Phase Behavior and Physical Properties of New Biobased Ionic Liquid

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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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The Journal of Physical Chemistry

Phase Behavior and Physical Properties of New Biobased Ionic Liquid Crystals 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, 13083862, Campinas, São Paulo, Brazil b

Department of Process and Products Design, School of Chemical Engineering, University of Campinas, 13083-852, Campinas, Saõ Paulo, Brazil

c

School of Pharmaceutical Sciences, Universidade Estadual Paulista, 14800-903, Araraquara, Saõ Paulo, Brazil

d

Department of Chemical Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, Perak 32610, Malaysia *[email protected] Tel.: (+55 19 3521 4056)

1 ACS Paragon Plus Environment


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KEYWORDS: ionic liquid, liquid crystal, fatty acid, solid-liquid equilibrium, rheology. ABSTRACT 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 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 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 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, especially as alternative solvents in extraction processes 1. However, their properties have attracted attention in several other fields, such as food, bioproducts and biomaterials industries 2. 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 right choice of ions 3. This functionality allows the synthesis of novel ILs for a given process, taking into account specific constraints and conditions. ILs are classically classified in two groups: aprotic and protic 4. The aprotic ILs (APILs) have been widely studied and reported in literature 5; 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 ethyl)ammonium carboxylates ([H2EA][CnOO]) described by Maximo, et al.

3c

and

Álvarez, et al. 6 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. Moreover, in the context of green chemistry 7, 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

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. 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 oils that will exceed 195 millions of tons until 2022 8. 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 their long alkyl chain

3c, 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 and dispersion forces

10

than the common aprotic ILs. This can also lead to the

obtainment of more stable mesophases

3c

, i.e. mesophases with high temperature

window, when compared to common ionic liquid crystals (ILCs)

11

. 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 delivery systems, electrical conducting media, friction reduction agent or thickener

12

.


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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. 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(2hydroxy 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 2hydroxy ethyl- ([HEA][CnOO]), bis(2-hydroxy ethyl)- ([H2EA][CnOO]) and tris(2hydroxy 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 𝑥1 = 0.5, fatty acid molar fraction, in a thermal fluid bath, at 10 K above the 5 ACS Paragon Plus Environment


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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.

Caprylic acid

Tris(2-hydroxy ethyl)amine

Tris(2-hydroxy ethyl)ammonium caprylate

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 least. The experimental Tm values were compared with COSMO-RS predicted data.

Table 1 Structures and abbreviations of PILs evaluated in this work Cations* Protic Ionic Liquids

2-hydroxy ethyl-

bis(2-hydroxy ethyl)-

tris(2-hydroxy ethyl)-

Caprylate

[HEA][C8OO]

[H2EA][C8OO]

[H3EA][C8OO]

Caprate

[HEA][C10OO]

[H2EA][C10OO]

[H3EA][C10OO]

Laurate

[HEA][C12OO]

[H2EA][C12OO]

[H3EA][C12OO]

Myristate

[HEA][C14OO]

[H2EA][C14OO]

[H3EA][C14OO]

Palmitate

[HEA][C16OO]

[H2EA][C16OO]

[H3EA][C16OO]

Stearate

[HEA][C18OO]**

[H2EA][C18OO]**

[H3EA][C18OO]

Oleate

[HEA][C18:1OO]**

[H2EA][C18:1OO]** [H3EA][C18:1OO]

Anions

Linoleate

[HEA][C18:2OO]

[H2EA][C18:2OO]

[H3EA][C18:2OO]

*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 7 ACS Paragon Plus Environment


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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. 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 (𝜎) – shear rate (γ̇ ) profile of the PILs.


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400,00

400

360,00

358 348 338

Temperature (K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


320,00

280,00

[HEA][CnOO]

MEACADSC MEACAMOLP DEACADSC

298

DEACAMOLP

[HE3A][CnOO]

[HE2A][CnOO]

250 240,00

TEACADSC TEACAMOLP

200

200,00

3,00

5,00

7,00 17,00Stearate19,00Oleate 21,00 23,00 Caprylate9,00Caprate11,00 Laurate13,00 Myristate15,00 Palmitate 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 liquidcrystal 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]).



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 bilayertype 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. 16

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.

