Phase Behavior and Physical Properties of New Biobased Ionic Liquid Crystals

Article abstract of DOI:10.1021/acs.jpcb.7b01384

Protic ionic liquids (PILs) have emerged as promising compounds and attracted the interest of the industry and the academy community, due to their easy preparation and unique properties. In the context of green chemistry, the use of biocompounds, such as fatty acids, for their synthesis could disclose a possible alternative way to produce ILs with a low or nontoxic effect and, consequently, expanding their applicability in biobased processes or in the development of bioproducts. This work addressed efforts to a better comprehension of the complex solid-[liquid crystal]-liquid thermodynamic equilibrium of 20 new PILs synthesized by using fatty acids commonly found in vegetable oils, as well as their rheological profile and self-assembling ability. The work revealed that their phase equilibrium and physical properties are significantly impacted by the structure of the ions used for their synthesis. The use of unsaturated fatty acids and bis(2-hydroxyethyl)ammonium for the synthesis of these biobased ILs led to a drastic decreasing of their melting temperatures. Also, the longest alkyl chain fatty acids promoted higher self-assembling and more stable mesophases. Besides their sustainable appeal, the marked high viscosity, non-Newtonian profile, and very low critical micellar concentration values of the PIL crystals here disclosed make them interesting renewable compounds with potential applications as emulsifiers, stabilizers, thickeners, or biolubricants.

Full text of DOI:10.1021/acs.jpcb.7b01384

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Article
Phase Behavior and Physical Properties
of New Biobased Ionic Liquid Crystals
Ariel Antonio Campos Toledo Hijo, Guilherme José Maximo, Mariana
Conceição Costa, Rosiane Lopes Cunha, Jorge Fernando Brandão Pereira,
Kiki Adi Kurnia, Eduardo A. Caldas Batista, and Antonio J. A. Meirelles
J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b01384 • Publication Date (Web): 23 Mar 2017
Downloaded from http://pubs.acs.org on March 28, 2017
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Phase Behavior and Physical Properties of New
Biobased Ionic Liquid Crystals
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Ariel A. C. Toledo Hijoa, Guilherme J. Maximoa, Mariana C. Costab, Rosiane L.
Cunhaa, Jorge F. B. Pereirac, Kiki A. Kurniad, Eduardo A. C. Batistaa, Antonio J. A.
Meirellesa,*
a School of Food Engineering, University of Campinas, R. Monteiro Lobato 80, 13083-
862, Campinas, São Paulo, Brazil
b Department of Process and Products Design, School of Chemical Engineering,
University of Campinas, 13083-852, Campinas, Sao Paulo, Brazil
̃
c School of Pharmaceutical Sciences, Universidade Estadual Paulista, 14800-903,
Araraquara, Sao Paulo, Brazil
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d Department of Chemical Engineering, Universiti Teknologi PETRONAS, Bandar Seri
Iskandar, Perak 32610, Malaysia
*tomze@unicamp.br Tel.: (+55 19 3521 4056)
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KEYWORDS: ionic liquid, liquid crystal, fatty acid, solid-liquid equilibrium,
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rheology.
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ABSTRACT
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Protic ionic liquids (PILs) have emerged as promising compounds and attracted the
interest of the industry and the academy community, due to their easy preparation and
unique properties. In the context of green chemistry, the use of biocompounds, such as
fatty acids for their synthesis could disclose a possible alternative way to produce ILs
with low or non-toxic effect and consequently, expanding their applicability in bio-
based processes or in the development of bioproducts. This work addressed efforts to a
better comprehension of the complex Solid-[Liquid Crystal]-Liquid thermodynamic
equilibrium of twenty new PILs synthesized by using fatty acids commonly found in
vegetable oils, as well as their rheological profile and self-assembling ability. The work
revealed that their phase equilibrium and physical properties are significantly impacted
by the structure of the ions used for their synthesis. The use of unsaturated fatty acids
and bis(2-hydroxy ethyl)ammonium for the synthesis of these biobased ILs led to a
drastic decreasing of their melting temperatures. Also, the longest alkyl chain fatty acids
promoted higher self-assembling and more stable mesophases. Besides their sustainable
appeal, the marked high viscosity, non-Newtonian profile and very low critical micellar
concentration values of the PIL crystals here disclosed make them interesting renewable
compounds with potential applications as emulsifiers, stabilizers, thickeners or
biolubricants.
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INTRODUCTION
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In the last two decades, literature has proved the promising role of ionic liquids
(ILs) in a wide range of applications for the chemical and pharmaceutical industries,
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especially as alternative solvents in extraction processes . However, their properties
have attracted attention in several other fields, such as food, bioproducts and
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biomaterials industries . In fact, such potential application of ILs is based on their
unique and tunable properties, such as low vapor pressure, non-flammability, thermal
stability, self-assembling ability, and wide liquid phase range, that can be tailored by the
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right choice of ions . This functionality allows the synthesis of novel ILs for a given
process, taking into account specific constraints and conditions.
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ILs are classically classified in two groups: aprotic and protic . The aprotic ILs
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(APILs) have been widely studied and reported in literature ; otherwise, protic ILs
(PILs) have emerged during past years as “a second generation IL”, easily synthesized
through a BrØnsted acid-base reaction. This is the case of some bis(2-hydroxy
3c
ethyl)ammonium carboxylates ([H2EA][CnOO]) described by Maximo, et al.
and
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Álvarez, et al. in which a proton is transferred from a BrØnsted acid (carboxylic acid)
to a BrØnsted base (2-hydroxy ethylammonium). Because of the large possible
combination of ions for the synthesis of PILs, there is still a lack of studies on their
production and applications.
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Moreover, in the context of green chemistry , PILs based on compounds from
natural sources are still demanded. This is the case of fatty acids, which can be used to
easily form PILs with peculiar properties, such as high self-assembling and structuration
ability, ability to form mesophases structures and wide liquid phase range. This opens
perspectives on the application of PILs as surfactants, cosmetic or pharmaceutical
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creams, biolubricants or drug delivery systems 3c. Fatty acids are lipidic compounds
present in vegetable oils and formed due to the hydrolysis of the triacylglycerols
molecules. Because of this, they must be removed during oils refining process in order
to avoid negative sensorial profiles. Therefore, they represent an important industrial
coproduct, taking into account the forecast of the worldwide production of vegetable
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oils that will exceed 195 millions of tons until 2022 . In this context, PILs synthesized
by using fatty acids are interesting substances for “sustainable chemistry and
engineering” with low environmental impact 4. Remarkably, fatty acids are classified as
GRAS substances - Generally Recognized as Safe - by U.S. Food and Drug
Administration (FDA). Thus, the use of such biocompounds could disclose an
alternative way to produce ILs with low or possible non-toxic effect, and consequently,
expand their applicability in food related processes, or even in the development of
bioproducts.
