Proton conduction, lateral

The xanthenes exist in solution in several different forms depending on pH, as shown in Figure 2 and Table 1 [18]. The emission quantum yield of fluorescein depends on the acidity of the solution, the fluorescence intensity decreasing as the protonated forms of the dye come to predominate with decreasing pH. This pH sensitivity allows fluorescein derivatives to be employed as pH indicators, to measure the pH inside living cells [19-22], at water-lipid interfaces [23], and in the interior of phospholipid vesicles [24]. The sensitivity of fluorescein emission to the pH of the medium has also been used to measure lateral proton conductances at water-lipid interfaces [25-28] and proton translocation across phospholipid vesicles [29] and to determine the electrostatic potential of macromolecules [30, 31]. The pheno-... 

Thermally or photochemically induced proton transfers represent bistable switching processes and are of interest for information storage. A lateral transfer of information on the surface of biological membranes is thought to occur by fast proton conduction through protonic networks [8.234]. 

The formation of spanning H-bonded water networks on the surface of biomolecules has been connected with the widely accepted view that a certain amount of hydration water is necessary for the dynamics and function of proteins. Its percolative nature had been suggested first by Careri et al. (59) on the basis of proton conductivity measurements on lysozyme this hypothesis was later supported by extensive computer simulations on the hydration of proteins like lysozyme and SNase, elastine like peptides, and DNA fragments (53). The extremely interesting... 

Advances in fuel cells were later accelerated by space and defense programs. Fuel cells found initial practical application with the Gemini (1962-1966) and the Apollo (1968-1972) spacecraft missions, and are still used to provide water and electricity for the Space Shuttle. The upgrade in fuel cell performance over the last four decades has been based on the development of new proton-conducting polymers, like Nafion and Gore-tex , ceramics and catalysts, as well as on greater insights into... 

Other elements of PEFC characeteristics which have been modeled recently are the variations of temperature and water content and the associated possible variations in protonic conductivity down the gas flow channel. These types of lateral thermal effects have been dealt with by Fuller and Newman [106] and by Nguyen and White [107]. 

Further modeling work done more recently reveals expected variations of both temperature and water content in a PEFC, together with the associated variations in protonic conductivity along the gas flow channel, that is, along the active surface area of the ME A. Such lateral distributions are evaluated and discussed in Ref. 38. 

In a later work, both the CuCl/KCl molten salt Wacker oxidation system and a [Bu4N][SnCl3] system (melting point 60 °C) was applied to the electrocatalytic generation of acetaldehyde from ethanol by co-generation of electricity in a fuel cell [56]. In the cell set-up, porous carbon electrodes supported with an ionic liquid catalyst electrolyte were separated by a proton conducting membrane (Fig. 5.6-4), and current efficiency and product selectivity up to 87% and 83%, respectively, were reported at 90 °C. 

Experimental studies of temperature-dependent proton mobility have a long and dramatic history. In a modern sense they date back to the works of Johnston [69] and Noyes [70,71 ], followed much later by the studies of the pressure dependence by Eucken [72,73], Gierer and Wirtz [74], Gierer [75], and Franck, Hartmaim and Hensel [76]. Reference [77] gives a comprehensive overview of aqueous proton conductivity and the early experimental data, based on the concept of the excess mobility, responsible for the difference of the observed proton mobihty from the one provided by the classical hydrodynamic motion of the hydronium ion. 

The hydrogen ion H" " cannot exist as a free species in condensed phases its hydration has long fascinated chemists and physicists. Existence of the hydrated proton was first postulated to explain the catalytic effect of the proton in esterification and later to rationalize the conduction of aqueous sulphuric acid solutions , The concept of electrolytic dissociation and consequent conduction in aqueous solutions is a forerunner of the modern notion of the salts themselves as solid electrolytes in the absence of any solvating medium. The parallel is particularly clear for strong mineral acid hydrates where several acid/water compositions of ionic character exist, many of which are proton conducting, and in which proton hydrates and H502 have been identified . 

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