Spectrum of
Emerging Sciences, 2 (2) 2023, 1728
Volumetric and Spectroscopic Studies of
1ethyl3methylimidazolium Ethylsulfate/Propane1ol Binary Mixtures at
Different Temperatures
Md. Ahad Ali^{1}, Md. Abu Bin
Hasan Susan^{1,2*}
Department of Chemistry^{1}
and Dhaka University Nanotechnology Center (DUNC)^{1,2}, University of
Dhaka, Dhaka 1000, Bangladesh^{}
*Corresponding
Author:
Email
Address: susan@du.ac.bd
Article
available online at: https://esciencesspectrum.com/AbstractView.aspx?PID=2022225
ARTICLE INFO


ABSTRACT

Original
Research Article
Received: 23 Feb 2023
Accepted: 24 March 2023
DOI
10.55878/SES2022225
KEYWORDS
Ionic liquid, binary mixture,
thermodynamic activation
parameters, NIR spectroscopy,
PCA and 2D correlation
spectroscopy


Binary mixtures of an ionic liquid,
1ethyl3methylimidazolium ethyl sulfate ([C_{2}mim]C_{2}H_{5}SO_{4})
with propane1ol were prepared over an entire composition range and density,
dynamic viscosity, and refractive index at T = 293.15 to T =
333.15 K at atmospheric pressure were measured. The
excess properties for the binary systems were determined and successfully
fitted to a polynomial equation of the Redlich–Kister type. The variation
of excess thermodynamic parameters predicted stronger intermolecular
interactions and effective packing in the binary system compared to
components. Thermodynamic activation parameters were also calculated from the
Eyring equation, which varied with the concentration of [C_{2}mim]C_{2}H_{5}SO_{4.}
The variation of these parameters also suggested the presence of strong
heteromolecular interactions. The nearinfrared (NIR) spectroscopic measurements
were conducted in the temperature range from
293.15 K to 333.15 K and spectral variations were analyzed. The NIR
data were further evaluated using principal component analysis (PCA) and
twodimensional (2D) correlation spectroscopy. The predicted molecularlevel
interactions mainly come from different types of HBs formed between unlike
molecules in the binary system. The binary mixture may open up a plethora of
possible uses due to its novel, distinctive molecularlevel interactions, and
favorable thermodynamic properties.