Literature has shown that under certain temperature conditions, especially just
above their solid melting temperature, fatty acids based PILs are liquid crystals, due to
3c,
their long alkyl chain
9. This makes them interesting additives in terms of
applicability by mixing the properties of both PILs and liquid crystals, and leading to
the formation of Protic Ionic Liquid Crystals. In fact, PILs can present stronger
molecular interactions resulted from the sum of hydrogen bonding, Coulombic charge
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and dispersion forces
than the common aprotic ILs. This can also lead to the
obtainment of more stable mesophases 3c, i.e. mesophases with high temperature
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window, when compared to common ionic liquid crystals (ILCs) . Stable mesophases
attract industrial interest since such a property combined with the amphiphilic nature of
PILs crystals make them able to several applications as chemical reaction media, drug
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delivery systems, electrical conducting media, friction reduction agent or thickener
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However, in order to expand their applicability, taking into account the development of
products and processes, a complete understanding of their thermodynamic phase
behavior and physicochemical properties is required. This is because some of them can
present high viscosity, marked non-Newtonian rheological profile, and self-structuration
ability, which are properties observed in emulsifiers, stabilizers, thickeners and
biolubricants 13.
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In this way, this work was aimed at the synthesis of twenty new PILs by using
the most representative fatty acids found in the vegetable oils and 2-hydroxy
ethylamines. Their Solid-[Liquid Crystal]-Liquid (SLcL) thermodynamic equilibrium
behavior was characterized, as well as their rheological profile and critical micellar
concentrations. Results were related to size effects for both anion and cation as well as
to the presence of unsaturations in the alkyl chain length of the fatty acids.
EXPERIMENTAL SECTION
Materials. The materials used for the PILs synthesis were caprylic, capric,
lauric, myristic, palmitic, stearic, oleic and linoleic acids, 2-hydroxy ethylamine, bis(2-
hydroxy ethyl)amine and tris(2-hydroxy ethyl)amine, all with purity higher than 99.9%
(w/w) (Sigma Aldrich, St. Louis).
Synthesis of the ionic liquids and their characterization. Twenty PILs of 2-
hydroxy ethyl- ([HEA][CnOO]), bis(2-hydroxy ethyl)- ([H2EA][CnOO]) and tris(2-
hydroxy ethyl)ammonium carboxylates ([H3EA][CnOO]) (Table 1) were designed and
synthesized in the present work. Compounds were previously dried under vacuum
before the synthesis procedure. Samples of approximately 1 g of PILs were obtained
through a BrØnsted acid-base reaction (Scheme 1) by stirring fatty acid + amine
mixtures at 푥1 = 0.5, fatty acid molar fraction, in a thermal fluid bath, at 10 K above the
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liquid crystal melting temperature for approximately 1 hour, under nitrogen atmosphere,
in order to avoid oxidation of the compounds, and then cooled down to room
temperature. The water content of the PILs was verified by Karl Fisher titration
showing values lower than 0.001 molar fraction. The new PILs were characterized by
proton Nuclear Magnetic Resonance (1H NMR). All NMR spectra were recorded using
a Bruker 300 Fourier 300 MHz . 1H NMR (300 MHz) was collected using CDCl3 as the
solvent, shifts reported in σ (ppm) and values are presented in Electronic Supplementary
Information. Spectrums could confirm the PILs’ structures and the effectiveness of the
synthesis.
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Tris(2-hydroxy ethyl)ammonium
caprylate
Caprylic acid
Tris(2-hydroxy ethyl)amine
Scheme 1. BrØnsted acid-base reaction between caprylic acid and tris(hydroxy
ethyl)amine.
Differential Scanning Calorimetry (DSC). The melting temperatures (Tm) and
enthalpies (ΔHm) of the PILs were determined using a DSC8500 calorimeter
(PerkinElmer, Waltham, MA, USA) in a cooling-heating cycle at 1 K min-1 using an
adapted methodology presented by Costa, et al. 14 The PILs were heated from 298.15 K
to 393.15 K, annealed for 15 min to remove the previous thermal history, cooled down
to 193.15 K and maintained at this temperature for 40 min, and heated again to 393.15
K. Tm and ΔHm were obtained in the last heating run. Tm was considered as the peak top
temperature and ΔHm the area of the peak. The equipment was previously calibrated
using the standards indium (PerkinElmer, Waltham), naphthalene and cyclohexane
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(Merck, Whitehouse Station, NJ). The measurements were performed in triplicate at
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least. The experimental Tm values were compared with COSMO-RS predicted data.
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Table 1 Structures and abbreviations of PILs evaluated in this work
Cations*
Protic Ionic Liquids
bis(2-hydroxy
ethyl)-
tris(2-hydroxy
Anions
2-hydroxy ethyl-
[HEA][C8OO]
[HEA][C10OO]
[HEA][C12OO]
[HEA][C14OO]
[HEA][C16OO]
[HEA][C18OO]**
ethyl)-
Caprylate
Caprate
Laurate
[H2EA][C8OO]
[H2EA][C10OO]
[H2EA][C12OO]
[H2EA][C14OO]
[H2EA][C16OO]
[H3EA][C8OO]
[H3EA][C10OO]
[H3EA][C12OO]
[H3EA][C14OO]
[H3EA][C16OO]
Myristate
Palmitate
Stearate
Oleate
[H2EA][C18OO]** [H3EA][C18OO]
[HEA][C18:1OO]** [H2EA][C18:1OO]** [H3EA][C18:1OO]
[HEA][C18:2OO] [H2EA][C18:2OO] [H3EA][C18:2OO]
Linoleate
*Cations = 2-hydroxy ethyl-, bis(2-hydroxy ethyl)-, and tris(hydroxy ethyl)ammonium;
** Synthesized by Maximo, et al. 3c.
Light-Polarized Optical Microscopy (POM). The liquid crystalline
mesophases (LCMs) of the PILs were evaluated using a polarized optical
thermomicroscope (Leica Mikrosysteme CMS GmbH, Frankfurt, Germany) equipped
with a LTS420 Linkam temperature controlled stage (Linkam Scientific Instruments
Ltd., Tadworth, U.K.). Samples of approximately 2.0 mg were put in concave slides
with coverslips and submitted to the following cooling-heating program: Samples were
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previously cooled from 298.15 K to 233.15 K and heated from 233.15 K to 10 K above
the temperature of the disappearance of the mesophases at a rate of 0.1 K min-1. The
measurements were performed in triplicate at least.
