Concentration-dependent apparent partition coefficients of ionic liquids possessing ethyl- and bi-sulphate anions

Article abstract of DOI:10.1039/c5cp06611e

This study deals with the concentration dependent apparent partition coefficients log P of the ethyl and bisulfate-based ionic liquids. It is observed that the bisulfate-based ionic liquids show different behaviour with respect to concentration as compared to ethyl sulphate-based ionic liquids. It is significant and useful analysis for the further implementation of alkyl sulfate based ionic liquids as solvents in extraction processes. The log P values of the ethyl sulphate-based ionic liquids were noted to vary linearly with the concentration of the ionic liquid, whereas a flip-flop trend with the concentration for the log P values of the bisulphate-based ionic liquids was observed due to the difference in hydrogen bond accepting basicity and possibility of aggregate formation of these anions. The π-π interactions between the imidazolium and pyridinium rings were seen to affect the log P values. The alkyl chain length of anions was also observed to influence the log P values. The hydrophobicity of ionic liquid changes with the alkyl chain in the anion in the order; [HSO₄]^- < [EtSO₄]^- < [BuSO₄]^-.

Full text of DOI:10.1039/c5cp06611e

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Concentration-dependent apparent partition
coefficients of ionic liquids possessing ethyl- and
Cite this:DOI: 10.1039/c5cp06611e
bi-sulphate anions†
Preeti Jain and Anil Kumar*
This study deals with the concentration dependent apparent partition coefficients log P of the ethyl and
bisulfate-based ionic liquids. It is observed that the bisulfate-based ionic liquids show different behaviour
with respect to concentration as compared to ethyl sulphate-based ionic liquids. It is significant and
useful analysis for the further implementation of alkyl sulfate based ionic liquids as solvents in extraction
processes. The log P values of the ethyl sulphate-based ionic liquids were noted to vary linearly with the
concentration of the ionic liquid, whereas a flip-flop trend with the concentration for the log P values of
the bisulphate-based ionic liquids was observed due to the difference in hydrogen bond accepting basicity
and possibility of aggregate formation of these anions. The p–p interactions between the imidazolium and
pyridinium rings were seen to affect the log P values. The alkyl chain length of anions was also observed to
influence the logP values. The hydrophobicity of ionic liquid changes with the alkyl chain in the anion in
the order; [HSO4]À o [EtSO4]À o [BuSO4]À.
Received 30th October 2015,
Accepted 25th November 2015
DOI: 10.1039/c5cp06611e
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The physico-chemical properties like viscosity, conductivity,
and surface tension of pure ionic liquids vary drastically in
1. Introduction
Recent years have witnessed an upsurge in the research the presence of small quantity of water present in ionic
activities in the area of ionic liquids. The field of ionic liquids liquids.16 This necessitates rigorous studies of the physico-
has drawn significant interest of researchers, including those chemical properties akin to the miscibility and fate of ionic
belonging to separation science as potential replacement to liquids in relation to the environment.
volatile organic compounds (VOCs). Ionic liquids possess
The determination of the octanol–water partition coefficient
several special physicochemical characteristics including a (Kow) of a chemical compound serves as a key parameter to
wide liquid range, high thermal stability, a broad solvation assess the environmental risk of that chemical and to explore
window ranging from water to non-polar organic solvents, low the properties of solvents in the extraction processes.17–21
flammability, etc.1–11 The most remarkable characteristic of The 1-octanol–water partition coefficient is a potential tool to
ionic liquids is that they are highly tunable with respect to assess the environmental acceptability of various chemical
the selection of cations and anions. These properties make compounds because 1-octanol being an amphiphilic solvent
ionic liquids beneficial from the perspective of separation possesses relative permittivity which is almost comparable to
science and technology and also used in many chemical and the comprehensive lipid phase. The relative permittivity of a
biochemical reactions.12–14 The overall awareness regarding compound plays a pivotal role in deciding its dissolution in any
greenness and sustainability in using ionic liquids as alternative solvent. Therefore, the 1-octanol–water partition coefficient of
reaction media to the currently used hazardous VOCs has been any biological compound is comparable with its partition in
considered by many researchers especially for industrial water and living systems.18
applications.15 It is normally considered that hydrophilic
Evaluation of the environmental acceptability of ionic
ionic liquid shows high miscibility with water, whereas liquids possessing different combinations of the cation and
hydrophobic ionic liquid absorbs a small amount of water. anion pair, through the measurements of Kow, is likely to
enhance their wide applications in industries, where biologi-
cally active compounds as well as medicinal compounds are
Physical & Materials Chemistry Division, CSIR – National Chemical laboratory,
being isolated from bulk reactants or solvents.21–23
Pune 411 008, India. E-mail: a.kumar@ncl.res.in; Fax: +91 2025902636
For ionisable species such as ionic liquids, the partition
coefficient is reported as the apparent partition coefficient
(log P). Unlike ionic liquids, most of the compounds report
† Electronic supplementary information (ESI) available: 1H-NMR spectra of ionic
liquids and their molar absorption coefficient values in water and 1-octanol. See
DOI: 10.1039/c5cp06611e
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the concentration-independent partition coefficient over a alkyl sulfate based ionic liquids. The study of the relationship
wide range of concentrations, whereas ionic liquids show a between the partition coefficient and the initial concentration
significant dependence on concentration24 because of their of alkyl sulfate based ionic liquids in the 1-octanol–water binary
distinct behaviour towards the water and octanol phase.
