From macroscopic phase separation to microscopic interactions: effects of pH, mass ratio, and ionic strength on phase...

This study investigated the effects of pH, mass ratio, and ionic strength on the phase behavior and thermodynamic interactions of whey protein isolate (WPI)-fucoidan (FUC) complexes. The mass ratio was identified as the primary factor governing phase behavior, with a stable homogeneous system observed at a 1:1 (w/w) ratio across pH 2-7. This work quantitatively demonstrated the dual role of ionic strength in regulating WPI/FUC complexation: low NaCl promoted complex formation via enhanced electrostatic interactions, whereas high salt induced phase modulation through combined electrostatic screening and excluded volume effects. Thermodynamic analysis revealed a pH-dependent transition in interaction mechanisms. At pH 4.0 and 5.5, complexation was spontaneous and enthalpy-driven, dominated by electrostatic interactions, while at pH 2.5 the driving force shifted to entropy-driven hydrophobic interactions. These findings provide new mechanistic insights into salt- and pH-regulated phase behavior in protein-polysaccharide systems.