Journal of Molecular Liquids 207 (2015) 60–66

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Journal of Molecular Liquids

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Ionic liquids derived from esters of Glycine Betaine: Synthesisand characterization

Yannick De Gaetano, Aminou Mohamadou, Stéphanie Boudesocque, Jane Hubert,Richard Plantier-Royon, Laurent Dupont ⁎Université de Reims Champagne-Ardenne, Institut de Chimie Moléculaire de Reims (ICMR), CNRS UMR 7312, UFR des Sciences Exactes et Naturelles, Bâtiment 18 Europol'Agro, BP 1039,F-51687 Reims Cedex 2, France

⁎ Corresponding author.E-mail address: laurent.dupont@univ-reims.fr (L. Dup

http://dx.doi.org/10.1016/j.molliq.2015.03.0160167-7322/© 2015 Published by Elsevier B.V.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 16 January 2015Accepted 9 March 2015Available online 12 March 2015

Keywords:Ionic liquidGlycine-betaineDicyanamideTetrafluoroborateThermal characterization

A series of new “green” ionic liquids was prepared in two or three steps from natural and low expense GlycineBetaine (GB). Esterification of GB was carried out with methanesulfonic acid and primary alcohols with alkylchains containing 6, 12, 14, 16 and 22 atoms of carbon. Anionic metathesis from these cationic esters with inor-ganic (ClO4

−, BF4−) or biobased ((S)-CH3-CHOH-COO− (Lac−)) anions was then achieved to afford a range of fullycharacterized ionic liquids. The influence of the alkyl chain length and the chemical nature of the counter anionon physicochemical properties such asmelting point, glass transition, decomposition temperatures and viscosityhas been investigated.

© 2015 Published by Elsevier B.V.

1. Introduction

Ionic liquids (ILs) are commonly defined as salts with a meltingpoint below 100 °C. Most of traditional ILs are constituted by the associ-ation of alkylphosphonium, alkylammonium, dialkylimidazolium,dialkylpyrrolidinium or alkylpyridinium cations with a bulky anionsuch as Cl−, AlCl4−, PF6−, BF4−, (CF3SO2)2N−, CF3CO2

− or CF3SO3−. Due to

their specific physicochemical properties including a low vapor pres-sure, high thermal and chemical stability, non-flammability, large elec-trochemical window, and outstanding ability to dissolve awide range ofcompounds [1–3], ILs have found a range of applications in various areasincluding for instance chemical synthesis, catalysis [4–9], electrochem-istry [10–12] or analytical chemistry [13,14].

ILs today are considered as “greener” alternatives to volatile organicsolvents, mainly due to their negligible vapor pressure. Their no-volatility is a common property to all ILs that enable them to generateonly little atmospheric pollution as compared to conventional solvents.For this reason, ILs are considered as “green solvents” but taking intoaccount the twelve principles of green chemistry, the terminology“green” is still open for debate. The most commonly used ILs, based onimidazolium cations and fluorinated anions are synthetic chemicals, andtherefore are not as green as desired. Chloroaluminate-based ILs are airand water sensitive. Hexafluorophosphate and tetrafluoroborate-basedILs are unstable towards hydrolysis, potentially releasing HF in contactwith moisture [15]. Although ILs cannot enter the environment by

ont).

evaporation, most of them are water soluble and could easily enter thebiosphere by this way [16].

Even the no-volatility of ILs has been questioned especially by thework of Earle et al. [17]which has shown that some ILs can be evaporat-ed and recondensed under relatively mild conditions. Recently Marlairet al. [18] have highlighted that ILs are flammable and have physico-chemical properties such as the heat of combustion that is close to con-ventional combustible materials such as wood or plastic. Regardingsafety towards human health and environment, it was shown that cer-tain conventional ILs are toxic towards a panel of micro-organisms rep-resentative of different ecosystems and could generate environmentaldamages in particular in the event of accidental release [19].

In order to obtain “green” ionic liquids, the starting materials mustbe at least non-toxic, while for a perfect solution, they should be renew-able. Moreover, the development of new “green” ILs still requires rela-tively low cost synthetic route and easy preparation. Bio-renewablenatural compounds are ideal materials from the viewpoints of both en-vironmental and economical concerns [20]. Some syntheses of ILs havebeen reported, using lactates [21,22], acesulfamates [23,24], levulinate[25], carbohydrate derivatives [26,27] or amino acids [28] for the anion-ic part, and choline [29,30] carbohydrates [31] or amino acids [32–34]for the cationic species.

In this work, Glycine Betaine (GB)was used for the IL synthesis. Gly-cine Betaine (GB) is a natural and cost-effective substance possessing aquaternary trimethylalkylammonium moiety and a carboxylate func-tion, useful to conceive new ILs. GB constitutes an abundant rawmate-rial representing 27% of molasses of sugar beet in weight. It is obtainedafter extraction of saccharose and currently remains few developed as

61Y. De Gaetano et al. / Journal of Molecular Liquids 207 (2015) 60–66

a by-product of the sugar industry. GB is very well known as one of themost powerful osmoprotectant identified so far, and has been widelystudied for this specific property [35–38]. More recently scientistshave also focused on its capacity to form deep eutectic solvents withurea, citric acid or malonic acid [39,40]. One example of protonated be-taine bis(trifluoromethylsulfonyl)imide [41,42] IL has been describedand used for the extraction of neodymium(III) and the selective solubi-lization of metal oxides.

Within this context, the aim of this study was to produce novel“green” ILs with GB esters as cationic moiety. Some of GB esters associ-ated with various counter ions such as chloride, hydroxide and lauratewere previously synthesized andused successfully as cationic surfactantin formulation of new emulsions with improved biodegradability prop-erties [43]. To our knowledge, their use as building block to generateionic liquids has never been described. This paper, deals with the syn-thesis and the characterization of 20 salts incorporating cationic estersof trimethyl(2-alkoxy-2-oxoethyl) ammonium (GBOCn

+) associatedwith inorganic (ClO4

−, BF4−, and Dca−) or biobased ((S)-CH3-CHOH-COO− (Lac−)) anions. The influence of the alkyl chain length (C6, C12,C14, C16, C22), grafted by esterification reactions on GB, and the natureof the anions on the physicochemical and thermal properties of the ILs(Scheme 1) were investigated to elucidate structure–propertiesrelationships.