phase is of high interest and significance. The purpose of using
A Kow value represents the distribution ratio of an ionic the bisulfate and ethyl sulphate-based ionic liquids is: (1) their
liquid in both water and 1-octanol. In both water and 1-octanol, unique physicochemical properties such as biodegradability
an ionic liquid can acquire three forms: intact ion pair, loose and acceptable level of toxicity, and (2) the ethyl sulphate anion
ion pair and the solvent separated ions under highly dilute (EtSO4À) possesses higher electron density as evident from the
conditions. Under such highly dilute conditions, the precise hydrogen bond acceptor ability or basicity denoted by a solvent
À
estimation of each individual form is extremely difficult. Some parameter called b. The b values for EtSO4 containing ionic
reports exist on the determining Kow values of ionic liquids and liquids (E0.788) is close to that of DMSO, a polar aprotic
on the effect of alkyl chain length of the cationic group of ionic solvent (E0.748).47,49 Due to the high charge density and thÀe
liquids on their partition in both the phases.25–29 Brennecke basicity, DMSO can solvate the cations. Similarly, the EtSO4
et al. have measured the 1-octanol–water partition coefficient of anion possesses high charge density and basicity, therefore it
dialkylimidazolium ionic liquids ranging between 0.003–11.1 at can interact with the imidazolium cation to a greater extent and
room temperature in order to describe the effect of anions on can be present as an intact ion pair.
log P values.28 The binary liquid–liquid equilibrium (LLE) data
In this work, we present the variation in the apparent partition
of alkylimidazolium chlorides with both water as well as coefficients (log P) with the variation in the substituents present
1-octanol have been measured and the solubility data used to in both cations and anions of ionic liquids. In this work, we have
estimate Kow values.30,31 Apart from experimental methods, taken the ratio of total concentrations of an ionic liquid present
many new different methods have been established to predict in each phase. The ionic liquids investigated in the present study
Kow such as a sum of fragment contributions or atom derived are [EMIM][EtSO4], [dEIM][EtSO4], [EBIM][EtSO4], [EOIM][EtSO4],
group equivalents.32,33 Most of the ionic liquids, the partition [BMIM][HSO4], [HMIM][HSO4], [OMIM][HSO4], [BPy][HSO4],
coefficient of which have been reported till date, contain [OPy][HSO4] and [BMIM][BuSO4]. A summary of the ionic
different halides as their anions. The use of halide as an anion liquids, the structures of cations and anions and the acronyms
in an ionic liquid gives rise to great limitation in industrial used in this study are given in Table 1. In this work, first the
applications. Recently, ionic liquids have been modified struc- apparent partition coefficients of these ionic liquids over a wide
turally for enhancing their biodegradability. It has been studied range of concentrations (0.01 to 0.05 M) have been described
that the imidazolium-based ionic liquids are biodegradable with respect to their concentrations. Then, the effect of the
and therefore the biodegradability of various classes of ionic cationic ring as well as anions analysed on the apparent
liquids should be explored predominantly on the basis of partition coefficient of ionic liquids by comparing the apparent
modification of the anion.34–36 The replacement of the halide- partition coefficient data of [BPy][HSO4], [OPy][HSO4],
based anion by the alkyl sulfate anion, enhances their [BMIM][HSO4], [BMIM][BuSO4], and [OMIM][HSO4]. Finally we
biodegradability to 54%. This is relatively close to the accep- investigate the effect of the number of carbon atoms on the
table 60% level for a substance to be categorised as readily log P values.
