Electrochemical preparation and kinetic study of poly(o-tolidine) in aqueous medium

Electrochimica Acta 52 (2007) 3883–3888

Electrochemical preparation and kinetic study of poly(o-tolidine) in aqueous medium Khaled M. Ismail ∗ Department of Chemistry, Faculty of Science, Cairo University, Giza, Egypt Received 10 August 2006; received in revised form 5 November 2006; accepted 7 November 2006 Available online 29 November 2006

Abstract Poly(o-tolidine), PoT, film was prepared by electrochemical oxidation of the monomer, oT, in 0.1 M HCl + 0.1 M KClO4 . The presence of KClO4 in the formation medium was found to be essential for the electropolymerization process to proceed. Increasing the upper potential limit up to +1.5 V, instead of +1.0 V, leads to appearance of a new anodic peak at +1.36 V and enhancement of the polymer formation of PoT without changing the film structure. The electrochemical behavior of the formed polymer films was investigated in 1.0 M HClO4 . The kinetic parameters were calculated from the values of the charge consumed during the electropolymerization process. The rate of the polymerization reaction was found to depend on the concentration of the monomer rather than the electrolyte. The polymerization rate is first order with respect to the monomer concentration and zero order with respect to the electrolyte. The electrolyte plays no active role in the kinetics of the electropolymerization process and its role is most likely limited to polymer doping. © 2006 Elsevier Ltd. All rights reserved. Keywords: Poly(o-tolidine); Electropolymerization; Kinetics; Chronoamperometry; Potential limit; Reaction order

1. Introduction Electrochemical polymerization of new monomers represents an attractive field because of the outstanding properties of the electroformed polymer films and their applications [1–7]. While the electropolymerization of mononuclear aromatic amines has received a great deal of attention [8–11], the electropolymerization of the polynuclear aromatic amines, especially polynuclear compounds with two NH2 groups, has got much less attention. Polynuclear amines containing one NH2 group such as 1-amino-9,10-anthraquinone [6,7], 5-amino-1naphthol [12], 1-naphthylamine [13] or two NH2 groups such as benzidine [14], 1,5-diaminonaphthalene [15], has been used for preparation of polymeric film coated electrodes. In general, the behavior of the polynuclear amines was found to follow the general rules of mononuclear amines [13]. The overall electrode process follows an ECE type of mechanism [16]. However, it is well known that, particularly in acidic solutions, the application of potentials higher than ca. 0.7 V versus

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SCE in the electrochemical synthesis of polyaniline at inert substrates gives rise to degradation of the polymer [17–19]. As a consequence of hydrolysis reactions, benzoquinone appears to be the major soluble degradation product [18]. The degradation of polyaniline films at potentials greater than 0.7 V versus SCE has been recorded and a pair of redox waves close to 0.5 V in the voltammetric response of the polymer has been attributed to a benzoquinone/hydroquinone couple [19]. Effect of upper potential limit on the rate of polymer formation of PAN and poly toluidines has been investigated [20]. Formation of PAN and poly toluidines was examined in the range of −0.2 to 1.2 V and it was revealed that the polymerization rate of PAN for the upper potential limit of 1.2 V was significantly promoted in comparison with the case of 0.8 V upper potential limit [21]. However, in contrast, the polymerization of o-toluidine was completely inhibited at the upper potential limit of 1.2 V. On the other hand, little attention has been paid to kinetic studies of electrochemical polymerization process. The growth rate of PAN and poly toluidines film grown by cyclic voltammetry scans of −0.2 to 0.8 V (versus SCE) has been examined [20,21]. It was found that the polymerization rate is first order in monomer concentration.