2. Experimental section

2.1. Materials

Glycine Betaine, methanesulfonic acid, the primary alcohols (n-hexanol, n-dodecanol, n-tetradecanol, n-hexadecanol, n-docosanol)and the sodium salts (dicyanamide, perchlorate and tetrafluoroborate)were purchased from Sigma Aldrich and used as received. Potassiumlactate was prepared by the reaction of potassium hydroxide and (S)-lactic acid. All aqueous solutions were prepared with distilled water.

2.2. Physicochemical analysis

Elemental analyses (C, H, N and S) were carried out on a Perkin-Elmer 2400 C ,H, N and S element analyzer in our University.

IR spectrum of liquid IL (GBOC6-Dca) was recorded on a BrukerAlpha-T FTIR spectrometer using NaCl plates, whereas those of solidcompounds were obtained in KBr pellets with a Nicolet Avatar 320apparatus.

1H and 13C NMR spectra were recorded at room temperature with aBruker AC 250 spectrometer (250MHz for 1H, 62.5MHz for 13C). Chem-ical shifts (in ppm) for 1H and 13C NMR spectra were referenced to re-sidual protic solvent peaks.

The decomposition temperatures of the ILs were measured using aNetzsch TG 209 F3 Tarsus thermogravimetric analyzer under argon at-mosphere, with the mass of TGA samples varying between 10 and20 mg. Samples were heated from 30 °C to 400 °C with a heating rateof 10 °C/min. The decomposition temperatures are reported in termsof Tonset (the intersection of the zero mass loss baseline and the tangentline through Tpeak, where Tpeak is the peak temperature of the time de-rivative of the mass loss curve d(mass) / dt).

Scheme 1. Ionic liquids based

Differential scanning calorimetry (DSC) experiments were carriedout on a TA Instruments Q100 under a nitrogen atmosphere with aheating rate of 10 °C min−1.

The viscosity was measured with a Brookfield Viscosimeter DV-II+Pro. The temperature of the sample was maintained to 80 or 90 °C ±0.1 °C by an external temperature control CPE 51.

Optical rotations were determined on a Perkin ElmerModel 341 po-larimeter at 589 nm.

2.3. Synthesis of ionic liquids

2.3.1. Hexylbetainium methanesulfonate (GBOC6-CH3SO3)A solution of n-hexanol (55 mL, 0.43 mol, 5 equiv.) and

methanesulfonic acid (8.4 mL, 128 mmol) was added dropwise to Gly-cine Betaine (10 g, 85.4 mmol). The reaction mixture was heated to160 °C with a Dean Stark apparatus to remove the water formed duringthe esterification. After 1 h, the reactionmixture became homogeneous.The heating was continued for 24 h and the orange solution was cooleddown to room temperature. Then, n-hexanol was distilled under re-duced pressure. The residual excess of alcohol was eliminated withtwo consecutive washes with Et2O (2 × 100 mL). The compound wasthen extracted with n-BuOH/EtOAc (1/1) (2 × 100 mL). The combinedorganic phaseswere evaporated under reduced pressure. The crudema-terial obtained was purified by recrystallisation from EtOAc/EtOH (1/1)to give awhite solid. Yield: (24.1 g, 81.1mol, 95%).Melting point 103 °C.1H NMR: δH (250 MHz; DMSO-d6): 0.97 (3H, t, J = 7.5 Hz, CH3), 1.38(6H, m), 1.73 (2H, q, J = 7.5 Hz, CH2CH2O), 2.51 (3H, s, CH3S), 3.36 (9H,s, (CH3)3N), 4.27 (2H, t, J = 7.5 Hz, CH2O), 4.61 (2H, s, CH2COO). 13CNMR: δC (62.5 MHz; DMSO-d6): 12.1 (CH3), 20.2, 23.1, 26.0, 29.0 (CH2),37.9 (CH3S), 51.2 ((CH3)3), 60.6 (CH2O), 63.9 (CH2CO), 163.3 (C_O).IR: ν(cm−1) 1743 (C_O). Analysis: Calculated for C12H27NO5S: C 48.46,H 9.15, N 4.70, S 10.78%. Found: C 48.73, H 8.81, N 4.40, S 10.97%.

2.3.2. Alkylbetainium methanesulfonate (GBOCn-CH3SO3)To a suspension of dried Glycine Betaine (8.3 g, 71 mmol, 1.0 equiv.)

in methanesulfonic acid (1.5 equiv.), an excess of n-alcohol (about 2equiv.) was added. The reaction mixture was gradually heated to130–140 °C under reduced pressure (50–100 mbars) to remove thewater formed during the reaction. After 1 h, the reaction mixture be-came homogeneous and after 7 h the brown mixture was cooleddown to room temperature and at atmospheric pressure. The crudema-terial was washed with Et2O (3 × 200mL) to remove the excess of fattyalcohol and then purified by recrystallisation from EtOAc/EtOH (1/1).For (GBOC16-CH3SO3) the purification was done by column chromatog-raphy over silica gel using EtOAc/iPrOH/H2O (62/30/8) as eluent.

2.3.2.1. Dodecylbetainium methanesulfonate (GBOC12-CH3SO3). Yield(23.4 g, 61.3 mmol, 86%). Melting point 104 °C. 1H NMR: δH(250 MHz; DMSO-d6): 0.95 (3H, t, J = 7.5 Hz, CH3), 1.34 (18H, m),1.70 (2H, q, J = 7.5 Hz, CH2CH2O), 2.52 (3H, s, CH3-S), 3.33 (9H, s,(CH3)3N), 4.27 (2H, t, J = 7.5 Hz, CH2O), 4.56 (2H, s, CH2COO). 13CNMR: δC (62.5 MHz; DMSO-d6): 14.2 (CH3), 22.4, 29.0, 29.2, 29.3, 29.4,31.6 (CH2), 40.0 (CH3S), 53.4 ((CH3)3), 62.8 (CH2O), 66.0 (CH2CO),165.3 (C_O). IR: ν(cm−1) 1750 (C_O). Analysis: Calculated forC18H39NO5S: C 56.66, H 10.30, N 3.67, S 8.40%. Found: C 56.28, H10.19, N 3.37, S 8.35%.

on Glycine Betaine esters.

