Cesium self-diffusivity

The self-diffusion coefficients of sodium and cesium ions have been measured for perfluorinated sulfonate (Nafion ) and carboxylate membranes of similar structure. The exchange-site con-... 

Self-diffusion coefficients for sodium ion, cesium ion, and water in these polymers are shown in Fig. 9, as Arrhenius plots from 0 to Diffusion of sodium ion and water is remarkably similar... 

Tracer self-diffusion coefficients for sodium ion and cesium ion have been measured for 1200 equivalent weight Nafion membranes 04-7). Results obtained at 25°C are listed in Table I, along with... 

Table I. Sodium Ion and Cesium Ion Self-Diffusion Coefficients, 25°C...

The self-diffusion coefficient for iodide ion was also measured at 25°C in the first sample of 1200 EW Nafion, before aging 04). The result, 9 10 8 cm2 sec"1, indicates that this anion is more mobile than cesium ion, but less than a sodium ion. This is also somewhat unusual. In general, co-ion diffusion coefficients are seen to be larger than those of counterions in ion exchangers, because no electrostatic attractions to the polymer phase exist. (The membrane concentration is only 5 x 10 3 mol L 1 though, reflecting the effects of Donnan exclusion processes (4 ).)... 

In order to interpret the Na" " and Cs" " diffusional results in terms of this model, it is assumed that both cations would be able to diffuse readily in both the ionic clusters and interfacial regions. Cesium ion would experience a more tortuous diffusion path compared to sodium ion, and thus would have a smaller measured self-diffusion coefficient. The insensitivity of this diffusion coefficient to increasing water sorption may then be because most of this water serves to increase the size of ionic clusters, which would have a relatively minor overall effect on the diffusion path length. 

Membrane Diffusion in Nonaqueous Solvent Environments. Self-diffusion coefficients of Na+ and Cs+ for 1200 EW Nafion membranes in dilute methanol and acetonitrile solutions have been measured (5). Arrhenius plots of these results are shown in Figure 7 along with corresponding results for aqueous experiments activation energies of diffusion are listed in Table IV. Diffusion coefficients of Na+ in methanol and water-equilibrated membranes are very similar, and the activation energy of diffusion for the methanol system is only slightly higher than the respective value for Na+ in pure methanol solvent, 12.9 kJ mol 1 (27). Thus a solution-like diffusion mechanism is inferred for both solvent systems. Cesium ion diffusion in the methanol equilibrated membrane is much slower than sodium ion diffusion in fact the... 

The variation of the self-diffusion coefficient of sodium and cesium counter-ions as a function of a is reported in Figure 3. The equivalent conductances of the same solutions are reported in Figure 4. 

The calculated viscosity, thermal conductivity and self-diffusion coefficients (the latter at 0.10 MPa) of nonionized monatomic lithium, sodium, potassium, rubidium and cesium vapors can be consulted for temperatures between 700 and 2000 K in Tables VIII to XII of the work of Fialho et al. (1993). 

This bimodal dynamics of hydration water is intriguing. A model based on dynamic equilibrium between quasi-bound and free water molecules on the surface of biomolecules (or self-assembly) predicts that the orientational relaxation at a macromolecular surface should indeed be biexponential, with a fast time component (few ps) nearly equal to that of the free water while the long time component is equal to the inverse of the rate of bound to free transition [4], In order to gain an in depth understanding of hydration dynamics, we have carried out detailed atomistic molecular dynamics (MD) simulation studies of water dynamics at the surface of an anionic micelle of cesium perfluorooctanoate (CsPFO), a cationic micelle of cetyl trimethy-lainmonium bromide (CTAB), and also at the surface of a small protein (enterotoxin) using classical, non-polarizable force fields. In particular we have studied the hydrogen bond lifetime dynamics, rotational and dielectric relaxation, translational diffusion and vibrational dynamics of the surface water molecules. In this article we discuss the water dynamics at the surface of CsPFO and of enterotoxin. 

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