Critical opalescence points to thermodynamic instability: relevance to small-angle X-ray scattering of resorcinol–formaldehyde gel formation at low pH

Abstract

Author(s) of this paper may load this reprint on their own web site or institutional repository provided that this cover page is retained. Republication of this article or its storage in electronic databases other than as specified above is not permitted without prior permission in writing from the IUCr.For further information see http://journals.iucr.org/services/authorrights.html Many research topics in condensed matter research, materials science and the life sciences make use of crystallographic methods to study crystalline and non-crystalline matter with neutrons, X-rays and electrons. Articles published in the Journal of Applied Crystallography focus on these methods and their use in identifying structural and diffusioncontrolled phase transformations, structure-property relationships, structural changes of defects, interfaces and surfaces, etc. Developments of instrumentation and crystallographic apparatus, theory and interpretation, numerical analysis and other related subjects are also covered. The journal is the primary place where crystallographic computer program information is published.Crystallography Journals Online is available from journals.iucr.org J. Appl. Cryst. (2008 During the formation at low pH of resorcinol-formaldehyde gels with a structure in the micrometre range, small-angle X-ray scattering exhibits a nonmonotonic intensity variation as a function of reaction time. The data are analyzed in terms of scattering by statistical fluctuations of polymer concentration, the amplitude of which is maximal close to the critical point for phase separation between polymer and solvent. The data do not carry any morphological information, but they unambiguously show that the driving force of the gel formation is a thermodynamic instability of the polymerizing solution.

Activated carbon xerogels for the removal of the anionic azo dyes Orange II and Chromotrope 2R by adsorption and catalytic wet peroxide oxidation

“…Recently, carbon xerogels have also proved to be excellent alternatives to activated carbons in various processes [14][15][16]. These materials are composed of microporous interconnected sphere-like nodules, formed during the gel synthesis via a microphase separation mechanism induced by polymer growth [17]. The size of these nodules is mainly regulated by the synthesis pH [18].…”

Toward a continuous synthesis of porous carbon xerogel beads

“…Finally, two other hypotheses could explain this skin phenomenon. First, SAXS measurements performed during the synthesis of low‐pH resorcinol and formaldehyde gels showed that the gel formation is driven by reaction‐induced phase separation processes . The relative velocity of the beads in oleic acid can generate friction and shear stresses at the beads surface that could locally disturb the phase separation of the gel formation and lead to such skin phenomenon.…”

“…The flask was left unstirred at 70 ∘ C for precuring during 45 min. In these conditions, the gelation time is approximately 50 min, thus, after 45 min, the precured solution was not gelled and could be handled as a liquid. The flask was quenched using tap water to slow down the gelation reaction and avoid any gelation in the nozzle that would instantly plug it.…”

Condensation-Induced Decrease of Small-Angle X-ray Scattering Intensity in Gelling Silica Solutions

“…This type of reaction-induced phase separation has been directly evidenced by scattering methods in quite a few instances: in hybrid silica gels, ,,− in curing polymer blends, − in hybrid organic−inorganic materials, and in resorcinol−formaldehyde gels. − Moreover, the fact that phase separation leads in a very natural way to the fiberlike morphologies ubiquitous to gels and many biological structures suggests that this mechanism is far more common than what is generally acknowledged. , …”