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

A method was developed for the determination of the protonation constants (KHnLnH,L), the stoichiometric factor of the protonation reaction (n) and pH of the half-color transition point (pH1/2) of three different types of electrochemically polymerized poly(aniline) (PANI) and poly(o-methylaniline) (PoMeANI) films. The protonation constants describe the reversible emeraldine base (EB) to emeraldine salt (ES) transition. The calculations of the constants were based on absorbance data obtained from UV–vis measurements of the polymer films equilibrated in pH buffer solutions between pH 2 and 12.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 KHnLnH,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 Po  MeANI(Cl) film has a protonation constant of log KHnLnH,L=3.2 (n = 0.61). It shows, in combination with pH1/2, that ring substitution lowers the stability of the ES form of PoMeANI(Cl) in comparison to the non-substituted PANI(Cl) film.