Solute-solvent interactions of amino acids in aqueous 1-propyl-3- methylimidazolium bromide ionic liquid solutions at 298.15 K

Article abstract of DOI:10.1007/s10953-010-9597-7

Densities, viscosities, and refractive indices of three amino acids (glycine, L-alanine, and L-valine) in aqueous solutions of an ionic liquid, 1-propyl-3-methylimidazolium bromide, have been measured at 298.15 K. These data have been used to calculate apparent molar volumes (V _φ ), viscosity B-coefficients, and molar refractions of these mixtures. The standard partial molar volumes (V_φ⁰) and standard partial molar volumes of transfer (Δ_tr V_φ⁰) have been determined for these amino acid solutions from these density data. The resulting values of V_φ⁰ and Δ_tr V_φ⁰ for transfer of amino acids from water to aqueous ionic liquid solutions have been interpreted in terms of solute + solvent interactions. These data also indicate that hydrophobic interactions predominate in L-alanine and L-valine solutions. Linear correlations were found for both V_φ⁰ and the viscosity B-coefficient with the number of carbon atoms in the alkyl chain of the amino acids, and have been used to estimate the contribution of the charged end groups ( NH⁺ ₃, COO^-), the CH₂ group, and other alkyl chains of the amino acids. The viscosity and molar refractivity results have been used to confirm the conclusions obtained from volumetric properties.

Full text of DOI:10.1007/s10953-010-9597-7

J Solution Chem (2010) 39: 1409–1427
DOI 10.1007/s10953-010-9597-7
Solute–Solvent Interactions of Amino Acids in Aqueous
1-Propyl-3-Methylimidazolium Bromide Ionic Liquid
Solutions at 298.15 K
Hemayat Shekaari · Fatemeh Jebali
Received: 4 March 2010 / Accepted: 26 April 2010 / Published online: 6 October 2010
© Springer Science+Business Media, LLC 2010
Abstract Densities, viscosities, and refractive indices of three amino acids (glycine,
L-alanine, and L-valine) in aqueous solutions of an ionic liquid, 1-propyl-3-methylimid-
azolium bromide, have been measured at 298.15 K. These data have been used to calculate
apparent molar volumes (Vφ), viscosity B-coefficients, and molar refractions of these mix-
tures. The standard partial molar volumes (Vφ0) and standard partial molar volumes of trans-
fer (ꢀtrVφ0) have been determined for these amino acid solutions from these density data.
The resulting values of Vφ0 and ꢀtrVφ0 for transfer of amino acids from water to aqueous
ionic liquid solutions have been interpreted in terms of solute + solvent interactions. These
data also indicate that hydrophobic interactions predominate in L-alanine and L-valine so-
lutions. Linear correlations were found for both Vφ0 and the viscosity B-coefficient with the
number of carbon atoms in the alkyl chain of the ami+no acids, and have been used to esti-
mate the contribution of the charged end groups (NH3 , COO−), the CH2 group, and other
alkyl chains of the amino acids. The viscosity and molar refractivity results have been used
to confirm the conclusions obtained from volumetric properties.
Keywords Amino acids · Ionic liquid · Apparent molar volume · Viscosity
1 Introduction
Thermodynamic properties of amino acids in aqueous electrolyte solutions can provide valu-
able information regarding the conformation stability of proteins in these solutions, their
solubility, denaturation, dissociation into subunits, the activity of enzymes, separation and
purification, solute–solvent and solute–solute interactions [1–4]. On the other hand, the use
of ionic liquids as reaction media for processes involving biologically relevant compounds is
promising in view of their successful application in kinetic resolution, biocatalysis, biosyn-
thesis, separation, and purification processes. In fact, to optimize and control the extraction
H. Shekaari ( ) · F. Jebali
ꢀ
Department of Chemistry, University of Mohaghegh Ardabili, Ardabil, Iran
e-mail: hemayatt@yahoo.com

