Molecular Simulations of Structure, Dynamics, and Thermodynamic Analyses of Counterion-Specific Collapse of Highly Charged Poly(Methacrylic Acid) in Methanol–Water Mixtures

The atomistic molecular dynamics simulations are performed to study the behavior of fully ionized poly(methacrylic acid) (PMA) in methanol–water mixtures, with a particular emphasis on its interactions with monovalent Na+ and Cs+ counterions across cosolvent volume fractions (Φ) ranging from 0.1 to 0.9. The counterion-specific collapse of the PMA chain occurs at distinct transition points for different counterions. The conformational transition of the PMA chain from an extended to a collapsed phase occurs at a lower Φ in the presence of Na+ counterions compared to Cs+. The conformational transition of the PMA chain also influences the surrounding water and methanol structure, as demonstrated by the hydrogen bonding behavior and intermolecular structure. At a higher methanol concentration, methanol replaces water in the PMA chain’s solvation shell, enhancing the number of hydrogen bonds between methanol and the PMA chain. The coordination number statistics reveal that Na+ ions preferentially condense along the carboxylate moieties of PMA compared to Cs+ ions, resulting in stronger charge screening and a more rapid chain collapse. In contrast, Cs+ ions primarily condense near the polymer backbone rather than near the carboxylate groups. The thermodynamic properties of the solution are evaluated using the second virial coefficient, excess entropy, and hydration enthalpy. Counterion binding, which facilitates chain collapse in poor solvents, also enhances randomness in the solution. Additionally, the self-diffusion coefficient of counterions progressively decreases with increasing Φ, driven by enhanced ion–pair interactions at higher methanol content in the solvent mixtures. This study offers valuable molecular-level insights into the conformational transitions of highly charged polyelectrolytes in water–alcohol mixtures, emphasizing counterion specificity.