Novel low viscous, green and amphiphilic N-oxides/phenylacetic acid based Deep Eutectic Solvents

Article abstract of DOI:10.1016/j.molliq.2017.05.084

Four novel Deep Eutectic Solvents (DESs) were prepared by mixing phenylacetic acid (which is a natural molecule present in honey) with amine N-oxides, which are molecules easily biodegradable in nature. Three of these N-oxides are amphiphilic. The novel DESs have very low freezing points (from ??34?°C to 20?°C) and they have very low viscosity, much lower than the most common and used DES in literature so far (choline chloride/urea mixture). The conductivity values resulted low and the ionicity analysis showed these DESs to be “super ionic”. Their polarity resulted high enough and it can be compared with other commonly used solvents or ionic liquids.

Full text of DOI:10.1016/j.molliq.2017.05.084

Accepted Manuscript
Novel low viscous, green and amphiphilic N-oxides/phenylacetic
acid based Deep Eutectic Solvents
Raimondo Germani, Matteo Orlandini, Matteo Tiecco, Tiziana
Del Giacco
PII:
S0167-7322(17)31248-5
doi: 10.1016/j.molliq.2017.05.084
MOLLIQ 7376
DOI:
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
22 March 2017
3 May 2017
17 May 2017
Please cite this article as: Raimondo Germani, Matteo Orlandini, Matteo Tiecco, Tiziana
Del Giacco , Novel low viscous, green and amphiphilic N-oxides/phenylacetic acid based
Deep Eutectic Solvents, Journal of Molecular Liquids (2017), doi: 10.1016/
j.molliq.2017.05.084
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ACCEPTED MANUSCRIPT
NOVEL LOW VISCOUS, GREEN AND AMPHIPHILIC N-
OXIDES/PHENYLACETIC ACID BASED
DEEP EUTECTIC SOLVENTS
a,b
a
a
a,b
Raimondo Germani* , Matteo Orlandini , Matteo Tiecco* , Tiziana Del Giacco
a
Department of Chemistry, Biology and Biotechnology, University of Perugia, via Elce di Sotto
-I, 06123, Perugia, ITALY
8
b
CEMIN Centre of Excellence on Nanostructured Innovative Materials, University of Perugia,
ITALY
Corresponding author
Department of Chemistry, Biology and Biotechnology, University of Perugia, via Elce di Sotto
8-I, 06123, Perugia, ITALY
Phone: +39 075 585 5548
Fax: +39 075 585 5560
E-mail: matteotiecco@gmail.com
1

ACCEPTED MANUSCRIPT
ABSTRACT
Four novel Deep Eutectic Solvents (DESs) were prepared by mixing phenylacetic acid (which
is a natural molecule present in honey) with amine N-oxides, which are molecules easily
biodegradable in nature. Three of these N-oxides are amphiphilic. The novel DESs have very
low freezing points (from -34°C to 20°C) and they have very low viscosity, much lower than
the most common and used DES in literature so far (choline chloride/urea mixture). The
conductivity values resulted low and the ionicity analysis showed these DESs to be “super
ionic”. Their polarity resulted high enough and it can be compared with other commonly used
solvents or ionic liquids.
KEYWORDS
Deep Eutectic Solvents (DESs), N-oxide, surfactants, low viscous liquids, green solvents.
2

