chapter – 6 iodine-iodide equilibrium in aqueous and...
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107
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 tri-
iodide (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 polymer-
iodine 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 poly-
iodines like I2, I4, I6, etc. or poly-iodide ions such as I3-, 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
108
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
(PEG200, PEG400, 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.
109
6.2 Experimental
Materials
Extra pure reagent grade sample of iodine (I2) 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 UV-
visible 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
20, 30 and 400C.
Determination of Equilibrium Constant, Keq
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.12
110
The formation of tri-iodide from iodine and iodide equilibrium is represented
as
(6.1)
( ) ( ) where a, b and x are the concentration of iodine, iodide and triiodide ion
respectively for which the equilibrium constant is given by
(6.2) The absorption, A at 350nm is given by the equation
(6.3)
where εo and εo are the molar absorption coefficients of I2 and I3- respectively.
From equations (6.1) and (6.2), we can define and compute ε as follows
(6.4) which when a>>b gives
(6.5) Then, from the slop of the linear plot of 1/ ε vs. 1/b, the equilibrium constant,
Keq 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.
−− →+ 32 III
xa − xb − x
x)x)(b(ax
]][I[I][I
K2
3eq −−
==−
−
xεx)(aεA 10 +−=
b]Kx)(aK[1)εb(εKε
aAε
eqeq
01eq0
+−+
−+=≡
1eq1 ε1b)1)(Kε1(ε1 +≈
111
Figure 6.1: Absorption spectra of iodine-iodide solution at 200C
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
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 200C with emphasis on the
changes in the tri-iodide absorption band at 350nm is shown Figure 6.2.
+−− ++→+ 2HIOI OHI 22 (6.6)
112
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 20, 30 and 40°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 (Keq) 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.13
113
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 Keq 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
114
slop of such plots. The values of Keq in the mixed media at three different
temperatures 20°C, 30°C and 40°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 Keq
∆G = ∆H - T∆S
We are not aware of any Keq value in any of the mixed aqueous-organic
solution under study to compare the values reported herein. The variation of
Keq with percentage amount of the organic solvent employed in the study at
200C has been graphically presented in Figure 6.6. In all the mixed media, it
was observed that Keq increases rather sharply with increased percentage of
the organic solvent in the mixed media. The increase in Keq 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 Keq with solvent % at 200C
115
Table 6.1: Table showing the variation of Keq with temperature along with the thermodynamic parameters in mixed solvent media containing ethylene glycol series
∆G, ∆H in kJmol-1 and ∆S in Jmol-1K-1
%
0C
EG ME EE Keq
x10-2
-∆G
-∆H ∆S Keq
x10-2
-∆G
-∆H ∆S Keq
x10-2
-∆G
0
20 30 40
6.94 5.26 5.06
15.9 15.8 16.2
15.5
14
1
20 30 40
6.75 5.48 4.81
15.8 15.9 16.1
15.7
10
7.13 6.39 4.97
16 16.3 16.2
15.9
7.75
3.64
14.4
5
20 30 40
7.17 5.98 5.49
16.0 16.1 16.4
15.6
20.1
8.91 8.16 7.44
16.6 16.9 17.2
16
30
4.03
14.6
10
20 30 40
9.13 8.44 7.06
16.6 16.9 17.1
16.2
23.3
13.8 11.5 11.2
17.6 17.8 18.3
16.8
34
8.13
16.3
15
20 30 40
10.4 9.64 9.03
16.9 17.3 17.7
16.2
39.2
17.2 15.2 13.3
18.1 18.4 18.7
17.5
38.1
12.1
17.3
116
Table 6.2: Table showing the variation of Keq with temperature along with the thermodynamic parameters in mixed solvent media containing polyethylene glycol series
∆G, ∆H in kJmol-1 and ∆S in Jmol-1K-1
%
0C
PEG200 PEG400
PEG600
Keq x10
-2 -∆G
-∆H ∆S Keq
x10-2
-∆G
-∆H ∆S Keq x10
-2 -∆G
-∆H ∆S
1
20 30 40
8.35 7.63 6.24
16.4 16.7 16.8
16.1
17.9
8.73 7.78 6.46
16.5 16.7 16.9
16.2
17.2
8.97 7.89 6.75
16.6 16.8 16.9
16.2
19.8
5
20 30 40
17.78 15.94 15.9
18.2 18.6 18.8
17.7
26
17.9 16.3 15
18.2 18.6 19.0
17.5
38.7
17.1 15.9 14.6
18.2 18.6 18.9
17.5
36.5
10
20 30 40
42.4 39.6 35.2
20.3 20.8 21.3
19.3
45
45.6 41.1 38.5
20.5 20.9 21.5
19.5
48.3
45.7 41.7 38.3
20.5 20.3 21.4
19.3
48.7
15
20 30 40
67.6 62.1 60.4
21.5
22 22.7
20.3
58.8
68.8 64.8 60.1
21.5 22.1 22.7
19.8
60.8
86.8 77.7 72.1
22.0 22.5 23.2
20.7
60
117
In the mixed media containing EG or its homologues, Keq in ME was found
to be relatively higher than those in EG. However, Keq 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 Keq in mixed media containing PEG was much higher as
compare to those containing EG or its homologues. The increase in Keq 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 Keq with changes in the chain length of PEG. The
results indicate that in addition to the hydrophobic factor, the sharp increase
in Keq 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
Keq value was found to decrease with temperature except for the one
containing EE, which could not been studied above 200C due to formation of
reddish brown color precipitate.
118
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
119
spectrophotometric method serves as a reliable method for the determination
of the equilibrium constant in presence of surfactant.
