Langmuir 2003, 19, 10703-10709
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Surface Electric Properties of Polypyrrole in Aqueous Solutions X. Zhang and Renbi Bai* Department of Chemical and Environmental Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 Received May 23, 2003 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.
Introduction Intrinsic conducting polymers (ICPs) have attracted great interest over the past two decades. Polypyrrole is one of the most commonly investigated ICPs due to its unique properties of high electrical conductivity, relatively good environmental stability, and ease of preparation.1-4 Because the oxidation potential of polypyrrole is lower than that of pyrrole monomer,5 the polymer can be simultaneously oxidized with pyrrole monomer during the polymerization reaction. Hence, polypyrrole prepared by both chemical and electrochemical polymerization reactions is usually in its oxidation state and carries charges in the polymer (i.e., some of the nitrogen atoms in polypyrrole are positively charged). To maintain charge neutrality, some of the counteranions present in the polymerization solution are incorporated into the growing polymer during the polymerization. The existence of positively charged nitrogen atoms in polypyrrole provides a good prospect for its applications in adsorption or filtration separation. For example, Minehan et al. reported the use of polypyrrole for binding of DNA.6,7 Zhang and * Corresponding author. Fax:
[email protected].
(65) 6779 1936. E-mail:
(1) Rodrı´guez, J.; Grande, H. J.; Otero, T. F. In Handbook of Organic Conductive Molecules and Polymers; Nalwa, H. S., Ed.; John Wiley & Sons Ltd.: New York, 1997; Vol. 2, Chapter 10. (2) Wang, L. X.; Li, X. G.; Yang, Y. L. React. Funct. Polym. 2001, 47, 125. (3) Deronzier, A.; Moutet, J. C. Coord. Chem. Rev. 1996, 147, 339. (4) Bhattacharya, A.; De, A. Prog. Solid State Chem. 1996, 24, 141. (5) Bre´das, J. L.; Street, G. B. Acc. Chem. Res. 1985, 18, 309. (6) Minehan, D. S.; Marx, K. A.; Tripathy, S. K. Macromolecules 1994, 27, 777. (7) Minehan, D. S.; Marx, K. A.; Tripathy, S. K. J. Macromol. Sci., Pure Appl. Chem. 2001, 38, 1245.
Bai studied the applications of polypyrrole for the removal of humic acid or clay particles from aqueous solutions.8-10 They found that the positively charged surface of polypyrrole largely enhanced the adsorption of negatively charged colloidal particles or other substances through electrostatic interaction or formation of surface complexes.8-10 From material research, polypyrrole has been known to undergo protonation or deprotonation reactions when it was immersed in an alkaline or acid solution, resulting in the change of its surface charges (the surface changed between a charged and a neutral state).11 The protonation/ deprotonation process of polypyrrole may be presented in Scheme 1.8,9 The capability of polypyrrole to reversibly transform between its oxidized (or charged) and neutral state has made it possible for polypyrrole to be used as a functional material in the manufacture of ion-selective membranes,2 in the fabrication of pH sensors and biosensors, and so forth.12-15 For all the engineering applications mentioned above, the surface electric properties of polypyrrole in an aqueous solution play an important role. The surface electric properties of a solid in an aqueous solution can be characterized by its ζ potentials and surface charges, both (8) Bai, R. B.; Zhang, X. J. Colloid Interface Sci. 2001, 243, 52. (9) Zhang, X.; Bai, R. B. Langmuir 2001, 18, 3459. (10) Zhang, X.; Bai, R. B. J. Mater. Chem. 2002, 12, 2733. (11) Pei, Q.; Qian, R. Synth. Met. 1991, 45, 35. (12) Nishizawa, M.; Matsue, T.; Uchida, I. Anal. Chem. 1992, 64, 2642. (13) Nishizawa, M.; Matsue, T.; Uchida, I. Sens. Actuators, B 1993, 13, 53. (14) Ge, H.; Liu, Y. Sens. Actuators, B 1994, 21, 57. (15) Fang, Y.; Tan, S. N.; Ge, H. Sens. Actuators, B 1996, 32, 33.