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of biomolecules by ionic liquids from aqueous media, knowledge of their thermophysical
and thermodynamic properties is required.
Ionic liquids (ILs) are organic electrolytes that are liquids at or near room temperature
in their pure state. They have been widely used in a number of fields in both academia and
industry and exhibit many advantages such as a low melting point (<373 K), are liquid over
a wide temperature range, and have suitable viscosity, thermal stability, and the ability to
dissolve a variety of materials, and most importantly, negligible vapor pressure [5, 6]. Ionic
liquids have been suggested as green and benign replacements for traditional volatile organic
solvents, and a growing number of applications in the fields of catalysis, chemical reactions,
separations, electrochemistry and nanoscience investigations have been studied [7, 8].
There are extensive volumetric and viscometric studies of amino acids in aqueous elec-
trolyte solutions [9, 10]. Consequently, the study of the volumetric properties of amino acids
in aqueous ionic liquid solutions will be very useful for obtaining information about various
types of interactions occurring in these solutions, which are mostly hydrophobic and elec-
trostatic. Studying these interactions can provide important insight into the conformational
stability and unfolding behavior of globular proteins.
In the present study, experiments have been performed to measure the density, viscosity,
and refractive indices of three amino acids, glycine, L-alanine, and L-valine, in an aque-
ous ionic liquid, 1-propyl-3-methylimidazolium bromide, [PMIm]Br, at T = 298.15 K. The
apparent molar volumes (Vφ), standard partial molar volumes (Vφ0), standard partial molar
volumes of transfer (ꢀtrVφ0), and side-chain contributions of the amino acids Vφ0(R) have
been computed from the experimental densities (d). The viscosity B-coefficients were cal-
culated by using the Jones-Dole equation [11]. All these parameters were analyzed in terms
of solute–solute and solute–solvent interactions occurring among the various components
of the ionic liquid and amino acid. The experimental refractive indices and density data for
these mixtures were used for the calculation of molar refractions.
2 Experimental
2.1 Materials
Reagents used in this work were N-methylimidazole (>99%), 1-bromopropane (>99%),
acetonitrile (GR, >99.8%), toluene (>99%), glycine (>99.7%), and L-alanine (>99%), all
obtained from Merck. L-valine (>99% in mass fraction) was purchased from Lobachemie
(India). The amino acids were dried in vacuum over blue silica gel for at least 72 h at room
temperature. The other chemicals were used without further purification. Doubly distilled
deionized water with a conductivity of approximately 0.7 µS·cm−1 was used for preparation
of the solutions.
2.2 Synthesis of Ionic Liquids
The ionic liquid [PMIm]Br was prepared and purified by using a procedure described in
the literature [12, 13]. Briefly, [PMIm]Br was synthesized in a round bottom flask by di-
rect alkylation of N-methylimidazole with an excess of 1-bromopropane under a nitrogen
atmosphere, for 48 h at T = 353 K. The crude product was dissolved in acetonitrile and
crystallized (in an ice bath) by adding toluene dropwise. The product was then dried under
high vacuum (0.7 kPa) at T = 333, K using a rotary evaporator for at least 4 h. The reaction
yield was 0.99 in mass fraction. The ionic liquid was used after being vacuum dried for at

J Solution Chem (2010) 39: 1409–1427
1411
least 48 h to remove trace amounts of moisture. Water contents in the [PMIm]Br were found
to be less than 0.02 mass fraction by the Karl Fischer method. This synthesized ionic liquid
was analyzed by 1HNMR (FT-NMR Brucker Av-300) and FT-IR (Perkin Elmer, Spectrum
RXI) to confirm the absence of any major impurities and the spectra were found to be in
good agreement with those reported in the literature [14, 15].
2.3 Apparatus and Procedure
The samples were prepared by filling glass vials with fixed compositions of ionic liquid and
amino acid and then closing them tightly. A sample was taken from the vial with a syringe
and immediately injected to into the apparatus. The mass of chemicals used to prepare the
mixtures was determined with an uncertainty 1 × 10−7 kg using a Sartrious analytical
balance AG TE214S. The uncertainty in molalities of prepared solutions is approximately
2 × 10−4 mol·kg−1
.
The densities of solutions were measured using a vibrating-tube densimeter DMA 4500M
(Anton Paar, Austria). The apparatus was calibrated with doubly distilled deionized and de-
gassed water, and with dry air at atmospheric pressure. Because densities are extremely
sensitive to temperature, it was kept constant within 1.0 ×10−3 K using the built in Peltier
device. The viscosity correction for density measurements was made automatically by the
densimeter. The apparatus was also tested by measuring the density of an aqueous NaCl
solution of known molality using the data given by Pitzer et al. [16] as standard. The un-
certainties of the density measurements are less than 1.0 × 10−5 g·cm−3. The average of
triplicate measurements is reported as the final result.
Viscosities were measured using an Ubbelohde-type viscometer, which has a flow time
of about 200 s for water at 298.15 K. The viscometer was calibrated with doubly distilled
deionized water. For this viscometer the viscosity of a solution η is given by the following
equation:
η
d
K
t
= Lt −
(1)
where d is the density, t is the flow time of the solution, and L and K are viscometer
constants. A digital stopwatch with a resolution of 0.01 s was used for the flow time mea-
surements. The estimated uncertainty of the experimental viscosities is 0.02 mPa·s, with
a temperature control of 0.01 K.
Refractive indices of the mixtures were determined using a digital refractometer
(ATAGO-DRA1, Japan) with an uncertainty of 1 × 10−4, calibrated with doubly distilled
water before each series of measurements. A procedure called “zero setting” was always
performed before the actual measurement of a sample’s refractive index, to ensure that the
refractometer is working properly. Calibration was checked with pure liquids of known re-
fractive index. The temperature was controlled with a circulating bath thermostat Julabo NP
(Germany) with a temperature stability of 0.01 K.
3 Results and Discussion
The experimental densities d of amino acid + [PMIm]Br + water ternary mixtures as a
function of the amino acid molality m (glycine, L-alanine, or L-valine) are reported in Ta-
ble 1. The apparent molar volumes Vφ the of amino acid in aqueous [PMIm]Br solutions

