CHAPTER 6 Iodine-Iodide Equilibrium in Aqueous and Mixed Aqueous Organic Solvent Media with or without a Surfactant

Transcription

1 CHAPTER 6 Iodine-Iodide Equilibrium in Aqueous and Mixed Aqueous Organic Solvent Media with or without a Surfactant 6.1 Introduction The classical age old iodine-iodide equilibrium leading to formation of triiodide (an aggregate) in solution has gained renewed interest especially with reference to the understanding of the interactions of iodine with water soluble polymers like starch or PVA leading to formation of the well known blue complex in aqueous as well as aqueous micellar media. 1-7 Because of the low solubility of the iodine, iodine is always used in combination with iodide ions and hence understanding the tri-iodide formation in the system assumes considerable significance in the study of formation of polymeriodine complex. In fact, the formation of the blue complex and the type and the nature of iodine or iodide present in the complex are truly a complex phenomena since there is a strong possibility of the involvement of polyiodines like I 2, I 4, I 6, etc. or poly-iodide ions such as I 3, I5, I7, etc. in the complexation. 1,3,8 Besides, because of the bactericidal properties associated with iodine, systems containing iodine/iodide have potential uses and in fact find applications in many biomedical areas. 9,10 Any change in the nature of the solvent media such as hydrophobicity of the media or the presence of surfactant is likely to influence the iodine-iodide equilibrium and the complexation, if any. It is surprising to note that despite the popularity of the iodine-iodide equilibrium, there is hardly any information available in the 107

2 literature on how the presence of an organic solvent or a surfactant would influence formation of tri-iodide though it has long been reported the formation of tri-iodide is indeed influenced by the presence of the organic solvent, which has been ascribed to the changes in the solvation properties. 11 Hayakawa et al 12 from the study of formation constant of the tri-iodide in mixed alcohol media reported similar observation that the equilibrium constant value increases with increase in the alcohol content. They also reported that in presence of a cationic surfactant DTAB, the iodine and the iodide ions are solubilized in the hydrophilic surface region of the micelles of DTAB. 13 Keeping in view of the role of the solvent media in the tri-iodide formation, it is of considered worthwhile to study the formation of tri-iodide in aqueous and mixed aqueous organic media including a surfactant. The present work is an attempt to study the influence of the solvent hydrophobicity on the iodine-iodide equilibrium in different mixed media including polymer with or without a surfactant. The organic solvents chosen for the present study are ethylene glycol (EG), 2-methoxy ethanol (ME), 2- ethoxy ethanol (EE) and the polymers include poly ethylene glycols (PEG0, PEG0, PEG 600), hydroxy propyl cellulose (HPC) and poly ethylene oxide (PEO) while the surfactant employed in the study are an anionic surfactant, SDS and a nonionic surfactant, TX-100 respectively. 108

3 6.2 Experimental Materials Extra pure reagent grade sample of iodine (I 2 ) was obtained from Merck (India) and potassium iodide (KI) having purity of over 99.5 % was procured from Loba Chemie (India). The organic solvents and the surfactant employed in the study are essentially the same as described elsewhere in the thesis. All the solvents were purified following the standard procedures. 14 Freshly prepared saturated stock solutions of Iodine were used in the study. The iodine solution was standardized by titrating against standard sodium thiosulphate solution with starch as indicator. 15 Double distilled water was used all through the study. Method The absorption spectra were recorded with a Perkin Elmer Lamda 35 UVvisible spectrophotometer using a pair of quartz cuvete of 1cm optical length kept in a cell holder to which a pelteir temperature programmer PTP-1 is connected. The measurements were performed at three different temperatures, and 0 C. Determination of Equilibrium Constant, K eq The equilibrium constant of Iodine-iodide equilibrium was determined spectrophotometrically from the changes of the tri-iodide band at 350nm in aqueous and mixed aqueous organic media in absence and in presence of surfactant

