The Anomalous Diffusion of Protons
The Grotthuss mechanism, along with the relative lightness and small size of the proton, explains the unusually high diffusion of the proton relative to other the typical ionic diffusion of other cations (Table 1) which is due simply to random thermal motion i.e. Brownian motion. Quantum tunnelling becomes more probable the smaller the mass of the cation is, and the proton is the lightest possible stable cation, thus there is a minor effect from quantum tunnelling also, although it dominates at low temperatures only.
Table 1
| Cation | Mobility / cm2 V−1 s−1 |
| NH4+ | 0.763×10−3 |
| Na+ | 0.519×10−3 |
| K+ | 0.762×10−3 |
| H+ | 3.62×10−3 |
Read more about this topic: Grotthuss Mechanism
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