Synthesis of hydrophobic resorcinol–formaldehyde xerogels by grafting with silanes

Hydrophobic Aerogels and Xerogels based on Trimethoxybenzene-Formaldehyde

Phenolic aerogels based on resorcinol-formaldehyde (RF) are among the best thermally insulating materials. However, the hydrophilicity inherent to the free phenolic moiety of RF gels generally limits their actual range of applications. Prior efforts to render phenolic gels hydrophobic are restricted to post-synthetic functionalizations of hydrophilic gels, processes that are often limited in efficiency, scope, and/or longevity. Here, an acid-mediated conversion of 1,3,5-trimethoxybenzene with formaldehyde is reported, yielding monolithic trimethoxybenzene-formaldehyde (TMBF) aerogels and xerogels with low density (0.11-0.30 g cm-3), high porosity (74-92 %), inner surface areas (SBET) of up to 284 m2 g-1, and thermal conductivity of 34.5-43.9 mW m-1 K-1. For a monolithic xerogel based on TMBF xerogels an unprecedently low thermal conductivity of 34.5 mW m-1 K-1 could be achieved. In addition, all TMBF gels are thermally stable (degradation >280-310 °C) and highly hydrophobic (water contact angles 130°-156°). As such, TMBF serves as a new class of inherently hydrophobic aerogels and xerogels and useful complement to RF materials.

The Development of Fe3O4-Monolithic Resorcinol-Formaldehyde Carbon Xerogels Using Ultrasonic-Assisted Synthesis for Arsenic Removal of Drinking Water

Inorganic arsenic in drinking water from groundwater sources is one of the potential causes of arsenic-contaminated environments, and it is highly toxic to human health even at low concentrations. The purpose of this study was to develop a magnetic adsorbent capable of removing arsenic from water. Fe3O4-monolithic resorcinol-formaldehyde carbon xerogels are a type of porous material that forms when resorcinol and formaldehyde (RF) react to form a polymer network, which is then cross-linked with magnetite. Sonication-assisted direct and indirect methods were investigated for loading Fe3O4 and achieving optimal mixing and dispersion of Fe3O4 in the RF solution. Variations of the molar ratios of the catalyst (R/C = 50, 100, 150, and 200), water (R/W = 0.04 and 0.05), and Fe3O4 (M/R = 0.01, 0.03, 0.05, 0.1, 0.15, and 0.2), and thermal treatment were applied to evaluate their textural properties and adsorption capacities. Magnetic carbon xerogel monoliths (MXRF600) using indirect sonication were pyrolyzed at 600 °C for 6 h with a nitrogen gas flow in the tube furnace. Nanoporous carbon xerogels with a high surface area (292 m2/g) and magnetic properties were obtained. The maximum monolayer adsorption capacity of As(III) and As(V) was 694.3 µg/g and 1720.3 µg/g, respectively. The incorporation of magnetite in the xerogel structure was physical, without participation in the polycondensation reaction, as confirmed by XRD, FTIR, and SEM analysis. Therefore, Fe3O4-monolithic resorcinol-formaldehyde carbon xerogels were developed as a potential adsorbent for the effective removal of arsenic with low and high ranges of As(III) and As(V) concentrations from groundwater.

Hydrophobization of Monolithic Resorcinol-Formaldehyde Xerogels by Means of Silylation

In materials research, the control of wettability is important for many applications. Since they are typically based on phenolics, organic aerogels, and xerogels are intrinsically hydrophilic in nature, and examples of the chemical functionalization of such gels are scarce and often limited to powders. This study reports on the silylation of monolithic resorcinol-formaldehyde (RF) xerogels using solutions of silyl chlorides and triflates, respectively, in combination with an amine base. The resulting gels are structurally characterized by means of elemental analysis, X-ray photoelectron spectroscopy, pycnometry, sorption analysis, and scanning electron microscopy with electron-dispersive X-ray spectroscopy. The wetting behavior of the silylated gels was studied by the determination of the contact angle to water after exposure of the gels to ambient air. Additionally, the uptake of liquid water and aqueous acids and bases was investigated. As a result, processes for the functionalization of RF xerogels with sterically demanding silyl moieties have been established. Although the analyses indicate that silylation occurred to a rather small extent, highly hydrophobic gels resulted which retained the wetting behavior over the course of several months with contact angles of >130°. Monoliths bearing sterically demanding silyl groups showed higher stability towards aqueous acid than trimethylsilylated RF gels.

Mimicking flower petals to fabricate self-cleaning and antireflective polymer surfaces

This work explored a facile way to fabricate superhydrophobic and antireflective polymer surfaces using Canna indica flower petals. A simple, inexpensive two-step soft-lithography technique was employed to replicate the surface structures of C. indica flower petals onto two different polymer surfaces namely, polydimethylsiloxane (PDMS) — a hydrophobic polymer and resorcinol–formaldehyde (RF) xerogel — a hydrophilic polymer. First, a negative replica was prepared in PDMS using the flower petal as a template. Second, the negative PDMS replica was later used as a stamp to prepare a positive replica in both PDMS and RF xerogel. These replicated structured surfaces greatly influenced the wettability properties. The contact angle of the replicated PDMS surfaces increased to the nearly superhydrophobic values of 145° (negative PDMS replica) and 144° (positive PDMS replica) compared with that of the plain surface, which is 101°. Interestingly, the contact angle of the replicated RF surface significantly increased to 151° (superhydrophobic) compared with that of the planar RF surface, which is hydrophilic (68°). Furthermore, the replicated polymer surfaces exhibited not only excellent superhydrophobic properties but also antireflective properties. These multifunctional surfaces with superhydrophobicity as well as an omnidirectional antireflective property may find a wide range of applications, such as solar energy harvesting, optical displays and marine applications.