Salt‐Controlled Multistep Phase Separation in Hydroxypropyl Cellulose Colloids for Sustainable Adhesives

ABSTRACT Phase separation is an evergreen subject that shapes our understanding of how order and functionality emerge from disordered complex soft matter systems. Here, we report on the salt‐regulated transition from isotropic liquid‐liquid phase separation to anisotropic liquid‐liquid crystalline phase separation within an aqueous mixture of hydroxypropyl cellulose (HPC) and kosmotropic salt sodium citrate (Na 3 Cit). The phase behavior of the HPC‐Na 3 Cit system is controlled by the competition for water molecules between the two components, which tunes the intermolecular interactions and drives the multistep self‐assembly from a homogeneous solution into a disordered aqueous HPC‐rich/salt‐rich biphasic system, and ultimately the HPC‐rich phase matures into ordered cholesteric liquid crystalline structure as the polymer chains align under the influence of intensified excluded volume effects. Mechanistic investigations reveal that the Na 3 Cit‐induced confinement enhances the intermolecular hydrogen bonding and electrostatic interactions, while modulating water mobility and viscoelastic relaxation. This results in structurally heterogeneous colloidal network that integrates shear‐resistant and energy‐dissipating domains. Consequently, the resulting HPC‐Na 3 Cit composite exhibits exceptional interfacial adhesion, particularly on wood surfaces, with strength exceeding 3.5 MPa and the structural integrity to support a 30 kg load across a minimal contact area. Our findings establish a robust strategy for bottom‐up design of sustainable adhesive system.