62 Y. De Gaetano et al. / Journal of Molecular Liquids 207 (2015) 60–66

2.3.2.2. Tetradecylbetainium methanesulfonate (GBOC14-CH3SO3). Yield(27.0 g, 66 mmol, 93%). Melting point 104 °C. 1H NMR: δH (250 MHz;DMSO-d6): 0.95 (3H, t, J = 7.5 Hz, CH3), 1.34 (22H, m), 1.71 (2H, q,J = 7.5 Hz, CH2CH2O), 2.40 (3H, s, CH3-S), 3.32 (9H, s, (CH3)3N), 4.27(2H, t, J = 7.5Hz, CH2O), 4.54 (2H, s, CH2COO). 13C NMR: δC(62.5 MHz; DMSO-d6): 14.3 (CH3), 22.5, 25.6, 28.9, 29.1, 29.3, 29.4,31.7 (CH2), 40.2 (CH3S), 53.4 (CH3)3, 63.0 (CH2O), 66.1 (CH2CO), 165.3(C_O). IR: ν(cm−1) 1751 (C_O). Analysis: Calculated forC20H43NO5S: C 58.64, H 10.58, N 3.42, S 7.84%. Found: C 58.17, H10.72, N 3.33, S 7.86%.

2.3.2.3. Hexadecylbetainium methanesulfonate (GBOC16-CH3SO3). Yield(18.9 g, 43.3 mmol, 61%). Melting point 105 °C. 1H NMR: δH(250 MHz; DMSO-d6): 0.95 (3H, t, J = 7.5 Hz, CH3), 1.33 (26H, m),1.70 (2H, q, J = 7.5 Hz, CH2CH2O), 2.39 (3H, s, CH3-S), 3.31 (9H, s,(CH3)3N), 4.27 (2H, t, J = 7.5Hz, CH2O), 4.54 (2H, s, CH2COO). 13CNMR: δC (62.5 MHz; DMSO-d6): 14.2 (CH3), 28.4, 29.4, 29.6, 29.7, 29.8,29.8, 32.0 (CH2), 39.6 (CH3S), 54.0 (CH3)3, 63.1 (CH2O), 66.8 (CH2CO),165.2 (C_O). IR: ν(cm−1) 1751 (C_O). Analysis: Calculated forC22H47NO5S: C 60.37, H 10.82, N 3.20, S 7.33%. Found: C 60.17, H10.72, N 3.23, S 7.36%.

2.3.3. Docosylbetainium methanesulfonate (GBOC22-CH3SO3)To a suspension of dried Glycine Betaine (7.8 g, 67 mmol, 1.1 equiv.)

in methanesulfonic acid (8.83 g, 92 mmol, 1.5 equiv.), n-docosanol(20 g, 61.3 mmol, 1 equiv.) was added. The reaction mixture was grad-ually heated to 140 °C under reduced pressure (50mbar) to remove thewater formed during the reaction. After 17 h, the reaction mixture be-came homogeneous and the dark brown mixture was cooled down toroom temperature and to atmospheric pressure. The crude materialwas dissolved in CHCl3 (300mL)washedwithwater (3× 150mL) to re-move the excess of Glycine Betaine andmethane sulfonic acid. The com-bined organic phases were evaporated under reduced pressure toobtain pure compound as a pale brown solid. Yield (25.8 g, 57 mmol,83%). Melting point 108 °C. 1H NMR: δH (250 MHz; CDCl3): 0.95 (3H,t, J = 7.5 Hz, CH3), 1.36 (38H, m), 1.72 (2H, q, CH2CH2O), 2.45 (3H, s,CH3S), 3.34 (9H, s, (CH3)3N), 4.29 (2H, t, J = 7.5 Hz, CH2O), 4.49 (2H,s, CH2COO). 13C NMR: δC (62.5 MHz; CDCl3): 13.8 (CH3), 22.4, 22.5,28.9, 29.0, 29.1, 29.3, 29.4, 29.5, 31.6 (CH2), 39.4 (CH3S) 53.8 ((CH3)3),62.8 (CH2O), 66.5 (CH2CO), 164.6 (C_O). IR: ν(cm−1) 1751 (C_O).Analysis: Calculated for C28H59NO5S: C 63.86, H 11.31, N 2.76, S 6.31%.Found: C 63.64, H 11.07, N 2.53, S 6.45%.

2.3.4. Alkylbetainium tetrafluoroborate (GBOCn-BF4)A slight excess of sodium tetrafluoroborate (2.03 g, 18.5 mmol, 1.1

equiv.) was added to a solution of GBOCn-CH3SO3 (16.8 mmol, 1equiv.) in water (150–200 mL). The reaction mixture was heated(30–40 °C) and stirred for 5 h. GBOCn-BF4 which precipitated wasthen filtered off, washed with Et2O (3 × 100 mL) and dried in vacuofor 24 h to obtain pure compound as a white solid.

2.3.4.1. Hexylbetainium tetrafluoroborate (GBOC6-BF4). Yield (4.3 g,15 mmol, 89%). Melting point 58 °C. 1H NMR: δH (250 MHz; DMSO-d6): 0.95 (3H, t, J = 7.5 Hz, CH3), 1.36 (6H, m), 1.72 (2H, q, J = 7.5 Hz,CH2CH2O), 3.33 (9H, s, (CH3)3N), 4.29 (2H, t, J = 7.5 Hz, CH2O), 4.53(2H, s, CH2COO). 13C NMR: δC (62.5 MHz; DMSO-d6): 13.8 (CH3), 21.9,24.8, 27.7, 30.8 (CH2), 53.1 ((CH3)3), 62.5 (CH2O), 65.8 (CH2CO), 164.8(C = O). IR: ν(cm−1) 1750 (C_O). Analysis: Calculated forC11H24BF4NO2: C 45.68, H 8.30, N 4.79%. Found: C 45.53, H 8.50, N 4.62%.

