Phase separation phenomena of polyelectrolyte solutions were reviewed. The follwing three systems were discussed;
1) liquid-liquid phase separation of water-polyelectrolyte-neutral salt systems,
2) water-polyelectrolyte systems with addition of divalent counter ions, and
3) complex coacervation of polycation and polyanion.
For the system 1), a brief account of phase diagrams and plait points, especially their molecular weight dependence, was presented. The system is important not only for molecular weight fractionation but also to determine the Theta point of polyelectrolyte salt solutions. The precipitation or gelation phenomena of the system 2) were explained by specific site binding of counter ions on polyian WhiCh eventually leads to three dimensional network formation, thus, precipitation ar gelation. Recent experimental research of the complex coacervation were reviewed, and the comparison between the Voorn-Overbeek theory and the phase diagrams leads to the conclusion that electrostatic free energy term is not sufficient to cause phase separation, but heat therm must be also taken into account.