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 Keq in presence of
SDS. Similar behavior was observed in case of the mixed aqueous organic
solvent media. The values of Keq in presence of SDS in different mixed
aqueous organic media at 20, 30 and 40oC 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
a b
120
Table 6.3: Table showing the variation of Keq with temperature along with the thermodynamic parameters in presence of SDS at different media
Medium (5%)
0C Keq x10-2
-∆G (kJmol-1)
-∆H (kJmol-1)
∆S (Jmol-1K-
1)
Aqueous
4mM 20 30 40
6.87 5.77 5.41
15.99 16.01 16.39
15.53
19.95
12mM 20 30 40
6.62 5.36 5.32
15.83 15.84 16.35
15.23
25.55
EG
4mM 20 30 40
7.82 6.85 6.10
16.24 16.46 16.70
15.77
23.10
12mM 20 30 40
7.46 6.59 6.38
16.12 16.36 16.82
15.39
34.73
ME
4mM 20 30 40
9.70 9.19 8.57
16.59 16.88 17.32
15.8
36.50
12mM 20 30 40
8.81 7.95 7.43
16.53 16.83 17.21
15.83
34.35
PEG200
4mM 20 30 40
12.73 10.56 9.71
17.43 17.55 17.91
16.9
24.25
12mM 20 30 40
11.23 9.56 9.01
17.12 17.30 17.72
1648
29.75
PEG400
4mM 20 30 40
13.41 11.96 10.45
17.54 18.03 18.88
16.14
27.50
12mM 20 30 40
11.49 10.77 9.14
17.17 17.50 17.75
16.61
28.90
PEG600
4mM 20 30 40
13.63 12.74 11.56
17.59 18.02 18.36
16.83
38.55
12mM 20 30 40
13.37 12.66 11.32
17.54 18.00 18.31
16.79
38.50
121
As evident from Table 6.3, presence of the SDS monomer (4mM SDS)
increases the Keq 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 Keq 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
Keq, which indicates the preferential adsorption SDS molecules on the
polymer (PEG). Presence of SDS micelles would further decrease the tri-
iodide formation due to micellar solubilization of iodine.
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
a b
122
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 Keq 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 6.10. The Keq 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 Keq in monomers of TX-100 both
in aqueous and mixed aqueous organic solvent media is consistently higher
than those in SDS. The increase in Keq may indicate that the organic solvents
123
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 Keq with temperature along with the thermodynamic parameters in presence of 0.1mM TX-100 at different media
Medium (5 %)
0C Keq x10-2 -∆G (kJmol-1)
-∆H (kJmol-1)
∆S (Jmol-1K-1)
Aqueous 20 30 40
8.10 7.30 6.38
16.32 16.52 16.81
15.81 24.6
EG 20 30 40
9.46 8.34 7.22
16.69 16.94 17.13
16.27 21.85
ME 20 30 40
9.77 8.99 8.03
16.77 17.14 17.39
16.17 34.6
PEG200 20 30 40
13.41 9.74 8.97
17.54 17.34 17.69
17.29 74.5
PEG400 20 30 40
14.11 13.65 13.33
17.66 18.19 18.73
16.61 52.9
PEG600 20 30 40
15.3 14.91 14.73
17.88 18.41 18.98
16.77 55.3
124
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
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.
a b
125
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.
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
is shown in Figure 6.13a while the linear plots of 1/є verses 1/b of the iodine-
b a
126
iodide system in presence of different percentage of PEO are shown in
Figure 6.13.
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 Keq 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 Keq 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, Keq 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.
a b
127
Table 6.5: Table showing the variation of Keq with temperature along with the thermodynamic
parameters in presence HPC
Table 6.6: Table showing the variation of Keq with temperature along with the thermodynamic parameters in presence PEO
PEO %
0C Keq x102 -∆G (kJmol-1)
-∆H (kJmol-1)
∆S (Jmol-1K-1)
0.02 20 30 40
6.95 6.82 6.22
15.94 16.44 16.74
15.17 0.02
0.05 20 30 40
7.28 6.89 6.75
16.06 16.46 16.95
15.14 0.05
0.3 20 30 40
9.89 8.95 8.72
16.80 17.12 17.62
15.95 0.3
HPC %
0C Keq x102 -∆G (kJmol-1)
-∆H (kJmol-1)
∆S (Jmol-1K-1)
0.02 20 30 40
7.70 5.41 5.28
16.2 16
16.3
15.9
6.2
0.05
20 30 40
8.60 6.48 5.66
16.4 16.3 16.5
16.4
1.8
0.1 20 30 40
8.76 8.10 5.83
16.5 16.8 16.6
16.6
3.3
128
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129
Supplementary Data
A. Absorption band of iodine at 350nm in presence of different
concentration of KI in different mixed media
1% EG
5% ME
10% ME
15% ME 1% EE
5% PEG200
10% EG
15% EG
1% ME
5% EE
10% EE 1% PEG200
130
B. Iodine-iodide system in different mixed media in presence of SDS
10% PEG200
15% PEG200
1% PEG400
5% PEG400 10% PEG400
4mM EG
4mM ME
15% PEG400
1% PEG600 5% PEG600
10% PEG600
15% PEG600
12mM EG
131
C. Iodine-iodide system in different mixed media in presence of Tx-100 (0.1mM)
EG ME PEG200
4mM PEG 200
12mM PEG 200
4mM PEG 400
12mM PEG 400
4mM PEG 600
12mM PEG 600
12mM ME
PEG 400 PEG 600
132
D. Absorption band of iodine at 350nm at different concentration of KI in presence of polymer
0.02% PEO
0.05% PEO
0.3% PEO
0.02% HPC
0.05% HPC
0.1% HPC