The use of ionic liquids (ILs) is one
of the most exciting developments in solvent chemistry. "ILs" refer
to solvent materials that are liquids below 373.15 K and consist only
of ions. They are known as
"designer solvents" because of their ability to modify and optimize
the physical properties of the IL for a specific purpose by adjusting the ions.
Because of their superior physicochemical characteristics, which include a wide
liquidus range, a relatively low melting point, imperceptible vapor pressure, high
electrical conductivity, a wide electrochemical potential window,
nonflammability, good thermal stability, recyclability, and an increased
capacity to dissolve a variety of inorganic and organic substances, ILs have
attracted the attention of scientists [1–5]. Numerous works on the
characteristics of ILs or their uses in chemical synthesis [6,7], catalytic
reactions [8, 9, separation techniques [10], membrane technology [11],
batteries [12], capacitors [13], solar cells [14], fuel cells [15], or as lubricants
[16] have been reported. The molecularlevel interactions of ILs remain poorly
understood since, in general, these novel solvents are extremely hygroscopic
[17], and even minute amounts of water or other substances can induce
significant changes in molecularlevel interactions in an IL [18–20]. Forming
binary IL mixtures using common molecular solvents is one method of adjusting
physicochemical characteristics. In addition to enhancing the features of a
medium for taskspecific applications, binary mixtures of ILs significantly
reduce costs.
One of the most used molecular
solvents for the binary combination with ILs is alcohol. The main reason for
using alcohol is the presence of both hydrogen bond (HB) acceptor and donor
sites in the alcohol structure as well as prospects of using numbers of
alcohols with variable numbers of OH groups and alkyl groups [21. Although in
the last decade, many experimental and theoretical researches have been
conducted on ILmolecular solvent binary systems, understanding of
the interactions between IL and molecular solvents, and now the influence
physicochemical parameters is still in its infancy. The dissolution behavior of
IL in molecular solvents and the interaction between IL and molecular solvents
in contrast to the interactions in ILs
have been examined through thermodynamic investigations and physicochemical
attributes of binary systems of ILs [22–25]. Welton and coworkers used infrared
(IR) spectroscopy to investigate the molecular interaction of a number of 1alkyl3methylimidazolium
ILs with water in binary mixtures and revealed that the main reason for water
absorption by ILs in humidifying conditions is HBs between an anion and water
[26]. Using near infrared (NIR) spectroscopy, Marium et al. investigated
interactions between 1ethyl3methylimidazolium tetrafluroborate ([C_{2}mim]BF_{4})
and water in binary mixtures at the molecular level and reported that the number of ILIL and
waterwater clusters and the strength of ILwater bonds dictate the
physicochemical properties [27]. Kiefer et al. measured IR spectra and
analyzed solventinduced line shifts and surplus IR spectra to understand the
molecularlevel interaction between 1ethyl3methylimidazolium ethylsulfate([C_{2}mim]C_{2}H_{5}SO_{4})
and acetone. Instead of interacting with ion pairs, they demonstrated that
acetone forms HBs with the hydrogen of the imidazolium ring [28]. Using IR
spectroscopy, Chang et al. studied the structural organization of the IL in
binary systems and reported that the IL is likely to aggregate in the alkyl
region. When high pressures were applied to binary mixtures of 1butyl3methylimidazolium
tetrafluroborate ([C_{4}mim]BF_{4}) with water, the free OH
species were converted to the bound OH species. For binary mixtures of this IL
with methanol, the free OH is stable at high pressures [29]. The majority of
current research focuses on the investigation of the OH band in the MIR
region, where analysis is challenging due to the substantial overlap between
the bands for various hydrogenbonded species. In the NIR region, the OH bands
of different types of clusters are well separated compared to MIR. [27]
Although IR spectra are used to investigate molecularlevel interaction between
several ILs and conventional molecular solvents, No work has yet been reported
on the investigation of molecular level interaction of [C_{2}mim]C_{2}H_{5}SO_{4}
and alcohol system using NIR spectroscopy.
We, therefore, aim at using NIR
spectroscopy to understand interactions between the various hydrogenbonded
species present in IL/alcohol, [C_{2}mim]C_{2}H_{5}SO_{4})/propane1ol
binary mixtures, both qualitatively and quantitatively. The changes in
,
,
, and
for the viscous flow of binary
mixtures with composition have been examined in detail. The ultimate goal was
to correlate solution parameters with NIR spectroscopic results to comprehend
the aggregation behavior and interactions in [C_{2}mim]C_{2}H_{5}SO_{4})/propane1ol
binary mixtures.
Experimental
Propane1ol from LabScan Analytical Sciences,
Thailand and [C_{2}mim]C_{2}H_{5}SO_{4} from
Merck were used without further
purification. The compositions used to prepare binary mixtures using
gravimetric methods are: 0 (propane1ol), 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7,
0.8, 0.9, and 1 ( [C_{2}mim]C_{2}H_{5}SO_{4}).
A DMA 4500 M densimeter (Anton Paar) and
a Lovis 2000 ME microviscometer (Anton Paar), each equipped with an
integrated Peltier controlled thermostat, were used to measure densityies and
