  Recent studies have pointed the importance of polyelectrolyte assembly in the
elaboration of innovative nanomaterials. Beyond their structures, many
important questions on the thermodynamics of association remain to be answered.
Here, we investigate the complexation between poly(diallyldimethylammonium
chloride) (PDADMAC) and poly(sodium acrylate) (PANa) chains using a combination
of three techniques: isothermal titration calorimetry (ITC), static and dynamic
light scattering and electrophoresis. Upon addition of PDADMAC to PANa or
vice-versa, the results obtained by the different techniques agree well with
each other, and reveal a two-step process. The primary process is the formation
of highly charged polyelectrolyte complexes of sizes 100 nm. The secondary
process is the transition towards a coacervate phase made of rich and poor
polymer droplets. The binding isotherms measured are accounted for using a
phenomenological model that provides the thermodynamic parameters for each
reaction. Small positive enthalpies and large positive entropies consistent
with a counterion release scenario are found throughout this study. Beyond,
this work stresses the importance of the underestimated formulation pathway or
mixing order in polyelectrolyte complexation.
