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Novel Method for the Preparation of Anionic Surfactant-Selective Electrodes Imre Varga,*,† Ro´bert Me´sza´ros,† Zolta´n Szaka´cs,‡ and Tibor Gila´nyi† Department of Colloid Chemistry, Lora´ nd Eo¨ tvo¨ s University, H-1518 Budapest 112, P.O. Box 32, Hungary, and Department of Inorganic and Analytical Chemistry, Lora´ nd Eo¨ tvo¨ s University, H-1518 Budapest 112, P.O. Box 32, Hungary Received March 9, 2005. In Final Form: May 26, 2005 A simple one-step synthesis is described for the functionalization of poly(vinyl chloride) used for the preparation of anionic surfactant-selective membrane electrodes. The method is based on the nucleophilic substitution of a fraction of the chlorine atoms bound to the poly(vinyl chloride) backbone by trimethylamine. The prepared slightly charged polymer gave rise to high-quality surfactant-selective electrode membranes, which had a Nernstian response, short response time, and appropriate stability.
Introduction Since the appearance of the first surfactant ion-selective electrodes,1,2 they have been improved significantly and used extensively for direct measurements of surfactant activity. According to the literature, basically four types of electrodes have been developed: the ion-exchange membrane, the solid state, the liquid ion-exchange membrane, and the polymeric membrane electrodes.3 Overall, the polymeric membrane electrodes have appeared to be much better compared to the three other types of surfactant ion-selective electrodes in terms of the nature and speed of the response, as well as the lifetime and the selectivity of the prepared electrodes.3 Earlier versions of the polymeric membrane electrodes were based on a poly(vinyl chloride) (PVC) membrane containing active ionic species (ionophores), generally large charged hydrophobic molecules of opposite charge to that of the investigated surfactant, and a plasticizer.4-6 It was assumed that the surfactant may form an immobile complex with the ionophore; therefore, the membrane in its conditioned state can be used for the construction of electrodes with a Nernstian response in the function of the equilibrium surfactant activity. There has been some progress in these types of membrane designs because very recently a new type of ionophore (aza-oxa cycloalkanes)7 as well as liquid plasticizers8 were tested for polymeric membrane surfactant electrodes. However, it turned out that although this design revealed good sensitivity and a Nernstian response its long-term stability was poor because of the dissolution of the (not completely immobile) ionophore/surfactant complex in the solution.4 The crucial steps toward a more sophisticated version of surfactant-selective polymeric membrane electrodes were made by Cutler et al.,4 Davidson,9 and Bloor et al.,10,11 * Corresponding author. E-mail:
[email protected]. † Department of Colloid Chemistry. ‡ Department of Inorganic and Analytical Chemistry. (1) Botre, C.; Crescenzi, V.; Mele, A J. Phys. Chem. 1959, 63, 650. (2) Malik W.; Jain, A. Indian J. Chem. 1968, 6, 140. (3) Surfactant Solutions. Zana, R., Ed.; Surfactant Science Series; Marcel Dekker: New York, 1987; Vol. 22, Chapter 9. (4) Cutler S. G.; Meares, P.; Hall, D. G. J. Electroanal. Chem. 1977, 85, 145. (5) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1982, 86, 3866. (6) Schefer, U.; Ammann, D.; Pretsch, E.; Oesch, U.; Simon, W. Anal. Chem. 1986, 58, 2282. (7) Segui, M. J.; Lizondo-Sabater, J.; Martinez-Manez, R. Anal. Chim. Acta 2004, 525, 83. (8) Masadome, T.; Yang, J. G.; Imato, T. Microchim. Acta 2004, 144, 217.