(A)

[HEA][C8OO] [HEA][C10OO]

313.38 K

(B)

[H2EA][C8OO]

(C)

[H3EA][C8OO]

257.88 K

[H2EA][C10OO] 322.63 K

Heat Flow Endo Down (mW)

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308.44 K

[H3EA][C10OO]

280.12 K

[HEA][C12OO]

[H2EA][C12OO]

316.71 K

[H3EA][C12OO]

292.40 K 323.32 K

[HEA][C14OO] 329.16 K

294.31 K

323.30 K

[H2EA][C14OO]

[H3EA][C14OO]

[H2EA][C16OO]

[H3EA][C16OO]

[HEA][C16OO] 337.70 K

315.21 K

329.44 K

335.58 K

[H3EA][C18OO]

339.23 K

[H3EA][C18:1OO]

[HEA][C18:2OO]

[H2EA][C18:2OO]

277.71 K

294.35 K

[H3EA][C18:2OO] 268.51 K

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(2hydroxy 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 min1

. DSC thermograms for [HEA][C18OO], [HEA][C18:1OO], [H2EA][C18OO] and

[H2EA][C18:1OO] were not reported by Maximo, et al. 3c. 10 ACS Paragon Plus Environment

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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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Temperature (K)

1 2 3 400,00 400 MEACADSC 4 Tm – [HEA][CnOO] 5 MEACAMOLP TLCm – [HEA][CnOO] 6 Tm – [H2EA][CnOO] DEACADSC 360,00 7 350 TLCm – [H2EA][CnOO] DEACAMOLP 8 9 T TEACADSC m – [H3EA][Cn OO] 10 TLCm – [H3EA][CnOO] 320,00 TEACAMOLP 11 12 300 13 280,00 14 15 22,00 24,00 16 250 17 240,00 18 19 20 200 21 200,00 22 3,00 5,00 7,00 15,00Palmitate 17,00Stearate19,00Oleate 21,00Linoleate 23,00 Caprylate9,00Caprate11,00Laurate13,00 Myristate 23 Carboxylate based PILs 24 25 26 Figure 3. Solid-[Liquid-Crystal]-Liquid phase transition for PILs. Tm and TLCm 27 28 represent the melting temperature of the solid phase and the transition temperature of 29 the liquid crystal phase, respectively. 30 31 32 In a thermodynamic point of view, the impact of the symmetry of the cations and 33 34 the alkyl chain length of the anions on the Tm of the PILs could be explained in terms of 35 36 37 their melting Gibbs energy. The disordering of the system due to the asymmetry of the 38 39 cation probably leads to systems with higher entropy values. This is also observed in 40 41 case of presence of unsaturations in the anion chain. Moreover, results showed an 42 43 44 increasing in ΔHm as Tm increased (Table 2). It means that PILs presenting high values 45 46 of Tm presented high values of ΔHm. Also, since ΔHm of the [H2EA][CnOO] series were 47 48 49 lower than for the other series, one can probably assume that the asymmetry of the 50 51 cation decreased both Tm and ΔHm. Therefore, with high entropy values and low ΔHm 52 53 values one can assume that the melting Gibbs energy tends to be negative 19, denoting 54 55 56 that the melting of the system is thermodynamically favorable, resulting in low-melting 57 58 ILs with lower melting enthalpies. 59 60


MEAC

MEAC

DEACA

DEACA

TEACA

TEACA

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Table 2. Phase transition temperatures and thermodynamic data for different PILs 2-hydroxy ethylammonium carboxylates PILs

a

Tm/K

TLCm/K

ΔHm/kJ mol-1

[HEA][C8OO] [HEA][C10OO] [HEA][C12OO] [HEA][C14OO] [HEA][C16OO] [HEA][C18OO] a [HEA][C18:1OO] a [HEA][C18:2OO]

313.38 ± 0.22 342.32 ± 0.71 27.10 ± 0.64 322.63 ± 0.39 375.18 ± 0.31 28.51 ± 0.76 323.32 ± 0.18 369.28 ± 0.65 31.14 ± 0.65 329.16 ± 0.12 355.92 ± 0.31 33.40 ± 1.07 337.70 ± 0.08 364.88 ± 0.35 35.22 ± 1.55 351.65 ± 0.40 373.15 ± 1.00 * 288.08 ± 0.40 358.15 ± 1.00 * 277.71 ± 0.01 349.15 ± 1.00 39.58 ± 2.60 bis(2-hydroxy ethyl)ammonium carboxylates

PILs [H2EA][C8OO] [H2EA][C10OO] [H2EA][C12OO] [H2EA][C14OO] [H2EA][C16OO] [H2EA][C18OO] a [H2EA][C18:1OO] a [H2EA][C18:2OO]

Tm/K TLCm/K ΔHm/kJ mol-1 257.88 ± 0.40 3.41 ± 1.03 280.12 ± 0.35 303.95 ± 0.71 9.92 ± 0.39 292.40 ± 0.05 341.15 ± 0.46 13.55 ± 0.24 294.31 ± 0.18 346.58 ± 0.49 22.05 ± 0.27 315.21 ± 0.11 378.35 ± 0.40 28.35 ± 1.01 339.48 ± 0.40 380.15 ± 1.00 * 283.60 ± 0.40 360.95 ± 1.00 * 339.55 ± 1.00 tris(2-hydroxy ethyl)ammonium carboxylates

