An electrochemical study of two water-soluble aniline derivatives,
Polyaniline (PANI) is one of the most promising materials from a technological standpoint because it hacs a high conductivity, can be easily synthesized, has well-behaved electrochemistry (the colour changes depending on the redox state), and is stable under ambient conditions. However, the low solubility in common organic solvents and the strong dependence of its electrical conductivity on the pH of the electrolytic medium (which falls almost to zero at pH higher than 3) [
The (electro)oxidation mechanism and the kinetics of the poly(
In order to obtain sulfonated self-doped polyanilines, possessing different solubility and enhanced properties, the chemical or electrochemical copolymerization reactions are the best ways, by choosing the comonomer ratio and appropriate (electro)polymerization conditions [
In this paper we present an electrochemical study of two aniline derivatives,
Aniline was double-distilled under reduced pressure just before use and stored in the dark and in a nitrogen atmosphere.
Electrochemical studies of aniline derivatives were carried out using a bioanalytical systems, Potentiostat-Galvanostat (BAS 100 B/W). The experiments were performed in a one-compartment cell using a standard three-electrode cell arrangement, including a working electrode, an auxiliary electrode (platinum wire), and a reference electrode (consisted of a silver wire coated with AgCl). A disk-shape Pt electrode (1.6 mm diameter), Pt rectangular plate (1.0 × 0.5 cm2 area) and a square transparent glass-ITO electrode (2.5 × 2.5 cm2 area), were used as working electrodes. Before each experiment, the Pt working electrodes were cleaned by polishing successively with 0.3 and 0.05
XRD measurements were performed with a Bruker AD8 ADVANCE diffractometer. The X-ray beam was CuK
The morphology of the films was investigated by atomic force microscopy (AFM) with a MultiView 4000 Nanonics System, working in noncontact mode using probes with the resonance frequency of 38–40 kHz.
The detailed synthesis procedure and structural characterization of the AnPS was reported in the literature [
(a) The chemical structures and (b) the optimized structural geometry of the aniline derivatives, DAnPS and AnPS.
The DAnPS structure can be viewed as a dimer of aniline with an AnPS unit. Although the presence of the sulfopropyl substituent attached to the nitrogen atom increases the water solubility of AnPS, the DAnPS is insoluble in water due to the hydrophobic effect of the second phenyl substituent. The solubility of DAnPS sample follows the following order: DMSO > DMF
The multicyclic voltammograms of DAnPS and AnPS in bidistillated water (free of any supporting electrolyte) on the disk-shaped Pt electrode were recorded by sweeping the working electrode potential between −0.2 V (0.0 V) and 1.6 V (1.2 V), respectively, at a constant rate of 50 mV s−1 (Figure
Multicyclic voltammograms of (a) 1.0 mM DAnPS (water dispersion) and (b) 1.0 mM AnPS on Pt electrode (disk-shaped, 1.6 mm diameter) recorded in bidistillated water; the dotted line represents the first cycle. Scan rate: 50 mV s−1.
By increasing the number of the scans, the anodic current peak intensity decreases. Thus, the first forward scan shows that the onset of oxidation process of DAnPS occurs at 0.118 V, and the peak is centred at 0.569 V (
Unlike DAnPS, the water-soluble AnPS, exhibits only one broad anodic peak on the subsequent cycles (Figure
Using a solution containing 1.0 mM DAnPS in organic solvent (DMF or DMSO), free of any electrolyte support, the monomer is free of electrochemical activity. This behaviour can be explained by the fact that in water some sulfonated groups (–SO3H) exist in a dissociated form giving to monomers zwitterionic structures, while in the organic solvent the sulfonated groups are nondissociated (Figure
The zwitterionic form of the AnPS and DAnPS monomers in aqueous solution.
By using LiClO4 as supporting electrolyte, DAnPS exhibits reversible cyclic voltammogram curves (Figure
(a) The CV of 1.0 mM DAnPS at Pt electrode (disk shape, 1.6 mm diameter) recorded in DMF and LiClO4 as supporting electrolyte; (b) the CV of 1.0 mM DAnPS in DMF-free electrolyte support (LiClO4); (c) the CV of the DMF-LiClO4 monomer-free solution; scan rate: 50 mV s−1.
The second anodic peak is located at 1.027 V and can be assigned to the second oxidation process of DAnPS. No oxidation peak was observed up to a potential of 1.400 V on the anodic scan.