2.3.4.2. Dodecylbetainium tetrafluoroborate (GBOC12-BF4). Yield (5.8 g,15.5 mmol, 92%). Melting point 89 °C. 1H NMR: δH (250 MHz; DMSO-d6): 0.95 (3H, t, J = 7.5 Hz, CH3), 1.34 (18H, m), 1.71 (2H, q, J =7.5 Hz, CH2CH2O), 3.31 (9H, s, (CH3)3N), 4.27 (2H, t, J = 7.5 Hz,CH2O), 4.51 (2H, s, CH2COO). 13C NMR: δC (62.5 MHz; DMSO-d6): 14.3(CH3), 22.5, 25.6, 28.2, 29.0, 29.1, 29.3, 29.4, 29.5, 31.7 (CH2), 53.6

((CH3)3), 62.9 (CH2O), 66.1 (CH2CO), 165.2 (C_O). IR: ν(cm−1) 1750(C_O). Analysis: Calculated for C17H36BF4NO2: C 54.70, H 9.72, N3.75%. Found: C 55.03, H 10.10, N 3.40%.

2.3.4.3. Tetradecylbetainium tetrafluoroborate (GBOC14-BF4). Yield (6.0 g,15.1 mmol, 90%). Melting point 96 °C. 1H NMR: δH (250 MHz; DMSO-d6): 0.94 (3H, t, J = 7.5 Hz, CH3), 1.33 (22H, m), 1.69 (2H, q, J =7.5 Hz, CH2CH2O), 3.30 (9H, s, (CH3)3N), 4.26 (2H, t, J = 7.5 Hz,CH2O), 4.51 (2H, s, CH2COO). 13C NMR: δC (62.5 MHz; DMSO-d6): 14.3(CH3), 22.5, 25.6, 28.2, 29.0, 29.1, 29.3, 29.4, 29.5, 31.7 (CH2), 53.6((CH3)3), 62.9 (CH2O), 66.2 (CH2CO), 165.2 (C_O). IR: ν(cm−1) 1751(C_O). Analysis: Calculated for C19H40BF4NO2: C 56.90, H 10.00, N3.48%. Found: C 56.90, H 10.14, N 3.17%.

2.3.4.4. Hexadecylbetainium tetrafluoroborate (GBOC16-BF4). Yield (6.7 g,15.6 mmol, 93%). Melting point 104 °C. 1H NMR: δH (250 MHz; DMSO-d6): 0.93 (3H, t, J = 7.5 Hz, CH3), 1.34 (26H, m), 1.71 (2H, q, J =7.5 Hz, CH2CH2O), 3.31 (9H, s, (CH3)3N), 4.26 (2H, t, J = 7.5 Hz,CH2O), 4.52 (2H, s, CH2COO). 13C NMR: δC (62.5 MHz; DMSO-d6): 14.1(CH3), 22.3, 28.0, 28.9, 29.1, 29.2, 31.5 (CH2), 53.4 ((CH3)3), 62.8(CH2O), 65.9 (CH2CO), 165.0 (C_O). IR: ν(cm−1) 1754 (C_O).Analysis: Calculated for C21H44BF4NO2: C 58.74, H 10.33, N 3.26%.Found: C 58.62, H 10.38, N 3.15%.

2.3.5. Alkylbetainium perchlorate (GBOCn-ClO4)An excess of sodium perchlorate (5.5 g, 39.4 mmol, 1.5 equiv.) was

added to a solution of GBOCn-CH3SO3 (26.2 mmol, 1 equiv.) in water(150–200 mL). The reaction mixture was stirred at room temperaturefor 6 h. The solid precipitate GBOCn-ClO4 was filtered off and dried invacuo for 2 days to obtain pure compound as a white solid.

2.3.5.1. Dodecylbetainium perchlorate (GBOC12-ClO4). Yield (9.6 g,24.9 mmol, 95%). Melting point 84 °C. 1H NMR: δH (250 MHz; DMSO-d6): 0.95 (3H, t, J = 7.5 Hz, CH3), 1.36 (18H, m), 1.68 (2H, q, J =7.5 Hz, CH2CH2O), 3.31 (9H, s, (CH3)3N), 4.26 (2H, t, J = 7.5 Hz,CH2O), 4.52 (2H, s, CH2COO). 13C NMR: δC (62.5 MHz; DMSO-d6): 14.3(CH3), 22.5, 25.6, 28.2, 29.0, 29.1, 29.2, 29.3, 29.3, 29.4, 31.7 (CH2),53.6 ((CH3)3), 63.0 (CH2O), 66.2 (CH2CO), 165.2 (C_O). IR: ν(cm−1)1092 (C-ClO4), 1750 (C_O). Analysis: Calculated for C17H36 ClNO6: C52.91, H 9.40, N 3.63%. Found: C 52.76, H 9.13, N 3.36%.

2.3.5.2. Tetradecylbetainium perchlorate (GBOC14-ClO4). Yield (10.1 g,24.6 mmol, 94%). Melting point 94 °C. 1H NMR: δH (250 MHz; DMSO-d6): 0.95 (3H, t, J = 7.5 Hz, CH3), 1.34 (22H, m), 1.68 (2H, q, J =7.5 Hz, CH2CH2O), 3.30 (9H, s, (CH3)3N), 4.26 (2H, t, J = 7.5Hz, CH2O),4.52 (2H, s, CH2COO). 13C NMR: δC (62.5 MHz; DMSO-d6): 14.3 (CH3),28.2, 29.0, 29.1, 29.3, 29.4, 31.7 (CH2), 53.6 ((CH3)3), 63.0 (CH2O), 66.2(CH2CO), 165.3 (C_O). IR: ν(cm−1) 1090 (C-ClO4), 1750 (C_O).Analysis: Calculated for C19H40ClNO6: C 55.63, H 9.74, N 3.38%. Found:C 55.74, H 9.63, N 3.30%.

