Selected Diffusion Constants

In view of the difficulty of measuring the diffusivity of o-xylene at the reaction temperature, 482°c, we have used the diffusivity determined at 120°C. For a series of ZSM-5 catalysts, the two D-values should be proportional to each other. Para-xylene selectivities at constant toluene conversion for catalysts prepared from the same zeolite preparation (constant r) with two different modifiers are shown in Figure 11. The large effect of the modifier on diffusivity, and on para-selectivity, is apparent. 

The dilutions arc too groat to alter sensibly the diffusion constant of the medium, Friend suggeata that the extraneous material or addition agent is selectively adsorbed and thus protects the iron from attack. An alternative explanation is the formation of an insoluble compound on the metal surface such as obtains in many cases of passivity. 

Fig. 18. Generalized diffusion constants for diffusion in sequence space (solid line) and diffusion in structure space (dotted line) as a function of the fitness cutoff Fciit. As / increases, the selection strength increases. Diffusion in structure space is more dependent on Fcnt than diffusion in sequence space. Reprinted from Govindarajan and Goldstein (1997b) with permission.

A simple one-dimensional spectrum is always recorded at the start of any structural analysis. Small modifications to the ID pulse sequence enables the selection of groups of signals or singlet signals, the separation of multiplets for determining coupling constants or multiplicity and the determination of molecular parameters such as relaxation times or diffusion constant. In addition ID pulse sequence can also be converted into 2D sequences (and vice versa) enabling homonuclear and heteronuclear correlation. Table... 

Fig. 11.5 Correlation of catalyst selectivity with diffusion constants for propane ( ) and n-butane ( ) measured on four catalyst samples.

Permselectivity is the ability of a membrane to allow a high flux of one chemical species (typically the analyte) while reducing or eliminating the flux of other species (chemical or sensor interferences). An extreme example of permselectivity is the liquid polymer membranes used in ion selective electrodes. The ionophores contained in these membranes bind selectively to a specific ion. However, this binding is often so strong that the diffusion constant for these ions through the membrane is vanishingly small [15]. 

There is good experimental evidence indicating that the lipid bilayer region of the membrane is interrupted at rare intervals (of the order of 1 m) by channels that ions traverse easily ( one-half the diffusion constant in water). The low ion permeability is ascribed to the paucity of channels. There are several kinds of channels named for the ion species that normally traverse them most readily as mentioned above. There are Na+ channels, K+ channels, and at least one other type of channel in excitable membranes. These channels are separable by their kinetics, ion selectivities, and pharmacology. ... 

Table 3. Selected self-diffusion constants D-ao, 0, and related activation energies E of some homologues of the liquid crystalline n-alkyl-cyano-biphenyl (nCB, n = 5) and n-alkyloxy-cyano-biphenyl (nOCB, n = 5S) series obtained at different temperatures AT Tc - T relative to the clearing point Tc by FC-PFG diffusion experiments. The error limit estimated from the experimental standard deviation is 10%.

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