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

Abstract

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.

Complex role of chemical nature and tacticity in the adsorption free energy of carboxylic acid polymers at the oil–water interface: molecular dynamics simulations

“…The GROMOS96 force field was shown to yield results in good agreement with experimental behavior for PAA and PMA in bulk aqueous solution 52 as well as at the CCl 4 -water interface. 42 The aliphatic atoms of the polymer are represented as united atoms in this force field and help in reducing the computational cost when compared to all-atom models. The SPC/E model gives accurate hydration free energies for amino acids and solution properties of polymers.…”

“…40 MD and DFT simulations showed that isotactic poly(methacrylic acid) adsorbs at the CCl 4 -water interface, and is hydrophobic, in agreement with VSFS experiments. 41,42 Coarse-grained MD simulations showed a change in the morphology of aggregates of poly(styrene)block-poly(2-vinylpyridine) chains at the chloroform-water interface in going from loosely packed to densely packed structure with the increase in the change fraction of the polymer. 43 Hydrophobic interactions are considered to be the major driving force for adsorption of amphiphilic molecules, such as proteins and surfactants, at the oil-water interface.…”

“…51 The conformations of poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMA) in bulk aqueous solution and at the oil-water interface are known. 42,52,53 The free energy change associated with various processes such as dissolution of solute in solvent, binding of ligands to proteins and transfer of solute across the liquid-air interface (or free surface of liquid) has been studied by MD simulations. [54][55][56] Adsorption of polymers from aqueous solution to the solidwater interface has been studied for PAA, polyacrylamide (PAM) and polyethylene glycol (PEG) at the b-dicalcium silicate surface 57 and for PAA on the calcite surface.…”

“…51 The conformations of poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMA) in bulk aqueous solution and at the oil–water interface are known. 42,52,53…”

Molecular-Level Insights into the Improvement Mechanism of the Bonding Agent MAPO on the Mechanical Properties of the AP/HTPB-TDI Cross-Linked System

“…This capability enables the theoretical evaluation of the interface interaction mechanisms of bonding agents and facilitates targeted development. In recent years, MD methods have been widely applied in various fields to investigate material mechanics, separation processes, adsorption phenomena, and diffusion characteristics. − Especially in propellants, Wang et al discussed the phase separation process of NPBA in a nitrate system. Zheng et al used the MD method to study the rheological behavior of the HTPB binder on the surface of Cu/Al nanoparticles.…”