Effect of ionic and nonionic solutes on the transient and steady states inthe settling of charged colloidal particles

Centrifugation was applied to study the transport phenomena in various susp ensions of charged colloidal silica particles. Isopycnic focusing of colore d density marker beads was used to view the evolution of the density gradie nt and to determine its steady-state shape from the measured positions of t he focused zones. The effect of the ionic and nonionic solutes on the trans ient and steady states was studied. The experimental results are discussed in relation with the published theoretical dependence of the transport coef ficients on the volume fraction of the settling particles, The understandin g of the transport phenomena and of the interfering particle interactions g overning the formation of the density gradient and the subsequent isopycnic focusing under model static conditions is necessary for the optimization o f the isoperichoric focusing held-how fractionation dynamic experiments, bu t it should allow a more accurate description of the relaxation phenomena a nd retention in polarization field-flow fractionation as well. Some previou s experimental results concerning the study of the transport phenomena of t he charged silica particles in an electric field are exploited here to demo nstrate the general character of the observed behavior, independent of the field nature. (C) 2000 Academic Press.