dynamic viscosity from 293.15 K to 333.15 K temperature range with a 5 K
interval with a sensitivity of ±0.01 K. The NIR spectra of each sample were
taken using a Fourier transform spectrophotometer (FTIR/NIR, PerkinElmer, USA)
in absorbance mode with 64 scans in the range of 4000–10000 cm^{1 }at
a resolution of 2 cm^{1}. A very sensitive liquid sample cell was used
to record the temperaturedependent NIR spectra with the help of two
rectangular CaF_{2} windows with curved edges (Specac model no.
GS20522). The path length was maintained 0.02 mm by using a rectangular
polytetrafluoroethylene spacer. Statistical analysis and mathematical
calculations were performed using OriginPro 2019b. The unprocessed NIR data
were baseline corrected and smoothed before processing. All mathematical and
statistical operations were carried out for NIR data between 6000 and 7500 cm^{1}.
Figure
1. Temperaturedependent changes in (a) density,
(b) refractive index, and (c) viscosity at different mole fractions of [C_{2}mim]C_{2}H_{5}SO_{4}.
Result
& Discussion
The density, refractive index, and viscosity of [C_{2}mim]C_{2}H_{5}SO_{4}
and propane1ol were measured along with [C_{2}mim]C_{2}H_{5}SO_{4}/propane1ol
binary mixtures at a mole fraction of [C_{2}mim]C_{2}H_{5}SO_{4}]
of 0.1 to 0.9 at T = 293.15 to T = 333.15 K under atmospheric pressure
(Figure 1). For the binary mixtures, both density and refractive index increase
with increasing mole fraction of [C_{2}mim]C_{2}H_{5}SO_{4}.
At a lower mole fraction of [C_{2}mim]C_{2}H_{5}SO_{4,
}the system behaves like a very dilute ionic solution. As the mole
fraction of [C_{2}mim]C_{2}H_{5}SO_{4 }increases,
the system gradually becomes a concentrated ionic solution with more or less
conspicuous ion pairing. [30] Both parameters for the [C_{2}mim]C_{2}H_{5}SO_{4}/_{
}propane1ol binary mixtures decrease with increasing temperature for
each system.
The viscosities of [C_{2}mim]C_{2}H_{5}SO_{4}
and propane1ol at 293.15 K are 96.0240 mPa.s and 2.2996 mPa.s, respectively
(Figure 1(c)). [C_{2}mim]C_{2}H_{5}SO_{4} has a
greater viscosity, which indicates stronger intermolecular association forces.
The primary pathways of intermolecular association for [C_{2}mim]C_{2}H_{5}SO_{4}
are coulombic interaction and intermolecular hydrogen bonding. HBs are the
dominant contributor of intermolecular force for alcohols. As the mole fraction
of propane1ol increases, a binary mixture becomes less viscous due to the
solvation of the ions by propane1ol, which lowers the strong ionion
interaction in [C_{2}mim]C_{2}H_{5}SO_{4}.
As the temperature increases, the dynamic viscosity
decreases, indicating a gradual drop in structural relaxation. At lower
temperatures, the temperature dependence of viscosity is more prominent.
Standard models can describe the temperature dependencies of binary mixtures.
The most popular model is the twoparameter Arrhenius one. To account for the temperature
dependence of the viscosity of the three binary mixtures and their pristine
components, the VogelFulcherTammann (VFT) and modified VFT (mVFT) equations
are also used among the threeparameter models [31–33]. The fitting parameters
are estimated from each equation and listed in Table S1, along with their
correlation coefficient values (R^{2}).
Temperature dependencies of the excess thermodynamic
parameters of [C_{2}mim]C_{2}H_{5}SO_{4}/propane1ol
binary mixtures with mole fraction of [C_{2}mim]C_{2}H_{5}SO_{4}
are depicted in Figure 2. Except for [C_{2}mim]C_{2}H_{5}SO_{4
}mole fraction of 0.7 and 0.8, the
are negative in the entire composition range. In
contrast to the individual homomolecular associations between the molecules,
this suggests contraction of volume, which denotes a strong heteromolecular
association between [C_{2}mim]C_{2}H_{5}SO_{4 }and
propane1ol. In other words, the solvation of each ion by polar propane1ol
solvent by iondipole interaction causes ionion interactions between positive
[C_{2}mim]^{+} and negative C_{2}H_{5}SO_{4}^{}
ions to weaken. The large value of the
originates
from contraction due to free volume differences between unlike molecules and
the HB formation between [C_{2}mim]C_{2}H_{5}SO_{4 }and
propane1ol molecules. The
Figure
2. (a)
, (b)
, (c)
, and (d)
of [C_{2}mim]C_{2}H_{5}SO_{4}/propane1ol
binary mixtures as a function of mole fraction of [C_{2}mim]C_{2}H_{5}SO_{4}
at different temperatures (Lines are predicted by RK type polynomial equation).
significant negative value at or near 0.4 IL mole
fraction suggests that HBs, particularly at this composition, predominate
between [C_{2}mim]C_{2}H_{5}SO_{4 }and
propane1ol molecules. However, for [C_{2}mim]C_{2}H_{5}SO_{4
}mole fractions of 0.7 and 0.8, the system has a greater intermolecular
void since there are fewer propane1ol molecules present. As a result,
values at these
mole fractions are positive. The values of
are progressively
negative as the temperature rises. The competition for packing efficiency and
the hydrogen bonding interaction (HBI) in the binary mixtures can be accounted
for the variation in the
with
temperature. As temperature increases, the stronger intermolecular HBIs
typically deteriorate. This trend is consistent with increasing
values. However, as
the temperature rises, the packing efficiency between the asymmetric positive
and negative ions improves in comparison to similar molecules, which causes the
to decrease [34].
Heteromolecular HBIs and overall packing efficiency compete with one another. sAs shown in Figure 2(b), except 0.9 [C_{2}mim]C_{2}H_{5}SO_{4
