Superhydrophobic Silica Aerogels Research Articles

Preparation of super hydrophobic silica aerogel and study on its fractal structure

The experimental results on silica aerogels with super hydrophobic property are reported. Silica alcogels were prepared via a two-step acid/base process by keeping the molar ratio of tetraethyoxysiliane (TEOS), ethanol (EtOH), water (H2O), hydrochloric acid (HCl) and ammonia (NH4OH) constant at 1:6:8:1.0×10−3:1.1×10−2, respectively, and varying the molar ratio of N,N-dimethylformamide (DMF)/TEOS (G) from 0 to 1.2. After two aging treatment steps, they were modified by isopropyl alcohol (IPA)/trimethylchlorosilane (TMCS)/n-hexane solution at 60°C. It was found that G value at 0.8 resulted in low density (∼0.2gcm−3) and the minimum volume shrinkage (∼6%), with the total water adsorption ratio ∼5.1% when exposed to water for 3 months and the contact angle θ≈178°. Besides, the aerogels (G=0.8) had higher volume fractal dimension (∼1.8), which indicted that it possessed better connectivity and more uniform particle sizes.

Studies on rheological properties of methyltriethoxysilane (MTES) based flexible superhydrophobic silica aerogels

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 2H 2O 4) and ammonium hydroxide (NH 4OH), 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.

Superhydrophobic silica aerogel powders with simultaneous surface modification, solvent exchange and sodium ion removal from hydrogels

We report a novel method for the rapid synthesis of superhydrophobic silica aerogel powders based on an inexpensive precursor such as sodium silicate via ambient pressure drying. The sodium silicate was directly polymerized without prior ion exchange by the addition of nitric acid and hexamethyldisilazane (HMDS) to yield silylated hydrogels. The indispensable steps such as surface modification, solvent exchange and sodium ion removal were simultaneously accomplished in one-step by immersing the silylated hydrogel in a non-polar solvent such as n-hexane. The organic modification in the aqueous phase, i.e. in the hydrogel essentially led to the displacement of pore water and simultaneous intrusion of n-hexane transformed the hydrogel into an organo-gel. During this process, sodium ions present in the hydrogel were removed with the displaced pore water. Using this process, it was possible to produce superhydrophobic silica aerogel powders with contact angle as high as 150°, very low tapping density of 0.08 g/cm 3, high specific surface area (778 m 2/g) and cumulative pore volume of 2.34 cm 3/g. The total processing time of the aerogel powders could be restricted to 5 h. The textural properties of the aerogel powders have been investigated and discussed.

Absorption and desorption of organic liquids in elastic superhydrophobic silica aerogels

The experimental results of the studies on the absorption and desorption of organic liquids in elastic superhydrophobic silica aerogels, are reported. The elastic superhydrophobic aerogels were prepared using methyltrimethoxysilane (MTMS) precursor by a two-step sol–gel process followed by supercritical drying. Monolithic superhydrophobic silica aerogels were used as the absorbents. In all, four alkanes, three aromatic compounds, four alcohols and three oils were used. The absorption property of the aerogel was quantified by the mass and moles of the organic liquid absorbed by unit mass of the aerogel. The superhydrophobic aerogels showed a very high uptake capacity and high rate of uptake. The desorption of solvents and oils was studied by maintaining the as-absorbed aerogel samples at various temperatures and weighing them at regular time intervals until all the absorbed liquid got totally desorbed. This was verified by measuring the weights of the aerogel samples before and after desorption. The transmission electron micrograph observations showed that the aerogel structure was not much affected by the solvent absorption, while the oil absorption led to the shrinkage resulting in a dense structure after the desorption. In all the cases, the aerogels retained hydrophobicity and could be re-used as absorbents.

Synthesis of flexible silica aerogels using methyltrimethoxysilane (MTMS) precursor

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/m 3. 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.

Transport of liquids using superhydrophobic aerogels

The experimental results of the studies on the transportation of water droplets on a superhydrophobic silica aerogel-powder-coated surface are reported. The superhydrophobic silica aerogels were prepared using sol–gel processing of methyltrimethoxysilane (MTMS) precursor, methanol (MeOH) solvent, and base (NH 4OH)-catalyzed water followed by supercritical drying using methanol solvent. The molar ratio of NH 4OH/MTMS, H 2O/MTMS, and MeOH/MTMS were varied from 1.7 × 10 −1 to 3.5 × 10 −1 , 2 to 8, and 1.7 to 14, respectively, to find out the best-quality aerogels in terms of higher hydrophobicity and high droplet velocity. A specially built device was used for the measurement of velocity of water droplet of size 2.8 mm ( ± 0.2 mm ) on an inclined surface coated with superhydrophobic aerogel powder. Liquid marbles were prepared by rolling water droplets on aerogel powder and the marble(s) velocities on a noncoated inclined surface were compared with that of the water droplets. It was observed that the microstructure of the aerogel affects the droplet as well as marble velocities considerably. For an aerogel with uniform and smaller particles, the water droplet and marble velocities were observed to be maximum, i.e., 144 and 123 cm/s, respectively, whereas for the aerogels with bigger and nonuniform particles, the water droplet and marble velocities were observed to be minimum, i.e., 92 and 82 cm/s, respectively. The results have been discussed by taking into account the contact angles and microstructural observations.

Superhydrophobic silica aerogels by fluorination at the gel stage

Superhydrophobic silica aerogels are obtained by surface modification of standard silica gels prepared by a two-step process with a heavily fluorinated silyl chloride followed by supercritical evacuation of the solvent.

Superhydrophobic silica aerogels based on methyltrimethoxysilane precursor

The experimental results on the synthesis and physicochemical properties of superhydrophobic silica aerogels, with the highest ever obtained contact angle (∼173°), using methyltrimethoxysilane (MTMS) precursor, methanol (MeOH) solvent and ammonium hydroxide (NH 4OH) catalyst, are reported. The molar ratios of NH 4OH/MTMS ( N), H 2O/MTMS ( H) and MeOH/MTMS ( M) were varied from 4.25 × 10 −2 to 3.5 × 10 −1, 2 to 10 and 1.75 to 17, respectively. It has been found that the gelation time decreases with increase in N and H values and it increases with increase in M values. The bulk density of the aerogels was found to decrease with increase in N, H and M values. It has been observed that the volume shrinkage increases with decrease in N and H values and increases with M values. In the case of catalyst concentration variation, the contact angle ( θ) increases slightly from 159° to 167° with increase in N values. On the other hand, in the case of H 2O and MeOH variations, the θ first increases from 162° and 160° up to the values of 173° and 167° and then decreases to 160° and 159° with increase in H and M values, respectively. All the MTMS aerogels are opaque to the visible light. The aerogels retain their hydrophobicity up to a temperature of ∼480 ° C. The thermal conductivity of the aerogels was found to be around 0.095 W/m K except for the aerogels with higher bulk density (>0.3 g/cm 3, at a lower H value of 2) whose thermal conductivity was around 0.109 W/m K. The aerogels have been characterized by Fourier transform infrared spectra (FTIR), thermogravimetric and differential thermal analyses (TGA-DTA) and scanning electron microscopy (SEM) techniques. The results have been discussed by taking into account the hydrolysis and condensation reactions and SEM observations.