Determination of the protonation constants of electrochemically polymerized poly(aniline) and poly(o-methylaniline) films

It is shown that the protonation constant of the PANI film depends on the size of the acid counter ion, which is incorporated in the polymer film during electropolymerization in aqueous solution. Protonation constants of log K-HnL(nH,L) = 3.5 (n = 0.57) and 5.4 (n = 0.56) were calculated for the PANI films prepared from a monomer solution containing 1.0 M HCl [small Cl- anion; PANI(Cl)] and 0.1 M DBSA [bulky DBS- anion; PANI(DBS)], respectively. The UV-vis spectra and pH1/2 shows that the electrically conducting protonated ES form of the PANI(DBS) film has a high stability and stays in the electrically conducting ES form between pH 2 and 7. In contrast, the PANI(Cl) film is mainly in the non-conducting EB form already at pH 7. The PoMeANI(Cl) film has a protonation constant of log K-HnL(nH,L) = 3.2 (n = 0.61). It shows, in combination with pH(1/2), that ring substitution lowers the stability of the ES form of PoMeANI(Cl) in comparison to the non-substituted PANI(Cl) film.