Grotthuss conduction

Although the Grotthuss conduction mechanism is also possible for HO ions in aqueous media, the ionic conductivity of AEM membranes are expected to be lower than the PEM for the same ion-exchange capacity. In fact, the ionic conductivity of ammonium quaternized AEM at room temperatiue spreads over a wide range of values, from a few mS.cm for QPEK-C [206], QSEBS [212], and QPVA [213], up to 60 and 140 mS.cm for QPAES [209] and QPPEKS [207], respectively. 

Extended chains of hydrogen-bonded water molecules proposed by Christian von Grotthuss in the early 19th century to explain electrolyte conductance in water. ... 

Ab initio methods provide elegant solutions to the problem of simulating proton diffusion and conduction with the vehicular and Grotthuss mechanism. Modeling of water and representative Nation clusters has been readily performed. Notable findings include the formation of a defect structure in the ordered liquid water cluster. The activation energy for the defect formation is similar to that for conduction of proton in Nafion membrane. Classical MD methods can only account for physical diffusion of proton but can create very realistic model... 

To conclude this section on proton transfer, we have examined an alternative mechanism to that of von Grotthuss for proton conduction [105-110], We carried out B3LYP/6-31+G and PW91 DFT calculations on model compounds (1,2,3,4-tetrasubstituted benzenes, e.g., 99) showing that these compounds could play the role of proton conductors [104],... 

This mechanism of prototropic conduction is similar to that proposed by -> Grotthuss for the electrolytic conduction, and therefore is sometimes also... 

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

These results emphasize the fact that ions produced by self-ionization of the solvent, e.g., HsO and 0H in water, ROHt and R0 in alcohols, and NHi" and NHJ in liquid ammonia, do not of necessity possess abnormal conductance, although they frequently do so. It is seen from Table XIX that the conductance of the hydrogen ion in liquid ammonia, i.e., NHi", is normal the same is true for the NHi" ion. The anilinium and pyridinium ions also have normal conductances in the corresponding solvents. The conductance of the HSOr ion in sulfuric acid as solvent is, however, abnormally high it is probable that a Grotthuss type of... 

Noda et al. [ 168] reported the details of Bronsted acid-based ionic liquids consisting of a monoprotonic acid and an organic base, in particular solid bis(trifluorometha-nesulfonyl)amide (HTFSI) and solid imidazole (Im) mixed at various molar ratios to form liquid fractions. Studies of the conductivity, H NMR chemical shift, selfdiffusion coefficient, and electrochemical polarization results indicated that, for the Im excess compositions, the proton conductivity increased with an increasing Im molar fraction, with rapid proton-exchange reactions taking place between the protonated Im cation and Im. Proton conduction was found to occur via a combination of Grotthuss- and vehicle-type mechanisms. Recently, Nakamoto [169] reported the... 

Figure 4. Leakage vs, conduction models for the permeation of protons. Left transient water wires form very infrequently in the nonpolar environment provided by lipid bilayers, but when they do, they could translocate just one proton very rapidly before breaking up only the hop step ofGrotthuss takes place. Right in polar channels such as GA, water wires are much more long-lived, which is consistent with their rapid relay of proton via a complete Grotthuss mechanism involving both hop and turn steps (l(f s ).

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. 

---