SEDIMENTATION-VELOCITY OF COLLOIDAL SPHERES IN DEIONIZED SUSPENSION

Sedimentation velocities (S) of silica spheres (diameter 311 and 507 nm) and heavy polystyrene spheres (diameter 937 nm) are measured in the completely deionized suspensions and in the presence of sodium chloride. S decreases slightly and then starts to increase as the ionic concentration of the suspension decreases. The first decrease is explained with the retarded Brownian movement of colloidal spheres coated with the electrical double layers. Increase of S in the completely deionized suspension is much more significant for the small spheres. One of the most plausible reasons for the increase is the strengthened gravitational forces on the colloidal particles coated with the expanded double layers. The other factor will be the reduced surface charges on the colloidal spheres in the deionized suspension. These experimental results show clearly that the expanded electrical double layers play an important role on the translational self-diffusion of colloidal spheres especially in the exhaustively deionized suspension.