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Critical Micellar Concentration (CMC) Measurements. The CMC of the
PILs were characterized by conductivity using an Orion 3 Star Model Conductivimeter
(Thermo Scientific, Beverly, MA, USA). Aqueous mixtures were prepared with
deionized water at a concentration of 0.001 M of PIL. The conductivity at each
concentration was obtained by successive dilutions. The relationship between the
conductivity and the concentration was plotted, and the CMC values were obtained
through the intersection of two successive linear behaviors.
Viscosity and Rheological Measurements. The rheological behavior and the
viscosity profile of the PILs were determined by using an AR 1500ex stress controlled
rotational rheometer (TA Instruments, New Castle, DE, USA). Flow curves were
obtained by using a stainless steel cone and plate geometry (diameter of 2 cm, angle of
2° and cone truncation of 57 µm) with an up-down-up step program, in which the shear
rate ranged from 0 to 300 s-1. Data were obtained at temperatures of 298.15 K, 338.15
K, 348.15 K and 358.15 K, taking into account the PILs Tm values and their liquid
crystal temperature domain as showed in Figure 1. Results obtained at the third step
program were used to characterize the steady-state behavior of the shear stress (휎) –
shear rate (γ̇) profile of the PILs.
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400
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[HEA][CnOO]
358
348
338
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200
[HE3A][CnOO]
[HE2A][CnOO]
Caprylate Caprate Laurate Myristate Palmitate Stearate Oleate
Linoleate
Carboxylate based PILs
Figure 1. Temperatures for the rheological measurements: 298.15 K, 338.15 K, 348.15
K and 358.15 K (dashed lines). Colored regions highlight the liquid crystal temperature
domain of the PILs: red for [HEA][CnOO], gray for [HE2A][CnOO] and green for
[HE3A][CnOO].
RESULTS AND DISCUSSION
Phase behavior and ionic liquid crystal characterization. The evaluation of
the phase behavior of the PILs synthesized revealed a SLcL phase transition profile with
a clear dependence on the structures of the ions used for the synthesis. The thermograms
of the three series of PILs, namely [HEA][CnOO], [H2EA][CnOO] and [H3EA][CnOO],
obtained by DSC, are shown in Figure 2. Melting temperatures (Tm) and enthalpies
(ΔHm) of the solid phase, obtained by DSC, and the transition temperature of the liquid-
crystal phase (TLCm), obtained by POM, are presented in Table 2 and sketched in Figure
3. One endotherm phase-transition peak was observed for each PIL thermogram (Figure
2), representing the melting point for each PIL, that varied from 257.88 K
([H2EA][C8OO]) to 339.23 K ([H3EA][C18OO]).
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According to the results, the melting temperature was directly related to the alkyl
chain length of the anion, which was evident for the three series of PILs. It means that,
the increase of the alkyl chain length of the anion promoted an increasing of the melting
temperatures. This increasing can be explained by the formation of more regular
packing arrangements, when long alkyl carbon chains induce the formation of bilayer-
type crystal structures with higher molecular interactions. A similar effect on the
melting behavior was also reported for N-alkyl-N-methylimidazolium based ILs 15. In
these cases, ILs with more than 8 carbons led to melting temperature increasing.
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Biswas, et al. reported that, for imidazolium based ILs the melting temperature can
increase by 18 K when the chain length of the carboxylate anion is increased by two
carbon atoms.
[H3EA][C8OO]
[H3EA][C10OO]
[HEA][C8OO]
[HEA][C10OO]
(A)
(B)
(C)
[H2EA][C8OO]
[H2EA][C10OO]
[H2EA][C12OO]
257.88K
313.38K
322.63K
308.44K
280.12K
316.71K
[H3EA][C12OO]
[H3EA][C14OO]
[H3EA][C16OO]
[HEA][C12OO]
292.40K
323.30K
323.32K
[H2EA][C14OO]
[H2EA][C16OO]
[HEA][C14OO]
[HEA][C16OO]
329.44K
294.31K
329.16K
335.58K
315.21K
337.70K
[H3EA][C18OO]
[H3EA][C18:1OO]
339.23K
294.35K
[H3EA][C18:2OO]
[HEA][C18:2OO]
[H2EA][C18:2OO]
277.71K
268.51K
253 263 273 283 293 303 313 323 333 343 353 253 263 273 283 293 303 313 323 333 343 353 253 263 273 283 293 303 313 323 333 343 353
Temperature(K) Temperature(K) Temperature(K)
Figure 2. DSC thermograms for (A) 2-hydroxy ethyl- ([HEA][CnOO]), (B) bis(2-
hydroxy ethyl)- ([H2EA][CnOO]) and (C) tris(2-hydroxy ethyl)ammonium carboxylates
([H3EA][CnOO]), synthesized in this work, during the second heating ramp at 1 K min-
1. DSC thermograms for [HEA][C18OO], [HEA][C18:1OO], [H2EA][C18OO] and
[H2EA][C18:1OO] were not reported by Maximo, et al. 3c.
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Figure 3 shows that the alkyl chain length effect on the Tm was more pronounced
for the [H2EA][CnOO] series, where the increase of the anion from caprylate (C10) to
stearate (C18) resulted in a Tm increasing of 81.6 K. The [HEA][CnOO] and
[H3EA][CnOO] series showed an increasing of 38.27 K and 30.79 K. On the other hand,
the increasing of the cation size from HEA to H2EA resulted in a reduction of the
melting temperature of up to 28.98 K and from HEA to H3EA in a reduction of up to 3.5
K. It means that the H2EA cation was able of lowering considerably the Tm of
carboxylates based ILs. The probable more asymmetric nature of the H2EA cation in the
PIL molecular structure when compared with HEA and H3EA ions (Table 1), probably
promoted a disordered system, minimizing the anion-cation interactions, and inducing a
further reduction of the Tm 17. Similar effects of the symmetry of the cation on the
melting behavior of ILs were reported in literature 18. This is the case of low melting ILs
based on aliphatic quaternary ammonium, reported by Zhou, et al. 18b, synthesized by
using cations with low symmetry. In addition, it is worth notice that the reduction of the
PILs Tm was more intense for carboxylates with shorter chain lengths. This indicates
that the molecular disordering effect of the cation increased as the alkyl chain length of
the anion decreased.
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6
7
8
400
350
Tm – [HEA][CnOO]
TLCm – [HEA][CnOO]
Tm – [H2EA][CnOO]
TLCm – [H2EA][CnOO]
Tm – [H3EA][CnOO]
TLCm – [H3EA][CnOO]
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59
60
300
250
200
Caprylate Caprate Laurate Myristate Palmitate Stearate Oleate
Linoleate
Carboxylatebased PILs
Figure 3. Solid-[Liquid-Crystal]-Liquid phase transition for PILs. Tm and TLCm
represent the melting temperature of the solid phase and the transition temperature of
the liquid crystal phase, respectively.