10.1021/la034893v CCC: $25.00 © 2003 American Chemical Society Published on Web 11/18/2003
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Scheme 1. Schematic Presentation of Protonation/ Deprotonation Behavior of Polypyrrole
of which are also dependent on the pH value and the ionic strength of the solution. It has been suggested that the different mechanisms of surface charge formation may be recognized by ζ potential versus pH plots at a constant ionic concentration,16 and ζ potential study may provide information on the surface complex formation at the solid/ liquid interface.17,18 However, a more detailed study of the electric properties of polypyrrole in aqueous solutions of different pH values has not been reported in the literature. In this work, the ζ potentials of chemically synthesized polypyrrole under various solution pH conditions (at a constant ionic strength) were investigated; the apparent surface charge density of polypyrrole was estimated from X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analyses, the Gouy-Chapman theory, and the experimentally measured ζ potentials; and, as an application assessment, the effect of the surface electric properties of polypyrrole in aqueous solutions on the adsorption behavior of humic acid was examined through a series of batch adsorption experiments under various solution pH conditions. Experimental Section Synthesis and Preparation of Polypyrrole Particles. Pyrrole (99%) and FeCl3‚6H2O (97%) were purchased from the Aldrich Chemical Co. Pyrrole was distilled twice prior to use. Pyrrole (1.75 mL, 0.025 mol) was added in droplets into 150 mL of FeCl3‚6H2O solution (0.05 M) in a 250 mL beaker at room temperature. The contents in the beaker were continuously stirred, and the polymerization of pyrrole to polypyrrole was allowed to proceed for 3 h. Then, the resultant black precipitates, that is, polypyrrole particles, were filtered, thoroughly washed with deionized (DI) water, methanol, and DI water again (pH about 6.5), and dried in a vacuum desiccator to a constant weight. The polypyrrole particles prepared in this way are referred to as PPyCl (i.e., chloride-doped polypyrrole) in this paper. To study the effect of deprotonation of PPyCl under an extreme pH condition, some PPyCl particles were treated in a sodium hydroxide solution (1 M, pH ) 14) for a time up to 12 h. Then, the treated polypyrrole particles were removed from the solution, washed with DI water until the washing liquid reached about pH 6.5, and dried in a vacuum desiccator to a constant weight. This type of polypyrrole particle is denoted as D-PPy in the discussion (no matter if the treatment was 12 h or shorter). Again, some of the D-PPy particles were further treated in hydrochloric acid (1 M, pH ) 0) for a time up to 12 h to examine their behavior of reprotonation. After the treatment, the particles were separated from the acid solution, washed with DI water till the washing liquid reached about pH 6.5, and dried in a vacuum desiccator to a constant weight. The polypyrrole particles prepared from this process are labeled as R-PPy in this study (no matter if the treatment was 12 h or shorter). (16) Schwarz, S.; Buchhammer, H. M.; Lunkwitz, K.; Jacobasch, H. J. Colloids Surf., A 1998, 140, 377. (17) Matsumoto, H.; Koyama, Y.; Tanioka, A. Langmuir 2001, 17, 3375. (18) Sarmiento, F.; Ruso, J. M.; Prieto, G.; Mosquera, V. Langmuir 1998, 14, 5725.
