Spectrophotometric determination of the formation constant of triiodide ions in aqueous-organic solvent or polymer mixed media both in absence and presence of a surfactant

Article abstract of DOI:10.1016/j.saa.2012.09.058

The formation constant of triiodide ions from iodine-iodide equilibrium in aqueous-organic solvent or polymer mixed media have been determined spectrophotometrically at three different temperatures 20, 30 and 40 °C. The organic solvents chosen for the study are ethylene glycol, 2-methoxy ethanol, and 2-ethoxy ethanol while the polymers include polyethylene glycol (PEG), hydroxypropyl cellulose (HPC) or polyethylene oxide (PEO). Effect of a surfactant on the formation of triiodide ions in the mixed media has also been investigated. Though presence of the organic solvents led to an increase in the triiodide formation except for 2-ethoxy ethanol mixed media at low percentage, the increase in presence of PEG or the other polymer mixed media even at very low percentage was much higher as compared to those in ethylene glycol or its homologues. The increase in the formation constant has been discussed in terms of changes in its solvation properties and the hydrophobic character of the mixed media besides the solvent dielectric effects. The sharp increase in triiodide formation in presence of the polymer under study suggests the possibility of iodine being present as triiodide or other higher polyiodides in the inclusion complexes of iodine with other polymers like starch or PVA. The decrease in the triiodide formation in presence of surfactant micelles may, however, be attributed to solubilization of iodine by the micelles. Iodine was found to be better solubilized in TX-100 micelles as compared to SDS micelles.

Full text of DOI:10.1016/j.saa.2012.09.058

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 67–73
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Spectrophotometric determination of the formation constant of triiodide ions
in aqueous-organic solvent or polymer mixed media both in absence and
presence of a surfactant
⇑
Homendra Naorem , Seram Dushila Devi
Department of Chemistry, Manipur University, Canchipur, Imphal, Manipur 795 003, India
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
Triiodide ions formation in mixed
aqueous organic solvent or polymer
media.
"
"
"
Sharp increase in triiodide formation
in presence of polymers.
Solubilzation of iodine by surfactant
micelles.
Possibility of iodine present as
triiodide or higher polyiodides in
polymer complexes.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 June 2012
Received in revised form 29 August 2012
Accepted 20 September 2012
Available online 28 September 2012
The formation constant of triiodide ions from iodine–iodide equilibrium in aqueous-organic solvent or
polymer mixed media have been determined spectrophotometrically at three different temperatures
20, 30 and 40 °C. The organic solvents chosen for the study are ethylene glycol, 2-methoxy ethanol,
and 2-ethoxy ethanol while the polymers include polyethylene glycol (PEG), hydroxypropyl cellulose
(HPC) or polyethylene oxide (PEO). Effect of a surfactant on the formation of triiodide ions in the mixed
media has also been investigated. Though presence of the organic solvents led to an increase in the tri-
iodide formation except for 2-ethoxy ethanol mixed media at low percentage, the increase in presence of
PEG or the other polymer mixed media even at very low percentage was much higher as compared to
those in ethylene glycol or its homologues. The increase in the formation constant has been discussed
in terms of changes in its solvation properties and the hydrophobic character of the mixed media besides
the solvent dielectric effects. The sharp increase in triiodide formation in presence of the polymer under
study suggests the possibility of iodine being present as triiodide or other higher polyiodides in the inclu-
sion complexes of iodine with other polymers like starch or PVA. The decrease in the triiodide formation
in presence of surfactant micelles may, however, be attributed to solubilization of iodine by the micelles.
Iodine was found to be better solubilized in TX-100 micelles as compared to SDS micelles.