In a thermodynamic point of view, the impact of the symmetry of the cations and
the alkyl chain length of the anions on the Tm of the PILs could be explained in terms of
their melting Gibbs energy. The disordering of the system due to the asymmetry of the
cation probably leads to systems with higher entropy values. This is also observed in
case of presence of unsaturations in the anion chain. Moreover, results showed an
increasing in ΔHm as Tm increased (Table 2). It means that PILs presenting high values
of Tm presented high values of ΔHm. Also, since ΔHm of the [H2EA][CnOO] series were
lower than for the other series, one can probably assume that the asymmetry of the
cation decreased both Tm and ΔHm. Therefore, with high entropy values and low ΔHm
19
values one can assume that the melting Gibbs energy tends to be negative , denoting
that the melting of the system is thermodynamically favorable, resulting in low-melting
ILs with lower melting enthalpies.
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Table 2. Phase transition temperatures and thermodynamic data for different PILs
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2-hydroxy ethylammonium carboxylates
Tm/K
PILs
Tm/K
TLCm/K
ΔHm/kJ mol-1
(Cosmo)
174.26
180.31
185.12
188.90
192.11
194.72
198.26
202.42
9
[HEA][C8OO]
[HEA][C10OO]
[HEA][C12OO]
[HEA][C14OO]
[HEA][C16OO]
[HEA][C18OO] a
[HEA][C18:1OO] a
[HEA][C18:2OO]
313.38 ± 0.22
322.63 ± 0.39
323.32 ± 0.18
329.16 ± 0.12
337.70 ± 0.08
351.65 ± 0.40
288.08 ± 0.40
277.71 ± 0.01
342.32 ± 0.71
375.18 ± 0.31
369.28 ± 0.65
355.92 ± 0.31
364.88 ± 0.35
373.15 ± 1.00
358.15 ± 1.00
349.15 ± 1.00
27.10 ± 0.64
28.51 ± 0.76
31.14 ± 0.65
33.40 ± 1.07
35.22 ± 1.55
*
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58
59
60
*
39.58 ± 2.60
bis(2-hydroxy ethyl)ammonium carboxylates
Tm/K
(Cosmo)
PILs
Tm/K
TLCm/K
ΔHm/kJ mol-1
3.41 ± 1.03
9.92 ± 0.39
13.55 ± 0.24
22.05 ± 0.27
28.35 ± 1.01
*
190.13
195.57
199.85
203.18
205.99
208.25
211.95
216.23
[H2EA][C8OO]
[H2EA][C10OO]
[H2EA][C12OO]
[H2EA][C14OO]
[H2EA][C16OO]
[H2EA][C18OO] a
257.88 ± 0.40
280.12 ± 0.35
292.40 ± 0.05
294.31 ± 0.18
315.21 ± 0.11
339.48 ± 0.40
-
303.95 ± 0.71
341.15 ± 0.46
346.58 ± 0.49
378.35 ± 0.40
380.15 ± 1.00
360.95 ± 1.00
339.55 ± 1.00
[H2EA][C18:1OO] a 283.60 ± 0.40
*
[H2EA][C18:2OO]
-
-
tris(2-hydroxy ethyl)ammonium carboxylates
Tm/K
(Cosmo)
199.77
PILs
Tm/K
TLCm/K
-
ΔHm/kJ mol-1
23.75 ± 1.70
27.95 ± 1.31
32.55 ± 1.82
35.67 ± 2.23
41.30 ± 0.64
48.44 ± 1.87
38.06 ± 2.21
10.76 ± 1.37
[H3EA][C8OO]
[H3EA][C10OO]
[H3EA][C12OO]
[H3EA][C14OO]
[H3EA][C16OO]
[H3EA][C18OO]
[H3EA][C18:1OO]
[H3EA][C18:2OO]
308.44 ± 0.18
316.71 ± 0.29
323.30 ± 0.36
329.44 ± 0.25
335.58 ± 0.45
339.23 ± 0.29
294.35 ± 0.39
268.51 ± 0.06
204.89
208.94
212.07
214.70
216.80
220.43
224.61
-
326.12 ± 0.06
344.78 ± 0.83
358.95 ± 1.00
366.65 ± 1.04
359.55 ± 1.00
347.35 ± 1.00
a Maximo, et al. 3c. * Values not reported by the authors.
Another notable effect of the alkyl chain nature of the anion on the Tm was due
to the unsaturation for [HEA][C18:1OO], [HEA][C18:2OO], [H2EA][C18:1OO],
[H2EA][C18:2OO], [H3EA][C18:1OO] and [H3EA][C18:2OO]. The unsaturations in the
anions oleate (C18:1) and linoleate (C18:2) compared with the saturated stearate (C18)
anion, were responsible for a quite reduction of the Tm, promoting a decreasing of up to
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54.77 K and 72.33 K, respectively. The influence of the double bonds in the structure of
the unsaturated anion based PILs is related to the decreasing of the chain symmetry, and
a consequent destabilization of their packing arrangement, similarly to what was
previously discussed. In addition, the [H2EA][C18:2OO] thermogram did not show any
melting endotherm peak in the range evaluated (Figure 2), indicating a significant
influence of the H2EA and the unsaturations in the disordering of the molecule in this
case, affecting the crystalline nature of the PIL 16, or even inducing a deep reduction of
the Tm below the temperature range used for the DSC analysis.
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Among the 24 evaluated PILs, being 20 synthesized here and 4 presented by
Maximo, et al. 3c, [H2EA][C8OO] and [H3EA][C18:2OO] presented the lower Tm of
257.88 K and 268.51 K, respectively. The higher Tm value (351.65 K) was observed for
3c
[HEA][C18OO] . These results are in agreement with the previous analysis. It means
that [H2EA][C8OO] contain the most asymmetric cation, and the shortest alkyl chain
anion; [H3EA][C18:2OO] is the PIL that contain the most bulky cation and higher
unsaturated structure. Otherwise, [HEA][C18OO] contains the smallest cation and the
longest anion, which increased their stability and consequently their melting
temperature.
The Tm profile of the ILs was also predicted by COSMO-RS as presented in
Table 2. However, results showed low accuracy. This behavior may be related to the
formation of mesophases after the melting of the solid phase of the PIL, what will be
discussed later, that could have probably influenced the prediction of the PILs physical
properties. This is relevant because COSMO-RS have been successfully used for the
prediction of APILs’ physical properties, highlighting that improvements of the
modeling in case of ionic liquid crystals are still required.
