Protonation effects in polyaniline Langmuir films investigated by surface potential measurements

Doping parent polyaniline (PANI) and its derivatives has been proven essential for obtaining stable Langmuir monolayers that are amenable to transfer onto solid substrates by the Langmuir-Blodgett (LB) technique. The doping process is generally carried out on the Langmuir trough, by spreading the polymer on an acidic subphase. The degree of doping affects the monolayer surface potential as the positive contribution from the double-layer increases with increasing protonation of the polymer molecules. One could then expect that the surface potential should increase with the acidic concentration in the subphase, but instead the potential diminishes for increasing concentration in the range 10(-4) - 10(-1) M. Interestingly, the results may be discussed in terms of the simple Gouy-Chapman theory, despite the complexity of the macromolecules involved.