Hydrophobic Silica Gel Research Articles

In English

The use of hydrophobic silicas as medical adsorbents in comparison with hydroxyl silicas allows to increase the adsorption of some toxins on their surface due to the reduction of water adsorption. The purpose of this study was to find a co-adsorbate that, adsorbed on a hydrophobic surface, would allow the preparation of aqueous suspensions of an enterosorbent based on methylsilica, and, once inside the body, would be easily destroyed by the enzymes of the gastrointestinal tract, freeing its surface for the adsorption of toxins on it. The structure of the hydrate shell and the adsorption capacity of composite materials based on methyl silica and gelatin obtained by different methods were investigated by a set of physical and chemical methods. Low-temperature 1H NMR-spectroscopy has been used to study of water clusters bound to composite surface. It has been found that the water in the composite on the basis of hydrophobic methyl silica and gelatin gel is present in the form of clusters with a radius of 0.5–15 nm and is in a strongly associated state when measured in air. When a liquid hydrophobic medium is added, the water partially passes into a weakly associated state. The bound water reacts to the presence of chloroform by changing the radial distribution of the adsorbed water clusters. It has been shown that for the composite system methyl silica AM-1/gelatin (5/1), the introduction of chloroform into the interfacial space leads to a significant decrease in the interfacial energy, which indicates a partial displacement of water by the hydrophobic solvent at the interface. At the same time, for composites made on the basis of dry powders, this effect is not observed and its interfacial energy has an intermediate value between the interfacial energies of methyl silica and gelatin containing the same amount of water. Adsorption of Congo red as the medium molecular weight toxins marker from aqueous solutions on the studied composites was studied in comparison with methylsilica. It has been found that gelatin in the composition of composites contributes to increasing dye adsorption. The amount of adsorbed Congo red depends on the method of preparation of the composite and the ratio of silica to gelatin. It is concluded that AM-1/gelatin composite systems can serve as effective adsorbents for removing medium molecular weight molecules from aqueous solutions.

A facile two-step method to construct environmental-friendly Janus coconut wood membrane for oil/water separation

The Janus structure and the super hydrophilicity of cellulose inspired to develop of low-cost and environmentally friendly Janus membrane materials with opposite wetting behavior for unidirectional water/oil transport and selective water/oil separation. However, it remains a challenge to prepare pollution-free, fluoride-free, low-cost Janus membranes in a simple way. A facile two-step top-down approach is utilized to prepare the Janus coconut wood (JCW) membrane using natural coconut wood in the transverse direction as raw material. The lignin and hemicellulose were removed from the coconut wood to prepare super-hydrophilic coconut wood (SCW). The SCW was then sprayed with hydrophobic silica gel to form a super-hydrophobic surface on one side which the water contact angle reached 153.4°. It was observed that the Janus membrane maintained chemical stability in highly saline and varied pH conditions, as well as at a long friction distance. Meanwhile, Janus coconut wood membrane oil/water separation devices readily achieve on-demand unidirectional oil/water transportation due to their superior separation efficiency of over 98 % and separation flux of over 4125 L/m2·h. The Janus coconut wood membrane provides a practical and effective solution for separating the oil/water mixture.

Significance of wettability of porous media and its implication for hydrate-based pre-combustion CO2 capture

Gas hydrate-based pre-combustion CO2 capture is considered a promising technology for CO2 separation. To further improve hydrate-based CO2 capture from gaseous mixtures (i.e., CO2 + H2) generated from pre-combustion processes, it is necessary to enhance the hydrate thermodynamic stability and formation kinetics. Surfactants and interstitial pore materials have been suggested to enhance formation kinetics, but the optimum confined space and wettability of the porous media, which are the key factors affecting kinetic performance, have not been fully determined when using thermodynamic promoters. In this study, the thermodynamic stability and formation kinetics (e.g., induction time, formation rate, gas composition, and gas uptake) of THF (5.56 mol%) hydrates of CO2 (40%) + H2 (60%) mixtures in the presence of SDS (0, 100, or 500 ppm) were evaluated in silica gels of various hydrophobicities. The results revealed that the hydrophobicity of the porous media significantly affects both the thermodynamic phase equilibria and kinetic properties. Interestingly, the weak hydrophobic silica gel enhanced the thermodynamic stability, induction time, formation rate, and gas uptake with 96% CO2 composition. The SDS (particularly 500 ppm) also improved the kinetic properties of the hydrate. The present findings provide a new guideline for designing the optimum hydrophobicity of porous media to enhance the thermodynamic stability and formation kinetics for hydrate-based pre-combustion CO2 capture.

