Influence of the Type of Oxidant on Anion ... - ACS Publications

As it has previously been shown(22) that cellulose extracted from Cladophora sp. algae has a specific surface area close to that of industrial absorbe...
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J. Phys. Chem. B 2009, 113, 426–433

Influence of the Type of Oxidant on Anion Exchange Properties of Fibrous Cladophora Cellulose/Polypyrrole Composites Aamir Razaq,† Albert Mihranyan,† Ken Welch,† Leif Nyholm,*,‡ and Maria Strømme*,† Nanotechnology and Functional Materials, Department of Engineering Sciences, The Ångstro¨m Laboratory, Box 534, 75121 Uppsala, Sweden, and Department of Materials Chemistry, The Ångstro¨m Laboratory, Box 538, 75121 Uppsala, Sweden ReceiVed: July 23, 2008; ReVised Manuscript ReceiVed: October 14, 2008

The electrochemically controlled anion absorption properties of a novel large surface area composite paper material composed of polypyrrole (PPy) and cellulose derived from Cladophora sp. algae, synthesized with two oxidizing agents, iron(III) chloride and phosphomolybdic acid (PMo), were analyzed in four different electrolytes containing anions (i.e., chloride, aspartate, glutamate, and p-toluenesulfonate) of varying size. The composites were characterized with scanning and transmission electron microscopy, N2 gas adsorption, and conductivity measurements. The potential-controlled ion exchange properties of the materials were studied by cyclic voltammetry and chronoamperometry at varying potentials. The surface area and conductivity of the iron(III) chloride synthesized sample were 58.8 m2/g and 0.65 S/cm, respectively, while the corresponding values for the PMo synthesized sample were 31.3 m2/g and 0.12 S/cm. The number of absorbed ions per sample mass was found to be larger for the iron(III) chloride synthesized sample than for the PMo synthesized one in all four electrolytes. Although the largest extraction yields were obtained in the presence of the smallest anion (i.e., chloride) for both samples, the relative degree of extraction for the largest ions (i.e., glutamate and p-toluenesulfonate) was higher for the PMo sample. This clearly shows that it is possible to increase the extraction yield of large anions by carrying out the PPy polymerization in the presence of large anions. The results likewise show that high ion exchange capacities, as well as extraction and desorption rates, can be obtained for large anions with high surface area composites coated with relatively thin layers of PPy. 1. Introduction The inherent properties of conducting polymers make them useful in a number of applications including polymer batteries,1 coatingsforelectromagneticshielding,2 actuators,3 andbiosensors.4,5 Polypyrrole (PPy) is currently one of the most commonly used conducting polymers due to its facile synthesis and relatively high electrical conductivity. PPy can be synthesized either chemically or electrochemically.6-8 While the electrochemical synthesis of PPy only can be carried out on electrically conducting substrates, chemical polymerization employing various oxidants enables one to combine PPy with various nonconducting substrates to form composites with interesting tailored properties. Chemical synthesis of PPy is also advantageous due to the straightforward synthesis merely involving the mixing of pyrrole with an oxidizing agent, the possibility of production scale-up, and cost savings. Chemical polymerization of pyrrole is generally based on the use of oxidizing agents such as various iron(III) salts, e.g., FeCl3 or Fe2(SO4)3, sodium persulfate (Na2S2O8), or phosphomolybdic acid (H3[P(Mo3O10)4] · 29H2O).9 The chemical nature of the oxidant as well as the ratio between the concentrations of the monomer and the oxidant then determine the properties (e.g., the electrical conductivity and morphology) of the resultant PPy material.10 Chemical polymerization of PPy on cellulose fibers gives rise to new possibilities for the manufacturing of conductive paper * Corresponding authors. E-mail: [email protected] (M.S.); [email protected] (L.F.). † Nanotechnology and Functional Materials, Department of Engineering Sciences. ‡ Department of Materials Chemistry.

