Properties of deep eutectic solvents

Abstract

Deep eutectic solvents (DESs) present several promising potential applications. In the industrial manufacture of chemicals, and organic synthesis, catalysis, and electrochemistry and in sample preparation for analytical chemistry, removal of pollutants from extraction processes, isolation and fractionation, the preparation of biofuels and also in separating azeotropic mixtures.Measured properties of deep eutectic solvents are very helpful in next research area focused on using DESs in biomass pretreatment or in extraction of valuable compounds from plant biomass. For these applications is important seemly low density and viscosity and based on thermal stability is chosen reaction temperature.

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1 Introduction
Deep eutectic solvents (DESs) present several promising
potential applications. In the industrial manufacture of
chemicals, and organic synthesis, catalysis, and
electrochemistry and in sample preparation for analytical
chemistry, removal of pollutants from extraction processes,
isolation and fractionation, the preparation of biofuels
and also in separating azeotropic mixtures. [1, 2]
Applications of DESs are limited by properties
of prepared green solvents. Properties of
DESs depend on the composition of eutectic
mixtures which is variable. DESs can consist
of two or more compounds and the melting
point of mixture is below melting points of
compounds of which it is formed. They can be
formed by organic acids, aminoacids, saccharides,
alcohols, aldehydes, polyols and ammonium salts
bonded with hydrogen bonds.
2 Experiment
Eutectic solvents were based on choline
chloride and malonic acid (1:1), malic acid
(1:1), glycolic acid (1:3), glycerol (1:2),
citric acid (monohydrate, 1:1), oxalic acid
(dihydrate, 1:1) and lactic acid (1:10). The
solutions were stirred in an oil bath to form a
homogeneous liquid. The viscosity of the deep
eutectic solvents reagent was measured using
Ubbelohde viscometer at atmospheric pressure
and temperature 60°C. The density of these
mixtures was determined by specific gravity
bottles, and refractive index was measured
by Abbe refractometer. Thermogravimetric
analysis of eutectic mixtures was carried out
using Mettler Toledo TGA/DSC 1 instruments
(atmosphere – nitrogen). The kinetic analysis
of the thermal decomposition reaction was
done applying the fitting function and
isoconversional method.
3 Results and discussion
The density and viscosity is one the most important
physical properties of solvents. The accessibility of the
lignocellulose material for delignification by DES depends on
several factors. The essential factors limiting the access of
the fibers are: macro and microstructure of the fibres [3]. In
the case of solid matrix samples, DESs applications have
been limited, especially due to their viscosity.
This work was supported by the Slovak Research and Development Agency under the contract No. APVV-0850-11 and
under the contract No. APVV- 0393-14. This article was realized also thanks to the support for infrastructure
equipment by the Operation Program Research and Development for the project "National Center for Research and
Application of renewable energy sources" (ITMS 26240120016, ITMS 26240120028) for the project "Competence
center for new materials, advanced technologies and energy "(ITMS 26240220073) and for the project" University
science park STU Bratislava "(ITMS 26240220084), co-financed by the European regional Development Fund.
5 References
[1] M. Jablonský, A. Škulcová, L. Kamenská, M. Vrška, J. Šima,
BioResources, 10, 8039 (2015).
[2] A. Škulcová, L. Kamenská, F. Kalman, A. Ház, M. Jablonský, K.
Čížová, I. Šurina, Key Engineering Materials, 688, 17 (2016).
[3] A. Wojcak, A. Pekarovicova, Cellulose Chemistry and
Technology, 35, 361 (2001).
In Tab. 1 are summarized used eutectic solvents,
molar ratios of hydrogen bond acceptor and donor,
refractive indexes of DESs, their viscosity at 60 °C and
activation energy. Fig. 1 shows scans from TGA
analysis of mixture choline chloride: citric acid (1:1)
taken at different heating rates. Fig. 2 compares
densities of several DESs mixtures as a function of
temperature. Eutectic mixtures which contain
hydrogen bond donor with more –OH groups
(citric acid, malic acid, oxalic acid) have
higher density than mixtures based on
glycolic acid or lactic acid or glycerol.
Thermal stability of several eutectic
mixtures was measured too using long-
term method at 60, 80 and 100 °C for 10
hours. From industrial application point of
DESs must endure certain high temperature for
a period of time. In that case, long-term thermal
stability is needed, which is measured isothermally
at different temperature interval. The selection of
temperature interval of DESs is chosen based
on possible industrial application of DESs.
In Fig. 3 is seen dependence of mass in %
at the heating time for mixture choline
chloride: oxalic acid (1:1). We can say that
already at 60 °C the occurs weight loss
due to the removal of moisture. At 100 °C
is observed weight loss higher and
dihydrate becomes anhydrous.
4 Conclusion
Measured properties of deep eutectic solvents are
very helpful in next research area focused on using
DESs in biomass pretreatment or in extraction of
valuable compounds from plant biomass. For these
applications is important seemly low density and
viscosity and based on thermal stability is chosen
reaction temperature.
Reagent Molar ratio
Refractive
index
at 25 °c
Viscosity
[mPa∙s
]
at 60 °C EA [kJ/mol]
malonic
acid 1:1 1.4861 * 252.7
malic
acid 1:1 1.4813 * 166.2
glycolic
acid 1:3 1.4709 39.47 138.0
citric
acid 1:1 1.4968 * 99.7
glycerol
1:2 1.4781 21.43 107.7
lactic
acid 1:10 1.4426 13.90 118.1
oxalic
acid 1:1 1.4662 35.09 115.1
* solvents can
´t be measured by Ubbelohde viscometer because of their high viscosity
Škulcová A., Jablonský M., Kamenská L., Kalman F., Ház A., Sládková A., Vrška M.
Slovak University of Technology in Bratislava, Faculty of Chemical and Food Technology, Institute of Natural and Synthetic Polymers,
Department of Wood, Pulp and Paper, Radlinského 9, 812 37 Bratislava
xskulcova@stuba.sk
Properties of deep eutectic solvents
Table 1: Measured properties of prepared deep eutectic solvents based on
choline chloride with reagent
20 30 40 50 60 70 80
1160
1180
1200
1220
1240
1260
1280
1300
1320
1340
1360
(kg/m3)
T (°C)
choline chloride with
●citric acid monohydrate,
▼malic acid, ◄oxalic acid
dihydrate, ■glycolic acid,
lactic acid, ▲glycerol
choline chloride:citric acid
monohydrate (1:1)
FIGURE 1
FIGURE 2
FIGURE 3
05000 10000 15000 20000 25000 30000 35000
86
88
90
92
94
96
98
100
mass (%)
time (s)
T = 60 °C
T = 80 °C
T = 100 °C
choline chloride:oxalic
acid (1:1)
Reagent Molar
ratio
Weight loss in % after 10 h
60 °C 80 °C 100 °C
malic
acid 1:1 4.1 4.4 *
glycolic
acid
1:3 5.9 11.6 *
citric
acid 1:1 3.1 5.1 *
glycerol
1:2 4.7 3.6 *
lactic
acid 1:10 4.8 7.9 12.5
oxalic
acid 1:1 4.9 7.3 10.3
*
Not measured yet
Table 2: Observed weight loss in % after 10 h
for mixtures based on choline chloride with
reagent from table