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Depending on the alkyl chain length and established interactions, PILs can
present oriented microstructures with crystalline arrangements but with flow properties
at temperatures above the melting point of the solid phase 3c, 11a. This ionic liquid
crystalline state is called “mesophase” and could be qualified by POM. The textures (as
micrographs of liquid crystals are often called) of the PILs mesophases are shown in
Figure 4 and 5. Most of the mesophases presented layered structures, what is typical for
smectic (Sm) phases (for mesophases types see Axenov and Laschat 20) and represented
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21
by focal conic fan-shaped textures . According to some authors, amphiphilic ILs with
long alkyl chain lengths tend to form smectic type phases, since they can align
themselves in layers due to a microsegregation effect (an intermolecular rearrangement
of polar and apolar sites of the molecule) 20, 22. Lamellar structures, possibly related to
bilayered Smectic A (SmA) phases, represented by typical marble textures and/or
maltase crosses, were also observed among the POMs. This was probably due to the
size and linearity of the alkyl-chain of the anion that could self-assembly similarly to
the behavior observed in organic lipid membranes. This could be interesting for the
design of active pharmaceutical ingredients or lipid-based food structuration agents.
These results are in agreement with some works published in literature on the
characterization of ionic liquid crystals (ILCs) with long alkyl chain length, in which
11a
smectic phases were observed 11c, 20, especially SmA phases 11a, 11b, 22b. Wang, et al.
identified focal conic textures and maltese crosses for 1,3-dialkylimidazolium
tetrafluoroborate [CnCnim][BF4] with long alkyl chain. Rohini, et al. 11b also found SmA
mesophase structures for 1,3-dialkylimidazolium ILs ([(C8)2-Im][Cl.H2O]). However,
what is quite relevant is that the liquid PILs of this work presented a larger LC window
(temperature domain) in comparison to these common APILs, what will be further
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discussed later. This is probably due to the weak molecular interactions of the APILs
compared to the PILs.
7
8
9
The longest alkyl chain length PILs presented a higher ability to self-organize
and packing, forming stronger mesophases structures, i.e. mesophases with high
temperature window. This is because the alkyl chain length increasing probably leads to
stronger van der Waals molecular interactions between long carboxylates, favoring the
microsegregation of the structures, it means, their pairing. Otherwise, short alkyl chain
length PILs formed weak mesophases structures. This was more evident for [HE3A]-
based PILs (Figure 4). Mosaic-like textures was clearly observed in case of
[H3EA][C16OO] (Figure 4-I) and little evident as the alkyl chain length decreased (see
[H3EA][C14OO], Figure 4-F and [H3EA][C12OO], Figure 4-C). On the other hand,
[HEA]- and [H2EA]-based PILs showed more compacted and structured mesophases.
Thus, the use of a bigger and symmetric cation such as [H3EA] and short alkyl chain
length anions was not apparently the right choice for the synthesis of more structured
ILCs.
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59
60
The ILC temperature domain, i.e. the temperature range in which the mesophase
still appears above the Tm is presented in Table 2 and Figure 3. Figure 5 shows
successive micrographs taken during the POM evaluation for [HEA][C18:2OO],
[H2EA][C14OO] and [H3EA][C18:2OO], in which the solid-PIL is melted and further
heated to form an isotropic liquid-PIL, and thus determining the TLCm. POM showed
four phase profiles during heating: a marble texture, a marble texture with maltese
crosses, a marble texture with an isotropic background, and maltese crosses in an
isotropic background. The analysis showed that the mesophase range above Tm varied
from 3 to 79 K, depending on the structure of the ions and a dependence on the anion
alkyl chain length was observed for [H2EA][CnOO] and [H3EA][CnOO] series. Among
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the saturated ILs, [H2EA][CnOO] presented the higher ILC domain. Otherwise,
[H3EA][CnOO] presented the shorter ILC temperature domain in which mesophases
were not observed for [H3EA][C8OO] and [H3EA][C10OO]. These results corroborate
with the fact that the PILs ability to form mesophases are dependent on the symmetry of
the cation and the size of the alkyl chain of the anion, which was previously discussed.
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Unsaturated ILs presented the highest ILC temperature domain of up to 73 K. In
fact, the ILC temperature range deeply increased from stearate to oleate (ΔT = 41 K)
and to linoleate (ΔT = 45 K). In fact, unsaturated PILs presented well-structured
mesophases at room temperature, which is an interesting behavior taking into account
that their non-Newtonian profile was maintained throughout such a temperature range,
which is interesting in some low and/or high temperature application. This will be
discussed later.
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2-hydroxy ethylammonium bis(2-hydroxy ethyl)ammonium tris(2-hydroxy ethyl)ammonium
(C)
(B)
(A)
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50 µm
50 µm
50 µm
(F)
(D)
(E)
50 µm
50 µm
50 µm
(I)
(H)
(G)
50 µm
50 µm
50 µm
Figure 4. Thermomicrographs showing the mesophases textures of (A) 2-hydroxy
ethylammonium laurate ([HEA][C12OO]) at 351.95 K, (B) bis(2-hydroxy ethyl)ammonium
laurate ([H2EA][C12OO]) at 338.55 K, (C) tris(2-hydroxy ethyl)ammonium laurate
([H3EA][C12OO]) at 325.75 K, (D) 2-hydroxy ethylammonium myristate ([HEA][C14OO]) at
351.25 K, (E) bis(2-hydroxy ethyl)ammonium myristate ([H2EA][C14OO]) at 340.55 K, (F)
tris(2-hydroxy ethyl)ammonium myristate ([H3EA][C14OO]) at 334.75 K, (G) 2-hydroxy
ethylammonium palmitate ([HEA][C16OO]) at 353.15 K, (H) bis(2-hydroxy ethyl)ammonium
palmitate ([H2EA][C16OO]) at 346.85 K and (I) tris(2-hydroxy ethyl)ammonium palmitate
([H3EA][C16OO]) at 343.15 K.
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(B)
(A)
(C)
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50 µm
50 µm
50 µm
(F)
(D)
(E)
50 µm
50 µm
50 µm
(I)
(G)
(H)
50 µm
50 µm
50 µm
Figure 5. Thermomicrographs showing the Liquid-crystal-Liquid phase transition: (A)
marble texture of [HEA][C18:2OO] at 323.15 K; (B) marble texture with maltese crosses
of [HEA][C18:2OO] at 349.05 K; (C) marble texture with maltese crosses in an isotropic
background of [HEA][C18:2OO] at 349.05 K; (D) marble texture of [H2EA][C14OO] at
340.55 K; (E) marble texture with maltese crosses of [H2EA][C14OO] at 345.25 K; (F)
maltese crosses in an isotropic background of [H2EA][C14OO] at 346.05 K; (G) marble
texture of [H3EA][C18:2OO] at 346.15 K; (H) marble texture with an isotropic
background of [H3EA][C18:2OO] at 359.55 K; (I) isotropic background and marble
texture of [H3EA][C18:2OO] at 359.55 K.
