  Phase separation of several different overall neutral polyampholyte species
(with zero net charge) is studied in solution with two oppositely charged ion
species that can form ion-pairs through an association reaction. A field theory
description of the system, that treats polyampholyte charge sequence dependent
electrostatic interactions as well as excluded volume effects, is hereby given.
Interestingly, analysis of the model using random phase approximation and field
theoretic simulation consistently show evidence of a re-entrant polyampholyte
phase separation at high ion concentrations when there is an overall decrease
of volume upon ion-association. As an illustration of the ramifications of our
theoretical framework, several polyampholyte concentration vs ion concentration
phase diagrams under constant temperature conditions are presented to elucidate
the dependence of phase separation behavior on polyampholyte sequence charge
pattern as well as ion-pair dissociation constant, volumetric effects on ion
association, solvent quality, and temperature.