biodegradable.37–39 Alkyl sulfate anion-based ionic liquids
are widely used in the petrochemical field for extraction
processes,40–42 alternatives to the routine hydrodesulfurization,43
2. Experimental section
2.1 Materials
as an azeotrope breaker in extractive distillation,44 and can
be synthesized efficiently as well as in a halide free way at
reasonable costs.45 Recent studies have shown that 1-ethyl-3-
1-Methylimidazole, 1-ethylimidazole, 1-butylimidazole,
1-octylimidazole, and pyridine were procured from M/s Sigma
methylimidazolium ethyl sulfate possesses an acceptable level
of toxicity.46 Besides their application in extraction processes,
Aldrich Co. These chemicals were distilled prior to their use in
experiments. 1-Octanol (with >99.5% purity) was purchased
the alkyl sulphate-based ionic liquids are used as solvents for
from M/s Fluka Co. and used without further purification.
organic reactions resulting in many remarkable and successful
outcomes.47–49 As the alkyl sulphate-based ionic liquids possess
Toluene, diethyl sulfate, dibutyl sulfate, sulphuric acid, and
ethyl acetate were obtained from M/s Merck Co. and used as
unique physico-chemical properties like biodegradability and
procured. Deionised water (resistivity 18.2 MO cm) was used
throughout the work.
adequate levels of toxicity, these must be further explored and
analysed in depth for their better applications such as in
separation, liquid–liquid extraction and for the removal of
nitrogen and sulfur containing compounds from currently used
2.2 Synthesis of ionic liquids
fuels. The lack of partition data of alkyl sulfate anion based All ionic liquids employed in this study, were synthesized by the
ionic liquids diminishes their use as alternative solvents in the following established procedures available in the literature.45,50
industries.
2.2.1 Synthesis of [EtSO4]À-based ionic liquids.
No reports are available in the literature on the concen- 1-Methylimidazole (1 mol) was taken in an ice cold bath
tration dependent study of apparent partition coefficients of containing 10 mL of toluene; diethyl sulfate (1 mol) dropwise
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Table 1 The acronyms, structures of cations and anions of ionic liquids employed in the present study
Name of ionic liquids
Structure of cations
Structure of anions
Acronym
1-Ethyl-3-methylimidazolium ethylsulphate
[EMIM][EtSO4]
1,3-Diethylimidazolium ethylsulphate
1-Butyl-3-ethylimidazolium ethylsulphate
1-Ethyl-3-octylimidazolium ethylsulphate
[dEIM][EtSO4]
[BEIM][EtSO4]
[EOIM][EtSO4]
1-Butyl-3-methylimidazolium bisulphate
1-Hexyyl-3-methylimidazolium bisulphate
1-Octyl-3-methylimidazolium bisulphate
1-Butyl-3-methylimidazolium butylsulphate
[BMIM][HSO4]
[HMIM][HSO4]
[OMIM][HSO4]
[BMIM][BuSO4]
N-Butyl-pyridinium bisulphate
N-Octyl-pyridinium bisulphate
[Bpy][HSO4]
[OPy][HSO4]
to pre-cooled 1-methylimidazole solution in toluene in order was removed by a rotary evaporator and dried under high
to maintain the temperature of reaction at 313 K. Following vacuum.
continuous stirring for 4 h, the upper organic layer was decanted
All the freshly prepared ionic liquids were dried under high
carefully. For the removal of trace amounts of solvent impurity, vacuum for 24 h prior to each experiment. The water content of
the product i.e. the ionic liquid was dried by a rotary vacuum at the ionic liquids was measured by the Karl–Fischer Coulometer,
the reduced pressure and 368 K. Other ethylsulfate-based ionic and was observed below to 50 ppm. The purity of all the
1
liquids were also prepared by the following similar procedure.
synthesised ionic liquids was ensured with the help of H-NMR
2.2.2 Synthesis of [HSO4]À-based ionic liquids. The synth- spectra (see ESI†).
esis involved two steps.
2.3 Determination of 1-octanol/water partition coefficients
(1) Quaternization reaction. 1-Methylimidazole to excess (log P)
haloalkane (1 : 1.2) was added into a round bottom flask fitted
By the definition ‘octanol–water partition coefficient’ represents
with a reflux condenser for 12 h and the temperature was
maintained at 343 K. The unreacted starting material was
removed by washing with ethyl acetate 3–4 times. The excess
solvent was removed by using a rotary evaporator under the
reduced pressure. Thereafter, the quaternized product was
further dried in a vacuum for 10 h in order to remove trace
amounts of volatile residues.
