Proton transport Grotthuss mechanism

Structure diffusion (i.e., the Grotthuss mechanism) of protons in bulk water requires formation and cleavage of hydrogen bonds of water molecules in the second hydration shell of the hydrated proton (see Section 3.1) therefore, any constraint to the dynamics of the water molecules will decrease the mobility of the protons. Thus, knowledge of the state or nature of the water in the membrane is critical to understanding the mechanisms of proton transfer and transport in PEMs. 

The mechanism of proton diffusion, usually referred as the Grotthuss mechanism, differs from that of other ions. Instead of a self-diffusion process, where the mass and the charge are inseparable, the proton diffusion is the movement of the protonic charge independently from the transport of the particle. This mode of propagation leads to the diffusion of the proton 0 + = 9.3 x 10 cm s being faster than the diffusion of all other ions (D <2 x 10 cm s ), and of the selfdiffusion of the water molecules in water as a solvent (for details see Ref [40]). 

Even if a proton jump along a bond is very rapid, the conductivity is explained by the Grotthuss mechanism (it is not the same proton that jumps). Please notice that this jump is not a rotation it takes place along the bond from one minimum of the potential to another, creating, for short time, an ion H30+ and a correspondent OH-. But, I insist, their concentration is very small. It is out of question to see such species, for example, in scattering experiments. Their impact on the transport properties (self-diffusion, molecular rotations) is totally negligible. 

Recently, we studied the water dependence in mixtures of water and the protonated form of Nafion [53] using both standard force field models and an empirical valence bond model to account for the Grotthuss structural diffusion mechanism of aqueous proton transport. Results showed a transition of an irregularly shaped filamentous (cylindrical) structure in the case of low water (A = 5, where A is defined as the ratio of water molecules to sulfonate groups) to a structure more in accord with the above-discussed models of nano-separation, where larger clusters form which are connected by narrow bridges. A comparison of aqueous cluster sizes indicated, for a simulation time of 30 ns, no percolating clusters for A = 5, whereas at A = 10 most water molecules were located in a connected cluster (see [53]). Other structural... 

The most important property of PVPA is conductivity based on hydrogen bonding, which allovt proton transport via a Grotthuss type mechanism (structural diffusion). The conductivity is the key property of PVPA, which is successfully used to obtain polymer electrolyte membranes by copolymerization or blending with appropriate materials. The applications of these materials have greatly expanded and now have many roles in the modern industrial economy. 

Kreuer etal. [21] provided an in-depth review of the basic mechanisms of transport in proton conductors. Transport of the proton can occur by two mechanisms structural diffusion and vehicular diSusion. Vehicular diffusion is the classical Einstein diffusive motion. The structural diffusion is associated with hopping of the proton along water molecules (the so-called Grotthuss mechanism). In the nanosized confined hydrophihc spaces within the membrane, both mechanisms are operative. What is important here is that the underlying mechanism of transport in PEMs changes as a function the level of hydration. Understanding the nature of these mechanisms and their dependence on the level of hydration and molecular structure is important in the development of advanced PEM materials that are more tolerant of higher temperatures and lower levels of saturation. 

Biochemical ion-transfer reactions, and especially proton-transfer reactions, are enormously important for life. They are used in catalysis of acid-base reactions and enzymatic catalysis, or to establish concentration gradients in living cells. Many biomolecules exhibit proton-transport chaimels through which protons may be transported. These ehannels are designed by the molecule s structure, and generally filled with a small amount of water molecules. The water in this eonfined system is used to transport the proton via a Grotthuss-type mechanism. 

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