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Special Issue "Engineering of Aerogels and Their Applications"

A special issue of Molecules (ISSN 1420-3049).

Deadline for manuscript submissions: closed (30 June 2019).

Special Issue Editors

Prof. Dr. Carlos A. A. García-González
E-Mail Website
Guest Editor
Pharmacology, Pharmacy and Pharmaceutical Technology Dept. Faculty of Pharmacy Universidade de Santiago de Compostela 15782 - Santiago de Compostela, Spain
Interests: aerogels; supercritical fluids; regenerative medicine; pharmaceutical technology; 3D-bioprinting; porous materials; scaffolds; biomedical applications
Special Issues and Collections in MDPI journals
Prof. Dr. Luísa Durães
E-Mail Website
Guest Editor
Department of Chemical Engineering, Faculty of Sciences and Technology, University of Coimbra, 3030-790 - Coimbra, Portugal
Interests: aerogels; nanoparticles; soft-solution synthesis; thermal insulation materials; environmental remediation; adsorption; heavy metals; VOC
Special Issues and Collections in MDPI journals

Special Issue Information

Dear Colleagues,

Aerogels are a unique class of light-weight nanoporous materials of interest in advanced applications for different fields. The advent of aerogels in the 1930s marked the starting point of an intense research on these materials firstly for aerospace engineering and building materials and nowadays for a broad portfolio of applications, including reaction catalysis, environmental and biomedical applications.

Interest on aerogels prompted the design and development of materials from different sources (inorganic, organic, hybrid), formats (monoliths, beads, powder), chemical functionalities (hydrophilic, hydrophobic) and sizes (insulation boards, micron-sized particles). This material’s research pace was aligned with the engineering of innovative and viable processes and unit operations to tackle the production of the ever-growing demand of aerogel quantities and varieties. Recently, the research on aerogels has particularly grown to target environmental and biomedical applications with the prospect of novel aerogel sources (biopolymers, biomass), innovative composite materials containing aerogels and environmentally friendly processing approaches.

In this changing and expanding scenario for aerogels, we are grateful to be the Guest Editors of this Special Issue on "Engineering of Aerogels and Their Applications". This Special Issue aims to assemble notable recent contributions on the engineering of aerogels in terms of sources, chemical functionalities and morphology as well as process design and optimization with a clear application-oriented focus.

This Special Issue is an initiative of the AERoGELS (CA18125 - Advanced Engineering and Research of aeroGels for Environment and Life Sciences) Action (https://cost-aerogels.eu) by COST (European Cooperation in Science and Technology) that aims to boost the development of aerogels for biomedical and environmental applications by setting up a multidisciplinary knowledge-based network from technological, scientific and market points of view.

Dr. Eng. Carlos A. García-González
Prof. Dr. Luísa Durães
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Molecules is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1800 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • Aerogels
  • Aerogel sources
  • Aerogel functionalization
  • Process engineering
  • Composite aerogels
  • Building applications
  • Environmental applications
  • Biomedical applications

Published Papers (10 papers)