2.3.5.3. Hexadecylbetainium perchlorate (GBOC16-ClO4). Yield (10.5 g,23.8 mmol, 91%). Melting point 99 °C. 1H NMR: δH (250 MHz; DMSO-d6): 0.95 (3H, t, J = 7.5 Hz, CH3), 1.34 (26H, m), 1.70 (2H, q, J =7.5 Hz, CH2CH2O), 3.31 (9H, s, (CH3)3N), 4.27 (2H, t, J = 7.5 Hz,CH2O), 4.52 (2H, s, CH2COO). 13C NMR: δC (62.5 MHz; DMSO-d6): 12.6(CH3), 20.8, 23.9, 27.4, 27.6, 27.7, 29.3, 29.9 (CH2), 51.9 (CH3)3, 61.3(CH2O), 64.4 (CH2CO), 163.8 (C_O). IR: ν(cm−1) 1100 (C-ClO4),1752 (C_O). Analysis: Calculated for C21H44ClNO6: C 57.06, H 10.03, N3.17, %. Found: C 56.96, H 10.01, N 3.02%.

2.3.6. Alkylbetainium dicyanamide (GBOCn-Dca)A slight excess of sodium dicyanamide (1.28 g, 14.4 mmol, 1.1

equiv.) was added to a solution of GBOCn-CH3SO3 (13.1 mmol, 1equiv.) in water (150 mL). The reaction mixture was heated(40–50 °C) and stirred for 4 h. GBOCn-Dca was then extracted with

63Y. De Gaetano et al. / Journal of Molecular Liquids 207 (2015) 60–66

EtOAc (3 × 100 mL). The combined organic phases were evaporatedunder reduced pressure to obtain white solid.

2.3.6.1. Hexylbetainium dicyanamide (GBOC6-Dca). Yield (3.4 g,12.4 mmol, 95%) as a pale yellow liquid. 1H NMR: δH (250 MHz;DMSO-d6): 0.92 (3H, t, J = 7.5 Hz, CH3), 1.33 (6H, m), 1.68 (2H, q,J = 7.5 Hz, CH2CH2O), 3.29 (9H, s, (CH3)3N), 4.26 (2H, t, J = 7.5 Hz,CH2O), 4.51 (2H, s, CH2COO). 13C NMR: δC (62.5 MHz; DMSO-d6): 14.2(CH3), 22.4, 25.3, 28.2, 31.2 (CH2), 53.6 ((CH3)3), 62.9 (CH2O), 66.2(CH2CO), 119.3 (CN), 165.3 (C_O). IR: ν(cm−1) 1749 (C_O), 2240(CN). Analysis: Calculated for C14H24N3O2: C 56.31, H 9.07, N 20.10%.Found: C 56.48, H 8.98, N 19.98%.

2.3.6.2. Dodecylbetainium dicyanamide (GBOC12-Dca). Yield (4.4 g,12.4 mmol, 95%). Melting point 58 °C. 1H NMR: δH (250 MHz; DMSO-d6): 0.95 (3H, t, J = 7.5 Hz, CH3), 1.34 (18H, m), 1.71 (2H, q, J =7.5 Hz, CH2CH2O), 3.32 (9H, s, (CH3)3N), 4.28 (2H, t, J = 7.5 Hz,CH2O), 4.53 (2H, s, CH2COO). 13C NMR: δC (62.5 MHz; DMSO-d6): 14.3(CH3), 29.1, 29.3, 29.3, 29.4, 31.7 (CH2), 53.6 ((CH3)3), 63.0 (CH2O),66.2 (CH2CO), 119.5 (CN), 165.2 (C_O). IR: ν(cm−1) 1749 (C_O),2240 (CN). Analysis: Calculated for C20H36N3O2: C 63.74, H 10.29, N15.90%. Found: C 63.51, H 10.17, N 16.23%.

2.3.6.3. Tetradecylbetainium dicyanamide (GBOC14-Dca). Yield (4.6 g,12 mmol, 92%). Melting point 61 °C. 1H NMR: δH (250 MHz; DMSO-d6): 0.96 (3H, t, J = 7.5 Hz, CH3), 1.34 (22H, m, CH2), 1.71 (2H, q, J =7.5 Hz, CH2CH2O), 3.33 (9H, s, (CH3)3N), 4.28 (2H, t, J = 7.5 Hz,CH2O), 4.55 (2H, s, CH2COO). 13C NMR: δC (62.5 MHz; DMSO-d6): 14.3(CH3), 22.5, 29.1, 29.3, 29.4, 29.5, 31.7 (CH2), 53.6 ((CH3)3), 63.0(CH2O), 66.2 (CH2CO), 119.5 (CN), 165.2 (C_O). IR: ν(cm−1) 1751(C_O), 2236 (CN). Analysis: Calculated for C22H40N3O2: C 66.29, H10.67, N 14.67%. Found: C 65.97%, H 10.67, N 14.32%.

2.3.6.4. Hexadecylbetainium dicyanamide (GBOC16-Dca). Yield (4.9 g,12 mmol, 92%). Melting point 63 °C. 1H NMR: δH (250 MHz; DMSO-d6): 0.95 (3H, t, J = 7.5 Hz, CH3), 1.34 (26H, m, CH2), 1.67 (2H, q, J =7.5 Hz, CH2CH2O), 3.32 (9H, s, (CH3)3N), 4.28 (2H, t, J = 7.5 Hz,CH2O), 4.53 (2H, s, CH2COO). 13C NMR: δC (62.5 MHz; DMSO-d6): 14.3(CH3), 22.5, 29.0, 29.1, 29.3, 29.4, 29.5, 31.7 (CH2), 53.6 ((CH3)3), 63.0(CH2O), 66.1 (CH2CO), 119.4 (CN), 165.2 (C_O). IR: ν(cm−1) 1751(C_O), 2236 (CN). Analysis: Calculated for C24H44N3O2: C 67.61, H10.85, N 13.71%. Found: C 67.56, H 10.81, N 13.60%.

2.3.6.5. Docosylbetainium dicyanamide (GBOC22-Dca). Yield (6.2 g,12.6 mmol, 96%). Melting point 80 °C. 1H NMR: δH (250 MHz; CDCl3):0.88 (3H, t, J = 7.5 Hz, CH3), 1.32 (38H, m, CH2), 1.69 (2H, q, J =7.5 Hz, CH2CH2O), 3.50 (9H, s, (CH3)3N), 4.24 (2H, t, J = 7.5 Hz,CH2O), 4.48 (2H, s, CH2COO). 13C NMR: δC (62.5 MHz; CDCl3): 14.3(CH3), 22.5, 25.6, 28.2, 29.0, 29.1, 29.4, 29.5, 31.7 (CH2), 53.6 ((CH3)3),62.9 (CH2O), 66.1 (CH2CO), 119.5 (CN), 165.2 (C_O). IR: ν(cm−1)1750 (C_O), 2236 (CN). Analysis: Calculated for C30H56N3O2: C 70.68,H 11.45, N 11,37%. Found: C 70.37%, H 11.72, N 11,62%.