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J Solution Chem (2010) 39: 1409–1427
1413

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J Solution Chem (2010) 39: 1409–1427
Fig. 1 Apparent molar volumes
of glycine in ([PMIm]Br +
water) mixtures with different
−1
molalities/(mol·kg ) of
[PMIm]Br at 298.15 K:
F, 0.0000; P, 0.0963;
2, 0.1962; E, 0.2995; Q, 0.3961
Fig. 2 Apparent molar volumes
of L-alanine in ([PMIm]Br +
water) mixtures with different
−1
molalities/(mol·kg ) of
[PMIm]Br at 298.15 K:
F, 0.0000; P, 0.1520;
2, 0.1996; E, 0.2420; Q, 0.3368
were calculated from the densities of the solutions using the following equation [17]:
M
d
d − d0
Vφ =
−
(2)
mdd0
where M is the molar mass (kg·mol−1) of the amino acid, m is the molality (mol·kg−1) of
the amino acid in a [PMIm]Br + water mixture, d and d0 are the densities (kg·m−3) of an
(amino acid + [PMIm]Br + water) ternary mixture and reference solvent (at the desired
molality of aqueous [PMIm]Br), respectively. Figures 1–3 show the dependence of Vφ of
glycine, L-alanine, and L-valine in aqueous [PMIm]Br solutions of approximately 0.1, 0.2,
0.3, and 0.4 mol·kg−1, respectively, at T = 298.15 K. The values of Vφ for glycine, L-
alanine, and L-valine in aqueous 0.1 mol·kg−1 [PMIm]Br are compared in Fig. 4 and follow
the order: glycine < L-alanine < L-valine.
The value of the standard partial molar volume (Vφ0) was obtained by least-squares fitting
to the following equation:
Vφ = Vφ0 + Sv · m
(3)

J Solution Chem (2010) 39: 1409–1427
1415
Fig. 3 Apparent molar volumes
of L-valine in ([PMIm]Br +
water) mixtures with different
−1
molalities/(mol·kg ) of
[PMIm]Br at 298.15 K:
F, 0.0000; P, 0.0965;
2, 0.1969; E, 0.2946; Q, 0.3949
Fig. 4 Comparison of the
apparent molar volumes (V ) of
φ
−1
amino acids in 0.1 mol·kg
aqueous [PMIm]Br solutions at
298.15 K: F, glycine; P,
L-alanine; 2, L-valine
where Sν is the experimental slope which is considered to be a pair wise interaction, volu-
metric virial coefficient. The sign of Sν indicates the nature of the solute–solute interactions,
while Vφ0 reflects the presence of solute–solvent interactions [18, 19]. The values of Vφ0
and Sν are given in Table 2. This table shows that for all of mixtures studied, the Sν val-
ues are positive and are less than Vφ0, thus suggesting that solute–solute interactions are
weak. Further, the values of Sν decrease with increasing molality of the ionic liquid in so-
lutions containing glycine, which indicates that the strength of glycine–water interactions
is decreasing. Therefore, the ionic liquid has a dehydration effect on aqueous solutions of
glycine and it strongly interacts with the zwitterionic end group of glycine.
The values of Vφ0 are positive for all studied amino acids at all [PMIm]Br molalities.
The order of increasing Vφ0 values is: glycine < L-alanine < L-valine. This order shows
that, in general, values of Vφ0 increase with increasing molar mass of the amino acid, as
has been reported in literature [20]. This table also shows that the Vφ0 values increase for

