A Chemically Accurate
Implicit-Solvent Coarse-Grained
Model for Polystyrenesulfonate Solutions
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Abstract
A systematic molecular coarse-graining (CG) approach
for aqueous
polyelectrolyte solutions is presented with sodium polystyrenesulfonate
(NaPSS) with different chain tacticities as example systems. The styrenesulfonate
repeat unit is mapped on a three-site CG representation with the counterion
being modeled explicitly while the solvent is modeled implicitly.
The CG force field discriminates between bonded and nonbonded forces,
which have been developed independently. The bonded interactions correspond
to the potentials of mean force of CG bond, angle, and torsion degrees
of freedom obtained from sampling isolated chains with an atomistic
force field that includes only the local interactions along the chain.
The nonbonded interactions correspond to bead–bead potentials
of mean force, obtained from simulations of small molecule or ion
pairs in explicit water. The CG model reproduces the local and global
conformations of polyelectrolyte chains in good agreement with the
parent atomistic chains in aqueous solution. By using a relative dielectric
permittivity based on the local concentration of counterions around
the polyelectrolyte chain, the quality of our CG models can be further
improved substantially. The effect of added salt (NaCl) on the radius
of gyration of PSS chains with different tacticities has also been
studied and results show the transferability of the CG NaPSS model
to regimes with different electrostatic conditions. We furthermore
show that the CG procedure presented here can easily be extended to
CG models for partially sulfonated polystyrene systems