The Viscosity of Certain ``Ferric Oxide'' Hydrosols

Abstract

(1) The changes produced in the viscosities of ``ferric oxide'' hydrosols by aging, heating, and the addition of a number of electrolytes and a non-electrolyte have been studied, in some instances together with the attendant changes in the pH values and specific conductances of the systems. (2) Agreement with Poiseuille's law has been demonstrated for typical cationic and anionic hydrosols. (3) The decrease in viscosity of hydrosols on aging and heating is accompanied by an increase in specific conductance. This conductance increase in the case of cationic sols is due almost entirely to the increase in hydrogen ion activity which occurs during the reaction. This is not so for anionic hydrosols, where the conductance increase is explained by the conversion of coordinated citrato groups to citrate ions. (4) The addition of salts and acids to these hydrosols first results in a viscosity decrease, followed by an increase in viscosity at higher electrolyte concentrations. (5) The viscosity increase occurs at lower salt concentrations for those salts which are potent precipitants than for those which are weak. For cationic hydrosols, salts of both weakly and strongly coordinating anions have the same effect in depressing the viscosity; in the case of anionic hydrosols, potassium citrate is less effective in this respect than potassium sulfate and nitrate. (6) The effects of acids in decreasing the viscosity of a sol, either cationic or anionic, was found to be the same for all acids used, when compared at the same pH values. The more powerful precipitants caused the subsequent viscosity increase to occur at lower electrolyte concentrations. (7) Acids are less effective than salts in decreasing the viscosities of cationic hydrosols. (8) Large concentrations of acetone produced a decrease in the relative viscosity of cationic hydrosols; this effect is, however, negligible in comparison with the magnitude of the changes caused by electrolytes. (9) The viscosity decrease produced by aging, heating, and the addition of electrolytes may be attributed in each case partly to the increase in electrolyte concentration of the system. (10) The viscosity resulting from these reactions is considered to be dependent also upon the micellar charge: the lower the charge, the lower the viscosity. (11) The viscosity increase at higher electrolyte concentrations may be ascribed to agglomeration of dispersed particles. (12) A mechanism based upon a Donnan equilibrium between the dispersed particle and the dispersion medium has been proposed to account for the effects observed.