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Spectrophotometry
Triiodide formation
Hydrophobic character
Solvation
Solubilization
Micelles
Introduction
complexes of iodine with polymers like starch or PVA since iodine
is always used in combination with iodide ions in the complexa-
The iodine–iodide equilibrium leading to formation of triiodide
ions in solution has gained renewed interest especially with refer-
ence to the type and the nature of iodine present in the inclusion
tion [1–8]. It has been reported that besides the dielectric of the
solvent media, the formation of triiodide is greatly influenced by
presence of organic solvent, which has been understood in terms
of the changes in the solvation properties of the mixed media [9–
11]. A sharp increase in the formation constant of triiodide ions
has been reported in water–alcohol or acetonitrile mixed media
⇑
Corresponding author. Tel.: +91 (0) 9436020590.
E-mail address: naorem.homendra@gmail.com (H. Naorem).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.09.058

68
H. Naorem, S.D. Devi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 67–73
[9,12,13]. The formation constant of triiodide ion in micellar solu-
tion of dodecyltrimethylamonium chloride was found to be much
larger than that in water but less than the water–alcohol mixed
solvents, which has been attributed to the solubilization of both io-
dine and triiodide ions in the hydrophilic surface region of the sur-
factant micelles [14]. A change in the hydrophobicity of the solvent
media is, therefore, expected to affect the triiodide ion formation.
In absence of such studies, it was considered worthwhile to study
the formation of triiodide ions in mixed aqueous media containing
a homologue of ethylene glycol or a polymer that does not form a
complex with iodine. The present work is an attempt to study how
changes in the hydrophobic character of the solvent media would
influence the triiodide formation. The organic solvents chosen for
the present study are ethylene glycol (EG), 2-methoxy ethanol
(ME), 2-ethoxy ethanol (EE) while the polymers include poly ethyl-
ene glycols of different lengths namely, PEG-200, PEG-400, and
PEG-600, poly ethylene oxide (PEO) and hydroxy propyl cellulose
(HPC). Since HPC, PEO or PEG interacts weakly with iodine, there
is hardly any possibility of formation of a polymer-iodine adduct
or complex and interfering with the triiodide equilibrium. We have
also evaluated the formation constant of triiodide ions in presence
of an anionic surfactant, sodium dodecyl sulfate (SDS) or a nonionic
surfactant, tritonX-100 (TX-100).
Method of determination of equilibrium constant, Keq
The formation constant of triiodide ions was determined spec-
trophotometrically from the dependence of the absorbance of the
triiodide band at 350 nm on the concentration of iodide ions in a
fixed amount of iodine in aqueous or mixed aqueous media [13].
For the iodine–iodide equilibrium leading to formation of triiodide
ions as shown in Eq. (1), the
I2 þ ðbIꢀꢀxÞ ! Iꢀ3
ð1Þ
ð2Þ
ðaꢀxÞ
x
½Iꢁꢀ3
x
Keq
¼
¼
½I2ꢁ½Iꢁꢀ ða ꢀ xÞðb ꢀ xÞ
apparent formation constant Keq is given by Eq. (2) where the brack-
ets represent the respective concentrations and a, b and x are the
initial concentrations of iodine, potassium iodide and the equilib-
rium concentration of triiodide ion. Since potassium iodide does
not absorb at 350 nm, the total absorbance, A at 350 nm due to io-
dine and triiodide may be expressed as:
A ¼ e0ða ꢀ xÞ þ
e1x
ð3Þ
where e0 and e1 are the molar absorption coefficients of I2 and I3ꢀ
ꢀ
e
respectively. From Eqs. (2) and (3), we can define
as follows
[13,16]:
A
a
e0 þ Keqbðe1
ꢀ
e0
Þ
eB
¼
ð4Þ
Experimental
½1 þ Keqða ꢀ xÞ þ Keqbꢁ
Since e1 is much larger than e0 at 350 nm and when b is much larger
than (a ꢀ x) as in the present study, Eq. (4) may be reduced to [13]:
Materials
Extra pure reagent grade sample of sublimed iodine (I2) was ob-
tained from Merck (India) and potassium iodide (KI) having purity
of over 99.5% was procured from Loba Chemie (India). The sample