Effect of Hydrophobic Silica Nanochannel Structure on the Running Speed of a Colloidal Damper

A colloidal damper (CD) can dissipate a significant amount of vibrations and impact energy owing to the interface power that is generated when it is used. It is of great practical significance to study the influence of the nanochannel structure of hydrophobic silica gel in the CD damping medium on the running speed of the CD. The fractal theory was applied to observe the characteristics of the micropore structure of the hydrophobic silica gel by scanning electron microscopy (SEM), the primary particles were selected to carry out fractal analysis, and the two-dimensional fractal dimension of the pore area and the tortuous fractal dimension of the hydrophobic silica gel pore structure were calculated. The fractal percolation model of water in hydrophobic silica nanochannels based on the slip theory could thus be obtained. This model revealed the relationship between the micropore structure parameters of the silica gel and the running speed of the CD. The CD running speed increases with the addition of grafted molecules and the reduction in pore size of the silica gel particles. Continuous loading velocity testing of the CD loaded with hydrophobic silica gels with different pore structures was conducted. By comparing the experimental results with the calculation results of the fractal percolation model, it was determined that the fractal percolation model can better characterize the change trend of the CD running velocity for the first loading, but the fractal dimension was changed from the second loading, caused by the small amount of water retained in the nanochannel, leading to the failure of fractal characterization.

Recovery of rare earth elements from uranium leach liquors by adsorption with diglycolamic acid ligands and ionic liquids

A series of extraction chromatographic resins containing various phosphorus-containing and DGA-based extractants and ionic liquids was synthesized and their sorption characteristics for rare earth elements (REE) in sulfate solutions were studied. The effects of the type and content of ionic liquid in the stationary phase as well as solid support type on the sorption capacity of the resins for La, Nd and Dy were evaluated. It was shown that the quantity of the stationary organic phase in the resins was 30–35% independent of the matrix type. In the case of the DODGAA: TODGA: [C4mim][Tf2N] system, the matrix type did not affect the resin capacity for REE in sulfuric acid solutions. Acidity of the aqueous phase was one of the most important factors affecting lanthanides sorption from sulfate solutions. The capacity of the sorbents for lanthanides decreased sharply with the increase of acidity of the solution, this decrease was stronger for light REE. The process of REE separation from uranium leach liquors was studied using real leach liquors after separation of uranium. It was shown that a resin based on hydrophobic silica gel impregnated by DODGAA:TODGA:[C4mim][Tf2N] mixture can be recommended for concentration and primary separation of lanthanides from multicomponent industrial solutions. The advantage of this sorbent is a high selectivity for REE over macro amounts of Al and Fe as compared with strong acid cation exchange resins and phosphorus-containing organic extractants, which are widely used for preconcentration. Using this sorbent for selective recovery of REE from uranium leach liquors provided concentration factors of 10–15 for light REE group and 150–300 for heavy REE group.

Rapid Fabrication of Superhydrophobic Virtual Walls for Microfluidic Gas Extraction and Sensing.

Based on the virtual walls concept, where fluids are guided by wettability, we demonstrate the application of a gas phase extraction microfluidic chip. Unlike in previous work, the chip is prepared using a simple, rapid, and low-cost fabrication method. Channels were cut into double-sided adhesive tape (280 µm thick) and bonded to hydrophilic glass slides. The tape was selectively made superhydrophobic by ‘dusting’ with hydrophobic silica gel to enhance the wettability contrast at the virtual walls. Finally, the two glass slides were bonded using tape, which acts as a spacer for gas transport from/to the guided liquids. In our example, the virtual walls create a stable liquid–vapor–liquid flow configuration for the extraction of a volatile analyte (ammonia), from one liquid stream to the other through the intermediate vapor phase. The collector stream contained a pH indicator to visualize the mass transport. Quantitative analysis of ammonium hydroxide in the sample stream (<1 mM) was possible using a characteristic onset time, where the first pH change in the collector stream was detected. The effect of gap length, flow rates, and pH of the collector stream on the onset time is demonstrated. Finally, we demonstrate the analysis of ammonium hydroxide in artificial human saliva to show that the virtual walls chip is suitable for extracting volatile analytes from biofluids.

Resistance to Growth of Molds for Wood Modified with Hydrophobic Hybrid Silica Gel Containing Copper Amine Complexes.

