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J. Phys. Chem. B 2007, 111, 4098-4102
Effect of an Anionic Surfactant on the Complexation of Some Nonionic Polymers with Iodine in Aqueous Media Homendra Naorem* and N. Shubhaschandra Singh Department of Chemistry, Manipur UniVersity, Canchipur, Imphal, Manipur-795003, India ReceiVed: NoVember 11, 2006; In Final Form: January 25, 2007
Complexation of some water soluble nonionic polymers, namely, polyvinylalcohol (PVA), polyvinylpyrrolidone (PVP), and hydroxypropyl cellulose (HPC), with iodine has been studied in aqueous and aqueous sodiumdodecylsulfate (SDS) solution. While the complexation was indicated by a red shift of the tri-iodide band in case of PVP or HPC, the PVA-iodine complex showed its characteristic band around 500 nm. It was observed for the first time that presence of SDS led to complete break down of the PVA-iodine complex and its characteristic blue color. The presence of monomers of SDS, however, appeared to favor the formation of the iodine complex with PVP or HPC. Addition of n-propanol, which is known to prevent the formation of gels or microgels in polymer solutions, was found to enhance the polymer-iodine complex. Gels of pure HPC and HPC with iodine both in presence and absence of SDS have been prepared and studied.
Introduction Some water-soluble nonionic polymers are known to form complexes with iodine, and such complexes are rapidly becoming interesting in view of their possible applications in biomedical area because of the bactericidal properties associated with the iodine atoms. For example, iodine complexes of amylose, polyvinylalcohol (PVA), or polyvinylpyrrolidone (PVP), etc., have shown promising applications in the treatment of female breast lesions or other related conditions.1-4 The bactericidal activities of the iodine atoms released from such complexes, however, have been reported to be dependent on the type and the nature of the iodine containing group in the polymer.3,4 The complexation process is generally understood to be initiated by sorption of iodine on the polymer surface, which brings about changes in the native conformation of the polymer, and the iodine molecules then enter into the spaces in the polymer aggregates where it gets converted into polyiodines or polyiodides.5,6 From diffraction studies,7 it was shown that iodine penetrates into the crystalline phase of polyacrylonitrile (PAN) chain uniaxially expanding the pseudohexagonal crystals of PAN by replacement of PAN molecular cylinders by polyiodine columns. In case of polyvinyl octal complex,8 iodine has been reported to induce widening of the polymeric chain changing the order in the molecular lattice. From detailed investigations on the amylose-iodine complex, Calabrese and Khan9 have reported the presence of I6 polyiodine units in the complex stabilized within the amylose helix cavity whereas presence of polyiodides in complexes of PVA,5 nylon-6,10 PVP,11 ethyl cellulose,12 etc. has been well established which clearly indicate that type and the nature of the iodine species present in the polymer-iodine complexes depend on the nature of the polymer. Because of low solubility of iodine in pure water, polymeriodine complexes have generally been studied in presence of added potassium iodide, which enhances the solubility of iodine in water. Investigations on the PVA-iodine complex in aqueous media revealed that while increase in intermolecular hydrogen bonding favors the complex formation, the presence of gels or * Corresponding author. E-mail:
[email protected].
microgels inhibits the complex.5,13 It has also been reported that presence of nonaqueous solvent in which iodine is highly soluble does not favor complexation of iodine with polymers.9,14 It is well-established that polymers can induce micellization of anionic surfactants at much lower concentration than their normal critical micelle concentration (cmc) values.15-17 On the other hand, addition of ionic surfactants to moderately concentrated solutions of nonionic polymers has been reported to substantially increase the viscosity of the solution even leading to formation of gels.18,19 Typically, a maximum in the viscosity has been observed at a certain surfactant concentration, beyond which the viscosity decreases. The increase in the viscosity has been attributed to the expansion of the polymer coils and increased intermolecular interaction between the polymer and the surfactant molecules.16,18 In view of the fact that presence of an anionic surfactant can bring about changes in the conformation as well as the aggregate structure of the polymer, presence of a surfactant is likely to affect the formation of polymer-iodine complex. The present paper reports the result of the studies on the complexation of some water soluble nonionic polymers with iodine in presence of an anionic surfactant, sodiumdodecylsulfate (SDS). The polymers chosen for the study are PVA, PVP, and hydroxypropyl cellulose (HPC). The complexation has also been studied in presence of n-propanol, which is known to minimize the formation of gels or microgels in polymer solutions. Since the HPC-iodine complex has comparatively been less studied, we have prepared and studied the gels of pure HPC and HPC with iodine both in absence and presence of SDS. Experimental Section The polymers PVP (Mw 40 000) and HPC (Mw 100 000) were obtained from Aldrich while PVA (Mw 15 000) was procured from Fluka. Polymer samples were used as received. High purity sample of SDS (Loba Chemie) was extracted from ether and finally re-crystallized from methanol. Chemical structures of the polymer, PVA, PVP, and HPC and the surfactant molecules are presented in Figure 1. Extra pure
10.1021/jp067471d CCC: $37.00 © 2007 American Chemical Society Published on Web 04/03/2007
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Figure 1. Schematic representation of the chemical structure of PVA, PVP, SDS, and HPC molecules.