}mole fraction, the
of the binary
mixtures of [C_{2}mim]C_{2}H_{5}SO_{4 }and
propane1ol are all positive over the whole composition range, and the
deviations rise to a more positive value with increasing temperature.
Figure 2(c) shows that for the binary mixtures, the
Δη values are asymmetrical and all negative over the entire composition
range at each temperature. The
values
decrease till the mole fraction of [C_{2}mim]C_{2}H_{5}SO_{4
}approaches 0.80 as the composition of [C_{2}mim]C_{2}H_{5}SO_{4
}increases. The value eventually becomes zero. The
values
decrease with increasing temperature. The viscosity of a binary liquid mixture
is greatly influenced by the composition of the two liquids. As a result, in
addition to molecular interactions, the size and shape of both types of species
also influence the viscosity. Intermolecular interactions and molecular sizes
and shapes are two factors that compete with one another. Negative
values
indicate that intermolecular interactions predominate in this scenario [35].
Furthermore, the negative values suggests lowering of interaction between
positive [C_{2}mim]^{+ }cations and C_{2}H_{5}SO_{4}^{}
anions when propane1ol is introduced. Since propane1ol molecules may
dissolve both cations and anions, this is the case. The negative values
observed for binary mixtures, on the other hand, show that the viscosities for
associated species formed between components are significantly higher than
those of dissimilar molecules. Positive
may also
indicate that dispersion forces are dominant, particularly in mixtures with
various molecular sizes [36]. On the contrary, the values of
increase as
more propane1ol is introduced. This is due to the fact that [C_{2}mim]C_{2}H_{5}SO_{4
}and propane1ol interact via a HBI that is stronger in combination with
a high propane1ol content than in the mixture with a low propane1ol level.
The negative
value shown in Figure 2 may be caused by
strong selfassociation and dispersion forces, as well as comparatively weak
HBI between [C_{2}mim]C_{2}H_{5}SO_{4 }and
propane1ol.
As shown in Figure 2(d), at all temperatures, the
of the binary mixture is positive for mole fraction
of [C_{2}mim]C_{2}H_{5}SO_{4 }of 0.1 to 0.6 and
negative at higher [C_{2}mim]C_{2}H_{5}SO_{4 }mole
fractions. The value of
determines whether or not new interactions between
distinct molecules are present in a binary mixing system [37]. The significant
positive value of
indicates the
existence of more heteromolecular HBs compared to homomolecular ones. The
negative readings of
signify a
reduction in the heteromolecular interaction in the system. Homomolecular
association between [C_{2}mim]C_{2}H_{5}SO_{4 }predominates
when the amount of propane1ol in the system decreases due to a shortage of
propane1ol in the system. The data points of all excess thermodynamic
properties for the binary mixtures were fitted with the RK type polynomial
expression [38].
Y^{E }is the excess
thermodynamic properties, n is the polynomial order, and x_{1}
is the mole fraction of [C_{2}mim]C_{2}H_{5}SO_{4 }while_{
}x_{2} is for propane1ol. A_{0} –A_{n }are
RK parameters optimized by the leastsquares method. The coefficients
are tabulated in Table S2.
From the Eyring Equation, the enthalpy of activation
for viscous flow (ΔH*) and the entropy of activation of viscous flow (ΔS*)
were calculated using the least square fit method for a plot of ln
vs. T. The molar Gibbs free energy of
activation of the viscous flow (ΔG*) was calculated from the values of ΔH*
and ΔS* using,
(2)
The activation parameters for the binary mixtures
are summarized in Table S3 along with R^{2} values. The
thermodynamic parameters for the viscous flow of the binary mixture are plotted
against mole fraction of [C_{2}mim]C_{2}H_{5}SO_{4 }in
Figure 3.
Figure 3(c) shows that throughout the entire
component range, all values of
are positive.
The system is not spontaneous if
has a positive value. Molecules absorb energy to
carry out useful work. The value of ΔG* increases as the mole fraction
of [C_{2}mim]C_{2}H_{5}SO_{4} increases. The
change of
is more
significant at a lower mole fraction of [C_{2}mim]C_{2}H_{5}SO_{4}
up to 0.5 before decreasing up to 0.8, and again increases as the mole
fraction increases further. The ability of molecules and ions to go into a
structural hole and the capacity to form another are determined by the
, which controls the flow of the fluid. Increased
intermolecular attraction forces between dissimilar molecules are indicated by
an increase in the value of
.
Figure
3. (a) Enthalpy, (b) entropy, and (c) Gibbs free energy
of activation for the viscous flow of the [C_{2}mim]C_{2}H_{5}SO_{4}/propane1ol
binary mixtures as a function of mole fraction of [C_{2}mim]C_{2}H_{5}SO_{4}
at different temperatures (lines are for visual aid only).
For a binary mixture, the ΔS* is negative in
the range of composition. As a result of the strong crossassociation in and
selfassociation into the solution via HBs, the ΔS* values were all
negative to indicate that contiguous liquid layers may be involved in an
ordered process to cause the viscous flow. Even when moving in a stationary
steady state, these solutions must keep their structural configuration [39].
The maximum entropy value observed for components suggests that they are more
organized than binary mixtures. Entropy is measured at its lowest point at 0.8
[C_{2}mim]C_{2}H_{5}SO_{4} mole fraction.
The fact that [C_{2}mim]C_{2}H_{5}SO_{4}
is mixed with propane1ol by an endothermic process is supported by the
