Abstract-- The aim of the study is to determine the effect of process parameters on dielectric properties of deep eutectic solvents (DESs), which have been widely used in recent years. In this study, DESs were prepared using choline-chloride as the hydrogen-bond acceptor and glycerol, formic acid and acetic acid as the hydrogen-bond donor. The effect of different process parameters such as molar ratio (1:2, 1:3 and 1:4), water content (15%, 30% and 45%), temperature (25, 50 and 75 °C) and frequency on dielectric properties of the DESs were examined. The highest dielectric constant values were found to be 38.13±0.54, 44.96±0.07 and 46.74±0.02 for glycerol, formic acid and acetic acid, respectively for the process conditions of 25 °C temperature, 45% water content and 1:4 molar ratio (at 2450 MHz). Moreover, for all DESs, it was found that a decrease in water content resulted in a decrease in both dielectric constant and loss factor values. This can be explained by the absence of free water molecules which are responsible from dipole rotation mechanism. In light of the results, if DES will be used in microwave processes, formic or acetic acid containing DESs might give more successful results compared to the ones with glycerol.
Keywords-- dielectric properties, deep eutectic solvent, frequency, water content, temperature.
I. INTRODUCTION
As environmental mandates became stricter, “green technology” concept has been more popular in recent years. Green technology represents the development and extension of processes, practices, and applications that improve or replace the existing technologies to decrease the impact of human on the planet, and reduce environmental risks and ecological scarcities (Singh and Kumar, 2017). Examples of green technologies include recycling, renewable energy, biotechnology or alternative approaches (ultrasound, microwave, radio frequency, pulsed electrical field, etc.) for traditional processes (Maczulak, 2010).
Solvents are used in many different areas such as paints, cosmetics, chemicals, pharmaceutical or food industries. The most commonly used solvents in industries are hexane, ethanol, methanol, n-butanol, acetone and iso-propanol. However, traditional solvents may have negative effects on health and the environment. Therefore, use of green solvents has been found to be promising in the last decades. The most commonly used green solvents are water, deep eutectic solvents, ionic liquids, supercritical fluids, etc. (Pena-Pereira and Tobiszewski, 2017). The utilization of different green solvents for various extraction processes and food materials were reported in literature. Some of the examples are polyphenol extraction from pomegranate peel using supercritical CO2 (Mushtaq et al., 2015), polyphenol extraction from potato peel using bio-solvent (glycerol) (Paleologou et al., 2016), polyphenol extraction from goji berry using water (Skenderidis et al., 2017), polysaccharide extraction from microalgae (Arthrospira platensis) using water (Silva et al., 2018), synthetic food dye extraction using ionic liquid (Salamat et al., 2018), bioactive compounds extraction from onion skin using supercritical CO2 (Campone et al., 2018), pesticide residue extraction in honey using ionic liquids (Zheng et al., 2019).
Deep eutectic solvents (DESs) have emerged as an alternative to existing solvents which have some disadvantages such as high cost, non-biodegradability and toxic effects. DESs, like ionic liquids, have low vapor pressure and high thermal stability. Moreover, unlike ionic liquids, they are cheap and easy to prepare. Moreover, synthesis of DESs is 100% atom economic (Zhang et al., 2012). DESs, which can be named as eutectic mixture, can be formed by establishing a hydrogen bond between the organic salts of ammonium or phosphonium (hydrogen bond acceptor-HBA) and the hydrogen bond donor (HBD) such as alcohol, acid or amide. The DESs have a lower melting point than the starting chemicals, which make it possible to be used at room temperatures (Dai et al., 2013). Choline-chloride (ChCl) as the organic salt and urea, glycerol, carbohydrate-derived polyols or carboxylic acids as the hydrogen bond donor are the most commonly used chemicals to form eutectic mixture (Zhang et al., 2012). DESs are successfully used as powerful solvents in many studies; such as extraction of phenolic metabolites from Carthamus tinctorius L. (Dai et al., 2013), extraction of major catechins from Camellia sinensis leaves (Li et al., 2015), extraction of Ochratoxin A from wheat (Piemontese et al., 2017), extraction of isoflavones from soy products (Bajkacz and Adamek, 2017), extraction of polyphenolic antioxidants from orange peel (Ozturk et al., 2018), extraction of bioactive compounds from various food products (Fernandez et al., 2018; Jablonsky et al., 2018; Choi and Verpoorte, 2019), extraction of soluble sugars from banana puree (Gomez et al., 2019), extraction of antioxidants from eucalyptus leaves (Gullon et al., 2019) and extraction of anthocyanin from mulberry (Guo et al., 2019 ).