and applications based on such materials. A few attempts have so far been made to improve the mechanical stability of conducting polymers by employing different forms of cellulose papers.11,12 As was recently shown by Mihranyan et al.,13 composites of PPy with cellulose in the form of paper sheets can function as ion exchange membranes by applying an electrical potential to the composite and switching the PPy between its oxidized and reduced states. Such electrochemically controlled extractions of anions and cations have previously been described by several groups based on the use of electrochemically synthesized conducting polymer coatings.14-20 In electrochemically controlled extraction, the extraction and desorption of anions is obtained by oxidizing and subsequently reducing the polymer as this will cause the anions to enter and leave the polymer as the electroneutrality of the polymer must be maintained (a similar approach can also be used for the extraction of cations based on PPy films polymerized in the presence of a sufficiently large anion).15 The mobility of the ions in the polymer will predominantly depend on their size, charge, and shape.21 To improve the ion exchange capacity of conductive paper, materials with very large surface areas should be used. Since the rate of mass transport of the ions in the polymers generally is low (particularly for large ions), it can be expected that it is more advantageous to use a large surface area material with a relatively thin coating of conducting polymer13 than the rather thick electropolymerized coatings (with small surface areas) used in electrochemically controlled extraction so far. As it has previously been shown22 that cellulose extracted from Cladophora sp. algae has a specific surface area close to that of industrial absorbents, the coating of such a cellulose substrate

10.1021/jp806517h CCC: $40.75  2009 American Chemical Society Published on Web 12/19/2008

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TABLE 1: Chemical Structures and Molecular Weights for the Employed Electrolytes and Oxidants

with a thin layer of a conducting polymer should therefore be very useful for the present type of extractions. Such a novel composite material of PPy with Cladophora cellulose was in fact recently described by Mihranyan et al.13 This novel composite, which was prepared in the form of a paper sheet, exhibited excellent mechanical stability and could be bent, twisted, or folded without disrupting its mechanical integrity. Due to its large surface area, the composite exhibited a high exchange capacity for chloride ions.13 Chloride ions are, however, very small in comparison with many biologically interesting ions and it is therefore important to also study the possibilities to extract larger ions. To optimize the composite for extractions of large ions and biomolecules (e.g., amino acids, oligopeptides, oligonucleotides, etc.), it is likely that it is necessary to modify the PPy coating by carrying out the polymerization in the presence of larger anions than chloride. One important question is then to what extent such a change in the polymerization conditions affects the extraction yields of larger ions. In the present work, we investigate how the electrochemically controlled ion exchange properties of PPy/Cladophora cellulose composites are affected by the use of oxidants with anions of different sizes during the polymerization of the PPy coating. By employing an oxidizing agent with a large anion, the network spacing of PPy chains should increase as a result of the incorporation of the anions during the polymerization. For this purpose, we use phosphomolybdic acid (PMo) and iron(III) chloride as oxidizing agents as PMo has a molecular weight that is 11 times larger than that of FeCl3. We hence compare the ion exchange properties of PMo and iron(III) chloride synthesized PPy/Cladophora cellulose composites at different oxidation potentials in electrolytes containing chloride, aspartate, glutamate, and p-toluenesulfonate. The composites are further characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and N2 adsorption analysis.

2. Materials and Methods 2.1. Chemicals and Reagents. Cladophora algae were collected from the Baltic Sea. The cellulose was extracted from Cladophora algae as described previously.22 Pyrrole (Py), iron chloride (FeCl3), phosphomolybdic acid (PMo) hydrate, sodium chloride, and hydrochloric acid were used as supplied by VWR, Sweden. DL-aspartic acid (99%), DL-glutamic acid (98%), and sodium p-toluenesulfonate (95%) were purchased from Sigma Aldrich. The chemical structures and molecular weights of the used oxidants and electrolytes are summarized in Table 1. 2.2. Preparation of Composites. Three hundred milligrams of cellulose powder was dispersed in 50 mL of water using high-energy ultrasonic treatment (VibraCell 750W, Sonics, USA) for 8 min, and the dispersion was collected on a filter paper. Three milliliters of Py was put in a volumetric flask and the total volume was brought to 100 mL. The collected cellulose cake was mixed with Py solution and redispersed by ultrasonication for 1 min. The dispersion was next allowed to stand for 30 min and was then collected on a filter paper. Eight grams of iron(III) chloride was dissolved in 100 mL of water and run through the filter cake to induce polymerization (the reaction was allowed to continue for 10 min prior to filtration). A fluffy spongelike cake was formed. 100 mL of 0.1 M HCl was subsequently run through the cake. The product was then thoroughly washed with water and dried (the cake was redispersed using ultrasonication to form a homogeneous layer). A similar procedure was used to prepare PMo synthesized composites by using PMo rather than iron(III) chloride as the oxidant. For that purpose, 34 g of PMo was dissolved in 100 mL of water and run through the filter cake to induce polymerization. No HCl was, however, run through the cake during the production of the PMo synthesized sample. 2.3. Primary Characterization. 2.3.1. SEM. Micrographs were taken with an environmental SEM (FEI/Philips XL 30, The Netherlands) in the high-vacuum mode. The samples were

