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. © 2011 American Chemical Society.

Upendra Natarajan

Department Of Chemical Engineering

Muralidharan S. Sulatha

ATOMISTIC SIMULATIONS

BACKBONE STRUCTURES

CARBOXYLATE GROUPS

CONFORMATIONAL BEHAVIOR

CORRELATION DISTANCE

Counterions

DILUTE AQUEOUS SOLUTION

FORCE FIELD PARAMETERS

Hydration behaviors

MD simulation

POLY (METHACRYLIC ACID)

Poly(acrylic acid )

Polyacrylic acids

POLYELECTROLYTE CHAIN

POLYIONS

POOR SOLVENTS

Solvent quality

STRUCTURAL BEHAVIORS

Carboxylation

Carboxylic acids

Polyacrylates

Solutions

Solvents

Behavioral research

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Industrial and Engineering Chemistry Research

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 &lt; c*) chains are expanded and in semi-dilute regime (c*&lt; c &lt; 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 &lt; 1 and exponential decrease for f = 1. The PAA self-diffusion coefficient shows power law decrease with concentration for f &lt; 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. © 2019, © 2019 Informa UK Limited, trading as Taylor &amp; Francis Group.

Molecular Simulation

Structure and dynamics of atactic Na+-poly(acrylic) acid (PAA) polyelectrolyte in aqueous solution in dilute, semi-dilute and concentrated regimes

Molecular dynamic simulations of anionic polyelectrolyte poly(acrylic acid) (PAA) in water–ethanol solution, specifically Li+-PAA and Cs+-PAA, were carried out across the solvent composition range 0&nbsp;≤&nbsp;фeth&nbsp;≤&nbsp;0.9. Chain collapse (i.e. shrinkage) occurs with increase in фeth for both types of counter-ion systems and in agreement with the experiments. The qualitative difference in the collapse point is in agreement with experimental results, with counter-ion specific chain collapse of PAA following the order Li+&nbsp;&gt;&nbsp;Cs+. With increase in фeth the number of hydrogen-bonds between PAA and water decreases while that between PAA and ethanol increases. At higher level of ethanol content in solution, ethanol molecules displace water molecules from the vicinity of the chain. The analysis of the radial distribution functions shows that counter-ion binding distance of Li+ to chain is lesser as compared to that of Cs+, as well as a higher coordination number exhibited by Li+. Thus, as compared to Cs+-PAA, greater number of contact ion pairs formed between Li+ and PAA induce chain collapse more easily. The coordination of Li+ to PAA is better than that of Cs+ throughout the фeth range, which could be the reason for the greater extent of PAA chain shrinkage observed in the case of Li+. Binding of water molecule to PAA units is stronger in the case of Cs+. The backbone dihedral trans probability of both systems displayed a decrease with фeth indicating chain shrinkage. The relaxation time of H-bonds between PAA and EtOH is greater for Li+-PAA as compared to Cs+-PAA system. The enhancement of counter-ion pairs formation is found to be directly responsible for the solvent composition at which chain collapse occurs in the particular system. © 2017 Informa UK Limited, trading as Taylor &amp; Francis Group.

Anionic polyelectrolyte poly(acrylic acid) (PAA) chain shrinkage in water–ethanol solution in presence of Li+ and Cs+ metal ions studied by molecular dynamics simulations

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. © 2017 Informa UK Limited, trading as Taylor &amp; Francis Group.

