Determination of the Ion Distribution in Aqueous Dispersions of Untreated and Surface-Modified SiO sub 2 Particles by zeta-Potential Measurements

摘要

The electrophoretic migration of turbidity boundaries between dispersions of untreated and surface-phenylated pyrogenic SiO sub 2 (Aerosil) and overlying layers of clear solutions was measured at various pH values. The zeta potentials were calculated from these results. Similar studies were performed on dispersions of pure and surface-methylated quartz (Min-O-Sil). Electrophoretic mobilities of individual particles of quartz were also determined microscopically. For both pyrogenic SiO sub 2 and quartz, substitution of the silanol groups or of the hydrogen atoms of the silanol groups resulted in no marked shift of the zeta potentials. The ion distributions in dispersions of pyrogenic SiO sub 2 were calculated from the concentrations of the initial solutions, the change in pH on adding the disperse phase and the zeta potentials. This calculation yields information on the number of ionized silanol groups existing on the particle surface, a number which determines the electrokinetic behavior of the particle. Similarly, information is also obtained on the distribution of all other ions in solution and in the double layer. The similar electrokinetic behavior of pure and surface-modified particles is obviously due to the fact that sufficient silanol groups remain unprotected, i.e. dissociable, even in the treated product. Thus, only 1.5 percent at most of the silanol groups are ionized in the pH range 2-9. In the pH range 9-11, the number of ionized silanol groups increases to approximately 100 percent. Thus, in this pH range, additional silanol groups must have been formed in the case of phenylated SiO sub 2 . (ERA citation 03:019318)