Physicochemical study on molecular interactions in ternary aqueous solutions of the pharmaceutically active ionic liquid cetylpyridinium salicylate and amino acid/glycylglycine at different temperatures

Article abstract of DOI:10.1016/j.molliq.2020.115258

The pharmaceutically active form of an ionic liquid, cetylpyridinium salicylate ([CetPy][Sal]), was synthesized, and the intermolecular interactions of [CetPy][Sal] with amino acids (glycine, L-alanine, L-valine, and L-leucine) and glycylglycine (AAGG) in

Full text of DOI:10.1016/j.molliq.2020.115258

Journal of Molecular Liquids 326 (2021) 115258
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Physicochemical study on molecular interactions in ternary aqueous
solutions of the pharmaceutically active ionic liquid cetylpyridinium
salicylate and amino acid/glycylglycine at different temperatures
⁎
Zhenning Yan , Xingxing Cao, Meng Sun, Lulu Zhang
College of Chemistry, Zhengzhou University, Zhengzhou, Henan 450001, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The pharmaceutically active form of an ionic liquid, cetylpyridinium salicylate ([CetPy][Sal]), was synthesized,
and the intermolecular interactions of [CetPy][Sal] with amino acids (glycine, L-alanine, L-valine, and L-
leucine) and glycylglycine (AAGG) in aqueous media were investigated by measuring the density, conductivity
and UV–visible spectra at different temperatures. The measured density data was used to compute the apparent
molar volume at infinite dilution, V02, φ, the hydration number, nH, the transfer volume, ΔtV0, and the apparent
molar expansibility at infinite dilution, E ∅ o, of AAGG in aqueous [CetPy][Sal] solution. The measured electrical
conductivity was used to calculate the critical micelle concentration, cmc, and the relative thermodynamic quan-
tities for the micellization of [CetPy][Sal] in an AAGG solution, i.e. the changes in the Gibbs free energy, the en-
thalpy, and the entropy. The binding constants between [CetPy][Sal] and AAGG were derived from UV–vis
spectroscopic data. The aforementioned properties were analyzed in terms of the molecular interactions and
structural changes existing in the studied ternary solutions of (AAGG +[CetPy][Sal]+water).
Received 15 September 2020
Received in revised form 19 December 2020
Accepted 29 December 2020
Available online 2 January 2021
Keywords:
Cetylpyridinium salicylate
Amino acid and glycylglycine
Solute-solvent interaction
Thermodynamic parameters
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
between these compounds and API-ILs can provide a wealth of informa-
tion concerning solute-solvent and solute-cosolute interactions on pro-
Numerous researchers worldwide are intensively studying ionic liq-
uids in pharmaceutically active forms (API-ILs). API-ILs are the third
generation ILs that can be easily synthesized by combining appropriate
active cations and anions. These ionic liquids exhibit improved charac-
teristics and biological activities [1,2] compared with the corresponding
prodrugs. For example, ionic liquids of ampicillin salts have potent anti-
proliferative effects against various tumor cell lines [3]. A methotrexate
(MTX) based API-IL produced with the proline ethyl ester exhibits the
desired in vitro antitumor activity and is more soluble than free MTX
and its sodium salt in both water and simulated body fluids [4]. M.
tein. However, few such studies have been performed. Shekaari and
coworkers made a great contribution to this respect [8–13]. They devel-
oped the synthesis of six API-ILs using 1-butyl (1-hexyl or 1-octyl)-3-
methylimidazolium as cations and salicylate/ibuprofenate as anions,
and investigated the thermodynamic properties of aqueous solutions
of (API-ILs+amino acids), such as density, sound velocity, viscosity,
conductivity and refractive index. All these properties were analyzed
in the terms of the molecular interactions between API-ILs and amino
acids. In the present work, a new API-IL, cetylpyridinium salicylate
([CetPy][Sal]), was synthesized, and the interactions of [CetPy][Sal]
with selected amino acids and glycylglycine (AAGG) were studied by
volumetric, conductometric and UV–vis absorption spectroscopy
methods. [CetPy][Sal] has a high affinity for human serum and is effec-
tive as an antimicrobial pharmaceutical [14]. The experimentally mea-
sured densities were utilized to compute the apparent molar volume,
the apparent molar volume at infinite dilution, the transfer volume
and the hydration number. The measured conductivity was used to de-
termine the micellization conditions for aqueous [CetPy][Sal] with
added AAGG. A UV spectroscopy study of ternary systems was also car-
ried out. All the calculated quantities were analyzed in terms of the pos-
sible molecular interactions present in the ternary solutions. This
Goto et al. synthesized
a new API-IL comprising N-methyl-2-
pyrrolidonium as a cation and ibuprofen as an anion. This API-IL has en-
hanced ability to penetrate skin and accumulate in the target tissues of
pig skin [5]. Some API-ILs have the applications in drug delivery [6].
Studying the intermolecular interactions between API-ILs and bio-
molecules is essential for modeling the performance and elucidating
the molecular mechanism of API-ILs [7]. Amino acids and dipeptides
are the basic structural entities of proteins. Studying the interactions
⁎
Corresponding author.
E-mail address: yanzzn@zzu.edu.cn (Z. Yan).
https://doi.org/10.1016/j.molliq.2020.115258
0167-7322/© 2020 Elsevier B.V. All rights reserved.

Z. Yan, X. Cao, M. Sun et al.
Journal of Molecular Liquids 326 (2021) 115258
Table 1
Specification of the studied chemicals.
Chemical
Glycine
CAS No.