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mounted on aluminum stubs using double-sided adhesive tape. Prior to imaging, Au/Pt was sputtered on the samples to minimize charging. 2.3.2. TEM. TEM was conducted with a JEOL-3010 microscope, operating at 300 kV (Cs 0.6 mm, resolution 1.7 Å). Images were recorded using a CCD camera (model Keen View, SIS analysis, size 1024 × 1024, pixel size 23.5-23.5 µm) at 30 000-100 000 times magnification using low-dose conditions on as-synthesized samples. Samples were sonicated for 5 min prior to transfer to the TEM grid. 2.3.3. Surface Area and Pore Volume. N2 gas adsorption and desorption isotherms were obtained with an ASAP 2020 (Micromeritics, USA). The specific surface areas were obtained according to the BET method.23 The total pore volume of the samples was measured as the single-point adsorption volume of pores with a diameter less than ∼1200 Å at a relative pressure of ∼0.98. 2.3.4. ConductiWity Measurements. The resistances of the samples were measured at room temperature using a semiconductor device analyzer (B1500A, Agilent Technologies, USA). Prior to the measurements, silver paint was pasted at the ends of rectangular samples to ensure good contacts. The voltage U, which was scanned between -1 and +1 V, was applied and the resulting dc current I was measured. The conductivity was then calculated as

σ)

∆I L ∆U wd

Figure 1. Schematic presentation of pyrrole polymerization with (a, top) iron(III) chloride and (b, bottom) phosphomolybdic acid.

(1)

where ∆I/∆U denotes the conductance of the sample obtained as the slope of the current versus voltage curve, L is the length (typically ∼2 cm), w is the width (typically ∼1 cm), and d is the thickness (typically ∼0.1-0.2 cm) of the sample. 2.4. Electrochemical Measurements. Cyclic voltammetric and chronoamperometric (potential step) measurements were performed in a standard three-electrode electrochemical cell utilizing an Autolab/GPES interface (ECO Chemie, The Netherlands) with the sample as the working electrode, a Pt wire as the counter electrode, and a Ag/AgCl electrode as the reference electrode. All measurements were carried out in 2.0 M solutions of sodium chloride, DL-aspartic acid, DL-glutamic acid, or sodium p-toluenesulfonate solutions at room temperature. The solubility of DL-aspartic acid and DL-glutamic acid was increased by increasing the pH of the solutions to 10.4 and 9.0, respectively, by addition of NaOH. Prior to the electrochemical measurements, the samples were rinsed in the electrolyte solution to be used for the analysis by first applying a reduction potential of -0.8 V for 300 s, followed by oxidation at +0.7 V for 300 s and then reduction at -0.8 V for 600 s. Three cyclic voltammograms were then recorded between -0.8 and +1.2V in a new portion of the electrolyte using a scan rate of 4.5 mV/s. Chronoamperometric step experiments were performed for each sample in all four electrolytes. The oxidation potential was varied between +0.7 and +1.2 V. In these experiments, the reduction potential was held constant at -0.8 V. Each step procedure was performed for a total of 1800 s by assigning 300 s for each step of reduction and oxidation, respectively. The same sample was used for all oxidation potentials in a specific electrolyte. Thus, a typical chronoamperometric step procedure was as follows: the step procedure was started by first reducing the sample at -0.8 V for 300 s and thereafter the potential was maintained at +0.7 V for 300 s. After two additional and

Figure 2. SEM micrographs of polypyrrole/Cladophora cellulose composites synthesized with (a) iron(III) chloride and (b) PMo.

consecutive reduction and oxidation steps, using the potentials given above, the step procedure was repeated with higher potential, i.e., +0.8 V up to +1.2 V. To investigate the reproducibility of the results, the entire procedure was repeated for three different samples. The typical dimensions of the samples used in the electrochemical measurements were 0.6 cm × 0.25 cm × (0.06-0.2 cm), which corresponded to sample weights of ∼10 mg. 3. Results and Discussion 3.1. Primary Characterization. Figure 1 shows schematically the pyrrole polymerization processes with iron(III) chloride and phosphomolybdic acid. Figure 2 shows SEM images of the iron(III) chloride and PMo synthesized composites. The iron(III)

