Rheological evidence for asymmetric phase separation mechanisms in soy glycinin's reentrant coacervation

Abstract

We mapped the isothermal (25 °C), isohydric (pH 7.0) phase diagram of soy-glycinin/NaCl/water and combined it with small-strain and large-amplitude oscillatory shear (LAOS) rheometry to relate salt-driven structural transitions to viscoelastic behavior. The diagram shows classic re-entrance—two single-phase regions separated by a protein-rich coacervate corridor—yet its boundaries are asymmetric: coacervates harvested at low salt (50 mM–100 mM) contain percolating 7S-trimer networks, whereas those formed at high salt (150 mM–200 mM) are populated by dispersed 11S-hexamers. Consequently, low-salt samples are solid-like (G′ > G″ for ω ≳ 10 rad/s) and exhibit pronounced non-linearities (I_3/1 ≥ 0.1), while high-salt samples behave as weak power-law fluids (G′ ∝ G″ ∝ ω^0.8) with frequency-dependent shear thinning. Pipkin diagrams confirm a shift from frequency-independent plasticity at 50 mM to viscous flow at 200 mM. These results establish a direct link between glycinin's salt-controlled quaternary structure and the mechanics of its self-coacervates, providing design guidelines for plant-protein adhesives, encapsulants and other soft, sustainable materials.