Source
Purity (mass fraction)
>0.99
Structure
Purification method
56–40-6
J&K Chemicals, China
Dried over P2O
L-alanine
56–41-7
J&K Chemicals, China
>0.99
Dried over P2O
L-valine
72–18-4
J&K Chemicals, China
>0.99
Dried over P2O
L-leucine
61–90-5
J&K Chemicals, China
>0.99
>0.99
Dried over P2O
Glycylglycine
556–50-3
Sigma-Aldrich, USA
Dried over P2O
CetPyCl·H2O
Sodium salicylate
[Cetpy][Sal]
6004-24-6
54–21-7
Aladdin Chemicals, China
J&K Chemicals, China
Synthesized
0.98
0.99
>0.96
None
None
Rotary/evaporator and vacuum
Dichloromethane
Potassium chloride
75–09-2
7447-40-7
J&K Chemicals, China
Sigma-Aldrich, USA
0.999
0.99999
None
Drying
investigation provides useful data for exploring the pharmaceutical pro-
file and performance of API-ILs.
a 20 ml acetone and water (1:1) mixture. The reaction mixture was
stirred overnight at room temperature. The solution was then extracted
with dichloromethane, the organic phase was washed with water several
times, the remaining water in the organic phase was removed with anhy-
drous magnesium sulfate, and the organic solvent was removed by rotary
evaporation after filtration. The obtained sample was dried under vac-
uum at 80 °C to obtain the final product (a white waxy solid). The synthe-
sized [CetPy][Sal] was characterized by 1H and 13C NMR (Bruker DPX,
400 MHz), and the NMR spectra are shown in Fig. S1 in the Supplemen-
tary Material. The water content for [CetPy][Sal] was about 510 ppm, as
determined using a Karl–Fischer titrator (Metrohm 815). The structure
of [CetPy][Sal] is graphically depicted in Table 1.
2. Experimental
2.1. Chemicals
A detailed description for the current materials is specified in
Table 1. The investigated amino acids and glycylglycine were vacuum
dried over P2O5 before use. The mixed solution was prepared with
fresh distilled and deionized water (conductance < 1 μS·cm−1).
2.2. Synthesis of [CetPy][Sal]
2.3. Apparatus and methods
Cetylpyridinium salicylate was prepared according to the literature
method [15]. Typically, cetylpyridinium chloride (0.0112 mol) and so-
dium salicylate (0.0112 mol) were weighed separately and dissolved in
A Sartorius BP 211D analytical balance was used to weigh the sam-
ples to a precision of
0.01 mg. The standard uncertainty in the
2

Z. Yan, X. Cao, M. Sun et al.
Journal of Molecular Liquids 326 (2021) 115258
molality of the solution is u(m) = 2.0×10−4 mol·kg−1. We utilized a
vibrating-tube digital densimeter (Anton Paar DMA 4500 M) to mea-
sure the density of the solutions at temperatures from 293.15 to
308.15 K. The precision in density is 1.0×10−5 g·cm−3. Before each se-
ries of measurements was made, the densimeter was calibrated accord-
ing to the instructions in the instrument manual by using doubly
distilled deionized water and dry air in the investigated temperature
range. The instrument was automatically thermostatted within
0.01 K using the built-in Peltier technique. Triplicate measurements
of each data point were done to obtain the average density. Uncertainty
calculations were carried out following the procedure described in
JCGM100: 2008 [16]. Taking into account the purity of the materials
and the uncertainty of the instrument, the combined expanded uncer-
structure of solute and [CetPy][Sal] getting more close [17]. Fig. 1a and
b clearly show that density increases with the AAGG concentration.
This trend reflects structural enhancement in water in the presence of
biomolecules. In addition, ρ decreases with the ascension of tempera-
ture. This phenomenon implies that the kinetic energy of the molecules
is strengthened with temperature.
Using the obtained ρ values of the ternary mixtures, apparent molar
volumes of AAGG have been derived first based on Eq. (1) [18]:
V2,φ ¼ M=ρ−1000ðρ−ρ0Þ=ðmρρ0Þ
ð1Þ
The symbol ρo stands for the densities of ([CetPy][Sal] + water)
mixed solvent, and m and M denote the molality and molar mass of
AAGG, respectively. Table S1 presents the resulting values of V2,φ. The
results clearly show that V2,φ value increases with temperature.
The apparent molar volume at infinite dilution, V02, φ was computed
from the least squares fit given by Eq. (2) [19]:
tainty in the density was estimated to be u(ρ) = 2.8×10−4 g·cm−3
.
The conductivity of the ternary mixtures was obtained by utilizing a
Thermo Orion digital conductivity meter (145A+) in the temperature
range of 293.15–313.15 K. The instrumental measurement has a relative
uncertainty of
3%. Prior to performing the experimental measure-
ments, the conductivity cell was calibrated by alignment in a standard
KCl solution. A DC-2006 circulated water thermostat manufactured by
Hengping Shanghai Ltd. was used to sustain the temperature within
0.02 K.
The UV–vis absorbance of sample solutions containing different
[CetPy][Sal] concentrations and 1.0×10−4 mol·kg−1 AAGG was mea-
sured at room temperature. The absorbance data was collected on a
UV–vis spectrometer (Lambda 365, PerkinElmer) at wavelengths of
190 to 350 nm using a 1 cm silica cuvette.
V2,φ ¼ V02,φ þ SV ∙m
ð2Þ
Here, SV is the slope of the regressed equation. The linear relation-
ship of V2,φ vs. m for glycine/glycylglycine in a 0.005 mol·kg−1 aqueous
[CetPy][Sal] solution is plotted in Fig. 2 as an example. The evaluated
V02,φ and the corresponding standard deviation are presented in
Table 2. The V02, φ values of AAGG in [CetPy][Sal] aqueous solution in-
crease with the [CetPy][Sal] concentration and temperature. This trend
implies the dehydration of amino acids and glycylglycine, indicating en-
hanced solute-solvent interactions. Furthermore, the V02, φ values de-
crease in the following order: leucine > valine > alanine > glycine.
This behavior is attributed to the increasing molar mass and hydropho-
bicity of the amino acids. The [CetPy][Sal]-amino acid interactions are
enhanced by the longer alkyl side chain of the amino acids.
The hydration behavior of AAGG in [CetPy][Sal]+water mixtures
was further analyzed by calculating the hydration number nH using
Millero's method [20], as described in the Supplementary Material.
