Investigation of the role of ionic liquids in tuning the pK a values of some anionic indicators

Article abstract of DOI:10.1007/s10953-009-9407-2

The effect of imidazolium-based ionic liquids, ([C₁₂mim][Cl] and [C₈mim][Cl]), on the acid-base equilibria of two sulfonated indicators has been studied. The presence of ILs leads to decreased pK _a values because of the st

Full text of DOI:10.1007/s10953-009-9407-2

J Solution Chem (2009) 38: 753–761
DOI 10.1007/s10953-009-9407-2
Investigation of the Role of Ionic Liquids in Tuning
the pKa Values of Some Anionic Indicators
A. Safavi · H. Abdollahi · N. Maleki · S. Zeinali
Received: 30 November 2008 / Accepted: 22 January 2009 / Published online: 17 April 2009
© Springer Science+Business Media, LLC 2009
Abstract The effect of imidazolium-based ionic liquids, ([C12mim][Cl] and [C8mim][Cl]),
on the acid-base equilibria of two sulfonated indicators has been studied. The presence of
ILs leads to decreased pKa values because of the stronger electrostatic interaction of cationic
ILs with the basic forms of the indicators with more negative charge. The longer alkyl side
chain of [C12mim][Cl] compared to [C8mim][Cl] results in stronger hydrophobic interaction
of this IL with the basic forms of the dyes leading to a more effective decrease in the pKa
values. Also, the transition points and transition intervals of the acid-base titration curves of
the indicators were affected by the presence of ILs. It was found that the IL interaction with
acid-base indicators also results in sharpening the acid-base titration curves of the indicators.
From these observations, it is concluded that the presence of ILs can tune the pKa values of
indicators. All the experiments were performed spectrophotometrically and the results were
obtained using curve fitting methods.
Keywords Ionic liquids · Imidazolium-based · Acid-base equilibrium · Titration · pKa
1 Introduction
Micelles of amphiphilic compounds can inhibit or accelerate reaction rates (by up to sev-
eral orders of magnitude) and also shift the equilibria (acid-base, redox and complexation).
These changes are generally due to the hydrophobic and electrostatic interactions of the re-
actants with micellar aggregates. The proper selection of an ionic surfactant can provide a
structure with an interior, an exterior and a charged interface, with an electrical potential at
or near the micellar surface that can differ by a few hundred millivolts from that of the bulk.
This allows ionic, as well as uncharged, species to interact with micellar aggregates, thus
A. Safavi ( ) · N. Maleki · S. Zeinali
ꢀ
Department of Chemistry, College of Sciences, Shiraz University, Shiraz, 71454, Iran
e-mail: safavi@chem.susc.ac.ir
H. Abdollahi
Department of Chemistry, Institute for Advanced Studies in Basic Sciences, Zanjan, 45195-159, Iran

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J Solution Chem (2009) 38: 753–761
greatly influencing equilibria and rates of reactions [1]. The acid-base equilibria of indica-
tors and thus the pKa values are significantly affected by the presence of micelles [2, 3]. As
knowledge of the shifts in the pKa values of indicators is of great value in analytical chem-
istry, considerable attention has been paid to understanding the phenomenon [4–9]. This
shift can be explained in terms of differences between the properties of the bulk solvent and
the interfacial region and perturbation of the acid-base equilibria by the electrostatic field
effect of the charged interface. The dissociation equilibria of substituted benzoic acids in
cationic and anionic surfactant micelles have been investigated potentiometrically [10]. The
acid-base equilibria of a number of phenols, amines, carboxylic acids, dyes, amino acids,
medicinal compounds and solvatochromic acid-base indicators in aqueous micellar solutions
have been examined [11–16]. The association constants for the interaction of indicators with
micelles may be quite informative with regard to the pKa shifts of such indicators.
Ionic liquids (IL) are a new class of ionic solvents that have gained much attention in
many fields of chemistry in the recent years. These compounds have low melting points,
negligible vapor pressures, and are stable over a wide range of temperature [17–24]. Re-
cently, ILs based on the 1-alkyl-3-methylimidazolium cation ([Cnmim]) have received much
attention as green solvents or additives in aqueous solutions. The amphiphilic nature of the
[Cnmim] cation means that, in many respects, these ILs will act analogously to cationic sur-
factants [25]. Recently many reports have been published to prove the surfactant-like and
aggregation behavior of ILs [26–30], but to the best of our knowledge there is no report on
the investigation of the effect of these ILs on acid-base equilibria. As ionic liquids are gain-
ing major importance as green solvents, the study of acid-base behavior in ionic liquid media
is important to the understanding of the mechanisms of reactions in analytical and pharma-
ceutical applications. These studies may also suggest the possibility of analogous pKa shifts
for biologically active compounds induced by their incorporation into membranes, lipo-
somes, or micelles. In the present paper, we focused our attention on studying the effects
of some 1-alkyl-3-methylimidazolium ionic liquids on the dissociation constants, transition
points and transition intervals of the acid-base titration curves of two sulfonated acid-base
indicators, namely, methyl orange (MO) and azocarmine G (AZG).
2 Experimental
2.1 Materials
The ILs were synthesized according to standard methods by the reaction of 1-methylimida-
zole with an excess of the appropriate haloalkane [17]. The reactants were stirred without
any additional solvent at 70 °C for 72 h. The [Cnmim][Cl] salt, where n is the carbon number
of the alkyl chain, was then purified by repeated washing with ethyl acetate before drying
1
overnight at 70 °C under a vacuum. The purity of the products was assessed by H NMR
spectroscopy and washing was continued until the characteristic peaks for the starting ma-
terials (1-haloalkane and imidazole) were not observed. All ILs should be dried at 70 °C
and stored in a dry place before use [26]. KCl that was used to fix the ionic strength was
purchased from Merck and used without any purification. At all titration steps the pH was
adjusted by the addition of few µL of concentrated HCl and NaOH solutions to be sure that
the total volume of the solution was not affected.