the ratio of the solubility of a given compound between 1-octanol
and water at a given temperature. If compound ‘X’ is dissolved in
the 1-octanol/water biphasic system, an equilibrium will be
reached after a certain period of time for both the phases at
certain temperature. It is known that during the prevalence of
equilibrium of distribution of the compound X between the
1-octanol-rich phase and the water-rich phase, activity will be
the same in both the phases. Activity can be expressed as ‘a’ =
(2) Metathesis. To a stirred solution of the halogenated c  g i.e. multiplication of concentration, c and the activity
product of the first step (1 mol) dissolved in 10 mL of acetonitrile; coefficient, g. Since the activity of the compound ‘X’ is the same
sulphuric acid (1.07 mol) was added in a drop wise manner. The in both the phases, then Xaw = Xao (Xaw indicates the activity
reaction mixture was refluxed for 48–72 h and the temperature of at water; Xao indicates activity at the 1-octanol phase of the
the reaction mixture was maintained at 343 K. The excess solvent compound X). At equilibrium,
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Xaw = Xao
(1)
(2)
or,
cwgw = cogo or, co/cw = gw/go
At constant temperature and pressure, if the compound ‘X’ is
present in both the water and 1-octanol phases under extremely
dilute conditions, the ‘infinite dilute’ condition activity coefficients
remain invariable with small variation in the concentrations.
It implies that the ratio of concentrations (co/cw) at equilibrium
remain constant i.e. Kow = co/cw represents constant unit less
value, independent of composition at constant temperature.
1-Octanol and water are not completely immiscible at 298 K.
The solubility of water in 1-octanol is quite high approximately
up to xw = 0.275 of water, but the solubility of 1-octanol in water
is xo = 7.5 Â 10À5 51,52
.
We used the shake flask method with
mutually saturated 1-octanol and water to determine the log P
value. For estimation of log P of ionic liquids, 1 : 1 (v : v) ratio of
water saturated with 1-octanol and 1-octanol saturated with
water was taken. The solutions of ionic liquids with the known
concentration (0.01–0.05 M) were made in 1-octanol and then
an equal volume of water was added to the solution. The
sample solution was stirred for 24 h with 170 rpm by an orbital
shaker at room temperature (298 Æ 2 K). The sample solutions
were kept for 24 h to get the equilibrium.
Following equilibration, water and 1-octanol phases were
collected separately with a syringe. The molar absorption
coefficient (e) was measured (Table S5, ESI†) in both phases
(water saturated with 1-octanol and 1-octanol saturated with
water) in order to calculate the concentration in both the
phases by applying the Lambert–Beer Law. The UV-visible
spectrophotometer (Varian make, Cary 50) was employed to
measure absorbance at absorption maxima (lmax) of the
substances in the extracted solvents. Water saturated with
1-octanol and 1-octanol saturated with water were used in the
sample cell. The concentrations of the dissolved solute was
determined from the absorbance value obtained from the
UV/vis spectrophotometer through a pre-calibrated graph. The
accuracy of all the measurements was estimated to be Æ3%.
The measured data, an average of triplicate measurements, are
precise to within Æ1%.
Fig. 1 Variation of log P with concentration (a) for [EMIM][EtSO4] ( ),
[dEIM][EtSO4] ( ), [BEIM][EtSO4] (&), and [EOIM][EtSO4] ( ). (b) For
[BMIM][HSO4] (E), [HMIM][HSO4] ( ), and [OMIM][HSO4] ( ).
the positive log P value shows the hydrophobic nature of a
solute. In the present study, we have observed that the
hydrophobicity or hydrophilicity of an ionic liquid linearly
increases by increasing the concentration of the ethyl
sulphate-based ionic liquids. The ionic liquids with a lower
alkyl group are hydrophilic in nature, which increases with
concentration as shown in Fig. 1(a). The log P values vary from
À2.04 to À2.82, À2.12 to À2.63, and À1.64 to À1.92 for
[EMIM][EtSO4], [dEIM][EtSO4] and [BEIM][EtSO4], respectively
over a wide range of concentrations from 0.01 to 0.05 M
(Table S1, ESI†). On the other hand, the hydrophobicity of
[EOIM][EtSO4] varies from 0.19 to 1.65 as a function of concen-
tration in the range of 0.01 to 0.05 M as shown in Fig. 1(a).
The log P values obtained for [HSO4]-based ionic liquids
3. Results and discussion
For the convenience of reporting, the log P data of ionic liquids
has been divided into three parts.
