Thermophysical properties and acute toxicity towards ...

Journal of Molecular Liquids 212 (2015) 352–359

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Thermophysical properties and acute toxicity towards green algae and Vibrio fischeri of amino acid-based ionic liquids Ouahid Ben Ghanem a,⁎, Nicolas Papaiconomou b,c,d, M.I. Abdul Mutalib a, Sylvie Viboud e, Mohanad El-Harbawi f, Yoshimitsu Uemura a, Girma Gonfa a, M. Azmi Bustam a, Jean-Marc Lévêque g,⁎ a

Faculty of Chemical Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 31750 Tronoh, Perak Malaysia Univ. Savoie, LEPMI, 73000, Chambéry, France Univ. Grenoble-Alpes, LEPMI, 38000, Grenoble, France d CNRS, LEPMI, 38000, Grenoble, France e Laboratoire CARRTEL, UMR INRA 42, CISM, Université de Savoie, 73376 Le Bourget-du-Lac Cedex, France f Chemical Engineering Department, King Saud University, Riyadh 11421, Saudi Arabia g Fundamental and Applied Sciences Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, Tronoh, Perak 31750, Malaysia b c

a r t i c l e

i n f o

Article history: Received 6 July 2015 Received in revised form 1 September 2015 Accepted 9 September 2015 Available online xxxx Keywords: Amino acids Ionic liquids Thermo-physical properties Toxicity Scenedesmus quadricauda (green algae) Vibrio fischeri

a b s t r a c t Four new ionic liquids (ILs) based on 1-(2-hydroxyethyl-3-methylimidazolium) cation with glycinate, serinate, alaninate, and prolinate amino acid anions have been synthesized, and their thermo-physical properties (density, viscosity, surface tension, and heat capacity) were measured. Data were described using empirical expressions to determine other physical–chemical properties, such as molecular volume, standard molar entropy, and lattice energy. Moreover, acute toxicity tests toward Scenedesmus quadricauda (green algae) and Vibrio fischeri have been conducted. The high values obtained for the 50% effective concentration (EC50) indicated that these new ionic liquids can be considered eco-friendly. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Ionic liquids (ILs) are organic salts with melting points typically below 100 °C. ILs are composed solely of bulky asymmetric organic cations and organic or inorganic anions. For more than a decade, ILs have been the subject of increasing interest in the scientific community because of their remarkable physical and chemical properties, notably, negligible vapor pressure, high thermal and chemical stability, wide liquid range, and electrochemical windows [1,2]. Moreover, the high number of possible combinations between cations and anions allows the properties of ILs to be developed for a specific task. Hence, ILs can be strategically designed for different applications that range from solvents in organic chemistry to lubricating agents or electrochemical media [2,3]. However, recent investigations indicate that most ILs are toxic to aquatic organisms and human cells [4–6]. Nowadays, the toxicity of ILs remains one of the main obstacles that limit potential commercial applications. Hence, the aim of this work is the design and synthesis

⁎ Corresponding authors. E-mail addresses: [email protected] (O.B. Ghanem), [email protected] (J.-M. Lévêque).

http://dx.doi.org/10.1016/j.molliq.2015.09.017 0167-7322/© 2015 Elsevier B.V. All rights reserved.

of the least toxic ILs to be used in high-scale processes without damaging the environment. Several groups have prepared different types of ILs that bear natural compounds or an oxygenated functional group to decrease the overall toxicity of these moieties compared with classical ILs that only bear saturated alkyl chains. Ionic liquids embedding hydroxyl groups onto the alkyl chain of the cation, such as [C 2 OHmim] (1-(2-hydroxylethyl)3-methylimidazolium), were first synthesized and characterized by Branco et al. [7] in 2002. These ionic liquids, which contain different anions such as chloride [Cl], trifluoroacetic acid [TFA], hexafluorophosphate [PF6], and tetrafluoroborate [BF4], exhibit higher polarity/solvation properties [8] than analogues without the hydroxyl group. Furthermore, introducing oxygen in the form of hydroxyl, ether, or ester functional groups on imidazolium side chains decreased toxicity [4,9]. Fukumoto et al. [10] studied amino acid based ionic liquids (AAILs) by combining 1-ethyl-3-methylimidazolium with amino acid anions. Tao et al. [11] reported also AAILs based on nitrate anion and cations functionalized with an amino acid functional group. Since then, many more AAILs have been studied, including some AAILs embedded with two amino acid functional groups appended in both the cation and anion moieties [12].