PILs [H3EA][C8OO] [H3EA][C10OO] [H3EA][C12OO] [H3EA][C14OO] [H3EA][C16OO] [H3EA][C18OO] [H3EA][C18:1OO] [H3EA][C18:2OO]

Tm/K 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

TLCm/K 326.12 ± 0.06 344.78 ± 0.83 358.95 ± 1.00 366.65 ± 1.04 359.55 ± 1.00 347.35 ± 1.00

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

Tm/K (Cosmo) 174.26 180.31 185.12 188.90 192.11 194.72 198.26 202.42 Tm/K (Cosmo) 190.13 195.57 199.85 203.18 205.99 208.25 211.95 216.23 Tm/K (Cosmo) 199.77 204.89 208.94 212.07 214.70 216.80 220.43 224.61

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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Page 14 of 35

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. 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 [HEA][C18OO] 3c. 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.


Page 15 of 35

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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 by focal conic fan-shaped textures 21. 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 smectic phases were observed

11c, 20

, especially SmA phases

11a, 11b, 22b

. Wang, et al.

11a

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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Page 16 of 35

discussed later. This is probably due to the weak molecular interactions of the APILs compared to the PILs. 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. 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 16 ACS Paragon Plus Environment

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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. 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.



2-hydroxy ethylammonium

bis(2-hydroxy ethyl)ammonium tris(2-hydroxy ethyl)ammonium

(C)

(B)

Laurate

(A)

50 µm

50 µm

(F)

(E)

Myristate

(D)

50 µm

50 µm

(G)

50 µm

50 µm

(I)

(H)

Palmitate

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 35

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.


[H2EA][C14OO]


(B)

(A)

50 µm

50 µm

(D)

(C)

50 µm

(F)

(E)

50 µm

50 µm

[H3EA][C18:1OO]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[HEA][C18:2OO]

Page 19 of 35

(G)

50 µm

(I)

(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. 19 ACS Paragon Plus Environment


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Page 20 of 35

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 2hydroxy 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 selfassembling and LC profile of the PILs of this work, their critical micellar concentration (CMC) was determined. 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 parameter (CPP)

25

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 (2hydroxy ethylammonium) are interacting with the hydrophilic medium. After the second CMC (CMC2) value they probably self-organize in more complex structures, 20 ACS Paragon Plus Environment

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such as cylindrical or bilayer. In fact, bilayer micellar structures were already reported for stearate based PILs

3c

and cylindrical micelles for unsaturated oleate based PILs.

Interestingly, as the alkyl chain length of PIL structures decreased, the CMC 1 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. 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.



Page 22 of 35

MEAAC

100 100 1 PIL CMC1 CMC2 2 90 [HEA][C 0.2621 MEACAPL 8 OO] 3 [HEA][C OO] 0.2041 MEACAP 10 4 80 [HEA][C OO] 0.1958 0.9100 MEALAU 12 5 75 [HEA][C 0.1764 0.7020 MEAMYR 14 OO] 6 70 [HEA][C OO] 0.1688 0.4866 MEAPALM 7 16 60 [HEA][C OO] 0.2330 0.6807 MEALIN 8 18:2 9 50 10 50 11 40 12 30 13 14 25 20 15 16 0,7 0,8 0,9 1,0 10 17 18 00 19 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0,0 0,1 0,2 0,3 0,4 0,5 0,6 DEAAC 20 PIL / mM PIL / mM 21 120 120 PIL CMC1 CMC2 22 [H 0.3174 DEACAPL 23 2 EA][C8 OO] [H DEACAP 24 100 2 EA][C10 OO] 0.1741 DEALAU [H 25 2 EA][C12 OO] 0.1183 0.8519 90 DEAMYR [H 26 2 EA][C14 OO] 0.1147 0.4123 80 DEAPALM [H 27 2 EA][C16 OO] 0.0992 0.4667 DEALIN [H 28 2 EA][C18:2 OO] 0.1378 0.5853 29 60 30 60 31 32 40 33 0,634 0,7 0,8 0,9 1,0 30 35 20 36 37 38 00 39 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0.0 0.1 0.2 0.3 0.4 0.5 0.6 40 PIL /TEAAC mM PIL / mM 41 60 60 42 PIL CMC1 CMC2 PIL 43 [TEA][C [H TEACAPL 8OO] 3EA][C 8OO] 0.5281 50 44 [TEA][C OO] 0.4819 [H EA][C OO] TEACAP 1010 3 45 [TEA][C OO] [H OO] 0.2779 0.6826 TEALAU 1212 3EA][C 45 46 [TEA][C OO] [H OO] 0.0688 0.3356 TEAMYR 1414 3EA][C 40 47 [TEA][C OO] [H OO] 0.0352 0.2074 TEAPALM 1616 3EA][C [TEA][C OO] 0.0607 0.4074 48 [H EA][C TEAEST 18:0 3 18OO] [TEA][C OO] [H OO]0.0927 0.4780 TEAOLE 49 18:1 3EA][C 18:1 30 [TEA][C OO] [H OO]0.0940 0.3505 TEALIN 18:2 3EA][C 18:2 50 30 51 52 20 53 54 15 55 10 56 0,7 0,8 0,9 1,0 57 58 00 59 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0.0 0.1 0.2 0.3 0.4 0.5 0.6 60 PIL / mM