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Open AccessArticle
Amine Modification of Silica Aerogels/Xerogels for Removal of Relevant Environmental Pollutants
Molecules 2019, 24(20), 3701; https://doi.org/10.3390/molecules24203701 - 15 Oct 2019
Abstract
Serious environmental and health problems arise from the everyday release of industrial wastewater effluents. A wide range of pollutants, such as volatile organic compounds, heavy metals or textile dyes, may be efficiently removed by silica materials advanced solutions such as aerogels. This option [...] Read more.
Serious environmental and health problems arise from the everyday release of industrial wastewater effluents. A wide range of pollutants, such as volatile organic compounds, heavy metals or textile dyes, may be efficiently removed by silica materials advanced solutions such as aerogels. This option is related to their exceptional characteristics that favors the adsorption of different contaminants. The aerogels performance can be selectively tuned by an appropriate chemical or physical modification of the aerogel’s surface. Therefore, the introduction of amine groups enhances the affinity between different organic and inorganic contaminants and the silica aerogels. In this work, different case studies are reported to investigate and better understand the role of these functional groups in the adsorption process, since the properties of the synthesized aerogels were significantly affected, regarding their microstructure and surface area. In general, an improvement of the removal efficiency after functionalization of aerogels with amine groups was found, with removal efficiencies higher than 90% for lead and Rubi Levafix CA. To explain the adsorption mechanism, both Langmuir and Freundlich models were applied; chemisorption is most likely the sorption type taking place in the studied cases. Full article
(This article belongs to the Special Issue Engineering of Aerogels and Their Applications)
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Open AccessArticle
Investigation of Hydrodynamic Behavior of Alginate Aerogel Particles in a Laboratory Scale Wurster Fluidized Bed
Molecules 2019, 24(16), 2915; https://doi.org/10.3390/molecules24162915 - 11 Aug 2019
Abstract
The effects of design and operating parameters on the superficial velocity at the onset of circulatory motion and the residence time of alginate aerogel particles in a laboratory scale Wurster fluidized bed were investigated. Several sets of experiments were conducted by varying Wurster [...] Read more.
The effects of design and operating parameters on the superficial velocity at the onset of circulatory motion and the residence time of alginate aerogel particles in a laboratory scale Wurster fluidized bed were investigated. Several sets of experiments were conducted by varying Wurster tube diameter, Wurster tube length, batch volume and partition gap height. The superficial velocities for Wurster tube with 10 cm diameter were lower than the tube with 8 cm diameter. Superficial velocities increased with increasing batch volume and partition gap height. Moreover, increasing batch volume and partition gap height led to a decrease in the particle residence time in the Wurster tube. The results showed that there is an upper limit for each parameter in order to obtain a circulatory motion of the particles. It was found that the partition gap height should be 2 cm for proper particle circulation. Maximum batch volume for the tube with 10 cm diameter was found as 500 mL whereas maximum batch volume was 250 mL for the tube with 8 cm diameter. The fluidization behavior of the aerogel particles investigated in this study could be described by the general fluidization diagrams in the literature. Full article
(This article belongs to the Special Issue Engineering of Aerogels and Their Applications)
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Open AccessArticle
Modelling of Mechanical Behavior of Biopolymer Alginate Aerogels Using the Bonded-Particle Model
Molecules 2019, 24(14), 2543; https://doi.org/10.3390/molecules24142543 - 12 Jul 2019
Abstract
A novel mesoscale modelling approach for the investigation of mechanical properties of alginate aerogels is proposed. This method is based on the discrete element method and bonded-particle model. The nanostructure of aerogel is not directly considered, instead the highly porous structure of aerogels [...] Read more.
