(574d) Molecular Dynamics Simulations of Complex Formation Between Poly(styrene sulfonate) and Poly(allylamine hydrochloride), and Poly(acrylic acid) and Poly(allylamine hydrochloride)

Explicit- and implicit-solvent molecular dynamics simulations are performed to study complexation in two polyelectrolyte systems: poly(styrene sulfonate)/poly(allylamine hydrochloride) (PSS/PAH) and poly(acrylic acid)/poly(allylamine hydrochloride) (PAA/PAH). In both the systems, the polyions have the same number of monomers and are present in stochiometric proportion, and water is used as solvent. Simulations give important insights into the structure and composition of the complexes. We found that the PSS/PAH complex is more compact and has a smaller fraction of water than the PAA/PAH complex due to the presence of phenyl ring in PSS and the hydrophillic nature of the charged group in PAA. The effects of salt concentration and partial charge fraction were also probed. Either an increase in salt concentration or a decrease in partial charge fraction increases complex swelling and the water content in the complex. To overcome the computational limitations associated with explicit-solvent simulations, we performed implicit-solvent simulations. A simple strategy to develop the force field for implicit-solvent simulations is proposed. In the absence of salt and for fully charged polyions, the radius-of-gyration as well as the various radial distribution functions predicted by the implicit-solvent simulations are in excellent agreement with the explicit-solvent simulations, while reasonable agreement is obtained in the other cases. The implicit-solvent simulations are performed for bigger, realistic systems, and we found that the general trends observed in these simulations agree with those for smaller system sizes, suggesting that the explicit-solvent simulation results are independent of system size. Finally, the findings of the simulations are used to explain the origin of exponential growth in layer-by-layer assembly.