Table 3 and Fig. 3 show that the nHvalues decrease with increased
[CetPy][Sal] molarity and temperature. This result confirms that
[CetPy][Sal] and high temperature result in the dehydration of the
3. Results and discussion
3.1. Volumetric properties of amino acids and glycylglycine
3.1.1. Apparent molar volumes at infinite dilution and hydration number
The experimentally measured densities (ρ) of different AAGG con-
centrations in 0.005 and 0.010 mol·kg−1 [CetPy][Sal] solutions are pre-
sented in Table S1 of the Supplementary Material. Fig. 1a shows that the
ρ values become large with the increased concentrations of [CetPy][Sal]
in the solutions, which implies the presence of interactions between
AAGG and [CetPy][Sal]. These interactions will result in the molecular
Fig. 1. (a) Density data of glycine in (■) 0.005 mol·kg−1 and (▲) 0.010 mol·kg−1 [CetPy][Sal] solutions vs. molality of glycine at 298.15 K; (b) Density data of glycine in 0.010 mol·kg−1
[CetPy][Sal] solution vs. molality of glycine at (■) 293.15 K, (●) 298.15 K, (▲) 303.15 K and (▼) 308.15 K.
3

Z. Yan, X. Cao, M. Sun et al.
Journal of Molecular Liquids 326 (2021) 115258
Fig. 2. Apparent molar volumes vs. molality of (a) glycine and (b) glycylglycine in aqueous 0.005 mol·kg−1 [CetPy][Sal] solutions at (■) 293.15 K, (●) 298.15 K, (▲)303.15 K and (▼)
308.15 K.
Table 2
ΔtV0 ¼ V02,φ ðaqueous ½CetPyꢀ½SalꢀÞ−V20,φ ðpure waterÞ
ð3Þ
Apparent molar volume at infinite dilution, V02, φ of AAGG in aqueous [CetPy][Sal] solution
at different temperatures and pressure p = 101 kPa.a
V02, φ/(cm3·mol−1
)
The values of V02, φ (pure water) for AAGG were taken from the liter-
ature [21–23]. The estimated ΔtV0 values are listed in Table 4. The ΔtV0
values are negative and increase with the enhancing temperature and
concentration of [CetPy][Sal]. This effect has also been observed for
amino acids in many ionic liquid solutions, such as [Emim][Br] [24], 3-
HPAF ionic liquid [25], [Phpi][BF4] [26], [Emim][HSO4] [27], and
[Bmim][BF4] [28] solutions. The cosphere overlap model [29] can be
used to explain the above mentioned effect. The following interactions
may occur between AAGG and [CetPy][Sal]:
293.15 K
298.15 K
303.15 K
308.15 K
0.005 mol·kg−1 [CetPy][Sal]
glycine
L-alanine
L-valine
L-leucine
glycylglycine
42.28 0.01
59.34 0.01
89.93 0.01
106.31 0.02
75.38 0.03
42.68 0.02
59.76 0.01
90.30 0.01
106.89 0.01
75.92 0.03
43.14 0.02
60.11 0.02
90.69 0.02
107.32 0.02
76.47 0.01
43.62 0.01
60.58 0.01
91.07 0.01
107.99 0.01
77.08 0.01
0.010 mol·kg−1 [CetPy][Sal]
glycine
L-alanine
L-valine
L-leucine
glycylglycine
42.36 0.01
59.59 0.02
89.95 0.01
106.51 0.01
75.55 0.01
42.74 0.02
60.01 0.01
90.46 0.03
107.09 0.03
75.96 0.02
43.31 0.02
60.39 0.01
90.83 0.01
107.62 0.01
76.55 0.01
43.75 0.01
60.85 0.01
91.32 0.01
108.30 0.01
77.12 0.01
(a) ionic-polar group interactions between ions of [CetPy][Sal] and
polar groups ((NH3+,COO−) and (CH2CONH)) of AAGG;
(b) ionic-apolar group interactions between ions of [CetPy][Sal] and
hydrophobic side chains of AAGG as well as between charged
groups of AAGG and hydrophobic side chains of [CetPy][Sal]; and.
(c) apolar-apolar group interactions between hydrophobic groups of
[CetPy][Sal] and hydrophobic side chains of AAGG.
a
Standard uncertainties u are: u(T) = 0.01 K, u(p) = 5 kPa.
Table 3
Values of hydration number, nH nHof AAGG in aqueous [CetPy][Sal] solution at different
temperatures and pressure p = 101 kPa.a
In accordance with the cosphere overlap model [29], the negative
ΔtV0 values are indicative of potent interactions (b) and (c). Apolar-
apolar group interactions can be heightened by the interactions of
alkyl groups in AAGG with the large hydrophobic aliphatic chain in
[CetPy][Sal]. Furthermore, the ΔtV0 values become less negative as the
molality of [CetPy][Sal] increases, implying the enhancement of ionic-
polar group interactions.
nH
293.15 K
298.15 K
303.15 K
308.15 K
0.005 mol·kg−1 [CetPy][Sal]
glycine
L-alanine
L-valine
9.40
3.49
4.91
1.84
6.36
8.71
3.15
4.50
1.55
5.81
7.64
2.72
3.91
1.27
5.03
6.95
2.39
3.52
1.00
4.50
L-leucine
glycylglycine
3.1.3. Apparent molar expansibility at infinite dilution
0.010 mol·kg−1 [CetPy][Sal]
The results presented above show that the V02, φ values are sensitive
to temperature. This temperature dependence can be represented as
glycine
L-alanine
L-valine
L-leucine
glycylglycine
9.37
3.41
4.91
1.77
6.30
8.69
3.08
4.45
1.49
5.80
7.59
2.64
3.87
1.19
5.01
6.91
2.33
3.46
0.93
4.49
2
V02,φ ¼ v1 þ v2ðT−273:15Þ þ v3ðT−273:15Þ
ð4Þ
a
Standard uncertainties u are: u(T) = 0.01 K, u(p) = 5 kPa.
where v1, v2 and v3 are the parameters. The temperature derivative
of Eq. (4) gives the apparent molar expansibility at infinite dilution, Eo∅:
ꢀ
ꢁ
amino acids/glycylglycine, which further supports the conclusions
E∅o
¼
∂V02,φ=∂T ¼ v2 þ 2v3ðT−273:15Þ
ð5Þ
drawn from the V02, φ values.
p
3.1.2. Transfer volume
Eq. (5) can also be utilized to compute (∂Eo∅/∂T) p, which reflects the
structure-forming or structure-breaking features of amino acids/
glycylglycine [30].
