Effective charge saturation in colloidal suspensions

Because micro-ions accumulate around highly charged colloidal particles in electrolyte solutions, the relevant parameter to compute their interactions is not the bare charge, but an effective ~or renormalized! quantity, whose value is sensitive to the geometry of the colloid, the temperature or the presence of added-salt. This nonlinear screening effect is a central feature in the field of colloidal suspensions or polyelectrolyte solutions. We propose a simple method to predict effective charges of highly charged macro-ions, that is reliable for monovalent electrolytes ~and counterions! in the colloidal limit ~large size compared to both screening length and Bjerrum length!. Taking reference to the non linear Poisson–Boltzmann theory, the method is successfully tested against the geometry of the macro-ions, the possible confinement in a Wigner–Seitz cell, and the presence of added salt. Moreover, our results are corroborated by various experimental measures reported in the literature. This approach provides a useful route to incorporate the nonlinear effects of charge renormalization within a linear theory for systems where electrostatic interactions play an important role. © 2002 American Institute of Physics. @DOI: 10.1063/1.1511507#