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National Science Foundation, NSF: DMR-0520513
Entropic effects in the electric double layer of model colloids with size-asymmetric monovalent ions
The structure of the electric double layer of charged nanoparticles and colloids in monovalent salts is crucial to determine their thermodynamics, solubility, and polyion adsorption. In this work, we explore the double layer structure and the possibility of charge reversal in relation to the size of both counterions and coions. We examine systems with various size-ratios between counterions and coions (ion size asymmetries) as well as different total ion volume fractions. Using Monte Carlo simulations and integral equations of a primitive-model electric double layer, we determine the highest charge neutralization and electrostatic screening near the electrified surface. Specifically, for two binary monovalent electrolytes with the same counterion properties but differing only in the coions size surrounding a charged nanoparticle, the one with largest coion size is found to have the largest charge neutralization and screening. That is, in size-asymmetric double layers with a given counterions size the excluded volume of the coions dictates the adsorption of the ionic charge close to the colloidal surface for monovalent salts. Furthermore, we demonstrate that charge reversal can occur at low surface charge densities, given a large enough total ion concentration, for systems of monovalent salts in a wide range of ion size asymmetries. In addition, we find a non-monotonic behavior for the corresponding maximum charge reversal, as a function of the colloidal bare charge. We also find that the reversal effect disappears for binary salts with large-size counterions and small-size coions at high surface charge densities. Lastly, we observe a good agreement between results from both Monte Carlo simulations and the integral equation theory across different colloidal charge densities and 1:1-elec-trolytes with different ion sizes. © 2011 American Institute of Physics.
Binary salts; Charge neutralization; Charge reversal; Charged nanoparticles; Colloidal charge; Colloidal surface; Counterions; Double layer structure; Double layers; Electric double layer; Electrostatic screening; Entropic effects; Excluded volume; Integral-equation theory; Ion concentrations; Ion size; Ion size asymmetry; Ionic charge; Large sizes; Model colloids; Monovalent ions; Monovalent salt; Monte Carlo Simulation; Polyion adsorption; Adsorption; Charge density; Colloid chemistry; Colloids; Electrochemistry; Entropy; Integral equations; Ions; Monte Carlo methods; Nanoparticles; Salts; Thermodynamics; Computer simulation
Consejo Nacional de Ciencia y Tecnología, CONACYT: CB-2006-01/58470
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