Dielectric properties of material indicate the ability of material interaction with electromagnetic field hence the heating feasibility (Tang et al., 2002). Parallel plate, lumped circuit, coaxial probe, transmission line, cavity resonator, free space, and time-domain spectroscopy are common measurement techniques to determine dielectric properties. Measurement device has to be selected based on the type of the material, the frequency range and the degree of accuracy required. Among these methods, a network analyzer, which is also used in this study, is expensive, but very versatile and easier if the number of samples are high (Nelson and Kraszewski, 1990; Içier and Baysal, 2004). The information about dielectric properties and the parameters studied on provides desired heating pattern in especially microwave assisted processes and moreover, it avoids hot and cold spots (Barba and d’Amore, 2012). Dielectric properties include dielectric constant and dielectric loss factor. The dielectric constant (ε') is a measure of the ability of a substance to absorb electrical energy. Dielectric loss factor (ε'') refers to the ratio of the conversion of electrical energy to heat energy (Wang et al., 2003). Frequency, temperature and water content are the main factors affecting the dielectric properties of the materials. Even though the dielectric properties of some of the DESs have also been studied in literature (Reuter et al., 2019; Muley et al., 2019; Boyko et al., 2020), the effect of different process conditions on the dielectric properties of DES have not been reported. Therefore, in the present study, deep eutectic solvents were prepared using choline-chloride (ChCl) as the hydrogen bond acceptor and glycerol (Gly), formic acid (FA) and acetic acid (AA) as the hydrogen bond donor. The effect of different process parameters (such as molar ratio of DESs (1:2, 1:3, 1:4), water content of the solvents (15%, 30% and 45%), temperature (25, 50 and 75 °C) and frequency) on ε' and ε'' were examined.
II. METHODS
A. Materials
Choline chloride (ChCl, ≥99%), glycerol (Gly, 85%), formic acid (FA, 98-100%) and acetic acid (AA, ≥99%) were obtained from Sigma-Aldrich, Isolab and Merck. All chemicals were used without further purification.
B. Preparation of DESs
The DESs (ChCl-Gly, ChCl-FA and ChCl-AA) were prepared by simply mixing the HBD and HBA at different molar ratios (1:2, 1:3 and 1:4). The mixture was then heated at 30°C and 180 rpm by using shaking incubator until a homogenous colorless liquid was obtained. Subsequently, the mixture was kept at 80 °C for 12 h to remove the unreacted free acids (Xu et al., 2016). The DESs were also prepared at different water contents (15%, 30% and 45%) in order to determine the effect of water content on dielectric properties.
C. Determination of dielectric properties
Dielectric properties of samples were measured with an Agilent 85070E open-ended coaxial probe connected to an Agilent E8362B Vector Network Analyzer (Agilent Technologies ES061B ENA Series Network Analyzer, ABD) within the range of 500-3000 MHz of frequency. The network analyzer was calibrated with air, short block and distilled water (Sipahioglu and Barringer, 2003). The HBA and HBDs are mixed in glass tubes and DESs at different molar ratios and water contents, as mentioned in part B, are obtained. The tubes are heated in a water bath until the target temperature was achieved. Subsequently, the tubes are taken out of the water bath and covered with an insulating material to avoid heat loss during measurement of dielectric properties. The measurement of dielectric properties was carried out within seconds. All measurements were done in triplicates.
D. Statistical analysis
Data were analyzed by the program MINITAB 16.1.1.0 (Minitab, State Collage, PA, USA) by one-way analysis of variance (ANOVA) followed by Tukey’s test, considering p<0.05 as to be statistically significant.
