Molecular dynamics simulation and X-ray absorption spectroscopy to characterize the local structure and dynamics of calcium ion in sulfonated polystyrene

Abstract

Molecular Dynamics (MD) Simulation and the Extended X-ray Absorption Fine Structure spectroscopy (EXAFS) were combined to characterize the “averaged” coordination structures and short-time dynamics of the calcium-doped sulfonated polystyrene (CaSPS) at the atomistic level. MD simulation of the CaSPS ionomer implies that there is evidence for the Ca...O coordination from both the sulfonate group and oxygen atoms. The EXAFS fitting procedure was compared between the conventional technique using the atomic coordinates from the standard CaO crystal and the method using selected snapshots from MD trajectories. The averaged Ca…O distance and the coordination number in the first solvation shell from the simulation are in good agreement with the EXAFS spectra (2.5, 2.4 Å and CN = 5.8, 5.7 from MD, EXAFS analysis, respectively). From MD simulation, the cases of 5–7 oxygen atoms from 4 to 5 sulfonated groups coordinating with calcium are the most frequently seen. The best fit of EXAFS data using the atomic coordinates from MD snapshot suggests that there is only one dominant shell for Ca…O coordination which is six oxygen atoms from five different sulfonated groups and the other one from a water molecule. The interaction between Ca2+ ion and sulfonate group strongly slows down the dynamics of these species in the cooperative manner whereas water molecules exhibit much faster dynamics.