4 The formation of tri-iodide from iodine and iodide equilibrium is represented as I 2 + I I3 (6.1) ( a x) ( b x) x where a, b and x are the concentration of iodine, iodide and triiodide ion respectively for which the equilibrium constant is given by K eq [I3 ] = [I ][I 2 = ] x (a x)(b x) (6.2) The absorption, A at 350nm is given by the equation A = ε 0 (a x) + ε1x (6.3) where ε o and ε o are the molar absorption coefficients of I 2 and I 3 - respectively. From equations (6.1) and (6.2), we can define and compute ε as follows which when a>>b gives A ε0 + Keqb(ε1 ε0) ε = a [1+ K (a x) + K b] eq eq 1 K eq )(1 b) 1 ε1 1 ε (1 ε + (6.4) (6.5) Then, from the slop of the linear plot of 1/ ε vs. 1/b, the equilibrium constant, K eq can be evaluated. 6.3 Results and Discussion The spectra of aqueous iodine solution shown in Figure 6.1 showed the characteristic iodine band at around 460 nm along with a band appearing at about 350 nm, which is ascribed to tri-iodide ions. The presence of an isobestic point in the spectra clearly indicates the presence of iodine-iodide equilibrium in the system. 110

5 Figure 6.1: Absorption spectra of iodine-iodide solution at 0 C It may, however, be noted that the spectra of pure aqueous iodine solution is always conspicuous by the presence of the tri-iodide band at 350 nm even after purification of iodine through a number of evaporation-condensation cycles. This has been attributed to the fact that when iodine is dissolved in the water there is formation of appreciable amount of iodide ions as represented in Equation (6.6), which then forms tri-iodide ions in the solution. 6 I H O OI + I + 2H (6.6) In the present study with very low concentration of iodine, the intensity of the tri-iodide band at 350nm in the pure iodine solution was negligibly low and hence the formation of other polyiodide species are neglected. + A typical spectra of iodine solution (0.1mM) with increasing amounts of potassium iodide in pure aqueous media at 0 C with emphasis on the changes in the tri-iodide absorption band at 350nm is shown Figure

6 Figure 6.2: Absorbance at 350 nm of iodine in presence of KI It is observed from Figure 6.2 that the intensity of the tri-iodide band increases with increase in the KI concentration with no spectral shift in the absorption band at 350nm. For the iodine-iodide systems in aqueous media, 1/є has been plotted against 1/b at, and C in Figure 6.3, which showed that they to a large extent yield a linear plot at all the temperatures. Figure 6.3: Spectrophotometric determination of equilibrium constant of I-KI system in aqueous medium at different temperatures. The tri-iodide equilibrium constant (K eq ) may, therefore, be computed from the slop of the linear plots and the values thus obtained were in close agreement with the value obtained from the partition co-efficient or other methods

7 A representative spectra of iodine (0.1mM) in presence of iodide in mixed aqueous media containing 5% EG by volume is shown in Figure 6.4. Though the absorption spectrum of the iodine is known to depend on the polarity of the surrounding medium, no significant change was observed in case of the tri-iodide band at least in the concentration range of EG under study. Figure 6.4: Absorbance at 350 nm of iodine at different concentration of KI in mixed media containing 5% EG Similar trend was observed in the other mixed media (supplementary data at the end of the Chapter). In all the mixed aqueous organic media, plots of 1/є vs 1/b were found to be linear, a typical plot is shown in Figure 6.5 (in EG media). Figure 6.5: Spectrophotometric determination of K eq in presence of different percentage of EG Similar behavior was observed in the other mixed media as well. The equilibrium constant in the mixed media was computed as usual from the 113

8 slop of such plots. The values of K eq in the mixed media at three different temperatures C, C and C are recorded in Tables 6.1 and 6.2 along with values of the standard thermodynamic functions G, H, and S, which have been determined using the following relations: G = -RT ln K eq G = H - T S We are not aware of any K eq value in any of the mixed aqueous-organic solution under study to compare the values reported herein. The variation of K eq with percentage amount of the organic solvent employed in the study at 0 C has been graphically presented in Figure 6.6. In all the mixed media, it was observed that K eq increases rather sharply with increased percentage of the organic solvent in the mixed media. The increase in K eq with amount of the organic solvent has been ascribed to re-arrangement of the structure of the solvent causing subsequent change in its solvation properties. 11,12 Figure 6.6: Variation of K eq with solvent % at 0 C 114

9 Table 6.1: Table showing the variation of K eq with temperature along with the thermodynamic parameters in mixed solvent media containing ethylene glycol series % 0 0 C EG ME EE K eq - G - G x10-2 x10-2 x H S K eq - H S K eq G G, H in kjmol -1 and S in Jmol -1 K

