Understanding polyelectrolyte solutions: macroion condensation with emphasis on the presence of neutral co-solutes

The multi-faceted applications of polyelectrolyte solution systems to a kaleidoscope of technological and biological processes make the understanding of these systems important and of interest. The highly relevant issue of instabilities that may occur in a polyelectrolyte solution and the ensuing macroion condensation constitute the premise of this review. An abundance of experimental and numerical simulation results in recent years provide evidence that a net electrostatic attractive force may exist between macroions and may lead to a phase separation. Specifically, in this review, three different types of instability involving macroions of spherical geometry are discussed. (i) The instability arising out of strong Coulomb correlations between counterions in the solution; this is most likely to occur in solutions containing multivalent counterions and/or in the presence of solvents of low relative permittivity. (ii) The instability caused by the macroion surface-charge fluctuations; the resultant charge correlations may induce an effective attraction between the weakly charged macroions. (iii) The instability due to the combined effect of electrostatic and crowding interactions when an inert co-solute is added to the solution. A sufficient increase in the concentration of the neutral species leads to a gradual change in the nature of the interaction between two macroions, from being repulsive to less repulsive and ultimately attractive. The structural features and thermodynamics in these complex systems are shaped by the collective and often competing effects of the species.