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J Solution Chem (2010) 39: 1409–1427
0
φ
Table 2 Standard partial molar volumes, V , for amino acids in aqueous [PMIm]Br solutions at 298.15 K
a
0
6
6
2
R
Mixture
m
/
V
× 10 /
S
× 10 /
σ(V )
φ
v
IL
φ
−1
3
−1
3
−2
mol·kg
m ·mol
m ·mol ·kg
1.16 0.06
Glycine + water
0.0000
43.10 0.01,
43.2 [34]
0.9805
0.03
[PMIm]Br + Gly + water
[PMIm]Br + Gly + water
[PMIm]Br + Gly + water
[PMIm]Br + Gly + water
L-Alanine + water
0.0963
0.1962
0.2995
0.3961
0.0000
43.43 0.01
43.44 0.01
43.62 0.01
43.65 0.01
0.75 0.04
1.01 0.02
0.75 0.02
0.66 0.03
1.85 0.06
0.9644
0.9961
0.9850
0.9873
0.9900
0.02
0.01
0.01
0.01
0.03
59.96 0.01,
60.5 [35]
[PMIm]Br + L-Ala + water
[PMIm]Br + L-Ala + water
[PMIm]Br + L-Ala + water
[PMIm]Br + L-Ala + water
L-Valine + water
0.1520
0.1833
0.2863
0.3651
0.0000
52.59 0.15
51.77 0.18
51.16 0.17
50.26 0.23
13.80 0.47
15.29 0.57
14.75 0.53
18.18 0.85
2.14 0.05
0.9970
0.9891
0.9896
0.9847
0.9967
0.23
0.27
0.25
0.33
0.02
90.12 0.01,
91.0 [36]
[PMIm]Br + L-Val + water
[PMIm]Br + L-Val + water
[PMIm]Br + L-Val + water
[PMIm]Br + L-Val + water
0.0965
0.1969
0.2946
0.3949
89.77 0.02
89.57 0.01
89.22 0.02
76.13 0.30
3.23 0.07
2.30 0.04
2.60 0.09
27.78 1.00
0.9970
0.9977
0.9921
0.9923
0.03
0.01
0.03
0.33
a
m
is the molality of the ionic liquid [PMIm]Br
IL
glycine with increasing molality of [PMIm]Br, whereas decreases occur for L-alanine and
L-valine. Possible reasons for this behavior could be the larger alkyl group of L-alanine and
L-valine that are undergoing hydrophobic hydration, or the caging effect of water molecules
[21] which reflects hydrophobic interactions in L-alanine and L-valine that cause volume
contractions are rather larger for glycine solutions. Similar linear correlations have been
observed earlier for homologous series of amino acids in aqueous electrolytes [22].
At each molality, the Vφ0 values show a linear variation with the number of carbon atoms
in the alkyl chain of the amino acids. This linear variation is represented by:
Vφ0 = Vφ0(NH+3 , COO−) + nCVφ0(CH2)
(4)
where nC is the number of carbon atoms in the alkyl chain of the amino acid, and Vφ0(NH3+,
COO−) and Vφ0(CH2) are the zwitterionic end groups and methylene group contributions
to Vφ0, respectively. The values of Vφ0(NH+3 , COO−) and Vφ0(CH2), calculated by a least-
squares regression analysis are listed in Table 3. Since the alkyl chains of the homologous
series of the amino acids studied in this work are CH2 (glycine), CH3CH (L-alanine), and
CH3CH3CHCH (L-valine), the values of Vφ0(CH2) obtained by this procedure characterize
the mean contribution of CH and CH3 groups to the total Vφ0 of the amino acids. The con-
tributions of the other alkyl chains of the amino acids reported in Table 3 were calculated as
follows [23]:
Vφ0(CH3) = 1.5Vφ0(CH2)
Vφ0(CH) = 0.5Vφ0(CH2)
(5)
(6)