reagent grade samples of Iodine (I2) and potassium iodide (KI) having purity of over 99.5% were procured from Loba Chemie. Reagent grade sample of n-propanol (Ranbaxy) was fractionally distilled prior to its use. Stock solution of the polymers (about 2% w/v) was prepared by dissolving the polymer in double distilled water, the dissolution was carried out by standing the solutions for about 48 h at 25 °C. Mixture of iodine and potassium iodide solution containing 0.2 mM iodine and 0.8 mM potassium iodide in aqueous media was prepared from a stock solution of iodine (2 mM) in potassium iodide solution (8 mM). The polymer-iodine reaction mixture was prepared in 10 mL volumetric flask by mixing 0.5 mL of the polymer (0.75 mL in case of HPC) solution with 5 mL of the I2-KI mixture and finally making up the volume with double distilled water or SDS solution such that the resultant mixture contains 0.1% polymer (except for HPC for which it was 0.15%), 0.1 mM iodine in different molar concentrations of SDS. HPC Gel Preparation. An aqueous solution of HPC (5% w/v) was prepared by dissolving the HPC polymer in double distilled water at about 50 °C in a water bath under constant stirring for 24 h. The mixture was then slowly cooled to room temperature and kept at this temperature for one night to eliminate air bubbles. Following a similar procedure, HPCiodine gels in aqueous or surfactant media were prepared by mixing adequate amounts of the polymer and the I2-KI solutions in aqueous or aqueous surfactant media such that the resultant gels have a polymer content of 2% and 1 mM iodine and 6 mM SDS. For simplicity, concentrations of the HPC, I2, or SDS in the gels were expressed based on their respective aqueous solutions before mixing. The gels thus prepared were then subjected to several cycles of heating (to melt) and cooling in order to obtain a uniform gel. It was then finally washed several times with water and dried in air at room temperature. The UV-visible absorption spectra of the polymer-iodine reaction mixtures both in aqueous and aqueous surfactant media were recorded against the polymer solution (reference) in a Shimadzu UV-2456 UV-vis scanning spectrophotometer in the wavelength range from 300 to 600 nm at 303.15K. For identification of the absorption due to the polymer-iodine complex, the spectra of the reaction mixtures were recorded using I2-KI solution containing 0.1 mM iodine and 0.4 mM potassium iodide as reference. All the measurements were carried out at 303.15 K, and the temperature of the solution
Figure 2. Absorption spectra of polymer-iodine mixtures in aqueous and aqueous surfactant solutions at 303.15 K with PVA solution as reference (spectra of the PVA-iodine complex recorded with I2-KI solution as reference are shown in the inset): I2-KI solution in water (a), PVA-iodine mixture in water (b), in 4 mM SDS (c), and in 12 mM SDS (d).