3.1 Concentration-dependent log P
It is interesting to know that how the apparent partition demonstrated interesting results about the concentration
coefficient of ionic liquids varies with the concentration of a dependent hydrophilicity or hydrophobicity. We have observed
given ionic liquid. The logP values as a function of concentration
a flip-flop behaviour in hydrophobicity or hydrophilicity
of the ionic liquid are shown in Fig. 1 for [EMIM][EtSO4], for [HSO4]À based ionic liquids with respect to the concen-
[dEIM][EtSO4], [BEIM][EtSO4], [EOIM][EtSO4], [BMIM][HSO4], tration as seen in Fig. 1(b). These ionic liquids show unusual
[HMIM][HSO4] and [OMIM][HSO4]. As seen from Fig. 1, the logP behaviour beyond a particular concentration. In the case of
values of these ionic liquids depend upon their concentrations. [BMIM][HSO4], upon increasing the concentration from 0.01 to
The negative log P value indicates the hydrophilic nature whereas 0.03 M, the log P value decreases from À2.01 to À2.11 and
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beyond this concentration, the log P value increases to À2.10
and À2.09 at 0.04 and 0.05 M, respectively (Table S2, ESI†).
These results reveal that [BMIM][HSO4] possesses hydro-
philic behavior at lower concentrations and shifts towards
hydrophobic behavior upon increasing concentrations (see
Fig. S1(a), ESI†).
The log P values of [HMIM][HSO4] increase from À1.76 to
À1.62 with increasing the concentrations from 0.01 to 0.02 M
and above that concentration, the log P values decreases up to
À1.87. It is observed that up to 0.02 M concentration,
the hydrophilicity decreases and beyond that concentration,
the hydrophilicity of the ionic liquids increases. Similarly, in
the case of [OMIM][HSO4], the log P value increases from 0.12 to
0.83 upon an increase in the concentration from 0.01 to 0.03 M.
Then it decreases from 0.83 to 0.64 as a concentration increases
from 0.03 to 0.05 M. On moving towards [HMIM][HSO4], the
polarity of ionic liquid decreases so the interactions with water
become less favorable.
[HMIM][HSO4] shows higher log P values as compared to
[BMIM][HSO4] and lower to [OMIM][HSO4] as shown in
Fig. S1(b) and (c) (ESI†). The change from hydrophobic to
hydrophilic nature and vice versa of these ionic liquids may occur
due to self-aggregation. It has also been reported that self-
aggregation of ionic liquids may occur in aqueous solution.53,54
The log P data obtained in the study revealed the formation
of aggregates in the aqueous phase during the distribution of
ionic liquids between the 1-octanol–water binary system.
The [HSO4]À-based ionic liquids show different behaviour as
compared to [EtSO4]À-based ionic liquids, because of the less
basicity and the possibility of formation of aggregates. At lower
concentrations, ionic liquids exist as intact ion pairs in the
aqueous phase. At lower concentrations of ionic liquids, strong
van der Waals interactions take place between the hydrophobic
Fig. 2 Variation of log P with concentration for (a) [BPy][HSO4], and (b)
[OPy][HSO4].
alkyl chain of ionic liquids and the hydrophobic part of the beyond this concentration, the log P value increases to À2.42 at
1-octanol thereby decreasing the hydrophilicity. Moving 0.05 M (Table S3, ESI†) in [BPy][HSO4] as shown in Fig. 2(a). It is
towards the higher concentration the possibility of aggregate shown in Fig. 2(b) that upon increasing the concentration from
formation increases. The aggregate formation takes place due 0.01 M to 0.05 M, the log P value increases from À0.39 to 1.22,
to the van der Waals interactions between the hydrophobic respectively. In the case of the pyridinium-based ionic liquids,
parts of ionic liquids. The flip-flopping in hydrophilicity/ the structure of aggregates is different from those observed for
hydrophobicity depends on the orientation of aggregates. the imidazolium-based ionic liquids. The pyridinium-based
Orientation of the imidazolium-based ionic liquid in an aggre- ionic liquids show favourable arrangement for p–p stacking
gate form favours the hydrogen bonding between [HSO4]À and between rings. At lower concentrations of [BPy][HSO4], the
the aqueous phase. In the case of imidazolium-based ionic ionic liquid remains as intact ion pair and thus shows favourable
liquids, the hydrophobic tail of ionic liquids reside away from interactions with the aqueous phase through hydrogen bonding
the water molecules, whereas the hydrophilic head of ionic between [HSO4]À and water. With an increase in the concen-
liquid and counter ions resides towards the aqueous phase due tration of the ionic liquid, the possibility of aggregate formation
to lesser possibility of p–p stacking. The orientation of an ionic also increases. The p–p stacking between rings orients the hydro-
liquid above the critical aggregate concentration (cac) favors its philic head reside away from the water molecules and the hydro-
interactions in the aqueous phase over the organic phase, phobic part towards it. Similarly, the orientation of aggregates of
leading to an increase in the hydrophilicity of the ionic liquid the pyridinium-based ionic liquids favours the interaction with
beyond the cac. These observations are consistent with the the 1-octanol phase, as van der Waals interactions are
earlier reports that the partition coefficient of ionic species is enhanced between the hydrophobic part of aggregates and
concentration dependent.24 A similar trend has also been the hydrophobic tail of 1-octanol. The observed log P value
observed in the case of [OPy][HSO4] and [BPy][HSO4] as shown reveals that the hydrophilicity of [BPy][HSO4] increases in
in Fig. 2. The value of log P decreases from À2.45 to À2.63, as lower concentration, but beyond cac (0.02 M), the hydrophili-
the concentration increases from 0.01 M to 0.02 M. However, city of the ionic liquid decreases as the orientation of the
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aggregate favours the interactions with the organic phase. anion interacts with imidazolium cations to a greater extent as
The hydrophobicity of [OPy][HSO4] increases linearly with the compared to the [HSO4]À anion. Simultaneously, hydrophobi-
concentration.
