COLLOIDAL STABILITY OF STOBER SILICA IN ACETONE-WATER MIXTURES

The colloidal stability and electrokinetic properties of Stober silica dispersed in acetone-water mixtures containing NaI and CaI2 were inve stigated. The relative permittivity, epsilon(r), of the dispersion med ium was varied between 21 and 79 by controlling the acetone:water rati o. In the presence of NaI, the coagulation concentrations (c(c)) incre ased from 19.6 to 125 mM as epsilon(r) of the mixtures was raised from 20.7 to 33. However, for epsilon(r) > 33, the particles could not be coagulated using NaI concentrations up to 0.5 M. The silica was sensit ive toward Cal(2) additions when epsilon(r) < 24.3 as indicated by the observation of two c(c) values at 0.045 and 2.5 mM when epsilon(r) wa s 20.7. As epsilon(r) was raised from 24.3 to 78, only relatively smal l changes in the c(c) were observed from 3 to 7.5 mM CaI2, respectivel y. The experimental c(c) results could not be predicted by calculation s using classic DLVO theory. Zeta potentials (zeta-potentials) for the silica were between -45 and -50 mV throughout the entire epsilon(r) r ange using 1.5 mM NaI solutions. In the absence of salt, the zeta-pote ntials remained constant at approximately -50 mV when 24.3 < epsilon(r ) < 78.5 but increased to -80 mV for epsilon(r) < 24.3. The zeta-poten tials using 1 mM CaI2 solutions remained at about -20 mV from epsilon( r) 78.5 to 33 but changed from -3 to +20 mV as epsilon(r) decreased fr om 24.3 to 20.7. Theoretical zeta-potentials calculated using the sing le-site dissociation model were compared with the experimental data. T he model predicted that the zeta-potentials should decrease with incre asing acetone content (decreasing epsilon(r)). The inability of the th eoretical models to predict the colloidal stability of silica in aceto ne-water mixtures, especially when epsilon(r) > 33, was believed to be due to the presence of a silica gel surface layer which acted as a st eric barrier at short-range interparticle distances. (C) 1996 Academic Press, Inc.