Adsorption suppression and viscosity transition in semidilute PEO/silica nanoparticle mixtures under the protein limit

Abstract

Understanding the interplay between polymer adsorption and colloidal interactions is essential for designing advanced materials with tailored properties. This study investigates the adsorption-driven aggregation and rheological transitions in semidilute mixtures of silica nanoparticles and high-molecular-weight poly(ethylene oxide) (PEO) in the protein limit, where the polymer’s size exceeds that of the particles. By systematically varying the ratio of the particle hydrodynamic size to the polymer’s hydrodynamic screening length (R_h,silica/ξ_h,PEO), distinct regimes of adsorption suppression, aggregation onset, and saturation were identified. Below R_h,silica/ξ_h,PEO = 1, adsorption was suppressed due to the entropic penalty of polymer distortion, resulting in negligible viscosity changes and stable particle dispersions. Near R_h,silica/ξ_h,PEO = 1, the adsorption energy overcame the entropy loss, triggering rapid aggregation and a sharp increase in viscosity, accompanied by the emergence of a slow relaxation mode in dynamic light scattering. At higher ratios (R_h,silica/ξ_h,PEO > 2), adsorption saturated, forming dense PEO-silica aggregates, as confirmed by small-angle neutron scattering. These findings challenge conventional theories of polymer adsorption and emphasize the critical role of polymer conformational entropy and adsorption energy balance. This study provides a framework for understanding polymer-mediated colloidal interactions in semidilute regimes, with implications for the rational design of polymer-colloid composites in materials science, biophysics, and industrial formulations.