Overlap Concentration of Sodium Polystyrene Sulfonate in Solution*

The overlap concentration c* of sodium polystyrene sulfonate in water is calculated using multichain atomistic and coarse grained (CG) simulations for a range of chain lengths. Fully atomistic molecular dynamics simulations are carried out for N = 32 to 192 monomers. The CG model was parameterized to match the end-to-end distance from the atomistic  simulations at small N and allows us to simulate much larger N. Treating the hydrophobic backbone by inclusion of attraction between monomers is an essential addition to the CG model. The simulation c* are in agreement with experimental data, yet at c*, the chains are not fully stretched even for N as large as 1200. This implies that none of the experimental systems are in the scaling regime, and to reach the scaling regime for NaPSS chains much longer than N=1200 are required.