Charge-regulated complexation and multiscale structural organization of poly(D-lysine) hydrobromide/poly(sodium 4-styrenesulfonate) complexes: effects of pH, ionic strength and temperature

Understanding the multiscale assembly of peptide–polyelectrolyte complexes is essential for designing responsive soft materials. Here, we investigate the charge-regulated complexation of poly(D-lysine) hydrobromide (PDLHB) and poly(sodium 4-styrenesulfonate) (PSSNa) under varying pH, ionic strength, and temperature. Dynamic light scattering (DLS), ζ-potential measurements, rheology, and ultra-small-angle light scattering (USALS) show behavior consistent with coacervate formation near electroneutrality (pHopt ≈ 2.9), with counterion release expected to contribute entropically. The pH dependence is governed by charge regulation, while salt suppresses phase separation through electrostatic screening; specific divalent-ion interactions may also contribute. USALS gives a nearly temperature-independent fractal dimension (Df ≈ 2.1), consistent with reaction-limited cluster aggregation (RLCA)-like organization and preserved mesoscale topology. In contrast, the hydrodynamic radius increases with temperature while viscosity decreases, indicating decoupling between intra-cluster swelling and inter-cluster connectivity. Overall, the results provide a multiscale description of PDLHB/PSSNa coacervation and show how charge regulation, ionic screening, and temperature modulate structural organization and rheological behavior.