Complexation copper with amine provides an effective strategy for fixation copper in wood, while hydrophobic modification improves the dimensional stability of wood. Thus, a combination of complexation and hydrophobization is expected to enhance the efficiency of copper-based biocides. In this study, hydrophobic hybrid silica gel containing copper amine complexes (MACu) was prepared through an in situ sol-gel process in wood using methyltriethoxysilane (MTES), 3-amino-propyltriethoxysilane (APTES), and copper chloride. The resistance to growth of molds for MACu modified wood (Populus tomentosa) was measured according to ASTM D3273-16. A leaching resistance test was carried out in accordance with AWPA E11-16. The results showed that only Aspergillus niger covered the surface of untreated wood blocks and no mold grew on the MACu surface even after the leaching test. MACu xerogel and MACu wood were further characterized by SEM-EDS, FTIR, and XPS. A possible schematic diagram of the reaction mechanism was proposed to explain the high-efficiency anti-mold performance of MACu wood.

Local Configurations of Pores that Course Non-Wetting Liquid Non Outflow

It is known that for the most nanoporous medium - non-wetting liquid systems are observed non-outflow phenomenon. It is shown that this phenomenon may depend on many factors, one of which is a waiting time. The observed anomalously slow relaxation (non-wetting liquid outflow) caused by formation of strongly interacting states of liquid clusters in pores. Previously was shown that relaxation of such states can occur due to relaxation of metastable local configurations of filled and empty pores. In the present work the relaxation of non-wetting liquid (water) dispersed in disordered nanoporous medium (hydrophobic silica gel Libersorb 23) for 6 and 9° C is considered. The pore size distribution functions of captured liquid for 9° C are obtained. Some local configurations for 6 and 9° C are analyzed and their lifetimes are defined.

Preparation of Nano-Porous Carbon-Silica Composites and Its Adsorption Capacity to Volatile Organic Compounds

Carbon-silica composites with nanoporous structures were synthesized for the adsorption of volatile organic compounds (VOCs), taking tetraethyl orthosilicate (TEOS) as the silicon source and activated carbon powder as the carbon source. The preparation conditions were as follows: the pH of the reaction system was 5.5, the hydrophobic modification time was 50 h, and the dosage of activated carbon was 2 wt%. Infrared spectrum analysis showed that the activated carbon was dispersed in the pores of aerogel to form the carbon-silica composites material. The static adsorption experiments, dynamic adsorption-desorption experiments, and regeneration experiments show that the prepared carbon-silica composites have microporous and mesoporous structures, the adsorption capacity for n-hexane is better than that of conventional hydrophobic silica gel, and the desorption performance is better than that of activated carbon. It still has a high retention rate of adsorption capacity after multiple adsorption-desorption cycles. The prepared carbon-silica composites material has good industrial application prospects in oil vapor recovery, providing a new alternative for solving organic waste gas pollution.

Preparation of Silica Aerogels Monoliths from Hydrophobic Silica Gels and Pluronic10R5 via Sol–Gel Process

Silica aerogel, the most common type of aerogels, comprised of 95% air in its structure which made the aerogel has a high surface area, high porosity, low density, and low thermal conductivity. Because of its structure and high porosity, one of its major weakness compared to other materials is being very brittle. This study aims at strengthening the connection points between silica nanoparticles using Pluronic10R5 (poly(propylene oxide)8–poly(ethylene oxide)22–poly(propylene oxide)8) where the Pluronic10R5 was used to reduce phase separation during the silica condensation reaction in the sol–gel process. Silica aerogel monoliths were prepared via a sol–gel process from hydrophobic silica gels and Pluronic10R5 with an ambient pressure drying (APD) process. Results from the compression test showed that the Pluronic10R5/silica aerogels have improved mechanical property by ten times that of unmodified silica aerogels. A thermogravimetric analysis (TGA) showed a mass loss at 300–400 °C that is attributed to the surface methyl group, while a mass loss at 200 °C refers to the loss of Pluronic10R5 which confirms the incorporation of Pluronic10R5 into the monolith. Moreover, infrared (IR) images revealed that the top surface temperature of Pluronic10R5/silica aerogels monolith is about 80 °C differs from the bottom heat source temperature of 160 °C.