J Solution Chem (2010) 39: 1409–1427
1417
+
−
Table 3 Contributions of the zwitterionic group (NH , COO ), CH group and other alkyl chains to the
2
3
0
infinite dilution apparent molar volume V in aqueous [PMIm]Br solutions in different molalities m at
IL
φ
298.15 K
0
6
3
−1
−1
Group
V
× 10 /m ·mol
φ
−1
−1
−1
m
= 0.1 mol·kg
m
= 0.2 mol·kg
m
= 0.3 mol·kg
m
IL
= 0.4 mol·kg
IL
IL
IL
+
−
(NH , COO
3
)
24.840
15.896
31.792
24.540
15.880
31.750
63.500
24.590
15.747
31.493
62.986
30.715
11.128
22.256
44.512
CH –
2
CH CH–
3
(CH ) CHCH– 63.584
3 2
0
φ
Table 4 Standard partial molar volumes of transfer of amino acids ꢀ V in different molalities of aqueous
tr
[PMIm]Br m at 298.15 K
IL
0a
6
3
−1
Amino acids ꢀ V × 10 /m ·mol
tr
φ
−1
−1
−1
−1
m
= 0.1 mol·kg
m
= 0.2 mol·kg
m
= 0.3 mol·kg
m
IL
= 0.4 mol·kg
IL
IL
IL
Glycine
0.33
0.34
0.53
0.56
L-Alanine
L-Valine
−7.37
−0.34
−5.10
−0.54
−8.79
−0.89
−9.70
−13.97
Larger values of Vφ0(NH3+, COO−) compared with Vφ0(CH2) at different molalities of
[PMIm]Br indicate that the interactions of the ions of [PMIm]Br with the zwitterionic groups
of the amino acids dominate over those resulting from the hydrophobic group [PMIm]Br
interactions. The difference in the molar masses of the (NH+3 , COO−) and CH2 groups also
accounts for the larger Vφ0 values of (NH3+, COO−).
Limiting thermodynamic properties of transfer give information regarding the interac-
tions of a cosolvent with a solute without having to take into account the effects of solute–
solute interactions. The standard partial molar volumes of transfer ꢀtrVφ0 for the amino acids
from water to [PMIm]Br solutions were calculated as follows:
ꢀtrVφ0 = Vφ0([PMIm]Br + water) − Vφ0(water)
(7)
The uncertainties in the values of volumes of transfer were calculated as the root mean
squares of the uncertainties associated with the apparent molar properties at infinite dilution.
These values are presented in Table 4 and have positive values for glycine and negative
values for L-valine and L-alanine. The values of ꢀtrVφ0 for glycine increase in the positive
direction with increasing molality of [PMIm]Br.
In general, several types of interactions can occur in ternary mixtures of amino acid +
[PMIm]Br + water: (a) ion–ion interactions between the Br− anion of [PMIm]Br and the
−NH+3 group of the amino acid, and between the [PMIm]+ cation and the COO− group of
the amino acid; (b) ion–hydrophobic interactions between [PMIm]+/Br− of the ionic liquid
and a nonpolar group of an amino acid, and between (NH+3 , COO−) of an amino acid and the
hydrophobic group [PMIm]+ of the ionic liquid; (c) hydrophobic–hydrophobic interactions
between the alkyl groups of [PMIm]Br and the hydrophobic group of the amino acid. Taking
the cosphere overlap model [24] as a guideline, the (a) and (b) types of interactions should
lead to positive ꢀtrVφ0 values since there is a reduction in the electrostriction effect and the
overall water structure is enhanced. Interactions of type (c) should lead to negative ꢀtrVφ0

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J Solution Chem (2010) 39: 1409–1427
values because the alkyl group provides an additional tendency for hydrophilic–hydrophobic
and hydrophobic–hydrophobic group interactions and there will be a reduction in the struc-
ture of water as their cospheres overlap. An increase in the concentration of the cosolute
[PMIm]Br results in a positive contribution to ꢀtrVφ0, indicating an enhancement of ion–ion
interactions between the zwitterionic centers of the amino acids and the ions of the ionic
liquid. The number of hydrophobic groups increases in the order: glycine < L-alanine <
L-valine. The increased number of hydrophobic groups in L-alanine and L-valine interact
strongly with the hydrophobic groups of [PMIm]Br, thereby leading to negative volumes of
transfer that increase with increasing molality of [PMIm]Br [25].
Franks et al. [26] explained the standard partial molar volume of a nonelectrolyte as being
a combination of the intrinsic volume of the solute and volume changes due to its interac-
tions with the solvent. The intrinsic volume is considered to consist of two contributions
[27]:
Vφ0(int) = VvW + Vvoid
(8)
where VvW is the volume occupied by the solute due to its Van der Waals volume [28] and
Vvoid is the volume associated with the voids and empty spaces present therein. The above
equation was modified by Shahidi et al. [29] in order to include the contribution of a solute
molecule to its standard partial molar volume as
Vφ0(int) = VvW + Vvoid − nσs
(9)
where σs is the shrinkage in volume produced by the interaction of hydrogen bonding groups
of the solute with water molecules, and n is the number of potential hydrogen bonding sites
in the molecule. Thus, Vφ0 of an amino acid can be written as
Vφ0 = VvW + Vvoid − Vshrinkage
(10)
If it is assumed that VvW and Vvoid remain approximately the same in water and in aqueous
[PMIm]Br solutions, then the positive volume of transfer for glycine can be rationalized in
terms of a decrease in the volume of shrinkage in the presence of [PMIm]Br.
The measured viscosities η for (amino acid + [PMIm]Br + water) ternary mixtures at
T = 298.15 K are tabulated in Table 5. The results show that the viscosity of the solutions
increases with increasing molality of [PMIm]Br. This is due to the fact that upon increas-
ing the molality of the ionic liquid, the number of collisions among the molecules also in-
creases, resulting in a loss of kinetic energy. Therefore, the molecules tend to stick together
with increasing viscosity. Figures 5–7 show the concentration dependence of η of glycine,
L-alanine, and L-valine, respectively, in aqueous [PMIm]Br solutions at T = 298.15 K. The
values of η for glycine, L-alanine, and L-valine in aqueous 0.1 mol·kg−1 [PMIm]Br solu-
tions are compared in Fig. 8 and follow the order glycine < L-alanine < L-valine. This order
is possibly due to stronger hydrophobic– hydrophobic interactions for longer alkyl chains
of amino acids.
Most studies on viscosity are confined to the description of dilute solutions of electrolytes
via determination of the viscosity B-coefficients of the Jones-Dole equation [30]:
η
= 1 + BC
(11)
η0
where η and η0 are the viscosities of the (amino acid + [PMIm]Br + water) ternary mixture
and solvent ([PMIm]Br + water), respectively, and C is the molar concentration of amino

