Structural study of the effect of mineral additives on the transparency, stability, and aging of silicate gels
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ORIGINAL PAPER: CHARACTERIZATION METHODS OF SOL-GEL AND HYBRID MATERIALS
Structural study of the effect of mineral additives on the transparency, stability, and aging of silicate gels N. El Felss1 A. Gharzouni1 M. Colas1 J. Cornette1 I. Sobrados2 S. Rossignol ●
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Abstract This work evaluates the structural evolution of silicate gel over time with mineral additives. A comparison between fresh and aged silicate gel synthesized from silica and potassium hydroxide was carried out using FTIR, Raman, and 29Si MAS-NMR spectroscopies. Then, different additives, including NaOH, AlOOH, Li2B4O7, and HCl, were incorporated into the silica solution. The effects of these additives on the aspect, transparency, Qn polymerization, and stability of the silicate gels were studied. Structural studies showed that the quantification of Qn species was difficult using FTIR and Raman spectroscopies because of the existence of several contributions in the same spectral region of silicate species. More precise information was provided by 29Si NMR spectroscopy, which demonstrated the increase in Q3 species to the detriment of Q4, revealing the depolymerization of the structure over time. Further study of the effect of additives revealed that HCl, AlOOH, and Li2B4O7 did not improve the stability of the gel due to the formation of several networks. However, promising stability and transparency results were obtained using NaOH with a silicate/modifier molar ratio of 2.52. Graphical Abstract
Gel with NaOH Silica/modifier = 2.52
Creep resistance
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Received: 14 May 2020 / Accepted: 30 July 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Q4/Q3 ratio
Gel without NaOH Silica/modifier = 2.48 Polymerization
Keywords
Silicate
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Si MAS-NMR Qn Mineral additive Aging pH value
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* S. Rossignol [email protected] 1
Institut de Recherche sur les Céramiques (IRCER), 12 rue Atlantis, 87068 Limoges, France
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Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas (CSIC), C/Sor Juana Inés de la Cruz, 3, 28049 Madrid, Spain
Journal of Sol-Gel Science and Technology
Highlights ● The decrease of Q4/Q3 ratio after ageing reveals the depolymerization of the structure of silicate gel. ● Adding NaOH to the silicate gel preserves the transparency and improves its stability over time. ● Polymerization of siliceous species with nSi/(nNa + nK) ratio equal to 2.52.
1 Introduction Silicate gels have desirable properties such as transparency, high adsorption capacity [1], acid/base properties, insolubility in different solvents, and thermal stability [2]. Because of these properties, they can be used in applications such as optical sensors and biosensors [3], heavy metal removal from low-content effluents [4], the removal of high concentrations of toluene vapor from gas [5], and other needs in the pharmaceutical industry [6]. The processes used to synthesize silica gels include the sol–gel process [7], the acidification or precipitation
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