of iodine was resublimed at least three times and potassium iodide
was dried at 110 °C under vacuum for about 24 h [15,16]. Analyti-
cal reagent grade samples of ethylene glycol, 2-methoxy ethanol
(ME), 2-ethoxy ethanol (EE) from SD Fine Chemicals (Mumbai)
were purified following standard procedures [17]. High purity
samples of poly ethylene glycols namely PEG-200, PEG-400 and
PEG-600 procured from CDH, Mumbai were used as received. The
samples of SDS, TX-100, PEO (50,000) and HPC (100,000) employed
in the study were obtained from Sigma–Aldrich (USA). Double dis-
tilled water was used all through the study. Saturated iodine solu-
tions were prepared freshly by dissolving an excess amount of
iodine in water in a volumetric flask. The resultant supersaturated
solution was continuously shaken for nearly 3 h and the liquid was
then drained out. The last fraction collected after a few cycles of
‘shaking and drain out’ was employed as saturated iodine solution
which was then standardized by titrating against standard sodium
thiosulfate solution using starch as indicator [18]. The concentra-
tion of the saturated solution of iodine thus obtained was always
found to be 1.10 0.02 mM.
ꢀ
1=
e
ꢂ ð1=e1 ꢃ KeqÞð1=bÞ þ 1=e1
ð5Þ
From Eq. (5), both Keq and
the slope and intercept of 1/
e
1 may be simultaneously calculated from
ꢀ
e
vs. 1/b plot.
In a typical experiment, iodine solutions (0.1 mM) in aqueous or
mixed aqueous media containing different amounts of KI were pre-
pared in separate 10 ml volumetric flasks and their spectra were
recorded. Using the absorbance at 350 nm for the iodine solution
ꢀ
at different KI, 1/e was plotted against 1/b. For all the systems un-
der study and within the concentration range employed, all the
plots were found to be linear and the Keq values were determined
from the slope of such linear plots. The maximum error involved in
the Keq values thus obtained is expected to be less than 1% of the
reported value.
Results and discussion
The spectra of aqueous solution of iodine (0.8 mM) with
increasing amount of potassium iodide at 20 °C are shown in
Fig. 1. Iodine solution showed its characteristic band at 460 nm
along with a low intensity band at 350 nm, which has been attrib-
uted to triiodide ions [6,13]. The spectra of aqueous solution of io-
dine are always conspicuous by the presence of weak band at
350 nm even after purification of iodine through a number of evap-
oration–condensation cycles. When iodine dissolves in water,
there is formation of iodide ions, however small it may be, as rep-
resented in Eq. (6), forming triiodide ions in the solution, which ac-
counts for the low intensity band at 350 nm [6,13]. The intensity
however is not proportional to the concentration of iodine.
Instrumentations
The absorption spectra were recorded with a Perkin Elmer Lam-
da-35 UV–visible spectrophotometer using a pair of quartz cuvette
of 1 cm optical length kept in a cell holder with built in Peltier tem-
perature programmer PTP-1. The measurements were performed
at three different temperatures 20, 30 and 40 °C. Cyclic voltagram
of the iodine–iodide in aqueous and aqueous surfactant media
were measured using electrochemical analyzer (600-CH Instru-
ment). The measurements were carried out using a classic cell
equipped with three electrodes – platinium as working, platinium
as auxillary and an Ag/AgCl electrode as reference electrode.
I2 þ H2O ! OIꢀ þ Iꢀ þ 2Hþ
ð6Þ
With gradual addition of iodide ions, the intensity of the 350 nm
band increases rapidly without any shift at the cost of the 460 nm
band giving rise to an isobestic point at 430 nm which is indicative
of the presence of an equilibrium in the system. In view of the low
concentration of iodine and iodide employed in the study, the equi-

H. Naorem, S.D. Devi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 67–73
69
tively is shown in Fig. 2 and the value of the formation constant has
been computed from the slope of the linear plots. The Keq in pure
water was found to be 694 5 at 20 °C, which is in close agreement
with the other reported values [16].