Zhang and Bai XPS and XRD Analyses. XPS was used to characterize the various types of polypyrrole particles (PPyCl, D-PPy, or R-PPy) in this study. XPS analyses were carried out on a VG ESCALAB MKII spectrometer with an Al KR X-ray source (1486.6 eV photons). To compensate for surface charging effects, all binding energies were referenced to the C 1s neutral carbon peak at 284.6 eV. Surface elemental stoichiometries were determined from sensitivity-factor-corrected peak area ratios, and the software XPSpeak 4.1 was used to fit the XPS spectral peaks. XRD measurements were performed on a Shimadzu XRD6000 diffractometer using Ni-filtered Cu (KR) radiation operated at 40 kV and 30 mA. The 2θ range started from 2θ of 1.5°. The prepared polypyrrole particles were carefully ground with an agate mortar and pestle before the analyses. ζ Potential Measurements. ζ Potentials were measured with a Zeta Plus4 instrument (Brookhaven Corp.). A 0.25 g amount of PPyCl, D-PPy, or R-PPy particles was added to a vial with 25 mL of 0.001 M NaCl solution. The pH value of the solution was adjusted with 0.1 M HCl or 0.1 M NaOH solution to a desired value. The contents in the vial were vibrated in an ultrasonic bath for 20 s, and the supernatant with small fragments from the particles in it was then decanted and used for ζ potential measurement. The ζ potentials measured in this way have been assumed to be the ζ potentials of the polypyrrole particles in the same solution conditions.19 To study the kinetic changes of ζ potentials of polypyrrole particles during the base or acid treatment, PPyCl or D-PPy particles were placed into 1 M NaOH or 1 M HCl solution, respectively, for different durations (from 15 s to 30 min). Then, similar procedures as above were used to analyze the ζ potentials of the D-PPy or R-PPy particles in a solution of pH ) 6.5 and with 0.001 M NaCl for comparison. Adsorption Experiments. As an assessment of polypyrrole application, batch adsorption experiments were conducted to evaluate how the electric properties of polypyrrole particles in aqueous solutions affected its adsorption performance for organic pollutants in water. Humic acid (supplied by Aldrich Chemical Co.) was used as a model organic compound because of its negative ζ potentials in solutions at pHs above 1.8.8-10 Polypyrrole was coated on glass beads by a method described elsewhere8 and was used as an adsorbent. The adsorbent with the PPyCl surface is referred to as PPyCl adsorbent, and the adsorbent with the D-PPy surface (i.e., treated in 1 M NaOH solution) as D-PPy adsorbent in the discussion that follows. In the batch adsorption experiments, a 10 g amount of the polypyrrole adsorbent was added into 75 mL of humic acid solution (15 mg L-1 concentration in a 150 mL flask) at room temperature and stirred on an orbit shaker operated at 100 rpm. Small volumes of HCl and NaOH solutions were added to adjust the initial solution pH value to a desired level. The concentrations of humic acid solution were determined by an ultraviolet-visible spectrometer (Hitachi UV2000). The amount of humic acid adsorbed per unit weight of the polypyrrole adsorbent at time t, q(t) (mg g-1), was calculated from the mass balance equation as
q(t) )
(C0 - Ct)V m
(1)
where C0 and Ct (mg L-1) are the initial humic acid concentration and the humic acid concentrations at any time t, respectively, V is the volume of the solution, and m is the weight of the polypyrrole adsorbent.
Results and Discussion ζ Potentials of Polypyrrole Particles. The conversion of ζ potentials from the measured electrophoretic mobilities is usually done through the Smoluchowski equation,20 that is,
ζ ) ηµe/
(2)
(19) Bai, R. B.; Tien, C. J. Colloid Interface Sci. 1999, 218, 488. (20) Hunter, R. J. Zeta Potential in Colloid Science: Principles and Applications; Academic Press: New York, 1981.
Surface Electric Properties of Polypyrrole
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Figure 1. ζ Potentials of PPyCl, D-PPy, and R-PPy as a function of solution pH values (in 0.001 M NaCl solution).
where η is the absolute viscosity of the aqueous solution, is the dielectric permittivity of the aqueous solution (equal to 0r, 0 being the permittivity of a vacuum and r the relative permittivity of the aqueous solution), and µe is the electrophoretic mobility of the dispersed solid particles. The Smoluchowski equation is usually valid when the Debye length, κ-1, is much smaller than the mean radius of the particles, R0 (i.e., κR0 . 1). κ is the reciprocal of the electrical double-layer (EDL) thickness of the particles in the aqueous solution and is defined as
κ)
x
∑cizi2
1000NAe2 kT
(3)
where NA is Avogadro’s number, ci and zi are the molar concentration and the valence of the ith ion species in the bulk solution, k is the Boltzmann constant, and T is the absolute temperature. The κ-1 value for water with 0.001 M NaCl at 25 °C may be calculated to be 9.6 nm. The sizes of polypyrrole particle fragments in the supernatants for ζ potential analyses, as estimated from a Coulter LS230 Multisizer (Coulter Corp.) in the study, were in the range from several hundred nanometers to a few micrometers. So, the use of the Smoluchowski equation is generally valid in this study. The measured ζ potentials of PPyCl at various solution pH values are shown in Figure 1. The point of zero ζ potential appeared at about pH ) 10. At pHs below 10, the ζ potentials were positive, and over the range of pH ) 8 to pH ) 3, they remained relatively constant (with only minor increases). The positive ζ potentials however increased dramatically with the decrease of solution pH values at pHs below 3. From pH ) 9, the positive ζ potentials decayed rapidly with the increase of solution pH values, and the ζ potentials became negative at pHs above 10 and reached a substantial value of about -50 mV at pH ) 11. The doping density of Cl- in PPyCl prepared through chemical polymerization has often been reported to be at about 0.25-0.3 per pyrrole monomer unit.21 In this study, XPS measurements were made to determine the doping density, that is, the proportion of positively charged nitrogen atoms in PPyCl. As shown in Figure 2a, the N 1s core-level spectra of PPyCl can be decomposed into four components. The peaks of imine (-Nd) and amine (-NH-) nitrogen atoms are centered at binding energies (BEs) of 397.6 and 399.4 eV, respectively. The peaks at (21) Kang, E. T.; Neoh, K. G.; Ong, Y. K.; Tan, K. L.; Tan, B. T. G. Macromolecules 1991, 24, 2822.