was maintained by circulating thermostated water from a HaakeThermostat around the sample holder. Infrared spectra of the gels of HPC were recorded using a Shidmazu FTIR8400S spectrophotometer in the region 500-4000 cm-1. DSC scan for the HPC gels were performed on a Perkin-Elmer DSC-2 from 37° to 170 °C at a heating rate of 10 °C/min in an atmosphere of dry oxygen free nitrogen. Results and Discussion Aqueous mixture of I2 and KI solution shows the characteristic tri-iodide (I3-) band around 350 nm.5,13 Formation of polymer-iodine complex is generally indicated by a shift in the triodide band and sometimes by the appearance of a new band around 450-650 nm as in case of starch-iodine9 or PVAiodine13 complexes, which has been assigned to polyiodines or polyiodides present in the complex. The absorption spectra of PVA, PVP, and HPC in I2-KI in aqueous as well as aqueous SDS solutions are shown in Figures 2, 3, and 4 respectively. It is observed from the Figures that while formation of PVP or HPC complex with iodine was indicated by a red shift of the I3- band, formation of the PVA-
4100 J. Phys. Chem. B, Vol. 111, No. 16, 2007
Figure 3. Absorption spectra of polymer-iodine mixtures in aqueous and aqueous surfactant solutions at 303.15 K with PVP solution as reference (spectra of the PVP-iodine complex recorded with I2-KI solution as reference are shown in the inset): I2-KI solution in water (a), PVP-iodine mixture in water (b), in 4 mM SDS (c), and in 12 mM SDS (d).
Figure 4. Absorption spectra of polymer-iodine mixtures in aqueous and aqueous surfactant solutions at 303.15K with HPC solution as reference (spectra of the HPC-iodine complex recorded with I2-KI solution as reference are shown in the inset): I2-KI solution in water (a), HPC-iodine mixture in water (b), in 4 mM SDS (c), and in 12 mM SDS (d).
iodine complex was characterized by the appearance of a new band at around 500 nm, which has been attributed to presence of I5- in the complex.13,20 For the PVA-iodine complex in aqueous solution, the absorption and the extent of color development have been reported to be greatly influenced by the syndiotacticity of PVA.5 From the absorption spectra shown in the inset of the Figures 2-4, λmax for the iodine complex of PVA, PVP, and HPC were observed at 495, 375, and 365 nm respectively. While the PVA complex is characterized by presence of both I3- and I5- polyiodides, PVP or HPC complex appeared to have I3- only in it. While our results for PVP or PVA iodine complexes are generally in good agreement with those reported in the literature,11,13 there are no report available for the HPC-iodine complex to compare our results. Polymers in solution exist as aggregates and the iodine molecules get adsorbed on the aggregate surface, which then enter into the aggregate spaces where it is converted into polyiodides through interactions with the hydroxyl or other groups present in the polymer molecule.13 The type of polyiodides present in the complex largely depends on the state of aggregation of the polymer, the size of the space in the polymer aggregate and also on the number of iodine molecules able to
Naorem and Singh
Figure 5. Absorption spectra of PVA-iodine complex in aqueous solution containing different percentage of n-propanol (vol/vol): 2% (a), 5% (b), and 8% (c).
enter into the aggregate space. The fact that bigger polyiodides are present in the PVA complex suggest that PVA provides relatively larger aggregate space as compared to those of PVP or HPC wherein only I3- are present. It is evident from Figure 2 that the characteristic absorption of the PVA-iodine complex at 495 nm, and hence, the blue color of the complex disappeared completely in presence of even small amounts of the anionic surfactant. To the best of our knowledge, the disappearance of the blue color of the PVAiodine complex in presence of an anionic surfactant has not been reported earlier. The molecules of SDS are known to micellize on PVA surface at much lower concentration than its usual critical micelle concentration (cmc) values, and the presence of SDS, on the other hand, enhances the viscosity of PVA solution due to increased network of intermolecular hydrogen bonding, even leading to formation of gels.17,21 The disappearance of the blue color or the break down of the PVA-iodine complex in presence of an anionic surfactant may, therefore, be attributed to the preferential interaction of PVA with SDS. In case of PVP or HPC complexes, the presence of SDS resulted in further red shifting of the tri-iodide band (Figures 3 and 4) along with an increase in absorption at low concentration of SDS (4 mM), but at higher concentration (12 mM) the absorption was found to decrease. The results would suggest that while presence of SDS monomers led to increasing the PVP or HPC complex, SDS micelles appeared to inhibit the complex formation. Since n-propanol is known to prevent gelation of polymer solutions by breaking the polymer aggregates and binding with the polymer through hydrogen bond interactions,13 the complexation of the polymers under investigation with iodine has also been studied in presence of n-propanol. The absorption spectra of the PVA-iodine complex in presence of n-propanol is presented in Figure 5 while those of PVP and HPC complexes are shown in Figure 6 respectively. The figures revealed that addition of n-propanol led to an increase in the absorbance of I3- for all the complexes, but in case of PVA complex, the increase in I3- was found to be accompanied by a proportionate decrease in the absorbance of the I5- band with an isosbestic point around 393 nm. The result is in conformity with the proposed mechanism for formation of polyiodides in the complexes:13
I2 + I- h I3I3- + I2 h I5-
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J. Phys. Chem. B, Vol. 111, No. 16, 2007 4101
Figure 6. Absorption spectra of PVP-iodine and HPC-iodine complexes in aqueous solution containing different percentage of n-propanol (vol/vol): 2% (a), 5% (b), and 8% (c).