J Solution Chem (2010) 39: 1409–1427
1419
Table 5 Viscosities η of some solutions of amino acids in aqueous [PMIm]Br at 298.15 K
a
3
a
3
a
3
a
3
a
3
C/
η × 10 / C/
η × 10 / C/
η × 10 / C/
η × 10 / C/
η × 10 /
−3
−3
−3
−3
−3
mol·dm
Pa·s
mol·dm
Pa· s
mol·dm
Pa·s
mol·dm
Pa·s
mol·dm
Pa·s
Glycine
a
m
= 0.0000
−1
m
= 0.0963
−1
m
= 0.1962
m
= 0.2995
−1
m
IL
= 0.3961
−1
IL
IL
IL
IL
−1
mol·kg
0.0520
0.0997
0.1580
0.1999
0.2244
0.2920
0.3289
0.4544
0.5246
mol·kg
0.0529
0.1036
0.1487
0.1962
0.2640
0.3040
0.3551
0.4178
0.4507
mol·kg
0.0424
0.1122
0.1443
0.1950
0.2671
0.2890
0.3520
0.4099
0.4450
mol·kg
0.0504
0.1022
0.1442
0.2050
0.2582
0.3011
0.3520
0.4177
0.4512
mol·kg
0.0485
0.1027
0.1516
0.1938
0.2572
0.2978
0.3653
0.4179
0.4508
0.899
0.909
0.919
0.925
0.929
0.939
0.945
0.965
0.980
0.924
0.930
0.936
0.942
0.951
0.957
0.963
0.974
0.978
0.964
0.970
0.975
0.981
0.988
0.991
1.002
1.008
1.013
0.998
1.004
1.009
1.019
1.029
1.035
1.045
1.055
1.060
1.019
1.025
1.031
1.037
1.048
1.052
1.063
1.069
1.075
L-Alanine
= 0.1833
m
= 0.0000
m
= 0.1520
m
m
= 0.2863
m
Il
= 0.3651
IL
Il
Il
Il
−1
−1
−1
−1
−1
mol·kg
0.0532
0.0910
0.1498
0.1918
0.2526
0.2941
0.3537
0.3827
0.4512
0.4876
mol·kg
0.0579
0.0987
0.1473
0.2017
0.2464
0.2966
0.3554
0.4147
0.4968
0.5217
mol·kg
0.0470
0.0970
0.1441
0.1903
0.2524
0.3122
0.3413
0.4082
0.4873
0.5017
mol·kg
0.0604
0.0972
0.1493
0.2032
0.2543
0.2934
0.3523
0.4091
0.4628
0.5431
mol·kg
0.0500
0.1017
0.1496
0.1961
0.2508
0.3001
0.3506
0.3914
0.4683
0.886
0.891
0.897
0.902
0.912
0.921
0.928
0.936
0.943
0.952
0.944
0.954
0.968
0.983
0.992
1.006
1.021
1.039
1.063
1.068
0.969
0.985
0.999
1.017
1.040
1.060
1.069
1.087
1.112
1.116
0.988
1.002
1.021
1.036
1.054
1.069
1.087
1.107
1.125
1.153
1.019
1.033
1.052
1.067
1.082
1.100
1.119
1.137
1.161
L-Valine
m
= 0.0000
−1
m
= 0.0965
−1
m
= 0.1969
m
= 0.2946
−1
m
Il
= 0.3949
−1
Il
Il
Il
Il
−1
mol·kg
0.1173
0.1403
0.1898
0.2521
0.2987
0.3228
0.3867
0.4413
mol·kg
0.0827
0.1360
0.1803
0.2499
0.3352
0.4064
0.4716
0.5337
mol·kg
0.0836
0.1475
mol·kg
0.1037
0.2109
0.2476
0.2962
0.3424
0.4055
0.4777
mol·kg
0.1010
0.1568
0.2631
0.3202
0.3537
0.3968
0.4891
0.892
0.900
0.909
0.916
0.921
0.926
0.941
0.954
0.965
0.987
0.996
1.017
1.049
1.073
1.096
1.142
1.173
1.200
1.028
1.066
1.084
1.107
1.130
1.170
1.215
1.054
1.077
1.127
1.155
1.169
1.192
1.243
1.0050 0.2267
1.032
1.064
1.095
1.123
1.159
0.2711
0.3213
0.4277
0.4854
0.5503
a
m
is the molality of the ionic liquid [PMIm]Br and C is molarity of the amino acid
IL

