Surface electric properties of polypyrrole in aqueous solutions

The electric properties, for example, zeta potential and surface charge, of a solid in contact with an aqueous solution play an important role in various interfacial and colloidal phenomena, as well as in adsorption and filtration processes. In this study, the zeta potentials of chloride-doped polypyrrole (PPyCl) particles were investigated as a function of solution pH values. It was found that PPyCl particles had a zero point of potential at about pH = 10, and the zeta potentials varied with solution pH values, which also greatly depended on the different pH ranges. With a treatment of PPyCl in a sodium hydroxide solution at pH = 14 (denoted as D-PPY), the zeta potentials of D-PPy at various solution pH values became significantly different from those of PPyCl and the zero point of zeta potential of D-PPy appeared at about pH = 3.5. A further treatment of D-PPy in a hydrochloric acid solution at pH = 0 (denoted as R-PPy), however, restored the zeta potentials of R-PPy at various solution pH values to almost the same as those of PPyCl. The changes of zeta potentials of polypyrrole (PPyCl, D-PPy, or R-PPy) with solution pH values can be attributed to the dissociation of the dopant anions (i.e., Cl-), the protonation/deprotonation of the nitrogen atoms, and the selective adsorption of OH- from the bulk solution, but polypyrrole is also believed to undergo molecular structure or composition changes at the solid/solution interface under extreme solution pH conditions, which causes PPyCl, D-PPy, or R-PPy to exhibit different surface electric properties in aqueous solutions of the same pH values. The surface charge densities of PPyCl and D-PPy at pH = 6.5 were evaluated from X-ray photoelectron spectroscopy and X-ray diffraction surface analyses, the Gouy-Chapman theory, and the experimental zeta potentials. The results indicated that only a small fraction of the positively charged nitrogen atoms on the PPyCl surface contributed to the positive zeta potential. Adsorption experiments also showed that the surface electric properties of polypyrrole in aqueous solutions of different pH values greatly affected its performance as an adsorbent in removing a substance, such as humic acid, from water.