Tacticity and Ionization Effects on Adsorption Behavior of Poly(acrylic acid) and Poly(methacrylic acid) at the CCl4-H2O Interface Revealed by MD Simulations

Atomistic molecular dynamic simulations were performed to investigate the adsorption behavior of poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMA) at the CCl4-H2O interface for isotactic, atactic, and syndiotactic forms. The conformational, orientation, and solvation behaviors of PAA and PMA chains at the interface were studied as a function of the degree of ionization (f). The calculated density profiles show that adsorption occurs only when degree of ionization is less than a critical value (ionization of 20% groups). The density profiles of different groups show the existence of carboxylic acid and carboxylate groups toward the aqueous phase and methyl groups toward the oil phase, relative to the interface. The radius of gyration values and dihedral distributions of completely adsorbed chains (i.e., for f = 0) reveal their existence in an extended conformation at the interface, in contrast to their coiled structure in bulk aqueous solution. The size of adsorbed chains (f < 0.2) decreases with increase in degree-of-ionization due to looping of chain toward water; the extent of looping depends on the distribution of charge on the chain. The carbonyl and methyl groups of uncharged PAA and PMA show two set of orientations corresponding to direction toward water and oil phases and this preferential orientation decreases with increase in degree-of-ionization. Significant differences in orientation distribution, dihedral angle, and hydration were observed among different tacticities of PMA which primarily reflect the hydrophobic nature of isotactic PMA as compared to other tacticity. The number of hydrogen bonds between the polyelectrolyte and water is much lower at the interface relative to the bulk aqueous phase as determined by the population of water in the interface region as well as charge on the polyelectrolyte.