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Gels
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Advances in Xerogels
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Advances in Xerogels

A special issue of Gels (ISSN 2310-2861).

Deadline for manuscript submissions: closed (26 November 2021) | Viewed by 17064

Special Issue Editor

Prof. Dr. Ashleigh Fletcher

E-Mail Website
Guest Editor
Department of Chemical and Process Engineering, University of Strathclyde, James Weir Building, Glasgow G1 1XJ, UK
Interests: organic gels; xerogels; dynamic light scattering; adsorption; post-synthetic modification; porous structures

Special Issue Information

Dear Colleagues,

Xerogels, which can be obtained from a variety of systems, include inorganic materials such as silica xerogels and organic backbones arising from organic gel structures, e.g., phenol-formaldehyde gels. The precursor gel materials are often sols with a liquid matrix, which may not be practical for use in the chosen application. Xerogels, which are processed using sub-atmospheric drying methods often offer a more useful product, and obtained via a more achievable and economic route in comparison with aerogels. Due to their tunability, both texturally and chemically, these materials have seen increased interest in recent years and application myriad fields, including gas separation and storage, catalysis, water remediation, thermal application, and in their carbonised form, end uses requiring electrical conductivity. This Special Issue will celebrate recent advances in xerogel sciences, including their manufacture, modification, characterisation and application, as well as systems modelling. We encourage submissions covering key aspects of xerogel systems from all facets of the xerogel community, including fundamental studies, as well as application-focused or modelling-oriented research.

Prof. Ashleigh Fletcher
Guest Editor

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 submissions that pass pre-check are 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. Gels is an international peer-reviewed open access monthly 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 2100 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

  • characterising xerogels
  • synthesis of xerogels
  • novel routes to xerogels
  • pathways to novel xerogels
  • applications of xerogels
  • green route to xerogels
  • modification of xerogels
  • modelling xerogel formation
  • modelling xerogel properties

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Published Papers (6 papers)