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1. INTRODUCTION
The design of innovative organic solvents plays a key-role in the green chemistry
framework[1–3]. The novel liquids have advantageous green properties compared with
typical organic solvents: low vapour pressure, low flammability, recycling capabilities
(
whenever used as reaction media) and so on. From this point of view Ionic Liquids (ILs) have
played an important role for many years, thanks to their unique properties[4–6] ILs have
been used in many topics such as extraction media, solubilizing agents and so on, but the most
studied property of these liquids is represented by their use as reaction media for a wide
number of chemical reactions[7–9]. Unfortunately, ILs showed high toxic properties, and this
represents a disadvantage in terms of their “greenness”[10,11]. The onium salts present in the
liquids has effects on the cell membranes of living cells, therefore, they determine the toxicity
of these liquids[12,13].
Deep Eutectic Solvents (DESs) represent a step ahead in the design of novel green organic
solvents[14–17]. These liquids are formed by simply mixing two (or more) high melting point
compounds; the interactions occurring between these molecules lead to a significant decrease
of the melting points of the mixtures, often to room temperature (RTDESs, Room
Temperature Deep Eutectic Solvents)[18]. The properties of the DESs are similar to the ones
of ILs. They have in fact low melting points (spanning from room temperature or lower to 60-
80°C), low vapour pressure, low flammability, high recycle capability whenever used as
reaction media and so on, but they have significant green advantages compared to ILs. The
first green advantage of these mixtures is that their syntheses do not require any solvents or
any other reactants except for the ones composing them: they can be easily obtained in fact by
simply mixing and gently heating two or more solid compounds. The second green advantage
of the DESs is represented by their non-toxicity and their biodegradability as demonstrated in
many recent papers and reviews[19–21].
DESs are classified in literature as type I, II, II and IV depending on the kind of molecules
involved in the liquid mixtures formation[17]. Considering the most used and studied
mixtures in literature (choline chloride / urea and similar mixtures), this kind of classification
can be simplified interpreting the DESs as mixtures of a hydrogen bond donor molecule
(
HBD) and a hydrogen bond acceptor molecule (HBA)[22,23]. However, changes in the
molecules composing the DESs can provoke significant changes in their properties, and this
leads to the realization and the studies of novel DESs mixtures, specifically designed for
specific roles[24–30].
3

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DESs were studied in literature as reaction media for a wide number of reactions[16,31,32],
and they can act as “innocent DESs” (if their role is unspecified or not clearly understood in
the reaction) or “active DESs” (if the liquid mixture has a role in the reaction i.e. acid
catalyst)[33]. There are also several applications of these novel liquids reported in literature
such as: extraction media, solubilization of nanoparticles, separation media, use for carbon
dioxide capture, use for drug delivery in biomedical applications, and so on[17,34–37].
Starting from our experience on the synthesis and on the characterization of specific
molecules for specific roles and of novel ionic liquids[38–40], recently we prepared novel
aromatic, aliphatic and amphiphilic DESs[18,41]. In this work, we show the preparation and
the characterization of novel N-oxide-based zwitterionic DESs. These mixtures are formed by
differently structured N-oxides as HBA molecules (also amphiphilic ones) and phenylacetic
acid (PhAA) as HBD compound. There are many green advantages on the choice of these
molecules: N-oxide compounds (particularly N-oxide surfactants) are environmentally
friendly as they can be easily degraded in nature both via aerobic and anaerobic
conditions[42,43]; PhAA is a compound largely present in honey and plants[44–46].
Therefore, these DESs could possess relevant green properties.
2. EXPERIMENTAL
2.1 REAGENTS AND DESs REALIZATION
Benzoic acid, salicylic acid, 2-chlorobenzoic acid, 3-chlorobenzoic acid, glycolic acid, 2-furoic
acid, phenylacetic acid were purchased from Sigma-Aldrich and were used without further
purifications.
-
Synthesis of N-oxides (AO-12, AO-18, MO, MO-12): N-oxides can be synthesized with
different procedures[47,48]. In this work, we used a simple and cheap procedure based on the
use of hydrogen peroxide: the corresponding amine was put at 65-70°C in EtOH with 1.5
equivalents of H
and the solvent was removed under vacuum. The resulting semi-solids were recrystallized
twice in acetone/Et O mixtures.
N,N-dimethyldodecyl-N-amine oxide (AO-12): m.p. = 89-91°C; H-NMR (400 MHz, CDCl
2 2
O . After 6h MnO
2
was added to destroy H
O
2 2
excess, the liquid was filtered
2
1
3
) δ
=
3.33-3.29 (m, 2H) 3.27 (s, 6H), 1.91-1.77 (m, 2H), 1.33-1.24 (m, 18H), 0.87 (t, 3H, J=8 Hz);
13C-NMR (101 MHz, CDCl
3
) δ = 71.50, 58.10, 32.03, 29.73, 29.72, 29.62, 29.54, 29.45, 29.44,
+
2
6.71, 23.95, 22.81, 14.25; [M+H] = 230.2484 (calc.: 230.2484).
4

