Overcharging and charge reversal in the electrical double layer around the point of zero charge

Abstract

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 (PM) 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 (CR) and overcharging (OC). Accordingly, colloidal CR, due to an excessive attachment of counterions, 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, the central macroion acquires additional like-charge (coions) and hence becomes "overcharged," a feature theoretically predicted in the past [F. Jiménez-Angeles and M. Lozada-Cassou, J. Phys. Chem. B 108, 7286 (2004)]. In other words, here we present the first simulation data on OC in the PM electrical double layer, showing that close to the point of zero charge, this novel effect surges as a consequence of the ionic size asymmetry. We also find that the HNC/MSA theory captures well the CR and OC phenomena exhibited by the computer experiments, especially as the macroion's charge increases. On the contrary, even if URMGC also displays CR and OC, its predictions do not compare favorably with the Monte Carlo data, evidencing that the inclusion of hard-core correlations in Monte Carlo and HNC/MSA enhances and extends those effects. We explain our findings in terms of the energy-entropy balance. In the field of electrophoresis, it has been generally agreed that the charge of a colloid in motion is partially decreased by counterion adsorption. Depending on the location of the macroion's slipping surface, the OC results of this paper could imply an increase in the expected electrophoretic mobility. These observations aware about the interpretation of electrokinetic measurements using the standard Poisson-Boltzmann approximation beyond its validity region.

Regimes of streaming potential in cylindrical nano-pores in presence of finite sized ions and charge induced thickening: An analytical approach

“…Numerous models have been recently reported in the literature in an effort to resolve these drawbacks. [18][19][20][21][22][23][24][25][26][27][28] Towards this, significant efforts have been directed to aptly account for finitesized effects of the ionic species. The Poisson-Boltzmann model, which considers the ions to be point sized, grossly over-predicts the surface charge density and overall concentrations for high surface charges and highly concentrated solutions.…”

Exploring charge inversion in electrolyte mixtures of trivalent and monovalent ions

“…As we know,the spatial correlation of ions is one reason for charge inversion, while on the other hand, ions adsorption selectivity and counterion volume effects are also make a contribution to this phenomenon occurs. Early works have shown that ion size and specie play an important role on the distribution of ions near charged surface [3,5,8,14,21], yet the complex interaction in mixtures has not been revealed completely. Normally, at room temperature Coulomb interactions are important in almost all electrolyte solution systems and in the past decade, concerted efforts have been invested in order to understand the statistical behavior of aqueous solutions in the presence of polyvalent ions.…”

“…However it is evident that the behavior of dielectric inhomogeneities can differ significantly from that of trivalent ones. The reason lies in the different strengths of electrostatic interactions between constituents of multicomponent ionic solutions [3,5,8,14].…”

“…This specific counterion adsorption is the aim of this paper to explore whether it is another fact to promote the charge inversion level [10,11]. While various of other experiments including electrophoresis mobile and molecular simulations(MD) were performed [11][12][13] there was no direct experimental evidence using surface force apparatus to confirm it [14][15][16].In this article, we investigate the interaction of mica surfaces in two different concentrations of LaCl 3 solution:5×10 -7 and 10 -4 mol/L. Then we add in 10 -3 NaCl to observe the change of the charge inversion level.…”

Mechanism and behavior of silver nanoparticles in aqueous medium as adsorbent

“…Thus, the pH of suspension may determine particle's mobility [98,99]. Nanoparticle stabilization and aggregation are influenced by the particle charge, and the surface charge in the vicinity of pH pzc is close to zero.…”