O.B. Ghanem et al. / Journal of Molecular Liquids 212 (2015) 352–359

Amino acids can be considered a promising component for the synthesis of new ionic liquids that exhibit low toxicities because of their biological nature, low cost [12,13], and ability to incorporate two functional groups, namely, amino and carboxylic acid groups. Amino acids are also known to exhibit strong hydrogen-bonding ability, which makes them potential media for various applications, such as biomass dissolution reaction medium and gas separation [12–15]. Therefore, this work reported on the synthesis and characterization of four new AAILs that incorporate four different AA anions with 1(2-hydroxylethyl-3-methylimidazolium) cation, offering a new set of RTILs, namely, 1-(2-hydroxyethyl)-3-methylimidazolium glycinate ([C2OHmim][Gly]), 1-(2-hydroxyethyl)-3-methylimidazolium alaninate ([C2OHmim][Ala]), 1-(2-hydroxyethyl)-3-methylimidazolium Serinate ([C2OHmim][Ser]), and 1-(2-hydroxyethyl)-3-methylimida zolium prolinate ([C2OHmim][Pro]). The structures of the studied AAILs are depicted in Fig. 1. Physicochemical properties such as density, viscosity, surface tension, and heat capacity in various temperatures and atmospheric pressures are reported. Thermal decomposition temperature was also investigated. Molecular volume, standard molar entropy, and lattice energy were calculated using empirical correlations. In addition, the toxicity of AAILs toward Senedesmus quadricauda (S. quadricauda) (green algae) and Vibrio fischeri (V. fischeri) was studied to determine if these are environment friendly. Green algae and V. fischeri were chosen because these are two of the most common test organisms in the Aquatic Toxicity Information Retrieval database (AQUIRE) published by the US Environmental Protection Agency (EPA). In addition, several organizations have recommended these species for aquatic toxicity assessment [16,17].

2. Experimental section 2.1. Materials All the starting materials were used as received without any further treatment. These materials include 1-methylimidazole (Merck, ≥99%), 2-bromoethanol (Merck, N99.8%), L-glycine (Merck, ≥ 99%), L-alanine (Merck, ≥ 99%), L-serine (Merck, ≥ 99.2%), L-proline (Merck, ≥ 99%), ethyl acetate (Fisher Scientific UK, ≥99.99%), ethanol (Merck, ≥99.9%), and ion exchange resin, Amberlite IRA-402 (OH) (Alfa Aesar).

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2.2. Synthesis of AAILs The AAILs were prepared according to established methods [10,11]. The process is described briefly as follows: 2-bromoethanol was carefully added dropwise to 1-methylimidazole in a three-necked, roundbottom flask equipped with a reflux condenser. The system was mixed under nitrogen for 24 h at room temperature. The resulting viscous liquid (1-(2-hydroxyethyl)-3-methylimidazolium bromide) solidified after being washed with ethyl acetate and then was dried under vacuum. The 1-(2-hydroxyethyl)-3-methylimidazolium bromide was dissolved in distilled water and passed through an anion-exchange column filled with excess of Amberlite IRA-402 to minimize bromide content. An excess amount of amino acid salt was added to the solution of 1-(2-hydroxyethyl)-3-methylimidazolium hydroxide that was collected from the resin. The solution was gently mixed for 12 h, and water was removed under low vacuum. The resulting AAILs were dissolved in dry ethanol to separate the unreacted amino acids, and the solutions were filtered. Finally, ethanol was evaporated, and the AAILs were dried in a vacuum line for 72 h. 2.3. Water and halide content Before measuring physical–chemical and thermal properties, all AAILs were further dried under low pressure by being kept in a vacuum oven for few hours at 80 °C. The water content of all AAILs was determined using a coulometer Karl Fischer titrator, DL 39 (Mettler Toledo) with the Hydranal Coulomat AG reagent (Riedel-de Haen). For each AAIL, water content was determined by calculating the average of three measurements. The bromide content in the final products were determined using Metrohm model 761 Compact IC. For the measurement, ionic liquid solutions were prepared by dissolving the AAILs in ultrapure water, where the eluent was prepared using mixtures of Na2CO3 and NaHCO3. 2.4. Density and viscosity Density and viscosity were measured over a temperature of 293.15 K to 373.15 K at atmospheric pressure using an Anton Paar densitometer (SVM3000). The instrument was calibrated with Millipore-grade water to establish the data.

