A series of RF gels were synthesised with varying compositions, each gel formed through an established procedure requiring four reagents: resorcinol (
ReagentPlus, 99%, Sigma-Aldrich, Poole, UK), formalin solution (37 wt % formaldehyde in water and methanol, Sigma-Aldrich, Poole, UK), deionised water (produced in-house with Millipore Elix 5, Progard 2), and a catalyst. In this work, the catalysts used were sodium carbonate (Na
2CO
3, anhydrous, ≥99.5%, Sigma-Aldrich, Poole, UK), sodium hydrogen carbonate (NaHCO
3, anhydrous, ≥99.5%, Sigma-Aldrich, Poole, UK), ammonium hydrogen carbonate (NH
4HCO
3, 99%, Fisher Scientific, Loughborough, UK), sodium sulphate (Na
2SO
4, anhydrous, ≥99.0%, Sigma-Aldrich, Poole, UK), and sodium chloride (NaCl, Redi-Dri, anhydrous, ≥99%, Sigma-Aldrich, Poole, UK).
In the preparation of the gels investigated here, the volume of liquid added per gel in the initial solution was kept constant at 60 mL, which included the water and methanol content of the formalin solution. The total solids content of the initial mixture was kept constant at 12 g, which included the mass of formaldehyde contained within the formalin solution. The mass of resorcinol (R), formaldehyde (F), and catalyst (C) was varied according to the R/C ratio being prepared, all the while maintaining a constant R/F ratio of 0.5 in accordance with the stoichiometry of the resorcinol–formaldehyde reaction. Data for the mass of individual components of each reagent are included within the
Supplementary Materials, in addition to details of how the volume of formalin required for each gel was calculated.
The RF gel synthesis follows a standard procedure, with the reagents initially combined in individual circular glass jars according to the desired catalyst concentration and mixture, forming the initial RF solution. For gels synthesized using a combination of two catalysts, the two compounds were weighed into separate crucibles and added to the mixture simultaneously. The resulting solution pH was measured using a Hanna Instruments
benchtop pH meter (Leighton Buzzard, Bedfordshire, UK), after which point the jar was sealed and placed into a Memmert ULE-500 oven (Büchenbach, Germany) at 85 °C and left to gel for a 3-day period.
After the gelation period was complete, the gels underwent a 3-day solvent exchange process, where the water within the pores was exchanged for acetone. Acetone was selected as a solvent for exchange, due to its low surface tension value (23.46 mN/m for acetone in comparison to 71.99 mN/m for water; both values taken at 25 °C [38 (
link)]), therefore, minimizing the extent of pore shrinkage upon drying, in addition to its miscibility with water.
Finally, following the completion of the 3-day solvent exchange step, the jars containing RF gel samples were drained, covered with perforated aluminum foil, and placed into a Towson and Mercer 1425 Digital Vacuum Oven (Stretford, UK). After closing the vacuum oven door, the oven heating was turned on and the temperature was set to 110 °C, which corresponds to 85 ± 5 °C inside the oven (monitored using a thermometer placed inside the oven). The vacuum pump (Vacuubrand MZ 2C NT, Wertheim, Germany) attached to the oven was turned on, with two solvent traps with water/ice mixture placed between the oven and the pump, to condense acetone evaporating from the gel samples. The solvent traps were used to preserve the vacuum pump, limiting the amount of solvent vapor that came into contact with the membranes, as well as monitoring sample drying. For safety purposes, the oven and the pump were not left running overnight but were turned on at the beginning of each working day for 8 h. The gels were dried for 2 days, after which they were transferred into labelled sample containers.
Martin E., Prostredny M, & Fletcher A. (2021). Investigating the Role of the Catalyst within Resorcinol–Formaldehyde Gel Synthesis. Gels, 7(3), 142.
