Proton conduction mechanism hydronium ions

The mechanism by which the hydronium ion moves in water is very diflferent than that by which most other ions move. As shown in fig. 6.13, the conductance process involves the transfer of a proton between two adjacent water molecules with the proton in a hydrogen bond between these molecules. Because of the relative orientation of the water molecules with respect to one another, the net motion of the proton is not exactly in the direction of the electrical field but rather in zig-zag motion about this direction. This feature of the net forward motion can also be described in terms of rotation of the water molecule to which the proton is transferred so that the orbitals on this molecule are favorably oriented with respect to the direction of the field. The net effect is that the value of at 25°C is 349.8 cm moU, that is, much higher than any other monovalent ion. 

Protons can propagate by two mechanisms. One is by the viscous flow of a complex H+(H20) where n = 1, 2, 3, 4, the second is by the Grotthuss mechanism, hopping of a proton from one hydronium H3O+ to a nearby water molecule. The latter mechanism has some similarity to hopping conduction of ions in solids as discussed in the following text. 

In the vehicle mechanism, proton movement occurs with the aid of moving carriers (e.g. H2O, classically in the form of hydronium ions, H30 ), and both water and protons diffuse at a similar rate. The overall proton conductivity is mainly determined by the diffusion coefficient of the vehicles. 

Because the proton in water is strongly hydrated, the hydronium ion, HjO+Caq), is formed in an aqueous solutiou, and conductivity in H+(aq)-containing solutions has a special mechanism. There is not any hydrodynamic movement of the hydronium ion when a potential is applied. Actually, the charge is moving from one hydronium structure to another one. This type of conductivity is called the hopping (Grotthuss) mechanism and is schanaticaUy illustrated in Figure 3.21. 

The WU of the manbranes is a key factor for sustaining the proton conductivity because water molecules play an important role as proton transportation carriers in manbranes. As protons migrate with the water as hydronium ions in the vehicle mechanism or via exchange of hydrogen bonding with water molecules in the... 

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