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

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 <= phi(eth) = 0.9. Chain collapse (i.e. shrinkage) occurs with increase in phi(eth) 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+ > Cs+. With increase in phi(eth) 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 phi(eth) 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 phi(eth) 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.