Journal of Chemical & Engineering Data
Article
After that, Zong et al. had further explored the properties of
K. The excess amino acid was precipitated by adding ethanol.
[
Ch][AA] ILs and found that the selective extraction of lignin
After filtration the AAIL was in a vacuum oven containing P O
2
5
from lignocellulose was enhanced significantly using the
[
at 363.15 K for 48 h to remove the residual water prior to use.
Five [Ch][AA] ILs were then obtained with the yields of more
than 80 % (on the base of [Ch]Cl), though it consumed too
much time for synthesis. Thus, the structures of [Ch][AA] ILs
18
Ch][Gly] ILs as a solvent. Therefore, to study the potential
of such AAILs to be applied in various fields of chemical
industry, the physicochemical properties of AAILs are very
important for analysis. Even though a great number of research
groups are working on this subject and the number of
published papers on physical properties of AAILs has increased
exponentially during the past decade, the effect of temperature
and anion size on the physicochemical properties of [Ch][AA]
ILs has not been studied systematically yet. It is very worth
noting that to date the properties of such AAILs remain
unknown.
1
were confirmed by H NMR, elemental analysis, and FT-IR
spectra (available as Supporting Information) and are in good
17
agreement with the literature. Then the purities of these
AAILs were estimated to be greater than 99 % (mass fraction)
1
from H NMR and elemental analysis. In addition, the water
content was less than 150 ppm in all of these [Ch][AA] ILs,
which was analyzed by Karl Fischer titration (Metrohm 756 KF
−
coulometer). The concentration of Cl in each [Ch][AA] IL
In this work, five AAILs, cholinium glycinate ([Ch][Gly]),
cholinium L-alaninate ([Ch][L-Ala]), cholinium β-alaninate
was measured by Mohr titration, and the related impurity was
less than 0.02 wt %. The [Ch][AA] ILs also had a calculated
+
(
[Ch][β-Ala]), cholinium prolinate ([Ch][Pro]), and choli-
Ag concentration of parts per billion level from the limited
nium serinate ([Ch][Ser]), were synthesized from choline
hydroxide and amino acids via simple neutralization reactions.
The general synthetic route of the [Ch][AA] ILs was depicted
in Figure 1, and they were characterized by NMR spectroscopy,
solubility product of AgCl.
Physical Properties. Before measuring the properties such
as density, viscosity, refractive index, conductivity, and thermal
stability, all of the [Ch][AA] ILs were drying in vacuum at
3
53.15 K. All instruments used for physical property measure-
ments were calibrated using Millipore-quality water as
19,20
described elsewhere.
An Anton Paar densimeter (model
DMA4500) and cone−plate viscometer (Brookfield DV II+
Pro) were used to measure the densities and viscosities,
respectively, over the temperature range (298.15 to 353.15) K
with a temperature control of ± 0.05 K. The precision of the
−
3
density apparatus was ± 0.001 g·cm , which was also
calibrated with dry air before each series of measurements.
The attaining thermal equilibrium time in the viscometer was
about 30 min, and the uncertainties were estimated to be ± 5.5
%. Refractive indices were determined at temperatures from
(
293.15 to 343.15) K using a Rudolph research analytical J357
refractometer with a measuring accuracy of ± 0.001 and a
temperature precision of ± 0.05 K. The apparatus was
calibrated before each series of measurements and checked
Figure 1. General route for the synthesis of [Ch][AA] ILs.
21
Fourier transform infrared (FT-IR) spectroscopy, elemental
analysis, thermogravimetry, and differential scanning calorim-
etry (DSC) analysis. Some key physical properties (density,
viscosity, refractive index, and conductivity) have been
measured as a function of temperature at atmosphere pressure,
while the influence of anion size on those properties was also
discussed.
using pure organic solvents with known refractive index.
Conductivity was determined by a conductivity meter (DDJS-
308A, Shanghai Leici Company) with a DJS-1C electrode.
Caution was taken to prevent evaporation, and the electrode
and the solution were sealed in typical glassware, which was
immersed into a thermostatic bath with an accuracy of ± 0.05
K. The uncertainty of the conductivity data was ± 3 %. In
2
0
addition, the optical rotations, [α] , for the three chiral ILs
D
EXPERIMENTAL SECTION
(
[Ch][L-Ala], [Ch][Ser], and [Ch][Pro]) were also measured
■
9
Materials. The glycine (purity ≥ 99 %), L-alanine (purity ≥
in aqueous solution (c = 2) using an automatic indication
polarimeter (Atago-AP300) with a measuring accuracy of ±
0.03.
9 %), β-alanine (purity ≥ 99 %), L-serine (purity ≥ 99 %), L-
proline (purity ≥ 99 %), and choline chloride (purity ≥ 99 %)
were purchased from Aladdin Chemical Reagent Co. Ltd.
Thermal Properties. The thermal stability of [Ch][AA]
ILs and [Ch]Cl was determined by a PerkinElmer Diamond
TG/DTA thermal gravimetric analyzer. The samples were
placed in an aluminum pan under nitrogen atmosphere at a
heating rate of 10 °C·min− with temperature accuracy better
(
Shanghai, China). Silver oxide and ethanol were of analytical
grade and used without any further purification. Doubly
distilled water was used in all experiments.
1
Preparation and Characterization of [Ch][AA] ILs. Five
AAILs, [Ch][Gly], [Ch][L-Ala], [Ch][β-Ala], [Ch][Ser], and
than ± 3 °C. The decomposition temperatures (T ) were then
d
[
Ch][Pro], were synthesized by the procedure as follows
Figure 1). In the first step, [Ch]OH aqueous solution was
obtained from the metathesis of [Ch]Cl (0.05 mol) with Ag O
taken as the onset of mass loss, defined as the intersection of
(
the baseline before decomposition and the tangent to the mass
loss afterward. Further, the glass transition temperatures (T ) of
2
g
(
0.025 mol) by using water as the solvent. In the second step, it
these [Ch][AA] ILs were determined with a differential thermal
was centrifuged and filtered; [Ch]OH containing filtrate was
then neutralized with an equimolar aqueous solution of amino
acid by stirring at room temperature for 12 h. After
neutralization, water was evaporated under vacuum at 323.15
analyzer (Netzsch DSC 200F3) with a heating rate of 10
−1
°C·min , after cooling samples to −80 °C under nitrogen. The
DSC instrument was calibrated by the standard reference
indium and zinc sample, and the uncertainty was ± 0.1 °C.
1
543
dx.doi.org/10.1021/je301103d | J. Chem. Eng. Data 2013, 58, 1542−1548