The transfer volume, ΔtV0, from water to the aqueous [CetPy][Sal]
solutions was computed as follows:
4

Z. Yan, X. Cao, M. Sun et al.
Journal of Molecular Liquids 326 (2021) 115258
Fig. 3. Hydration number nH of alanine (a) and leucine (b) vs. temperature in aqueous (■) 0.005 mol·kg−1 and (●) 0.010 mol·kg−1 [CetPy][Sal] solutions.
Table 4
Table 5
Transfer volumes, ΔtV0 of AAGG in aqueous [CetPy][Sal] solution at different temperatures
and pressure p = 101 kPa.a
Partial molar expansibility at infinite dilution, E0∅ and (∂E∅0 /∂T)p values of AAGG in aqueous
[CetPy][Sal] solutions.a
ΔtV0 (cm3·mol−1
)
E0∅/(cm3·mol−1·K−1
293.15 K 298.15 K 303.15 K 308.15 K
0.005 mol·kg−1 [CetPy][Sal]
)
(∂E0∅/∂T)p/
(cm3·mol−1·K−2
)
293.15 K
298.15 K
303.15 K
308.15 K
0.005 mol·kg−1 [CetPy][Sal]
glycine
L-alanine
L-valine
−0.62
−0.63
−0.61
−0.82
−0.41
−0.56
−0.60
−0.57
−0.80
−0.24
−0.52
−0.58
−0.53
−0.75
−0.18
−0.26
−0.39
−0.41
−0.40
−0.11
glycine
0.0776
0.0739
0.0747
0.0959
0.0856
0.0789
0.0757
0.1049
0.1095
0.0936
0.0839
0.0767
0.1139
0.1165
0.1016
0.0889
0.0777
0.1229
0.1235
0.0016
0.0010
0.0008
0.0018
0.0014
L-alanine
L-valine
L-leucine
L-leucine
glycylglycine
glycylglycine 0.1025
0.010 mol·kg−1 [CetPy][Sal]
0.010 mol·kg−1 [CetPy][Sal]
glycine
L-alanine
L-valine
L-leucine
glycylglycine
−0.54
−0.38
−0.59
−0.62
−0.24
−0.50
−0.35
−0.41
−0.60
−0.20
−0.36
−0.30
−0.39
−0.45
−0.10
−0.13
−0.12
−0.16
−0.09
−0.07
glycine
0.0858
0.0772
0.0848
0.1030
0.0918
0.0812
0.0888
0.1130
0.0980
0.0978
0.0852
0.0928
0.1230
0.1140
0.1038
0.0892
0.0918
0.1330
0.1300
0.0012
0.0008
0.0008
0.0020
0.0032
L-alanine
L-valine
L-leucine
glycylglycine 0.0820
a
a
Standard uncertainties u are: u(T) = 0.01 K, u(p) = 5 kPa.
Standard uncertainties u are: u(T) = 0.01 K, u(p) = 5 kPa.
ꢂ
ꢃ
1 þ eðc−c Þ=Δc
0
ꢀ
ꢁ
f ðcÞ ¼ f ð0Þ þ S1c þ ΔcðS2−S1Þ ln
ð7Þ
À
Á
1 þ e−c=Δc
∂Eo∅=∂T
¼
∂2V02,φ=∂T2 ¼ 2v3
ð6Þ
p
p
where f(0) is the conductivity at the [CetPy][Sal] concentration c=0, S1
and S2 are the slopes for the lower and upper cmc values, respectively.
Δc is the width of a sudden transition in f(c). c0 is [CetPy][Sal] concentra-
tion at the midpoint of the transition, which corresponds to the cmc. The
obtained cmc is shown in Table 6. Saraiva et al. [33] measured the cmc
value of [CetPy][Sal] in water using fluorescence assay at room temper-
ature. There is no significant deviation between our cmc value in water
and the literature value of 0.19 mmol.dm−3. The calculated cmc in water
is much smaller than that of cetylpyridinium chloride, 0.99 mmol·kg−1
[15]. This fact is not surprising because salicylate anion has been re-
ported to promotes the micellization of some cationic surfactants
[14,33,34]. This behavior is mainly attributed to -OH group being lo-
cated at the ortho-position in [Sal]−. The strong hydrophobicity of
[Sal]− relative to Cl− also contributes to this behavior.
The calculated Eo∅ and (∂E∅o /∂T) values are reported in Table 5.
p
Note that the Eo∅ values are all positive. This phenomenon implies
the presence of interactions between the solute and solvent. Further-
more, increasing the temperature and molarity of [CetPy][Sal] causes
Eo∅ to increase (Table 5), corresponding to the easier release of
electrostricted water from loose hydration layers of small biomole-
cules and a subsequent expansion in volume. The positive sign of
(∂Eo∅/∂T) p indicates the structure-forming behavior of AAGG in the in-
vestigated cases [30].
3.2. Conductivity properties
The influence of the concentration of AAGG and temperature on
the cmc values of [CetPy][Sal] is shown in Fig. 4. The cmc increases
with temperature. This phenomenon could be attributed to the defor-
mation of the water structure under intensified molecular motion
[35]. Fig. 4 also shows that the cmc decreases upon AAGG addition.
These results indicate that the addition of small biomolecules to an
aqueous solution promotes the micellization of [CetPy][Sal]. This
3.2.1. Critical micelle concentration of [CetPy][Sal]
The long alkyl chains in [CetPy][Sal] enable the formation of micellar
aggregates in pure water and (AAGG + water) systems. The conductiv-
ity was measured to analyze the micellization conditions for [CetPy]
[Sal]. The measured conductance (Table S2) was used in Eq. (7) to calcu-
late the critical micelle concentration (cmc) [31,32]:
5

Z. Yan, X. Cao, M. Sun et al.
Journal of Molecular Liquids 326 (2021) 115258
Table 6
Various micellization and thermodynamic parameters of [CetPy][Sal] in AAGG-water mixed media at different temperatures a
.