Figure 2. XPS N 1s core-level spectra of (a) PPyCl and (b) D-PPy.
400.6 and 402.2 eV can be assigned to two high oxidation states of the nitrogen atoms or protonated nitrogen atoms with positive charges (i.e., NI+ and NII+) in PPyCl. The proportion of positively charged nitrogen atoms was found to be about 26% (in terms of [N+]/[N]). The positively charged nitrogen atoms in PPyCl can be expected to contribute to the positive surface charges and therefore the positive ζ potentials of PPyCl in aqueous solution because the doped ionizable counteranions, Cl-, on the surface of PPyCl may transfer into the bulk solution, as shown in eq 4:
where δ ()0.25-0.3) is the doping density, n is the average number of pyrrole monomers in PPyCl that contain one unit of positive charge (with nδ ) 1), and X- represents the counteranion, that is, Cl-, in this case. The dissociation of the Cl- from the surface of PPyCl in the aqueous solutions may be assumed to be primarily responsible for the positive ζ potentials of PPyCl particles observed at a solution pH of about 6.5 (which is equal to the pH when the PPyCl particles were washed in the synthesis), due to the generation of PPy+ (PPy+ denotes PPyCl with Cldissociated). The decrease of solution pH values will exponentially increase the H+ concentrations and can lead to the protonation of the nitrogen atoms or the selective adsorption of the H+ ions on the surfaces of PPyCl. As a result, the positive ζ potentials of PPyCl increased gradually with the decrease of solution pH values from 6.5 to 3 but significantly at pHs below 3. The reaction may be represented by
PPyCl + H+ + Cl- T PPyClH+ + Cl-
(5)
Pei and Qian also proposed that PPyCl may be protonated in a strong acid as follows:11 + -Py+H- + H + Cl T -Py+HH+Cl 2Cl-
(6)
where -Py+H- represents a PPy chain segment of 3-4 pyrrole rings with a positively charged nitrogen atom. The reactions in eqs 5 and 6 can be used to explain the observed increase of the positive ζ potentials of PPyCl at pH < 6.5 in Figure 1.
