Covalently Cross-Linked Polyelectrolyte Complex Nanoparticles with Enhanced Stability against Dissociation at High Ionic Strength

Polyelectrolyte complex nanoparticles (PECNPs) often fully dissociate into individual polycations (PC) and polyanions (PA) at high salinities. Herein, we introduce a novel type of colloidally stable PECNP in which the PC is cross-linked, in this case branched polyethylenimine (PEI) to limit this dissociation, even in solutions up to 5.2 M NaCl or 5.4 M CaCl₂. For cross-linked PECNPs at specified conditions, the size and the PC (poly(vinylsulfonate)) partition coefficient reach equilibrium within the first 24 h and change very little for 7 weeks. From the determination of the released polyanion concentration over a wide range in PEI protonation degree (<i>f</i>), it was found that strong nonelectrostatic (hydrophobic) as well as electrostatic interactions between the PC and PA control the degree of dissociation. The electrostatic repulsion from the PEI chains on the surface provided long-term colloidal stability with PECNP hydrodynamic diameters on the order of 200 to 300 nm. The ability to achieve partial dissociation of a PECNP up to ultrahigh salinity creates new opportunities in fundamental experimental and theoretical studies of PECNP with relevance to controlled release in subsurface energy and environmental applications.