Simple Model for Attraction between Like-Charged Polyions
  We present a simple model for the possible mechanism of appearance of attraction between like charged polyions inside a polyelectrolyte solution. The attraction is found to be short ranged, and exists only in presence of multivalent counterions. The attraction is produced by the correlations in the condensed layers of counterions surrounding each polyion, and appears only if the number of condensed counterions exceeds the threshold, $ n > Z/2 \alpha $, where $\alpha$ is the valence of counterions and $Z$ is the polyion charge. 
Jeferson J. Arenzon, Juergen F. Stilck, and Yan Levin