a= nm. The silica particles are dispersed in aqueous NaN0solutions and the

With increasing pH, the mobility is no longer monotonic and a plateau appears. As the density of ionized sites increases, effects from the diffuse charge cloud begin to compensate. A maximum charge can be expected and all charges added to the particle above this maximum value will condense on the surface. The position of the plateau depends on the ionic strength of the dispersion and the size of silica particles and is also correlated with the parameter Ka. These results indicate that there are different surface states and different surface reactions in these two ranges of pH.