Vol. 26, No. 12 (2014)
A Novel Deep Eutectic Solvents Synthesized by Solid Organic Compounds 3627
applications in fiber, paper, film, polymer and paints industries.
However, natural cellulose is high crystalloid, with strong inter-
and intra-molecular H bonding caused by the hydroxyl groups
and is insoluble in water and common organic solvents. The
searching for “Green solvents” on cellulose has become a high
priority. Swatloski et al.20 led the first research on developing
uses of ionic liquids as cellulose solvents. They reported in
2002 that the ionic liquid 1-butyl-3-methyl imidazole chloride
([C4mim]Cl) exhibited super solubility for cellulose. Recently,
more than 20 ionic liquids have shown promise as a class of
new solvents which can dissolve cellulose. Cationic types of
ionic liquids which have been studied include: alkyl quaternary
ammonium cation [NRxH4-x]+, alkyl quaternary phosphonium
cation [RRxH4-x]+, N-alkyl pyridine cation [RPy]+ and N,N-2-
alkyl imidazole cation [R1R3im]+, where "im" represents the
imidazole structure21. To our best of knowledge, deep eutectic
solvent as a solvent for cellulose has not been reported. In the
current work, we focus on a novel kind of eutectic solvents
that is synthesized by two organic solids compounds, such as
caprolactam, acetamide and urea. These eutectic mixtures are
easy to prepare in a pure state. The first application as solvent
for cellulose is also investigated. This research not only extends
the field of ionic liquids and deep eutectic solvent but also
provides a new approach for the researching of solvent on
cellulose.
was added into the dissolved cellulose solution. The above
experimental procedure was repeated for several cyclic numbers.
The solubility of cellulose corresponding to dissolution times
in ionic liquids for each cyclic number was recorded.
Detection method: FTIR spectra of the synthesized deep
eutectic solvent were recorded on a FTS-135 spectrometer
(BIO-RAD, America) using the KBr wafer technique. The
conductivity and its temperature dependence were determined
using a DDS-II conductivity meter with temperature and
conductivity probes (probe cell constant 1 cm-1). The disso-
lution process of cellulose was monitored by using a polarizing
microscope (XP-203, Shanghai Changfang Optical Instrument
Co., Ltd., China).
RESULTS AND DISCUSSION
Melting point: Melting point is a key parameter to evaluate
deep eutectic solvent. The melting point depended on the molar
ratio of two compounds. With the molar ratio increasing or
decreasing to the optimal value, the melting point increase.
The optimal molar ratio of different deep eutectic solvent and
the corresponding melting point were showed in Table-1. For
every compound, it could be seen that the melting point was
much lower than that of the raw materials. It was evidence
that caprolactam, urea and acetamide combine of two and
produce new specie. In addition, according to the definition
of ionic liquid, it was present liquid at wide range of molar
ratio of two materials, but a lowest melt point at optimal ratio.
We deduced that two organic molecule combined by H-
bonding and formed super-molecule. Organic solid compounds
containing H-bonding are polar molecule, which is formed by
regularly arranging of H-bonding. For the simple discussion,
deep eutectic solvent of caprolactam/urea was chosen as an
example. When the mixing-system heated, the intramolecular
H-bonding of each precursor compound was destroyed and
new intermolecular H-bondings between a urea molecule and
more caprolactam molecules were formed, which led to the
decrease of some physical factors, such as energy of H-bonding,
the degree of molecular arrange and the electrostatic attraction
between hydrogen and organic molecule. Generally, low sym-
metry, weak ion interactions (such as suppressing H bonding)
and effective charge distribution over the cation and/or anion
tends to reduce the crystal lattice energy of the salts, thus resul-
ting in low-melting salts24. Theoretically, two organic materials
with H-bonding can synthesize this type of deep eutectic
solvent. It must be noted that in this type deep eutectic solvent,
only the solid organic materials such as solid alcohol, sugar,
solid aldehyde, etc., were considered.
EXPERIMENTAL
Caprolactam, urea and acetamide were purchased from
China of Tianjin Chemical Reagent Factory and further dried
under vacuum prior to use. The melt point of three reagents is
68-70, 133 and 78-81 °C, respectively. Alcohol was used as
received.
Cotton-ramie pulp obtained from commercial sources was
used as the cellulose sample, with a degree of polymerization
(DP) of 517.All cotton-ramie pulps were cut into small pieces
and dried at 100 ºC for 12 h without activation treatment before
use.
Preparation of deep eutectic solvent: The urea and
caprolactam were added to the beaker at different molar ratio.
The mixture was placed on heating jacket and stirred until a
homogeneous colorless liquid was formed. All the operation
was processing in glove box with N2. After that, the compound
was dried in vacuum at 25 °C. The final compound was sealed
at vessel. The procedures of preparing caprolactam/acetamide
and acetamide/urea deep eutectic solvent were same with
above. The obtained liquids were cooled at a rate of 1 °C min-1
and the freezing point was taken as the temperature at which
the first solid began to form.
TABLE-1
MELTING POINT OF SYNTHESIZED
DEEP EUTECTIC SOLVENT
Dissolution of cellulose: The dissolution of cellulose was
processed using Zhao et al.22 and Liu et al.23 as reference. In
the dissolving process, a certain amount of cellulose sample
was added into the ionic liquids. The flask was continuously
purged with gaseous N2 and the cellulose solution was stirred
continuously in an oil bath at 120 °C. Because of high crysta-
llinity of cellulose, the process of dissolution of cellulose was
monitored with a polarizing optical microscope.After the emer-
gence of a blackburst in the cellulose solution, as determined
on a polarization microscope, the next level of cellulose sample
Urea/
Caprolactam
Caprolactam/
Acetamide
Urea/
Acetamide
Systems
Optimal mole ratio
Melting point (°C)
1:3
30
1:1
18
1:2
48
Conductivity:The conductivity of synthesized deep eutectic
solvent at optimal molar ratio was about 10-5-10-4 S/m.
Compared with ionic liquids, the conductivity of deep eutectic