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2.2 Apparatus
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The UV-Vis absorption spectrophotometric measurements were carried out with a Perkin-
Elmer (Lambda 2) spectrophotometer using a 1-cm quartz cell. The temperature in all ex-
periments was kept constant at 25 °C using a LO-Temprol 154 thermostat.
3 Results and Discussion
3.1 Study of the Effect of ILs on the pKa Values of the Indicator
The acid-base equilibrium of a protonated indicator in the experimental pH ranges can be
represented as
K
a
HIn ꢀ In− + H+
(1)
(2)
(3)
[In−][H+]
Ka =
[HIn]
Ct = [HIn] + [In−]
where HIn, In− and Ct refer to the acidic and basic forms and total concentration of the
indicators, respectively.
The equilibrium concentration of the acidic (HIn) and basic (In−) forms of indicators can
be calculated using Eqs. 2 and 3 as follows:
[H+]
[HIn] = Ct
[In−] = Ct
(4)
(5)
([H+] + Ka)
Ka
([H+] + Ka)
Beer’s law can be expressed as:
So at any wavelength:
AHIn = εHInb[HIn]
(6)
(7)
−
−
−
AIn = εIn b[In ]
−
A = AHIn + AIn
(8)
−
εHIn and εIn can be obtained by dividing the absorbance value at a certain wavelength by
the total concentration of indicator in completely acidic and basic media, respectively.
A curve-fitting method was applied to calculate the Ka values (and then the pKa values)
of the indicators in aqueous solutions.
In the present work, we are primarily concerned with the effect of different concen-
trations of two imidazolium-based ionic liquids, 1-octyl-3-methylimidazolium chloride
[C8mim][Cl] and 1-dodecyl-3-methylimidazolium chloride [C12mim][Cl], on the pKa val-
ues of two sulfonated indicators. As mentioned previously, the aggregation behavior of these
ILs have been investigated before and their critical aggregation concentrations (CAC) have
been reported [26–31]. Azocarmine G (AZG) and methyl orange (MO) were chosen as two

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sulfonated indicators in this work. The type of dye–IL interaction was investigated in our
previous work [31]. It is notable that these sulfonated dyes have negative charges in both
acidic and basic pH because of presence of the −SO−3 group. Therefore, both forms can
interact with ILs, but as mentioned previously [31], this interaction is stronger in the case
of the basic form, because of the higher negative charge. Figures 1 and 2 show the spectral
changes of AZG and MO by varying pH in the presence and absence of [C12mim][Cl]. All
the experiments were performed in 1 mol·dm−3 KCl to prevent the change in ionic strength
upon addition of ILs. However it should be noted that the spectral changes of the indicators
in the presence of ILs are the results of IL–dye interaction, not the effect of varying ionic
strength [31]. A curve-fitting method was applied to calculate the apparent pKa values of the
indicators in different concentrations of ILs. The fitting procedure is described as follows.
3.1.1 Fitting Procedure
The experimental absorbance values were chosen at the maximum wavelength of the acidic
or basic form of the indicators whereas the calculated absorbance values were obtained by
Eqs. 4–8. The main goal in every fitting procedure is to approach the best fit or minimum
differences between the experimental and calculated absorbance values by changing the fit-
ting parameter(s). The procedure is started by considering an initial estimate for the Ka
values (as the fitting parameter) and the best fit is obtained for the minimum difference be-
tween experimental and calculated absorbance values (by calculating the sum of the square
errors between them). The fitting procedure was carried out in Excel (Solver program). The
calculated pKa values are reported in Table 1. Good agreement between the calculated and
experimental absorbance values against pH were observed for both indicators with ILs. As
an example, Fig. 3 shows the absorbance variation against pH for Azocarmine G in the
presence of different concentrations of [C8mim][Cl].
As can be seen by increasing the IL concentration the pKa values decrease. These pKa
variations can be related to the difference between the properties of the bulk solvent and of
the interfacial region. These variations in behavior can originate from two different sources.
One of them is related to the electrostatic interaction of the ILs and indicators, and the other
is the hydrophobic interaction. The anion of the indicator (In−) is strongly attracted to the
interfacial layer of the IL with positive charge due to the electrostatic attraction between
opposite charges. The stronger electrostatic interaction of In−, compared to HIn, with the
positive head group of IL [31] can influence the acid-base equilibrium. In other words, the
dissociation equilibrium (Eq. 1) moves towards the right upon addition of IL, causing a
decrease in the pKaapp values compared to the pKa value in water (pKw).
a
Table 1 The pK values of two
indicators obtained by curve
fitting method in different
a
[C mim][Cl]
8
pK
[C mim][Cl]
12
pK
a
a
−3
−3
(mol·dm
)
(mol·dm
)
concentrations of [C mim][Cl]
8
and [C mim][Cl]
12
Azocarmine G
Methyl orange
–
7.80
7.40
6.07
5.50
–
7.80
4.58
4.28
4.67
0.05
0.27
0.67
0.011
0.063
0.110
[C mim][Cl]
8
pK
[C mim][Cl]
12
pK
a
a
–
3.35
2.41
2.26
–
3.35
3.03
2.07
0.18
0.34
0.0011
0.0032