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Research

15 pages, 4263 KiB  
Article
Optical Properties and Upconversion Luminescence of BaTiO3 Xerogel Structures Doped with Erbium and Ytterbium
by Ekaterina I. Lashkovskaya, Nikolai V. Gaponenko, Margarita V. Stepikhova, Artem N. Yablonskiy, Boris A. Andreev, Vadim D. Zhivulko, Alexander V. Mudryi, Igor L. Martynov, Alexander A. Chistyakov, Nikolai I. Kargin, Vladimir A. Labunov, Tamara F. Raichenok, Sergey A. Tikhomirov and Victor Yu. Timoshenko
Gels 2022, 8(6), 347; https://doi.org/10.3390/gels8060347 - 2 Jun 2022
Cited by 6 | Viewed by 2297
Abstract
Erbium upconversion (UC) photoluminescence (PL) from sol-gel derived barium titanate (BaTiO3:Er) xerogel structures fabricated on silicon, glass or fused silica substrates has been studied. Under continuous-wave excitation at 980 nm and nanosecond pulsed excitation at 980 and 1540 nm, the fabricated [...] Read more.
Erbium upconversion (UC) photoluminescence (PL) from sol-gel derived barium titanate (BaTiO3:Er) xerogel structures fabricated on silicon, glass or fused silica substrates has been studied. Under continuous-wave excitation at 980 nm and nanosecond pulsed excitation at 980 and 1540 nm, the fabricated structures demonstrate room temperature PL with several bands at 410, 523, 546, 658, 800 and 830 nm, corresponding to the 2H9/2 → 4I15/2, 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, 4F9/2→ 4I15/2 and 4I9/2→ 4I15/2 transitions of Er3+ ions. The intensity of erbium UC PL increases when an additional macroporous layer of strontium titanate is used beneath the BaTiO3 xerogel layer. It is also enhanced in BaTiO3 xerogel films codoped with erbium and ytterbium (BaTiO3:(Er,Yb)). For the latter, a redistribution of the intensity of the PL bands is observed depending on the excitation conditions. A multilayer BaTiO3:(Er,Yb)/SiO2 microcavity structure was formed on a fused silica substrate with a cavity mode in the range of 650–680 nm corresponding to one of the UC PL bands of Er3+ ions. The obtained cavity structure annealed at 450 °C provides tuning of the cavity mode by 10 nm in the temperature range from 20 °C to 130 °C. Photonic application of BaTiO3 xerogel structures doped with lanthanides is discussed. Full article
(This article belongs to the Special Issue Advances in Xerogels)
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11 pages, 2546 KiB  
Article
Hydrophobization of Monolithic Resorcinol-Formaldehyde Xerogels by Means of Silylation
by Fabian Henn and René Tannert
Gels 2022, 8(5), 304; https://doi.org/10.3390/gels8050304 - 16 May 2022
Cited by 3 | Viewed by 2095
Abstract
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 [...] Read more.
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. Full article
(This article belongs to the Special Issue Advances in Xerogels)
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15 pages, 4364 KiB  
Article
Kinetics of Resorcinol-Formaldehyde Condensation—Comparison of Common Experimental Techniques
by Eva Kinnertová, Václav Slovák, Roman Maršálek and Martin Mucha
Gels 2022, 8(1), 8; https://doi.org/10.3390/gels8010008 - 23 Dec 2021
Cited by 1 | Viewed by 2842
Abstract
Porous carbons, originated from resorcinol-formaldehyde (RF) gels, show high application potential. However, the kinetics and mechanism of RF condensation are still not well described. In this work, different methods (dynamic light scattering–DLS, Fourier transform infrared spectroscopy–FTIR, low field 1H nuclear magnetic resonance [...] Read more.
Porous carbons, originated from resorcinol-formaldehyde (RF) gels, show high application potential. However, the kinetics and mechanism of RF condensation are still not well described. In this work, different methods (dynamic light scattering–DLS, Fourier transform infrared spectroscopy–FTIR, low field 1H nuclear magnetic resonance relaxometry–1H-NMR, and differential scanning calorimetry–DSC) were used to follow the isothermal RF condensation of mixtures varying in catalyst content (Na2CO3) and reactant concentration. The applicability and results obtained by the methods used differ significantly. The changes in functional groups can be followed by FTIR only at very early stages of the reaction. DLS enables the estimate of the growth of particles in reaction solution, but only before the solution becomes more viscous. Following the relaxation of 1H nuclei in water during RF condensation brings a different view on the system—this technique follows the properties of the present water that is gradually captured in polymeric gel. From this side, the process behaves similarly to the nucleation reaction, which is in contradiction to the n-order mechanism confirmed by other techniques. The widest range of applicability was found for DSC measurement of the freezing/melting behavior of the reaction mixture, which is possible to use without any limitations until full solidification. Furthermore, this approach enables us to follow the gradual formation and development of the gel through the intermediate undergoing glass transition. Full article
(This article belongs to the Special Issue Advances in Xerogels)
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12 pages, 1646 KiB  
Article
Silica Xerogel Doped with Iron(III) as Sensor Material for Salicylhydroxamic Acid Determination in Urine
by Maria A. Morosanova, Vitaliy A. Kovalev and Elena I. Morosanova
Gels 2021, 7(3), 143; https://doi.org/10.3390/gels7030143 - 16 Sep 2021
Cited by 2 | Viewed by 2052
Abstract