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On the other hand, the increases of solution pH at pH > 6.5 will reduce the H+ concentrations and increase the OH- concentrations in the aqueous solution. This can lead to the deprotonation of the protonated nitrogen atoms on PPyCl, the selective adsorption of OH- ions on PPy+, and possibly the replacement of a Cl- in the vicinities of PPyCl or PPy+ by one or more of the OH- ions from the solution (OH- being physically smaller than Cl-). These reactions may be described as
PPyClH+ + Na+ + OH- T PPyCl + H2O + Na+ (7) PPy+ + Na+ + OH- T PPy+OH- + Na+
(8)
PPyCl + Na+ + OH- T PPyClOH- + Na+
(9)
-
+
-
PPyClOH + Na + OH T PPy+(OH-)2 + Cl- + Na+ (10) Equations 7 and 8 can explain the decay of the positive ζ potentials of PPyCl at pHs greater than 6.5, and eqs 9 and 10 explain the negative ζ potentials of PPyCl retained at pH > 10, as shown in Figure 1. The ζ potentials of D-PPy at various solution pH values are also shown in Figure 1. As can be seen from the figure, the ζ potentials of D-PPy are significantly different from those of PPyCl. The point of zero ζ potential of D-PPy particles was at about pH ) 3.5, in contrast with pH ) 10 for PPyCl. At each solution pH value below 11, the ζ potential of D-PPy was always lower or more negative than that of PPyCl. Since the ζ potentials of D-PPy were not restored to those of PPyCl at the same solution pH values, the results imply that the treatment of PPyCl in a strong sodium hydroxide solution has resulted in a molecular structure or composition change of the D-PPy surface from that of PPyCl. This may be due to the introduction and therefore the presence of oxygen atoms in D-PPy, as suggested for example in eq 10. Elemental analyses of D-PPy typically showed the presence of oxygen. The electronegativity of the oxygen atoms on the surfaces of D-PPy can make the water molecules or the OH- ions near the solid-liquid interface arrange in such a way that the hydrogen atoms face the D-PPy surface and the oxygen atoms face the bulk solution. This is the so-called flopdown state, and the flopped-down dipoles can create greater negative ζ potentials.22 The N 1s XPS spectrum of the D-PPy particles in Figure 2b showed the complete disappearance of the positively charged nitrogen atoms on D-PPy, and the XPS wide survey scan of D-PPy particles in Figure 3 revealed the presence of oxygen atoms and the absence of Cl- (Cl 2p at about 197 eV was not observed) on D-PPy. These results suggest that the strong base treatment caused a series of sequential reactions as indicated from eq 7 to eq 10 and the surface of D-PPy after the treatment was eventually in a state dominated by PPy+(OH-)2, which made D-PPy show completely different surface electric properties from those of PPyCl, even under the same solution pH values. Figure 1 also includes the ζ potentials of R-PPy. The ζ potentials of R-PPy versus solution pH values are almost the same as those of PPyCl. The treatment of D-PPy in a strong HCl solution hence appeared to have reversed the sequential reactions from eq 10 to eq 7 and restored the surface of R-PPy to that of PPyCl (or PPyClH+), resulting in R-PPy with the same surface electric proper(22) Bockris, J. O’M.; Reddy, A. K. N. Modern Electrochemistry; Plenum Press: New York, 1970; Vol. 2, Chapter 7.
Figure 3. XPS survey scan showing the major elements of D-PPy.
Figure 4. Variations of the ζ potentials of PPyCl immersed in a 1.0 M NaOH solution and D-PPy immersed in a 1.0 M HCl solution versus immersion time (ζ potentials were measured in a 0.001 M NaCl solution at pH ) 6.5).
ties as that of PPyCl. In fact, the polymerization of pyrrole to polypyrrole during the preparation of PPyCl actually took place in a solution pH of about 1-2 (the pH of the FeCl3 solution) that was quite close to the pH in the acid treatment for obtaining R-PPy. In Figure 4, greater details are presented for the dynamic changes of the ζ potentials of D-PPy and R-PPy particles versus the times at which the particles were placed in the strong sodium hydroxide solution (1 M NaOH) or strong hydrochloric acid solution (1 M HCl). All the measurements of the ζ potentials in this case were made in a near neutral pH (pH ) 6.5) solution for comparison. It is observed from Figure 4 that the ζ potentials of the particles changed rapidly with the treatment time in the strong sodium hydroxide solution or strong hydrochloric acid solution. For example, the zeta potentials of D-PPy changed from about 42 mV for PPyCl at t ) 0 to about -50 mV after 5-15 min of treatment in the strong sodium hydroxide solution. Similarly, the ζ potentials of R-PPy changed from about -40 mV for D-PPy at t ) 0 to about 45 mV after 5-10 min of treatment in the strong hydrochloric acid solution. The changes were especially fast within the first 1 or 2 min. Hence, the treatment in a strong base solution or a strong acid solution at least intensified the deprotonation or reprotonation process on the surface, although it may also cause possible structural or molecular changes at the polypyrrole/water interface. Both of them can affect the surface charges or ζ potentials of polypyrrole particles in an aqueous solution.