10 Table 6.2: Table showing the variation of K eq with temperature along with the thermodynamic parameters in mixed solvent media containing polyethylene glycol series % 0 C PEG0 PEG0 PEG600 Keq - G - H S Keq - G - H S Keq - G - H S x10-2 x10-2 x G, H in kjmol -1 and S in Jmol -1 K

11 In the mixed media containing EG or its homologues, K eq in ME was found to be relatively higher than those in EG. However, K eq in EE was found to initially decrease up to about 5% and then increased with increase percentage. Since the dielectric constant of EE is much lower as compared to that of EG or ME, the solvation effect perhaps is not very prominent at lower percentage of EE and hence the initial decrease. This suggests that the formation of tri-iodide ion is enhanced not only by the solvent dielectric but more significantly by the hydrophobic character of the solvent media. It is also evident that K eq in mixed media containing PEG was much higher as compare to those containing EG or its homologues. The increase in K eq is more likely to be due to increase in the hydrophobic character of the solvent media since the dielectric factor in the mixed media containing PEG or EG homologous will remain more or less similar. There is, however, no appreciable change in K eq with changes in the chain length of PEG. The results indicate that in addition to the hydrophobic factor, the sharp increase in K eq in PEG mixed media may also be due to the fact that PEG can provide an effective surface for the iodine to interact with the iodide ions that will facilitate the formation of tri-iodide ions. In all the system under study, the K eq value was found to decrease with temperature except for the one containing EE, which could not been studied above 0 C due to formation of reddish brown color precipitate. 117

12 Tables 6.1 and 6.2 also present the thermodynamic parameters of the I-KI system in the mixed media. The H values (which are negative) are nearly constant in system containing EG homoloques whereas there is a slight decrease with increased percentage of the solvent in the systems containing PEG. On the other hand, there is increase in S in all the system with increased amount of the organic solvent. The changes in H and S in the mixed media may also be attributed to the variation in the solvation properties rather than the variation in the activity co-efficients of tri-iodide and iodide ions in presence of the organic solvent. 12 Negative values of the G in all the mixed media indicates the spontaneity of the formation of the tri-iodide and the formation is apparently entropy controlled rather than enthalpy controlled. In order to further examine the formation of tri-iodide in heterogenous media, similar studies have also been carried out in presence of a surfactant, which can provide not only a hydrophobic environment but also a surface on which the reactants can be adsorbed. There is hardly any report on the formation of the tri-iodide in presence of a surfactant. This may be due to the complicacies involved in the classical method of determination of the equilibrium constant of the iodine/iodide system via partition coefficients in presence of a surfactant since surfactant also is distributed between the two layers besides emulsification of the water/oil media. In such cases, the 118

13 spectrophotometric method serves as a reliable method for the determination of the equilibrium constant in presence of surfactant. a b Figure 6.7: Absorbance at 350 nm of iodine at different concentration of KI in aqueous media in presence of SDS. (a) 4mM (b) 12mM Figure 6.7 shows the typical spectra Iodine (0.1mM) at different KI concentration in presence of monomers (4mM) as well as micellar (12mM) of SDS. In both the cases, the plots of 1/є verses 1/b are found to be linear as shown in Figure 6.8, which justifies the computation of K eq in presence of SDS. Similar behavior was observed in case of the mixed aqueous organic solvent media. The values of K eq in presence of SDS in different mixed aqueous organic media at, and o C along with the thermodynamic parameters G, H, and S are presented in Table 6.3. Figure 6.8: Plot of 1/ε vs 1/b for iodine-iodide system in aqueous media in presence of SDS 119

14 Table 6.3: Table showing the variation of K eq with temperature along with the thermodynamic parameters in presence of SDS at different media Medium (5%) 0 C K eq x G (kjmol -1 ) - H (kjmol -1 ) S (Jmol -1 K - 1 ) Aqueous EG ME PEG0 PEG0 PEG600 4mM 12mM 4mM 12mM 4mM 12mM 4mM 12mM 4mM 12mM 4mM 12mM