MEACAPL 60

(A)

-1 -1 mC  cm kk// mC  cm

k / mC  cm-1

50

40 30

MEACA

20

Série2

10

Série3

k / mC  cm-1

MEACAPL MEAPALM MEACAP 0 MEALAU 60 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 MEAMYR [HEA][C 16OO] 50 MEAPALM PIL / mM

40

MEALIN

MEAPA

30

Série1

20

Série2

10

0.7 0,7

0.8 0,8

0.9 0,9

k / mC  cm-1

100

 cm-1cm -1 kk //mCmC

Série3

0 0.00,1 0,2 0,3 0,4 0,5 0.5 0,6 0,7 0,8 0,9 1.0 1.0 1,00,0 1,0 DEACAPL PIL / mM 120

(B)

[H2EA][C8OO]

80 60

DEACAPL

40

Série2

20 DEACAPL

Série3

DEAPALM

k / mC  cm-1

0 DEACAP DEALAU 40 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 35 DEAMYR [H2EA][C16OO] PIL / mM 30 DEAPALM 25 DEALIN 20 15 10 5 0 1,0 0,0 0,5 0,6 0,7 0,8 0,9 1.0 1,0 1.0 0.0 0,1 0,2 0,3 0,4 0.5 TEACAPL PIL / mM 60

0,7 0.7

0,8 0.8

0,9 0.9

(C)

/ mC  cm-1

k / mC  cm-1

50

Série1 Série2 Série3

[H3EA][C8OO] TEACAPL

30 20

0

15 k / mC  cm-1

DEAPALM

40

10

k / mC  cm-1

PIL / mM

[HEA][C8OO]

TEACAPL TEACAP TEALAU TEAPALM TEAMYR TEAPALM 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 TEAEST [HTEAOLE OO]/ mM 3EA][C16PIL TEALIN

10

Série2 Série3

TEAPALM Série1

5

Série2 Série3

0,7 0.7

0,8 0.8

0,9 0.9

0 1,00.0 0,0 0,1 0,2 0,3 0,4 0.5 0,5 0,6 0,7 0,8 0,9 1.0 1,0 1.0 PIL / mM

Figure 6 Conductivity (κ) as function of concentration (mM) of (A) 2-hydroxy ethyl- ([HEA][CnOO]), (B) bis(2hydroxy ethyl)- ([H2EA][CnOO]) and (C) tris(2-hydroxy ethyl)ammonium carboxylates ([H3EA][CnOO]) aqueous 22 micellar solutions. Solid lines represent the linear fitting of the data (R2 > 0.98) for the determination of critical concentration (CMC). ACS Paragon Plus Environment

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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. Data were fitted to the Herschel-Bulkley (eq 1) model 27 and their parameters are presented in Table 3. 𝜎 = 𝜎0 + 𝐾γ̇ 𝑛

(1)

where 𝜎 is the shear stress (Pa), γ̇ is the shear rate (s-1); 𝜎0 is the yield stress (Pa); 𝐾 is the consistency index; 𝑛 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 shearthickening 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 23 ACS Paragon Plus Environment

Page 24 of 35

[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.

Insaturados 1400 1400

(A)

MEALIN 18:2OO] [HEA][C DEALIN [HE 2A][C18:2OO] [HE TEAOLE 3A][C18:1OO] [HE TEALIN 3A][C18:2OO]

1200 1200

1000 1000

800 800

σ0 /Pa

σ/Pa

MODMEALIN

MEALIN

MODDEALIN

DEALIN

MODTEAOLE

TEAOLE

MODTEALIN

TEALIN

MODMEALIN

600 600

MODDEALIN

MODTEAOLE MODTEALIN

400 400

200 200

250

300

00 0

50

0 65 °C 500 500

100

50

150

100

γ̇/ s

200

250

300

250

300

65 °C

65 °C

100

90

450

80

N

70 60

σ0/Pa

400 400

50

50

N

40

350

30

N N

20

10

300 300 0

200

150-1

100

σ/Pa σ /Pa

1 2 os34 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 200 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 50 55 56 57 58 59 60