city of the anion also increases upon increasing the number of
At 0.01 M concentration, logP (À0.39) of [OPy][HSO4] shows carbon atoms and hence [HSO4]À anions are more hydrophilic
hydrophilic behavior but upon increasing the concentration, the in nature as compared to [EtSO4]À and [BuSO4]À. As we move
hydrophobicity of ionic liquid increases linearly. In the case of from [HSO4]À to [EtSO4]À it is noted that the ionic liquids
[OPy][HSO4], at a lower concentration of 0.01 M, it shows high possessing the [HSO4]À anion shows higher interactions with
interactions with water but as we increase the concentration of water as compared to [EtSO4]À due to the absence of the –C2H5
the ionic liquid, the longer alkyl group resides towards the group and the possibility of aggregate formation. Hydrophobi-
aqueous phase due to the p–p stacking between the pyridinium city is induced by the –C2H5 group in [EtSO4]À, whereas it is not
ring leading to a decrease in the interaction with water. As the observed in the case of [HSO4]À. The electronegativity of oxygen
concentration of [OPy][HSO4] increases, it results in favorable atoms attached to the alkyl chain (hydrogen or ethyl) is
interaction with 1-octanol owing to the orientation of increased upon increasing the alkyl group length leading to
aggregates.
an enhanced electrostatic interaction. It is reported that
upon increasing the number of carbon atoms in cations as
well as, anions, a favorable interactions with 1-octanol or high
solubility in 1-octanol is seen. Fig. 1 depicts that [HSO4]À based
ionic liquids behave differently as a function of concentration
as compared to [EtSO4]À-based ionic liquids. Therefore the
[HSO4]À containing ionic liquids show greater ionic liquid–
water interactions due to less hydrophobicity and large negative
charge on the anion oxygen atom. Increasing the alkyl chain
length in the sulfate anion increases the negative charge of the
oxygen atom and holds a strong electrostatic interaction
between cations and anions. This lowers the ionic liquid/
water interaction and hence the solubility or interaction with
1-octanol increases. The log P values is mentioned in Table S4
(ESI†) for the ionic liquids having different anions and cationic
rings. [BMIM][HSO4] and [BMIM][BuSO4] have a similar cationic
ring but different alkyl groups in anions. Since both the anions
possess higher capability of hydrogen bonding with water,
negative log P values are obtained. However, in the case of the
[BuSO4]À anion, the presence of the butyl group imparts some
degree of hydrophobicity in comparison to that of the [HSO4]À
anion. This enhanced hydrophobicity of the [BuSO4]À ion is
also reflected in the log P values of [BMIM][BuSO4] i.e. log P
values reside on the higher side as compared to [BMIM][HSO4],
as shown in Fig. 4.
3.2 Effect of cationic rings and anions on log P
As demonstrated in the first section, the hydrophobicity of
[OPy][HSO4] increases linearly with an increase in concentration,
whereas flip-flopping observed in the case of [OMIM][HSO4].
In order to compare the effect of the cationic ring on log P
values, we have chosen imidazolium and pyridinium cationic
rings. The imidazolium ring is smaller in size and less aromatic
in nature as compared to pyridinium. The imidazolium ring
shows strong interactions with water as compared to pyridine.