15 As evident from Table 6.3, presence of the SDS monomer (4mM SDS) increases the K eq while there is decrease in presence of SDS micelles (12mM SDS). The increase in presence of SDS monomers is perhaps through (i) enhanced interaction of iodine/iodide on the surfactant surface and (ii) increased ionization of iodine to iodide in mixed media containing EG or ME, both of which would favor the formation of tri-iodide ion. The decrease in K eq in presence of the SDS micelles may indicate that iodine is preferentially solubilized by the SDS micelles. However, in system containing PEG, presence of the SDS monomer or micelles led to decreasing K eq, which indicates the preferential adsorption SDS molecules on the polymer (PEG). Presence of SDS micelles would further decrease the triiodide formation due to micellar solubilization of iodine. a b Figure 6.9: Absorbance at 350 nm of iodine at different concentration of KI in aqueous media in presence of TX100. (a) 0.1mM (b) 0.4mM Shown in the Figure 6.9 is the spectra of iodine-iodide system in presence of 0.1mM (pre-micellar) and 0.4 mm (post-micellar) of TX-100. Similar results were observed in the mixed aqueous organic solvent media also. It is obvious 121

16 from Figure 6.9 that in presence of TX-100 micelles the 460 nm band suffered a blue while the 350 nm band undergoes relatively smaller red shift eventually giving rise to a merged band at about 370 nm. This clearly indicates the better solubilization of iodine by the micelles of Tx-100 as compare to that of SDS micelles. Hence, the K eq could not be computed in presence of micelles of Tx-100. In presence of monomers of TX-100, the iodine-iodide system in mixed aqueous organic solvent showed good linearity between 1/є and 1/b in as shown in Figure The K eq values along with the thermodynamic parameters in presence of TX-100(0.1mM) in mixed organic media at three different temperatures are given in Table 6.4. Figure 6.10: Plot of 1/ε vs 1/b for iodine-iodide system in aqueous media in presence of TX-100 From Table 6.3 and 6.4, it is apparent that K eq in monomers of TX-100 both in aqueous and mixed aqueous organic solvent media is consistently higher than those in SDS. The increase in K eq may indicate that the organic solvents 122

17 under study has lesser affinity for TX-100 as compare to SDS that led to increased interactions between iodine and iodide ions on the surface of the non-ionic surfactant, TX-100. Table 6.4: Table showing the variation of K eq with temperature along with the thermodynamic parameters in presence of 0.1mM TX-100 at different media Medium (5 %) Aqueous EG ME PEG0 PEG0 PEG600 0 C K eq x G (kjmol -1 ) H (kjmol -1 ) S (Jmol -1 K -1 )

18 The iodine-iodide system in presence of different concentration of SDS or TX-100 has also been investigated using Cyclic Voltameter and the respective cyclic voltagrams are shown in the Figures 6.11a and 6.11b. The a b Figure 6.11: Cyclic voltagram of iodine-iodide (1:4) system in presence of (a) SDS (b) TX-100 voltagram of iodine-iodide system in pure aqueous system showed the oxidation and the reduction potentials at around 0.573V and 0.793V. Presence of the SDS caused relatively much larger shift in both the ionization potentials than those in presence TX-100. However, in presence of TX-100, the shift is accompanied by rapid decrease of the current intensity while the decrease was gradual in case of SDS. The results indicate that while the formation of tri-iodide is initially enhanced at lower concentration of the surfactant, iodine is subsequently solubilized by the surfactant micelles especially the TX-100 micelles that would lead to decreasing the tri-iodide formation. This is in agreement with the findings from spectrophotometric study that iodine is better solubilized by TX-100 micelles. 124

19 Further, with a view to critically analyzing the role of the solvent hydrophobicity on the equilibrium, we have studied the iodine-iodide equilibrium in presence of low percentage of HPC or PEO, which would impart more hydrophobic environment in the solvent media as compare to the organic solvents chosen for the study. Since polymers are known to form complexes with iodine in presence of iodide ion, the study was confined to very low concentration of the polymer in order to minimize the complex formation. Within the polymer concentration range used in the study, no significant shift in tri-iodide band was observed, nor was there any new band due to formation of any complex. Figure 6.12a shows the representative spectra of the iodine-iodide system in presence of 0.02%HPC while the corresponding plots of 1/є verses 1/b in presence of different percentage of HPC is shown in 6.12b. a is shown in Figure 6.13a while the linear plots of 1/є verses 1/b of the iodineb Figure 6.12: (a) Absorbance at 350 nm of iodine at different concentration of KI in presence of 0.02%HPC (b) Plot of 1/ε vs 1/b for the system in presence of different HPC % Representative spectra of the iodine-iodide system in presence of 0.02% PEO 125

