Binding of Monovalent Ions to PEO in Solution: Relevant Parameters and Structural Transitions

Abstract

We present a comparative small-angle neutron scattering and theoretical study of the charging effects of poly(ethylene oxide) (PEO) in the presence of monovalent ions as a function of the solvent characteristics and cation strength, and we demonstrate that the classical “salting-in” condition is constituted by distinct subregimes depending on salt and solvent characteristics. In solvents with moderate dielectric constant and moderate hydrogen- bonding capacity, two distinct regimes can be distinguished: (1) at low ionic strengths, the addition of salts primarily results in the formation of polymer−cation association complexes and thus in charging of the polymer chain (binding regime); (2) at higher salt concentrations, the binding of ions becomes increasingly unfavorable, resulting in the screening of electrostatic interactions and neutral polymer-like characteristics of PEO are observed (screening regime). The extent of ion-binding is compared to predictions based on mean-field theory in the thermodynamic limit, and it is shown that the pseudo-polyelectrolyte type changes in the polymer's conformation can successfully be described by the random phase approximation. In solvents of high dielectric constant or pronounced hydrogen-bonding capacity, the solvation of ions counteracts the complex formation process, and PEO effectively remains a neutral polymer in solution.