In Figure
Multicyclic voltammograms (20 cycles) of 1.0 mM DAnPS in (a) DMF-HCl (1 M), (b) DMF-H2SO4 (1 M) and (c) DMF-HClO4 (1 M), and 1.0 mM AnPS at Pt electrode (disk shape,1.6 mm diameter) in (a′) HCl (1 M), (b′) H2SO4 (1 M) and (c′) HClO4 (1 M); the dotted line represents the first cycle. Scan rate: 50 mV s−1.
In DMF-HCl (1 M) mixture, the CV of the DAnPS (1.0 mM) exhibits on the first scan two anodic peaks associated to monomer oxidation processes. The second anodic peak cannot be observed in this CV because its current intensity is very high (Figure
Cyclic voltammetry data of DAnPS and AnPS recorded at Pt electrode in different electrolyte media.
DAnPS (1.0 mM) | AnPS (1.0 mM) | |||||||
H2O (dispersion) | DMF-LiClO4 | DM-H2SO4 (1 M) | DM-HClO4 (1 M) | DMF-HCl (1 M) | H2SO4 (1 M) | HClO4 (1 M) | HCl (1 M) | |
0.118/0.30 | 0.409/1.6 | 0.445/2.29 | 0.437/2.20 | 0.432/2.59 | 0.805/3.17 | 0.773/3.51 | 0.777/0.43 × 103 | |
0.569/9.01 | 0.525/33.5 | 0.552/29.33 | 0.544/30.62 | 0.529/21.68 | 0.983/26.78 | 0.960/49.06 | 1.051/12.59 × 103 | |
— | 1.027/22.8 | 1.075/19.96 | 1.064/23.62 | 1.474/−4.88 | — | — | 1.363/12.36 × 103 | |
0.424/1.42 | 0.560/1.3 | 0.593/−0.61 | 0.583/2.06 | 0.925/18.29 | 0.580/−2.66 | 0.594/−3.92 | 0.552/−0.27 × 103 | |
0.145/−3.22 | 0.435/−30.0 | 0.454/−25.28 | 0.455/−27.13 | 0.937/−18.22 | 0.399/−13.60 | 0.522/−11.68 | 0.390/−1.76 × 103 | |
−0.091/−5.14 | — | after 3rd cycle 0.207 | — | 0.431/−20.87 | — | 0.426/−22.57 | — | |
1.75 | 1.11 | 1.16 | 1.17 | 0.99 | 1.97 | 4.20 | — |
In H2SO4 and in HClO4 aqueous solutions (1 M), DAnPS exhibits two anodic peaks at 0.552 V and 1.075 V, and 0.544 V and 1.064 V, respectively, on the first scan. On the reverse scanning, only one cathodic peak appears at 0.454 V (in H2SO4 aqueous solution) and 0.455 V (in HClO4 aqueous solution). It can be noticed that the first oxidation-reduction couple is reversible, and this is confirmed by the value of the ratio between the first anodic and cathodic peaks currents intensities of 1.16 (Figure
Taking into account the electrooxidation mechanism of 4-aminodiphenylamine in acetonitrile, it can be stated that the first one-electron transfer peak of DAnPS gives rise to a cation radical that later undergoes irreversible follow-up deprotonation to give a radical which then undergoes another electron transfer involving an ECE mechanism [
Scheme
Possible anodic oxidation process of the DAnPS monomer, at the Pt electrode surface.
The negative shift of the first redox couple compared to
In Figure
It is well known that the potential value needed for a polymer, oligomer, or monomer to be oxidized follows this order:
The cyclic voltammograms of AnPS recorded in H2SO4 (1 M) and HClO4 (1 M) show approximately the same behaviour as in HCl electrolyte medium, except the fact that the second anodic peaks are not very well distinguished, more broad, and overlapped on the first anodic one. This suggests the low stability of the cation-radical intermediate (AnPS+*) formed in the first oxidation. The radical cation intermediate generated at the surface of electrode can react with another cation-radical to form a dimer. The dimer is then oxidized in one step to diimine quinoid form with participation of two electrons, at a lower oxidation potential in comparison with AnPS monomer.
The low stability of the cation-radical intermediate can be related with the presence in the electrolyte medium of
The oxidation processes of sulfonated
During the oxidation process, the AnPS monomer exhibits an electrochromic behaviour, being yellow at low potential value and passing through green and blue to black at higher potentials (up to 1.0 V) (Figure
The colour changes by electrooxidation of AnPS in acidic aqueous medium.