20 iodide system in presence of different percentage of PEO are shown in Figure a b Figure 6.13: (a) Absorbance at 350 nm of iodine at different concentration of KI in presence of 0.02%PEO (b) Plot of 1/ε vs 1/b for the system in presence of different PEO % The K eq values for the iodine-iodide system in presence of different percentage of HPC computed from the slop of the linear plots at three different temperatures along with the thermodynamic functions G, H, and S are given in Table 6.5 while those in presence of different percentage of PEO are given in Table 6.6. It is evident from the Tables 6.5 and Table 6.6 that at a given temperature, the K eq values increases with increased percentage of HPC or PEO and formation of tri-iodide ion decreases with increased temperature. At the same level of incorporation, K eq in system containing HPC is relatively higher than that in system containing PEO which is in agreement with the fact that HPC provides relatively better hydrophobic environment as compare to PEO. The results further confirm that as the hydrophobic character of the solvent media increases the tendency of the formation of the tri-iodide ion increases. 126

21 Table 6.5: Table showing the variation of K eq with temperature along with the thermodynamic parameters in presence HPC HPC % C K eq x G (kjmol -1 ) H (kjmol -1 ) S (Jmol -1 K -1 ) Table 6.6: Table showing the variation of K eq with temperature along with the thermodynamic parameters in presence PEO PEO % C K eq x G (kjmol -1 ) H (kjmol -1 ) S (Jmol -1 K -1 )

22 Reference 1. J. Liu, L. Fei, M. Maladen, B.R. Hamaker, G. Zhang, Carbohydrate Polymers, 75, 351, H. Naorem, S. N. Singh, J. Phys. Chem. B, 111, 98, T. Yokota, Y. Kimura, Makromol.Chem., 190, 939, H. Noguchi, H. Jyodai, S. Matsuzawa, J. Polym. Sci.B: Polym. Phys., 35, 1701, H. Noguchi, H. Jyodai, Y. Ito, S.Tamura, S. Matsuzawa, Polymer International, 42, 315, 1997, 6. M. Minick, K. Fotta, A. Khan, Biopolymers, 31, 57, H. Sukuwa, Y. Yoda, H. Sugimoto, S. Yoshida, T. Yamamoto, S. Kuruda, K. Sanechika, M. Hishinuma, Polymer Journal, 21, 3, V.T. Calabrese, A. Khan, J. Phys. Chem. A, 104, 1287, S. Messager, P. D. Goddard, P. W. Dettmar, J. Y. Maillard, J. Medical Microbiology, 50, 284, J. P. Singhal, A. R. Ray, Trends Biomater. Artiff. Organs, 16, 46, A. A. Ramadan, P. K. Agasyan, S. I. Petrov, Zh. Cbshch. Kim, 44, 983, K. Hayakawa, S. Nakamura, Bull. Chem. Soc. Japan, 50, 566, K. Hayakawa, M. Kanda, I. Satake, Bull. Chem. Soc. Japan, 52, 3171, J. A. Riddick, W. B. Bunger, Sakano, Organic Solvents, Physical Properties and Methods of Purification, Vol II 4 th Ed, John Wiley and Sons, Inc., J. Mendham, R. C Denney, J. D. Barnes, M. J. K. Thomas, Vogel s text book of Quantitative Chemical Analysis, Pearson Education Ltd., 6 th Ed,

23 Supplementary Data A. Absorption band of iodine at 350nm in presence of different concentration of KI in different mixed media 1% EG 10% EG 15% EG 1% ME 5% ME 10% ME 15% ME 1% EE 5% EE 10% EE 1% PEG0 5% PEG0 129

24 10% PEG0 15% PEG0 1% PEG0 5% PEG0 10% PEG0 15% PEG0 1% PEG600 5% PEG600 10% PEG600 15% PEG600 B. Iodine-iodide system in different mixed media in presence of SDS 4mM EG 12mM EG 4mM ME 1

25 12mM ME 4mM PEG 0 12mM PEG 0 4mM PEG 0 12mM PEG 0 4mM PEG mM PEG 600 C. Iodine-iodide system in different mixed media in presence of Tx-100 (0.1mM) EG ME PEG0 PEG 0 PEG

26 D. Absorption band of iodine at 350nm at different concentration of KI in presence of polymer 0.02% HPC 0.05% HPC 0.1% HPC 0.02% PEO 0.05% PEO 0.3% PEO 132