The conformational and hydration behavior of poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMA) in dilute aqueous solution, as a function of charge density, is studied using two different sets of force field parameters: FF-1 and FF-2. The two sets of force field parameters predict similar correlation distances between carboxylate groups and water. The simulations bring about the difference in the interaction of the counterions with the polyions PAA and PMA. FF-1 predicts that upon neutralization PMA shows stronger correlation with Na + ions in comparison to PAA in aqueous solution. This difference in behavior agrees with the fact that, in poor solvents, the polyelectrolyte chain and the counterions interact strongly. The atomistic simulations using FF-1 parameters presented here bring about the difference in the conformational behaviors of PAA and PMA and the effect of solvent quality on the polyacrylate chain backbone structure.

Atomistic molecular dynamics simulations of copolymers of poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMA) in dilute aqueous solution were performed as a function of charge density, in explicit solvent medium and counterions. The studied polyelectrolytes follow a general behavior of chain expansion with charge density until a point where the repulsion between the electrostatic charges between the anionic residues is effectively neutralized by the counterions. The average persistence length is found to increase and levels off at higher charge densities, and the values imply the chains to be flexible. With increase in PMA content in the chain, counterions show increased correlation with chain backbone and a systematic reduction in the number of water molecules in the first hydration shell. The intermittent hydrogen-bond correlation function for the hydrogen bonds between the chain residues and water decays faster for PAA chain as compared to PMA, indicating a rigid hydration layer for the latter. The shorter H-bond lifetimes coupled with the slower relaxation indicate that MA–water H-bonds break more easily than those of AA–water H-bonds, but the water molecules remain in the vicinity of the chain because of slow diffusivity and can easily reform the bonds.

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We have investigated the interaction of dodecyltrimethylammonium chloride (DoTA) micelle with weak polyelectrolytes, poly(acrylic acid) and poly(methacrylic acid). Anionic as well as un-ionized forms of the polyelectrolytes were studied. Polyelectrolyte-surfactant complexes were formed within 5-11 ns of the simulation time and were found to be stable. Association is driven purely by electrostatic interactions for anionic chains whereas dispersion interactions also play a dominant role in the case of un-ionized chains. Surfactant headgroup nitrogen atoms are in close contact with the carboxylic oxygens of the polyelectrolyte chain at a distance of 0.35 nm. In the complexes, the polyelectrolyte chains are adsorbed on to the hydrophilic micellar surface and do not penetrate into the hydrophobic core of the micelle. Polyacrylate chain shows higher affinity for complex formation with DoTA as compared to polymethacrylate chain. Anionic polyelectrolyte chains show higher interaction strength as compared to corresponding un-ionized chains. Anionic chains act as polymeric counterion in the complexes, resulting in the displacement of counterions (Na(+) and Cl(-)) into the bulk solution. Anionic chains show distinct shrinkage upon adsorption onto the micelle. Detailed information about the microscopic structure and binding characteristics of these complexes is in agreement with available experimental literature.