positive values of ΔH*. The values of ΔH* increase up to the mole
fraction of [C_{2}mim]C_{2}H_{5}SO_{4} of 0.5
and subsequently decrease up to mole fraction of 0.8 before increasing once again.
The inclusion of a larger molecule inside the smaller propane1ol molecules
causes the value of ΔH* to increase. These larger species sterically
hinder the mobility of the molecule. Therefore, making the holes for viscous
flow requires more energy. However, the system starts to exhibit some disorder
when the mole fraction exceeds 0.5, as evidenced by the entropy, and some holes
begin to appear without using any more energy from the system, which lowers the
ΔH* values up to a mole fraction of 0.8. By taking into account the
aforementioned thermodynamic activation parameter, it follows that for smaller
mole fractions of [C_{2}mim]C_{2}H_{5}SO_{4} up
to 0.5, the intermolecular interaction between similar propane1ol molecules
reduces, and the solvation of the ions in [C_{2}mim]C_{2}H_{5}SO_{4}
by propane1ol takes place. However, as more [C_{2}mim]C_{2}H_{5}SO_{4}
is added, the system generates more ions and cannot be solvated by propane1ol
due to the lack of sufficient solvent molecules to do so.
The NIR spectra of propane1ol are depicted in
Figure 4(a). The first overtone, corresponding to stretching vibration
of OH, is reflected by a cluster of peaks close to between 6000 and 7100 cm^{1}
[4043]. At lower temperatures, the peak at 7095 cm^{1 }is quite
feeble, but as the temperature rises, the peak intensity leaps to the highest
at 313.15 K. A descending peak close to 6300 cm^{1 }is present
alongside this ascending peak. To highlight an inverse relationship between
these two peaks, there is an isosbestic point near 6950 cm^{1}. These
two peaks represent two distinct OH groups in the system as clusters. As the
temperature rises, the peak at 7095 cm^{1} does not change to indicate
that the vibrational energy of the corresponding bond is not much susceptible
to temperature. While a blue shift at the peak near 6300 cm^{1}
indicates that vibrational energy increases as the temperature rises. Hence
this set of peaks suggested a clustered OH group; with temperature, the HBs
inside the cluster break down, and as a result, the vibrational frequency
increases for the OH stretching band. When compared to the pure
hydroxyl peaks, both hydrogenbonded hydroxyl peaks are noticeably broader. The
hydrogenbonded hydroxyl absorption peak may broaden, and the mixing ratio may
alter when different HBs are coupled to cause a shift in peak frequency.
Figure 4. NIR spectra of (a)
propane1ol and (b) [C_{2}mim]C_{2}H_{5}SO_{4}
at different temperatures.
Figure 5 shows 2D correlation spectral analysis of
propane1ol in the wavenumber range of of 60007500 cm^{1}. The
synchronous spectra (Figure 5a) show a strong auto peak near 6300 cm^{1}
(red shade) and a weak auto peak at 7095 cm^{1} (cyan shade),
indicating that spectral features vary significantly in these positions,
especially at the peak near 6300 cm^{1}. A negative cross peak appears
at 7095 and 6300 cm^{1 }(blueshaded) between the auto peaks to
indicate that spectral changes at these positions are negatively correlated to
each other. In addition, since the cross peak of 6300 and 7095 cm^{1}
in the asynchronous map (Figure 5b) is negative, the variance near 6300 cm^{1}
occurs after the variance at 7080 cm^{1} in accordance with the
principle of 2D correlation spectra. As the temperature rises, the vibration
for a peak near 6300 cm^{1} transforms into the vibration at 7080 cm^{1}.
In addition, the peak near 6300 cm^{1 }diverges to the offdiagonal
position at the synchronous correlation contour map to indicate the peak near
6300 cm^{1 }to be not a single peak, rather consisting of several
peaks. The asynchronous contour map shows that there are also several peaks at
6843, 6735, 6621, 6547, 6341, and 6288 cm^{1}, etc. These peaks
correspond to the overtone peaks of OH bonds in different hydrogen bonding
environments.
Figure 5. Synchronous and
asynchronous 2D correlation spectra of
propane1ol.
Figure 6. Loadings and
scores of the first two principal components of propane1ol.
PCA was used to obtain quantitative data in the same
region where the 2D correlation was performed. The loadings and scores of these
components against wavenumber and temperature are depicted in Figure 6.
Table 1 displays the principal component eigenvalues
and variance. Four primary components make up propane1ol, with the first
principal component contributing to 98.76% of the overall spectrum and the
second principal component 1.03%. The loadings of the latter two components
practically produce a straight line close to zero, indicating a baseline shift
during measurement. The PC1 shows a large peak at about 6300 cm^{1},
indicating that intermolecular HBs make up most of the propane1ol, and a
smaller signal at 7095 cm^{1}, indicating the presence of some free
OH groups. Below 308.15 K, the PC1 score becomes steady, and above 313.15 K,
it drops significantly. This indicates that some PC1 breaks at higher
temperatures. The PC2 has a prominent peak at 7095 cm^{1}, indicating
that PC2 is composed of molecules of propane1ol that are not hydrogen bonded.
The result demonstrates that its quantity increases with temperature,
particularly around 313.15 K. Certain HBs disintegrate and free propane1ol
molecules are formed at higher temperatures.
Table 1: Eigenvalues and percentage of the variance
of the principal components of propane1ol.
Principal Component Number

