Diffusivities significant discrepancies

The scaling dependence of the diffusion coefficient on N and Cobs Iso poses a number of questions. While the original scaling predictions, based on reptation dynamics [26,38], oc N, have been verified by some measurements [91,98], significant discrepancies have been reported too [95,96]. Attempts to interpret existing data in terms of alternative models, e.g., by the so-called hydrodynamic scaling model [96], fail to describe observations [100,101]. 

The mixed side-pore diffusion model also reasonably simulated the experimental data (Figures 5a and 5b). This model was slightly more accurate than the reaction-rate model in simulating breakthrough curves for a range of input concentrations (Figure 5a-5c) however, significant discrepancies also were observed between experimental data and model simulations at concentrations of less than 0.01 mmol/1 Mo(VI). 

The techniques outlined above have been used to study diffusion in a wide range of zeolite systems. In general we find that there is reasonable agreement between the different macroscopic methods and also between the microscopic methods (QENS, and PFG NMR). However, although for several systems the macroscopic and microscopic measurements are also consistent, there are many systems for which we see significant discrepancies between the two classes of measurements. 

The diffusivity of water vapor into PEFC electrolyte Nation is a function of the material s water uptake, since the vapor must diffuse through the entire media. There is significant discrepancy between various authors on the measured values of the water diffusivity coefficient, since it is a difficult parameter to accurately measure, and the membrane itself swells with water uptake. The diffusion coefficient of water in 1 lOO-EW Nation PFS A polymer with A, > 4 has been correlated as [12] ... 

Indeed, it is worth noting that by itself, a permeation rate proportional to p°50 could be consistent with any value whatever for the ratio of monatomic to diatomic species in the solid, if the diatomic species is very immobile. For in such case, the permeation flux would be carried entirely by the monatomic species, whose concentration always goes as p0 50. However, a sizable diatomic fraction would significantly modify the transient behavior of the permeation after a change in gas pressure. Although neither Van Wieringen and Warmholtz nor Frank and Thomas published details of the fit of their observed transients to the predictions of diffusion theory, it is unlikely that any large discrepancies would have escaped their attention. 

The activation energies calculated for Rb, Cs and Sr in the present study (Table III and Figure 8) are considerably lower than those calculated for high temperature diffusion in both crystalline and glass silicates. This discrepancy in the latter case implies that the glass matrix may be significantly different in high and low temperature diffusion studies. 

The electrode size is another important factor to be considered since it affects the magnitude of the diffusive transport, as shown in Fig. 7.14 for totally irreversible processes. At planar and spherical electrodes significant differences are found between double pulse and multipulse modes, with the discrepancy diminishing when the electrode radius decreases, since the system loses the memory of the previous pulses while approaching the stationary response. Thus, the relative difference in the peak current of a given double pulse technique and the corresponding multipulse variant is always smaller than 2 % when... 

Finally, we also find several discrepancies between our results and experiment. For example, surface-immobilization is experimentally observed to slow down conformational dynamics in the unfolded state, which would be consistent with our results in the case of an attractive surface. However, as mentioned above, the effect of surface-immobilization on the distribution of end-to-end distance is clearly inconsistent with this scenario. Furthermore, (D)tw is experimentally observed to be about an order of magnitude smaller in the unfolded state in comparison to the folded state. [33] Our results in the case of the freely diffusing polypeptide suggest that the value of (D)tw >n the unfolded state should actually be larger than in the folded state. It is also interesting to note that while the authors of Ref. [33] argue that low values of (D)(Tw) are indicative of slow conformational dynamics, we find that the opposite is true. Thus, while our results are consistent with the idea that conformational dynamics in the unfolded state is slower than in the folded state, we find that this would lead to a smaller value of (D)(Tw) in the folded state. One may speculate that this discrepancy results from surface-immobilization. Indeed, we found that the value of (D)(Tw) obtained in the case of the attractive surface case can be significantly lower than in free solution (Cf. Fig. 5). Unfortunately, the results for the end-to-end distance distributions appear to be inconsistent with this possibility. 

On another hand, migration energies calculated in Ref. [6] differ significantly from the isothermal residual resistance recovery measurements obtained by authors in Refs. [5] (0.45 eV), [10] (0.43 eV) and from the Gorsky effect determination with self-diffusion of H in Lu [11] (0.575 eV). Why does such a discrepancy exist ... 

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