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Figure 3. TEM images of polypyrrole/Cladophora cellulose composites synthesized with (a) iron(III) chloride and (b) PMo.

chloride synthesized sample (panel a) displays the fine fibril structure which is typical for Cladophora cellulose.22 For the PMo synthesized composites, this fibril structure does not seem to be fully preserved as the samples exhibit a more nodular cauliflower-like morphology (panel b). As is seen in Figure 3, which displays TEM images of iron(III) chloride and PMo synthesized samples, the PPy layer covering the cellulose fibrils is approximately 50 nm thick for both samples thus creating fibers with a diameter slightly larger than 100 nm. The measured specific surface areas of the iron(III) chloride and PMo synthesized samples were 58.8 and 31.3 m2/g, respectively, while the corresponding total pore volumes were 0.186 and 0.128 cm3/g. From the weight in combination with the macroscopically measured sample volume, a bulk density of ∼1.1 g/cm3 was obtained for the PMo synthesized samples. The corresponding value for the iron(III) chloride synthesized sample was, however, only ∼0.32 g/cm3. This suggests that the porosity in the pore size range above the maximum analysis limit of the N2 gas adsorption technique may be larger for the iron(III) chloride synthesized samples than for the PMo synthesized ones. This is supported by a comparison of the SEM micrographs in Figure 2 which indicate a larger portion of pores with diameters larger than 300 nm for the iron(III) chloride synthesized sample. The conductivity of the iron(III) chloride synthesized sample was 0.65 S/cm whereas the corresponding value for the PMo synthesized sample was 0.12 S/cm. The data presented above indicate that while the thickness of the PPy coating on the cellulose fibrils was similar for both types of samples, the iron(III) chloride synthesized samples were more porous and had higher surface areas and higher electrical conductivities than the PMo synthesized samples. 3.2. Electrochemical Characterization. Cyclic voltammograms recorded for the two sample types under study in electrolytes containing chloride (a), aspartate (b), glutamate (c), and p-toluenesulfonate (d) are displayed in Figure 4. In these voltammograms, the current was normalized with respect to the mass of the composite used as the working electrode in the experiments. The shapes of the voltammograms clearly differ for the two samples as well as for different electrolytes with the same type of sample. For all electrolytes, the current was found to be higher for the iron(III) chloride synthesized samples than for the PMo synthesized ones. This suggests that the transport rates of the ions were lower in the PMo samples as a result of a more compact structure in good agreement with the SEM micrographs in Figure 2. In the chloride-containing electrolyte, oxidation and reduction peaks are clearly seen for

the iron(III) chloride synthesized sample while the shape of the corresponding voltammogram for the PMo sample indicates that the current was limited by the (higher) resistance associated with this sample. The fact that less well-defined voltammograms were obtained for the iron(III) chloride samples in the three other electrolytes can be explained by the lower conductivities of these electrolytes. This effect also explains the more positive peak potentials in these solutions for both samples. It can thus be concluded that the peak potentials and shape of the voltammograms depend both on the conductivity of the sample and that of the electrolyte. To minimize the effects of the ohmic drop, the samples used as the working electrode were therefore kept as small as possible. Figure 5 shows the result of a chronoamperometric experiment in which the potential was repeatedly stepped between a reducing potential (-0.8 V) and increasing oxidizing potentials to study the oxidative behavior of the two sample types under investigation in the chloride containing electrolyte. For the iron(III) chloride synthesized sample (panel a), it is seen that the oxidation current remained approximately constant during the first three step cycles at oxidation potentials lower than +1.0 V. However, for oxidation potentials of +1.0 V and higher, a gradual decrease during stepping at the same potential together with an overall decrease with increasing potential was seen in the oxidation current, indicating a partial loss of electroactivity due to the onset of overoxidation of the sample. For the PMo synthesized sample (panel b), the oxidation current remained approximately constant during the first three step cycles at oxidation potentials lower than +1.1 V. At the two highest oxidation potentials the overall current magnitude increased while a gradual decrease during repeated stepping to the same potential was observed. The increasing current magnitude at these large potentials shows that the PMo synthesized sample is not fully oxidized at lower potentials as is the iron(III) synthesized sample due to the lower conductivity of the former. The gradual decrease in the current observed during repeated stepping to 1.1 and 1.2 V can most likely also be ascribed to overoxidation giving rise to a gradually decreasing sample capacity. The results thus indicate that the samples can be reversibly oxidized and reduced when the reduction is carried out at -0.8 V for 300 s provided that the oxidation potential is kept below approximately +1.1 V, thus avoiding both trapping of charges in the samples and overoxidation. To evaluate the cycling capacity of the two different samples under study in the various electrolytes, the number of unit charges participating in the oxidation and reduction process, reflecting the number of anions absorbed and expelled respec-