III. RESULTS
The dielectric properties of a pure polar liquid, such as water, can
be represented by Debye model that has three parameters (
, 
and 
) (Siguemoto and
Gut, 2016);
(1)
(2)
where 
is the angular frequency of the field (rad/s), is the
relative permittivity at very high frequencies (no effective molecular
polarization), is the
relative permittivity at zero frequency (DC circuit), and is the
relaxation time (s), which can be defined as the time required for the polar
molecules to revert to their original orientation after the electric field is
removed.
The 
is called the
ability of the material to store electrical energy. Thus, for deep eutectic
solvents heated using microwave energy, the is a relative
measure of polarity. Since dipole polarization results in microwave heating of
polar molecules, higher ε' values are desired for efficient dielectric heating
(Muley et al., 2019). The loss factor of distilled water depends on only
dipolar rotational loss (
). However, in
case of salt solutions as our solvents another mechanism contributes to the
loss factor, namely ionic conduction. Free ions have the ability to move with
electric field and consequently generate heat but they do not contribute to the
polarization (energy storage). On the contrary, they bound to water molecules
and cause a reduction in permittivity. In the light of these two mechanisms,
the effects of temperature, frequency, water content, molar ratio and DES type
will be discussed in details.
A. Effect of temperature and frequency on dielectric properties of DES
The effect of temperature on dielectric properties were shown in
Fig. 1a-c at constant molar ratio and water content (1:2 and 30%). Similar results
were also obtained for other molar ratios and water contents, therefore not
presented. The highest value was
measured at 25 °C for all frequencies. Dielectric constant gives information
about the electromagnetic energy storing capacity of a material (DES in our
case) (Ibarz and Barbosa-Canovas, 2014). In general, it can be said that there
was a negative correlation between temperature of DES and . This finding
can be explained by increasing molecular mobility with increasing temperatures
causing a decrement in viscosity (Celebi et al., 2019). However, the values of
DESs at 50 and 75 °C were found to be similar except for ChCl-FA. This could be
due to differences in hydrogen bond strength of DES constituents (Gautam et
al., 2020). Megriche et al. (2012) have also reported a decrement in
of
ethanol-water mixture with increasing temperature. This was explained by
dipole-dipole interactions under the influence of hydrogen bonds, which were
dominant in the medium. It was emphasized that with an increase in temperature,
hydrogen bonds were broken and caused a decrease in the . Similarly,
Singh et al. (2014) reported a decrement in for pure
water and 30% methanol, with increasing temperature.
The increase in temperature had a positive effect on loss
factor (
) and the
maximum values were obtained at the highest temperature (75 °C). The mobility
of the ions increased due to decreasing viscosity of the liquid as the
temperature increased (Datta et al., 2014). Reuter et al. (2019)
studied the dielectric properties of DESs at different temperatures. They found
that as the temperature of DESs increased, the dielectric loss factors were
also increased for ethaline and reline.
The dielectric properties of most materials vary
considerably with the frequency of the applied electric fields. There is an
inverse relation between frequency and wavelength. In addition, frequency is
effective on dielectric properties due to variable angular frequency (
) and
relaxation time (). It was observed that both the and 
were
decreased with an increase in frequency (Fig. 1a-c). Figure 2a-c also presents
the dielectric properties of different DESs at different molar ratios and
temperatures for 915 and 2450 MHz (at constant water content of 45%). The
values of and at 915 MHz
were higher than the values at 2450 MHz for all temperatures and DESs (except
for values of
ChCl-FA at 75 °C).
Similar results were also reported in literature. Muley et
al. (2019) have prepared different DESs using lactic acid, oxalic acid,
formic acid and choline chloride. They have measured the dielectric properties
of all three DESs at different frequencies. It was reported that as the
frequency increased, both of the dielectric properties were decreased.
Horikoshi et al. (2012) have also examined the dielectric properties of
several solvents (ethanol, ethylene glycol, 1-propanol and 2-propanol) at
different frequencies (915 MHz, 2.45 GHz and 5.8 GHz). In parallel with our
findings, higher and values were
noted as the frequency was decreased from 5.8 GHz to 915 MHz. In general,
microwave heating of polar solvents has been reported to be highly effective at
lower frequencies.
