Interaction of the calcium ion with poly( acrylic acid) as investigated by a combination of molecular dynamics simulation and X-ray absorption spectroscopy

A combination of molecular dynamics (MD) simulation and extended X-ray absorption fine structure (EXAFS) spectroscopy was employed to investigate the structure and interaction of the calcium (Ca2+) ion with poly(acrylic acid) (PAA). The results indicated that there is only one dominant shell from PAA oxygen atoms coordinated with calcium ions. The distance between the Ca2+ ion and PAA oxygen atoms in the first solvation shell from simulation is in good agreement with EXAFS measurement (within +/- 0.1 angstrom). The Ca K-edge EXAFS spectra of CaCl2-PAA complexes indicated that there is no evidence for the formation of Ca2+center dot center dot center dot Cl- contact ion pairs and the Ca2+ ion environment is nearly independent of salt concentration and anion type. The first coordination shell of the Ca2+ ion (within a distance of 2.85 angstrom) in the PAA matrix is mainly from carbonyl oxygens with about 7.6 oxygen atoms. From MD trajectories, the cases of seven or eight oxygen atoms from four or three PAA chains, respectively, coordinated to Ca2+ are the most frequently seen. PAA backbones adopt more gauche conformations to facilitate the interaction of carbonyl oxygen atoms with the Ca2+ ion. The complexation of the Ca2+ ion with PAA chain oxygens weakly influences the local structure adopted by the polymer molecule.