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Self-assembling ability of PILs. The synthesis of PILs through the BrØnsted
acid-base reaction promotes the formation of amphiphilic compounds (Scheme 1), since
their structure presents hydrophilic and hydrophobic moieties (represented by the 2-
hydroxy ethylammonium ion as cation and the carboxylate ion as anion, respectively).
As observed, their self-assembling ability promotes the formation of liquid crystals
matrices 21, 23, at a given temperature condition. Authors in literature have shown that
PILs in a solution with hydrophilic or hydrophobic solvents tend to self-organize at a
given concentration in different micelles structures, such as spherical, cylindrical,
hexagonal or in bilayers 24. Such behavior is commonly observed for amphiphilic
compounds in aqueous solutions, such as for surfactants. Considering the self-
assembling and LC profile of the PILs of this work, their critical micellar concentration
(CMC) was determined.
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The dependence of conductivity with the concentration of the diluted PILs in
water is shown in Figure 6, as well as their CMC values. A linear dependence with two
or three shifts in their slope was observed for all series of PILs, depending on their
structures. PILs with long alkyl chain length presented two CMC values, which mean
that those compounds tend to self-aggregate in two different micelles arrangements
during the concentration increasing. For the qualification of the micellar structure
formed, their molecular geometry constraints, also known as critical packaging
25
parameter (CPP) was determined. The CPP is an indicator for the preferred type of
micellar structure formed when an amphiphilic compound is diluted 24a. In this work,
PILs aggregated in spherical micelles after the first CMC (CMC1) value, since their
CPP values were always lower than 1/3. This means that the polar head groups (2-
hydroxy ethylammonium) are interacting with the hydrophilic medium. After the
second CMC (CMC2) value they probably self-organize in more complex structures,
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such as cylindrical or bilayer. In fact, bilayer micellar structures were already reported
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3c
for stearate based PILs and cylindrical micelles for unsaturated oleate based PILs.
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Interestingly, as the alkyl chain length of PIL structures decreased, the CMC1 value
increased. This behavior has been also reported by other authors 26. Considering that the
lower the CMC1 value, the higher the PIL ability to form spherical micelles, and thus,
PILs with long alkyl chain length presented better surfactant ability. Among the 24
evaluated PILs, [H3EA][C16OO] showed the lowest CMC1 value of 0.0352 mM. Also,
among the saturated [H3EA][CnOO] series, the CMC1 values decreased up to
[H3EA][C16OO] when they started to increase. This means that despite the fact that
[H2EA]-based PILs presented the most structured mesophase, [H3EA][C16OO] showed
the best micellar forming ability. This shows the clear relationship between the cation
(length of the hydrophilic moiety) and the anion (size of the hydrophobic moiety),
which rules the surfactant efficiency of such compounds.
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Most of the evaluated PILs presented two CMC values, excepted for shorter
alkyl chain based PILs, that do not tend to form more complex micellar structures. In
fact, PILs with short alkyl chain have lower ability to self-aggregate and higher
concentration of PILs is required for obtaining spherical micelle structures when diluted
in water. Among all evaluated PILs, [H3EA][C8OO] showed the higher CMC1 value of
0.5281, which are the PIL composed by the largest cation and the smallest anion.
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PIL
CMC1 CMC2
[HEA][C8OO]
(A) [HEA][C OO]
0.2621
0.2041
-
-
8
[HEA][C OO]
10
[HEA][C OO]
0.1958 0.9100
0.1764 0.7020
0.1688 0.4866
12
[HEA][C OO]
14
[HEA][C OO]
16
[HEA][C18:2OO] 0.2330 0.6807
8
9
10
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25
[HEA][C16OO]
0
0.0
1.0
0.5
1.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
PIL / mM
120
PIL
CMC1 CMC2
[H2EA][C8OO]
(B)
[H2EA][C8OO] 0.3174
[H2EA][C10OO] 0.1741
-
-
[H2EA][C12OO] 0.1183 0.8519
[H2EA][C14OO] 0.1147 0.4123
[H2EA][C16OO] 0.0992 0.4667
[H2EA][C18:2OO] 0.1378 0.5853
90
60
30
[H2EA][C16OO]
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.0
0.5
1.0
PIL / mM
60
[H3EA][C8OO]
(C)PIL CMC1 CMC2
0.5281
0.4819
-
-
[H3EA][C8OO]
[H3EA][C10OO]
[H3EA][C12OO]
[H3EA][C14OO]
[H3EA][C16OO]
0.27790.6826
0.06880.3356
0.03520.2074
45
30
[H3EA][C18OO]] 0.06070.4074
[H3EA][C18:1OO
0.09270.4780
[H3EA][C18:2OO 0.09400.3505
]
]
]
]
[H3EA][C16OO]
15
0
0.0
0.5
1.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
PIL / mM
Figure 6 Conductivity (κ) as function of concentration (mM) of (A) 2-hydroxy ethyl- ([HEA][CnOO]), (B) bis(2-
hydroxy ethyl)- ([H2EA][CnOO]) and (C) tris(2-hydroxy ethyl)ammonium carboxylates ([H3EA][CnOO]) aqueous
2
22
solutions. Solid lines represent the linear fitting of the data (R > 0.98) for the determination of critical micellar
concentration (CMC).
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Rheological characterization of the PILs. For the rheological characterization
of the PILs synthesized in this work, flow curves were obtained at different
temperatures above PILs Tm, ranging from 298.15 K to 358.15 K and presented in
Figure 7 and 8. The temperatures were chosen taking into account the PILs Tm values
and the liquid crystal temperature domain determined by POM (this was sketched in
Figure 1). Thus, groups of PILs presenting similar Tm and LC domain were studied at
the same temperature values.
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Data were fitted to the Herschel-Bulkley (eq 1) model 27 and their parameters are
presented in Table 3.
휎 = 휎0 + 퐾γ푛
̇
(1)
where 휎 is the shear stress (Pa), γ
̇
is the shear rate (s-1); 휎0 is the yield stress (Pa); 퐾 is
the consistency index; 푛 is the flow behavior index.
According to the results, PILs with shorter alkyl chain tend to present a
Newtonian rheological profile. All caprylate (C8) and caprate (C10) based PILs
presented Newtonian behavior (Figure 7B, Table 3), i.e. n = 1 and σ0 = 0 (eq 1), except
for [HEA][C10OO]. [H3EA][C12OO] and [H3EA][C14OO] also showed a Newtonian
profile at 338.15 K. Otherwise, most of the PILs with alkyl chain length higher than
C10 presented non-Newtonian rheological behavior with a marked yield stress (σ0) and
n < 1, i.e. they are Herschel-Bulkley (HB) fluids. This is probably due to the strong
molecular interactions, leading to packaged structures that require initial shear stresses
for flowing. The exception is for [HEA][C16OO] with σ0 = 0, showing a shear-
thickening profile. Moreover, results showed that the increase of the size of the cation
from HEA to H2EA or H3EA also clearly impacted their rheological profile. The
[H3EA][CnOO] series presented Newtonian behavior more pronounced than for
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[H2EA][CnOO] and [HEA][CnOO] series. This corroborates with the ILC temperature
window profile. Otherwise, unsaturated based PILs were less influenced by the size of
the cation, and were all well-adjusted by the non-Newtonian HB model.
