The conditions of reversible sol-gel transitions were studied. For three oxide sols (SiO2, Al2O3, and ferric hydroxide), it was shown that the higher the specific surface area of the particles and the lower their surface charge density, the lower is the (critical) gel formation concentration, i.e the lowest is the concentration of solid at which a cohesive structure forms from the sol. Due to the low specific surface area and the anisometric shape of the particles, from the Al2O3 sol a flocculum-like (loose) sediment (liogel) may only form. Liogels were prepared by concentrating the ferric hydroxide sols and by adding an electrolyte (KCl solution) to the SiO2 sols. Surface charge density plays a role in the formation of liogels; the gel formation concentration is directly proportional to the charge density. By fractionally drying the liogels over H2SO4 - water mixtures of various vapour pressures, samples were prepared with different amounts of adsorbed water. The "presorbed" water was removed by further drying and from the loss of the weights and the corresponding vapour pressures, (vapour) adsorption isotherms for the solids have been determined. The specific surface area and the adsorption heats for the xerogels have been evalulated but the immersion heats measured, and the interaction energies between the solid and water, as a function of the thickness of adsorbed water layer, have been calculated. No direct correlation could be detected between the (evalulated) adsorption heats or (measured) immersion heats and the hydrophilic character of the sol particles.
|Translated title of the contribution||Reversibility of liosol ↔ liogel ↔ xerogel transformations, II|
|Number of pages||6|
|Journal||Magyar Kemiai Folyoirat, Kemiai Kozlemenyek|
|Publication status||Published - Dec 1 2000|
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