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1
N,N-dimethylottadecyl-N-amine oxide (AO-18): m.p. = 91-97°C; H-NMR (400 MHz, CDCl
3
)
δ = 3.32-3.28 (m, 2H) 3.27 (s, 6H), 1.90-1.76 (m, 2H), 1.32-1.24 (m, 30H), 0.86 (t, 3H, J=8 Hz);
13C-NMR (101 MHz, CDCl
3
) δ = 71.49, 58.06, 32.06, 29.83, 29.80, 29.79, 29.74, 29.64, 29.55,
+
2
9.45, 26.71, 23.95, 22.82, 14.25; [M+H] = 286.3122 (calc.: 286.3104).
1
N-dodecylmorpholine-N-oxide (MO-12): m.p. = 71-78°C; H-NMR (400 MHz, CDCl
3
) δ =
4
2
2
.47-4.41 (m, 2H), 3.81-3.78 (m, 2H), 3.36-3.29 (m, 7H), 1.98-1.86 (m, 2H), 1.35-1.26 (m,
13
0H), 0.89 (t, 3H, J=8 Hz). C-NMR (101 MHz, CDCl ) δ = 72.02, 63.66, 61.69, 32.02, 29.72,
3
+
9.63, 29.47, 26.79, 22.80, 21.99, 14.23; [M+H] = 272.2554 (calc.: 272.2589).
1
N-methylmorpholine-N-oxide (MO): m.p. = 180-184°C; H-NMR (400 MHz, CDCl
3
) δ = 4.41-
13
4
.36 (m, 2H), 3.72-3.75 (m, 2H), 3.32-3.37 (m, 2H), 3.22 (s, 3H), 3.11-3.08 (m, 2H). C-NMR
+
(
-
101 MHz, CDCl
3
) δ = 66.13, 61.73, 61.25; [M+H] = 118.0841 (calc.: 118.0868).
DESs were prepared by mixing the solid compounds at the proper HBD/HBA molar ratio and
heating the mixtures at 90°C when necessary, up to obtain a liquid phase.
The water content of the novel mixtures is about 5% w/w as resulted via Karl-Fischer
titration.
-
2.2 MELTING POINT DETERMINATION OF DESs
In a 10 mL round-bottomed flask, equipped with a thermometer, were introduced weighed
amounts of N-oxides and phenylacetic acid. The binary systems were gently heated and mixed
until became homogenous liquids, and then were gradually cooled first to room temperature
then immersed in an acetone/liquid nitrogen mixture.
2.3 DENSITY MEASUREMENTS
The density values at eutectic ratio at different temperatures were obtained by measuring the
weight of the sample in a 2.00 mL volumetric flask. The flasks were held in a thermostated
bath for 1 hour then brought to volume by eliminating the amount of liquid with a Pasteur
pipette. The flask was then thermostated at room temperature for 1 hour and then weighed
on analytical balance.
2.4 VISCOSITY MEASUREMENTS
The viscosity of the eutectic mixtures was measured using a Fungilab Expert L viscometer,
fitted with a thermostatic jacket and a temperature probe. The viscometer jacket was
connected to an external thermostated bath. The viscosity measurements were obtained
using a spindle attachment.
5

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2.5 CONDUCTIVITY MEASUREMENTS
Conductivity measures were performed with an Analytical Control - Model 120
-1
Microprocessor conductivity meter with a platinum cell (Cell constant K = 1.05 cm ). The
mixtures were thermostated with a glass-jacked beaker connected to a thermostatic bath. The
readings were taken after 20–25 minutes for each temperature selected.
2.6 POLARITY MEASUREMENTS
The absorbance spectra of Nile Red (Sigma-Aldrich) in the DESs were registered in a
Shimadzu UV-2401 PC spectrophotometer with a wavelength resolution of λ ± 0.2. The dye
solutions (10 mL for each sample) in the DESs were prepared putting 10 μL of a CHCl
3
-3
solution of Nile Red (2  10 M) in a 10 mL flask; the chloroform was carefully evaporated
using a nitrogen flux and the DESs were added to the dried dye. Three samples were acquired
for each reading point, and the standard deviation between the absorbance of the three
samples was considered as the precision of the measures.
2.7 KARL-FISCHER TITRATION MEASURES
The water content of all the DESs was evaluated via Karl-Fischer titration with Metrohm 684
KF Coulometer.
2.8 NMR AND MASS SPECTROMETRY MEASURES
1
13
H-NMR and C-NMR spectra were recorded on a Bruker Avance III 400 MHz.
ESI MS measurements were performed on a Quadrupole Time-of-Flight mass spectrometer
®
instrument (Xevo G2 QTof; Waters Corporation, Milford, USA), equipped with a high-
performance Z SprayTM source and operating with MassLynxTM Software package (Version 4.1,
Waters Corporation). The instrument was calibrated using a 5 mM sodium formate solution in
90:10 2-propanol:water, in the m/z range 50-1200. The spectra were recorded in positive
polarity, sensitivity mode. Source parameters: Source Temperature: 150°C; desolvation
temperature: 250 °C; capillary voltage: 3.0 kV cone Voltage: 15V; Extraction Cone: 2.0V.
6

