Monte Carlo simulations of a charged dendrimer with explicit counterions and salt ions

Static properties of a dendrimer with generation g = 5 with positively charged terminal groups in an athermal solvent are studied by lattice Monte Carlo simulations using the cooperative motion algorithm as the tossing scheme. The calculations are performed both for a salt-free system with neutralizing counterions and for a small amount of added monovalent and divalent salt. The full Coulomb potential and the excluded volume interactions between ions and beads are taken explicitly into account with the reduced temperature τ, the number of salt cations (anions) ns, and salt valence zs as the simulation parameters. The bahaviour of the systems is analyzed by the mean effective charge per end-bead 〈Q〉, Coulomb mean energy 〈E〉, mean-square radius of gyration 〈R2g〉, pair correlation functions gαβ, and charge density ρch. The simulations show that for ns ≥ 0 and decreasing τ: (a) there is encapsulation in the dendrimer and condensation onto the terminal groups of anions accompanied by a monotonic decrease in 〈Q〉 and 〈E〉 and by subsequent swelling and shrinking of the molecule; (b) encapsulation, condensation and shrinking are the most significant and swelling weaker for |zs| = 2; (c) penetration of salt cations into the dendrimer is minor when compared to that of anions; (d) ρch is reduced and becomes negative close to the center of mass of the dendrimer and on its periphery; (e) for the considered ns > 0, unlike divalent salt ions the monovalent ones cause slight effects when compared to the salt-free case.