Condensation of Rodlike Counterions on a Charged Cylinder

We study the condensations of dumbbell-like counterions onto an oppositely charged cylinder at weak and strong electrostatic coupling strengths. Performing extensive Monte Carlo simulations, we show that the condensation characteristics of the dumbbell ions can be understood by viewing dumbbell ions of length d as point ions with an effective valency depending on the radial distance r from the cylinder’s axis. In terms of the condensation behavior, dumbbell ions behave as two independent point ions if r ≪ d , but act as a single point ion with twofold valency when r ≫ d . Such a distance-dependent effective valency as a characteristic of ions of extended structures explains a unique feature in the condensed counterion fraction and heat capacity at both weak and strong coupling strengths.