h and found that the pores were continuous in SiO2 network. Visible spectra of R6G in water, ethanol, SiO2 alcosol and xerogel were taken for 1.6×10−4 M R6G and observed that there were two absorption peaks at 499 and 525 nm in the spectrum of R6G in water due to dimerization of R6G molecules and only one absorption peak at 530 nm in the spectra of ethanol, SiO2 alcosol and xerogel because of monomerization of R6G molecules. Visible spectra of the R6G doped silica xerogels for varying R6G/TEOS molar ratios from 9.2×10−8 to 9.2×10−5 were taken and found the red shift (5–10 nm) with increasing the R6G/TEOS molar ratio from 9.2×10−8 to 9.2×10−5. The effect of temperature on these sample was studied by varying the temperature from 50 to 300 °C and found that the R6G doped silica samples were stable up to 200 °C. IR spectra were taken for pure R6G powder and R6G doped silica xerogels of 9.2×10−8 and 9.2×10−5 R6G/TEOS molar ratios and found that most of the peaks present in pure R6G powder spectrum were absent in the spectra of trapped R6G SiO2 xerogels. This shows that, the SiO2 network hinders the rotational and vibrational transitions of R6G when it is caged in the SiO2 network. The peaks related to bending motion in R6G molecules were not disturbed by the SiO2 network
Science and Technology of Advanced Materials.
-
[show abstract]
[hide abstract]
ABSTRACT: Malachite green (MG) laser dye doped silica matrix was prepared by sol–gel processing by keeping the molar ratio of tetramethylorthosilicate [TMOS,Si(OCH3)4]:methanol:catalytic water:methanolic MG constant at 1:4:5:7 × 10−4:3 × 10−6, respectively and varying the catalyst. The effect of acidic and basic catalysts, MG/TMOS molar ratio and co-precursors on the optical properties of the MG doped silica xerogels has been studied. Colourless transparent samples were obtained with strong acids, [hydrochloric (HCl)], [nitric acid (HNO3)], strong base [sodiumhydroxide (NaOH)] and strong acid–weak base [ammoniumhydroxide (NH4OH)] mixture and whereas turbid, opaque and light blue coloured samples were obtained with weak base. However, transparent, monolithic and blue coloured MG doped silica xerogels with longitivity of 6 months were obtained with weak acids [acetic acid (CH3COOH, ACT), citricacid (C6H8O7·H2O, CTA)]. The MG/TMOS molar ratio was varied from 3 × 10−8 to 3 × 10−5 (10−6 to 10−3 M of MG) and the absorbance and luminiscence of the MG doped silica xerogels increased with increase in the MG/TMOS molar ratio to 1 × 10−5 and remained constant for further increase of MG/TMOS molar ratio. Various co-precursors used such as methyltrimethoxysilane (MTMS), ethyltrimethoxysilane (ETMS), methyltriethoxysilane (MTES), ethyltriethoxysilane (ETES), phenyltrimethoxysilane (PTMS), phenyltriethoxysilane (PTES), dimethyldodecyl chlorosilane (DDCS) and dimethyloctadecylmethoxysilane (DOMS) and the absorbance and luminescence of the MG doped silica xerogels increased with addition of MTMS, ETMS, MTES, ETES, PTMS and PTES but decreased with DDCS and DOMS.
Materials Chemistry and Physics.
-
[show abstract]
[hide 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/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.
Applied Surface Science.
