Competition between condensation of monovalent and multivalent ions in   DNA aggregation
  We discuss the distribution of ions around highly charged PEs when there is competition between monovalent and multivalent ions, pointing out that in this case the number of condensed ions is sensitive to short-range interactions, salt, and model-dependent approximations. This sensitivity is discussed in the context of recent experiments on DNA aggregation, induced by multivalent counterions such as spermine and spermidine. 
Yoram Burak, Gil Ariel, David Andelman