Fig. 1. Structure of the studied AAILs.

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Table 1 Chemical formula, molar mass, water and halide content for the synthesized AAILs.

Chemical formula Molar mass, g·mol−1 Halide, ppm w, ppm

[C2OHmim][Gly]

[C2OHmim][Ala]

[C2OHmim][Ser]

[C2OHmim][Pro]

C8H15N3O3 201.22 66 510

C9H17N3O3 215.25 78 255

C9H17N3O4 231.25 55 476

C11H19N3O3 241.29 76 850

2.5. Surface tension Surface tensions of the AAILs were determined using pendant drop method. Drops generated by syringe were photographed using a camera (OCA 20). The shapes of the generated drops were evaluated using SCA 22 software. The measurements were recorded over a temperature range of 293.15 K to 353.15 K. 2.6. Thermal decomposition temperature The onset temperatures of the AAILs were measured using a PerkinElmer, Pyris V-3.81 thermal gravimetric analyzer. Samples were heated from 298.15 K to 873.15 K in a crucible under a nitrogen atmosphere. The ILs samples were heated at a rate of 10 K/min. 2.7. Glass transition and heat capacity measurements Glass transition temperatures and heat capacity were determined using a differential scanning calorimeter [Mettler Toledo. MODEL (DSC1/500)]. For glass transition, the samples were kept in sealed aluminum pans and heated at 10 K/min from room temperature to 423.15 K under liquid nitrogen. The samples were cooled to 143.15 K and reheated again (10 K/min) to 423.15 K. The heat capacities of the AAILs were determined using the same instrument. The samples were sealed in an aluminum pan and kept at constant temperature (258.15 K) for 15 min, followed by a constant heating rate of 20 K/min to 358.15 K, at which point the temperature was kept constant for 15 min. 2.8. Acute toxicity test toward green algae and V. fischeri Acute toxicity test toward S. quadricauda (green algae) was conducted according to (OECD-201) [18] to investigate the effect of the newly synthesized AAILs on the aquatic organisms. The strain was obtained from Algaetech International@ Sdn Bhd (Technology Park Malaysia). The green algae was cultured for seven days in B11 medium [19] on distilled water. After the culture period, optical density was checked, and the strain was diluted using a fresh medium to allow exponential growth [20]. The AAILs were dissolved in 50 ml of cultured medium in conical flasks. All the studied AAILs were found to be completely soluble in the medium, and no co-solvent was used. The flasks were placed in