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3. RESULTS AND DISCUSSION
N,N-dimethyldodecyl-N-amine oxide (AO-12) was chosen as HBA molecule for a preliminary
screening with different solid carboxylic acids as HBD molecules. The freezing points of
different mixtures with different carboxylic acids at different molar ratios were examined. In
Table 1 the acids tested for this screening with their melting points, the optimal molar ratios
acid/AO-12 (in terms of the lower melting points observed) and the freezing points of the
DESs mixtures are reported.
Table 1: Acids tested as HBD, melting points (m.p.) of the acids, acid/AO-12 molar ratio,
freezing points of the DESs mixtures.
Acid m.p.
°C)
Acid/AO-12
DES freezing
Acid
(
molar ratio
point (°C)a
Benzoic
Salicylic
121-125
158-161
138-140
153-157
75-80
1.5
1.5
1.5
1.5
2
55
50
45
5
2
3
-Chlorobenzoic
-Chlorobenzoic
Glycolic
30
15
-34
2
-Furoic
128-132
76-78
2
Phenylacetic
1
aAO-12 m.p. = 89-91°C
From this screening on the acids as HBD, phenylacetic acid / N,N-dimethyldodecyl-N-amine
oxide (PhAA/AO-12) mixture showed the lowest melting point in the set. Therefore, PhAA
was chosen as HBD compound, considering also the natural source of the molecule and the
greenness of the resulting DESs.
As well as with AO-12, PhAA was then mixed with other three differently structured N-oxides
to form novel zwitterionic amphiphilic N-oxide based DESs: N,N-dimethyloctadecyl-N-amine
oxide (AO-18); N-dodecylmorpholine-N-oxide (MO-12); N-methylmorpholine-N-oxide (MO).
These HBA molecules were chosen for several reasons: AO-12 and AO-18 represent
amphiphilic compounds and therefore they can form DESs with peculiar amphiphilic
properties. MO and MO-12 were chosen in order to evaluate the effect of the chain length of
these molecules on the DESs, because N-methylmorpholine-N-oxide is very poorly
hygroscopic compared to the correspondent short-chain N-oxides of AO-12 and AO-18. The
low water content is important because it can act a significant structural role in the DESs
7

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mixtures. The synthetic procedure used to obtain these molecules is very easy and cheap: it is
made treating the correspondent amine with H
2 2
O . Moreover, also other techniques can be
efficaciously used for their preparation (i.e. O catalytic oxidation)[47,48]. The molar ratio of
2
N-oxide / PhAA was maintained as 1/1. In Figure 1 the structures of the molecules
composing the DESs are reported.
Figure 1: Structures of the molecules composing the novel DESs. PhAA: Phenylacetic acid; AO-
1
8: N,N-dimethyloctadecyl-N-amine oxide; AO-12: N,N-dimethyldodecyl-N-amine oxide; MO-
2: N-dodecylmorpholine-N-oxide; MO: N-methylmorpholine-N-oxide.
1
In Table 2 are reported the melting points of the N-oxides, the freezing points of the obtained
DESs and the ΔT of melting between the ones of N-oxides and of the DESs.
Table 2: Melting points (m.p.) of the N-oxides, freezing points of the obtained DESs and Δt of
melting between the ones of N-oxides and of the DESs. Molar ratio N-oxides/PhAA = 1/1.
N-oxide
m.p. (°C)
89-91
DES freezing
DES
Δt (°C)
point (°C)
-34
AO-12/PhAA
AO-18/PhAA
MO-12/PhAA
MO/PhAA
123
70
91-97
20
71-78
-21
92
180-184
-27
207
a
PhAA m.p. = 76-78°C
8

