Neutron scattering studies water protons

Tor of the LIB state actually represents the waiting time before the proton jumps. This molecular picture is supported of the experimental studies by Teixeira et al. [15] in high-quality quasi-elastic incoherent neutron scattering in water. The jump diffusion of the proton across the tetrahedral angle is rather temperature independent. A very useful interpretation of this experiment made recently by Teixeira [16] is reproduced in Appendix I. 

A primary hydration number of 6 for Fe + in aqueous (or D2O) solution has been indicated by neutron diffraction with isotopic substitution (NDIS), XRD, 16,1017 EXAFS, and for Fe " " by NDIS and EXAFS. Fe—O bond distances in aqueous solution have been determined, since 1984, for Fe(H20)/+ by EXAFS and neutron diffraction, for ternary Fe " "-aqua-anion species by XRD (in sulfate and in chloride media, and in bromide media ), for Fe(H20)g by neutron diffraction, and for ternary Fe -aqua-anion species. The NDIS studies hint at the second solvation shell in D2O solution high energy-resolution incoherent quasi-elastic neutron scattering (IQENS) can give some idea of the half-lives of water-protons in the secondary hydration shell of ions such as Fe aq. This is believed to be less than 5 X I0 s, whereas t>5x10 s for the binding time of protons in the primary hydration shell. X-Ray absorption spectroscopy (XAS—EXAFS and XANES) has been used... 

Another new approach to the water structure has been provided by Brookhouse s [9] study of inelastic neutron diffraction from water, Here he is measuring not, as in elastic neutron scattering, the relative positions of the proton in the water, but the velocity spectrum of the water molecules, He finds here that the molecules appear to behave very largely like those for gas, that is, they appear to be moving in relatively free space. 

These results show the power of the high resolution neutron quasi-elastic scattering technique to study the water mobility in these membranes. The main conclusion is that on a space scale of 8 A, the water protons move practically as freely as in bulk water, but the longer distance motion is much more difficult. Since the Vander Waals radius of the water protons is about 1 A, the space should be renormalized to 10 A for the water molecules. 

Incoherent neutron scattering (INS) can be used to study the translational, rotational, and vibrational motion of water protons on a time scale between 10" and 10 s. Thus INS provides data pertinent to the V structure and to the transition from the V structure to the D structure in liquid water. The principal use of INS has been to characterize the translational and rotational motion of water molecules through the interpretation of scattering data with model expressions. The three most important model parameters used are the self-diffusion coefficient, Ds, which can also be measured in an experiment involving isotope-labeled water molecules the residence time of a water molecule, tr, during which it vibrates about a fixed position before jumping to its next position and the correlation time, Ti, which is a time constant for the decay of correlation between the orientation of a water molecule at some initial time and at some later time. ... 

Nafion has been the subject of extensive characterization studies [1]. Its microstructure has been exhaustively studied by scattering methods, especially small angle X-ray scattering (SAXS) and small angle neutron scattering (SANS) [19-23]. Mechanical properties of Nafion as functions of temperature have been used to identify temperature-induced transitions [2,24—27]. Transport of protons and water has also been the subject of numerous studies over the past 25 years [28-34]. However, there has been limited progress in coimecting the chemical structure of Nafion to mechanical and transport properties, especially how these properties are altered due to environmental conditions. In this chapter, we will review recent studies of mechanical and transport properties of Nafion done under controlled conditions of water activity and temperature. 

The nature of the bottlenecks for proton conductance in the dry membrane state or on the way to it is, however, still the subject of debates. This wiU only be resolved after more detailed experimental studies (of macroscopic transport parameters such as proton conductance and electro-osmotic coefficients as a function of water content, or gas and liquid permeability before and after operation, and of microscopic structural probes such as small-angle neutron and X-ray scattering) will have discriminated between competing models. By and large, the direction of effects that go with dehydration is obvious enough to be introduced into phenomenological models of overall cell performance. 

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