Insights into structural difference between sodium polyacrylate PAA and sodium polymethacrylate PMA in salt solutions investigated by molecular simulations

The difference in the structural properties of highly charged polyacrylic acid (PAA) and polymethacrylic acid (PMA) is investigated by atomistic molecular dynamics (MD) simulations in the presence of divalent salt magnesium chloride. A united-atom model approach was used to represent the backbone methylene groups in both the polymers. The salt concentration (Cₛ) dependence of conformational radius of gyration (Rg) and chain persistence length (Lₚ) is discussed in detail for PAA and PMA. The Rg showed a decrease with Cₛ for both PAA and PMA. However, the PMA Rg is greater than PAA at the salt concentration range of 0.1 < Cₛ < 0.9 M. The chain stiffness calculated by Lₚ demonstrated a decrease with Cₛ for both PAA and PMA. However, the Lₚ of PMA at Cₛ ≥ 0.7 M is greater than PAA, rendering it stiffer than its counterpart at higher salt concentration. The total number of intermolecular hydrogen bonds between PAA and water is greater compared to PMA–water at all Cₛ values. This behavior may be attributed to methyl side groups in PMA, rendering it more hydrophobic. The coordination number value for PAA–water is greater than the PMA–water pair across the entire Cₛ range. However, the number of h-bonds does not change with variations in Cₛ for both PAA and PMA. The degree of counterion binding to the polyelectrolyte quantified by coordination number shows a decrease with Cₛ due to an increase in salt concentration and replacement of Na⁺ counterions by divalent Mg²⁺ salt ions for PAA and PMA. The spatial distribution of salt ions shows an increase in PAA-Mg²⁺ and PMA-Mg²⁺ coordination number with Cₛ. However, the coordination number for PAA-Mg²⁺ is greater than its counterpart at all Cₛ values indicating a significant electrostatic charge screening in the case of the PAA compared to PMA. Overall, the understanding pertaining to the differences in the microstructure, hydrogen bonding, intermolecular structure, and salt-ion distribution around PAA and PMA in the dilute aqueous divalent salt solution is significantly advanced in the present work.