Effects of Tetraethyl Orthosilicate on Improving Adhesion Between Polyurethane Sponge/Hydrophobic Silica Gel Composite Materials for Oily Wastewater Treatment

The environmental disposal of oily wastewater is a challenge to household and food industries. Cost-effective oil absorbent materials for oily wastewater treatment have been developed by various methods, one of which, the modification with hydrophobic particles. However, the oil absorbents still suffer high cost and low reusability because of the insufficient adhesion between supported material and the hydrophobic particles. Herein, a low cost and simple method to fabricate oil absorbents were reported by immersing commercial polyurethane (PU) sponge in hydrophobic silica gel. The adhesion between hydrophobic silica gel to the 3D surface of PU was assisted with the use of tetraethyl orthosilicate (TEOS) as a silane coupling agent. Only 0.5% TEOS was required for up to 73% improvement of adhered calculated from % silica loading. The use of TEOS showed adhesion enhancement between the surface of PU and the silica which in turn improve reusability. Reusability test mimics the real situation of squeezing the PU/silica sponge to demonstrate good mechanical properties and flexibility. Furthermore, the PU/silica sponge displayed good and rapid absorption capacity. Results from Water Contact Angle (WCA), Scanning Electron Microscopy (SEM) and Fourier-transform infrared spectroscopy (FT-IR) were also used to confirm successful modification of PU with hydrophobic silica gel.

Durable Superhydrophobic Silica Aerogel Coating from Hydrophobic Gel Synthesis

Silica aerogel is a nanostructured porous solid material. It has a low bulk density, low thermal conductivity and can be hydrophobic. In this work, hydrophobic silica gel, a material used to form aerogel, is used instead of its powder form to avoid ultrafine air particles pollution. It is used to surface modified materials to make the materials superhydrophobic and still withstand physical abrasion that ordinary aerogel would not able to do. The superhydrophobic silica aerogel coating was designed by mixing the hydrophobic gel with DOW CORNING® 2405 resin as binder and varying DOWSIL™ Z-6137 silane and tetraethyl orthosilicate (TEOS). The coating is characterized by the static contact angles (CA) and abrasion test. Scanning electron micrographs of different coating compositions were investigated. Results show that the hydrophobic gel mixed with resin and Z-6137 silane have contact angle &gt;179º. Superhydrophobic silica aerogel coating can be utilized as material coatings for glass, fiber, polymer, etc.

Impregnated type sorbents based on benzo-15-crown-5 for gold(III) extraction from hydrochloric solutions

A series of sorbents based on benzo-15-crown-5, various supports (styrene-divinylbenzene LPS-500, hydrophobized silica gel), and diluents (octan-1-ol, nitrobenzene, 1,1,7-trihydrododecafluoroheptanol (FHA), ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (С4mim+Tf2N-)) and without diluents was obtained. The sorbents without diluents with the benzo-15-crown-5 content 20–30% have the maximum sorption characteristics (recovery of gold higher than 90%). The influence of the diluent and support nature, procedure of support pretreatment, impregnation time and temperature, and the solvent for impregnation on the sorption characteristics was studied. The maximum sorption characteristics were found for the sorbents based on the styrene-divinylbenzene support LPS-500. A high selectivity of the obtained sorbents toward gold was established, and the recovery of gold higher than 95% was achieved by the optimization of the synthesis procedure of the sorbents.

Physicochemical regularities of strontium sorption by sorbents based on di(tert-butyldicyclohexano)-18-crown-6

A number of sorbents based on di(tert-butyldicyclohexano)-18-crown-6, 1,1,7-trihydrododecafluoroheptanol, and various supports (Porolas-T, LPS-500, hydrophobized silica gel) were prepared. The effect of the supports on strontium sorption from nitric acid solutions was estimated. The physicochemical regularities of strontium sorption (isotherm, kinetics, and selectivity) were studied.

Adsorption and Biodegradation of Aromatic Chemicals by Bacteria Encapsulated in a Hydrophobic Silica Gel.

An adsorbent silica biogel material was developed via silica gel encapsulation of Pseudomonas sp. NCIB 9816-4, a bacterium that degrades a broad spectrum of aromatic pollutants. The adsorbent matrix was synthesized using silica precursors methyltrimethoxysilane and tetramethoxysilane to maximize the adsorption capacity of the matrix while maintaining a highly networked and porous microstructure. The encapsulated bacteria enhanced the removal rate and capacity of the matrix for an aromatic chemical mixture. Repeated use of the material over four cycles was conducted to demonstrate that the removal capacity could be maintained with combined adsorption and biodegradation. The silica biogel can thus be used extensively without the need for disposal, as a result of continuous biodegradation by the encapsulated bacteria. However, an inverse trend was observed with the ratio of biodegradation to adsorption as a function of log Kow, suggesting increasing mass-transport limitation for the most hydrophobic chemicals used (log Kow > 4).

