Particle Size Dependence for Effective Charge Density of Ionic Colloids

Particle size dependence of the effective surface charge density, sgr e, of colloidal silica particles, having radius a ranging from 16 to 60 nm, was examined by applying electrical conductivity measurements. sgr e markedly increased with decreasing a, and approached to the case for small ions, namely, sgr e=analytical (net) charge density, sgr a. sgr e was roughly proportional to 1/a, as expected from the mean field theories. However, leveling off of sgr e at high sgr a, which was expected from charge renormalization approach, was not observed.