0

0

0

0

50

100

150

100

γ̇/ s-1

200

200

250

250

200 200 150

100 100

300

300

(B)

[HEA][C MEACAPL 8OO] [HEA][C MEACAP 10OO] [HEA][C MEALAU 12OO] [HEA][C MEAMIR 14OO] [HE DEAMIR 2A][C14OO] [HE TEACAP 3A][C10OO] [HE TEALAU 3A][C12OO] [HE TEAMIR 3A][C14OO] [HE TEAPALM 3A][C16OO] MEACAPL MEACAP MEALAU MEAMIR DEAMIR TEACAP TEALAU TEAMIR TEAPALM MODMEACAPL MODMEACAP MODMEALAU MODMEAMIR MODDEAMIR MODTEACAP

MEACAPL

MEACAP MEALAU MEAMIR

DEAMIR TEACAP

MODMEACAPL MODMEACAP MODMEALAU MODMEAMIR MODDEAMIR MODTEACAP MODTEALAU MOD TEAMIR MODTEAPALM

TEALAU

TEAMIR TEAPALM MODMEACAPL

MODMEACAP MODMEALAU

MODMEAMIR MODDEAMIR MODTEACAP

50

MODTEALAU

00 100

150

0 0

γ̇/ s-1

200

50 50

250

100 100

300

150 150

γ̇/ s-1

200 200

250 250

MOD TEAMIR

300 300

MODTEAPALM

γ̇/s-1

Figure 7. Shear stress (𝜎) 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 HerschelBulkley model. 24 ACS Paragon Plus Environment

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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. Unexpectedly, [HEA][C18OO] (Figure 8E) showed also an irreversible disrupting of its structure, in which the flow behavior was decharacterized at the 3 rd 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.


400


350

Page 26 of 35

300

1 MEALAU 250 2 500 500 400 400 3 1st step 1rst curve 200 450 1st step 1rst curve 4 nd step 350 2 2nd curve 1.47 Pa·s 2ndcurve step 2nd 5 150 400 3rd curve step 3rd rd curve 3rd 3 step 6 300 100 350 7 4.05 Pa·s 250 8 300 50 9 250 1rst curve 200 250 0 200 10 0 50 100 150 200 250 300 2nd curve 11 250 200 150 300 γ̇/ s-1 3rd curve 12 150 1.30 Pa·s 13 100 100 14 3.00 Pa·s 400 50 15 50 16 350 0 00 0 17 50 100 150 200 250 00 50 100 150 200 250 300 00 50 100 150 200 250 50 100 150 200 250 300 18 γ̇/ s 300 -1-1 -1 γ̇ / s γ̇/MEAPALM s γ̇/s 19 250400 400 40 40 20 (D) (C) 21 st step 1st step 1rst curve 11rst curve 200 350 35 22 nd nd step 2 curve step 2nd 22nd curve 150 23 rd step 3rd 3rd curve step 3rd curve 3 300 30 0.14 Pa·s 1.38 Pa·s 24 100 25 250 25 3.60 Pa·s 26 50 200 200 201rst curve 20 27 0 28250 2nd curve 0 50 100 150 200 250 300 300 15 150 29 -1 3rd curve 0.50 Pa·s γ̇/ s 30 0.03 Pa·s 10 100 0.05 Pa·s 31 1.04 Pa·s 2.52 Pa·s 32 5 50 33 00 00 34 0 50 100 150 200 250 50 100 150 200 250 300 0 50 100 150 200 250 300 0 50 100 150 200 250 35 γ̇/ s -1 36 γ̇γ̇//ss-1 γ̇/s-1 37 140 38 st step 140 11rst curve 1.28 Pa·s 39 nd 22nd step curve 40 2.09 Pa·s 120 rd step 33rd curve 41 42 100 43 70 44 80 1rst curve 45 2nd curve 46 60 0.90 Pa·s 150 200 250 300 3rd curve 47 γ̇/s-1 48 40 0.17 Pa·s 49 20 50 51 00 52 00 50 100 150 200 250 300 50 100 150 200 250 300 53 -1 -1 γ̇ / s γ̇/s 54 55 Figure 8. Typical shear stress (σ) versus shear rate (γ̇ ) curves with an up-down-up step 56 program of (A) protic ionic liquids (PILs), (B) 2-hydroxy ethylammonium laurate 57 ([HEA][C12OO]) at 338 K, (C) 2-hydroxy ethylammonium myristate ([HEA][C14OO]) 58 59 at 338 K, (D) 2-hydroxy ethylammonium palmitate ([HEA][C16OO]) at 358 K, (E) 260

(B)

σ0 /Pa

σ0 /Pa

σ/Pa

σ0 /Pa

(A)

2F-1S Re

2F-2S Re

300 300

σ0 /Pa

σ0 /Pa

σ/Pa

σ0 /Pa

-1

2F-3S Re

-1

σ/Pa σ0 /Pa

(E)

hydroxy ethylammonium stearate ([HEA][C18OO]) at 358 K. Arrows indicate apparent viscosity (η).


