Independent ion migration in suspensions of strongly interacting charged colloidal spheres

We report on systematic measurements of the low-frequency conductivity σ in aqueous suspensions of highly charged colloidal spheres. Sample preparation in a closed tubing system results in precisely controlled number densities of 1016 m-3≤n≤1019 m-3 (packing fractions of 10-7≤Φ≤102 2 ) and electrolyte concentrations of 10≤c≤10-3 mol l-1. Due to long-range Coulomb repulsion, some of the systems show a pronounced fluid or crystalline order. Under deionized conditions we find σ to depend linearly on the packing fraction with no detectable influence of the phase transitions. Further, at constant packing fraction σ increases sublinearly with the increasing number of dissociable surface groups N. As a function of c the conductivity shows pronounced differences depending on the kind of electrolyte used. We propose a simple yet powerful model based on the independent migration of all species present and the additivity of the respective conductivity contributions. It takes account of small ion macro-ion interactions in terms of an effectively transported charge. The model successfully describes our qualitatively complex experimental observations. It further facilitates quantitative estimates of σ over a wide range of particle and experimental parameters. ©2000 The American Physical Society.