Scheme 2. Synthetic route of G

2.3.7. Alkylbetainium lactate (GBOCn-Lac) xH2OA slight excess of potassium lactate (1.83 g, 14.3mmol, 1.1 equiv.) in

ethanol (50 mL) was added to a solution of GBOCn-ClO4 (13 mmol, 1equiv.) in ethanol (150 mL). The reaction mixture was stirred at roomtemperature for 17 h. The precipitated solid KClO4 was filtered off andthe filtrate was evaporated to dryness. The white solid obtained waswashed with a mixture of EtOH/Et2O (0.6/0.4) and dried in vacuo for2 days to obtain pure compound.

2.3.7.1. Dodecylbetainium lactate (GBOC12-Lac) 1,5H2O. Yield (4.8 g,12 mmol, 92%). Melting point 39 °C. 1H NMR: δH (250 MHz; DMSO-d6): 0.94 (3H, t, J = 7.5 Hz, CH3CH2), 1.16 (3H, d, J = 7.5Hz, CH3CH),1.34 (18H, m), 1.70 (2H, q, J = 7.5 Hz, CH2CH2O), 3.33 (9H, s,(CH3)3N), 3.61 (1H, q, J = 7.5 Hz, CH3CH), 4.27 (2H, t, J = 7.5 Hz,CH2O), 4.58 (2H, s, CH2COO). 13C NMR: δC (62.5 MHz; DMSO-d6): 14.3(CH3), 21.8 (CH3-CH), 22.4, 25.6, 28.2, 29.0, 29.1, 29.3, 29.4, 29.5, 31.7(CH2), 53.5 ((CH3)3), 62.9 (CH2O), 66.1 (CH2CO), 67.2 (CHOH),165.4 (C_O), 177.3 (COO−). IR: ν(cm−1) 1604 (COO−), 1750(C_O). [α]D20 = − 3.4° mL/g/dm. Analysis: Calculated forC20H44NO6.5: C 59.70, H 11.02, N 3.40%. Found: C 60.10%, H 10.84, N3.10%.

2.3.7.2. Tetradecylbetainium lactate (GBOC14-Lac) H2O. Yield (4.9 g,11.8 mmol, 91%). Melting point 48 °C. 1H NMR: δH (250 MHz; DMSO-d6): 0.95 (3H, t, J = 7.5 Hz, CH3CH2), 1.16 (3H, d, J = 7.5 Hz, CH3CH),1.36 (22H, m), 1.71 (2H, q, J = 7.5 Hz, CH2CH2O), 3.33 (9H, s,(CH3)3N), 3.63 (1H, q, J = 7 .5 Hz, CH3CH), 4.27 (2H, t, J = 7.5 Hz,CH2O), 4.57 (2H, s, CH2COO). 13C NMR: δC (62.5 MHz; DMSO-d6): 14.3(CH3), 21.7 (CH3-CH), 22.5, 25.6, 28.2, 29.0, 29.1, 29.3, 29.4, 29.5, 31.7(CH2), 53.4 ((CH3)3), 62.8 (CH2O), 66.0 (CH2CO), 67.3 (CHOH), 165.4(C = O), 177.5 (COO−). IR: ν(cm−1) 1604 (COO−), 1749 (C = O).[α]D20 = − 4.3° mL/g/dm. Analysis: Calculated for C22H47NO6: C 62.67,H 11.22, N 3.30%. Found: C 62.58, H 10.93, N 2.98%.

2.3.7.3. Hexadecylbetainium lactate (GBOC16-Lac) H2O. Yield (4.6 g,10.8 mmol, 95%). Melting point 58 °C. 1H NMR: δH (250 MHz; DMSO-d6): 0.95 (3H, t, J = 7.5 Hz, CH3CH2), 1.18 (3H, d, J = 7.5 Hz, CH3CH),1.34 (26H, m), 1.71 (2H, q, J = 7.5 Hz, CH2CH2O), 3.34 (9H, s,(CH3)3N), 3.63 (1H, q, J = 7.5Hz, CH3CH), 4.28 (2H, t, J = 7.5 Hz,CH2O), 4.57 (2H, s, CH2COO). 13C NMR: δC (62.5 MHz; DMSO-d6): 14.3(CH3), 21.7 (CH3-CH), 22.5, 25.6, 28.2, 29.0, 29.1, 29.3, 29.4, 29.5, 31.7(CH2), 53.4 ((CH3)3), 62.8 (CH2O), 66.0 (CH2CO), 67.2 (CHOH),165.4 (C_O), 177.5 (COO−). IR: ν(cm−1) 1604 (COO−), 1750(C_O). [α]D20 = − 4.3° mL/g/dm. Analysis: Calculated forC24H51NO6: C 63.97, H 11.23, N 2.98%. Found: C 64.11, H 11.43, N3.11%.

3. Results and discussion

3.1. Synthesis

To develop a greener approach for the synthesis of novel ILs, one ofthe criteria is tominimize the use of solvents. Indeed, the presence of or-ganic solvent traces (acetone, dichloromethane or acetonitrile) in ILsoccasionally leads to the formation of undesired compounds [44].

BOCn-X ionic liquids (ILs).

Scheme 3. Synthetic route of GBOCn-Lac ILs.

