Coloides

PHYSICAL REVIEW E 80, 021501 2009

Simulational and theoretical study of the spherical electrical double layer for a size-asymmetric electrolyte: The case of big coions
G. Iván Guerrero-García, Enrique González-Tovar, and Martín Chávez-Páez
Instituto de Física, Universidad Autónoma de San Luis Potosí, Álvaro Obregón 64, 78000 San Luis Potosí, S.L.P., México Received 26 April 2009; published12 August 2009 Monte Carlo simulations of a spherical macroion, surrounded by a size-asymmetric electrolyte in the primitive model, were performed. We considered 1:1 and 2:2 salts with a size ratio of 2 i.e., with coions twice the size of counterions , for several surface charge densities of the macrosphere. The radial distribution functions, electrostatic potential in all the space and at theHelmholtz surfaces, and integrated charge are reported. We compare these simulational data with original results obtained from the Ornstein-Zernike integral equation, supplemented by the hypernetted chain–hypernetted chain HNC-HNC and hypernetted chain–mean spherical approximation HNC-MSA closures, and with the corresponding calculations using the modified GouyChapman and unequal-radius modifiedGouy-Chapman theories. The HNC-HNC and HNC-MSA integral equations formalisms show good concordance with Monte Carlo “experiments,” whereas the notable limitations of point-ion approaches are evidenced. Most importantly, the simulations confirm our previous theoretical predictions of the nondominance of the counterions in the size-asymmetric spherical electrical double layer J. Chem. Phys. 123, 034703 2005, the appearance of anomalous curvatures at the outer Helmholtz plane, and the enhancement of the charge reversal and screening at high colloidal surface charge densities due to the ionic size asymmetry. DOI: 10.1103/PhysRevE.80.021501 PACS number s : 61.20.Ja, 61.20.Gy, 61.20.Ne, 61.20.Qg

I. INTRODUCTION

The study of charged colloidal solutions is very relevant for both basic research andtechnology due to the ubiquitous nature of these systems 1–9 . Accordingly, the attainment of a successful theoretical description of such state of matter should represent a keystone for later developments in colloid science. For many years, the scientific community has investigated the structural characteristics of these materials, trying to understand the role of the electrostatic and entropiccorrelations in their observable properties. In particular, the interest in charged suspensions has prompted the burgeoning of unprecedented experimental techniques and of numeric and statistical mechanics approaches of increasing complexity. On the theoretical side, and in spite of the notorious progress in the speed of machine calculations, at present, it is not yet possible to mimic a realdispersion without making several and important simplifications in order to establish a tractable problem. Thus, for example, one of the most elemental idealizations of a diluted charged colloidal suspension is the combination of the cell and primitive models. Within this scheme, the average distance between nonconcentrated macroions bathed by an electrolyte is very large, and therefore it is expectedthat the thermodynamics of the system will depend mainly on the ionic structure, or electrical double layer EDL , around a single macroparticle enclosed in an electroneutral cell. Complementarily, the so-called primitive model PM , in which the ions are treated as hard spheres with punctual charges embedded in their centers and the solvent is considered a continuous medium, stands as the mostthriving representation of a multicomponent electrolyte. A particular case of the PM is the restricted primitive model RPM , where all the ionic species are of equal size. This condition drastically facilitates the theoretical analysis and, as a consequence, a great amount of work has been performed in the
1539-3755/2009/80 2 /021501 10

RPM for the planar 10–15 , cylindrical 16–19 , and spherical...