Monitoring the gelation mechanism of resorcinol- formaldehyde gels by dynamic light scattering

Xerogels and porous materials for specific applications such as catalyst supports, CO2 capture, pollutant adsorption and selective membrane design require fine control of pore structure, which in turn requires improved understanding of the chemistry and physics of growth, aggregation and gelation processes governing nanostructure formation in these materials. We used time-resolved dynamic light scattering to study the formation of resorcinol-formaldehyde gels through a sol-gel process in the presence of Group I metal carbonates. We showed that an underlying nanoscale phase transition (independent of carbonate concentration or metal type) controls the size of primary clusters during the pre-aggregation phase; while the amount of carbonate determines the number concentration of clusters and, hence, the size to which clusters grow before filling space to form the gel. This novel physical insight, based on a close relationship between cluster size at the onset of gelation and average pore size in the final xerogel results in a well-defined master curve, directly linking final gel properties to process conditions, facilitating the rational design of porous gels with properties specifically tuned for particular applications. Interestingly, while results for lithium, sodium and potassium carbonate fall on the same master curve, cesium carbonate gels have significantly larger average pore size