Formation constant in mixed aqueous-organic solvent
Though the absorption spectrum of iodine is known to depend
on the polarity of the medium, no significant shift in the triiodide
band was observed in presence of the organic solvents under study,
at least in the concentration range used. For all the mixed solvents
ꢀ
under study, plots of 1/e vs. 1/b for the solutions containing 0.1 mM
iodine with different KI concentrations were found to be linear and
the Keq in the mixed media has been computed from the slope of
such plots. We are not aware of any Keq value in any of the mixed
aqueous media under study to compare the values reported herein.
The values of Keq in the mixed media containing homologues of
ethylene glycol or PEG at 20, 30 and 40 °C respectively are recorded
in Table 1. The variation of Keq with the percentage of EG homolo-
gous or PEG at 20 °C shown in Fig. 3 revealed that Keq in presence of
PEG is relatively much higher than those in EG homologous and it
increases with increased percentage of the organic solvent in the
mixed media. The increase in Keq in mixed media may be ascribed
to changes in its solvation properties and the hydrophobicity of the
mixed media besides the solvent dielectric effects [9,13]. The in-
crease in Keq in the order EE > ME > EG (at higher percentage),
which is also in accordance with the increasing basicity of the or-
ganic co-solvent indicates that the phenomenon is also related to
changes in the acidity or basicity of the mixed media. In the io-
dine–iodide system, increasing basicity of the organic co-solvent
would favor triiodide formation by stabilizing the iodide ions as
is obvious from Eq. (6) hence the increase in Keq in methyl or ethyl
substituted ethylene glycol.
Fig. 1. Absorption spectra of aqueous solution of 0.8 mM iodine in presence of
different molar concentration KI at 20 °C (a – 0, b – 0.02, c – 0.04, d – 0.06, e – 0.08,
f – 0.1, g – 0.14, h – 0.17, i – 0.21 mM).
librium may safely be assumed to be between iodine and iodide
ions leading to formation of triiodide ions and hence the possibility
of formation of other higher polyiodides may be ignored.
ꢀ
A typical linear plot of 1/e against 1/b for iodine–iodide system
containing 0.1 mM iodine in pure water at 20, 30 and 40 °C respec-
The consistently higher value of Keq in mixed media containing
ME as compared to those containing EG indicates that substitution
of the hydrogen of the –OH in ethylene glycol by a methyl group
led to increasing the triiodide formation. However, in mixed media
containing EE, an initial decrease in Keq was observed at lower per-
centage up to about 5% followed by a rapid increase thereafter.
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. Further,
Keq in mixed media containing PEG was much higher as compared
to those containing EG or its homologues. In addition to the
changes in the hydrophobic character as well as the solvation
properties of the mixed media, the sharp increase in Keq in PEG
ꢀ
Fig. 2. Linear plots of 1/
e
vs. 1/b for the 350 nm band of iodine (0.1 mM) in aqueous
medium in presence of different concentration of KI, b (0.18, 0.24, 0.3, 0.39, 0.48,
0.6, 0.72, 0.9 and 1.2 mM) at 20, 30 and 40 °C.
Table 1
Value of Keq in mixed aqueous media containing ethylene glycol homologues or PEG series at different temperatures.