Figure 8. DSC profile of gels of HPC (a), HPC in 6 mM SDS (b), HPC-iodine (c), and HPC-iodine in 6 mM SDS (d).
Figure 7. Infrared spectra of gels of HPC(a), HPC in 6 mM SDS (b), HPC-iodine (c), and HPC-iodine in 6 mM SDS (d).
According to this mechanism, the type of the polyiodides formed in the complex is influenced by the state of aggregation of the polymer in solution and any decrease or increase in I5will be compensated by a proportional increase or decrease in I3- concentration. The observation that addition of n-propanol led to increase in complexation confirms the presence of microgels in the polymer solutions. Since microgels or gels are formed due to increased network of intermolecular hydrogen bonds and microgels have many junction points with reduced aggregate spaces, microgels do not have the ability for complex formation.13 The decrease in I5- as in case of PVA complex indicates that while effectively preventing formation of microgels in the system, the hydroxyl group of n-propanol also interacts with the hydroxyl side of PVA lowering not only the formation of PVA aggregate but also the size of the aggregate spaces, which would result into conversion of the larger polyiodides I5- trapped in the polymer space into smaller polyiodides I3-. It may, however, be mentioned that in presence of SDS no appreciable ‘n-propanol effect’ was observed suggesting that SDS molecules preferentially interact with the polymer as compared to n-propanol molecules. The FTIR spectra of pure HPC, HPC-SDS, and HPC-iodine gels with and without SDS are given in Figure 7. The HPC gels are characterized by a broad band around 3500 cm-1 due to -OH stretching.22 A comparison of the spectra of the different gels indicate there was no significant structural change in the HPC gel by incorporation of SDS or iodine except for the broadening of -OH stretching band along with a small shift toward higher frequency. The broadening and the shifting of the IR band in the mixed gels as compared to that of pure HPC
gel are indicative of increased interactions due to intermolecular hydrogen bonding in the gel structure.22,23 Figure 8 shows the DSC profile of pure HPC gel along with HPC-iodine gels both in absence and presence of SDS. It is clear from the figure that pure HPC gel shows one endothermic peak, the gel melting point around 365K.23 It is also evident from the figure that while introduction of iodine into the HPC gel matrix led to an increase in the gel melting point, presence of SDS in the HPC gels lowered the gel melting point. But when both iodine and SDS are present in the gel, the resultant gel showed a decrease in the melting point. However, introduction of iodine or SDS in the HPC gels resulted in splitting the endothermic peak into two peaks indicating the presence of two phases in the systems.24 The shift to the higher temperature would suggest the presence of iodine molecules in the gel network. The broadening and the shifting of the peak toward lower temperature in presence of SDS would suggest that the polymer molecules preferentially form intermolecular hydrogen bond with the surfactant molecules as compared to the iodine molecules. Conclusion The formation of a complex between PVP or HPC and iodine was indicated by a red shift in the triodide band while PVAiodine complex showed its characterized band around 500 nm in pure aqueous media. Addition