2F-1S Repet

2F-2S Repet

2F-3S Repet

300 300

Page 27 of 35

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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 σ0/Pa

PILs

[HEA][C18:1OO] a, b 74.22 [HEA][C18:2OO]

K¥

nǂ

R2

η/ Pa∙s at 10 s-1

-

-

-

-

η/ Pa∙s at 50 s-1 η/ Pa∙s at 150 s-1 -

-

80.02 21.6517 0.6490 0.9672 14.8993 ± 0.4058 7.3575 ± 0.5551 4.2155 ± 0.0530

[HE2A][C8OO]

0.0

0.8968 1.0000 0.9999

0.9621 ± 0.0089

0.9235 ± 0.0083 0.8931 ± 0.0391

[HE2A][C10OO]

0.0

1.6832 1.0000 0.9999

1.7415 ± 0.0183

1.7110 ± 0.0099 1.6890 ± 0.0255

[HE2A][C12OO] [HE2A][C18:1OO] [HE2A][C18:2OO] [HE3A][C8OO]

79.41 17.7422 0.7282 0.9706 13.9506 ± 0.1612 7.2915 ± 0.2737 5.0055 ± 0.0587 a

14.34 10.8220 0.8730 0.9520

-

-

-

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

[HE3A][C18:1OO]

85.10 13.1666 0.7552 0.9723 14.5984 ± 0.0477 6.7035 ± 0.0163 4.5300 ± 0.0721

[HE3A][C18:2OO]

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


η/ Pa∙s at 50 s-1 η/ Pa∙s at 150 s-1

0.2062 1.0000 0.9999

0.2514 ± 0.0034

0.2144 ± 0.0052 0.2047 ± 0.0004

[HEA][C10OO]

38.48 14.0892 0.5803 0.9475

8.0467 ± 0.1676

3.7710 ± 0.5049 1.7235 ± 0.0064

[HEA][C12OO]

39.73 23.5866 0.3833 0.9141

7.3712 ± 0.3303

3.1545 ± 0.2171 1.2155 ± 0.1195

[HEA][C14OO]

38.69 13.7009 0.4441 0.8732

7.3830 ± 0.0995

2.6030 ± 0.1216 0.9599 ± 0.1148

[HE2A][C14OO]

30.69 7.3195 0.6615 0.9889

5.1689 ± 0.1042

2.7450 ± 0.0014 1.5465 ± 0.0219

[HEA][C8OO]

0.0

K¥

nǂ

R2

[HE3A][C10OO]

0.0

0.1250 1.0000 0.9689

0.1694 ± 0.0035

0.1313 ± 0.0035 0.1250 ± 0.0034

[HE3A][C12OO]

0.0

0.0333 1.0000 0.9999

0.0476 ± 0.0018

0.0360 ± 0.0004 0.0340 ± 0.0002

[HE3A][C14OO]

0.0

0.0520 1.0000 0.9982

0.1018 ± 0.0031

0.0639 ± 0.0025 0.0548 ± 0.0021

[HE3A][C16OO]

25.33 4.6804 0.7154 0.9924 4.3020 ± 0.2631 T = 348.15 K

2.1690 ± 0.0014 1.3030 ± 0.0028

PILs

σ0/Pa


η/ Pa∙s at 50 s-1 η/ Pa∙s at 150 s-1

[HE2A][C18OO]

28.75 13.7681 0.4780 0.9556

5.7635 ± 0.1977

2.8015 ± 0.0926 1.2015 ± 0.1280

[HE3A][C18OO]

14.18 4.8281 0.5636 0.9649 2.3828 ± 0.1561 T = 358.15 K

1.0820 ± 0.0269 0.7107 ± 0.0204

PILs

σ0/Pa


η/ Pa∙s at 50 s-1 η/ Pa∙s at 150 s-1

0.1356 ± 0.0250

0.0452 ± 0.0059 0.0284 ± 0.0038

3.4638 ± 0.3527

0.9290 ± 0.0427 0.2980 ± 0.0441

4.1711 ± 0.3152

2.0885 ± 0.0049 0.9991 ± 0.0183

[HEA][C16OO] [HEA][C18OO] * [HE2A][C16OO]

