Characterization of the Solvation and Transport of the Hydrated Proton in the Perfluorosulfonic Acid Membrane Nafion

TitleCharacterization of the Solvation and Transport of the Hydrated Proton in the Perfluorosulfonic Acid Membrane Nafion
Publication TypeJournal Article
Year of Publication2006
AuthorsPetersen, MK, Voth, GA
JournalJ Phys Chem B
KeywordsArtificial Models, Chemistry, Molecular Molecular Conformation Oxygen/chemistry Protein Structure, Physical/*methods Diffusion Fluorocarbon Polymers/*chemistry Hydrogen/*chemistry Ions Membranes, Tertiary *Protons Solvents/chemistry Thermodynamics Time Factors

The solvation and transport properties of the sulfonate-hydronium ion pair have been studied in hydrated Nafion through molecular dynamics simulation. Explicit proton and charge delocalization of the excess proton transport, via the Grotthuss hopping mechanism, were treated using the self-consistent multistate empirical valence bond (SCI-MS-EVB) method. The nature of the sulfonate-hydronium ion pair was characterized through analysis of free-energy profiles. It was found that, in general, the excess proton is solvated between two water molecules of a Zundel moiety while in the contact ion pair position, but then it transitions to an Eigen-like configuration in the solvent-separated pair position. Furthermore, the positive charge associated with the excess proton passes between the contact and solvent-separated ion pair positions through the Grotthuss mechanism rather than simple vehicular diffusion. The total proton diffusion was decomposed into vehicular and Grotthuss components and were found to be of the same relative magnitude, but with a strong negative correlation resulting in a smaller overall diffusion. Correlated motions between the ion pair were examined through the distinct portion of the van Hove correlation function, and a characteristic time scale of approximately 425 ps was observed. Additionally, the association of the hydrated proton with the hydrophobic polymer backbone suggests its amphiphile-like behavior (see Acc. Chem. Res. 2006, 39, 143; Phys. Rev. 1954, 95, 249; J. Chem. Phys. 2005, 123, 084309).