Surface Electric Properties of Polypyrrole
Langmuir, Vol. 19, No. 26, 2003 10707 Chart 1. Model Arrangement of the Chains in a Layer of Undoped Polypyrrole
Figure 5. A diagram schematically showing the various surface charge densities and electric potentials defined in the discussion for a polypyrrole particle in an aqueous solution.
Evaluation of Surface Charge Densities. ζ Potential is a dynamic surface electric property measured at a shear plane for a solid suspended in a solution (see Figure 5). Depending on the environmental conditions, the shear plane may be located at a position from the GouyChapman plane to, at the extreme case, the Stern plane. The electrostatic double-layer theory may be used to establish a correlation between the ζ potentials and the surface charge densities. As illustrated in Figure 5, for overall electrical neutrality of a solid enclosed by the double layers, there is
σs + σi + σd ) 0
(11)
where σs is the surface charges per unit area of the solid surface, σi is the charge density of the adsorbed counterions in the adsorbed layer (within the Stern plane), and σd is the charge density of the counterions in the diffusion layer (between the Stern plane and the Gouy-Chapman plane). The charge densities of σi and σd are in terms of per unit projected area on the solid surface. The diffusion charge density, σd, may be calculated using the Gouy-Chapman equation as23
σd ) -
zeψd 2κkT sinh ze 2kT
(12)
where κ, k, T, , and z have the same definitions as in eqs 2 and 3, and ψd is the electric potential at the interface between the adsorption layer and the diffusion layer (i.e., at the Stern plane). In general, the accurate determination of ψd is difficult, if not impossible, and ψd may be often approximated by the ζ potential. Since one is more interested in the electric properties at the shear plane for a solid in a solution, the charge density σd may be further divided into σdi and σdo (σd ) σdi + σdo, see Figure 5). Then, with the application of eq 12, one may have
simplified to give a relationship of σs + σi + σdi versus ζ as
σs + σi + σdi ) 0.3713 sinh(19.46ζ)
(15)
where σs, σi, and σdi are all expressed in µC cm-2 and ζ is in V. From eq 15, the charge densities of σs + σi + σdi can be calculated, for example, as 0.353 and -0.279 µC cm-2 for PPyCl and D-PPy at pH ) 6.5 with the ζ potentials given in Figure 1. These charge densities indicate the extent of the surface charges originating from the oxidized or protonated nitrogen atoms in polypyrrole counterbalanced by the dopant anions or the anions adsorbed from the bulk solution. For PPyCl at pH ) 6.5, the positive value of 0.353 µC cm-2 suggests that, at least, some of the dopant anions of Cl- on the PPyCl surface dissolved into the solution outside the shear plane, which led the polypyrrole particles in the solution to have net positive charges at the shear plane (more oxidized or protonated nitrogen atoms than Cl- within the shear plane). The negative value of -0.279 µC cm-2 for D-PPy however indicates that either the amount of oxidized or protonated nitrogen atoms on polypyrrole particles was reduced or some of the anions from the bulk solution were adsorbed onto the surface of polypyrrole or into the shear plane during the strong base treatment, which caused the polypyrrole particles in the solution to have net negative charges at the shear plane. The surface charge density, σs, may be further estimated from the structure of the polypyrrole molecule and the number of positively charged nitrogen atoms per unit surface area of polypyrrole particles. Geiss et al.24 proposed a layered structure of undoped polypyrrole (see Chart 1), in which the polymer chains lie in the (a, b) plane with a ) 8.2 Å and b ) 7.35 Å, and the interlayer spacing is equal to c ) 3.41 Å. In this study, the X-ray diffraction spectrum of PPyCl was obtained (see Figure 6) and the three peaks were centered at around 2θ ) 9°, 21.5°, and 26°. Bragg’s law may be used to relate the diffraction angles with the atom spacing distance as
n j λ ) 2d sin θ
(16)
(14)
where λ is the X-ray wavelength (1.54 Å), n j is an integer (often 1 in the analysis), d is the spacing of the lattice atoms, and θ is the diffraction angle. From the three peaks in Figure 6, the values of a, b, and c for PPyCl can be determined from eq 16 as a ) d ) 9.8 Å, b ) 2d ) 8.2 Å, and c ) d ) 3.4 Å, respectively. The values of a ) 9.8 Å (0.98 nm) and b ) 8.2 Å (0.82 nm) for PPyCl are greater than those of a ) 8.2 Å and b ) 7.35 Å for the undoped
where σs + σi + σdi represents the net or apparent charge density of the solid at the shear plane. At 25 °C and in a bulk solution with 0.001 M sodium chloride, eq 14 can be
(23) Hunter, R. J. Foundations of Colloid Science; Clarendon Press: Oxford, 1991; Vol. 1, Chapter 6. (24) Geiss, R. H.; Street, G. B.; Volksen, W.; Economy, J. IBM J. Res. Dev. 1983, 27, 321.
