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A Molecular Dynamics Study of the Specific Aspects of Counterion Condensation onto the Poly(styrene-sulfonate) Anion and Poly(allylammonium) Cation

The phenomena of counterion condensation onto the synthetic linear polyelectrolytes of high charge density in solution is known for decades. However, detailed understanding of the influence that structure, charge, and charge density of counterions have on their distribution within the polyion domene is still rather limited. In order to gain a deeper insight into the abovementioned aspects of ion condensation we have performed explicit solvent all-atom molecular dynamics simulations of two systems containing studied polyions, namely poly(styrene-sulfonate) anion (PSS ; Pn =100) and poly(allylammonium) cation (PAH ; Pn =100), counterbalanced with various monovalent cations (M+ ; M = Na, K, Cs, ) and anions (A- ; A= F, Cl, Br, I, NO3, ClO4) in TIP4P water. Polymer chain parameters were based on OPLS-AA forcefield. Gromacs 4.5.3 package was used to perform the simulations. The choice of counterions was based on their notably different charge density and, in some cases, structure (NO3-, ClO4-). In the case of PAH energetically most favorable condensation was obtained in the case of NO3with whom the polyion has the most direct contacts. Also, the notably favorable condesation of perchlorate anion was observed. The results for F–, Cl and Br were similar, both in terms of energetics and distribution around the PAH chain. These ions showed less affinitiy for PAH chain than NO3– and ClO4–. The condensation of bulky I- was the least favorable. The notable influence of cation charge density and structure on their distribution near the PSS chain was not observed. Moreover, the energetics of counterion condensation was similar for all investigated cations.

Molecular Dynamics; polyelectrolyte; ion condesation

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