Molecular dynamic simulations study of the effect of salt valency on structure and thermodynamic solvation behaviour of anionic polyacrylate PAA in aqueous solutions

Self-association (i.e. interchain aggregation) behavior of atactic poly(ethacrylic acid) PEA in dilute aqueous solution as function of degree-of-neutralization by Na+ counter-ions (i.e. charge fraction f) was investigated by molecular dynamics simulations. Aggregation is found to occur in the range 0&nbsp;≤&nbsp;f&nbsp;≤0.7 in agreement with experimental results compared at specified polymer concentration Cp&nbsp;=&nbsp;0.36&nbsp;mol/l in dilute solution. The macromolecular solution was characterized and analysed for radius-of-gyration, torsion angle distribution, inter and intra-molecular hydrogen bonds, radial distribution functions of intermolecular and inter-atomic pairs, inter-chain contacts and solvation enthalpy. The PEA chains form aggregate through attractive inter-chain interaction via hydrogen bonding, in the range f&nbsp;&lt;&nbsp;0.7, in agreement with experimental observation. The numbers of inter-chain contacts decreases with f. A critical structural transition occurs at f&nbsp;=&nbsp;0.7, observed via simulations for the first time, in Rg as well as inter-chain H-bonds. The inter-chain distance increases with f due to repulsive interactions between COO− groups on the chains. PEA-PEA electrostatic interactions dominant solvation enthalpy. The PEA solvation enthalpy becomes increasingly favorable with increase in f. The transition enthalpy change, in going from uncharged (acid) state to fully charged state (f&nbsp;=&nbsp;1) is unfavorable towards aggregate formation. © 2017 Elsevier Inc.

Journal of Molecular Graphics and Modelling

Factors responsible for the aggregation behavior of hydrophobic polyelectrolyte PEA in aqueous solution studied by molecular dynamics simulations

The influence of tacticity on chain dimensions, backbone and side-group conformational states and relaxation dynamics, intermolecular hydrogen bonding and its relaxation dynamics was investigated for 30 repeat unit poly(methacrylic acid) (PMA) as a function of the degree-of-neutralisation, f (i.e. charge density) [0 &lt; f &lt; 1 range] in explicit water with Na+ neutralising counter-ions, using molecular dynamics simulations. Chain expansion with increase in charge density is observed. Simulation results, where applicable, compare well with experimental results in the literature. Isotactic (i-PMA) chain exhibits the largest expansion (89% change in Rg), and syndiotactic (s-PMA-RR-0.7) chain shows 31%. For fully neutralised chain (high charge density), the probability for trans conformation at backbone bonds follows the trend i-PMA &gt; a-PMA &gt; s-PMA. At high charge density, a higher probability of trans state is obtained at meso dyads as compared to racemic dyads and the side group in i-PMA relative to other tacticity is conformationally more extended. RR triads impart better hydrogen bonding with water. In the COOH side group, greater rotational mobility is observed for i-PMA as compared to s-PMA. Water coordination to PMA is invariant with tacticity at low f. For f = 1, s-PMA exhibits the best level of H-bonding, due to its 3D chemical structural configuration. Binding distance between water and PMA atoms and their coordination number remain unchanged with respect to ionisation of COOH groups. Isotactic (i-PMA) exhibits the fastest relaxation of polymer-water H-bonds. While counter-ion coordination on to PMA is unaffected by tacticity, the intensity of ion condensation as well as hydrogen bond relaxation time at f = 1 varies in the order i-PMA &gt; a-PMA &gt; s-PMA. © 2015 Taylor &amp; Francis.

Tacticity effects on conformational structure and hydration of poly-(methacrylic acid) in aqueous solutions-a molecular dynamics simulation study

Journal of Chemical Theory and Computation

GROMACS 4:  Algorithms for Highly Efficient, Load-Balanced, and Scalable Molecular Simulation

Journal of Colloid and Interface Science

Ionization of short polymethacrylic acid: titration, DLS, and model calculations

The Journal of Physical Chemistry B

Single-Chain Properties of Polyelectrolytes in Poor Solvent†

The Journal of Chemical Physics

Strongly charged flexible polyelectrolytes in poor solvents: Molecular dynamics simulations with explicit solvent

How does the chain extension of poly (acrylic acid) scale in aqueous solution? A combined study with light scattering and computer simulation

Origin of the difference in structural behavior of poly(acrylic acid) and poly(methacrylic acid) in aqueous solution discerned by explicit-solvent explicit-ion MD simulations

Journal	Industrial and Engineering Chemistry Research
ISSN	08885885
Open Access	No