The simple and efficient way to obtain sulfonated self-doped polyanilines, possessing different solubility and enhanced properties, is chemical or electrochemical
In an attempt to synthesize AnPS, by taking an excess of aniline in reaction with 1, 3-propanesultone, a mixture of AnPS and aniline was obtained that couldnot be separated by ordinary separation techniques. An explication for this can be the coexistence of the AnPS and aniline monomers as an acid-base complex in which the NH2 group of aniline is protonated by the sulfonic acid of AnPS. The ratio between these two monomers was calculated from the 1H NMR spectra and is 1 : 0.75 AnPs to aniline. By submission of this monomer mixture to electrochemical measurements, the cyclic voltammograms recorded with a Pt plate and ITO-glass as working electrodes in H2SO4 (1 M) aqueous solution present on the first cycle one anodic peak and two cathodic peaks which correspond to oxidation and reduction processes of the AnPS monomer.
In case of a mixture containing both AnPS and aniline is expected that the electrooxidation and the polymerization process, respectively, be more complex because, besides the anodic oxidation and chemical coupling of AnPS monomer the redox processes characteristic of aniline must be taken into account. From the cyclic voltammograms recorded at Pt plate electrode (Figure
Multicyclic voltammograms (20 cycles) of AnPS-aniline mixture (10−2 M) in H2SO4 (1 M) aqueous solution at (a) Pt plate (1 × 0.5 cm2) and (b) ITO glass (2.5 × 2.5 cm2) working electrode; the dotted line represents the first cycle. Scan rate: 50 mV s−1.
Overall, the electrochemical growth of the copolymer on the working electrode employing cyclic voltammetry is significantly different in the electrochemical behaviour of the AnPS and aniline individually. Moreover, the appearance of new redox couple 0.330 V/0.250 V in the cyclic voltammograms of the AnPS: aniline mixture marks the formation of a thin layer of copolymer because this does not appear in the cyclic voltammograms of the deposited polyaniline. The cyclic voltammograms recorded at ITO-glass electrode are characterized by oxidation and reduction large peaks and their current intensities increase with the number of scans (Figure
The data regarding the morphology and roughness of the thin layer deposited on Pt electrode was provided by AFM technique. In Figure
AFM images (2D and 3D) and the section profile of the thin layer of poly(AnPS-
This thin green copolymer layer deposited on Pt electrode was not soluble in any of the organic solvents. Trying to obtain soluble polymer a mixture consisting of DAnPS and aniline as comonomers was submitted on cyclic voltammetry measurements, with 1 : 1 and 1 : 3 feed ratios. The cyclic voltammograms recorded at Pt electrode and transparent ITO-glass electrodes in DMF-HCl (1 M) aqueous solution are presented in Figure
Multicyclic voltammograms (20 cycles) of DAnPS-aniline (1 : 3 ratio mixture) (10−2 M) in DMF-HCl (1 M) aqueous solution at (a) Pt plate (1.0 × 0.5 cm2) and (b) ITO glass (2.5 × 2.5 cm2) as working electrodes; the dotted line represents the first cycle. Scan rate: 50 mV s−1.
The recorded cyclic voltammograms are irreversible. The mechanism of electrochemical polymerization of this mixture is very complex, and thus, it cannot be stated if DAnPS monomer is responsible for the appearance of these peaks. Continuing with the scanning on the same potential range, the redox current intensities fall gradually during the 10 cycles. At the end of the 10th cycles a blue-green thin layer copolymer was observed deposited on the Pt electrode surface.
All attempts to get an adherent copolymer layer at the electrode surface by using the same system but from the aqueous H2SO4 (1 M) solution failed because, at the end of 10 cycles, the reaction products formed at the electrode are dispersed in solution. In Figure
The poly(DAnPS-
AFM images (2D and 3D) and the section profile of the thin layer of poly(AnPS-
Analyzing the thin layers by X-ray technique, results that both copolymers (poly(AnPS-
XRD spectra of poly(AnPS-
The development of the highly ordered structures for self-doped poly(aniline-N alkanesulfonate)s was evidenced by Kim et al. [
In summary, the cyclic voltammetry was used to study the electrochemical behaviour of two
One of the authors (L. Vacareanu) acknowledges the financial support of the European Social Fund “Cristofor I. Simionescu” Postdoctoral Fellowship Programme (ID POSDRU/89/1.5/S/55216), and the Sectoral Operational Programme Human Resources Development 2007–2013.