Independent ion migration in suspensions of strongly interacting charged colloidal spheres

We report on systematic measurements of the low-frequency conductivity sigma in aqueous suspensions of highly charged colloidal spheres. Sample preparation in a closed tubing system results in precisely controlled number densities of 10(16) m-3 < or = n < or = 10(19) m-3 (packing fractions of 10(-7) < or = phi < or = 10(-2)) and electrolyte concentrations of 10 < or = c < or = 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 sigma to depend linearly on the packing fraction with no detectable influence of the phase transitions. Further, at constant packing fraction sigma 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 sigma over a wide range of particle and experimental parameters.