incubator orbital shakers (SI 600-Lab companion) at 25 °C ± 2 with a shaking speed of 150 rpm. During the test, light was supplied continuously by two fluorescent lamps (130 μmol·photons·m−2 s−1). Seven different concentrations ranging from (1000 to 10000) mg·L−1 with three replicates for each concentration were used in the study for each AAIL. After 96 h, optical density was checked using a Shimadzu UV-2600 Spectrophotometer, and the growth against AAIL concentration plots was constructed to determine EC50 values. The toxicity of AAILs was also assessed using a bioluminescent marine bacteria, V. fischeri, where light emission is inhibited when the bacteria is grown under toxic conditions. The adopted technique in this work follows the published international standard. [21,22] Tests were performed according to a previously reported procedure [23]. In this work, the toxicity values are indicated in tables expressed as (EC50 at 15 min using concentration unit of mg·L− 1). EC50 15 min refers to the toxicity values measured 15 min after the V. fischeri was in contact with the IL. 3. Results and discussion The structures of the synthesized AAILs were characterized by recording 1HNMR spectra utilizing a Bruker Advance 500 MHz spectrometer. Details are given below. [C2OHmim][Gly]: 1H NMR (500 MHz, DMSO) δ = 2.79 (s, 2H), 3.71 (t, 2H), 3.92 (s, 6H), 4.26 (t, 2H), 7.74 (s, 1H), 7.79 (s, 1H), 9.37 (s, 1H). [C2OHmim][Ala]: 1H NMR (500 MHz, DMSO) δ = 1.04 (d, 3H), 2.99 (quad, 1H), 3.6 (s, 2H), 3.72 (t, 2H), 3.87 (s, 4H), 4.27 (t, 2H), 7.68 (s, 1H), 7.75 (s, 1H), 9.38 (s, 1H). [C2OHmim][Ser]: 1H NMR (500 MHz, DMSO) δ = 2.89 (t, 1H), 3.31 (d, 2H), 3.6 (s, 2H), 3.72 (m, 3H), 3.87 (s, 4H), 4.23 (t, 2H), 7.68 (s, 1H), 7.73 (s, 1H), 9.24 (s, 1H). [C2OHmim][Pro]: 1H NMR (500 MHz, DMSO) δ = 1.62 (m, 2H), 1.88 (m, 2H), 2.7 (t, 2H), 3.3 (s, 1H), 3.69 (t, 1H), 3.71 (t, 2H), 3.87 (s, 4H), 4.22 (t, 2H), 7.69 (s, 1H), 7.75 (s, 1H), 9.29 (s, 1H). 3.1. Water and halide content The water and halide contents of the four AAILs studied in this work are reported in Table 1. The ionic liquid based on alaninate

Table 2 Experimental density, ρ, and dynamic viscosity, η for the studied AAILs at a temperature range of 293.15 K to 373.15 K and 0.1 MPaa. T

[C2OHmim][Gly] ρ/g·cm

293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 a

1.3736 1.3669 1.3603 1.3535 1.3468 1.3404 1.3341 1.3279 1.3218

−3

[C2OHmim][Ala] η/mPa·s

ρ/g·cm

6528.50 2283.50 921.75 428.23 219.22 124.41 76.54 50.28 34.81

1.2317 1.2254 1.2190 1.2124 1.2061 1.2000 1.1939 1.1879 1.1821

−3

[C2OHmim][Ser] η/mPa·s

ρ/g·cm

9964.30 3177.50 1205.90 526.60 259.61 142.77 85.62 55.13 37.63

1.3139 1.3075 1.3010 1.2946 1.2880 1.2815 1.2753 1.2693 1.2634

Standard uncertainties are u(T) = ±0.01 K, u(ρ) = ±0.0002 g/cm3 and u(η) = ±0.48% mPa·s.

−3

[C2OHmim][Pro] η/mPa·s

ρ/g·cm−3

η/mPa·s

31054.00 8813.40 3039.30 1218.00 555.82 283.51 158.94 96.32 62.32

1.2404 1.2341 1.2279 1.2217 1.2153 1.2093 1.2033 1.1973 1.1913

48680.80 10818.00 3513.51 1415.53 636.217 305.17 170.52 102.00 70.10


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Table 3 Fitting parameter values, correlation coefficient, R2 and standard deviation, SD of Eqs. (1) and (2)a. [C2OHmim][Gly] [C2OHmim][Ala] [C2OHmim][Ser] [C2OHmim][Pro] A0 −6.84777 −7.36322 −8.1496 −8.58218 3089.2511 3288.9792 3662.0042 3827.6813 A1 0.988 0.990 0.990 0.982 R2 SD 0.074503 0.0844 0.0882 0.1142 A2 1.5636 1.4139 1.4999 1.42018 4 −6.4950 −6.2250 −6.3517 −6.1400 A3 ∗ 10 0.9997 0.9997 0.9997 0.9999 R2 SD 3.1282 2.9107 2.7797 1.4435 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2 ∑ðZ exp −Z cal Þ . nDAT, Zexp, and Zcal are the number of experimental points, expera SD ¼ nDAT

imental and calculated data values, respectively.