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J Solution Chem (2010) 39: 1409–1427
Fig. 5 Viscosities of (glycine +
[PMIm]Br + water) solutions
with different
−1
molalities/(mol·kg ) of
[PMIm]Br at 298.15 K:
F, 0.0000; P, 0.0963;
2, 0.1962; E, 0.2995; Q, 0.3961
Fig. 6 Viscosities of
(L-alanine + [PMIm]Br + water)
solutions with different
−1
molalities/(mol·kg ) of
[PMIm]Br at 298.15 K:
F, 0.0000; P, 0.1520;
2, 0.1996; E, 0.2420; Q, 0.3368
Fig. 7 Viscosities of (L-valine +
[PMIm]Br + water) solutions
with different
−1
molalities/(mol·kg ) of
[PMIm]Br at 298.15 K:
F, 0.0000; P, 0.0965;
2, 0.1969; E, 0.2946; Q, 0.3949

J Solution Chem (2010) 39: 1409–1427
1421
Fig. 8 Comparison of viscosities
(η) of amino acids in
−1
0.1 mol·kg aqueous
[PMIm]Br solutions at 298.15 K:
F, glycine; P, L-alanine;
2, L-valine
Table 6 The B-coefficients of the Jones-Dole viscosity equation for amino acids in different molalities of
aqueous [PMIm]Br solutions at 298.15 K
3
3
−1
Amino acid
B × 10 dm ·mol
m
= 0.0
m
= 0.1
−1
m
= 0.2
−1
m
= 0.3
−1
m
IL
= 0.4
−1
IL
IL
IL
IL
−1
mol·kg
mol·kg
mol·kg
mol·kg
mol·kg
Glycine
0.118
0.174
0.205
0.149
0.291
0.451
0.131
0.341
0.471
0.163
0.349
0.524
0.140
0.350
0.541
L-Alanine
L-Valine
acid in the ternary mixture. The viscosity B-coefficient is an empirical constant which de-
pends on size and shape effects as well as structural effects induced by solute + solvent
interactions [31]. Values of the viscosity B-coefficient were calculated by the least-squares
method by fitting the experimental data to the Jones-Dole equation. These results are given
in Table 6. It is seen from Table 6 that the viscosity B-coefficients increase in the order
glycine < L-alanine < L-valine. This implies that L-valine has the strongest effect on the
water structure via hydrophobic interactions. Similar results have been reported for other
amino acids [9]. In all cases the viscosity B-coefficients are larger in the aqueous ionic liq-
uid solutions than the corresponding values in water. This indicates promotion of the liquid
structure in the presence of an ionic liquid, which supporting the conclusions obtained from
volumetric properties. The values of viscosity B-coefficients in inorganic salt solutions are
large in comparison with those of organic salts, indicating increasing interactions between
amino acid and inorganic salts [25].
The viscosity B-coefficients given in Table 6 reflect the net structural effects of charged
groups and hydrophobic CH2 groups on the amino acids. These two effects can be separated
by noting that the B-coefficients are linear in nC, that is
B = B(NH+3 , COO−) + nCB(CH2)
(12)
The regression parameters B(NH+3 , COO−), the zwitterionic group contribution, and
B(CH2), the methylene group contribution to the viscosity B-coefficient, are listed in Ta-
ble 7. It can be seen from Table 7 that the values of B(NH+3 , COO−) increase while the