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Figure 4. Cyclic voltammograms of polypyrrole/Cladophora cellulose composites synthesized with iron(III) chloride and PMo recorded at a scan rate of 4.5 mV/s in 2.0 M solutions of (a) chloride, (b) aspartate, (c) glutamate, and (d) p-toluenesulfonate. The current was normalized with respect to the mass of the sample used as the working electrode. Note that the current range on the vertical axis is different in the different figure panels.

tively, were plotted as a function of the oxidation potentials used in Figure 6. The number of absorbed and expelled ions per sample mass was then found to be larger for the iron(III) chloride synthesized sample than for the PMo synthesized one for all four electrolytes. As it is reasonable to assume that the oxidation current was limited by the rate of incorporation of the anion, this can be explained by the presence of larger pores in the iron(III) chloride synthesized sample. In Figure 6, it is also seen that the number of unit charges was significantly higher for chloride than for the other larger anions for both samples (this effect can incidentally also be seen in the voltammograms in Figure 4). This is not surprising considering the difference in size of the anions. In the chloride-containing electrolyte, the number of unit charges for the iron(III) chloride synthesized sample initially increased with increasing oxidation potential and reached a maximum of 3.3 × 1021 units/gram sample at an oxidation potential of 1.1 V. The lower value for oxidation at 1.2 V can be ascribed to overoxidation of the sample at this potential after which it is only possible to expel a small portion of the ions incorporated in the sample during the following reduction. A corresponding increase in the number of unit charges with increasing potential was also seen for the PMo sample in the chloride containing electrolyte for potentials larger than 1 V. The higher potential needed in this case can be explained by the higher iR drop for the less conducting PMo sample. For the larger anions, an increase in the number of unit charges absorbed and expelled with increasing potential was only seen for the glutamate anions at the lowest potentials with the iron(III) chloride synthesized sample. This suggests the presence of a significantly larger iR drop in the glutamate electrolyte than the chloride electrolyte, in good agreement with the fact that the corresponding voltammogram in Figure 4 shows that a higher potential was needed to oxidize the sample in the presence of glutamate. The constant number of unit charges for different oxidation potentials in Figure 6b-d clearly shows that the number of

anions taking part in the oxidation and reduction process was independent of the applied potential as long as it was sufficiently high to oxidize both samples. This means that the oxidation currents in these cases were practically independent of the oxidation potential. This finding indicates that the three largest anions were only able to access sites very close to the polymer electrolyte surface, at least for the 300 s oxidation time employed here. For the three largest anions, the iron(III) chloride synthesized sample absorbed approximately 1 × 1021 unit charges/gram of sample during an oxidation cycle. A simple calculation gives that, if these charges were all situated exactly on the sample surface, the sites would on the average be located about 2.4 Å from each other ([(1 × 1021/58.8 m2)1/2]-1). As this value is smaller than the diameters of the largest anions used here24,25 (e.g., the p-toluenesulfonate ion has a diameter of 7.40 Å in aqueous solution24), it is reasonable to assume that the anions were in fact able to penetrate a short distance into the polymer. For the p-toluenesulfonate ions, a distance of 6.9 nm perpendicular to the surface would be enough to accommodate 1 × 1021 ions ([(1 × 1021/(58.8 × 6.9 × 10-9 m3))1/3]-1 ) 7.40 Å). In Figure 6 it is also seen that the amount of unit charges participating in the reduction process was between 10 and 30% smaller than the corresponding value for the oxidation step for the iron(III) chloride synthesized samples and between 5 and 13% smaller for the PMo synthesized sample. This phenomenon cannot be explained by a trapping effect,26 in which some anions are trapped in the polymer during the reduction as a result of the formation of small islands of conductive zones within an insulating matrix, since the composite then rapidly would lose its ion incorporation capacity. In the experiments the composites were, on the other hand, found to maintain their capacity for many cycles under the experimental conditions employed here, provided that the samples were not overoxidized. This behavior is also in good agreement with our previous results.13 It is thus clear that the mismatch between the oxidation and reduction