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1400
(A)
[HEA][C18:2OO]
1200
[HE2A][C18:2OO]
[HE3A][C18:1OO]
[HE3A][C18:2OO]
1000
800
600
400
200
0
0
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500
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200
100
0
100
50
0
[HEA][C8OO]
[HEA][C10OO]
[HEA][C12OO]
[HEA][C14OO]
[HE2A][C14OO]
[HE3A][C10OO]
[HE3A][C12OO]
[HE3A][C14OO]
[HE3A][C16OO]
(B)
N
N
N
N
0
100
200
300
0
50
100
150
γ/̇
s-1
200
250
300
Figure 7. Shear stress (휎) versus shear rate (γ̇) curves of several protic ionic liquids
(PILs) at (A) 298.15, (B) 338.15 K. Solid lines are the fitted curves for Herschel-
Bulkley model.
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Figure 8 shows the flow curves with the up-down-up step program. Figure 8A
represents a typical profile for most of the PILs. It means that, in general, there was no
dependence with the time (no thixotropy), and the same shear rate/shear stress behavior
was maintained after two steps of shear stress variations. This behavior is quite
interesting, considering their use as lubricants in successive shear stresses processes,
which could promotes their reuse and improves their self-life. Otherwise, several
[HEA][CnOO] presented a marked dependence with the time (Figure 8B-E), i.e. a clear
thixotropic behavior. This means that the shear rate/shear stress curves changed at the
third step, presenting a reduction of the initial flow curve area of 26%, 24.36%, 75.02%
and 46.66% for [HEA][C12OO], [HEA][C14OO], [HEA][C16OO] and [HEA][C18OO],
respectively. Moreover, a slight variation of the stress-strain behavior was observed for
higher strain values. This was characterized by the “zig zag” behavior of the curves,
clearly observed at shear rates higher than 100 s-1 (Figure 8A-C). This phenomenon
may be attributed to a breaking-recovery process due to the packing and self-assembling
ability of these PILs, even after high shear rates. This is peculiarly interesting and
corroborates to what was previously observed about applying these compounds in
lubrication processes.
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Unexpectedly, [HEA][C18OO] (Figure 8E) showed also an irreversible
disrupting of its structure, in which the flow behavior was decharacterized at the 3rd
ramp. This shows that, although large anions presented higher self-assembling ability,
the thixotropy of PILs exhibit clear specific constraints related to the anion size.
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500
400
1st step
2nd step
3rd step
(B)
(A)
1st step
2nd step
3rd step
1.47 Pa·s
6
7
8
4.05 Pa·s
9
200
250
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1.30 Pa·s
3.00 Pa·s
0
0
0
50
100
150
γ/̇
s-1
200
250
300
0
50
100
150
γ/̇
s-1
200
250
300
400
40
(D)
(C)
1st step
2nd step
3rd step
1st step
2nd step
3rd step
1.38 Pa·s
0.14 Pa·s
3.60 Pa·s
200
20
0.50 Pa·s
0.05 Pa·s
0.03 Pa·s
1.04 Pa·s
2.52 Pa·s
0
0
0
50
100
150
200
250
300
0
50
100
150
200
250
300
γ/
̇
s-1
γ/̇
s-1
140
1st step
2nd step
3rd step
1.28 Pa·s
(E)
2.09 Pa·s
70
0.90 Pa·s
0.17 Pa·s
0
0
50
100
150
γ/̇
s-1
200
250
300
Figure 8. Typical shear stress (σ) versus shear rate (γ̇) curves with an up-down-up step
program of (A) protic ionic liquids (PILs), (B) 2-hydroxy ethylammonium laurate
([HEA][C12OO]) at 338 K, (C) 2-hydroxy ethylammonium myristate ([HEA][C14OO])
at 338 K, (D) 2-hydroxy ethylammonium palmitate ([HEA][C16OO]) at 358 K, (E) 2-
hydroxy ethylammonium stearate ([HEA][C18OO]) at 358 K. Arrows indicate apparent
viscosity (η).
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Table 3. Rheological models’ parameters related to shear stress/shear rate curves
fitted to flow experimental data and apparent viscosity for the PILs
T = 298.15 K
PILs
σ0/Pa
K¥
nǂ
R2
η/ Pa∙s at 10 s-1
η/ Pa∙s at 50 s-1 η/ Pa∙s at 150 s-1
10
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[HEA][C18:1OO] a, b 74.22
-
-
-
-
-
-
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[HEA][C18:2OO]
[HE2A][C8OO]
[HE2A][C10OO]
[HE2A][C12OO]
80.02 21.6517 0.6490 0.9672 14.8993 ± 0.4058 7.3575 ± 0.5551 4.2155 ± 0.0530
0.0 0.8968 1.0000 0.9999 0.9621 ± 0.0089 0.9235 ± 0.0083 0.8931 ± 0.0391
0.0 1.6832 1.0000 0.9999 1.7415 ± 0.0183 1.7110 ± 0.0099 1.6890 ± 0.0255
79.41 17.7422 0.7282 0.9706 13.9506 ± 0.1612 7.2915 ± 0.2737 5.0055 ± 0.0587
[HE2A][C18:1OO] a 14.34 10.8220 0.8730 0.9520
-
-
-
[HE2A][C18:2OO]
[HE3A][C8OO]
[HE3A][C18:1OO]
[HE3A][C18:2OO]
75.33 11.7840 0.7639 0.9739 13.1407 ± 0.7458 6.0395 ± 0.0318 4.5150 ± 0.0240
0.0 0.9051 1.0000 0.9999 0.9563 ± 0.0424 0.9163 ± 0.0438 0.9081 ± 0.0432
85.10 13.1666 0.7552 0.9723 14.5984 ± 0.0477 6.7035 ± 0.0163 4.5300 ± 0.0721
68.27 11.1810 0.7401 0.9791 12.8212 ± 0.3580 5.9740 ± 0.3776 3.5620 ± 0.1160
T = 338.15 K
PILs
σ0/Pa
K¥
nǂ
R2
η/ Pa∙s at 10 s-1
η/ Pa∙s at 50 s-1 η/ Pa∙s at 150 s-1
[HEA][C8OO]
[HEA][C10OO]
[HEA][C12OO]
[HEA][C14OO]
[HE2A][C14OO]
[HE3A][C10OO]
[HE3A][C12OO]
[HE3A][C14OO]
[HE3A][C16OO]
0.0 0.2062 1.0000 0.9999 0.2514 ± 0.0034 0.2144 ± 0.0052 0.2047 ± 0.0004
38.48 14.0892 0.5803 0.9475 8.0467 ± 0.1676 3.7710 ± 0.5049 1.7235 ± 0.0064
39.73 23.5866 0.3833 0.9141 7.3712 ± 0.3303 3.1545 ± 0.2171 1.2155 ± 0.1195
38.69 13.7009 0.4441 0.8732 7.3830 ± 0.0995 2.6030 ± 0.1216 0.9599 ± 0.1148
30.69 7.3195 0.6615 0.9889 5.1689 ± 0.1042 2.7450 ± 0.0014 1.5465 ± 0.0219
0.0 0.1250 1.0000 0.9689 0.1694 ± 0.0035 0.1313 ± 0.0035 0.1250 ± 0.0034