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All the mixtures are liquid at room temperature. The freezing point of AO-18/PhAA resulted
the highest of the set (20°C), just below room temperature. The other ones have very low
freezing points, spanning from -21°C to -34°C. AO-18/PhAA mixture showed a liquid-solid
transition, while the others showed transitions to semi-solid phases decreasing the
temperature.
These novel four systems were analyzed and characterized via measurements of density,
viscosity, conductivity, ionicity (by Walden Plot) and polarity (by Nile Red dye).
3
.1 DENSITY
The density values of these mixtures were measured in the range of temperatures between
0°C and 70°C. The values at 25°C and 65°C are reported in Table 3, all the measures are
2
reported in Supplementary data. The values of AO-18/PhAA and MO-12/PhAA DESs are
slightly under 1 g/mL, while the values of AO-12/PhAA and MO/PhAA DESs are slightly over 1
g/mL. These density values were used to measure the viscosity (in cST) and molar
conductivity values.
Table 3: Density values (ρ) at 25°C and 65°C of all the DESs mixtures.
DES
ρ (g/mL) at 25°C
1.156
ρ (g/mL) at 65°C
1.106
AO-12/PhAA
AO-18/PhAA
MO/PhAA
0.977
0.947
1.156
1.106
MO-12/PhAA
0.995
0.955
3.2 VISCOSITY
The viscosity (η) of these mixtures is exponentially dependant on the temperature, following
the Arrhenius equation.
ꢆꢇ
ꢀ
ꢁꢂ ꢃ ꢀꢁꢂ ꢅꢁ
ꢄ
ꢈꢉ
9

ACCEPTED MANUSCRIPT
The viscosities of the novel N-oxide DESs mixtures were measured in a temperature range
-1
between 20°C to 65°C; the profiles of ln η (in cST) vs. 1/T (in K ) are reported in Figure 2; all
the data are reported in Supplementary data.
7
6
5
4
3
2
.5
6
.5
5
.5
4
MO-12/PhAA
AO-18/PhAA
MO/PhAA
.5
3
AO-12/PhAA
.5
0
.0029 0.003 0.0031 0.0032 0.0033 0.0034 0.0035
-
1
1
/T (K )
Figure 2: Viscosity profiles of N-oxide DESs in the temperature range between 20°C and 65°C.
Black squares = MO-12/PhAA; grey triangles = AO-18/PhAA; black diamonds = MO/PhAA;
empty circles = AO-12/PhAA.
All the DESs showed low viscosity in all the temperature range analyzed and Newtonian
behaviours. In Table 4 the values at 25° and 40°C and the activation energies of viscosity E
η
(
calculated via Arrhenius equation) are reported. In the same table values of two other
zwitterionic DESs and of the most studied DESs in literature are reported: a) Gly/TMG,
glycolic acid/trimethylglicine; b) SB3-12/CSA, 3-(dodecyldimethylammonio)propane-1-
sulfonate/(1S)-(+)-10-camphorsulfonic acid (that is also amphiphilic) c) ChCl/U, choline
chloride/urea mixture[14,15,18,41,49].
1
0

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Table 4: Activation energies of viscosity (E
η
) and viscosity values (η at 25° and 40°C)
measured for all the novel DESs compared with those of and of ChCl/U DES and of other
zwitterionic DESs (Gly/TMG; SB3-12/CSA).
E
η
η (cST) at
25°C
η (cST) at
40°C
DES
(
kj/mol)
AO-12/PhAA
AO-18/PhAA
MO-12/PhAA
MO/PhAA
-41.6
-43.1
-49.8
-46.9
-59.6
-42.6
-38.2
a
175.07
314.32
412.36
214.34
600.00
83.20
144.73
167.15
86.66
ChCl/U14
140.92
745.72
3617.05a
Gly/TMG41
SB3-12/CSA18
T=80°C
The activation energies (that are determined by the packing capabilities of the molecules in
the liquid structures) are almost the same for all the novel DESs, and the values are similar to
the ones of the other DESs reported in literature. On the contrary, the values of viscosity,
especially the ones at room temperature, are much lower than those observed with the other
DESs systems (from 175 cST to 412 cST compared to 600 cST). At higher temperatures,
ChCl/U decreases its viscosity until a value similar to that of amine oxide-based systems. The
analyzed amphiphilic sulfobetaine-based DESs (SB3-12/CSA) showed much higher values
than the other systems (3617.05 cST, determined at 80°C). This suggested that the
interactions occurring in these novel systems are much different from the ones observed in
sulfobetaine-based DESs, even if the chain length is the same. Such low viscosity values
support these DESs as promising novel organic liquids for different applications, even at room
temperature.
3.3 CONDUCTIVITY
Conductivity is a technique that can give structural information about a liquid system. The
conductivity values are dependent on the amount of free charges in a system and on their
mobility, therefore on their dimensions. Small molecules (therefore with high mobility) with
high charge density lead in fact to higher conductivity values compared with the ones
observed with molecules having lower charge density, larger dimensions and therefore lower
mobility. The mobility of the charged molecules is a key factor when charged molecules can
1
1