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The aim of the present work is to study the electro-oxidation of o-tolidine (oT) in aqueous acid medium. The preparation of poly(o-tolidine) (PoT) films by the anodic oxidation of o-tolidine and the investigation of the formed films represent the main concern of this study. Moreover, chronoamperometry technique was used to establish the reaction orders with respect to the electrolyte and monomer. 2. Experimental details o-Tolidine (referred to as oT herein), Merck, was used without further purification. Aqueous solutions were prepared using triply distilled water, potassium perchlorate and other chemicals were analytical grade reagents and were used as received. The solutions of oT used to obtain the polymer film were prepared by dissolving oT in solvent-supporting electrolyte solution which consists of 0.1 M HCl + 0.1 M KClO4 . Both the synthesis and the electrochemical characterization of the films were carried out in an unstirred single compartment three-electrode cell.

A standard three-electrode cell was used with platinum disk (Metrohm) as working electrode, a silver/silver chloride (Ag/AgCl/3 M KCl) as reference and a platinum wire as counter-electrode. The working electrode had a surface area of 0.071 cm2 . Before each experiment the working electrode was polished mechanically with 1.0 ␮m alumina powder, washed and then rubbed against smooth cloth. All electrochemical measurements were carried out using the electrochemical system, Zahner elektrik IM6d (Germany). All experiments were carried out at room temperature (25 ± 1 ◦ C) and all potentials were referred to the silver/silver chloride reference electrode (E◦ = 0.19 V). Except otherwise stated, electropolymerizations were run at laboratory ambient temperature with cycling between −0.2 and +1.5 V. Both the synthesis and the electrochemical characterization of the films were carried out in an unstirred solution. FTIR spectra were obtained between 400 and 4000 cm−1 as KBr pellets on a Perkin-Elmer Model 1650 spectrometer. Thus, a film formed by potential cycling of a Pt electrode in 15 mM oT solution was carefully rinsed with water, dried and peeled off the electrode. 3. Results and discussion 3.1. Electropolymerization of oT Several aqueous and non-aqueous media were investigated for the electropolymerization of oT at the platinum electrode. However, the majority of the tested media showed either low solubility of oT or high solubility of oligomers at the initial stages of the electropolymerization process. In the case of HCl, although solubility of the monomer was good, up to about 25 mM, almost no polymer film was observed due to the high solubility of the

Fig. 1. Typical cyclic voltammograms of PoT film formation from 10 mM oT in 0.1 M HCl containing 0.1 M KClO4 on a Pt electrode at a scan rate of 100 mV/s, for 15 cycles in the potential range between (a) 0.0 and +1.0 V, and (b) −0.2 and +1.5 V.

initially formed polymer (oligomers) in this medium. The best medium for the electrochemical polymerization was found to be 0.1 M HCl with the addition of 0.1 M KClO4 . Typical cyclic voltammogram of 10 mM oT in 0.1 M HCl containing 0.1 M KClO4 on a Pt electrode is shown in Fig. 1a. The electrode potential was swept continuously at a rate of 100 mV/s between 0 and +1.0 V. A large anodic peak (Ia ) is followed by a smaller one (IIa ) in the first positive sweep. The monomer was electro-oxidized in two successive anodic peaks at ca. +0.6 and +0.7 V. These peaks could be attributed to the oxidation of the amino group to the radical cation and then to the dication [22]. The absence of a complementary cathodic peak can be attributed to a fast follow-up chemical reaction following the oxidation of the amino group [12,23]. It is worthwhile to mention that in the absence of KClO4 a quite large complementary cathodic peak was observed. This means the oxidized monomer is quite stable and/or the chemical follow-up reaction is very slow in the absence of KClO4 [14]. During the second forward scan, the anodic peak (IIa ) shifted towards less positive potential and overlapped with the first anodic peak (Ia ). In general, cation radicals produced from aromatic amine couple rapidly to give dimers, which are more easily oxidizable than the parent monomer. These dimers are produced by C–C, C–N and/or N–N coupling. The relative yields of these coupling depend on the experimental conditions [16]. As the potential scan is continued, the anodic currents decrease grad-