Eigenvalue

Variance (%)

Cumulative (%)

1

1.45062E4

98.76289

98.76289

2

1.51293E6

1.03005

99.79294

3

2.68905E7

0.18308

99.97602

4

3.52251E8

0.02398

100

5

3.91785E35

2.6674E29

100

The NIR spectroscopic measurements were performed
for [C_{2}mim]C_{2}H_{5}SO_{4 }at the temperature
range from 293.15 K to 333.15 K at 5 K intervals, as shown in Figure 4(b). The
second overtone of various types of CH bonds dominates the range between 5500
and 6400 cm^{1} in the NIR spectra of [C_{2}mim]C_{2}H_{5}SO_{4}.
However, most of the peaks in this region overlapped. The second overtone
of the CH stretching at the aromatic ring peaks at 6150 cm^{1} [42].
The 2D synchronous and asynchronous correlation spectra reveal that there is no
correlation between the peaks in this region.
The NIR spectra of the [C_{2}mim]C_{2}H_{5}SO_{4}/propane1ol
binary mixtures at various mole fractions of [C_{2}mim]C_{2}H_{5}SO_{4}
from 0.1 to 0.9 were recorded over 293.15 K to 333.15 K at 5 K interval. The
absorbance of the peak near 6300 cm^{1} decreases when [C_{2}mim]C_{2}H_{5}SO_{4}
is added to propane1ol, and a new peak is observed near 6200 cm^{1}.
But a noticeable peak shifting is observed here. The peak close to 6300 cm^{1}
experiences a blue shift from 6300 to 6333 cm^{1}, which suggests the
bond strengthening in the associated vibration. A substantial fraction of the
intermolecular HB is broken down when [C_{2}mim]C_{2}H_{5}SO_{4}
is added to propane1ol since this vibration corresponds to the stretching of
the intermolecularly bound dihydrogen OH group.
Additionally, the peak near 6200 cm^{1} tends
to blue shift, indicating that the cluster in [C_{2}mim]C_{2}H_{5}SO_{4}
was similarly broken down without considerable solvation by hydrogen bonding of
OH of propane1ol with H at the carbon2 of the imidazolium ring. Another
notable shift in the spectrum can be seen at about 6800 cm^{1} when a
new peak is observed for neither propane1ol nor [C_{2}mim]C_{2}H_{5}SO_{4}.
The mono hydrogenbonded hydroxyl group with a weak HB has a peak. This might
be caused by the weak intermolecular HBI that forms between the propane1ol
molecule and either aromatic hydrogen_{ }or oxygen in the ethylsulfate
anion. The system may only have a few free hydroxyl groups, based on the
extremely faint peak at 7095 cm^{1} that does not change. Since there
are strong peaks in [C_{2}mim]C_{2}H_{5}SO_{4}
in this region, the contribution of [C_{2}mim]C_{2}H_{5}SO_{4}
causes the formation of weak peaks in 7100 to 7400 cm^{1}. 2D
correlation spectral analysis of the raw spectrum data in the 6000–7500 cm^{1}
region was carried out to investigate the spectral features further. Figure S1
displays 2D contour maps of the synchronous and asynchronous spectra.
The synchronous contour map shows a strong auto peak
near 6200 cm^{1} as well as numerous smaller auto peaks at 6300, 6780,
6895, and 6970 cm^{1}. The auto peak at 6200 cm^{1} diverges
to an offdiagonal position to suggest that several peaks are buried within one
peak. The negative correlation between these peaks is indicated by the
offdiagonal negatively correlated peaks between 6194 cm^{1} and peaks
between 7100 to 7500 cm^{1}. The principle of 2D correlation
spectroscopy states that while one peak weakens, another strengthens,
indicating a positive association between these peaks are positive cross peaks
between 6200 and 6780 cm^{1}. It appears from the fact that
introducing [C_{2}mim]C_{2}H_{5}SO_{4} to
propane1ol encourages the production of monohydrogen bonded hydroxyl groups
from dihydrogen bound hydroxyl groups.
The PCA suggests that four different components
mainly cause spectral variations. The PC1 contributes 65.39257% of the total
spectral variation. The scores and loadings are presented in Figure S2. A
prominent peak at 6200 and 6300 cm^{1} in the PC1 loading indicates
that the PC1 is primarily composed of the imidazolium ring of [C_{2}mim]^{+}
and the dihydrogen linked OH of propane1ol with various extents of their
interactions. The results indicate that this system is only slightly influenced
by temperature. There are negative peaks in the loading of PC2 at 6200, 6895,
and 6970 cm^{1}. The score indicates that as the temperature rises,
the production of PC2 should increase as well. Although the peak loading of PC3
cannot be deciphered, PC4 has a positive peak at 6895 cm^{1} and a
negative one at 7095 cm^{1}. 1.02748% of the total spectral
contribution in PC4 thus, comes from free hydroxyl groups.
The peak shifts towards a lower wavenumber from 6200
to 6189 cm^{1} at 0.2 [C_{2}mim]C_{2}H_{5}SO_{4}
mole fraction as [C_{2}mim]C_{2}H_{5}SO_{4}
content is increased further. Surprisingly, the peak at ca. 6300 cm^{1}
is absent and the band at 6995 cm^{1} grows stronger. This suggests
that some of the highly hydrogenbonded clusters are disintegrated, resulting
in clusters with weaker HBs. The intensity rises while the peaks between 7100
and 7500 cm^{1} remain unchanged. 2D correlation spectral analysis is
carried out for indepth investigation. The synchronous and asynchronous
contour plots are displayed in Figure S3. Strong auto peaks at 7140 and
7330 cm^{1} and negative cross peaks between them are visible in the
synchronous spectra. This implies that there is a strong negative correlation
between these peaks. Along with these weaker auto peaks, some others are at
6189, 6688, and 6850 cm^{1}. There are numerous cross peaks in
addition to the auto peaks. Important cross peaks include the positive cross
