Effects of the ionic size-asymmetry around a charged nanoparticle: unequal charge neutralization and electrostatic screening

Abstract

We study the consistent inclusion of ionic size-asymmetry for a wide range of macroparticle charges in the primitive model of an electrical double layer around a spherical colloid using (1) Monte-Carlo simulations, (2) the hybrid integral-equation formalism of hypernetted-chain (HNC) and mean-spherical approximation (MSA), and (3) the Gouy–Chapman theory modified for unequal ionic radii. In our simulations, for a weakly charged macroion, we observe surface charge amplification from adsorption of like-charged ions, as well as charge reversal due to overcompensation of the bare nanoparticle charge by counterions. When the nanoparticle charge increases, we detect both asymmetric neutralization and asymmetric electrostatic screening that depend on the sign of the macroion's valence. Specifically, there exists a higher reduction of the original bare charge and a smaller electrostatic potential for the case of negative nanoparticles with positive small counterions, versus the case of positive nanoparticles with negative large counterions. These coarse-grained results are in agreement with the predictions of asymmetric charge renormalization (P. González-Mozuelos and M. Olvera de la Cruz, Phys. Rev. E, 2009, 79, 031901), in which the aqueous solvent is explicitly taken into account. Results from the Gouy–Chapman theory modified for unequal ionic radii differ notably from our obtained Monte-Carlo data, while good agreement exists between simulation results and HNC/MSA-treatment findings.