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Figure 5. Chronoamperometric step experiment carried out with an iron(III) chloride (a) and a PMo (b) synthesized sample in 2.0 M sodium chloride. The reduction potential was -0.8 V (for 300 s) while the oxidation potential was varied as indicated. The duration of each oxidation step was 300 s. The current was normalized with respect to the mass of the sample used as the working electrode. Note that the current range on the vertical axis is different in the different figure panels.

values must be due to another effect. A more likely explanation is that the charge associated with the charging of the double layer differed for the oxidation and reduction steps. In the above discussions, the influence of the charging of the double layer on the total current was neglected. However, since the specific surface areas of the composites were relatively large, the influence of the charging of the double layer capacitance was most likely significant in the present experiments. It is therefore likely that the observed difference in number of unit charges participating in the oxidation and reduction processes stems from the fact that the number of charges contributing to the double layer charging was different as a result of the potential of zero charge of the system being located closer to the reduction potential than to the oxidation potential. In the extreme scenario that the potential of zero charge is equal to the reduction potential, the double layer capacitance can be estimated to be between 10 and 60 F/g for the iron(III) chloride synthesized sample and between 1 and 14 F/g for the PMo synthesized sample (these values were calculated from the data recorded at an oxidation potential of +0.9 V as Cdl ) ∆Q/[+0.9 V - (-0.8 V)], where Cdl is the double layer capacitance and ∆Q the difference in number of charges participating in the oxidation and reduction process). In this case, the numbers of unit charges

J. Phys. Chem. B, Vol. 113, No. 2, 2009 431 actually participating in the oxidation process would be lower than in Figure 6 and, more importantly, these values would be almost equal to the reduction charges shown in the same figure (as required by the cycling stability). With an assumed potential of zero charge more positive than the reduction potential, but still closer to this potential than to the oxidation potential, the estimated double layer capacitance would be smaller and the oxidation charge would hence be less overestimated, but the oxidation value would still approximately equal the reduction value, which then is slightly underestimated in Figure 6. Above, it was also assumed that the oxidation current is controlled by the rate of the anion absorption process. This is a reasonable assumption since the solubility of the employed salts should be low in the reduced PPy film (which actually may be seen as an organic solvent). Charge compensation by expulsion of cations is therefore unlikely during the oxidation step. It has previously been found15 that no charge compensation by absorption of cations could be detected in electrolytes containing anions with a diameter smaller than or equal to that of PO43- (viz., ∼5 Å27) during the reduction of the oxidized polymer with PPy-covered electrodes synthesized in the presence of ClO4- ions. Given the small size of the chloride ion and the thin PPy coatings, it is reasonable to assume that cation absorption can be totally neglected during the reduction of the presently studied materials. Charge compensation by cation absorption can, however, not be completely ruled out for the reduction steps following the first oxidation in the other electrolytes employed in this study. However, as already mentioned, these large anions would most likely occupy sites at or in the close vicinity of the PPy surface and should therefore be able to readily leave the sample during the reduction step. The influence of cation absorption and desorption on the behavior of the PPy samples will therefore not be further considered here. To more clearly visualize the different anion extraction behavior of the iron(III) chloride and PMo synthesized materials, the number of unit charges participating in the oxidation process at an oxidation potential of +0.9 V has been plotted as a function of anionic species in Figure 7. It is clearly seen that the number of ions absorbed by the iron(III) chloride synthesized sample was higher than for the PMo sample for all ions, most likely due to the almost 2 times larger surface area of the former sample. It is, however, also seen that the relative yield for the PMo sample was better for the largest p-toluenesulfonate ions than for chloride. This indicates that the PMo sample is better suited for the extraction of larger ions. In Figure 7 it is also seen that while the iron(III) chloride synthesized sample did not seem to discriminate between the three types of larger anions (the normalized values are roughly the same for all three types), the corresponding results for the PMo synthesized sample differed. The value for the largest p-toluenesulfonate ions was thus 4.4 times higher than the value for aspartate and 1.2 times larger than the value for glutamate for the PMo sample. As the uncertainties in the displayed data are smaller than 6%, the effects are clearly statistically significant. The different values obtained for these ions with the PMo synthesized sample may be explained by different degrees of extraction of the ions within the bulk of the material. For the iron(III) chloride sample, these larger ions probably reside mainly on the surface of the material. This would explain the almost equal values obtained with the latter material for aspartate, glutamate, and p-toluenesulfonate, as well as the higher value found for chloride (which readily should be able to enter into the bulk of the material) than for the larger ions.