A novel mesoscale modelling approach for the investigation of mechanical properties of alginate aerogels is proposed. This method is based on the discrete element method and bonded-particle model. The nanostructure of aerogel is not directly considered, instead the highly porous structure of aerogels is represented on the mesoscale as a set of solid particles connected by solid bonds. To describe the rheological material behavior, a new elastic-plastic functional model for the solids bonds has been developed. This model has been derived based on the self-similarity principle for the material behavior on the macro and mesoscales. To analyze the effectiveness of the proposed method, the behavior of alginate aerogels with different crosslinking degrees (calcium content) was analyzed. The comparison between experimental and numerical results has shown that the proposed approach can be effectively used to predict the mechanical behavior of aerogels on the macroscale. Full article
(This article belongs to the Special Issue Engineering of Aerogels and Their Applications)
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Open AccessArticle
Symbiotic Aerogel Fibers Made via In-Situ Gelation of Aramid Nanofibers with Polyamidoxime for Uranium Extraction
Molecules 2019, 24(9), 1821; https://doi.org/10.3390/molecules24091821 - 11 May 2019
Abstract
The uranium reserve in seawater is enormous, but its concentration is extremely low and plenty of interfering ions exist; therefore, it is a great challenge to extract uranium from seawater with high efficiency and high selectivity. In this work, a symbiotic aerogel fiber [...] Read more.
The uranium reserve in seawater is enormous, but its concentration is extremely low and plenty of interfering ions exist; therefore, it is a great challenge to extract uranium from seawater with high efficiency and high selectivity. In this work, a symbiotic aerogel fiber (i.e., [email protected]) based on polyamidoxime (PAO) and aramid nanofiber (ANF) is designed and fabricated via in-situ gelation of ANF with PAO in dimethyl sulfoxide and subsequent freeze-drying of the corresponding fibrous gel precursor. The resulting flexible porous aerogel fiber possesses high specific surface area (up to 165 m2·g−1), excellent hydrophilicity and high tensile strength (up to 4.56 MPa) as determined by BET, contact angle, and stress-strain measurements. The batch adsorption experiments indicate that the [email protected] aerogel fibers possess a maximal adsorption capacity of uranium up to 262.5 mg·g−1, and the absorption process is better fitted by the pseudo-second-order kinetics model and Langmuir isotherm model, indicating an adsorption mechanism of the monolayer chemical adsorption. Moreover, the [email protected] aerogel fibers exhibit selective adsorption to uranium in the presence of coexisting ions, and they could well maintain good adsorption ability and integrated porous architecture after five cycles of adsorption–desorption process. It would be expected that the symbiotic aerogel fiber could be produced on a large scale and would find promising application in uranium ion extraction from seawater. Full article
(This article belongs to the Special Issue Engineering of Aerogels and Their Applications)
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Open AccessArticle
Nanoindentation of Graphene-Reinforced Silica Aerogel: A Molecular Dynamics Study
Molecules 2019, 24(7), 1336; https://doi.org/10.3390/molecules24071336 - 04 Apr 2019
Cited by 4
Abstract
In the present work, we performed nanoindentation tests using molecular dynamics (MD) simulations on graphene, native silica aerogels, and single- and multi-layered graphene-reinforced silica aerogel nanocomposites. This work mainly focused on the two aspects of nanoindentation simulations: first, the resultant indentation force–depth curves, [...] Read more.
In the present work, we performed nanoindentation tests using molecular dynamics (MD) simulations on graphene, native silica aerogels, and single- and multi-layered graphene-reinforced silica aerogel nanocomposites. This work mainly focused on the two aspects of nanoindentation simulations: first, the resultant indentation force–depth curves, and second, the associated mechanical deformation behavior. We found that in the single-layer graphene-reinforced silica aerogel nanocomposite, the indentation resistance was four-fold that of native silica aerogels. Moreover, the combined system proved to be higher in stiffness compared to the individual material. Furthermore, the indentation resistance was increased significantly as we proceeded from single- to two-layered graphene-reinforced silica aerogel nanocomposites. The results of the study provide a detailed understanding of the mechanical behavior during the indentation tests of nanocomposites, which helps to design advanced nanoscale multi-layered materials. Full article
(This article belongs to the Special Issue Engineering of Aerogels and Their Applications)
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Open AccessArticle
Fabrication and Characterization of MSQ Aerogel Coating on ePTFE Thin Films for Cable Sheaths
Molecules 2019, 24(7), 1246; https://doi.org/10.3390/molecules24071246 - 30 Mar 2019