Organic solvent %
1
Temp. (°C)
EG
ME
EEa
PEG-200
PEG-400
PEG-600
Keq ꢄ 10ꢀ2b (molꢀ1 L)
20
30
40
6.75
5.48
4.81
7.13
6.39
4.97
3.64
8.35
7.63
6.24
8.73
7.78
6.46
8.97
7.89
6.75
5
10
15
20
30
40
7.17
5.98
5.49
8.91
8.16
7.44
4.03
8.13
12.1
17.78
15.94
15.9
17.9
16.3
15
17.1
15.9
14.6
20
30
40
9.13
8.44
7.06
13.8
11.5
11.2
42.4
39.6
35.2
45.6
41.1
38.5
45.7
41.7
38.3
20
30
40
10.4
9.64
9.03
17.2
15.2
13.3
67.6
62.1
60.4
68.8
64.8
60.1
86.8
77.7
72.1
a
Keq could not be determined at higher temperature due to formation of reddish brown colored precipitate.
The maximum error limit is expected to be less than 1%.
b

70
H. Naorem, S.D. Devi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 67–73
Fig. 3. Variation of Keq with % of the organic solvents. (A) EG homologous, (B) PEG at 20 °C.
Fig. 4. (A) Absorption spectra of iodine (0.1 mM)-HPC (0.02%) mixture in presence of different concentration of KI (a – 0, b – 0.06, c – 0.12, d – 0.18, e – 0.24, f – 0.3,
ꢀ
g – 0.39 mM) and (B) the corresponding linear plots of 1/
e
vs. 1/b in presence of HPC.
mixed media may be due to the fact that PEG can provide an effec-
tive surface for the iodine to interact with the iodide ions facilitat-
ing the formation of triiodide ions. There is, however, no
appreciable change in Keq with change in the chain length of
PEG. In all the systems under study, the Keq was found to decrease
with temperature except for the one containing EE, which could
not been studied above 20 °C due to formation of reddish brown
colored precipitate.
range used nor was there any new band due to formation of any
adduct or complex in the system clearly rules out the possibility
Formation of triiodide in presence of HPC or PEO
With a view to further highlighting the effect of change in the
hydrophobicity of the solvent media, we have studied the forma-
tion of triiodide ions in presence of low percentage of HPC or
PEO, which would impart more hydrophobic environment as com-
pared to the organic solvents chosen for the study. Typical spectra
of HPC–iodine mixture containing 0.1 mM iodine with different io-
dide concentrations are presented in Fig. 4 along with the corre-
ꢀ
sponding linear plots of 1/
e against 1/b. Similar results were
obtained in presence of PEO. The isobestic point observed at
around 465 nm in such spectra confirms that the iodine–iodide
equilibrium is still present in the mixed aqueous media containing
HPC or PEO. The fact that no significant shift in the iodine or the
triiodide band was observed within the polymer concentration
Fig. 5. Absorption spectra of the benzene layer of the polymer-iodine/iodide
mixture extracted from a – starch, b – PVA, c – PEO, d – PEG-200, e – HPC. Band at
500 nm indicates the presence of iodine in the extract.

H. Naorem, S.D. Devi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 67–73
71
Table 2
Value of Keq in presence of different percentage of HPC or PEO at 20, 30 and 40 °C.
HPC %
0.02
Temp. (°C)
Keq ꢄ 10ꢀ2a (molꢀ1 L)
7.70
5.41
5.28
PEO%
0.02
Temp. (°C)
Keq ꢄ 10ꢀ2a (molꢀ1 L)
6.95
6.82
6.22
20
30
40
20
30
40
0.05
0.10
20
30
40
8.60
6.48
5.66
20
30
40
7.28
6.89
6.75
0.05
0.30
20
30
40
8.76
8.10
5.83
20
30
40
9.89
8.98
8.72
a
The maximum error limit is expected to be less than 1%.
Fig. 6. Absorption spectra of iodine (0.1 mM) in presence of anionic surfactant SDS with different concentration of KI, (A) 4 mM SDS, and (B) 12 mM SDS (a – 0, b – 0.12,
c – 0.3, d – 0.48, e – 0.72, f – 1.2 mM KI).
of complicating the triiodide band with formation of iodine adduct
or complex with HPC or PEO. This is further supported by the fact
that iodine could be detected in the benzene extract from HPC, PEO
or PEG-iodine/iodide mixture but not from PVA or starch-iodine/
iodide mixture (Fig. 5) since they form an adduct or inclusion com-
plex [4,5].
presence of a surfactant except for the report in presence of dode-
cyltrimethylammonium chloride (DTAC) micelles [14].