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ually indicating that the growing film has no activity in the formation medium. During the electrochemical polymerization, accumulation of the polymer was readily observable by naked eyes. Occasionally, during the first few cycles, small yellow tendrils of free-floating polymer were observed to break off from the formed polymer film and drift away from the vicinity of the working electrode surface. After long electrochemical polymerization times, oligomers with a certain chain length are formed on the electrode surface and the drift away of the yellow tendrils from the electrode surface stop. For electrochemical polymerization times that correspond to fifteen cycles, the resulting polymer was dark violet in color. The effect of upper potential limit on the polymerization process was also studied. The electrode potential was swept continuously at a rate of 100 mV/s between −0.2 and +1.5 V (Fig. 1b). Except another anodic peak (IIIa ) recorded at ca. +1.36 V, the cyclic voltammograms showed more or less the same features as those recorded at lower potential limit (+1.0 V). Although the reason for the appearance of this broad peak is unclear, it can be attributed to the electro-oxidation of the hydrazo group (NH–NH) of the dimer, hydrazo-o-tolidine, resulted from N–N coupling. The formation of the polymer probably occurs by cleavage of the hydrazo bond during oxidation, with the formation of a nitrenium cation (R-NH+ ) [24]. This cation most likely reacts with a neutral o-tolidine molecule through C–N coupling. The structure of the polymer formed at low or high upper potential limit was the same as shown from IR spectrum. Deposition of thick films of PoT on the Pt electrodes occurred rapidly, indicating a fairly efficient film formation process, especially when it is formed at higher upper potential limit (+1.5 V). This can be also seen from the fast decay of the anodic peak when the film is formed at higher upper potential limit (Fig. 1a and b). In general, high upper switch potentials could cause degradation and/or crosslinking reactions during the polymerization [17,24]. On the other hand, high applied potentials may lead to a fast rate of polymerization [21]. The oxidized monomer may undergo a cyclization reaction to give phenazine structure. Nevertheless, the possibility that the dication polymerizes without cyclization cannot be ignored. Thus, a composite of two different films may be obtained, one of linear chain structure and the other with ring chain structure, although the former seems to be more plausible. After the film formation, the electrode was thoroughly washed with distilled water and then transferred into 1.0 M HClO4 solution containing no monomer. Fig. 2 shows a typical cyclic voltammogram representing the electrochemical response for the oxidation–reduction reaction of the PoT film deposited at a scan rate of 100 mV/s in the potential range between +0.2 and +0.8 V. PoT did not display a symmetric charge–discharge wave as can be seen from Fig. 2. Moreover, a tailing of the wave was recorded which may be due to an increased contribution from capacitive charging or most probably due to the apparent diffusion [25]. The polymer film can be cycled between +0.2 and +0.8 V with negligible loss of activity, which reflects the high stability of the film in 1.0 M HClO4 acid solution.

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Fig. 2. Cyclic voltammogram of the electrochemical response of the PoT film in 1.0 M HClO4 solution, at a scan rate of 100 mV/s in the potential range between +0.2 and +0.8 V, recorded after 10 cycles.

3.2. Chronoamperometric kinetics Electropolymerization of oT under potentiostatic conditions (chronoamperometric) was accomplished using various monomer concentrations, polarizing the electrode at different positive potentials, namely +1.0 and +1.5 V, for different time lengths. The chronoamperometric study for the electropolymerization of films from oT solution was carried out by potential steps from open-circuit potential to more positive potentials than the beginning of its oxidation during the polarization time (Fig. 3). The typical current transient for the electropolymerization process under potentiostatic control, at +1.5 V, shows an initial abrupt decrease of the recoded current due to the diffusion controlled monomer oxidation in solution [26,27]. This followed by a slower current decrease due to a nucleation of oligomers produced in solution and growth of a film, which is electroinactive in the formation medium. The kinetic parameters were calculated from values of the polymerization charge consumed in the electrogeneration of the polymer. Assuming no collateral or secondary reactions, the charge consumed during polymerization (Q), calculated by integration of the experimental chronoamperograms, can be used to obtain the kinetic parameters of the electropolymerization

Fig. 3. I–t transients recorded during the formation of PoT on Pt electrode at +1.5 V.