of the anionic surfactant SDS resulted into disappearance of the characteristic blue color of the PVA-iodine complex indicating that the complex is not formed in aqueous surfactant media. However, in case of PVP or HPC, presence of the monomers of SDS favored the complex formation but at higher concentration, the micelles of SDS apparently decreased the complex. Complexation was found to increase with increasing content of n-propanol in the system since n-propanol inhibits the formation of gels or microgels in the polymer solution. But in case of PVA-iodine complex, addition of n-propanol led to conversion of bigger polyiodines into smaller ones, which perhaps is indicative of increased intermolecular hydrogen bond interaction between n-propanol and PVA effecting a decrease in the PVA aggregate space. FTIR
4102 J. Phys. Chem. B, Vol. 111, No. 16, 2007 spectra of the gels of pure HPC and HPC mixed with iodine in presence SDS showed that there was no significant structural change in the HPC gel by incorporation of SDS or iodine except for the broadening of -OH stretching band along with a small shift toward higher frequency. Acknowledgment. The authors thank Prof. T.N. Guru Row of the Solid State Chemistry Unit of IISc, Bangalore for giving access to the DSC. References and Notes (1) Ghent, W. R.; Eskin, B. A.; Low, D. A.; Hill, L. P. Can. J Surg. 1993, 36, 453. (2) Eskin, B. A.; Grotkowski, C. E.; Connolly, C. P.; Ghent, W. R. Biol. Trace Elem. Res. 1995, 49, 9. (3) Singhal, J. P.; Singh, J.; Ray, A. R.; Singh, H. Biomater. Artif. Cells Immobilization Biotech. 1991, 19, 631. (4) Pierard, G. E.; Franchimont, C. P.; Arrese, J. E. Eur. J. Clin. Pharmacol. 1997, 53, 101. (5) Noguchi, H.; Jodai, H.; Ito, Y.; Tamura, S.; Matsuzawa, S. Polym. Int. 1997, 42, 315. (6) Amiya, S.; Fujiwara, Y. Rep. PVAL Committee, 1975, 66, 94. (7) Choi, Y. S.; Oishi, Y.; Miyasaka, K. Polym. J. 1990, 22, 601. (8) Kenigsberg, T. P.; Ariko, N. G.; Agabekov, V. E. Ach-Models Chem. 1995, 132, 303. (9) Calabrese, V. T.; Khan, A. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 2711.
Naorem and Singh (10) Kawaguchi, A. Polymer 1996, 37, 4877-4880. (11) Guzenko, N. V.; Voronina, O. E.; Vlasova, N. N. J. Appl. Spectrosc. 2004, 71, 151. (12) Wang, Y.; Easteal, A. J. Appl. Polym. Sc. 1999, 71, 1303. (13) Noguchi, H.; Jyodai, H.; Matsuzawa, S. J. Polym. Sci., Part B: Polym. Phys. 1997, 35, 1701. (14) Barrett, A. J.; Barrett, K. L.; Khan, A. J. Macromol. Sci. Part A 1998, 35, 1603. (15) Meszaros, R.; Varga, I.; Gilanyi, T. J. Phys. Chem. B 2005, 109, 13538. (16) Griffiths, P. C.; Howe, A. M. Recent Res. DeV. Phys. Chem. 1998, 2, 893. (17) Frederic, G. F.; Freitag, R. Langmuir 2001, 17, 4711. (18) Chari, K.; Antalek, B.; Lin, M. Y.; Sinha, S. K. J. Chem. Phys. 1994, 100, 5294. (19) Holmberg, C.; Nilsson, S.; Singh, S. K.; Sundelof, L. O. J. Phys. Chem. 1992, 96, 871. (20) Hayashi, S.; Hirai, Y.; Hojo, N, Sugita, H.; Kyogoku, Y. J. Polym. Sci. Polym. Lett. ed. 1982, 20, 69. (21) Ogasawara, K.; Nakajima, T.; Yamaura, K.; Matsuzawa, S. Progr. Colloid Polym. Sci. 1975, 58, 145. (22) Socrates, G. In Infrared Characteristic Group Frequencies; Wiley: New York, 1980; p 46. (23) Chatterjee, J.; Haik, Y.; Chen, C.-J. J. Appl. Polym. Sci. 2004, 91, 3337. (24) Khutoryanskkiy, V. V.; Cascone, M. G.; Lazzeri, L.; Barbani, N. Z. S.; Mun, G. A.; Dubolazov, A. V. Polym. Int. 2004, 53, 307.