Fig. 6 shows the typical spectra of iodine (0.1 mM) at different
KI concentration in presence of monomers (4 mM) as well as mi-
celles (12 mM) of SDS respectively. In both the cases, the plots of
The value of Keq in presence of different percentage of HPC or
PEO computed from the slope of the linear plots at three different
temperatures are given in Table 2. At a given temperature, Keq in-
creased with percentage of HPC or PEO but decreased with in-
creased temperature. At the same level of incorporation, Keq in
system containing HPC was relatively higher than those in PEO,
which corroborates the fact that HPC provides relatively more
hydrophobic environment as compared to PEO. Considering the
much lower percentage of the polymer used, Keq in HPC or PEO is
relatively much higher as compared to those containing EG or
PEG. The results further confirmed that as the hydrophobic charac-
ter of the solvent media increases the tendency of the formation of
the triiodide ions also increases. This strongly indicates the possi-
bility of iodine being present as triiodide or other higher polyiod-
ides in the inclusion complexes of iodine with other polymers
like starch or PVA [2].
Table 3
Value of Keq in the mixed aqueous media containing 5% organic solvent in presence of
4 mM or 12 mM SDS at different temperatures.
Mixed media
Aqueous
Temp. (°C)
Keq ꢄ 10ꢀ2a (molꢀ1 L)
4 mM SDS
12 mM SDS
20
30
40
6.87
5.77
5.41
6.62
5.36
5.32
EG
20
30
40
7.82
6.85
6.10
7.46
6.59
6.38
ME
20
30
40
9.70
9.19
8.57
8.81
7.95
7.43
PEG-200
PEG-400
PEG-600
20
30
40
12.73
10.56
9.71
11.23
9.56
9.01
20
30
40
13.41
11.96
10.45
11.49
10.77
9.14
Effect of surfactant on the formation of triiodide
In order to further examine the formation of triiodide ions in
heterogeneous media, similar studies were carried out in presence
of a surfactant, which can provide not only a hydrophobic environ-
ment but also a surface on which the reactants can be adsorbed.
There is hardly any report on the formation of triiodide ions in
20
30
40
13.63
12.74
11.56
13.37
12.66
11.32
a
The maximum error limit is expected to be less than 1%.

72
H. Naorem, S.D. Devi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 67–73
ꢀ
1/e
vs. 1/b are found to be linear, which justifies the computation of
of the SDS may be due to preferential adsorption of SDS molecules
on the PEG surface [19].
Keq in presence of SDS as in the case of DTAC solution [14]. The va-
lue of Keq in presence of SDS in different mixed aqueous organic
media at 20, 30 and 40 °C are given in Table 3.
Shown in the Fig. 7 are the spectra of iodine (0.1 mM) in pres-
ence of 0.1 mM (pre-micellar) or 0.4 mM (post-micellar) of TX-
100 with different molar concentrations of iodide. In presence of
TX-100 micelles the 460 nm band of iodine suffered a blue shift
while the 350 nm band undergoes a 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 and hence Keq could not be computed in micel-
lar solution of TX-100. However, in presence of monomers of TX-
100, the iodine–iodide system in aqueous and mixed aqueous or-
It is evident from Table 3 that presence of the SDS led to mar-
ginal decrease in Keq in aqueous media, which indicate that iodine
or iodide ions are not solubilized in the hydrophilic surface region
of the SDS micelles unlike the micelles of cationic surfactant DTAC.