Abstract
With methylsilsesquioxane (MSQ) aerogels synthesized by the sol-gel method as a raw material and Si-Ti sol as a binder, an alcohol-based aerogel slurry consisting of only MSQ aerogel and Si-Ti sol was prepared and coated on expanded polytetrafluoroethylene (ePTFE) to form an MSQ [...] Read more.
With methylsilsesquioxane (MSQ) aerogels synthesized by the sol-gel method as a raw material and Si-Ti sol as a binder, an alcohol-based aerogel slurry consisting of only MSQ aerogel and Si-Ti sol was prepared and coated on expanded polytetrafluoroethylene (ePTFE) to form an MSQ aerogel coating layer, followed by low-temperature heat treatment. The effect of Si-Ti sol content on the microstructure of the MSQ aerogel coating layer was investigated, and the properties of a typical MSQ aerogel-layer-coated ePTFE film were evaluated. The results show that Si-Ti sol has an important role in terms of film-forming capability, surface smoothness, flexibility, and powder dropping of the MSQ aerogel coating layer. With a Si-Ti sol of 10.5 wt.% content as a binder and after heat treatment at 170 °C for 30 min, the coated ePTFE flexible thin film with a layer thickness of 30 μm shows high uniformity, integrity, and electrical insulation properties, with an elongation at break decrease over 130%, a thermal conductivity of 0.1753 W/(m·K) at 25 °C, a dielectric constant of 16.5674, and a dielectric loss of 0.06369, which can be promisingly applied in cable sheaths. Full article
(This article belongs to the Special Issue Engineering of Aerogels and Their Applications)
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Open AccessArticle
Design of Aerogels, Cryogels and Xerogels of Alginate: Effect of Molecular Weight, Gelation Conditions and Drying Method on Particles’ Micromeritics
Molecules 2019, 24(6), 1049; https://doi.org/10.3390/molecules24061049 - 17 Mar 2019
Cited by 1
Abstract
Processing and shaping of dried gels are of interest in several fields like alginate aerogel beads used as highly porous and nanostructured particles in biomedical applications. The physicochemical properties of the alginate source, the solvent used in the gelation solution and the gel [...] Read more.
Processing and shaping of dried gels are of interest in several fields like alginate aerogel beads used as highly porous and nanostructured particles in biomedical applications. The physicochemical properties of the alginate source, the solvent used in the gelation solution and the gel drying method are key parameters influencing the characteristics of the resulting dried gels. In this work, dried gel beads in the form of xerogels, cryogels or aerogels were prepared from alginates of different molecular weights (120 and 180 kDa) and concentrations (1.25, 1.50, 2.0 and 2.25% (w/v)) using different gelation conditions (aqueous and ethanolic CaCl2 solutions) and drying methods (supercritical drying, freeze-drying and oven drying) to obtain particles with a broad range of physicochemical and textural properties. The stability of physicochemical properties of alginate aerogels under storage conditions of 25 °C and 65% relative humidity (ICH-climatic zone II) during 1 and 3 months was studied. Results showed significant effects of the studied processing parameters on the resulting alginate dried gel properties. Stability studies showed small variations in aerogels weight and specific surface area after 3 months of storage, especially, in the case of aerogels produced with medium molecular weight alginate. Full article
(This article belongs to the Special Issue Engineering of Aerogels and Their Applications)
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Open AccessArticle
Sterile and Dual-Porous Aerogels Scaffolds Obtained through a Multistep Supercritical CO2-Based Approach
Molecules 2019, 24(5), 871; https://doi.org/10.3390/molecules24050871 - 01 Mar 2019
Cited by 3
Abstract
Aerogels from natural polymers are endowed with attractive textural and biological properties for biomedical applications due to their high open mesoporosity, low density, and reduced toxicity. Nevertheless, the lack of macroporosity in the aerogel structure and of a sterilization method suitable for these [...] Read more.