ACCEPTED MANUSCRIPT
interact with larger dimension molecules such as DNA, microbial cells or TiO suspensions;
2
this is because a decrease of the conductivity is observed as a consequence of interaction with
lower mobility systems[50–53]. In an organic liquid such as a DES, the conductivity values are
determined by the amount of free charges in the system, therefore they are usually low. In
Figure 3 are reported the plots of the conductivity values (ln σ, S) of the DESs measured at
-1
different temperatures (1/T, K ).
-
7
MO/PhAA
-
-
-
7.5
AO-12/PhAA
AO-18/PhAA
MO-12/PhAA
-
8
8.5
-
9
9.5
10
10.5
-
-
-
11
0
.0029 0.003 0.0031 0.0032 0.0033 0.0034 0.0035
-
1
1
/T (K )
-1
Figure 3: plot of conductivity (ln σ, S) vs 1/T (K ) of the N-oxide based DESs. Black diamonds
=
MO/PhAA; empty circles = AO-12/PhAA; grey triangles = AO-18/PhAA; black squares = MO-
2/PhAA.
1
In Table 5 the values of the activation energies (E
conductivity has an Arrhenius behavior) and the conductivity values (in mS/cm) observed at
5° and 60°C of the novel DESs are collected. All the data are reported in Supplementary data.
Λ
) calculated by these plots (even the
2
Table 5: Conductivity activation values (E ) and conductivity values (σ) at 25° and 60°C of the
Λ
novel DESs.
E
Λ
σ (mS/cm) σ (mS/cm)
DES
(
kj/mol)
25.03
at 25°C
0.180
at 60°C
0.586
MO/PhAA
MO-12/PhAA
30.26
0.029
0.104
1
2

ACCEPTED MANUSCRIPT
AO-12/PhAA
AO-18/PhAA
24.81
24.05
0.078
0.053
0.228
0.154
The values of activation energies (E ) observed are slightly under those reported for other
Λ
DESs mixtures in literature (ChCl/Oxalic acid = 34.6 kJ/mol; Gly/TMG = 31.1 kJ/mol; SB3-
2/CSA = 33.0 kJ/mol), except for MO-12/PhAA DES that is more similar to them[14,18,41].
1
The values of activation energies are related to the energies required for the formation or
expansions of the vacancies in the liquid structures[54]. These values suggested an easier
formation of vacancies in the liquid structures compared to the other DESs systems analyzed.
The conductivity values observed are low: in literature, the values reported for protic ionic
liquids (which are similar to HBD/HBA DESs but they are formed by a complete hydrogen
transfer) are much higher and they vary from 150 mS/cm at 25 °C up to 470 mS/cm at 100
°
C[55]. So, these novel N-oxide-based DESs cannot be considered as charged species mixtures
such as protic ionic liquids; this is because of their low conductivity values observed.
MO/PhAA DES showed conductivity values higher than the other ones in the set; this is
because of the higher mobility of morpholinium (for its smaller dimensions) compared with
the other amphiphilic HBA molecules.
3.4 IONICITY
The Walden plot is a simple semi-quantitative method to establish the ionicity extent of a
novel organic liquid and it is based on the Walden law that correlates the molar conductivity
(Λ) with the viscosity (η)[56,57]:
ꢊ ꢋ ꢁꢂ ꢃ ꢌꢍꢀꢎꢏꢐꢀꢏ
This method has been largely used for ILs and DESs in the recent years to establish the
relationships between their conductivity and viscosity and to define the extent of ion
association of a liquid compared to an ideal electrolyte solution. In fact, in the Walden plot the
diagonal indicates the molar conductivity vs. the logarithm of inverse viscosity of an aqueous
solution of 0.01 M KCl; the liquids with values on the lower side of the plot under this diagonal
are called “poor ionic” liquids due to the high ion association of their components; on the
1
3

