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Microscopic diffusion measurements

For the FTIR microscopic diffusion measurements (micro-FTIR spectroscopy) [30-32] the same Perkin-Elmer 1800 spectrometer was used, but a so-called IR microscope (Spectra Tech model IR-Plan) and an appropriate flow-through micro-cell were attached (see Figures 4a,b). [Pg.143]

J. Karger, In-Depth Study of Surface Resistances in Nanoporous Materials by Microscopic Diffusion Measurement, Microporous Mesoporous Mater., 2014, 189, 126. [Pg.50]

Comparison between xf a as determined on the basis of Eq. (3.1.15) from the microscopically determined crystallite radius and the intracrystalline diffusivity measured by PFG NMR for sufficiently short observation times t (top left of Figure 3.1.1), with the actual exchange time xintra resulting from the NMR tracer desorption technique, provides a simple means for quantifying possible surface barriers. In the case of coinciding values, any substantial influence of the surface barriers can be excluded. Any enhancement of xintra in comparison with x a, on the other side, may be considered as a quantitative measure of the surface barriers. [Pg.244]

Alkyl radicals react in solution very rapidly. The rate of their disappearance is limited by the frequency of their encounters. This situation is known as microscopic diffusion control or encounter control, when the measured rate is almost exactly equal to the rate of diffusion [230]. The rate of diffusion-controlled reaction of free radical disappearance is the following (the stoichiometric coefficient of reaction is two [233]) ... [Pg.99]

While microscopic techniques like PFG NMR and QENS measure diffusion paths that are no longer than dimensions of individual crystallites, macroscopic measurements like zero length column (ZLC) and Fourrier Transform infrared (FTIR) cover beds of zeolite crystals [18, 23]. In the case of the popular ZLC technique, desorption rate is measured from a small sample (thin layer, placed between two porous sinter discs) of previously equilibrated adsorbent subjected to a step change in the partial pressure of the sorbate. The slope of the semi-log plot of sorbate concentration versus time under an inert carrier stream then gives D/R. Provided micropore resistance dominates all other mass transfer resistances, D becomes equal to intracrystalline diffusivity while R is the crystal radius. It has been reported that the presence of other mass transfer resistances have been the most common cause of the discrepancies among intracrystaUine diffusivities measured by various techniques [18]. [Pg.419]

After the experiment, the experimental charge is prepared for analysis of the diffusion component or species. The analytical methods include microbeam methods such as electron microprobe, ion microprobe, Rutherford backscatter-ing, and infrared microscope to measure the concentration profile, as well as bulk methods (such as mass spectrometry, infrared spectrometry, or weighing) to determine the total gain or loss of the diffusion component or species. Often, the analysis of the diffusion profile is the most difficult step in obtaining diffusivity. [Pg.285]

C.J.Burton, IEC 35, 120-5(l943)(Electron microscope and cellulose) 19)E.Ott, "Cellulose and Derivatives , Interscience, NY(1943) 20)Hackh s(1944), 178 21)N.Gral6n, "Sedimentation and Diffusion Measurements on Cellulose and Cellulose Derivatives , Almqvist Wiksells, Uppsala(1944) 22)E.Heuser, "The Chemistry of Cellulose , Wiley, NY(1944) 22a)... [Pg.492]

Many of the macroscopic techniques can be apphed to the measurement of self-diffusion by using isotopically labeled tracers. Such methods, first introduced by Barrer and Fender [97], have been widely applied in order to obtain data which should be directly comparable with microscopic self-diffusion measurements. Such comparisons are presented in several of the chapters within the present volume. [Pg.29]

Since most microscopic techniques measure self-diffusion, whereas the macroscopic techniques generally measure transport diffusion, direct comparisons between the measured diffusivities are not meaningful, except in the... [Pg.47]

Diffusion measurements fall into two broad classes. Under macroscopic equilibrium, i.e. if the overall concentration within the sample remains constant, molecular diffusion can only be studied by following the diffusion path of the individual molecules ( microscopic measurement by quasielastic neutron scattering (QENS) [48,183,184], nuclear magnetic relaxation and line-shape analysis, PFG NMR) or by introducing differently labelled (but otherwise identical) molecules into the sample and monitoring their equilibration over the sample ( macroscopic measurements by tracer techniques) [185,186]. The process of molecular movement studied under such conditions is called self-diffusion. [Pg.121]


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