B. Effect of water content on dielectric properties of DES
The variations of dielectric properties with respect to water
content at constant temperature and molar ratio (75 ºC
Figure 1: The effect of temperature and frequency on dielectric properties of a) ChCl-AA, b) ChCl-FA, c) ChCl-Gly (The water content and molar ratio were kept constant as 30% and 1:2, respectively).
and 1:4) were given in Fig. 3a-c. For all DESs, it was found that a
decrease in water content resulted in a decrease in both and values. This
can be explained by the absence of free water molecules which are responsible
for dipole rotation mechanism and hence a decrease in both dielectric
properties were observed.
The and values
generally decreased with increasing frequency for all DESs and temperatures.
When Fig. 3a-c are examined, the highest value was
measured as 34.36 for the ChCl-AA. However, the highest value was
found to be 108.63 for the ChCl-FA. In a study using ChCl-FA (Muley et al.,
2019), it was stated that the variation of dielectric properties decreased with
increasing frequency. Dielectric constant and dielectric loss factor at 2450
MHz (room temperature) were found to be approximately 15 and 20, respectively.
Pandey et al. (2014) have also reported choline chloride-based deep
Figure 2. The effect of molar ratios and temperatures at 915 and 2450 MHz on dielectric properties of a) ChCl-AA, b) ChCl-FA, c) ChCl-Gly (The water content was kept constant as 45%)
eutectic solvents’ dielectric constants in the range of 12 to 32. The dissimilarities in the results can be attributed to differences in temperature, molar ratios, and water content.
Parallel findings were reported in literature. Alvarez
et al. (2017) studied the dielectric properties of solvents (ethanol-water
mixtures). The and were
decreased when water concentration was decreased. Bennett et al. (2019)
studied the dielectric properties of (2-hydoyethyl) ammonium formate prepared
at different water content values (0%, 10%, 25%, 50%). They reported a
significant effect of water content on overall complex permittivity due to
polarity of water molecule.
In addition to these studies, Dai et al. (2015)
investigated how the water content affects the conductivity of natural deep
eutectic solvents (NADES). Conductivity is an ideal tool to learn more about
the ionic charge-transport mechanism and possible additional dynamic processes.
This provides information on possible reorientation degrees of freedom of
dipolar molecules and ions (Reuter et al., 2019). The conductivity of
DESs can be related to the abundance and mobility of ionic species since the
movement of free electrons throughout the DESs is unlikely to occur. Moreover,
it provides knowledge about how lossy the dielectric material is. In other
words, it shows how well the dielectric material converts the energy into heat
when exposed to a certain amount of electric field. NADESs (glucose:choline
chlo-
Figure 3: The effect of water content and frequency on dielectric properties of a) ChCl-AA, b) ChCl-FA, c) ChCl-Gly (The temperature and molar ratio were kept constant as 75 °C and 1:4, respectively).
ride; 1,2-propanediol:choline chloride; sucrose:choline chloride; proline:malic acid; lactic acid:glucose) were prepared at different water contents. A certain increase in the water content of NADES caused a decrease in viscosity and thus conductivity of NADES have increased.
C. Effect of molar ratio on dielectric properties of DES
The dielectric properties of the chemicals used in this study were given in Table 1 for 915 and 2450 MHz at constant temperature (25 °C). It has been observed that formic acid has higher dielectric properties than other HBDs and hence ChCl-FA mixtures had also better dielectric properties.
