Monte Carlo simulations of ion activities in rodlike polyelectrolyte solutions

Canonical Monte Carlo simulations are applied to the investigation of the small ion distributions around a rodlike polyelectrolyte in aqueous solutions with or without added 1:1 salt. The mean and single ion activity coefficients are estimated and are compared with the experiments and the theories. The Monte Carlo cell systems are examined to measure the ion distribution adopting simple molecular models of the polyions, and the activity coefficients are obtained from the local ion concentrations at the cell boundary. The results are compared with the solutions of the Poisson-Boltzmann equation. On comparison with the experimental data in the literature, considerable agreement is observed in several cases. These comparisons provide the fundamental knowledge of our simple systems as well as of the essential properties of the polyion-small ion interactions. The applicability and the limitation of the method are discussed.