STABILITY OF DISPERSIONS IN POLAR MEDIA

The stability of dispersions in polar organic media was reviewed comparing with that of in aqueous and non-polar media. Structure and properties of the double layer of AgI in water ethylene glycol were studied and the stability of the dispersions was related to the study of the double layer. Coagulation concentrations of Ag, AgI and alpha-FeOOH dispersions in methanol, ethanol, 2-propanol and acetone by the addition of electrolytes were determined. Coagulation by monovalent counterions is due to double layer compression. With bi- and multivalent counterions or in low dielectric constant solvents coagulation is primarily caused by charge neutralization. The effect of an ionic surfactant, AOT on the stability of TiO2 or alpha-Fe2O3 in water-p-dioxane mixtures was studied. AOT behaved as a coagulating agent in polar media (epsilon = 10-50), though it worked as a dispersing agent in nonpolar (dioxane) and water-rich media. AOT accordingly coagulated the dispersion a carbon black in 2-butanone, though a nonionic surfactant did not so. Desolvation from the particle surface is considered to be an important origin of coagulation by AOT in polar media, because AOT is not adsorbed on particles and considerably dissociates into ions in 2-butanone.