64 Y. De Gaetano et al. / Journal of Molecular Liquids 207 (2015) 60–66

Moreover, none of these solvents is welcome in the chemical industrybecause of volatility, safety and environmental issues. The strategy de-veloped here considers the use of environmental friendly startingmate-rials. Experiments were carried out with ethyl acetate, ethanol andwater as solvents. The synthesis of ILs was performed through twosteps, except for ILs with lactate anions which require three steps. Thefirst step is the synthesis of the methanesulfonic salt of the cationicester of Glycine Betaine by esterification of Glycine Betaine with prima-ry alcohols using methanesulfonic acid (MSA) as catalyst followingthe methodology developed by Goursaud et al. [43]. The choice ofmethanesulfonic acid is compatible with the development of green syn-thetic route. Indeed, methanesulfonic acid is an easy-to-handle liquid,less aggressive than other inorganic acids such as sulfuric acid. MSAtakes part to the natural sulfur cycle [43] and is readily biodegradable,by forming sulfates and carbon dioxide. Themethane sulfonate ester de-rivatives of betaineGBOCn-CH3SO3were successfully synthesized by thereaction of the corresponding alcohols with GB, as revealed by givingthe good yields obtained (61–93%). The second step is the synthesis ofILs by anionic metathesis from the methanesulfonic salt using a salt ofthe anion constitutive of ionic liquids. The overall synthetic route isgiven in Scheme 2.

The anionic metathesis between lactate and methanesulfonateanion was incomplete with GBOCn-CH3SO3, due to a competition be-tween methane sulfonate and lactate anion. For this reason anotherstrategy was implemented for the synthesis of the ILs with lactateanion. In this case, the ILs were synthesized by anionic metathesis be-tween the corresponding perchlorate ILs and the lactate potassiumsalt. In this procedure, the potassium perchlorate formed was precipi-tated in ethanol and eliminated by filtration. The synthesis and purifica-tion procedure lead to yields higher than 85%with a high level of purity(Scheme 3).

Table 1Melting point (Tm), glass transition (Tg), ratio Tg/Tm and decomposition (Tdc) tempera-tures for Glycine Betaine derivatives ILs.

Ionic Liquids Tm (°C) Tg (°C) Ratio Tg/Tm Tdc (°C)

GBOC6-CH3SO3 103 – 203GBOC12-CH3SO3 104 –1 0.72 237GBOC14-CH3SO3 104 – – 254GBOC16-CH3SO3 105 – – 257GBOC22-CH3SO3 108 – – 273GBOC6-Dca – – – 181GBOC12-Dca 58 −44 0.69 231GBOC14-Dca 61 – – 268GBOC16-Dca 63 – – 281GBOC22-Dca 80 −4 0.76 323GBOC6-BF4 58 −34 0.72 305GBOC12-BF4 89 −18 0.70 316GBOC14-BF4 96 – – 319GBOC16-BF4 104 – – 332GBOC12-ClO4 84 −14 0.72 283GBOC14-ClO4 94 – – 288GBOC16-ClO4 99 – – 293GBOC12-Lac 39 – – 179GBOC14-Lac 48 – – 212GBOC16-Lac 58 −32 0.73 238

1H NMR chemical shifts of the CH2–C_O, CH2–O, CH3–N and CH3–Sare remarkably similar and cannot provide information regarding thepresence or the absence of hydrogen bonding between the anions andthe cations of the ILs. In addition, the IR spectra for all the ILs showonly weak absorption bands in the 3000–3100 cm−1 region, that indi-cates that the interaction between the anion and the cation of ILs via hy-drogen bonds is rather limited. In this case, only the anion–cationcoulombic attraction ensures the cohesion of the salts.

All the ILs except, GBOC6-Dca, were obtained as white powder. Themethanesulfonate salts GBOCn-CH3SO3 are hydrophilic. The anionicme-tathesis between GBOCn-CH3SO3 and perchlorate, tetrafluoroborate ordicyanamide anions generates hydrophobic ionic liquids whereas hy-drophilic lactate ionic liquids were obtained from GBOCn-ClO4. ForGBOCn-Dca, the miscibility with water depends on the alkyl chainlength: GBOC6-Dca is hydrophilic, GBOCn-Dca (n = 12, 14 or 16) areslightly soluble, and GBOC22-Dca is hydrophobic. All the solid ILs arewater- and air- stable. The lactate salts crystallize with one or morewater molecules. The presence of water molecules is confirmed by thelarge and intense band in the 3200–3600 cm−1 of the IR spectra corre-sponding to OH stretching.

All the salts, except GBOC22-Dca, are soluble in ethyl acetate andchloroform.

3.2. Physicochemical properties

3.2.1. Melting point and glass transitionAs pointed out by Tao et al. [44], the esterification of amino acid is a

successful strategy to prepare cationic synthons able to generate suc-cessfully various ILs by combining with a proper anion. Esterificationof amino acid suppresses the hydrogen bonds involving carboxylicgroups,while having little influence on the ammonium function. Conse-quently, the anion–cation associations generatedwith cationic ester de-rivatives exhibit lower melting points than their analogous generatedwith amino acids. It increases the likelihood of generating associations,incorporating biosourced molecules, with physicochemical propertiescorresponding to the required criteria to be defined as an ionic liquid.Furthermore, it should be noted, that esterification provides the possi-bility of adjusting the properties of the resulting ILs by varying the na-ture of the substituents or the length of the alkyl chain grafted byesterification reaction. The “designer solvent” character of ILs is thusenhanced.

It is clear that among the salt synthesized (Table 1), GBOCn-CH3SO3

(n = 6, 12, 14, 16 and 22) and GBOC16-BF4 compounds can be consid-ered as molten salts due to their melting point higher than 100 °C.But all other compounds are solid ILs at room temperature, exceptGBOC6-Dca which is liquid.

As shown in Fig. 1 and in Table 1, both cations and anions contributeto the lowmelting points of ionic liquids. Lowermelting points concernILs containing anions such as lactate and dicyanamide. For the samecation GBOCn+ (n = 12, 14 or 16), the melting point follows the trend:Lac− b Dca− b ClO4

− b BF4−.For the same anion, the melting points increase linearly with the

lengthening of alkyl chain of the cation as observed by Holbrey et al.with1-alkyl-3methyimidazolium salts [45]. This increase depends onthe nature of the anion. With inorganic anions (ClO4

− or BF4−), the

Table 2Viscosity for Glycine Betaine derivatives ILs.

