Upendra Natarajan

Structural and dynamic properties of aqueous solution of atactic poly(acrylic) acid (PAA) in dilute, semi-dilute and concentrated regimes were studied by fully atomistic molecular dynamics simulations with explicit solvent description, as a function of polymer concentration c (i.e. volume fraction φp) and charge density f. PAA size (Rg, R) decreases with φp in semi-dilute and concentrated regimes, due to increase in counter-ion condensation. For all values of f, in dilute regime (c < c*) chains are expanded and in semi-dilute regime (c*< c < c**) chains are in contact with each other, while for c ≅ c** aggregates comprising of few PAA chains occur (at f = 0.2, 0.4 and 0.7). Number of PAA intrachain h-bonds is greater than PAA–PAA interchain h-bonds at all values of f and φp. The number of h-bonds between carboxylic acid groups and carboxylate groups remain unaffected by φp. The Na+ ion self-diffusion coefficient shows linear decrease with concentration for f < 1 and exponential decrease for f = 1. The PAA self-diffusion coefficient shows power law decrease with concentration for f < 1 and exponential decrease for f = 1. Aggregation of chains is favoured due to PAA–PAA interactions with increase in concentration. Our simulation results are in agreement with experiments and coarse-grained simulations in the literature.

Polyelectrolytes are useful in a variety of applications such as detergents, consumer care, electrochemical devices, solid-state reference electrode, water treatment, paper production, contact lenses, drug delivery systems, tissue engineering and biosensors. The behaviour of polyelectrolytes in solution is influenced by stereochemistry. The influence of tacticity on conformations of carboxylate anionic polyelectrolytes isotactic poly(acrylic acid) (i-PAA) and syndiotactic poly(acrylic acid) (s-PAA) in water was investigated by explicit solvent atomistic molecular dynamics (MD) simulations using different generic force-fields (CHARMM27, GROMOS 53a6) and water models (SPC-E and TIP4P). Simulations with uniformly distributed charges were performed using 20, 30, and 60 repeat unit chains of i-PAA and s-PAA. CHARMM27 FF with SPC/E water model correctly depict the helical conformational structure of the i-PAA (tg + = 76%) in comparison with experimental data (tg + = 72%). Simulation results show i-PAA to be helical as compared to s-PAA which is extended (trans), in qualitative agreement with experimental data on i-PAA and s-PMA. Counter-ion condensation is stronger for i-PAA which may be partly responsible for its helical conformation. The novelty of the present work is the origin of the conformational behaviour of the anionic polyelectrolyte with such information having potential biological applications such as gene therapy. Abbreviations: i-PAA: isotactic-poly(acrylic acid); s-PAA: syndiotactic-poly(acrylic acid); s-PMA: syndiotactic-poly(methacrylic acid); PEA: Poly(ethacrylic acid); SPC: Simple Point Charge; SPC/E: Extended Simple Point Charge; ST2: Based on the Ben-Naim and Stillinger model; TIPS: A Set of Transferable Intermolecular Potential function; TIP3P: Three-point Transferable Intermolecular Potential; TIP4P, TIP4P-Ew, TIP4P/Ice, TIP4P/2005: Four-point Transferable Intermolecular Potential, for Ewald technique, for Ice, reparametrised original TIP4P model, respectively; TIP5P: Five-point Transferable Intermolecular Potential; R g: Radius-of-gyration; R: End-to-end distance; RDF: Radial Distribution Function; RB: Ryckaert-Bellemans; COM: Centre-of-Mass; CHARMM: Chemistry at HARward Macromolecular Mechanics; GROMOS: GROningen MOlecular Simulation; GROMACS: GROningen MAchine for Chemical Simulations; MD: Molecular Dynamics; PME: Particle Mesh Ewald; LINCS: LINear Constraint Solver; VR: Velocity Rescaling; NPT: Constant Number of particle, Pressure, and Temperature; NVT: Constant Number of particle, Volume, and Temperature; L-J: Lennard-Jones; vdW: van der Waals; FF, FF1, FF2: Force-Field, Force-Field 1, Force-Field 2 respectively; f Charge fraction or charge density or degree-of-ionisation; ACF: Autocorrelation Function.

The effect of salt concentration and valency on intermolecular structure and solvation thermodynamic properties of aqueous solution containing polyacrylicacid (PAA) chains and multi-valent salts calcium chloride (CaCl2) and aluminium chloride (AlCl3) as a function of charge density was investigated using atomistic molecular dynamic simulations with explicit solvent. Salt-free solution favours the self-association of uncharged (acidic form) PAA chains facilitated by inter-chain hydrogen bonds. The ionised (charged) PAA chains are not associated in salt-free aqueous solutions and undergo self-association in the salt solutions due to bridging effect induced by condensed salt ions in agreement with scattering investigations available in literature. The collapse behaviour of PAA in presence of CaCl2 and re-expansion behaviour of PAA chains in case of AlCl3 salt solutions are observed. The rigidity of PAA chains decrease with increase in salt concentration, in agreement with experimental results available in literature. The trivalent salt favours relatively the greater extent of shrinking of PAA chains as well as inter-chain interactions as compared to divalent salts as evident from radius-of-gyration, H-bond and pair-wise solvation enthalpy data. The conformation and hydration behaviour of the acid form of PAA chains are not significantly altered by added salt ions. The hydration behaviour of ionised PAA chains is significantly reduced by added salts due to screening effect of the condensed salt ions. The pair correlation functions of solutions species such as Ca2+, Al3+, Na+ and Cl− with respect to PAA oxygen show the greater affinity of PAA units with the higher valency Al3+ ions over Ca2+ and Na+ in solution. With increase in concentration of AlCl3 and CaCl2 salts, a decrease in effective charge density of ionised PAA chains is observed from the existence of unfavourable PAA–water, PAA–Ca2+ and PAA–Al3+ interactions.

Chain conformations, counter-ion structure, intermolecular hydrogen bonding structure and dynamics of atactic polyethacrylic acid (PEA) in salt-free aqueous dilute solution at 25°C are studied via molecular dynamics (MD) simulations with explicit-solvent and explicit-ion description for the first time. The intermolecular structure was analysed by the radial distribution functions (RDF) for specific atom types between PEA chain, water molecules and Na+ counter-ions, as well as by the hydration near the PEA chain in the solvated system. An increase in f provides an increase in 〈Rg〉 of the chain, consistent with the existence of the compact form of PEA. The simulations show expansion for radius-of-gyration with increase in f, as expected for flexible polyelectrolytes under salt-free condition. The extent of intermolecular hydrogen bonds (H-bonds) between PEA and water is enhanced by increase in f. Chains having a higher counter-ion density show higher values of 〈Rg〉, influenced by intermolecular interactions between PEA and water. The coordination of Na+ counter ions and water molecules to carboxyl oxygens of polyacrylic acid (PAA) increases with charge density of the chain. A comparison of the structure aspects is made with PAA and PMA polyelectrolytes in dilute solution, which brings out the hydrophobic effect of the ethyl side-groups in PEA on conformational properties and counter-ion condensation structure. © 2013 Taylor & Francis.