Fig. 2. Viscosities as a function of temperature for ■, [C2OHmim][Gly]; , [C8mim][Ala]; , [C2OHmim][Ser]; , [C2OHmim][Pro].

exhibits the lowest water content, and its prolinate homologue has the highest water content. Water and halide contents are low in all cases, which indicates high purity of approximately 99% for all AAILs. 3.2. Viscosity and density Viscosity and density were measured between 293.15 K and 373.15 K using an Anton Paar viscometer. The corresponding data are presented in Table 2 and plotted in Figs. 2 and 3. Experimental viscosities, η, were fitted using the following equation: logðηÞ ¼ A0 þ

A1 T

ð1Þ

where η stands for the viscosity in mPa·s. The values of A0 and A1 constants for each of the AAILs are summarized in Table 3 along with the standard deviation. Viscosity is observed to follow the trend ½C2 OHmim½Gly b ½C2 OHmim½Ala b ½C2 OHmim½Serb ½C2 OHmim½Pro: The presence of both amine and hydroxyl groups strengthens hydrogen bonding. Thus, the viscosities of the AAILs are significantly higher than those obtained for typical ionic liquids based on fluorinated anion and bis(trifluoromethylsulfonyl)imide [24,25].

Fig. 3. Densities as a function of temperature: ■, [C2OHmim][Gly]; [C2OHmim][Ser]; , [C2OHmim][Pro].

, [C8mim][Ala];

,

Interestingly, the presence of the hydroxyl functional group on the short C2 alkyl chain increases the viscosity of the AAILs than the ones pairing an alkyl chain length ranging from C2 to C8 without the hydroxyl group. For example, the viscosity of the AAILs at 293.15 K of [C2mim] analogue pairing the same studied amino acid anions was found to be between 93.15–611.87 mPa·s and increased to 626.95–2853.00 mPa·s for [C8mim] analogue [26,27]. By contrast, the least viscous AAIL is [C2OHmim][Gly] with a viscosity as high as 6500.00 mPa·s at the same temperature. In agreement with previous reports, the observed increase in viscosity can be linked to the presence of the hydroxyl group on the cation that leads to the formation of hydrogen bonds. The densities for the four AAILs were measured within the temperature range of 293.15 K to 373.15 K at atmospheric pressure. Results are indicated in Table 2. At any studied temperature, the density follows the trend [C2OHmim][Gly] N [C2OHmim][Ser] N [C2OHmim][Pro]–[C2OHmim][Ala]. As expected, these results show that [C2OHmim] cation with [Gly] anion, which is the smallest involved anion of all four AA, yields to the denser ionic liquid, which is the opposite of what was observed in viscosity. Alaninate anion corresponds to a glycinate anion with an additional methyl group. Hence, IL with a density lower than that of its glycinate homologue is expected to be produced. Adding an –OH group on the alaninate anion establishes the structure of the serinate anion, increases hydrogen bonding, and results in a higher density. Hence, the density of serinate-based ILs is expected to be always higher than that of alaninate homologues. On the contrary, prolinate anion that contains a large number of hydrocarbons yields to ionic liquids with reduced density. As reported for all ILs thus far, densities decrease with temperature. The densities reported here are higher than those for the other imidazolium homologues that contain the same anions. For instance, at 303 K, the densities for 1-ethyl-3-methyl-imidazolium glycinate ([C2mim][Gly]) and [C2OHmim][Gly] are 1.1559 and 1.3669, respectively. Densities for 1-ethyl-3-methyl-imidazolium alaninate ([C2mim][Ala]) and [C2OHmim][Ala] are 1.1180 and 1.2254 g·cm−3, respectively [28, 29]. As pointed out by Shokouhi, et al. [30] and Fakhraee et al. [31] through experimental and theoretical calculations, the hydroxyl functional group creates a hydrogen bond and yields in stronger interactions between the imidazolium cations and the amino acid anions. This, in turn, leads to more packed ionic liquids than their [C2mim] homologues. The densities of the four AAIL are lower than those of other hydroxyl-functionalized imidazolium homologues with the conventional anions. For example, the density values of [C2OHmim][PF6], [C2OHmim][TfO] and [C2OHmim][Tf2N] at 313.15 K reported by Fakhraee et al. [31] are 1.495, 1.593, and 1.649 g·cm−3 respectively. The lower density is caused by the significantly higher molecular weight of the fluorinated anions.