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process [28,29]. The charge consumed during the electropolymerization process (Q) is proportional to the electroformed amount of polymer (M): Q = kM

(1)

The charge consumed, Q, the electrolyte concentration and the monomer concentration can be correlated by means of the equation: RP =

dQ = k[E]n [oT]m dt

(2)

where RP is the polymerization rate; n and m are the reaction orders related to the electrolyte and monomer, respectively, and k is the kinetic constant. Eq. (2) can be expressed in logarithmic form as: log

dQ = log k + n log[E] + m log[oT] dt

(3)

Monomer and electrolyte concentrations were varied keeping one of them constant to evaluate their respective reaction orders. The monomer concentrations varied from 5 to 25 mM and the electrolyte concentration was 0.1 M HCl + 0.1 M KClO4 . Electrolyte concentration [KClO4 ] values ranged from 0.02 to 0.16 M and the constant monomer concentration was 15 mM. The synthesis potential applied was either +1.0 or +1.5 V and the polymerization time varied from 15 to 120 s. Fig. 4a and b shows the polymerization charge versus time plots associated with PoT formation from 0.1 M HCl + 0.1 M KClO4 concentration as a function of the monomer concentration at two different applied potentials +1.0 and +1.5 V, respectively. Both graphs show good linearity between the polymerization charges and the formation times. The slopes of the graphs resulting from the logarithmic plots log(dQ/dt) versus log[oT] yield reaction order value with respect to the monomer. The plot of log RP versus log[oT] was found to be linear with slope values of 1.02 and 0.92 at +1.0 and +1.5 V, respectively (Fig. 4c). This implies that the value of the reaction order associated to the monomer is practically the same, i.e. first order dependence of RP on [oT]. The influence of the electrolyte on the polymerization process was also studied by varying the concentration of each of the following ions ClO4 − and Cl− . Similarly, by varying the [ClO4 − ] for fixed value of [oT], two different sets of experiments (at +1.0 and +1.5 V) were performed. Fig. 5a and b shows the polymerization charge versus time plots associated with PoT formation from 15 mM oT concentration as a function of the electrolyte concentration at two different applied potentials +1.0 and +1.5 V, respectively. On varying the [ClO4 − ] by keeping [oT] as constant, RP was found to have very small dependence on [ClO4 − ]. The plot of log RP versus log[ClO4 − ] was found to be linear with slope values of 0.12 and 0.0 at +1.0 and +1.5 V, respectively (Fig. 5c). This means that the reaction order with respect to ClO4 − ion is almost zero. Also, the Cl− ion showed a similar behavior where the variation of its concentration has no effect on the kinetics of the electropolymerization process. The negligible value obtained for the reaction order associated to the electrolyte suggests that the electrolyte plays no active role in

Fig. 4. Polymerization charge vs. time as a function of the monomer concentration at two different applied potentials: (a) +1.0 V and (b) +1.5 V; (c) logarithmic plot of polymerization rate vs. monomer concentration.

the kinetics of the electropolymerization process. Therefore, the role of the electrolyte is probably limited to polymer doping. 3.3. IR absorption characterization of PoT film Fig. 6 shows the IR spectra of the PoT obtained by potentiodynamic oxidation of oT on Pt electrode at two different upper potential limits (+1.0 and +1.5 V) in comparison with that of the monomer. The IR spectra of the PoT (Fig. 6a and b) are almost identical, showing that there is almost no influence of the upper potential limit on the IR response of the polymer. While a strong absorption peak due to the N–H stretching vibrations of the imino groups of the PoT film was observed at ∼3340–3314 cm−1 , absorption peaks corresponding to the N–H stretching modes of the amino groups of the oT, were observed at 3468–3337 cm−1 . This fact suggests that the NH2 groups take part in the electropolymerization process [30]. The peak observed at ca. 1639–1624 cm−1 for both the polymer and the monomer is considered to the bending vibration of the N–H bond [22]. A band at 1373–1381 cm−1 appears in all cases, which is also present in the monomer, is due to methyl group symmetric deformation. The observed band in the region of 1080 cm−1 which is present only for the polymer is attributed to the presence of ClO4 − ions.

