Attractive interaction between macroanions mediated by multivalent cations in biological fluids

Attractive interaction between macroanions mediated by divalent cations is studied using an Ornstein-Zernike equation with hypernetted-chain closure. The calculated results indicate that the divalent cations mediate the strong attraction and that this attractive force is much stronger than that mediated by monovalent cations. In this article, we shed light on effects of the co-existing of monovalent cations because the attractive interaction mediated by multivalent cations is observed in various biological fluids that also contain monovalent cations. When the system also contains monovalent cations, they suppress the attraction mediated by the divalent cations. However, the attraction remains and the reentrant behavior of attraction appears due to the exchange of attraction-mediating divalent cations for monovalent cations if the charge of the macroanions is sufficiently large.