Influence of temperature and salt on coacervation in an aqueous mixture of poly-L-lysine (PLL) and poly-(sodium styrene sulfonate) (PSSNa).

The mixture of poly-L-lysine (PLL) and long-chain PSSNa can lead to the formation of soluble complexes depending on pH, PLL concentration, ionic strength, and temperature. The influence of these stimuli was studied by zetametry, dynamic and ultra-small-angle light scattering, and turbidimetric measurements. First of all, we studied the stoichiometry of complexation, and then considered the influence of salt concentration and temperature on the behavior of the mixture at different pH values. These findings have allowed us to conclude that the polyelectrolyte-polypeptide stoichiometry is controlled by electrostatic interactions between opposite charges. At mass ratios between 1.8 and 2.3 and with net charges close to neutrality, unstable complexes were formed and flocculated due to the hydrophobic attraction leading to macroscopic phase separation. The linear charge density of the complex is also controlled by the ionic strength. Higher CaCl concentrations reduce the complex stability and decrease the charge density, which leads to surface patch binding (SPB) at higher pH. Finally, the electrostatic interactions and strength of hydrogen bonds increased the stabilization of the complexes formed at temperatures lower than 45 °C. At temperatures higher than 45 °C, hydrophobic interactions became more dominant, causing a destabilization of the complexes.