ACCEPTED MANUSCRIPT
upper side of the diagonal “super ionic” liquids are found, indicating a charge separation in
their structures.
In Figure 4 the Walden plots of the novel DESs AO-12/PhAA, AO-18/PhAA, MO/PhAA, MO-
12/PhAA are reported. In the same plot the values of other similar organic liquids are
reported as comparison: a typical ionic liquid ([BMIM][BF ], 1-butyl-3-methylimidazolium
4
tetrafluoroborate, t=25°C), a choline chloride-based deep eutectic solvent (ChCl/Eg, choline
chloride/ethylene glycol, t=20°C), a zwitterionic betaine-based DES (Gly/TMG, glycolic
acid/trimethylglicine, t=45°C), a zwitterionic sulfobetaine-based DES (SB3-12/CSA, 3-
(
dodecyldimethylammonio)propane-1-sulfonate/(1S)-(+)-10-camphorsulfonic
acid,
t=75°C)[18,41,56,57].
7
5
3
1
1
3
5
7
MO/PhAA
AO-18/PhAA
AO-12/PhAA
MO-12/PhAA
BMIM BF4
ChCl/EG
-
-
-
-
Gly/TMG
SB3-12/CSA
5 7
1
3
-
7
-5
-3
-1
ln η-1 (P-1)
Figure 4: Walden plots of the novel N-oxide DESs (in the temperature range 25°C-70°C) and
of other similar organic liquids. Black diamonds = MO/PhAA; grey triangles = AO-18/PhAA;
empty circles = AO-12/PhAA; black squares = MO-12/PhAA; + = [BMIM] [BF
ChCl/Eg (t=20°C)[53]; empty square = Gly/TMG (t=45°C)[41]; circle = SB3-12/CSA
t=75°C)[18].
4
] (t=25°C)[54], X
(
All novel N-oxide based DESs showed “super ionic” nature and all of them showed linear
profiles, therefore Walden behavior. However, the low values of both molar conductivity and
viscosity observed lead to the position in the negative portion of the plot. This position is
interesting and uncommon considering the other similar DESs and organic liquids. The other
1
4

ACCEPTED MANUSCRIPT
zwitterionic DESs are closer (or slightly under) the diagonal, therefore they can be considered
“
poor ionic” liquids; this indicates a higher charge separation in the novel DESs. PILs (Protic
Ionic Liquids, with a complete proton transfer from the HBD to the HBA molecules) and ILs
with discrete ionic species) are on the contrary on the right portion on the Walden plot and
this suggests a different structure of the liquids.
(
3.5 POLARITY
UV-VIS measures with solvatochromic dyes were performed in literature using a wide
number of probes and on a wide number of solvents or ILs or DESs to evaluate the polarity of
these media[15,58–60]; in these works Reichardt’s dye represents one of the most used
molecule thanks to its properties[61,62]. Our systems are based on HBD – HBA interactions,
therefore Reichardt’s probe is not applicable due to its sensitivity to proton-donor species.
Nile Red dye was therefore used for its solvatochromic sensitivity even in presence of HBD
species[63].
This dye manifests a positive solvatochromism, thus the maximum of the wavelength
adsorption changes depending on the polarity of the media: higher polar media lead to longer
wavelengths of absorption. Nile Red has a large bathochromic shift: a change of over 100 nm
in λmax can be observed passing from water to pentane. For these reasons this dye is widely
used in determining empirical scales of polarity of different media[64].
In Table 6 are reported the values of λmax (in nm) observed for AO-12/PhAA, MO-12/PhAA
and MO/PhAA DESs and in another zwitterionic DES (glycolic acid/trimethylglicine mixture,
GA/TMG) at 25°C. Unfortunately, AO-18/PhAA DES was not studied with this dye because the
temperature studied (25°C) was close to its freezing point (20°C), therefore the dye was not
properly solubilized and gave unreproducible results. In the same table are reported the
values observed in literature in a choline-based DES (choline chloride/D-(+)-xylose mixture,
ChCl/Xyl), and in a typical ionic liquid ([BMIM][PF6], 1-butyl-3-methyl imidazolium
hexafluorophosphate)[65,66].
Table 6: Maximums of wavelength adsorption of Nile Red dye in the analyzed liquids.
λ
max, nm
546.7
549.7
561.3
AO-12/PhAA
MO-12/PhAA
MO/PhAA
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5

