Colloidal Stability of Stöber Silica in Acetone

The colloidal stability of Stöber silica dispersed in acetone (99.7 wt %) was investigated and compared to the results obtained in water using electrophoresis and coagulation experiments with NaI, CaI 2, and Bu 4 NI. A critical coagulation concentration ( c c ) of 19.6 mM NaI in acetone was observed whereas Bu 4 NI acetone solutions up to 145 mM had no effect on the stability of the silica organosols. In NaI and Bu 4 NI acetone solutions, the magnitude of the electrophoretic mobilities of the silica organosols decreased with increasing electrolyte concentration. From 0.11 to 13.5 mM NaI the electrophoretic mobilities decreased from −3.3 × 10 - 8 to −0.39 × 10 - 8 m 2 V - 1 s - 1; from 3.9 to 39 mM Bu 4 NI the mobilities decreased from −2.8 × 10 - 8 to −0.91 × 10 - 8 m 2 V - 1 s - 1 . Results using a single-site dissociation model were compared with experimental ζ-potentials for silica in NaI acetone solutions and showed adequate agreement when ΔpH was assumed to be 2. Experimental coagulation concentrations for NaI and Bu 4 NI did not agree with simple DLVO theory. The discrepancy was believed to be due to the presence of a steric barrier consisting of a silicic acid gel network. CaI 2 additions to the silica acetone solutions influenced the electrostatic potential of the particles and caused coagulation. At 0.06 and 1 mM CaI 2, the electrophoretic mobilities of the particles were −2.9 × 10 - 8 and +2.3 × 10 - 8 m 2 V - 1 s - 1, respectively. The charge reversal led to the restabilization of the particles as evidenced by two c c values 0.045 mM ( c c 1 ) and 2.75 mM ( c c2 ).