Neutron scattering residence time

These speciation concepts are illustrated in Fig. 3 for the idealized basal-plane surface of a smectite, such as montmorillonite. Also shown are the characteristic residence-time scales for a water molecule diffusing in the bulk liquid (L) for an ion in the diffuse swarm (DI) for an outer-sphere surface complex (OSQ and for an inner-sphere surface complex (ISC). These time scales, ranging from picosecond to nanosecond [20,21], can be compared with the molecular time scales that are probed by conventional optical, magnetic resonance, and neutron scattering spectroscopies (Fig. 3). For example, all three surface species remain immobile while being probed by optical spectroscopy, whereas only the surface complexes may remain immobile while being probed by electron spin resonance (ESR) spectroscopy [21-23]. 

Neutron-scattering and dielectric relaxation studies [23] both indicate that the water molecules solvating monovalent exchangeable cations on montmorillonite are a little less mobile, in respect to translational and reorientational motion, than are water molecules in the bulk liquid. For example, as with vermiculite, neutron-scattering data show that no water molecule is stationary on the neutron-scattering time scale. In the one-layer hydrate of Li-montmorillonite, the residence time of a water molecules is about six times longer than in the bulk liquid, with a diffusive jump distance of about 0.35 nm, and a water molecules reorients its dipole axis about half... 

The process of molecular diffusion may be viewed conceptionally as a sequence of jumps with statistically varying jump lengths and residence times. Information about the mean jump length /(P and the mean residence time t, which might be of particular interest for a deeper understanding of the elementary steps of catalysis, may be provided by spectroscopic methods, in particular by quasielastic neutron scattering (see next Section) and nuclear magnetic resonance (NMR). 

It has been demonstrated that the combined application of various NMR techniques for observing molecular rotations and migrations on different time scales can contribute to a deeper understanding of the elementary steps of molecular diffusion in zeolite catalysts. The NMR results (self-diffusion coefficients, anisotropic diffiisivities, jump lengths, and residence times) can be correlated with corresponding neutron scattering data and sorption kinetics as well as molecular dynamics calculations, thus giving a comprehensive picture of molecular motions in porous solids. 

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. ... 

Neutron scattering data for Li- and Na-vermiculite, on the other hand, gave no indication of water protons being immobile on the neutron scattering time scale.This result is consistent with the behavior of water molecules in aqueous solution, since the residence time in the primary solvation shell of a monovalent cation is about 10" s, well within the time scale probed by neutrons. However, as shown in Table 2.4, the self-diffusion coefficients of water molecules on Li- and Na-vermiculite were found to be much smaller than the bulk liquid value at 298 K. These data suggest that, even in the two-layer hydrate, the solvating water molecules exhibit only about 5 per cent of the mobility they have in the bulk liquid phase and about 10 per cent of that in the primary solvation shell of a monovalent cation in aqueous solution (Dg 1.3 x 10" m s" ) . This reduction in water molecule mobility is evidently produced by interactions with the charge distribution on the siloxane surface. 

Nowikow et al. (1999) measured the self diffusion of water in 0.94 m BU4NCI at room temperature using quasi-elastic neutron scattering. The residence times of water molecules in the hydration shell of the cation were twice longer than in bulk water. Still, (1 - Z>w(E)/-Dw) > 0 and the salt is a net structure maker. Sacco et al. (1994) measured the self diffusion coefficient of the water in aqueous CsCl in D2O... 

The quasi-elastic neutron scattering from H2O and D2O, in harmotome, has been further measured in samples with 2, 4, and 12 water molecules per unit cell at 294 K < T < 423 K [98S1]. From the observed coherent scattering relaxation time for D-D correlations, 2 to 11 ps were obtained. Fitting the incoherent quasi-elastic peaks from H2O to a two site jump model, yielded residence times of the protons in the range from 4 to 6 ps [98S1], close to the previous reported values. 

---