Stability of hydrophobically modified poly(p-phenylenesulfonate) bundles as observed by molecular dynamics simulation

The bundle formation in solution of hydrophobically modified sulfonated poly(p-phenylene) oligomers (PPP) is studied by molecular dynamics simulations. These oligomers form cylindrical micelles in water. Light scattering experiments suggest that the number of PPP oligomers in the cross section is around 10-19 when monovalent Na+ is used as counterion. On the other hand, if divalent Ca2+ counterions are used, this number dramatically increases to 60 oligomers. Here, we show that the basic packing of the PPP oligomers does not change upon increase of the counterion valency. However, the interaction among bundles goes from repulsive to attractive as the counterions are changed from Na+ to Ca2+. We propose that the observed aggregate size of 60 oligomers could be explained by aggregation of several bundles (i.e., bundle of bundles).