0.0 0.0333 1.0000 0.9999 0.0476 ± 0.0018 0.0360 ± 0.0004 0.0340 ± 0.0002
0.0 0.0520 1.0000 0.9982 0.1018 ± 0.0031 0.0639 ± 0.0025 0.0548 ± 0.0021
25.33 4.6804 0.7154 0.9924 4.3020 ± 0.2631 2.1690 ± 0.0014 1.3030 ± 0.0028
T = 348.15 K
PILs
σ0/Pa
K¥
nǂ
R2
η/ Pa∙s at 10 s-1
η/ Pa∙s at 50 s-1 η/ Pa∙s at 150 s-1
[HE2A][C18OO]
[HE3A][C18OO]
28.75 13.7681 0.4780 0.9556 5.7635 ± 0.1977 2.8015 ± 0.0926 1.2015 ± 0.1280
14.18 4.8281 0.5636 0.9649 2.3828 ± 0.1561 1.0820 ± 0.0269 0.7107 ± 0.0204
T = 358.15 K
PILs
σ0/Pa
0.0 0.3382 0.5351 0.9665 0.1356 ± 0.0250 0.0452 ± 0.0059 0.0284 ± 0.0038
3.4638 ± 0.3527 0.9290 ± 0.0427 0.2980 ± 0.0441
K¥
nǂ
R2
η/ Pa∙s at 10 s-1
η/ Pa∙s at 50 s-1 η/ Pa∙s at 150 s-1
[HEA][C16OO]
[HEA][C18OO] *
[HE2A][C16OO]
-
-
-
-
21.59 6.9517 0.5937 0.9768 4.1711 ± 0.3152 2.0885 ± 0.0049 0.9991 ± 0.0183
¥
ǂ
퐾 for Herschel-Bulkley model. 푛 for Herschel-Bulkley model. η is the apparent viscosity.
*[HEA][C18OO] flow curve did not show shear stress/shear rate dependence in the 3rd step.
a Maximo, et al. 3c. b data was obtained by using other model.
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Figure 9 sketched the apparent viscosity of the PILs as a function of the shear
rate and temperature (according to Table 3). For the non-Newtonian PILs an
exponential decay of the apparent viscosity was observed as the shear rate increased,
especially at lower values, up to 100 s-1. Above this shear rate the decay was lower or
non-pronounced. High apparent viscosity values were observed for PILs with long alkyl
chains, increasing from laurate (C12) to stearate (C18), including the unsaturated PILs.
This is noteworthy because oleate and linoleate based PILs presented lower melting
temperature and a very interesting viscosity profile at room temperature, similar to a
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gel-like behavior. In fact, high viscosity values are typically observed for ILs with
3c, 11a, 11c
longer alkyl chain (C12-C18)
and this is attributes to the alkyl chain length and
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the increasing of the van der Waal molecular interactions . On the other hand, in the
[HEA][CnOO] series, the caprate based PIL presented viscosity values higher than
laurate (C12) and myristate (C14). This corroborates with the fact that the lattice energy
of the molecules is related to a balance between the symmetry and size of the ions. It
means that in case of [HEA][C10OO], the symmetry/size of the anion and the cation are
such that they promoted the increase of the viscosity of the PIL. Otherwise, the decrease
of the anion from caprate (C10) to caprylate (C8), in this case decreased the values of
the viscosity, and this is also in agreement to the fact that the caprylate based PILs
presented low mesophase formation ability. The high viscosity, marked non-Newtonian
behavior in a wide temperature range and low melting temperatures, which is the case
of unsaturated PILs (Figure 9A) can be quite interesting for industry, since they can be
applied in process at room temperature, as previously mentioned. On the other hand,
fluids with low viscosity could be desired for pumping, stirring and mixing and also
required in case of using them as solvent in extraction processes.
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At low shear rates, the viscosity values could qualify the influence of the ions
structures on the rheological profile of the compounds. According to Figure 9 and Table
3, in general, the apparent viscosity at low shear rates increased with the increasing of
the alkyl chain length of the anion for [H2EA][CnOO] and [H3EA][CnOO] series. On the
other hand, for [HEA][CnOO] series the viscosity decreased as the alkyl chain length of
the anion increased, except for [HEA][C8OO]. Also, in terms of absolute values, the
viscosity decreased, in case of saturated PILs at the order [H2EA][CnOO] >
[HEA][CnOO] > [H3EA][CnOO]. On the other hand, in the case of linoleate PILs,
[HEA] > [H2EA] > [H3EA], showing that the effect of the cation also depends on the
presence of unsaturations in molecule. Again, these results clearly show that the balance
between symmetry and size of the ions has a significant impact on the established intra-
and intermolecular interactions, promoting a diversified viscosity profile.
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Among all evaluated substances, at room temperature (298.15 K), oleate,
linoleate and laurate based PILs displayed the highest viscosities of 20.0 Pa.s and 5.0
Pa.s, approximately, at lower (γ̇ ̇
< 5 s-1) and higher (γ > 50 s-1) shear rates, respectively.
This is quite interesting when compared to the viscosities of ILCs reported in literature.
11c
Wang, et al.
reported viscosity values close to 3 Pa.s for low shear rates (γ̇
< 5 s-1)
and non-Newtonian flow for 1,3-didodecylimidazolium tetrafluoroborate
([C12C12IM][BF4]) and 1,3-didodecylimidazolium perchlorate ([C12C12IM][ClO4]) at the
11a
liquid crystalline state. Wang, et al.
low shear rates (γ
also reported viscosity values close to 9 Pa.s at
̇
< 5 s-1) and non-Newtonian flow for 1,3-didohexylimidazolium
tetrafluoroborate ([C16C16IM][BF4]) and 1,3-didohexylimidazolium perchlorate
([C12C12IM][ClO4]) at the liquid crystalline state.
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