The interaction between like-charged nanoparticles mediated by rod-like ions

We present a brief review of recent studies of the interaction between charged nanoparticles immersed in a solution of oppositely charged rod-like counterions. To analyze these systems, we extend and employ an approximate field theory for point charges that has been shown to be accurate from the weak to the intermediate through to the strong coupling regimes. The resulting theory compares well with simulation data. We find that in the weak coupling limit, the interactions between the plates in the rod system are only repulsive. In the intermediate coupling regime, the multivalent rod-like counterions can mediate attractive interactions between the nanoparticles through two mechanisms. Charge correlations between different counterion rods lead to attractions between the plates, as in the case for the point charge counterions. For sufficiently long rods, however, bridging contributes to the attractive interaction, where the correlation of the charges within the rods is important. In the strong coupling limit, the inter-ionic charge correlations dominate the attractive interactions at short separations between the charged nanoparticles.