Fig. 3 shows that the log P value of [OPy][HSO4] is higher than
that of [OMIM][HSO4] at the concentrations higher than
B0.035 (mol LÀ1) indicating that pyridine is less hydrophilic
as compared to that of imidazole. In the present study, effects
of anions on the log P were also observed. The effect of anions
on log P values has been observed in sulfate-based ionic liquids
by changing the alkyl group in anions such as [HSO4]À,
[EtSO4]À, and butyl sulfate ([BuSO4]À). All the alkyl sulfate
anion containing ionic liquids show higher interactions
with water.
The basicity of anions increases upon increasing the carbon
number in anions. Upon increasing the basicity; electrostatic
interactions between cations and anions increase. It was also
noted that the [EtSO4]À anion has a similar behavior as that of
DMSO in terms of b values. Similar to DMSO, the [EtSO4]À
3.3 Effect of the number of carbon atoms attached to the
cationic ring of ionic liquids on log P:
As described in Fig. S2 (ESI†), the log P values increase with an
increase in size of the alkyl chain attached to the cationic ring.
[EtSO4]À is hydrophilic in nature. A change from [EMIM]+ to
[dEIM]+ leads to a decrease in the hydrophilicity of ionic liquids
by a marginal amount as identified by log P values (log P for
[EMIM][EtSO4] = À2.41; log P for [dEIM][EtSO4] = À2.35) at a
particular concentration of 0.02 M. However, a value of log P =
À1.72 for [BEIM][EtSO4] indicates that the hydrophilicity again
decreases (Table S1, ESI†) and shows the marginal behavior
lying between hydrophobicity and hydrophilicity of these ionic
liquids. From Fig. S2(a) (ESI†) it is clear that upon increasing
the alkyl chain length of the cationic ring of the [EtSO4]À-based
ionic liquids ([EMIM][EtSO4], [dEIM][EtSO4], [BEIM][EtSO4]
and [EOIM][EtSO4]) the log P value changes from negative to
positive. These changes occur because the non-polarity of the
Fig. 3 A comparative study of log P with concentration for [OPy][HSO4]
(&), and [OMIM][HSO4] ( ); also given [EOIM][EtSO4] (,).
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Fig. 4 A comparative study of log P with concentration for [BMIM][BuSO4]
(&), and [BMIM][HSO4] ( ).
Fig. 5 Variation of log P with the number of carbon atoms in alkyl groups
attached to the cationic ring of the ionic liquids.
above ionic liquids increase with the introduction of additional
alkyl groups, and thus, the solubility in 1-octanol of ionic liquid
increases. [EOIM][EtSO4] possess a positive log P value (0.59),
whereas for [EMIM][EtSO4], [dEIM][EtSO4], and [BEIM][EtSO4]
have negative log P values.
The increasing order of hydrophobicity by varying the alkyl
chain is as follows
[EOIM][EtSO4] is hydrophobic in nature. The effect of alkyl
chain length on the log P values for the [HSO4]À-based ionic
liquids were obtained as shown in Fig. S2(b) (ESI†). The log P
values of [BMIM][HSO4], [HMIM][HSO4], and [OMIM][HSO4] are
À2.01, À1.76, and 0.12, respectively at 0.01 M (Table S2, ESI†).
The log P values indicate that hydrophilicity of ionic liquid
decreases upon moving from the butyl to the hexyl group. In
the case of [OMIM][HSO4], due to the larger hydrophobic alkyl
group, its solubility increases in 1-octanol thus showing the
positive log P values. The [HSO4]À-based ionic liquids show a
similar trend as noted in the case of [EtSO4]À-based ionic
liquids. The log P values vary due to different behavior of
anions. Upon increasing alkyl chain length of cationic ring,
there is a possibility of strong van der Waals interactions
between the alkyl group of ionic liquid and the hydrophobic
part of 1-octanol. This behavior of hydrophobicity can also be
explained by their polarity values. As the size of the alkyl group
in ionic liquid increases; its polarity decreases.55 This inhibits
its ability to interact with water.56,57 The lengthening of the
aliphatic chain at positions 1 and 3 (in an imidazolium ring) on
the cation also affects the interaction of ionic liquids with
solvents. Many interactions like van der Waals interactions
(between the hydrophobic part of ionic liquids and solvents),
hydrogen bonding, coulombic interactions etc. are responsible
for solubility. Increasing the carbon atoms on cations show the
strong van der Waals interactions with the hydrophobic part of
1-octanol. Acidic C2 protons on the imidazolium cationic ring
shows hydrogen bonding with water but upon increasing the
alkyl chain length on the ring, the acidity of C2 proton
decreases thereby increasing the hydrophobicity of cationic
rings. Ionic liquids having smaller alkyl groups at 1 and 3
positions shows favorable interactions with water.58 This sup-
ports our experimental results that upon increasing the alkyl
chain length on imidazolium cation solubility of ionic liquids
in water decreases (Fig. 5).