m/(mol·kg−1
)
T/K
cmc/(mmol·dm−3
)
α
ΔG0m/(kJ·mol−1
)
ΔH0m/(kJ·mol−1
)
ΔS0m/(J·mol−1·K)
ΔΔG0m/(kJ·mol−1
)
water
0.00
293.15
298.15
303.15
308.15
313.15
0.15
0.16
0.17
0.18
0.19
0.7466
0.7181
0.7150
0.6798
0.6781
−39.17
−40.54
−41.12
−42.75
−43.32
−11.82
−11.85
−11.59
−11.58
−11.22
93.29
96.24
97.41
101.16
102.49
glycine + water
0.05
293.15
298.15
303.15
308.15
313.15
293.15
298.15
303.15
308.15
313.15
0.14
0.15
0.16
0.18
0.19
0.13
0.14
0.15
0.16
0.17
0.5832
0.5734
0.5582
0.5770
0.5557
0.5571
0.5570
0.5718
0.5616
0.5832
−44.52
−45.35
−46.36
−46.08
−47.33
−45.60
−46.12
−46.16
−47.02
−46.84
−14.71
−16.02
−17.48
−18.57
−20.25
−15.67
−15.25
−14.62
−14.20
−13.40
101.70
98.36
95.29
89.28
86.46
102.12
103.54
104.04
106.52
106.78
−5.35
−4.81
−5.24
−3.33
−4.01
−6.43
−5.58
−5.04
−4.27
−3.52
0.10
L-alanine + water
0.05
293.15
298.15
303.15
308.15
313.15
293.15
298.15
303.15
308.15
313.15
0.13
0.14
0.15
0.16
0.17
0.12
0.13
0.14
0.15
0.16
0.5729
0.5783
0.5655
0.5562
0.5805
0.5636
0.5614
0.5597
0.5677
0.5970
−45.10
−45.43
−46.36
−47.19
−46.93
−45.68
−46.24
−46.80
−47.06
−46.61
−15.49
−15.02
−14.69
−14.25
−13.43
−16.87
−16.38
−15.82
−15.09
−14.08
100.99
102.00
104.49
106.91
106.98
98.27
100.16
102.21
103.75
103.87
−5.93
−4.89
−5.24
−4.44
−3.61
−6.51
−5.70
−5.68
−4.31
−3.29
0.10
L-valine + water
0.05
293.15
298.15
303.15
308.15
313.15
293.15
298.15
303.15
308.15
313.15
0.12
0.13
0.14
0.15
0.16
0.11
0.12
0.13
0.14
0.15
0.5565
0.5632
0.5590
0.5776
0.5800
0.6186
0.5995
0.6141
0.5412
0.5529
−45.90
−46.18
−46.82
−46.73
−47.17
−44.22
−45.30
−45.30
−48.19
−48.32
−16.95
−16.36
−15.83
−14.98
−14.25
−17.66
−17.28
−16.41
−16.46
−15.45
98.74
−6.73
−5.64
−5.70
−3.98
−3.85
−5.05
−4.76
−4.18
−5.44
−5.00
100.02
102.25
103.03
105.12
90.61
93.97
95.30
102.95
104.96
0.10
L-leucine + water
0.05
293.15
298.15
303.15
308.15
313.15
293.15
298.15
303.15
308.15
313.15
0.11
0.12
0.13
0.14
0.15
0.10
0.11
0.12
0.13
0.14
0.5589
0.5667
0.5703
0.5501
0.5922
0.5503
0.5321
0.5488
0.5458
0.5400
−46.13
−46.35
−46.72
−47.89
−47.00
−46.74
−47.79
−47.72
−48.31
−49.01
−18.42
−17.68
−16.92
−16.36
−15.03
−20.33
−19.76
−18.63
−17.67
−16.64
94.51
96.16
98.29
102.32
102.09
90.12
94.02
95.96
99.43
103.36
−6.96
−5.81
−5.60
−5.14
−3.68
−7.57
−7.25
−6.60
−5.56
−5.69
0.10
glycylglycine + water
0.05
293.15
298.15
303.15
308.15
313.15
293.15
298.15
303.15
308.15
313.15
0.10
0.11
0.12
0.13
0.14
0.08
0.09
0.10
0.11
0.13
0.5956
0.5726
0.5869
0.5639
0.5538
0.5313
0.5242
0.5532
0.5076
0.4768
−45.28
−46.47
−46.47
−47.70
−48.54
−48.15
−48.78
−48.24
−50.22
−51.42
−19.69
−19.21
−18.14
−17.45
−16.48
−19.28
−22.80
−25.90
−30.59
−35.38
87.29
91.42
93.44
98.18
102.38
98.49
87.14
73.68
63.70
51.23
−6.11
−5.93
−5.35
−4.95
−5.22
−8.98
−8.24
−7.12
−7.47
−8.10
0.10
a
m is the molality of AAGG in water. Standard uncertainties u are u(T) = 0.02 K, u(P) =5 kPa, u(m) = 0.0002 mol∙kg−1, u(cmc) = (0.003–0.008) mol·dm−3, u(ΔGm0 ) = 0.02 kJ·mol−1
,
u(ΔH0m) = 0.06 kJ·mol−1, u(ΔSm0 ) = 0.1 J·mol−1·K, u(ΔΔG0m) = 0.03 kJ·mol−1
.
behavior may be attributed to the electrostatic attraction between
polar groups of AAGG and the cationic [CetPy]+ of API-IL, which de-
creases the electrostatic repulsive forces between the charged heads
of [CetPy]+. Consequently, micellization is promoted, and the counter-
ions are liberated from the aggregate. The length of the alkyl chain of
AAGG also affects the cmc. The cmc increases in the order:
glycylglycine < leucine < valine < alanine < glycine < water. This re-
sult has been previously reported in the literature [36–39]. This de-
crease in the cmc upon AAGG addition is due to the breaking of the
hydration film of [CetPy][Sal] because of the interactions between
AAGG and [CetPy][Sal]. The entropy value of the studied system in-
creases, and hence, micelles can easily form.
6

Z. Yan, X. Cao, M. Sun et al.
Journal of Molecular Liquids 326 (2021) 115258
Fig. 4. Variation in critical micelle concentration of [CetPy][Sal] in water (■) and aqueous alanine (a) and glycylglycine (b) solutions with temperature and molality of alanine and
glycylglycine: (●) 0.05 mol·kg−1, (▲) 0.10 mol·kg−1
.