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GA/TMG
ChCl/Xyl
564.2
563.0
547.5
[
BMIM] [PF ]
6
In Figure 5 the maximum of wavelength absorption of a wide series of typical organic liquids
as well as water and the ones reported in Table 6) are reported as comparison; all the values
(
are reported in Supplementary data.
Figure 5: Maximum of wavelength absorption of Nile Red dye in a series of common organic
solvents, in water, in an ionic liquid ([BMIM] [PF6]), in a typical DES (ChCl/Xyl), in a
zwitterionic DES (GA/TMG), in three of our novel Amine-oxide DESs (MO-12/PhAA, AO-
12/PhAA, MO/PhAA mixtures).
As it can be observed, the polarity of these mixtures resulted high enough, with values among
typical organic solvents. The amphiphilic mixtures (AO-12/PhAA and MO-12/PhAA) showed
polarities that are similar to the ones of methanol (with values almost overlapped with AO-
12/PhAA DES) and of the ionic liquid [BMIM][PF6]; the non-amphiphilic DES MO/PhAA
showed a higher polarity, also higher than the one of acetic acid. The three novel DESs
analyzed showed a polarity that is lower than the one observed for the other zwitterionic DES
GA/TMG and also of the one of ChCl/Xyl mixture.
4. CONCLUSIONS
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Four novel amine N-oxide-based DESs were prepared and characterized; three of them are
formed by amphiphilic molecules. These liquids are prepared with eco-compatible
components: N-oxides (which are biodegradable) and phenylacetic acid (which is a molecule
present in honey). The novel mixtures showed very low freezing points, spanning from -34°C
to 20°C. Their densities are slightly over (AO-12/PhAA and MO/PhAA) or slightly under (AO-
18/PhAA and MO-12/PhAA) the density of water. It is relevant to notice that their viscosities
are very low, much lower than the ones of other zwitterionic DESs and of the most common
and used DES described in literature so far (ChCl/U). These values are very low even at 25°C,
promoting these liquids for their use at room temperature. Their conductivity values are very
low, much lower than the ones reported in literature for protic ionic liquids; this indicates
that the proton is not completely transferred from HBD to HBA molecule in these novel
liquids. Their position in the Walden plot is peculiar: they are located in the negative portion
both of molar conductivity and viscosity, but they are “super ionic” for their charge
separation. The polarity of these mixtures resulted high enough, and it can be compared with
the one of typical organic solvents or ionic liquids, with AO-12/PhAA almost overlapping
methanol value and MO-12/PhAA much similar to the one of [BMIM][PF6] ionic liquid.
MO/PhAA mixture showed the highest value in the set, but its polarity resulted lower than the
one of other zwitterionic DESs (GA/TMG) or commonly used DES (ChCl/Xyl).
These characteristics promote a use of these novel liquids in the same manners of other DESs
in literature but with advantageous properties compared to them.
Measures of solubilization capabilities of various polymers in these DESs are currently in
progress.
5. FUNDING
This research did not receive any specific grant from funding agencies in the public,
commercial, or not-for-profit sectors.
6. ACKNOWLEDGEMENTS
The authors would like to thank Dr Samantha Reale for ESI MS measurements.
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Graphical abstract
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HIGHLIGHTS





The preparation and the characterizations of four novel DESs are reported
The DESs are formed mixing green molecules (Phenylacetic acid and N-oxides)
The novel DESs have very low freezing points, spanning from -36°C to 20°C
The novel DESs have very low viscosity, lower than other common DESs in literature
The novel DESs have low conductivity, high enough polarity and are “super ionic” (via
Walden Plot)
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