Viscosity (cP)

T = 80 °C T = 90 °C

GBOC6-BF4 118.0 45.1GBOC6-Dca 46.0 23.2GBOC12-Dca – 57.8GBOC12-Lac 150.7 30.7GBOC14-Lac 365.0 132.3GBOC16-Lac 672.1 195.0

Fig. 1.Melting point evolution of ionic liquids GBCn-X. (n= 6, 12, 14, 16 and 22). (circle)X = lactate, (cross) X = dicyanamide, (triangle) X = perchlorate and (square) X =tetrafluoroborate.

65Y. De Gaetano et al. / Journal of Molecular Liquids 207 (2015) 60–66

lengthening of alkyl group of two atoms of carbon leads to an averageincrease of melting point of 7.5 °C, whereaswith organic anion (lactate)this increase is 9.5 °C. It is to be noted that the melting point of ILs withDca− anion is almost between those of ILswith organic (Lac−) and inor-ganic (ClO4

− or BF4−) anions. However, the increasing of the number ofthe atoms of carbon of alkyl chain causes a small increase in meltingpoint of these Dca-based ILs (average value, 2.5 °C).

The solid–liquid phase transition of these hydrophobic ionic liquidshas been studied by differential scanning calorimetry (DSC). In allcases, the salts exhibited amarked tendency to supercool before crystal-lizing or to form glasses (Fig. 2).

Most of ionic liquids exhibit a weak DSC signature of a glass tran-sition and the corresponding temperature (Tg) has been determinedonly for a limited number of ionic liquids (see Table 1). The only ob-servation that can be made is that for ILs containing the same cationGBOCn

+ with different anions, the glass transition temperature in-creases as follow: Dca− b BF4− b ClO4

−. This trend is in agreementwith the results previously obtained with ethyl ester betaine deriva-tives [46]. Another property, related to the melting point, is the glasstransition temperature of a molecule. From the literature, it is knownthat the glass transition temperature is approximately two-thirds ofthe melting point value expressed in Kelvin degree [47]. The range of

Fig. 2. DSC scan fo

experimental Tg/Tm ratio was found between 0.58 and 0.78 for dif-ferent molecules and polymers [48]. The same ratio range is also ex-pected for the melting points and glass transition temperatures ofthe prepared ILs. The Tg/Tm ratios were determined where the ILsshowed a melting point and a glass transition temperature. Thevalues ranging between 0.69 and 0.76, match relatively well to thevalues reported in the literature [48].

3.2.2. Thermal stabilityThermal stability of the ionic liquids was studied by thermal gravi-

metric analysis (TGA) over the temperature range of 30–500 °C. Ther-mal degradation temperature (Tdc) of these salts lies in the range of217–332 °C (Table 1). Both cations and anions have a strong influenceon the thermal stability of these ILs. The results show that the stabilityis enhanced when the alkyl chain length of the cation increases. For ex-ample, with the GBOCn-Dca (n = 6, 12, 14, 16, 22) series, the degrada-tion temperature increases from 181 to 323 °C when the number of theatoms of carbons for alkyl groups grafted on Glycine Betaine increasesfrom 6 to 22 (Table 2). These Tdc values also show that the thermal sta-bility is widely influenced by the nature of the anion. With the sameGBOCn

+ cation, the thermal stability increases in the order of Lac−

bDca− b ClO4− b BF4−. Indeed, with fluorinated anion, the thermal stabil-

ity of the ILs is significantly enhanced. For example with the sameGBOCn+ cation, ILs with BF4− and Dca− anions display degradation tem-perature rangingbetween (305–332) °C and (181–281) °C, respectively.The higher stability of BF4− ionic liquid is related to the weaker coordi-nating properties of this anion. Indeed, it is generally observed thatthe weaker the coordinating anion, the higher the thermal stability ofILs [49].

r GBOC6-BF4.

66 Y. De Gaetano et al. / Journal of Molecular Liquids 207 (2015) 60–66

3.2.3. ViscosityThe relatively high melting point of ILs limits the temperature

range for viscosity measurements and the number of ILs that canbe studied by this technique. Thus, for ILs having a melting pointbelow 70 °C (GBOC6-Dca, GBOC6-BF4, GBOC12-Dca, GBOC12-Lac,GBOC14-Lac and GBOC16-Lac), the viscosity was measured for tem-perature ranging between 80 and 90 °C (Table 2).

As expected, the viscosity decreases slightly with the increase oftemperature. The higher viscosity of GBOC6-BF4, compared to thisof GBOC6-Dca is ascribed to the intermolecular friction generatedby the anion. The intermolecular friction generated by rod-shapedDca− anion is thought to be smaller than that of spherical BF4−

anion, resulting in their different viscosities [45]. Similar interpreta-tion can be given to explain the difference of viscosity betweenGBOC12-Dca and GBOC12-Lac. The determined viscosities ofGBOC12-Lac, GBOC14-Lac and GBOC16-Lac at 90 °C are 30.7 cP,132.3 cP and 195.0 cP, respectively. In accordance with previousworks [49–53], the lengthening of the alkyl chain leads to an increaseof viscosity. For the series of ILs with lactate anions, the increasecould be attributed to the interactions of Van der Waals betweenthe alkyl chains [53].

4. Conclusion

The data of physical properties on ionic liquids are essential for boththeoretical research and industrial applications. In this work, new“green” ionic liquids containing an esterified Glycine-Betaine with dif-ferent alkyl chains and different inorganic and organic anions havebeen synthesized. Their physicochemical properties such as glass transi-tion and decomposition temperature, viscosity, and melting point havebeen determined in order to study the relationship between structureand properties. In all cases, the lengthening of the alkyl chain enhancesthe thermal stability of these ionic liquids. The thermal and viscosityproperties of these ILs with the inorganic anions are totally differentcompared to those with organic anions.

Due to the inherent advantages of the ionic liquids, new analyticalmethods should be developed in thesemedia, especially for the organicand inorganic compounds. For example, studying the fundamental as-pects of metal ion or organic compound partitioning into ILs GBOCn-Xsolvent. Moreover, studies of interfacial properties to determine thepossibility of these compounds of forming self-assembly, that can beused for solvent-based applications or in extraction processes are inprogress.

Acknowledgments

We thank the Conseil Régional de la Marne for a grant to Y. DeGaetano and for its financial support.

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