Protonated inorganic anions

The anion exchange mechanism in Table 4.3 is the analog of the reaction in Eq. 4.45, wherein the signs of the valences are reversed B symbolizes a carboxylate group (COO ) and M is replaced by a univalent, inorganic anion that forms outer-sphere complexes with protonated surface hydroxyl or amine groups. This mechanism is not observed often, possibly because of the weakness of the surface complexes involved, but it should be prominent in acidic soils whose clay fraction comprises primarily metal oxides. 

Generally, the protonation of expanded porphyrins at the surface of the electrode membrane is a prerequisite for accommodating the guest anions and yielding their potential response. The electrodes strongly respond to benzoates, but rather less to inorganic anions and saturated aliphatic organic carboxylates. The selectivity sequences for the sapphyrin-based electrodes were found in some cases to deviate from the Hofmeister series. Fluoride preference over chloride and bromide is particularly noted. 

One of the current goals within the field of PEMFCs is to find a nonaqueous, proton-conducting solid electrolyte-membrane system for fuel cells. While these are not PlLs, they have similarities. Proton-exchange materials have been produced from imidazolium cations with inorganic anions, such as benzimidazole with TFSl, formed into a polymer with mai similarities to PlLs. 

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