peaks between 6189 and 7140 cm^{1} and 6850 and 7140 cm^{1},
as well as the negative cross peaks between 6189 and 6688 cm^{1}, 6688
and 7080 cm1, and 6189 and 7330 cm^{1}.
According to the PCA, the spectral features are
contributed by 8 major components. The first four main components contribute
97.90402% of the spectral variations. The PCA suggests that this peak is
present at a modest content in PC1 of the mole fraction of 0.2 of [C_{2}mim]C_{2}H_{5}SO_{4}
in the binary system, even though we anticipated that it would not be present
in the typical NIR spectrum based on the loadings of PC1, a little positive
peak near 6300 cm^{1} (Figure S4). Weak negative peaks at 6194 and
6850 cm^{1} as well as the strong broad peak at 7140 cm^{1}
are also noted. From this information, it can be assumed that PC1 is a cluster
made up of [C_{2}mim]C_{2}H_{5}SO_{4} cations
that are strongly solvated and include both mono hydrogen and dihydrogen linked
OH. The results show that PC1 declines as the temperature rises. Positively
and negatively correlated peaks on the PC2 are located at 6900 and 7330 cm^{1}
and 6194 and 7140 cm^{1}, respectively. The loadings demonstrate that
this component increases slightly with temperature before decreasing at higher
temperatures.
The peak of CH experiences a red shifting to 6185
cm^{1} at 0.3 [C_{2}mim]C_{2}H_{5}SO_{4}
mole fraction, indicating significant HBI at this point, i.e., cations are more
firmly solvated at this mole fraction. The redshift of the signal at 6995 cm^{1}
to 6960 cm^{1} suggests that some of the HBs in the OH group of
propane1ol have become weaker. The
auto peak is evident on the 2D synchronous contour map at 6185 cm^{1},
but the fact that it broadens to an offdiagonal position implies that it is
not a single peak (Figure S5). This peak is composed of many peaks to indicate
that HBs between propane1ol and CH are different from one another in
strength. Additional auto peaks are also observed at 6626, 6668, 7058, and
7282 cm^{1}. The large peak at 6226 cm^{1} broadens to
offdiagonal positions to suggest that there are multiple peaks present in this
region. The offdiagonal negative cross peak between 6185 and 6626 cm^{1}
shows a negative correlation between them or when the intensity of one peak
increases while the intensity of the other drops. This suggests that the more
frequently the intermolecular HBs between molecules of propane1ol break, the
more solvation takes place. The upward cross peaks between 6185 and 7058 cm^{1}
show that when one peak increases, the other also keeps increasing. This also
suggests that the system contains more free OH groups to solvate more [C_{2}mim]C_{2}H_{5}SO_{4}.
The asynchronous contour map shows a negative correlation between the peak at
6226 cm^{1} and the peaks at 6960 and 7282 cm^{1} (Figure
S5).
The PCA of the spectral data reveals eight
principal components, the first four of which account for 97.85613% of the
total spectral variation (Figure S6). A large positive peak at 6626 cm^{1}
and a small negative peak at 6185 cm^{1} are both present in the
loadings of the PC1, which accounts for 68.100% of the overall spectral
intensity change. The results indicate that when the temperature rises, the
contribution of PC1 decreases (Figure S6). The loadings of PC2 reveal a broad
negative peak at 6626 cm^{1}, a broad positive peak at 6226 cm^{1},
and a broad peak at 7282 cm^{1}. A higher temperature causes PC2 to
increase after decreasing up to 298.15 K, according to the PC2 scores. PC3 has
a positive peak at 7282 cm^{1} and negative peaks at 6226, 6626, and
7058 cm^{1}. The structure of the wide loading plot suggests the
presence of various cluster types with varying HB strengths.
The peak for CH does not change from 6185 cm^{1}
at [C_{2}mim]C_{2}H_{5}SO_{4} mole
fraction of 0.4, but the peak intensity does, indicating an expected rise in
the number of this bond. In contrast, the peak at 6985 cm^{1}
experiences a decline in intensity while exhibiting an increase in the peak
area. The 2D synchronous plot reveals highly correlated auto peaks at 6900,
7058, 7162, 7286, and 7345 cm^{1} instead of the expected auto peak at
6185 cm^{1} (Figure S7). Each pair is negatively correlated to one
another, as seen by the offdiagonal negative cross peaks between 7286, 7162,
7162, and 7058 cm^{1}, and 7058 and broad 6900 cm^{1}.
Although there are no auto peaks at 6185 cm^{1}, there are positive
cross peaks between 6185 and 7058 cm^{1}, which suggests that as the
number of solvated cations increases, there will also be free OH groups.
The strong intermolecular HBs formed by connecting propane1ol molecules inferred by
intermolecular crosspeaks between 6185 and 6900 cm^{1} indicate the
formation of clusters of solvated cations by the interconnection of
propane1ol molecules. The crosspeaks between 6185 and 6900 cm^{1},
which are negatively correlated, serve as evidence for this. The asynchronous
correlation clearly shows a negative correlation between the peaks at 7345 and
7058 cm^{1} and between 7058 and 6900 cm^{1}. This PCA
yields eight main components, with the first three contributing 99.1489% of the
total spectral variables (Table S7). Positive peaks can be seen in the loadings
of PC1 at 6200, 7058, 7080, and 7345 cm^{1}, while negative peaks can
be seen close to 6900 cm^{1} (Figure S8). This suggests that there is
a positive correlation between them and the scores, which suggests that as the
temperature rises, the spectral contribution of PC1 increases,
suggesting that PC1 will remain stable at higher temperatures. Additionally,