who revealed that PVC membranes with chemically bound charges are necessary to form completely immobile ionophores and therefore electrodes with the desired response and long-term stability. Because of the commercial availability of negatively charged, carboxylatemodified PVC, cationic surfactant-selective electrodes, they have been widely constructed and successfully applied in the literature.5,12-14 However, the construction of anionic surfactant electrodes remained a real challenge for the researchers. It was shown that PVC membranes with a few quaternized amine groups covalently bound to the end groups of the PVC molecules could be used for the preparation of anionic surfactant-selective electrodes with good characteristics.4,9-11,15 A further improvement was made by Davidson16 et al. and Bloor et al.10,11 with the application of polymeric plasticizers instead of liquid plasticizers that might also be solubilized in concentrated surfactant solutions. The latest development was presented by Xu et al.,11 who coated a silver wire with a membrane film containing the functionalized PVC and a polymeric plasticizer. In this electrode setup, there is no need for an inner reference solution; therefore, the risk of potential leakage between the inner reference solution and the outer surfactant solution via the polymeric membrane diminishes. Altogether, it can be concluded that anionic surfactantselective electrodes that are based on PVC membranes with a few bound cationic segments have proven to be superior to the earlier polymeric membrane constructions in every respect, but especially in long-term stability and electrode response time. These types of electrodes have been successfully applied recently in the studies of the nature of polymer/surfactant interaction as well as the equilibrium properties of surfactant solutions.17-20 However, the process of the functionalization of the PVC membrane with quaternized amine groups is a long, (9) Davidson, C. J. Ph.D. Thesis, University of Aberdeen, Aberdeen, U.K., 1983 (10) Bloor, D. M.; Li, Y.; Wyn-Jones E. Langmuir 1995, 11, 3778. (11) Xu, R.; Bloor, D. M. Langmuir 2000, 16, 9555. (12) Harrison, I. M.; Candau, F.; Zana, R. Colloid Polym. Sci. 1999, 277, 48. (13) Gharibi, H.; Rafati, A. A.; Feizollahi, A.; Razavizadeh, B. M.; Safar pour, M. A. Colloids Surf., A 1998, 145, 47. (14) Kosmella, S.; Kotz, J.; Shirahama, K.; Liu, J. J. Phys. Chem. B 1998, 102, 6459. (15) Fukui, H.; Kaminaga, A.; Maeda, T.; Hayakawa, K. Anal. Chim. Acta 2003, 481, 221. (16) Davidson, C. J.; Meares, P.; Hall, D. G. J. Membr. Sci. 1988, 36, 511.
10.1021/la050639m CCC: $30.25 © 2005 American Chemical Society Published on Web 06/10/2005
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Figure 1. Reaction scheme illustrating the functionalization of PVC.
tedious, and expensive procedure. The method involves the (co)polymerization of vinyl chloride in the presence of alkyl amines followed by the purification of the PVC samples and then the long process (3 days) of the quaternization of the amine groups with alkyl halogenides at elevated temperature. In this letter, a simple and more economic synthesis of a PVC membrane with a low level of bound cationic charges will be given. Furthermore, preliminary tests of the anionic surfactant-selective electrode made from these new types of polymeric membranes will also be presented and discussed. Experimental Section Materials. Sodium dodecyl sulfate was purchased from SigmaAldrich and was recrystallized twice from a 1:1 benzene/ethanol mixture. For the membrane preparations, a PVC sample (BorsodChem, S-5070; Mw ≈ 105) was used. All of the other chemicals (plasticizer: tritolyl phosphate, tetrahydrofuran (THF), tetrahydrofuran-d8 (THF-d8), acetone, and NaBr) were provided by Sigma-Aldrich. During the experiments, bidistilled water was used for the preparation of aqueous solutions. Functionalization of PVC. PVC (2 g) was dissolved in 100 cm3 of THF. The polymer solution and the trimethylamine were cooled to -20 °C by immersion in a mixture of acetone and carbon dioxide. Trimethylamine (∼11-12 g) was distilled into the PVC solution by means of the careful removal of the cooling mixture from below its container. Finally, the reaction vessel was sealed and left for 2 weeks at room temperature. After the 2-week reaction period, the reaction vessel was opened, and the excess amount of trimethylamine and the THF were removed by vacuum distillation. The resulting polymer film was dissolved in 50 cm3 of THF, and the vacuum distillation was repeated to remove any traces of trimethylamine. Finally, the polymer was dried in vacuum until reaching constant mass. Preparation and Conditioning of the Membrane Electrode. The polymer product was dissolved in 50 cm3 of THF. This solution (20 cm3) was mixed with the plasticizer solution (2.97 g of tritolyl phosphate in 40 cm3 of THF). The electrode membrane film was prepared by the careful drying of this solution for 2 days in a dust-free environment. The optimal membrane thickness was found to be around 0.1-0.2 mm. Circular membranes of appropriate sizes were cut from the polymeric membrane, and they were mounted into an electrode body in the same way as in ref 16. The inner electrode was a silver/silver bromide electrode immersed in a NaBr solution. The conditioning of the membrane electrodes was found to be crucial and essential to gain high-quality, stable electrodes. Prior to a series of potentiometric titrations, the electrode was conditioned in 5 mM SDS solution for 2 days. Having finished the measurements, we rinsed the electrode carefully and stored it dry in a dust-free environment. Applying this procedure, we could use the electrode in at least 80-100 subsequent titrations.
Methods NuclearMagneticResonanceSpectroscopy(NMR). H NMR measurements of the PVC and the functionalized
1
(17) Bloor, D. M.; Wan Yurnus, W. M. Z.; Wan-Badhi, W. A.; Li, Y.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1995, 11, 3395. (18) Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1995, 11, 2312. (19) Li, Y.; Ghoreshi, S. M.; Warr, J.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 3093. (20) Sidhu, J.; Bloor, D. M.; Couderc-Azouani, S.; Penfolld, J.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2004, 20, 9320.