0.0 -

K¥

K¥

nǂ

nǂ

R2

R2

0.3382 0.5351 0.9665 -

-

-

21.59 6.9517 0.5937 0.9768

𝐾 for Herschel-Bulkley model. ǂ 𝑛 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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Page 28 of 35

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 gel-like behavior. In fact, high viscosity values are typically observed for ILs with longer alkyl chain (C12-C18)

3c, 11a, 11c

and this is attributes to the alkyl chain length and

the increasing of the van der Waal molecular interactions

28

. 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 intraand intermolecular interactions, promoting a diversified viscosity profile. 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 (γ̇ > 50 s-1) shear rates, respectively. This is quite interesting when compared to the viscosities of ILCs reported in literature. Wang, et al. and

11c

reported viscosity values close to 3 Pa.s for low shear rates (γ̇ < 5 s-1)

non-Newtonian

flow

for

1,3-didodecylimidazolium

tetrafluoroborate

([C12C12IM][BF4]) and 1,3-didodecylimidazolium perchlorate ([C12C12IM][ClO4]) at the liquid crystalline state. Wang, et al.

11a

also reported viscosity values close to 9 Pa.s at

low shear rates (γ̇ < 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.



348.15 K 298.15 K 6 6,0

16 16,0

(A)

14,0

(B) 5,0

12 12,0 4,0

ɳ/Pas

10,0

3 3,0

8,08

10 s-1

10 s-1 50 s-1

50 s-1

150 s-1

6,0

150 s-1

2,0 4,04

1,0 2,0

0 0,0

0,00

1

2

3

4

5

6

7

1

8

2

358.15 K 338.15 K 9,0 9

5,0 5

(C)

8,0

(D)

4,5 4,0 4

7,0

3,5

6,0 6

3 3,0

ɳ/Pas

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 35

5,0

10 s-1

10 s-1

2,5

4,0

2 2,0

50 s-1

50 s-1

150 s-1

150 s-1

3,0 3

1,5

2,0

1 1,0

1,0

0,5

0 0,0

0,0 0 1

2

3

4

5

6

7

8

9

1

2

3

Figure 9. Apparent viscosity data of protic ionic liquids (PILs) at 10 s-1 (blue), 50 s-1 (red) and 100 s-1 (green) at (A) 298.15, (B) 348.15, (C) 338.15, (D) 358.15 K. CONCLUSIONS Twenty new protic ionic liquids were here synthesized by using fatty acids found in vegetable oils and 2-hydroxy ethylamines. Their melting profile exhibited the formation of very well-defined liquid crystalline mesophases, characterizing a SLcL phase transition. The size and symmetry of the ions showed a drastic influence on the physical properties investigated, such as melting temperature, liquid crystalline temperature range, viscosity and critical micellar concentration. The use of bis(230 ACS Paragon Plus Environment

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hydroxy ethyl)ammonium cation and the unsaturated carboxylate anions were the best choice for considerably lowering the melting temperature of the PILs and enhancing the liquid crystalline temperature window. PILs with the longest alkyl chain lengths presented higher ability for self-assembling in bilayer-structured Smectic mesophases, showing higher apparent viscosity values and non-Newtonian behavior. The lowest CMC value was obtained for [H3EA][C16OO], the lowest melting temperature for [H2EA][C8OO] and the largest liquid crystal temperature range for the unsaturated carboxylates. This specific behavior proposes that also the size of the cation clearly impacted the physical properties. It means that one must take into account a right proportion between the symmetry and the size of the ions, as well as the lipophilic/hydrophilic balance of the molecule, in order to meet the desired requirements for a given task specific application and process. Remarkably, such properties are significantly more accentuated when compared to the ionic liquid crystals reported up to date in literature. Therefore, the interesting characteristics of the lipidic protic ionic liquids here disclosed, together with their sustainable appeal, and possible low toxicity, open new possibilities for studying their use in the design of products or processes. ASSOCIATED CONTENT Supporting Information. The list of chemical shifts for all PILs obtained by Proton Nuclear Magnetic Resonance (1H NMR). AUTHOR INFORMATION Corresponding author. * Department of Food Engineering, University of Campinas, R. Monteiro Lobato 80, 13083-862, Campinas, São Paulo, Brazil., Fax: + 55 19 3521 4027 Tel: + 55 19 3521 4097; E-mail: [email protected]



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Page 32 of 35

ACKNOWLEDGMENTS The authors thank the national funding agencies CNPq (National Council for Scientific

and

Technological

Development)

