Intermolecular complexation and phase separation in aqueous solutions of oppositely charged biopolymers

Turbidity measurements performed at 450 nm were used to follow the process of complex formation, and phase separation in gelatin–agar aqueous solutions. Acid (Type-A) and alkali (Type-B) processed gelatin (polyampholyte) and agar (anionic polyelectrolyte) solutions, both having concentration of 0.1% (w/v) were mixed in various proportions, and the mixture was titrated (with 0.01 M HCl or NaOH) to initiate associative complexation that led to coacervation. The titration profiles clearly established observable transitions in terms of the solution pH corresponding to the first occurrence of turbidity (pHC, formation of soluble complexes), and a point of turbidity maximum (pHϕ, formation of insoluble complexes). Decreasing the pH beyond pHϕ drove the system towards precipitation. The values of pHC and pHϕ characterized the initiation of the formation of intermolecular charge neutralized soluble aggregates, and the subsequent formation of microscopic coacervate droplets. These aggregates were characterized by dynamic light scattering. It was found that Type-A and -B gelatin samples formed soluble intermolecular complexes (and coacervates) with agar molecules through electrostatic and patch-binding interactions, respectively.