Coacervation was defined as the phenomenon in which a colloidal dispersion 
separated into colloid-rich (the coacervate), and colloid-poor phases, both with 
the same solvent. Complex coacervation covered the situation in which a mixture 
of two polymeric polyions with opposite charge separated into liquid dilute and 
concentrated phases, in the same solvent, with both phases, at equilibrium, 
containing both polyions. Voorn and Overbeek provided the first theoretical 
analysis of complex coacervation by applying Flory-Huggins polymer statistics to 
model the random mixing of the polyions and their counter ions in solution, 
assuming completely random mixing of the polyions in each phase, with the 
electrostatic free energy, ΔG(elect), providing the driving force. However, 
experimentally complete randomness does not apply: polyion size, heterogeneity, 
chain stiffness and charge density (σ) all affect the equilibrium phase 
separation and phase concentrations. Moreover, in pauci-disperse systems 
multiple phases are often observed. As an alternative, Veis and Aranyi proposed 
the formation of charge paired Symmetrical Aggregates (SA) as an initial step, 
followed by phase separation driven by the interaction parameter, χ(23), 
combining both entropy and enthalpy factors other than the ΔG(elect) 
electrostatic term. This two stage path to equilibrium phase separation allows 
for understanding and quantifying and modeling the diverse aggregates produced 
by interactions between polyampholyte molecules of different charge density, σ, 
and intrinsic polyion structure.