Conformational Behavior of a Single Polyelectrolyte Chain with Bulky Counterions

We report a dissipative dynamic simulation study of a polyelectrolyte chain conformational behavior with small and bulky counterions. The size of the small counterions was equal to the size of a monomer unit, while the bulky counterions were modeled as big spherical particles with twice the radius of a monomer unit. Two cases of the charge location within the bulky counterions were studied: (i) in the center (symmetrical case) or (ii) on the counterion surface (asymmetrical case). The strength of electrostatic interactions, λ, as well as the fraction of charged beads on the polymer chain, <i>f</i>, was varied. At high charge fractions the chains with both small counterions and symmetrical bulky counterions collapsed at large λ. In contrast, the chain with asymmetrical bulky counterions did not collapse at all; instead, it adopted swollen conformations with counterions strongly attached to the chain backbone. At low charge fractions and high λ the multiplet structures observed in the systems with small and symmetrical bulky counterions were completely different: bulky counterions favored formation of significantly larger multiplets (mainly ionic rings). On the contrary, almost no multiplets were found for the case of asymmetrical bulky counterions. This distinction in behavior was explained by possible steric restrictions, entropic effects and differences in dipole–dipole interactions. Finally, it was shown that the mobility of small counterions in ionic aggregates formed at high λ depends significantly on the charge fraction and chain rigidity. It was found that at high <i>f</i> ion pairs are unstable within the polyelectrolyte globule and counterions can freely migrate within the globule volume. At low <i>f</i>, the counterion mobility is realized through “hopping” between ion pairs