Interface Counterion Localization Induces a Switch between Tight and Loose Configurations of Knotted Weak Polyacid Rings despite Intermonomer Coulomb Repulsions

Stochastic simulations have been used to investigate the conformational behavior of knotted weak polyacid rings as a function of pH. Different from
the commonly expected ionization−repulsion−expansion scheme upon increasing
pH, theoretical results suggest a nonmonotonic behavior of the gyration radius Rg
2
.
Polyelectrolyte recontraction at high ionization is induced by the weakening of
Coulomb repulsion due to counterions (CIs) localizing at the interphase between
the polymer and solvent, and the more marked it appears, the more complex is the
knot topology. Compared with strong polyelectrolytic species of identical
ionization, weak polyacids present tighter knots due to their ability to localize
neutral monomers inside the tangled part. Increasing the solvent Bjerrum length
enhances CIs localization, lowering the pH at which polyacids start decreasing their
average size. A similar effect is also obtained by increasing the amount of
“localizable” cations by adding salts.