Local Structure around Chloride Ion in Anion Exchange Resin

Neutron diffraction measurements on 35Cl/37Cl isotopically substituted anion exchange resins were carried out in order to obtain direct information on the local structure around the chloride ion absorbed in the resin. Structural parameters concerning the first nearest-neighbor interaction of chloride ions were determined through a least-squares fitting procedure of the observed first-order difference function, DeltaCl(Q). It has been revealed that the chloride ion is neighboring an ion exchange group (-CH2(CH3)3N+) with a Cl-...N distance of 3.10(3) A, and simultaneously bonded with 2.4(1) D2O molecules with a Cl-...D distance of 2.25(2) A. The second and third nearest water molecules around Cl- have also been observed. These results indicate that the direct ionic interaction between Cl- and -CH2(CH3)3N+ drastically reduces the number of first-neighbor water molecules around Cl- but enhances the long-distance structuring of the remaining water molecules in the environment surrounded by a hydrophobic polymer matrix.

Car-Parrinello Molecular Dynamics Study of DCl Hydrate Crystals

Three DCl hydrates have been studied using Car-Parrinello (CP) molecular dynamics. The structural results at low temperatures (T<350 K) agree with experiment and previously published CP results. The monohydrate has been shown to be built from D3O+ and Cl- ions. The dihydrate is built exclusively from D5O2+ and Cl- ions. The trihydrate consists of D2O, D5O2+, and Cl- ions. Thus, the DCl hydrates are excellent systems for studying ideal D3O+ and D5O2+ ions. The data from these ideal structures can be compared to high-concentration acid-liquid simulations. [ClD...Cl]- ions were detected at amorphous and fluid conditions after melting the di- and trihydrate crystals. The experimentally observed spectral continuum can be explained by combining the Eigen and Zundel spectra, and interpolating between the shared deuterium frequencies of the latter and the stretching band of the former.

Excess Proton Solvation and Delocalization in a Hydrophilic Pocket of the Proton Conducting Polymer Membrane Nafion

Solvation properties of the hydrated excess proton are studied in a hydrophilic pocket of Nafion 117 through a series of molecular dynamics simulations. The multistate empirical valence bond (MS-EVB) methodology, which enables the delocalization of the excess proton through the Grotthuss hopping mechanism, was employed for one of the excess protons in the simulation cell. Simulations were performed such that "classical" nondissociable hydronium cations and a single excess proton treated with the MS-EVB methodology were at a concentration ratio of 39:1. Two degrees of hydration of the Nafion polymer electrolyte membrane were simulated, each displaying the same marked difference between the solvation structures of the classical versus MS-EVB treated (Grotthuss shuttling) excess proton species. These differences are attributed to the solvent dynamics needed to transfer the cation between the solvent separated and contact pair positions about the sulfonic acid counterion. The results demonstrate that it is generally impossible to describe the low pH conditions in the hydrophilic domains of Nafion without the explicit treatment of Grotthuss delocalization in the underlying molecular dynamics model for the excess protons.

Proton mobilities in water and in different stereoisomers of covalently linked gramicidin A channels

Proton conductivities in bulk solution (lambda(H)) and single-channel proton conductances (g(H)) in two different stereoisomers of the dioxolane-linked gramicidin A channel (the SS and RR dimers) were measured in a wide range of bulk proton concentrations ([H], 0.1-8000 mM). Proton mobilities (micro(H)) in water as well as in the SS and RR dimers were calculated from the conductivity data. In the concentration range of 0.1-2000 mM, a straight line with a slope of 0.75 describes the log (g(H))-log ([H]) relationship in the SS dimer. At [H] > 2000 mM, saturation is followed by a decline in g(H). The g(H)-[H] relationship in the SS dimer is qualitatively similar to the [H] dependence of lambda(H). However, the slope of the straight line in the log(lambda(H))-log([H]) plot is 0.96, indicating that the rate-limiting step for proton conduction through the SS dimer is not the diffusion of protons in bulk solution. The significant difference between the slopes of those linear relationships accounts for the faster decline of micro(H) as a function of [H] in the SS dimer in relation to bulk solution. In the high range of [H], saturation and decline of g(H) in the SS dimer can be accounted for by the significant decrease of micro(H) in bulk solution. At any given [H], g(H) in the RR dimer is significantly smaller than in the SS. Moreover, the g(H)-[H] relationship in the RR stereoisomer is qualitatively different from that in the SS. Between 1 and 50 mM [H], g(H) can be fitted with an adsorption isotherm, suggesting the presence of a proton-binding site inside the pore (pK(a) approximately 2), which limits proton exit from the channel. At 100 mM < [H] < 3000 mM, g(H) increases linearly with [H]. The distinctive shape of the g(H)-[H] relationship in the RR dimer suggests that the channel can be occupied simultaneously by more than one proton. At higher [H], the saturation and decline of g(H) in the RR dimer reflect the properties of micro(H) in bulk solution. In the entire range of [H], protons seem to cross the SS and RR channels via a Grotthuss-like mechanism. The rate-limiting step for proton transfer in the SS dimer is probably the membrane-channel/bulk solution interface. It is also proposed that the smaller g(H) in the RR dimer is the consequence of a different organization and dynamics of the H-bonded network of water molecules inside the pore of the channel, resulting in a slower proton transfer and multiple pore occupancy by protons.