Effective forces between macroions: The cases of asymmetric macroions and added salt

The distance-resolved effective forces between two spherical, highly charged colloidal macroions are calculated by computer simulation within the primitive model of strongly asymmetric electrolytes. In particular we consider the case of two asymmetric macroions, i.e., two particles with different charges and different radii, as well as the case of added salt ions. Different parameter sets corresponding to typical experimental samples an investigated. The results are compared with the predictions of traditional linear screening theory of Derjaguin and Landau [Acta Physicochim. URSS 14, 633 (1941)] and of Verwey and Overbeek [Theory of the Stability of Lyophobic Colloids (Elsevier, Amsterdam, 1948)]. For moderate charge asymmetries we find a semiquantitative agreement and verify different scaling laws obtained from Derjaguin-Landau-Verwey-Overbeek (DLVO) theory justifying the DLVO description of binary mixtures and of charge- and size-polydisperse macroion samples. However for very large asymmetry, particularly for the mixture of charged and uncharged colloid particles, we obtain a nonzero repulsive interaction contrarily to DLVO theory.