Molecular dynamics simulations were employed in models of peripherally charged dendrimers in solutions of explicit solvent and monovalent counterions in order to explore aspects of the dynamic behavior of counterions. The present study explores the effects of varying strength of electrostatic interactions for models of two dendrimer generations, in explicit solvent solutions below the dendrimer overlap concentration. Counterion diffusional motion as well as residence lifetimes of pairs formed by charged dendrimer beads and condensed counterions is monitored in the different electrostatic regimes. Spatiotemporal characteristics of self- and collective counterion motion are explored by means of space-time Van Hove correlation functions. A characteristic scaling law is found to describe the counterion diffusion coefficient as a function of Bjerrum length in the strong electrostatic regime, independent of the size of the dendrimer molecules at the examined volume fractions. The change noted in the diffusional motion of counterions in the range of strong Coulombic interactions is also reflected to their relevant residence times. Development of dynamic heterogeneities in counterion self-motion is observed during the gradual increase in the strength of electrostatic interactions, characterized by the emergence of distinct counterion populations in terms of their mobility. The time scale for the development of such a mobility contrast in the self-motion of the counterions can be correlated with that describing their collective motion as well. The latter increases with Bjerrum length but remains shorter compared to the time scale at which free diffusional motion sets in. Findings from the present study provide further insight on the mechanisms pertinent to ion migration in macroion dispersions and may serve as a basis for the interpretation of ionic motion in a broader range of polyelectrolyte systems.

In this study we extend our previous work in the self-organization of dendrimer polyelectrolytes (Macromolecules, 2008, 41, 225) by examining the effects of dendrimer concentration and/or total volume fraction in the ordering process and the resulting structure, in the arrangement of counterions and dendrimer beads and in the diffusive motion of dendrimers at different strengths of Coulombic interactions. It is found that as long as the total volume fraction remains low (i.e. no jamming phenomena intervene) the symmetry of the resulted cubic phases is unaltered. At a higher volume fraction and at the strong electrostatic regime a kinetic arrest of the dendrimer molecules much in analogy to a colloidal glass-like transition is observed, inhibiting thus the ordering process. Changes in the strength of electrostatic interactions and dendrimer concentration induces a systematic variation of the counterion -counterion and the counterion -charged-dendrimerbead spatial arrangement. These findings are in qualitative agreement with previous studies in systems with very different structural details of the considered solutes, indicating a more general behaviour in charged macroion/counterion solutions.