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757
(a)
(b)
(c)
−5
−3
Fig. 1 Absorption spectra of AZG (5.8 × 10 mol·dm ) at different pH values, (a) in pure water (pH
−3
−3
from 3.1 to 12.1), (b) in 0.67 mol·dm
[C mim][Cl] (pH from 2.1 to 12.4)
[C mim][Cl] (pH from 1.6 to 11.4), and (c) in 0.11 mol·dm
8
12

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(a)
(b)
(c)
−5
−3
Fig. 2 Absorption spectra of MO (1.46×10 mol·dm ) at different pH values, (a) in pure water (pH from
−3
−3
1.4 to 12.6), (b) in 0.34 mol·dm of [C mim][Cl] (pH from 0.65 to 12.25), and (c) in 0.0032 mol·dm
8
[C mim][Cl] (pH from 1.4 to 12.7)
12

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Fig. 3 Plot of calculated (lines)
and experimental (dots)
absorbance values versus pH at
maximum wavelength of AZG in
the concentrations
−3
(1) 0 mol·dm
,
−3
(2) 0.27 mol·dm and
−3
(3) 0.67 mol·dm [C mim][Cl]
8
Table 2 The influence of
Azocarmine G
pH transition
interval
Methyl orange
pH transition
interval
[C mim][Cl] and [C mim][Cl]
8
12
on the transition points and the
transition intervals of acid-base
indicators
ꢀpH
ꢀpH
Water
[C mim][Cl]
5.90–9.82
3.82–6.84
3.33–5.76
3.92
3.02
2.23
1.39–4.85
0.97–3.77
1.37–3.43
3.46
2.80
1.91
8
[C mim][Cl]
12
As can be seen from Table 1, the decrease in the pKa values of the two indicators was
achieved at a lower concentration of [C12mim][Cl] compared to the [C8mim][Cl]. This can
be related to the longer alkyl chain of [C12mim][Cl] that leads to the formation of IL aggre-
gates at a lower concentration compared to [C8mim][Cl].
Also, a relatively sharp decrease in the pKaappvalues can be observed at IL concentrations
near or at the CAC. However, at IL concentrations well above the CAC, the pKa values
decrease slightly and the curve reaches a plateau.
3.2 Study of the Effect of ILs on the Transition Point and Transition Interval of the
Indicators
Since the IL solutions have remarkable effects on the dissociation equilibria of acid-base
indicators, the transition points and transition intervals of the acid-base titration curves of
the indicators should be affected. The effects of [C8mim][Cl] and [C12mim][Cl] on the tran-
sition points and transition intervals of AZG and MO are shown in Table 2. It can be seen
that the transition intervals of AZG and MO in [C8mim][Cl] and [C12mim][Cl] solutions
are reduced with respect to the case in pure water, so it results in an increase in the titra-
tion curve sharpness. Again, the effect of [C12mim][Cl] is more pronounced compared to
that of [C8mim][Cl] because the longer alkyl side chain in [C12mim][Cl] leads to stronger
hydrophobic interactions and its lower CAC value.
Therefore, the acid-base titration accuracy will increase if the titration is carried out in
an optimal IL concentration. According to this special characteristic of ILs, it is expected
that the pKaapp of a wide range of acid-base indicators can be tuned by changing the ILs’
concentration in aqueous solutions.

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4 Conclusion
It is evident that ionic liquids, especially imidazolium-based ILs, can form aggregates
that have considerable effects on acid-base equilibria. In this work, the influence of two
imidazolium-based ILs ([C12mim][Cl] and [C8mim][Cl]) on the acid-base equilibrium of
two anionic indicators (azocarmine G and methyl orange) was investigated and the results
show the effectiveness of the presence of ILs in tuning the pKa values of these indicators.
In addition, ILs can influence the transition interval and transition points of the indica-
tors that can be very useful, especially in the case of some weak acid-base indicators like
azocarmine G.
Acknowledgements The authors wish to acknowledge the support of this work by the Iranian Ministry of
Science, Research and Technology and Third World Academy of Sciences, Iran Chapter (TWASIC).
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