Salicylhydroxamic acid (SHA) is used as antimicrobic medicine and its concentration has to be monitored in urine. For the first time, silica xerogels doped with iron(III) have been proposed as sensor materials for SHA determination in biological samples. Three xerogels with iron(III) content [...] Read more.
Salicylhydroxamic acid (SHA) is used as antimicrobic medicine and its concentration has to be monitored in urine. For the first time, silica xerogels doped with iron(III) have been proposed as sensor materials for SHA determination in biological samples. Three xerogels with iron(III) content in the range of 0.04–1.74% wt have been synthesized. BET surface area of these xerogels has varied in the range of 696–529 m2/g and total pore volume has varied in the range of 0.92–0.23 cm3/g. Complex formation between immobilized iron(III) and salicylhydroxamic acid has been investigated with solid phase spectrophotometry and IR spectroscopy. Orange-brown iron(III)-SHA complex with 1:1 stoichiometry is formed at pH 1–4 with half-reaction time of 17 min. Silica xerogel doped with 0.33% wt iron(III)) has been used as sensor material for SHA solid phase spectrophotometric determination (LOD 1.4 mg/L (n = 3), analytical range 4–230 mg/L). Proposed sensor material has been applied for SHA determination in biological samples of synthetic and human urine. The proposed procedure is characterized by a good level of accuracy (recovery values 97–120%) and precision (RSD values 4–9%) and can be recommended for pharmacokinetic–pharmacodynamic studies of hydroxamic acid-based medications. Full article
(This article belongs to the Special Issue Advances in Xerogels)
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21 pages, 4578 KiB  
Article
Investigating the Role of the Catalyst within Resorcinol–Formaldehyde Gel Synthesis
by Elisha Martin, Martin Prostredny and Ashleigh Fletcher
Gels 2021, 7(3), 142; https://doi.org/10.3390/gels7030142 - 15 Sep 2021
Cited by 7 | Viewed by 2880
Abstract
Resorcinol–formaldehyde (RF) gels are porous materials synthesized via a sol–gel reaction and subsequently dried, producing structures with high surface areas and low densities—properties that are highly attractive for use in various applications. The RF gel reaction takes place in the presence of a [...] Read more.
Resorcinol–formaldehyde (RF) gels are porous materials synthesized via a sol–gel reaction and subsequently dried, producing structures with high surface areas and low densities—properties that are highly attractive for use in various applications. The RF gel reaction takes place in the presence of a catalyst, either acidic or basic in nature, the concentration of which significantly impacts final gel properties. The full extent of the catalyst’s role, however, has been subject to debate, with the general consensus within the field being that it is simply a pH-adjuster. The work presented here explores this theory, in addition to other theories postulated in the literature, through the synthesis and analysis of RF gels catalysed by mixtures of relevant compounds with varying concentrations. The relationship between catalyst concentration and initial solution pH is decoupled, and the individual roles of both the cation and the anion within the catalyst are investigated. The results presented here point towards the significance of the metal cation within the RF gel reaction, with similar structural properties observed for gels synthesized at constant Na+ concentrations, regardless of the initial solution pH. Furthermore, through the use of alternative cations and anions within catalyst compounds, the potential effects of ions on the stabilization of macromolecules in solution are explored, the results of which suggest a ‘Hofmeister-like’ series could be applicable within the catalysis of RF gel reactions. Full article
(This article belongs to the Special Issue Advances in Xerogels)
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13 pages, 3438 KiB  
Article
Acidic and Electrosurface Properties of Binary TiO2-SiO2 Xerogels Using EPR of pH-Sensitive Nitroxides
by Denis O. Antonov, Daria P. Tambasova, Andrey B. Shishmakov, Igor A. Kirilyuk and Elena G. Kovaleva
Gels 2021, 7(3), 119; https://doi.org/10.3390/gels7030119 - 11 Aug 2021
Cited by 7 | Viewed by 2620
Abstract
The binary xerogels TiO2-SiO2 are widely used as catalysts and their carriers in organic synthesis. Characterization and adjustment of the electrostatic properties of the surface and the local acidity inside the pores, are necessary for the further development of TiO [...] Read more.
The binary xerogels TiO2-SiO2 are widely used as catalysts and their carriers in organic synthesis. Characterization and adjustment of the electrostatic properties of the surface and the local acidity inside the pores, are necessary for the further development of TiO2-SiO2 xerogels applications. This research investigates acid–base equilibria in the pores, and the surface electrostatic potential (SEP) of binary TiO2-SiO2 xerogels, by the EPR of stable pH-sensitive nitroxide radicals. These radicals are small enough to penetrate directly into the pores, and to be adsorbed onto the surface of the material under study. This makes it possible to obtain valuable information on the acidic and electrosurface properties of the studied system. The highest negative surface electrical charge associated with surface electrical potential (SEP) was equal to −196 ± 6 mV. It was induced by the surface of the sample with a 7% TiO2 content. The local acidity inside the pores of this sample was found to be higher, by approximately 1.49 pH units, as compared to that in the external bulk solution. Full article
(This article belongs to the Special Issue Advances in Xerogels)
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