Colloidal Stability of Stöber Silica in Acetone–Water Mixtures

The colloidal stability and electrokinetic properties of Stöber silica dispersed in acetone–water mixtures containing NaI and CaI2were investigated. The relative permittivity, εr, of the dispersion medium was varied between 21 and 79 by controlling the acetone:water ratio. In the presence of NaI, the coagulation concentrations (cc) increased from 19.6 to 125 mMas εrof the mixtures was raised from 20.7 to 33. However, for εr> 33, the particles could not be coagulated using NaI concentrations up to 0.5M.The silica was sensitive toward CaI2additions when εr< 24.3 as indicated by the observation of twoccvalues at 0.045 and 2.5 mMwhen εrwas 20.7. As εrwas raised from 24.3 to 78, only relatively small changes in theccwere observed from 3 to 7.5 mMCaI2, respectively. The experimentalccresults could not be predicted by calculations using classic DLVO theory. Zeta potentials (ζ-potentials) for the silica were between -45 and -50 mV throughout the entire εrrange using 1.5 mMNaI solutions. In the absence of salt, the ζ-potentials remained constant at approximately −50 mV when 24.3 < εr< 78.5 but increased to −80 mV for εr< 24.3. The ζ-potentials using 1 mMCaI2solutions remained at about −20 mV from εr78.5 to 33 but changed from −3 to +20 mV as εrdecreased from 24.3 to 20.7. Theoretical ζ-potentials calculated using the single-site dissociation model were compared with the experimental data. The model predicted that the ζ-potentials should decrease with increasing acetone content (decreasing εr). The inability of the theoretical models to predict the colloidal stability of silica in acetone–water mixtures, especially when εr> 33, was believed to be due to the presence of a silica gel surface layer which acted as a steric barrier at short-range interparticle distances.