(133152/2014-6,

483340/2012-0,

305870/2014-9, 309780/2014-4, 143050/009-5, 406856/2013-3), UNICAMP/FAEPEX (PAPDIC 0125/16) and FAPESP (Research Support Foundation of the State of São Paulo) (2016/08566-1, 2014/21252-0, 2014/03992-7, 2012/05027-1, 2007/58017-5) for financial support and scholarships. Jorge F. B. Pereira acknowledges financial support by FAPESP through the research project (2014/16424-7). REFERENCES 1. (a) Das, R. N.; Roy, K., Advances in QSPR/QSTR models of ionic liquids for the design of greener solvents of the future. Molecular Diversity 2013, 17 (1), 151-196; (b) Zhou, Y.; Wu, D.; Cai, P.; Cheng, G.; Huang, C.; Pan, Y., Special effect of ionic liquids on the extraction of flavonoid glycosides from Chrysanthemum morifolium Ramat by microwave assistance. Molecules 2015, 20 (5), 7683-7699. 2. (a) Toledo Hijo, A. A. C.; Maximo, G. J.; Costa, M. C.; Batista, E. A. C.; Meirelles, A. J. A., Applications of Ionic Liquids in the Food and Bioproducts Industries. ACS Sustainable Chemistry & Engineering 2016, 4 (10), 5347-5369; (b) Guo, F.; Fang, Z.; Tian, X. F.; Long, Y. D.; Jiang, L. Q., One-step production of biodiesel from Jatropha oil with high-acid value in ionic liquids. Bioresource Technology 2011, 102 (11), 6469-6472; (c) Zhang, L.; Cui, Y.; Zhang, C.; Wang, L.; Wan, H.; Guan, G., Biodiesel production by esterification of oleic acid over brønsted acidic ionic liquid supported onto Fe-incorporated SBA-15. Industrial and Engineering Chemistry Research 2012, 51 (51), 16590-16596; (d) Sun, S. N.; Li, M. F.; Yuan, T. Q.; Xu, F.; Sun, R. C., Effect of ionic liquid/organic solvent pretreatment on the enzymatic hydrolysis of corncob for bioethanol production. Part 1: Structural characterization of the lignins. Industrial Crops and Products 2013, 43 (1), 570-577; (e) Bica, K.; Gaertner, P.; Rogers, R. D., Ionic liquids and fragrances Direct isolation of orange essential oil. Green Chemistry 2011, 13 (8), 1997-1999. 3. (a) Neves, C. M. S. S.; Batista, M. L. S.; Cláudio, A. F. M.; Santos, L. M. N. B. F.; Marrucho, I. M.; Freire, M. G.; Coutinho, J. A. P., Thermophysical properties and water saturation of [PF6]-based ionic liquids. Journal of Chemical and Engineering Data 2010, 55 (11), 5065-5073; (b) Gardas, R. L.; Costa, H. F.; Freire, M. G.; Carvalho, P. J.; Marrucho, I. M.; Fonseca, I. M. A.; Ferreira, A. G. M.; Coutinho, J. A. P., Densities and derived thermodynamic properties of imidazolium-, pyridinium-, pyrrolidinium-, and piperidinium-based ionic liquids. Journal of Chemical and Engineering Data 2008, 53 (3), 805-811; (c) Maximo, G. J.; Santos, R. J. B. N.; Lopes-Da-Silva, J. A.; Costa, M. C.; Meirelles, A. J. A.; Coutinho, J. A. P., Lipidic protic ionic liquid crystals. ACS Sustainable Chemistry and Engineering 2014, 2 (4), 672-682. 4. Peric, B.; Sierra, J.; Martí, E.; Cruañas, R.; Garau, M. A.; Arning, J.; Bottin-Weber, U.; Stolte, S., (Eco)toxicity and biodegradability of selected protic and aprotic ionic liquids. Journal of Hazardous Materials 2013, 261, 99-105.


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Page 35 of 35

For Table of Contents Use Only

Biobased Protic Ionic Liquid Crystals 400 400

High Viscosity & Stability

350

Natural Sources

300 250

σ0 /Pa

σ/Pa σ/Pa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1rst curve

200 200

2nd curve 150

3rd curve

100 50

00 00

50 50

100 100

150 150

200 200

250 250

300 300

-1

γ̇γ̇//ss-1

Manuscript title Phase Behavior and Physical Properties of New Biobased Ionic Liquid Crystals Authors 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, Saõ Paulo, Brazil c

School of Pharmaceutical Sciences, Universidade Estadual Paulista (UNESP), 14800-

903, Araraquara, Saõ Paulo, Brazil d

Department of Chemical Engineering, Universiti Teknologi PETRONAS, Bandar Seri

Iskandar, Perak 32610, Malaysia


Phase Behavior and Physical Properties of New Biobased Ionic Liquid

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