Aerogels from natural polymers are endowed with attractive textural and biological properties for biomedical applications due to their high open mesoporosity, low density, and reduced toxicity. Nevertheless, the lack of macroporosity in the aerogel structure and of a sterilization method suitable for these materials restrict their use for regenerative medicine purposes and prompt the research on getting ready-to-implant dual (macro + meso)porous aerogels. In this work, zein, a family of proteins present in materials for tissue engineering, was evaluated as a sacrificial porogen to obtain macroporous starch aerogels. This approach was particularly advantageous since it could be integrated in the conventional aerogel processing method without extra leaching steps. Physicochemical, morphological, and mechanical characterization were performed to study the effect of porogen zein at various proportions (0:1, 1:2, and 1:1 zein:starch weight ratio) on the properties of the obtained starch-based aerogels. From a forward-looking perspective for its clinical application, a supercritical CO2 sterilization treatment was implemented for these aerogels. The sterilization efficacy and the influence of the treatment on the aerogel final properties were evaluated mainly in terms of absence of microbial growth, cytocompatibility, as well as physicochemical, structural, and mechanical modifications. Full article
(This article belongs to the Special Issue Engineering of Aerogels and Their Applications)
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Open AccessArticle
Silica Aerogel Monoliths Derived from Silica Hydrosol with Various Surfactants
Molecules 2018, 23(12), 3192; https://doi.org/10.3390/molecules23123192 - 04 Dec 2018
Cited by 1
Abstract
Owing to their ultra-low thermal conductivity, silica aerogels are promising thermal insulators; however, their extensive application is limited by their high production cost. Thus, scientists have started to explore low-cost and easy preparation processes of silica aerogels. In this work, a low-cost method [...] Read more.
Owing to their ultra-low thermal conductivity, silica aerogels are promising thermal insulators; however, their extensive application is limited by their high production cost. Thus, scientists have started to explore low-cost and easy preparation processes of silica aerogels. In this work, a low-cost method was proposed to prepare silica aerogels with industrial silica hydrosol and a subsequent ambient pressure drying (APD) process. Various surfactants (cationic, amphoteric, or anionic) were added to avoid solvent exchange and surface modification during the APD process. The effects of various surfactants on the microstructure, thermal conductivity, and thermal stability of the silica aerogels were studied. The results showed that the silica aerogels prepared with a cationic or anionic surfactant have better thermal stability than that prepared with an amphoteric surfactant. After being heated at 600 °C, the silica aerogel prepared with a cationic surfactant showed the highest specific surface area of 131 m2∙g−1 and the lowest thermal conductivity of 0.038 W∙m−1∙K−1. The obtained low-cost silica aerogel with low thermal conductivity could be widely applied as a thermal insulator for building and industrial energy-saving applications. Full article
(This article belongs to the Special Issue Engineering of Aerogels and Their Applications)
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Open AccessOpinion
An Opinion Paper on Aerogels for Biomedical and Environmental Applications
Molecules 2019, 24(9), 1815; https://doi.org/10.3390/molecules24091815 - 10 May 2019
Cited by 1
Abstract
Aerogels are a special class of nanostructured materials with very high porosity and tunable physicochemical properties. Although a few types of aerogels have already reached the market in construction materials, textiles and aerospace engineering, the full potential of aerogels is still to be [...] Read more.
Aerogels are a special class of nanostructured materials with very high porosity and tunable physicochemical properties. Although a few types of aerogels have already reached the market in construction materials, textiles and aerospace engineering, the full potential of aerogels is still to be assessed for other technology sectors. Based on current efforts to address the material supply chain by a circular economy approach and longevity as well as quality of life with biotechnological methods, environmental and life science applications are two emerging market opportunities where the use of aerogels needs to be further explored and evaluated in a multidisciplinary approach. In this opinion paper, the relevance of the topic is put into context and the corresponding current research efforts on aerogel technology are outlined. Furthermore, key challenges to be solved in order to create materials by design, reproducible process technology and society-centered solutions specifically for the two abovementioned technology sectors are analyzed. Overall, advances in aerogel technology can yield innovative and integrated solutions for environmental and life sciences which in turn can help improve both the welfare of population and to move towards cleaner and smarter supply chain solutions. Full article
(This article belongs to the Special Issue Engineering of Aerogels and Their Applications)
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