1422
J Solution Chem (2010) 39: 1409–1427
+
−
Table 7 Viscosity B-coefficients of zwitterionic (NH , COO ) and CH groups of amino acids in aqueous
[PMIm]Br solutions at 298.15 K
2
3
3
3
−1
Amino acid
B(R) × 10 dm ·mol
m
= 0.0
m
= 0.1
m
= 0.2
−1
m
= 0.3
−1
m
IL
= 0.4
−1
IL
IL
IL
IL
−1
−1
mol·kg
mol·kg
mol·kg
mol·kg
mol·kg
+
−
(NH , COO
3
)
0.103
0.027
0.069
0.098
0.066
0.106
0.071
0.116
0.072
0.107
CH –
2
Table 8 The coefficients A and
1
−1
3
3
−1
2
5
m
/mol·kg
A × 10 dm ·mol
A
2
R
σ(B) × 10
IL
1
A
2
of Eq. 13, correlation
2
coefficients R and standard
deviations σ(B) for amino acids
in aqueous [PMIm]Br solutions
at 298.15 K
0.0
0.1
0.2
0.3
0.4
0.055
0.002 1.000 0.009
0.006 1.000 0.022
0.006 1.000 0.029
0.007 1.000 0.035
0.009 1.000 0.038
−0.067
−0.065
−0.074
−0.190
B(CH2) values decrease with increasing molality of [PMIm]Br, indicating that the zwitte-
rionic groups break the structure while CH2 groups enhance the structure of aqueous ionic
liquid solutions. The values of B(NH3+, COO−) are less than the B(CH2), confirming that
all of the investigated amino acids are net structure makers. Moreover, it is observed that
the B-coefficients show a linear correlation with the limiting partial molar volumes Vφ0 of
amino acids in aqueous [PMIm]Br solutions. The coefficients A1 and A2 of the equation:
B = A1 + A2Vφ0
(13)
are given in Table 8 together with the corresponding correlation coefficients. Similar corre-
lations are observed for amino acids in electrolyte solutions [32]. This is because the partial
molar volumes and B-coefficients both reflect solute–solvent interactions.
Experimental refractive index data nD for the investigated aqueous solutions of ionic
liquid are reported in Table 9. The molar refractivity RD of the mixtures under study can be
computed from the refractive index data using the Lorenz-Lorenz equation:
ꢂ
ꢆ
ꢀ
ꢁ
ꢄ
ꢅ
3
(n2D − 1)
(n2D + 1)
xiMi
d
ꢃ
RD =
(14)
i=1
where xi and Mi are the mole fraction and molecular weight of the ith component of the mix-
ture, respectively. The values of RD are found to increase linearly with increasing amount of
amino acids in aqueous [PMIm]Br, and RD is directly proportional to the molecular polar-
izability. Figure 9 reveals that the overall polarizability of the three mixture systems under
study increases with increasing molality of amino acid. This observation is in good agree-
ment with results reported in the literature for other amino acids in aqueous organic elec-
trolyte solutions [25].

J Solution Chem (2010) 39: 1409–1427
1423

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J Solution Chem (2010) 39: 1409–1427

J Solution Chem (2010) 39: 1409–1427
1425
Fig. 9 Comparison of molar
refractivities (R ) of amino acids
D
−1
in 0.4 mol·kg aqueous
[PMIm]Br solutions at 298.15:
F, glycine; P, L-alanine;
2, L-valine K
4 Conclusions
Density, viscosity, and refractive index data of the amino acids glycine, L-alanine, and
L-valine in aqueous 1-propyl-3-methylimidazolium bromide ([PMIm]Br) ionic liquid so-
lutions have been measured at T = 298.15 K. The apparent molar volumes (Vφ), stan-
dard partial molar volumes (Vφ0), standard partial molar volumes of transfer (ꢀtrVφ0),
and side-chain contributions Vφ0(R) of the amino acids have been computed. Amino
acids with a larger alkyl group (L-alanine and L-valine) undergo a greater degree of hy-
drophobic +hydration in the presence of [PMIm]Br as compared to glycine. Larger values
of Vφ0(NH3 , COO−) compared to Vφ0(CH2) indicate that the interactions of ions from
[PMIm]Br with the zwitterionic groups of the amino acids dominate over hydrophobic
group [PMIm]Br interactions. The viscosity B-coefficients were calculated by using the
Jones-Dole equation. All of these parameters were interpreted in terms of solute–solute
and solute–solvent interactions occurring among the various components of these ternary
mixtures. The results show that the values of B(NH+3 , COO−) increase while B(CH2) de-
creases with increasing molality of the ionic liquid, indicating that the zwitterionic groups
break structures while the CH2 groups enhance the structures of aqueous ionic liquid solu-
tions.
Acknowledgements The author wishes to thank the Graduate Council of the University of Mohaghegh
Ardabili for financial support.
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