Competitive Adsorption and Ordered Packing of Counterions near Highly Charged Surfaces:
From MeanField Theory to Monte Carlo Simulations
Mr. Jiayi Wen
Department of Mathematics and Center for Theoretical Biological Physics
UC San Diego
ABSTRACT
Competitive adsorption of counterions of multiple species to charged surfaces is studied by a sizeeffect included meanfield theory and Monte Carlo (MC) simulations. The meanfield electrostatic freeenergy functional of ionic concentrations, constrained by Poisson's equation, is numerically minimized by an augmented Lagrangian multiplier method. Unrestricted primitive models and canonical ensemble MC simulations with the Metropolis criterion are used to predict the ionic distributions around a charged surface. It is found that, for a low surface charge density, the adsorption of ions with a higher valence is preferable, agreeing with existing studies. For a highly charged surface, both of the meanfield theory and MC simulations demonstrate that the counterions bind tightly around the charged surface, resulting in a stratification of counterions of different species. The competition between mixed entropy and electrostatic energetics leads to a compromise that the ionic species with a higher valencetovolume ratio has a larger probability to form the first layer of stratification. In particular, the MC simulations confirm the crucial role of ionic valencetovolume ratios in the competitive adsorption to charged surfaces that had been previously predicted by the meanfield theory. The charge inversion for ionic systems with salt is predicted by the MC simulations but not by the meanfield theory. This work provides a better understanding of competitive adsorption of counterions to charged surfaces and calls for further studies on the ionic size effect with application to largescale biomolecular modeling.
This is joint work with Shenggao Zhou, Zhenli Xu, and Bo Li.
