  The ionic adsorption around a weakly charged spherical colloid, immersed in
size-asymmetric 1:1 and 2:2 salts, is studied. We use the primitive model of an
electrolyte to perform Monte Carlo simulations as well as theoretical
calculations by means of the hypernetted chain/mean spherical approximation
(HNC/MSA) and the unequal-radius modified Gouy-Chapman (URMGC) integral
equations. Structural quantities such as the radial distribution functions, the
integrated charge, and the mean electrostatic potential are reported. Our Monte
Carlo "experiments" evidence that near the point of zero charge the smallest
ionic species is preferentially adsorbed onto the macroparticle, independently
of the sign of the charge carried by this tiniest electrolytic component,
giving rise to the appearance of the phenomena of charge reversal and
overcharging. Accordingly, charge reversal is observed when the macroion is
slightly charged and the coions are larger than the counterions. In the
opposite situation, i.e. if the counterions are larger than the coions,
overcharging occurs. In other words, in this paper we present the first
simulational data on overcharging, showing that this novel effect surges close
to the point of zero charge as a consequence of the ionic size asymmetry.
Further, it is seen that the inclusion of hard-core correlations in HNC/MSA
leads to spatial regions near the macroion's surface in which the integrated
charge and/or the mean electrostatic potential can decrease when the colloidal
charge is augmented and vice versa. These observations aware about the
interpretation of electrophoretic mobility measurements using the standard
Poisson-Boltzmann approximation beyond its validity region.