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Fig. 5. Polymerization charge vs. time as a function of the perchlorate concentration at two different applied potentials: (a) +1.0 V and (b) +1.5 V; (c) logarithmic plot of polymerization rate vs. perchlorate concentration.

The peaks at 1308 cm−1 in the IR spectrum of the polymer are attributed to the stretching of the C–N bonds of the secondary amines or stretching vibration of C–N groups with partially double bonds characteristics [31]. The presence of relatively strong absorption peak corresponding to the N–H stretching vibrations may suggest that the PoT does not posses ring closed structure and/or that the degree of the polymerization is relatively low. The absence of the characteristic strong absorptions carbonyl group (1680 cm−1 ) in the spectra indicates the absence of any significant quantity of o-quinone structure may result from overoxidation of the PoT [22]. Based on the above results, the PoT is expected to have oT unit structure and such units are linked to each other via the C–NH–C and C–N C bonds. Some of the polymer units may have a ladder-like planar structure with phenazine rings cannot be ruled out. On the basis of the electrochemical results and the IR spectra, it appears that the mechanism for the polymerization process is similar to that proposed for benzidine in aqueous media [14]. This is shown in the following equations: oT ↔ oT2+ + 2e−

(4)

oT + oT2+ → D + 2H+

(5)

Fig. 6. FT-IR spectra of the PoT film electropolymerized potentiodynamically at two different upper potential limits: (a) +1.0 V, (b) +1.5 V and (c) monomer.

D → Oligomer → Polymer

(6)

The suggested mechanism follows an E(CE)n type in which the monomer, oT, is oxidized to a dication (Eq. (4)). The dication is transformed into a dimer, D, through a coupling reaction with the parent molecule (Eq. (5)), which is oxidized again to give an oligomer and then a polymer (Eq. (6)). The polymer film can be formed by C–C, C–N and/or N–N coupling. In view of

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the above discussion, the most probable is C–N coupling in the ortho position of the aromatic ring [14]. 4. Conclusions The successful preparation of PoT requires the presence of perchlorate ions in the formation medium. In the absence of KClO4 , formation of soluble products is predominant and no polymeric films can be detected. Increasing the upper potential limit up to +1.5 V, instead of +1.0 V, leads to appearance of a new anodic peak and enhancement of the polymer formation of PoT without changing the film structure. The kinetic data indicate that the electropolymerization rate depends on the monomer concentration rather than the electrolyte. While the reaction order with respect to the monomer was one, it was almost zero with respect to the electrolyte (perchlorate and chloride ions). This suggests that the electrolyte acted only as a dopant agent rather than contributing actively to the kinetics of the electropolymerization process. Acknowledgments The AvH foundation (Bonn-Germany) and University of Cairo (Egypt) are gratefully acknowledged for providing the electrochemical workstation. References [1] J. Xu, J. Hou, Q. Xiao, Q. Wei, R. Zhang, S. Zhang, S. Pu, Mater. Lett. 60 (2006) 1156. [2] P. Manisankar, C. Vedhi, G. Selvanathan, H.G. Prabu, Electrochim. Acta 51 (2006) 2964. [3] P.S. Murray, S.F. Ralph, C.O. Too, G.G. Wallace, Electrochim. Acta 51 (2006) 2471. [4] Ch. Lo, A. Adenier, K.I. Chane-Ching, F. Maurel, J.J. Aaron, B. Kosata, J. Svoboda, Synth. Met. 156 (2006) 256. [5] L. Wang, J. Ji, Y. Lin, S. Rwei, Synth. Met. 155 (2005) 677.

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