PC2 includes negative peaks at 6675 cm^{1} and numerous other negative
peaks between 7200 and 7500 cm^{1}.
The trend in spectral shifts is followed by an
additional increase in the composition of [C_{2}mim]C_{2}H_{5}SO_{4}.
The intensity of this peak increases up to mole fraction of 0.8 of [C_{2}mim]C_{2}H_{5}SO_{4},
decreases for 0.9, and then increases once more for [C_{2}mim]C_{2}H_{5}SO_{4}.
The peak at 6185 cm^{1} does not shift as composition changes, but the
intensity variation does not increase as the amount of [C_{2}mim]C_{2}H_{5}SO_{4}
increases. This occurs due to the fact that the cation solvation is the highest
at a mole fraction of 0.8 for [C_{2}mim]C_{2}H_{5}SO_{4}.
Due to a lack of propane1ol, further addition of [C_{2}mim]C_{2}H_{5}SO_{4}
results in excess cations without any solvation. However, an interionic HB
between the CH donor and the sulfate acceptor causes the intensity to
increase. When the mole fraction of [C_{2}mim]C_{2}H_{5}SO_{4}
increases, the intensity increases for a mole fraction up to 0.8 and drops for
0.9; a blue shift is marked at ca. 6900 cm^{1}. Due to a lack of
propane1ol, this peak is absent for [C_{2}mim]C_{2}H_{5}SO_{4}.
A positive correlation between these bonds is also evident in the 2D
correlation spectral analysis. Each composition, except the mole fraction of
0.9 of [C_{2}mim]C_{2}H_{5}SO_{4}, follows the
same pattern. The 2D synchronous and 2D asynchronous contour plots and the
loadings and scores of the first two components are given in Figure S1 – Figure
S18. The eigenvalues and percentage of variance from the PCA are tabulated in
Table S4 – Table S12.
Molecular
level interaction between [C_{2}mim]C_{2}H_{5}SO_{4 }and
propane1ol
It is evident from the NIR spectroscopic
investigations that propane1ol is made up of two major types of clusters:
hydrogenbonded and nonhydrogenbonded. Some of the hydrogenbonded clusters
disintegrate into free propane1ol molecules as the temperature rises. As a
result, as the temperature rises, the proportion of nonhydrogen bonded
propane1ol molecules in the system increases, leading to an increase in
molecular voids. Because of this, the density of propane1ol drops as the
temperature rises. The viscosity of the propane1ol system experienced the
same effect. When [C_{2}mim]C_{2}H_{5}SO_{4} is
introduced to propane1ol, the system is primarily constituted of the cations
and anions of [C_{2}mim]C_{2}H_{5}SO_{4}. Some
of the similar HBs between molecules of propane1ol are broken down by
solvation, and new, weak HBIs are generated between OH groups of propane1ol
and CH of cations and S=O of anions. The molar volume decreases due to the
breakdown of some structural voids caused by breaking HBs. Despite an increase
in the number of interactions, the intensity of each bond weakens, which causes
a reduction in the viscosity, as indicated by a negative
. The positive value of
also predicts more numbers of bonds between unlike
molecules. Due to the more bonds and more ordered structure created by the
increased number of bonds, the ΔS* also decreases. Due to the absence of
propane1ol molecules in the system, however, at mole fractions greater than
0.4, the clusters of solvated ions cannot be attached by hydrogen bonding. As a
result, the
increases further and has a positive value for
higher concentrations of [C_{2}mim]C_{2}H_{5}SO_{4}.
Due to a decrease in interactions compared to binary systems with smaller [C_{2}mim]C_{2}H_{5}SO_{4}
concentration, the
is more
negative for greater [C_{2}mim]C_{2}H_{5}SO_{4}
content. Such interaction is further supported by increasing entropy and
negative
values.
Conclusion
Physicochemical properties and molecular level interactions
of [C_{2}mim]C_{2}H_{5}SO/propane1ol binary mixtures
change markedly from the components. The density, refractive index, and
viscosity of the binary mixtures of [C_{2}mim]C_{2}H_{5}SO_{4}
and propane1ol increase with increasing mole fraction of [C_{2}mim]C_{2}H_{5}SO_{4}
and decreasing temperatures. The temperature dependence of the viscosity of [C_{2}mim]C_{2}H_{5}SO/propane1ol
binary mixtures gives a good fit at Arrhenius, VFT, and mVFT equations. The
of [C_{2}mim]C_{2}H_{5}SO/propane1ol
binary mixtures are all negative except the higher mole fraction of [C_{2}mim]C_{2}H_{5}SO_{4}
to indicate more compactness of the binary system compared to the components.
The
of the binary
mixture are all negative and becomes more negative at a higher mole fraction of
[C_{2}mim]C_{2}H_{5}SO_{4} and also with
increasing temperature. The
is positive
at lower [C_{2}mim]C_{2}H_{5}SO_{4} content up
to [C_{2}mim]C_{2}H_{5}SO_{4} mole fraction of
0.6 and negative for the rest. The NIR
spectra, 2D correlation analysis, and PCA analyses showed the intermolecular
bonding inside the binary system and its components. For the binary system with
increasing mole fraction of [C_{2}mim]C_{2}H_{5}SO_{4},
the intermolecular HBs are broken down; some of the OH groups become HB free
and some form weak intermolecular HBs with cations and anions of [C_{2}mim]C_{2}H_{5}SO_{4}.
These improved volumetric properties and molecular interaction may make the
binary system a potential medium for diverse applications.
Acknowledgement
Authors acknowledge the Ministry of Science and
Technology, Government of the People’s Republic of Bangladesh, for the National
Science & Technology (NST) Fellowship and Bose Center for Advanced Study
and Research in Natural Sciences for financial support.
Conflicts
of Interest
There are no
conflicts to declare.