Diffusion molecular displacements larger than

In general, for zeolitic self-diffusion at sufficiently high temperatures, the mean molecular displacements outside the crystal are much larger than those inside the zeolites that is to say, long-range self-diffusion, Di.r., is much faster than intracrystalline self-diffusion, Dintra- For observation times comparable with the mean lifetimes of the adsorbed molecules in the individual crystallites, the spin-echo attenuation can be approximated by the superposition of two exponentials of the type of Eq. (6)... 

Once the molecular root mean square displacements observed by PFG NMR are much larger than the mean crystallite diameters, a direct measurement of the rate of mass transfer through the assemblage of zeolite crystallites becomes possible. This rate is represented by the coefficient of long-range diffusion... 

If the mean molecular displacements in the interval between the two field-gradient pulses are much larger than the crystalhte diameters, the diffusivity resulting from PFG NMR measurements reflects the rate of molecular propagation through the bed of crystalhtes. This coefficient of long-range diffusion may be shown to be determined by [84]... 

Brownian motion of a panicle is a result of the thermal motion of the molecular agitation of the liquid medium. Much stronger random displacement of a particle is usually observed in a less viscous liquid, smaller particle size, and higher temperature. A particle of size larger than 1 pm doesn t show a remarkable Brownian motion. There is much literature available on Brownian motion [7-9], and the Brownian motion is regarded as a diffusion process. For an isolated particle, i.e., there is no intcrparticlc action, the diffusion coefficient D , can be expressed as the Stokes-Einstein equation ... 

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