Water-soluble soy protein contains a large amount of acidic amino acids, such as glutamic and asparticacids. Since these acidic amino acids possess the –COOH group in their side chain, the water solublesoy protein acts as an acidic polymer. Therefore, we prepared a composite material by the mixing ofacidic protonated soy protein (SPH) and a basic heterocyclic molecule, such as imidazole (Im). TheSPH-Im composite materials showed a thermal stability at ￡130 °C by the acid-base interaction betweenthe acidic –COOH group, related to the side chain of the acidic amino acids, and basic Im molecule.Additionally, the proton conductivity of the SPH-Im composite material increased with the mixing ofthe Im molecule and reached a maximum proton conductivity at R=0.2. This proton conductivity was4.4 ′ 10-4 S cm-1at 130 °C under flowing dry nitrogen. Furthermore, the Ea of the proton conduction inthe SPH-Im composite material was 0.27 – 0.76 eV. These results suggested that the proton conductivemechanism in composite material was based on the anhydrous proton transfer from the –COOH groupto the deprotonated –COOgroup.
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