3.2.2. Thermodynamics of [CetPy][Sal] micellization
Standard thermodynamic quantities of micellization were com-
puted using the following relations based on the mass action model:
illustrates the absence of an absorption peak in the 190–350 nm range
in the amino acid solutions, whereas strong absorption peaks appear
at ~190 nm for the glycylglycine solutions. In the presence of [CetPy]
[Sal], three absorbances were observed at approximately 200, 260 and
295 nm. In the UV absorption spectra, the absorbance increases and
the UV absorption at 200 nm is shifted to higher wavelengths as the
[CetPy][Sal] concentration increases. These changes are indications of
significant AAGG-[CetPy][Sal] interactions. The UV absorption data at
200 nm was used with the Scott equation to calculate the binding con-
stant (Kb) between AAGG and [CetPy][Sal] [42].
ΔG0m ¼ ð2−αÞ RT ln Xcmc
ð8Þ
ð9Þ
ΔH0m ¼ −ð2−αÞ RT2 d ln Xcmc=dT
ꢀ
ꢁ
ΔS0m
¼
ΔH0m−ΔG0m =T
ð10Þ
where ΔG0m, ΔHm0 and ΔS0m are changes in the standard Gibbs free energy,
the enthalpy, and the entropy for micellization, respectively. The cmc
appears as a molecular fraction (Xcmc). The term (dlnXcmc/dT) can be ob-
tained using the following second-degree polynomial equation:
CDCm
ΔA
l
Cm
1
¼
þ
ð12Þ
εm−ε0 ðεm−ε0ÞKb
Here ΔA and (εm-ε0) are the differences in the absorbance and molar
extinction coefficient, respectively, with and without [CetPy][Sal]. Cm
and CD are the concentrations of [CetPy][Sal] and AAGG, respectively,
and CD is 1.0×10−4 mol·kg−1. The Kb value is evaluated from the ratio
of the intercept to the slope of the straight lines of (CDCml/ΔA) against
Cm as shown in Fig. S3. The Kb values for the association of [CetPy]
[Sal]-AAGG (glycine, alanine, valine, leucine, and glycylglycine) are
3472, 3537, 3934, 4474 and 4530 m3·mol−1, respectively. These Kb
values show that increasing the side-chain length of an amino acids in-
creases the binding appetency of the amino acid towards [CetPy][Sal].
The order of the strength of the AAGG and [CetPy][Sal] interactions is at-
tributed to the difference in the hydrophobicities of the corresponding
alkyl chains. This result agrees with the results of the volumetric studies.
ln Xcmc ¼ k1 þ k2T þ k3T2
ð11Þ
The calculated thermodynamic quantities are presented in Table 6.
In all the studied cases, ΔG0m value is negative. The negative ΔG0m indi-
cates that the aggregation process is spontaneous. To further explore
the impact of AAGG addition on the micellization of [CetPy][Sal], the
Gibbs free energy of transfer, ΔΔG0m=ΔGm0 (in a mixed solvent) -ΔG0m
(in water), was calculated. The gotten results in Table 6 show that
ΔΔG0m values in the aqueous AAGG solution are negative. This result im-
plies that the micelle formations are more favorable in the presence of
the AAGG than in water. More negative ΔG0m values signify that the de-
hydration of [CetPy][Sal] occurs at high temperatures, which is a major
driver for the formation of micelles [40]. The negative value of ΔHm0
shows that micelle formation is exothermic. Micellization becomes
more exothermic in the presence of biomolecules in aqueous solution.
The large positive ΔS0m values state that the micellization process is
entropy-driven, that is, the transfer of the hydrophobic group from the
solvent to the micellar interior provides the driving force for the
[CetPy][Sal] micellization. The negative ΔH0m and positive ΔSm0 values
show that electrostatic interactions play an important role, in addition
to hydrophobic interactions, in micelle formation [41].
4. Conclusions
A pharmaceutically active ionic liquid [CetPy][Sal] was synthesized
and the volumetric, conductometric and UV–vis spectra properties
were determined for ternary aqueous solutions containing [CetPy][Sal]
and amino acids/glycylglycine (AAGG). The experimental data was
used to calculate several parameters, such as the apparent molar vol-
ume at infinite dilution, the hydration number, the transfer volume,
the apparent molar expansibility at infinite dilution of AAGG, the critical
micelle concentration, relative thermodynamic properties of [CetPy]
[Sal] and the binding constant between AAGG and [CetPy][Sal]. These
thermodynamic properties depend upon the temperature, composition,
and size of the AAGG alkyl chain. The hydrophilic-hydrophobic and
hydrophobic-hydrophobic interactions between AAGG and [CetPy]
3.3. UV–vis spectroscopy
To elucidate the intermolecular interactions in the studied systems,
UV–vis absorption measurements were performed in the mixed solu-
tions with a fixed AAGG concentration of 1.0×10−4 mol·kg−1. Fig. S2
7

Z. Yan, X. Cao, M. Sun et al.
Journal of Molecular Liquids 326 (2021) 115258
[12] H. Shekaari, M.T. Zafarani-Moattar, S.N. Mirheydari, E.M.H. Agha, The effect of phar-
maceutically active ionic liquids, 1-methyl-(3-hexyl or octyl) imidazolium
ibuprofenate on the thermodynamic and transport properties of aqueous solutions
of glycine at T = 298.2 K and p = 0.087 MPa, J. Mol. Liq. 288 (2019), 111009, .
[13] H. Shekaari, M.T. Zafarani-Moattar, S.N. Mirheydari, S. Faraji, Thermophysical prop-
erties of 1-hexyl-3-methylimidazolium salicylate as an active pharmaceutical ingre-
dient ionic liquid (API-IL) in aqueous solutions of glycine and l-alanine, J. Chem. Eng.
Data 45 (2019) 124–134.
[14] P.C.A.G. Pinto, D.M.G.P. Ribeiro, A.M.O. Azevedo, V.D. Justina, E. Cunha, K. Bica, M.
Vasiloiu, S. Reisa, M.L.M.F.S. Saraiva, Active pharmaceutical ingredients based on sa-
licylate ionic liquids: insights into the evaluation of pharmaceutical profiles, New J.