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Figure 6. Number of unit charges participating in the oxidation (OX) and reduction (RED) of the iron(III) chloride and PMo synthesized samples, respectively, as a function of the oxidation potential. Each data point represents an average of the number of charge in steps 2 and 3 for three-step procedures. The deviations between the values for each potential were always less than 6%. The number of charges was normalized with respect to the mass of sample material.

Figure 7. Number of unit charges participating in the oxidation process at an oxidation potential of +0.9 V normalized with respect to the sample mass. See Figure 6 for experimental details and estimates of the uncertainties.

A comparison of the results for aspartate and glutamate is complicated by the fact that these experiments were carried out at pH 10.4 and 9.0, respectively. At pH 10.4, the aspartate solution should contain a mixture of doubly charged and singly charged anions since the pKa for the aspartate NH3+ group is 10.0.28 The dominating species in the case of glutamate should, on the other hand, be the singly charged anion (as the corresponding pKa for the NH3+ group is 9.9.28 Based merely

on the sizes and charges of the aspartate and glutamate ions, one may thus expect a higher degree of ion uptake in the aspartate solution. However, as is seen in Figure 7, the uptake was higher in the glutamate case. There could be at least two reasons for this: the hydrophobicity difference between the aspartate and glutamate ions and the higher pH used in the aspartate experiments. Since glutamate has an additional -CH2 group, this ion should be more soluble in the polymer matrix. At pH 10.4, OH- will also compete with the aspartate ions to a higher degree than at pH 9.0 which would significantly decrease the degree of the uptake of aspartate. The higher degree of uptake of glutamate at pH 9.0 than of aspartate at pH 10.4 can consequently be explained by a combination of hydrophobicity and pH effects. As already indicated, the different ion extraction properties for the iron(III) chloride and the PMo synthesized samples depicted in Figure 7 are due to the very different size of the anions used in the polymerization reactions. The phosphomolybdate anion has been reported to form clusters with sizes of 10-11 Å,29 which means that the vacancies created when this sample is reduced (and the phosphomolybdate anion leaves the sample) is expected to be larger than the size of the ptoluenesulfonate anion (i.e., the largest anion used in this study). The fact that the ratio between the p-toluenesulfonate and chloride values was significantly higher for the PMo sample also clearly shows that the PMo sample is better suited for the extraction of larger ions than the iron(III) chloride synthesized sample. The latter should be particularly useful in extractions

Cladophora Cellulose/Polypyrrole Composites of large anions from solutions also containing small anions. It can thus be concluded that the surface area of the composite should be as large as possible to ensure a large contact area with the solution. Further, the size of the anion used in the polymerization step should be chosen in proportion to the size of the molecule to be extracted to ensure both a large absorption capacity and selectivity. 4. Conclusions It can be concluded that the fine fibril structure, typical for Cladophora cellulose, was preserved for the iron(III) chloride synthesized sample whereas the PMo synthesized sample exhibited a more nodular cauliflower-like morphology. For both samples, the PPy layer covering the cellulose fibrils was found to be about 50 nm thick, thus giving rise to composite fibers with a diameter slightly larger than 100 nm. It was established that the surface area of the iron(III) chloride synthesized sample was almost twice as large as that of the PMo synthesized sample. Both samples absorbed significantly higher amounts of chloride ions as compared to the larger anions investigated. The number of absorbed ions per sample mass was larger for the iron(III) chloride synthesized sample than for the PMo synthesized one for all four electrolytes studied, whereas the latter showed a higher selectivity toward the largest anions under study. The large specific surface areas of the presented materials entail a considerable absorption capacity for large anions, a feature that may prove useful in biotechnological applications involving extraction of proteins, DNA and other biomarkers. When tailoring this type of large surface area fibrous composites for ion extraction, the size of the anion used in the polymerization step as well as the surface area should be optimized. Acknowledgment. Dr. Alfonso E. Garcia Bennett is gratefully acknowledged for taking the TEM pictures. The Knut and Alice Wallenberg foundation as well as the Swedish Science Council are also acknowledged for financially supporting our work. References and Notes (1) Mermilliod, N.; Tanguy, J.; Petiot, F. J. Electrochem. Soc. 1986, 133, 1073.

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