Spinodal stability of a salt polyelectrolyte solution: Effect of formation of ion pairs and multiplets

For a salt polyelectrolyte solution, spinodal curves are constructed taking into account the possibility of formation of ion pairs and multiplets between ions of polymer chains and oppositely charged ions of a low-molecular-mass salt. The fraction of ion pairs and multiplets increases with increasing the salt concentration and the solvent polarity. It was shown that the attraction between ion pairs and the formation of multiplets lead to effective attraction between monomer units of polymer chains; as a result, the instability region of the homogeneous state of the solution widens. As the salt concentration in weakly polar media increases, the spinodal instability may develop even in a good (for uncharged polymer units) solvent. The region of solution parameters where the spinodal curves exhibit two maxima-that is, where the existence of the triple point on the phase diagram is possible-is discovered.