PVC were made on a Bruker DRX 250 MHz instrument in THF-d8. Potentiometric Measurements. The electromotive force (emf) values of the constructed galvanic cell (Ag/ AgBr/cNaBr solution/membrane/cSDS, cNaBr/AgBr/Ag) were determined by means of a Radelkis research pH meter at 25.00 ( 0.05 °C with an accuracy of (0.1 mV. A medium (10 cm3 of 10-4 or 0.1 M NaBr) was placed into the measuring cell and equilibrated for 30 min. After that, the medium was titrated with an SDS stock solution (of the same NaBr concentration). The equilibrium emf values were read at each titration step. Results and Discussion The targeted introduction of the cationic segments into the PVC backbone was easily achieved by substituting a small fraction of the chlorine atoms bound to the polymer backbone by trimethylamine in a THF solution at room temperature (Figure 1). The successful introduction of the trimethylammonium groups into the polymer chain was indicated by the appearance of a small peak at δ 3.3 in the 1H NMR spectrum of the functionalized PVC, with a relative integral of ∼0.5%, also taking into consideration the chemical equivalency of the N-methyl groups (9H). Furthermore, it should be noted that because the boiling point of the trimethylamine is ∼3 °C the product could be easily purified by simple vacuum distillation. The carefully prepared and conditioned electrodes gave rise to highly accurate potentiometric measurements (with (0.2 mV reproducibility of the repeated experiments). The typical response time of the electrodes was around 30-40 s. A set of potentiometric measurements are shown in Figure 2, at low (10-4 M NaBr) and high (0.1 M NaBr) ionic strength, as a function of the SDS concentration. According to Figure 2a, the sensitivity of the electrode (the detection limit of the SDS) was found to be =10-6 M SDS in 10-4 M NaBr solution, and the electrode gave a Nernstian response in the SDS concentration range of 10-5 M - cmc. The emf versus log(cSDS) functions are linear up to the cmc (with slopes of 58.1 and 58.6 mV) at both ionic strengths, and they become constant above the cmc. The experimental cmc’s of the SDS, determined by this potentiometric method, were 8.1 and 1.24 mM in 10-4 M NaBr and 0.1 M NaBr, respectively. These values are in very good agreement with the well-known literature data.21 The accuracy and reliability of the electrode in the presence of high salt concentration reveal the high selectivity of the electrode toward small inorganic anions. Having followed the previously discussed instructions referring to the storage and conditioning of the electrodes, the average lifetime of the membrane was found to be around 80-100 titration cycles. Altogether, the prepared anionic surfactant-selective electrode reveals all of the favorable characteristics of the earlier PVC membrane electrodes where the end groups of the PVC were functionalized by cationic groups. (21) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems; NSRDS-NBS 36; U.S. National Bureau of Standards: Washington, DC, 1971; p 52.
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the negative adsorption of these ions on the micelles22 leading to the decrease of the volume average surfactant concentration. Because in the presence of a large amount of inert electrolyte the electrostatic interactions of the surfactant ions and micelles are hindered, the minimum cannot be observed in Figure 2b. Finally, it should be noted that further improvements involving the application of polymeric plasticizers and the fabrication of coated wirelike electrodes are under intensive development. Furthermore, the application of the electrode, prepared by the novel functionalization method, for the study of polymer/anionic surfactant interaction is also in progress.
Figure 2. Electromotive force in the function of the total SDS concentration (A) in 10-4 M NaBr and (B) in 0.1 M NaBr.
It should be noted that the measured emf curves show a shallow minimum at the cmc in low ionic strength solutions (e.g., in 10-4 M NaBr), whereas the measured emf becomes practically constant in the presence of a large amount of supporting electrolyte. This could be interpreted in terms of the electrostatic interaction of the surfactant with the similarly charged micelles, which gives rise to
Conclusions A novel method for the preparation of anionic surfactantselective membrane electrodes is described. The prepared electrodes are based on a new type of PVC membrane in which a few cationic segments are incorporated statistically along the polymer chain. The synthesis of the functionalized polymer is based on the nucleophilic substitution of a fraction of the chlorine atoms bound to the poly(vinyl chloride) backbone by trimethylamine. The new method involves a simple one-step synthesis and a straightforward purification and offers a quick and cheap alternative to the preparation of the chain-end-functionalized PVC membranes described earlier in the literature. By means of these slightly charged polymeric membranes, high-quality surfactant-selective electrode membranes that have a Nernstian response, short response time, and appropriate stability can be prepared. Acknowledgment. This research was supported by a Marie Curie European Reintegration Grant within the 6th European Community RTD Framework Program and by the Hungarian Scientific Research Fund (OTKA No. F 034838 and T 043621). LA050639M (22) Gilanyi, T.; H. Szabo, G. Colloids Surf. 1991, 57, 273.
