Diffusion mesoscopic techniques

These results demand a reassessment of our basic ideas on sorption kinetics and the role of intracrystalhne diffusion in zeohte-based processes. It seems clear that intracrystalline diffusion can be reliably measured only by microscopic or mesoscopic techniques. In ideal crystals these values should correspond with the values derived from macroscopic measurements of sorption rates, but, since the majority of crystals that have been studied appear to be far from ideal, such a correspondence should not be assumed a priori. Conversely, the role of true intracrystalline diffusion in determining the rates of sorption and catalytic processes may be minimal and we may be forced to conclude that the rates of most large-scale processes are in fact largely influenced or even controlled by surface and internal barriers imre-lated to the ideal zeohte structure. 

The nature of polymer motion in semidilute and concentrated solutions remains a major question of macromolecular science. Extant models describe polymer dynamics very differently 3-11). Many experimental methods have been used to study polymer dynamics (12). One meAod is probe diffusion, in which inferences about polymer dynamics are made by observing the motions of dilute mesoscopic probe particles diffusing in the polymer solution of interest. Probe diffusion can be observed by several experimental techniques, for example, quasi-elastic light scattering spectroscopy (QELSS), fluorescence recovery after photobleaching (FRAP), and forced Rayleigh scattering (FRS). 

Two experimental methods, namely SANS and NSE spectroscopy, are presented here in some detail. Whereas NSE spectroscopy measines dynamic processes such as diffusion of mesoscopic large objects, SANS determines large stmctures of similar length scale and their kinetics during relaxation to equilibrium. Both techniques are complementary SANS yields a scattering pattern that corresponds to the ensemble coverage of the conformation at one instant, whereas NSE spectroscopy allows the tempered change at the stmcture to be followed. 

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