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 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.
Highlights
This, in addition to the results presented previously, suggests that both the presence of sodium ions and appropriate solution pH value are necessary in order to obtain a viable porous gel structure
The individual catalyst investigated, the results of which point towards a pivotal role beyond just pHinvestigated, the results of which point towards a pivotal role beyond just pHadjusting—the theory that is generally accepted within the gel research field
Gels gels trations prepared with a constant sodium ion concentration with varying carbonate concentrations all possessed similar textural properties, despite their differences in initial solution pH
Summary
Owing to their highly attractive and tuneable properties, resorcinol–formaldehyde (RF) gels have been the focus of numerous studies over the years [1,2,3,4] Their synthesis procedure, as established by Pekala [5], involves a sol–gel reaction, a subsequent solvent exchange step, and, lastly, a drying process to produce the lightweight, porous structure of the final gel. The addition reaction proceeds, typically with molecules of formaldehyde positioning themselves at two available carbon atoms on the benzene ring, with the resulting molecules being known as hydroxymethyl derivatives This addition reaction, under basic catalysis, is fast, producing numerous hydroxymethyl derivatives, which react to form many small clusters. Research conducted over the years has indicated that its role may be far more complex
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