zeζ 2κkT sinh σdo ) ze 2kT
(13)
Equation 11 can then be rewritten as
σs + σi + σdi )
2κkT zeζ sinh ze 2kT
( )
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Figure 6. XRD spectrum of PPyCl particles.
polypyrrole in Chart 1, indicating that a larger space was made available for the dopant ions of Cl- during the polymerization of pyrrole to polypyrrole. The density of the nitrogen atoms on the polypyrrole surface, [N], can be determined from the area they occupy as (refer to Chart 1)
[N] ) 2 ÷ 2÷
[x
[x
(2b) × b] ) 0.82 -( × 0.82] ) 2.74 nm 2 )
a2 -
0.982
2
2
-2
(17)
Since the percentage of the positively charged nitrogen atoms was at 26% (see Figure 2a) for PPyCl, the surface charge density, σs, can be obtained as
σs ) 0.26[N]e ) 11.4 µC cm-2
(18)
The value of σi + σdi can thus be calculated as 0.353 - 11.4 ) -11.047 µC cm-2 (the minus sign indicates negative charges). The calculation indicates that most of the positive charges from the oxidized or protonated nitrogen atoms on PPyCl did not contribute to the positive ζ potential as they were counterbalanced by the anions within the shear plane. The calculation also suggests that in this case the shear plane is probably quite close to the Gouy-Chapman plane rather than the Stern plane. For D-PPy at pH ) 6.5, σs from the oxidized or protonated nitrogen atoms was zero (see Figure 2b, there was no oxidized or protonated nitrogen) and σi + σdi can be given as -0.279 - 0 ) -0.279 µC cm-2. This suggests that the negative ζ potential of D-PPy was most likely from the adsorption of OH- anions on the D-PPy surface during the base treatment; see eq 10. (Due to the limitation in XPS analysis to obtain N 1s spectra at different solution pH values, the above calculation was not extended to cases with other solution pH values.) Effect on Humic Acid Adsorption. As mentioned earlier, polypyrrole can be potentially used as an adsorbent for the adsorption of macromolecules that carry negative surface charges.6-10 Figure 7 shows the typical results of humic acid adsorption onto the polypyrrole adsorbent prepared in this study at various solution pH values. Generally, the results indicate that the extent of humic acid adsorption improved with the decrease of solution pH values, and there was hardly any adsorption of humic
Figure 7. Adsorption of humic acid on (a) PPyCl adsorbent and (b) D-PPy adsorbent (initial humic acid concentration, 15 mg/L; solution volume, 75 mL; weight of adsorbent, 10 g).
