Complexation of alginate with linear and branched polyethyleneimine.

This study investigates the complexation behavior of alginate (ALG), a naturally occurring anionic polysaccharide, with two different molecular architectures of synthetic cationic polyethyleneimine (PEI): linear (l-PEI) and branched (b-PEI). While PEI/ALG interactions have been studied for various biomedical and industrial applications, a quantitative assessment of how PEI architecture affects phase behavior, charge compensation, and thermodynamics under varying ionic strengths remains unexplored. To fill this gap, we employ a combination of complementary techniques, including millifluidic droplet assays, isothermal titration calorimetry (ITC), conductometric titration, turbidity analysis, and optical microscopy. We reveal that b-PEI, due to its branched configuration and higher charge density, forms complexes and coacervates that are more stable and salt-resistant than l-PEI. Notably, b-PEI enables coacervation to persist up to 1.8 M NaCl, over twice the maximum salt concentration tolerated by complexes involving l-PEI. ITC thermograms reveal a biphasic, architecture-dependent binding mechanism and a pronounced enthalpic asymmetry on titration direction. These findings offer valuable mechanistic insights into how polymer structure governs polyelectrolyte complexation, enabling the rational design of customizable materials for applications in drug delivery, tissue engineering, and colloidal stabilization.