Removal of 90Sr from nitric acid solutions with sorbents based on di-tert-butyldicyclohexyl-18-crown-6

A series of impregnated sorbents based on di-tert-butyldicyclohexyl-18-crown-6 (DTBDCH18C6), various diluents (1-octanol, nitrobenzene, telomeric alcohol n3), and supports (Porolas-Т, LPS-500, hydrophobized silica gel) for removing tracer and weighable amounts of Sr ions from nitric acid solutions were prepared. The distribution coefficient of tracer amounts of 90Sr on all the synthesized sorbents increases with an increase in the HNO3 concentration in the range 1–7 M. The most pronounced increase in the sorption occurs with an increase in the HNO3 concentration to 3 M. The maximal distribution coefficient of 90Sr (K d = 158 cm3 g–1) is reached for the sorbent based on DTBDCH18C6 and 1-octanol in 7 M HNO3. Experiments on sorption of weighable amounts of Sr2+ ions show that the static capacity of all the prepared sorbents for Sr in nitric acid solutions amounts to 4.3, 9.2, and 8.4 mg g–1 for the sorbents based on 1-octanol, nitrobenzene, and telomeric alcohol n3, respectively. The synthesized sorbents are suitable for radioanalytical determination of Sr radionuclides in liquid radioactive waste and in radioactively contaminated natural objects.

Monitoring local configuration and anomalously slow relaxation of a nonergodic system of interacting liquid nanoclusters in a disordered confinement of a random porous medium

The relaxation of a confined nonwetting liquid dispersed in a disordered nanoporous medium has been experimentally studied in the system consisting of water and the L23 hydrophobized silica gel. Discovered that the relaxation of these states to study the system occurs as a result of the relaxation of local metastable configurations of filled and empty pores of the porous medium. Such relaxation occurs abnormally slowly back to the power law with the exponent α < 0.16. The observed anomalously slow relaxation of such a system and comparison with the time dependence of the volume of the confined liquid obtained in confirm the correctness of the description of disordered media on the basis of the notion of local metastable structures.

Observation of the Anomalously Slow Relaxation of a Nonergodic System of Interacting Liquid Nanoclusters in a Disordered Confinement of a Random Porous Medium

The relaxation of a confined nonwetting liquid dispersed in a disordered nanoporous medium has been experimentally studied in the system consisting of water and the L23 hydrophobized silica gel. It has been found that the relaxation of the system under study is anomalously slow, according to an inverse power law with the exponent α < 0.16. It has been also established that the exponent in this law in the temperature range under investigation has a maximum, which can indicate a transition from the regime of the accelerated decay of a quasi-nonergodic state with an increase in the temperature to the regime of the decay of a nonergodic state with its slowing down because of the breaking of paths for the escape of the liquid through the fractal percolation cluster.

Dispersion of a Nonwetting Liquid in a Disordered Nanoporous Medium

A dispersion transition in the Fluka 100 С8 and Fluka 100 С18 hydrophobic silica gels close in the structural characteristics has been studied. Nonlinear dependences of the volume of the trapped liquid on the degree of filling, which have not yet been observed at the interaction of nonwetting liquids with nanoporous media, have been revealed. Two critical temperatures Tc1 ≈ 330K and Tc2 ≈ 340K have been found for the Fluka 100 С18/water system. At a temperature above Tc1, a transition occurs at a degree of filling of 0.5 from the state of the system with a linear increase in the volume of the trapped liquid with the degree of filling to the state in which the dispersion transition occurs at a degree of filling of 0.5. At temperatures above Tc2, the fraction of the trapped liquid is small and independent of the temperature and degree of filling. When Fluka 100 С8 is filled with water, no abrupt change in the volume of the trapped liquid with an increase in the degree of filling has been detected in the temperature range under study T = 300–360K. In this case, a linear increase in the volume of the trapped liquid with the degree of filling has been observed.

Preparation and Activity of Ionic Liquid Catalysts Grafted on Hydrophobic Silica Gel

An ionic liquid 1- (triethoxysilyl) propyl-3-methylimidazolium hydroxide ([Smim]OH) was grafted on three kinds of mesoporous hydrophobic silica gel by means of post-grafting under mild conditions. The catalysts were [Smim]OH/SiO2-MTES (methyltriethoxysilane), [Smim]OH/SiO2-TMCS (trimethylchlorosilane), [Smim]OH/SiO2-CPTEO (triethoxysilyl propyl chloride). Such grafted ionic liquids (GILs), which were characterized by means of FTIR and N2 adsorption-desorption could be used as effective heterogeneous catalysts toward propylene carbonate synthesis through cycloaddition of carbon dioxide with propylene oxide under solventless and mild conditions.