Covalently\nCross-Linked Polyelectrolyte Complex Nanoparticles\nwith Enhanced Stability against Dissociation at High Ionic Strength

Polyelectrolyte complex nanoparticles\n(PECNPs) often\nfully dissociate\ninto individual polycations (PC) and polyanions (PA) at high salinities.\nHerein, we introduce a novel type of colloidally stable PECNP in which\nthe PC is cross-linked, in this case branched polyethylenimine (PEI)\nto limit this dissociation, even in solutions up to 5.2 M NaCl or\n5.4 M CaCl<sub>2</sub>. For cross-linked PECNPs at specified conditions,\nthe size and the PC (poly(vinylsulfonate)) partition coefficient reach\nequilibrium within the first 24 h and change very little for 7 weeks.\nFrom the determination of the released polyanion concentration over\na wide range in PEI protonation degree (<i>f</i>), it was\nfound that strong nonelectrostatic (hydrophobic) as well as electrostatic\ninteractions between the PC and PA control the degree of dissociation.\nThe electrostatic repulsion from the PEI chains on the surface provided\nlong-term colloidal stability with PECNP hydrodynamic diameters on\nthe order of 200 to 300 nm. The ability to achieve partial dissociation\nof a PECNP up to ultrahigh salinity creates new opportunities in fundamental\nexperimental and theoretical studies of PECNP with relevance to controlled\nrelease in subsurface energy and environmental applications.