acid on the adsorbent at a pH of about 12. These results are consistent with what is indicated by the ζ potential results in Figure 1. Since the ζ potentials of humic acid are known to be negative at pHs greater than 1.8,9 for the PPyCl adsorbent in Figure 7a, there was an electrostatic attraction between the PPyCl surface of the adsorbent and the humic acid molecules to be adsorbed at pH ) 1.89, 4.03, 6.55, and 8.22, resulting in significant amounts of adsorption of humic acid onto the PPyCl adsorbent at these solution pH values. At pH > 10, the surface charge of the PPyCl adsorbent becomes negative, leading to an electrostatic repulsion between the adsorbent and the humic acid molecules to be adsorbed. The strong electrostatic repulsion between the adsorbent and humic acid molecules at pH ) 12.06 actually prevented the humic acid molecules from being adsorbed at all. The adsorbed amounts of humic acid on the D-PPy adsorbent at various solution pH values are shown in Figure 7b. The amounts of humic acid adsorbed by the D-PPy adsorbent were significantly lower than those of the PPyCl adsorbent at any similar solution pH values in the pH range of about 2-12. This can be attributed to the lower positive or the negative ζ potentials of the D-PPy adsorbent in these solutions. The electrostatic interactions between the D-PPy adsorbent and the humic acid molecules were less attractive or even repulsive. As a result, much less humic acid was adsorbed, in comparison with the results for the PPyCl adsorbent as shown in Figure 7a. At the intermediate pH values (i.e., 4.05, 6.11, and 8.93), certain amounts of humic acid were still adsorbed by the D-PPy adsorbent although the electrostatic interactions were repulsive (both humic acid and the D-PPy
Surface Electric Properties of Polypyrrole
adsorbent had negative ζ potentials). This phenomenon may be explained by the protonation behavior of polypyrrole. The surface charges of the D-PPy adsorbent in an aqueous solution can be heterogeneously distributed; see eqs 7 and 10. There can be a number of protonated nitrogen sites carrying positive charges on the surface of the adsorbent although the overall surface charges may be negative in the weak acidic, neutral, and basic pH conditions. The positively charged sites on the surface provide favorable surface interactions for humic acid adsorption. After these positive sites are consumed, new sites with positive charges may be generated due to protonation of some of the nitrogen atoms on the D-PPy adsorbent surfaces.25 Since the concentration of H+ was relatively low at pH 4.05, 6.11, and 8.93, as compared with the concentration of humic acid, the whole adsorption process was limited by the protonation process. As a result, the adsorption of humic acid on the D-PPy adsorbent was a much slower process than on the PPyCl adsorbent. This is clearly observed from the experimental results in Figure 7 where the adsorption reached saturation after around 600 min for the PPyCl adsorbent and, in contrast, it took more than 7200 min to reach saturation when the D-PPy adsorbent was used. Conclusions The surface electric properties of polypyrrole in aqueous solutions greatly depend on the solution pH values. The ζ potentials of PPyCl particles are positive at pH < 10 and negative at pH > 10. The positive ζ potentials increase only slightly for pHs from 8 to 3 but significantly at pH < 3. There is also a rapid decay of the ζ potentials from positive to negative for pHs from 9 to 11. A strong base treatment of PPyCl can cause D-PPy to have a completely different ζ potential profile against the solution pH values, with positive ζ potentials at pH < 3.5 and negative ζ (25) Zhang, X.; Bai, R. B. J. Colloid Interface Sci. 2003, 264, 30.
Langmuir, Vol. 19, No. 26, 2003 10709
potentials at pH > 3.5. However, a strong acid treatment of D-PPy appears to restore the ζ potentials of R-PPy to those of PPyCl at various solution pH values. The changes of the ζ potentials of polypyrrole with solution pH values can be attributed to several factors, including the dissociation of the dopant anions (i.e., Cl-), the selective adsorption of OH- from the bulk solution, and the protonation/deprotonation of the nitrogen atoms on PPyCl, D-PPy, or R-PPy in the aqueous solutions. The results suggest that in extreme solution pH conditions, polypyrrole can possibly undergo molecular structure changes or rearrangements at the polypyrrole/water interface, which consequently affects the electric properties of polypyrrole in aqueous solutions. XPS analysis reveals that the positive surface charge of polypyrrole is originated from the oxidized or protonated nitrogen atoms on the polypyrrole surface. The evaluation of the surface charge densities from XPS and XRD analyses, the GouyChapman theory, and the experimental ζ potentials, however, indicates that only a fraction of the positively charged nitrogen atoms on the polypyrrole surface contribute to the positive ζ potentials of polypyrrole in an aqueous solution at a pH of about 6.5. The effect of solution pH on the electric properties of polypyrrole in aqueous solutions also largely influences the performance of polypyrrole as a potential adsorbent for the removal of macromolecules, such as humic acid, that carry negative charges. The study also implies that for other potential applications of polypyrrole as a functional material for ion-selective membranes, pH sensors or biosensors, and so forth, one should pay attention to the characteristic changes of the electric properties of polypyrrole with solution pH values and in different solution pH ranges. Acknowledgment. The financial support of the Academic Research Funds, National University of Singapore, is acknowledged. LA034893V