Chem. 37 (2013) 4095–4102.
[15] K. Bica, C. Rijksen, M. Nieuwenhuyzen, R.D. Rogers, In search of pure liquid salt
forms of aspirin: ionic liquid approaches with acetylsalicylic acid and salicylic
acid, Phys. Chem. Chem. Phys. 12 (2010) 2011–2017.
[16] JCGM, Evaluation of Measurement Data - Guide to the Expression of Uncertainty in
Measurement (JCGM 100:2008), BIPM, 2008.
[17] W.P. Jencks, Catalysis in chemistry and enzymology, Courier Corporation, New York:
Dover, 1987.
[18] I. Klotz, R.M. Rosenberg, Chemical Thermodynamics, Basic Theory and Methods, 3rd
ed. Benjamin Cummings, Upper Saddle River, NJ, 1972.
[19] J.F. Reading, G.R. Hedwig, Thermodynamic properties of peptide solutions 4. Partial
molar volumes and heat capacities of aqueous solutions of some glycyl dipeptides, J.
Solut. Chem. 18 (1989) 159–171.
[Sal] increase with the cosolute concentration. The presence of [CetPy]
[Sal] and high temperature result in the dehydration effect on AAGG.
The increase in alkyl chain length of AAGG enhances solute-solvent in-
teractions in the present ternary solutions. The critical micelle concen-
trations of [CetPy][Sal] increase with temperature and decrease with
the addition AAGG. The micellization process is spontaneous and exo-
thermic. Both hydrophobic interactions and electrostatic interactions
play an important role in the formation of micelle process.
Credit authorship contribution statement
Zhenning Yan: Methodology, Validation, Formal analysis, Data
curation, Writing - review & editing, Writing - original draft, Project ad-
ministration, Funding acquisition. Xingxing Cao: Conceptualization,
Validation, Formal analysis, Writing - review & editing. Meng Sun: Val-
idation, Investigation, Formal analysis. Lulu Zhang: Validation, Visuali-
zation, Investigation.
[20] F.J. Millero, A.L. Surdo, C. Shin, The apparent molal volumes and adiabatic compress-
ibilities of aqueous amino acids at 250C, J. Phys. Chem. 82 (1978) 784–792.
[21] C. Jolicoeur, R. Riedl, D. Desrochers, L.L. Lemelin, R. Zamojska, O. Enea, Solvation of
amino acid residues in water and urea-water mixtures: Volumes and heat capacities
of 20 amino acids in water and in 8 molar urea at 25°C, J. Solut. Chem. 15 (1986)
109–128.
[22] A. Pal, N. Chauhan, Volumetric behaviour of amino acids and their group contribu-
tions in aqueous lactose solutions at different temperatures, J. Chem. Thermodyn.
43 (2011) 140–146.
[23] Z.N. Yan, R.L. Liu, S.Y. Wu, X.R. Bai, J.J. Wang, Effect of temperature on the interac-
tions of glycyl dipeptides with sodium perfluorooctanoate in aqueous solution: vol-
umetric, conductometric, and spectroscopic study, J. Chem. Thermodyn. 57 (2013)
360–366.
[24] S. Fang, D.H. Ren, Effect of 1-ethyl-3-methylimidazolium bromide ionic liquid on the
volumetric behavior of some aqueous L-amino acids solutions, J. Chem. Eng. Data 58
(2013) 845–850.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the results reported in this paper.
Acknowledgment
This work was supported by the National Natural Science Founda-
tion of China (Grant No. 21573199).
Appendix A. Supplementary data
[25] V. Singh, P.K. Chhotaray, P.K. Banipal, T.S. Banipal, R.L. Gardas, Volumetric properties
of amino acids in aqueous solutions of ammonium based protic ionic liquids, Fluid
Phase Equilib. 385 (2015) 258–274.
[26] H. Xie, L. Zhao, C. Liu, Y. Cao, X. Lu, Q. Lei, W. Fang, Volumetric property of glycine, L-
serine, L-alanine and L-proline in aqueous solutions of 1-phenylpiperazinium tetra-
fluoroborate, J. Chem. Thermodyn. 99 (2016) 75–81.
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.molliq.2020.115258.
References
[27] H.R. Rafiee, F. Frouzesh, Volumetric properties for glycine and L-serine in aqueous
solutions of 1-ethyl-3-methylimidazolium hydrogen sulfate ([Emim][HSO4]) at T
= (293.15 to 313.15) K and ambient pressure, J. Chem. Thermodyn. 102 (2016)
398–405.
[28] H.R. Rafiee, F. Frouzesh, The study of solute–solvent interactions in the ternary
{Amino acid (Glycine or L-serine) + ionic liquid (1-butyl-3-methylimidazolium
tetra fluoroborate [Bmim][BF4]) + H2O} system at different temperatures and am-
bient pressure: volumetric study, J. Mol. Liq. 230 (2017) 6–14.
[29] R.W. Gurney, Ionic Processes in Solution, McGraw Hill, New York, 1953.
[30] L. Hepler, Thermal expansion and structure in water and aqueous solutions, Can. J.
Chem. 47 (1969) 4613–4617.
[31] P. Carpena, J. Aguiar, P. Bernaola-Galvan, C.C. Ruiz, Problems associated with the
treatment of conductivity−concentration data in surfactant solutions: simulations
and experiments, Langmuir 18 (2002) 6054–6058.
[32] J. Aguiar, P. Carpena, J.A. Molina-Bolivar, C.C. Ruiz, On the determination of the crit-
ical micelle concentration by the pyrene 1:3 ratio method, J. Colloid Interface Sci.
258 (2003) 116–122.
[33] T. Mukhim, J. Dey, S. Das, K. Ismail, Aggregation and adsorption behavior of
cetylpyridinium chloride in aqueous sodium salicylate and sodium benzoate solu-
tions, J. Colloid Interface Sci. 350 (2010) 511–515.
[34] Z.N. Yan, S.X. Shen, L.M. Ma, L.Y. Liu, X. Chen, Interaction between an active pharma-
ceutical ingredient ionic liquid benzalkonium salicylate and small biomolecules in
aqueous solution: UV absorption, conductivity, and volumetric study, J. Solut.