The effect of molar ratios of DESs on dielectric
properties were also shown in Fig 4a-f (at 50 °C 45% of water content). In Fig
4a and b (for ChCl-AA), it was observed
Table 1. The dielectric properties of used chemicals in this study for 915 and 2450 MHz (Temperature was kept constant as 25 °C)
Figure 4: The effect of different molar ratio and
frequency on dielectric properties a) 
-ChCl-AA, b) 
-ChCl-AA, c) -ChCl-FA, d) -ChCl-FA, e) -ChCl-Gly, f) -ChCl-Gly (The temperature and water content were kept
constant as 50 °C and 45%, respectively).
that the values were
higher at 1:4 molar ratio. However, the was found to
be maximum at 1:2 molar ratio. This may indicate that when acetic acid
concentration is lower in DES mixture, the solvent may absorb less microwave
energy, but can convert that energy into heat better compared to the mixtures
having higher acetic acid concentration. The and of ChCl-FA
was higher at 1:3 of molar ratio (Fig. 4c and d). For ChCl-Gly, 1:2 molar ratio
presented the highest values for and ' (Fig. 4e and
f). In literature, no studies were found regarding the dielectric properties of
DESs at different molar ratios.
D. Effect of hydrogen bond donor type on dielectric properties of DES
The variation of dielectric properties with
respect to frequency and DES type were given in Fig. 5a-b. It was observed that
ChCl-Gly mixture had the lowest values of dielectric properties compared to
ChCl-AA and ChCl-FA
Figure 5: The effect of different DESs and frequency on
dielectric properties of a) , b) (The water content, molar ratio and temperature were
kept constant as 30%, 1:3 and 25 °C, respectively).
at constant water content (30%), molar ratio
(1:3) and temperature (25 °C). The and decreased
with an increase in frequency (from 500 MHz to 3000 MHz) for all DESs. The
highest was found as
44 for ChCl-AA (at 500 MHz). This result showed that ChCl-AA had the highest
capability to store energy when it is exposed to an electric field. The highest
was as 240
for ChCl-FA (at 500 MHz). It could be said that presence of formic acid greatly
enhances the convertibility of electrical energy into heat.
Similar values for DESs were reported in literature.
Boyko et al. (2020) investigated the dielectric properties of
sorbitol-based natural deep eutectic solvents (NADES). The of
sorbitol:malic acid:water (1:1:3 in molar ratio) was found as 33±2 while of a modified
NADES which is sorbitol:malic acid:water:glycerin (1:1:1:1 in molar ratio) was
reported as 41±2 (at a constant temperature of 25 °C).
In conclusion, DES prepared with formic and acetic acid presented similar dielectric properties, which were higher than those of DES prepared with glycerol. Therefore, at 2450 MHz, use of acidic hydrogen bond donors (in DES) for microwave assisted experiments might give more successful results. The maximum value of the dielectric constant was found to be 46.74±0.02 for ChCl-AA at the highest water content (45%), molar ratio (1:4), and the lowest temperature (25 °C) (for 2450 MHz). The maximum value of the dielectric loss factor was found to be 122.90±3.04 for ChCl-FA at the highest water content (45%), temperature (75 °C), and the lowest molar ratio (1:2) (for 2450 MHz). DESs are generally used at high temperatures in microwave applications. Considering that ChCl-FA had higher dielectric loss factor at high temperatures, it can be said that ChCl-FA can convert microwave energy into heat energy much better compared to others. Therefore, ChCl-FA can give better results in microwave applications using DESs.
IV. CONCLUSIONS
A negative correlation between temperature and was
determined which may be due to decreasing the viscosity with increasing
temperature. In other words, at higher temperatures, molecular motion is
aggravated, weakening the dipole polarization and lowering the . However, the
increase in temperature had a positive effect on and the
maximum values were obtained at 75 °C. It was found that both the and slightly
decreased with increasing frequency. In addition, it was found that a decrease
in water content resulted in a decrease in both ε' and values. This
can be explained by the absence of free water molecules that are responsible
for the dipole rotation mechanism. It was observed that the ChCl-Gly mixture
had the lowest values of dielectric properties compared to ChCl-AA and ChCl-FA
at constant water content (30%), molar ratio (1:3) and temperature (25 °C).
Based on these results, DES prepared with acidic hydrogen bond donors are
recommended for microwave-assisted DES applications.
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Received: January 18, 2021
Sent to Subject Editor: March 18, 2021
Accepted: August 11, 2021
Recommended by Subject Editor Maria Laura Foresti