Unveiling the\nElectrostatically Driven Collapsing\nand Relaxation of Polyelectrolyte–Colloid Complexes: A Tunable\nPathway to Colloidal Assembly

Polyelectrolyte–colloid (PEC) complexes, ubiquitous\nacross\ndiverse fields, exhibit remarkable phase transitions, mimicking intricate\nbiological assemblies. While the role of electrostatic forces in the\nPEC complex assembly is undeniable, achieving a holistic comprehension\nremains an elusive goal. This study unveils a fascinating phenomenon:\nthe formation of highly collapsed coacervate structures in PEC complexes\nat elevated polyelectrolyte concentrations, followed by the swelling\nof complexes at even higher concentrations. Employing anionic silica\ncolloids and cationic chitosan as a model system, small-angle X-ray/neutron\n(SAXS/SANS) elucidates the transition from a bead-on-a-necklace-like\nphase to a dense packed coacervate state (with volume fraction ∼0.62)\nuntil 3 wt % concentration of the polyelectrolyte. However, beyond\n3 wt %, swelling of the dense collapsed assembly is observed. This\nstructural evolution of PEC complexes as a function of chitosan concentration\nis attributed to the interplay of electrostatically driven interactions\nand the Donnan effect. Notably, the critical concentration for coacervation, <i>C</i><sub>s</sub>*, demonstrates a linear dependence on the\ninitial colloid concentration. Interestingly, a complete expansion\nof the coacervate is observed at a high polyelectrolyte concentration,\nparticularly for dilute colloid solutions (2 wt %). Furthermore, the\naddition of an electrolyte sheds light on the delicate interplay of\nforces. While a low electrolyte concentration partially screens charges,\nleading to a shift in phase diagram, higher concentrations trigger\ncomplete coacervate dissolution beyond the critical electrolyte concentration\nof 0.2 M, due to the complete screening of electrostatic charges.