Chem. 47 (2018) 1514–1528.
[35] H. Kumar, C. Chadha, Conductometric and spectroscopic studies of
cetyltrimethylammonium bromide in aqueous solutions of imidazolium based
ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate, J. Mol. Liq. 211
(2015) 1018–1025.
[1] M. Smiglak, J.M. Pringle, X. Lu, L. Han, S. Zhang, H. Gao, D.R. MacFarlane, R.D. Rogers,
Ionic liquids for energy, materials, and medicine, Chem. Commun. 50 (2014)
9228–9250.
[2] R. Ferraz, L.C. Branco, C. Prudencio, J.P. Noronha, Z. Petrovski, Ionic liquids as active
pharmaceutical ingredients, Chem. Med. Chem. 6 (2011) 975–985.
[3] R. Ferraz, J. Costa-Rodrigues, M.H. Fernandes, M.M. Santos, I.M. Marrucho, L.P.N.
Rebelo, C. PrudÞncio, J.P. Noronha, Z. Petrovski, L.C. Branco, Antitumor activity of
ionic liquids based on ampicillin, Chem. Med. Chem 10 (2015) 1480–1483.
[4] R.Md. Moshikur, Md.R. Chowdhury, R. Wakabayashi, Y. Tahara, N.M. Moniruzzaman,
M. Goto, Ionic liquids with methotrexate moieties as a potential anticancer prodrug:
synthesis, characterization and solubility evaluation, J. Mol. Liq. 278 (2019)
226–233.
[5] R.Md. Moshikur, Md.R. Chowdhury, R. Wakabayashi, Y. Tahara, N.M. Moniruzzaman,
M. Goto, Ionic liquids with N-methyl-2-pyrrolidonium cation as an enhancer for
topical drug delivery: Synthesis, characterization, and skin-penetration evaluation,
J. Mol. Liq. 299 (2020) 112–166.
[6] W. Huang, X. Wu, J. Qi, Q. Zhu, W. Wu, Y. Lu, Z. Chen, Ionic liquids: green and tailor-
made solvents in drug delivery, Drug Discov. Today 25 (2020) 901–908.
[7] H. Shekaari, M.T. Zafarani-Moattar, S.N. Mirheydari, E.M.H. Agha, The effect of phar-
maceutically active ionic liquids, 1-methyl-(3-hexyl or octyl) imidazolium
ibuprofenate on the thermodynamic and transport properties of aqueous solutions
of glycine at T = 298.2 K and p = 0.087 MPa, J. Mol. Liq. 288 (2019), 111009, .
[8] H. Shekaari, M.T. Zafarani-Moattar, S.N. Mirheydari, Conductometric analysis of 1-
butyl-3-methylimidazolium ibuprofenate as an active pharmaceutical ingredient
ionic liquid (API-IL) in the aqueous amino acids solutions, J. Chem. Thermodyn.
103 (2016) 165–175.
[9] H. Shekaari, M.T. Zafarani-Moattar, S.N. Mirheydari, Thermodynamic properties of
1-butyl-3-methylimidazolium salicylate as an active pharmaceutical ingredient
ionic liquid (API-IL) in aqueous solutions of glycine and L-alanine at T =
(288.15–318.15) K, Thermochim. Acta 637 (2016) 51–68.
[10] H. Shekaari, M.T. Zafarani-Moattar, S.N. Mirheydari, Effect of 1-butyl-3-
methylimidazolium ibuprofenate as an active pharmaceutical ingredient ionic liq-
uid (API-IL) on the thermodynamic properties of glycine and l-alanine in aqueous
solutions at different temperatures, J. Solut. Chem. 45 (2016) 624–663.
[11] H. Shekaari, M.T. Zafarani-Moattar, S.N. Mirheydari, Study of interactions between l-
[36] K. Kumar, S. Chauhan, Surface tension and UV–visible investigations of aggregation
and adsorption behavior of NaC and NaDC in water–amino acid mixtures, Fluid
Phase Equilib. 394 (2015) 165–174.
[37] Z.N. Yan, Q. Zhang, W.W. Li, J.J. Wang, Effect of temperature on the interactions of
glycyl dipeptides with sodium dodecyl sulfate in aqueous solution: a volumetric,
conductometric, and fluorescence probe study, J. Chem. Eng. Data 55 (2010)
3560–3566.
alanine
and
1-octyl-3-methylimidazolium
salicylate
or
1-octyl-3-
[38] Z.N. Yan, X.M. Sun, W.W. Li, Y. Li, J.J. Wang, Interactions of glutamine dipeptides with
sodium dodecyl sulfate in aqueous solution measured by volume, conductivity, and
fluorescence spectra, J. Chem. Thermodyn. 43 (2011) 1468–1474.
methylimidazolium ibuprofenate using the thermophysical properties at T=
298.15K, J. Mol. Liq. 278 (2019) 105–114.
8

Z. Yan, X. Cao, M. Sun et al.
Journal of Molecular Liquids 326 (2021) 115258
[39] L. Yu, T. Lu, Y.X. Luan, J. Liu, G.Y. Xu, Studies on the effects of amino acids on micell-
ization of CTAB via surface tension measurements, Colloids Surf. A Physicochem.
Eng. Asp. 257–258 (2005) 375–379.
[40] H. Kumar, A. Katal, P. Rawat, FT-IR spectroscopic and micellization studies of
cetyltrimethylammonium bromide in aqueous and aqueous solution of ionic liquid
(1-butyl-3-methylimidazolium bromide) at different temperatures, J. Mol. Liq. 249
(2018) 227–232.
[41] A. Pal, A. Yadav, Mixed micellization of a trisubstituted surface active ionic liquid 1-
dodecyl-2,3-dimethylimidazolium chloride [C12bmim][Cl] with an amphiphilic drug
amitriptyline hydrochloride AMT: a detailed insights from conductance and surface
tension measurements, J. Mol. Liq. 279 (2019) 43–50.
[42] R.L. Scott, Some comments on the Benesi-Hildebrand equation, Recueil des Travaux
Chimiques des Pays-Bas. 75 (1956) 787–789.
9