[EMIM][EtSO4] o [dEIM][EtSO4] o [BEIM][EtSO4]
o [EOIM][EtSO4]
Similarly, for the [HSO4]À-based ionic liquids hydrophobicity
increases in the order as follows
[BMIM][HSO4] o [HMIM][HSO4] o [OMIM][HSO4]
For the purpose of convenience, log P values for the investigated
ionic liquids have been summarized in the form of adjustable
parameters of a polynomial equation (Table 2).
At the end, we state that the log P scale may not act as a
robust tool to predict hydrophobicity in ionic liquids as struc-
tural aspects of both cations and anions lead to complex
properties. In an excellent review published by van Rantwijk
and Sheldon13 the miscibility aspects of ionic liquids have been
discussed. The constituent anions of the ionic liquid seem
to reflect on the miscibility of the ionic liquids in water.
Table 2 Values of the adjustable parameters to describe concentration
dependence of log P by log P = q0 + q1c + q2c2 + . . .; c = molar
concentration (mol LÀ1
)
Parameters
q0
q1
q2
r2
Ionic liquids
[EMIM][EtSO4]
[dEIM][EtSO4]
[BEIM][EtSO4]
[EOIM][EtSO4]
À1.96 Æ 0.09 À17.70 Æ 2.89
À2.06 Æ 0.06 À11.8 Æ 1.7
—
—
—
—
0.9015
0.9238
0.9863
À1.57 Æ 0.01
À0.16 Æ 0.02
À6.8 Æ 0.4
36.5 Æ 0.7
0.9987
[BMIM][HSO4]
À1.92 Æ 0.01 À9.73 Æ 0.90
128 Æ 15 0.9810
[HMIM][HSO4] À1.86 Æ 0.11
16.24 Æ 8.59 À336 Æ 140 0.6971
74.2 Æ 16.9 À1057 Æ 277 0.8545
[OMIM][HSO4]
[BPy][HSO4]a
[OPy][HSO4]
À0.48 Æ 0.22
À1.93 Æ 0.18
À0.70 Æ 0.09
À76 Æ 24
2679 Æ 884 0.7910
38.80 Æ 2.93
—
0.9775
[BMIM][BuSO4]a À0.35 Æ 0.07 À29.06 Æ 8.24
693 Æ 266 0.8679
a
Contains q3 for [BPy][HSO4] = À27067 Æ 9734, and for [BMIM][BuSO4] =
À4907 Æ 2730.
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5 M. Deetlefs, K. R. Seddon and M. Shara, Phys. Chem. Chem.
Phys., 2006, 8, 642–649.
For example, 1-butyl-3-methylimidazolium tetrafluoroborate
([BMIM][BF4]) and 1-butyl-3-methylimidazolium methylsulfate
([BMIM][MeSO4]) are water-miscible, while 1-butyl-3-methyl-
imidazolium hexafluorophosphate ([BMIM][PF6]) and 1-butyl-
3-methylimidazolium bis(trifluoromethanesulfonyl) imide
([BMIM][Tf2N]) are not, though these ionic liquids possess
nearly similar polarity on the Reichardt scale.59 In addition, the
coordination strengths of the [BF4] and [PF6] anions are also
comparable.60 Examining the log P values of the ionic liquids
suggests that log P predicts the partitioning of ionic liquids
between water and 1-octanol.
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4. Conclusions
In short we noted that (1) the apparent partition coefficient of
ionic liquids is concentration dependent and predominantly
influenced by the nature of anions. (2) The [HSO4]À-based ionic
liquids show flipping in log P values as a function of concen-
tration due to the formation of aggregates, whereas [EtSO4]À-
based ionic liquids possess a linear relationship of log P with
concentration. (3) This different behavior of [HSO4]À and
[EtSO4]À-based ionic liquids can be correlated with the variations
in hydrogen bond accepting basicity (b) of these anions as well as
the possibility of aggregate formation. (4) A change in the cationic
core from imidazolium to pyridinium significantly modifies the
log P values due to: p–p interactions, the number of electronega-
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the cationic ring. (5) The alkyl chain length in the cationic and
the anionic part of ionic liquids also affects the logP values;
hydrophobicity of anions increases as [HSO4]À o [EtSO4]À o
[BuSO4]À. Ultimately, the apparent partition coefficients can
illustrate the scale of hydrophobicity or hydrophilicity of
ionic liquids.
¨
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PJ thanks CSIR, New Delhi, for awarding her a Senior Research
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