Equilibrium properties of a grafted polyelectrolyte with explicit counterions.

We study the equilibrium conformations of a grafted polyelectrolyte (PE) in the presence of explicit counterions (CIs) using Monte Carlo simulations. The interplay between attractive Lennard-Jones interactions (parametrized by epsilon) and electrostatics (parametrized by A=q(2)l(B)/a, where q is the CI valency, l(B) is the Bjerrum length, and a is the monomer diameter) results in a variety of conformations, characterized as extended (E), pearls with m beads (P(m)), sausage (S), and globular (G). For large epsilon, we observe a transition from G-->P(2)-->P(3)--> ... -->S-->G with increasing A, i.e., a change from poor to good, to re-entrant poor solvent, whereas, at lower epsilon, the sequence of transitions is E-->S-->G. The conformation changes are directly related to the nature of binding of CI onto the PE. The transition between S-->G is continuous and associated with critical fluctuations in the shape driven by fluctuations in the fraction of condensed CI.