The Grotthuss Mechanism

It was demonstrated earlier in several ways that although it may well apply to and Na, Stokes law certainly does not apply to the proton, the nonconforming ion. There may be a slow, viscous drifting of through the solvent. However, it does not explain the proton s movement. 

A Swedish worker, Grotthus, had noticed that in a row of marbles in contact, the collision of the marble at one end of the row with a new marble caused a marble at the far end to detach itself and go off alone (Fig. 4.121). This sort of movement would provide a rapid way for an ion to appear to travel through a solution. There would then be no need for a whole H3O to lumber along, taking the proton with it. Could a proton 

The Machinery of Nonconformity A Closer Look at How the Proton Moves 

If one is going to consider protons actually jumping from one quasi-stationary water molecule to the next, the classical view would be to ask what fraction of them would be sufficiently activated to get over the top of the corresponding energy barrier. Another possibility was discussed not long after the introduction of quantum mechanics in 1928 by Bernal and Fowler. In a famous paper of 1933, they applied quantum mechanics to the possibility of tunneling through a barrier. 

It is not reasonable just to say the proton transfers to a water molecule, because that is a fairly vague statement. One starts asking questions like Can it transfer to a water molecule when it arrives from any direction Hardly, for it has to find an orbital on O to form a bond with—and orbitals have direction. Considering that water molecules librate and sometimes break H bonds and rotate, the next question which will have to be researched is Does a water get into a proton-receiving posture all by 

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Agmon, N. 1995. The Grotthuss mechanism. Chemical Physics Letters 244 456—462. 

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. 

Similar thermal effects have also been noted in hydroxide water clusters [54]. While proton transfer is widely accepted to follow the "Grotthuss" mechanism... 

Figure 10 Proton conductivity according to the Grotthuss mechanism. (Reprinted from Ref. 131 2003, with permission from Elsevier)...

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]). 

Figure L Schematic representation of hop and turn steps in the Grotthuss mechanism along a hydrogen-bonded chain (see text).

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. 

We saw on p. 384 that the observed rate constant for the reaction between hydrogen and hydroxide ions in aqueous solution is 1.3 x 10 M s b This very large value is explained partly by the fact that the ions are oppositely charged, so that the di(fusion rates are enhanced by the electrostatic attraction. In addition, the hydrogen and hydroxide ions move by the Grotthuss mechanism described on p. 286, which enables them to move at abnormally high speeds. 

One of the most interesting systems is the class of imidazoles. They are selfdissociation compounds with high proton conductivity (>100mS cm ) without any acid doping. Further enhancement of the conductivity and also the thermal stability of the system can be realised by the addition of acidic components [45]. This is due to the proton transfer via the Grotthuss mechanism. 

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