The decrease in Keq in presence of the SDS micelles may, however,
be due to solubilization of iodine by the SDS micelles, which is
indicated by a small blue shift in the 460 nm iodine band. The sig-
nificant decrease in Keq in mixed media containing PEG in presence
Fig. 7. Absorption spectra of iodine in presence of (A) 0.1 mM and (B) 0.4 mM TX-100 with different iodide concentration. (a – iod and b – 0 refer to spectra of iodine in pure
water and in presence of TX-100 without any KI).
Table 4
Value of Keq in the mixed aqueous media containing 5% organic solvent in presence of 0.1 mM TX-100 at different temperatures.
Mixed media
Aqueous
Temp. (°C)
Keq ꢄ 10ꢀ2a (molꢀ1 L)
8.10
7.30
6.38
Mixed media
Temp. (°C)
Keq ꢄ 10ꢀ2a (molꢀ1 L)
13.41
9.74
8.97
20
30
40
20
30
40
PEG-200
EG
20
30
40
9.46
8.34
7.22
20
30
40
14.11
13.65
13.33
PEG-400
PEG-600
ME
20
30
40
9.77
8.99
8.03
20
30
40
15.3
14.91
14.73
a
The maximum error limit is expected to be less than 1%.
Fig. 8. Cyclic voltagrams of iodine–iodide (0.01: 0.04 M) system in presence of a surfactant: (A) SDS and (B) TX-100.

H. Naorem, S.D. Devi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 67–73
73
ꢀ
ganic solvent showed good linearity between 1/
e
and 1/b. The Keq
tion SDS molecules on the polymer surface. The formation constant
could not be determined in presence of TX-100 micelles due to
effective solubilization of iodine in the TX-100 micelles as indi-
cated by the emergence of a merged band at about 370 nm. Pres-
ence of very low concentration of HPC or PEO was found to favor
the triiodide formation and Keq in system containing HPC is rela-
tively higher than that those containing PEO which is in agreement
with the fact that HPC provides relatively better hydrophobic envi-
ronment as compare to PEO. The results would strongly indicate
the possibility of iodine being present as triiodide or other higher
polyiodides in the inclusion complexes of iodine with other poly-
mers like starch or PVA.
in presence of 0.1 mM TX-100 at three different temperatures are
given in Table 4. It is apparent that Keq in presence of monomers
of TX-100 both in aqueous and mixed aqueous media is consis-
tently higher than those in SDS.
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 voltagram is shown in the Fig. 8. The volt-
agram of iodine–iodide system in pure aqueous media showed the
oxidation and the reduction potentials at around 0.513 V and
0.734 V respectively. 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 forma-
tion of triiodide ions is initially enhanced at lower concentration
of the surfactant, it becomes less favorable in the presence of the
surfactant micelles especially the TX-100 micelles due to solubili-
zation of iodine by the micelles impacting a decrease in the triio-
dide ion formation. This is in agreement with the findings from
spectrophotometric studies.
Acknowledgement
The financial support from the Department of Science and Tech-
nology, New Delhi in the form of a Research Project No. SR/S1/PC-
34/2007 dated 15/12/2008 is gratefully acknowledged.
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Conclusion
Spectrophotometric technique is a convenient method for the
determination of Keq in mixed aqueous-organic system and also
in presence of a surfactant. It has been observed that Keq in the
mixed aqueous media was always higher than those in pure aque-
ous media which has been attributed to the increased hydrophobic
character of the mixed media and also changes in its solvation
properties. Further, the relatively much larger values of Keq in
mixed media containing a polymer as compared to those contain-
ing EG or its homologues indicates that the polymer can provide an
effective surface for the iodine to interact with the iodide ions facil-
itating the formation of triiodide ions. Presence of the SDS mono-
mer increases Keq while there is decrease in presence of SDS
micelles. Iodine may be preferentially solubilized by the SDS mi-
celles thereby decreasing Keq in presence of the SDS micelles. How-
ever, in system containing PEG, presence of the SDS monomer or
micelles led to decreasing Keq, perhaps due to preferential adsorp-