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Surface Electric Properties of Polypyrrole in Aqueous Solutions

2003; American Chemical Society; Volume: 19; Issue: 26 Linguagem: Inglês

10.1021/la034893v

1520-5827

X. Zhang, Renbi Bai,

Advanced Sensor and Energy Harvesting Materials

The electric properties, for example, ζ 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 ζ 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 ζ potential at about pH = 10, and the ζ 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 ζ potentials of D-PPy at various solution pH values became significantly different from those of PPyCl and the zero point of ζ 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 ζ potentials of R-PPy at various solution pH values to almost the same as those of PPyCl. The changes of ζ 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 ζ potentials. The results indicated that only a small fraction of the positively charged nitrogen atoms on the PPyCl surface contributed to the positive ζ 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.