Including Ion Solvation in Coarse-Grained Polymer Models with Drude Oscillators

Starting from a simple model of salt-doped polymers, with bead-spring chains and single site ions, we compare different ways of accounting for the strong solvation of ions by polar polymers. Beyond Lennard-Jones (LJ) and Coulomb interactions, solvation must be included as an additional feature because a simple coarse-grained bead is a neutral sphere, though it represents a small group of partially charged and polarizable atoms which have an overall dipole moment. We use Drude oscillators so the polymer beads will solvate ions; each polymer bead is then composed of two particles of opposite charge connected by a stiff spring, where one particle is kept within the LJ radius of the main bead. We compare this with an approach from prior work of adding an ion-polymer potential representing averaged ion-induced dipole effects, also with a background dielectric constant. We find that the Drude oscillator approach effectively increases the local ion-ion clustering relative to long-range structural features. The effect of using a Drude oscillator-like model but with a nonzero equilibrium bond length (representing a permanent dipole plus inducible dipole) will also be discussed.