Water proton mobility

During cerebral ischemia, water proton mobility declines rapidly, which may be measured with diffusion-weighted imaging (DWI) or other methods (Nicholson and Sykova 1998). In animal models of focal ischemia, the ADC declines by about 30%-50% starting within a few minutes following onset... 

Tapping mode atomic force microscopy Water molecules per acid site, H2O/SO3H Fickian diffusion coefficient of water Self-diffusion coefficient of water Proton mobility Electroosmotic drag coefficient Bulk membrane proton resistance Uncompensated resistance Glass transition temperatme Water volume fraction... 

In many products, the spin-relaxation properties of the components can be different due to molecular sizes, local viscosity and interaction with other molecules. Macromolecules often exhibit rapid FID decay and short T2 relaxation time due to its large molecular weight and reduced rotational dynamics [18]. Mobile water protons, on the other hand, are often found to have long relaxation times due to their small molecular weight and rapid diffusion. As a result, relaxation properties, such as T2, have been used extensively to quantify water/moisture content, fat contents, etc. [20]. For example, oil content in seeds is determined via the spin-echo technique as described according to international standards [64]. 

Eigen pointed out that the mobility of a proton in ice at 0°C is about 50 times larger than in water. In ice, H20 molecules already occupy fixed positions suitable for accepting a proton, so that the proton mobility is directly proportional to the rate of tunnelling. 

This technique is also used as a very common diagnostic tool, since the chemical shift of water protons depends also on the mobility of the water molecules. Therefore, it is possible to discriminate between different tissues. A computer analyses the emissions from the hydrogen nuclei of water molecules... 

Cukierman, S. (2000) Proton mobilities in water and in different stereoisomers of covalently linked gramicidin A channels. Biophysical Journal, 78, 1825-1834. 

Apart from the data of thermonephelometry and HS-DSC,1H NMR studies have also revealed [27] some properties that allowed the attribution of such s-type copolymers to the protein-like ones. A marked broadening of the water proton signal was observed caused by the decreased mobility of bound water just in the vicinity of the temperature of HS-DSC peak. These data indicated the heat-induced compaction of the interior of the polymer coils, as would occur with protein-like macromolecules. Figure 5 demonstrates the experimental data, viz., the temperature dependences of signal width at half-height for the peaks of water protons recorded in D2 O-solutions of p- and s-fractions of the copolymer synthesized from the feed with an initial comonomer ratio of 85 15 (mole/mole). 

At low water contents, Dh o decreases more rapidly for SPEEKK than for Nafion. Proton mobility, D, behaves in a similar fashion. When water content is high, is higher than a result of the influence of inter-... 

Proton mobility (D J and water self-diffusion coefficient (D q) as a function of the water volume fraction (X ) in Nafion and SPEEKK, where X, = volume of water in membrane divided by volume of wet membrane. (From Kreuer, K. D. 2001. Journal of Membrane Science 185 29-39.)... 

It is interesting to note, however, that even though the SPEEKK sample shown in Figure 3.22 has a higher lEC content (1.46 meq/g) than sPS02-781 (1.28 meq/g), it exhibits overall lower conductivity for a given water content. Based on an examination of the proton mobility values for these polymers, it was suggested that this may be due to some microstructural differences between the two different systems. Proton conductivity as a function of... 

Peckham, T. J., Schmeisser, J., Rodgers, M. and Holdcroft, S. 2007. Main-chain, statistically sulfonated proton exchange membranes The relationships of acid concentration and proton mobility to water content and their effect upon proton conductivity. Journal of Materials Chemistry 17 3255-3268. 

Proton Mobility near the Polymer-Water Interface.385... 

Proton conductivities of 0.1 S cm at high excess water contents in current PEMs stem from the concerted effect of a high concentration of free protons, high liquid-like proton mobility, and a well-connected cluster network of hydrated pathways. i i i i Correspondingly, the detrimental effects of membrane dehydration are multifold. It triggers morphological transitions that have been studied recently in experiment and theory.2 .i29.i ,i62 water contents below the percolation threshold, the well-hydrated pathways cease to span the complete sample, and poorly hydrated channels control the overall transports ll Moreover, the structure of water and the molecular mechanisms of proton transport change at low water contents. 

The complications for fhe fheoretical description of proton fransporf in the interfacial region befween polymer and water are caused by the flexibility of fhe side chains, fheir random distributions at polymeric aggregates, and their partial penetration into the bulk of water-filled pores. The importance of an appropriate flexibilify of hydrated side chains has been explored recently in extensive molecular modeling studies. Continuum dielectric approaches and molecular dynamics simulations have been utilized to explore the effects of sfafic inferfacial charge distributions on proton mobility in single-pore environments of Molecular level simulations were employed... 

On the other hand, the merits of such insights are obvious. It would become possible to evaluate the relative importance of surface and bulk mechanisms of PT. The transition from high to low proton mobility upon dehydration could be related to molecular parameters that are variable in chemical synthesis. It could become feasible to determine conditions for which high rates of interfacial PT could be attained with a minimal amount of hghtly bound water. As an outcome of great practical value, this understanding could direct the design of membranes that operate well at minimal hydration and T > 100°C. 

The total electro-osmotic coefficient = Whydr + mo includes a contribution of hydrodynamic coupling (Whydr) and a molecular contribution related to the diffusion of mobile protonated complexes—namely, H3O. The relative importance, n ydr and depends on the prevailing mode of proton transport in pores. If structural diffusion of protons prevails (see Section 6.7.1), is expected to be small and Whydr- If/ ori the other hand, proton mobility is mainly due to the diffusion of protonated water clusters via the so-called "vehicle mechanism," a significant molecular contribution to n can be expected. The value of is thus closely tied to the relative contributions to proton mobility of structural diffusion and vehicle mechanism. ... 

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