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Table 4 Molecular volume, Vm, standard entropy, S0, and crystal energy, UPOT of the AAILs at 298.15 K and at 0.1 MPa.

V (nm3) S° (J·K−1·mol−1) UPOT (kJ·mol−1)

[C2OHmim][Gly]

[C2OHmim][Ala]

[C2OHmim][Ser]

[C2OHmim][Pro]

0.2443 334 479

0.2915 393 458

0.2935 395 457

0.3244 434 445

The experimental density values (ρ) for the AAILs with temperature (T) were fitted by applying the following equation: ρ g:cm−3 ¼ A2 þ A3 T

ð2Þ

where ρ denotes to the density of the AAILs, A2 and A3 are correlation coefficients, and T is temperature in Kelvin. The estimated values for the correlation coefficients are presented together with standard deviation values, SD in Table 3. The density of all the AAILs shows a linear correlation with temperature. Parameter A1 appears to be more negative than that obtained for other ionic liquids, which indicate the sensitivity of the AAILs to temperature. The experimental values of density, ρ, for the AAILs were used to calculate the molecular volumes, Vm, molar entropy, S°, and lattice energy, UPOT, using Eqs. (3), (4), and (5). M Vm cm3 ¼ ρNA

ð3Þ

−1 ≈1246:5 V nm3 þ 29:5 S0 J:K−1 mol

ð4Þ

1 −1 ¼ 1981:2 ρ =M =3 þ 103:8 UPOT kJ:mol

ð5Þ

where M is the molecular weight (g·mol−1), and NA is the Avogadro's constant. Vm, S°, and UPOT values calculated at 303.15 K for the AAILs are listed in Table 4. The Vm value increases in the order [C2OHmim][Gly] b [C2OHmim] [Ala] b [C2OHmim][Ser] b [C2OHmim][Pro]. In general, the molecular volume of the current hydroxyl functionalized AAILs are lower than those reported for non-functionalized AAILs. For example, the molecular volume of [Cnmim][Ala] (for n = 2,3,4,5,6,8) are 0.2948, 0.3222, 0.3492, 0.3772, 0.4062, and 0.4784 nm3 [27,28]. The decrease in molar volume for the four AAILs is most likely caused by the stronger interactions between the cation and anion, possibly intramolecular hydrogen bonding, which results in more compact molecules. The molecular volume and standard entropy follow the same trend. The values of the standard entropy of the AAILs are lower than those for AAILs with non-functionalized alkyl chain. For instance, in this study, the standard entropy for [C2OHmim][Ala] is 393 J·K−1·mol−1, where the values of standard entropy reported for [Cnmim][Ala] (for n = 2,3,4,5,6,8) are 396.9, 431.1, 464.8, 499.7, 535.8, and 625.79, respectively [27,28]. The lower entropy value suggests better organization for the AAILs in the presence of hydroxyl functional group introduced in the alkyl spacer. This finding is consistent with the assumption of increased hydrogen bonding network within such ionic liquids.

The crystal energies of the present AAILs are significantly lower than those of inorganic fused salts. The crystal energy for cesium iodide, which has the lowest crystal energy among the alkali-chlorides, is 613 kJ·mol−1 [32]. The low crystal energy of AAILs is the underlying reason for the formation of liquid salts at room temperature. UPOT of the present AAILs decreases in the order [C2OHmim][Gly] N [C2OHmim][Ala] ≈ [C2OHmim][Ser] N [C2OHmim][Pro], which is the reverse order of molar volume. 3.3. Surface tension Surface tension was measured over a temperature range of 293.15 K to 353.15 K. Values are reported in Table 5 and plotted in Fig. 4. The surface tension of the AAILs increases in the order [C2OHmim] [Ser] b [C2OHmim][Pro] b [C2OHmim][Ala] b [C2OHmim][Gly]. The surface tension of the current hydroxyl functionalized AAILs is higher than that of the non-functionalized AAILs with the same anion [28,29]. The surface tension for all the AAILs is higher than that of common conventional solvents, such as methanol, acetone (23.5 mN·m− 1), (22.07 mN·m− 1), and alkanes, but lower than that of water (71.98 mN·m−1) [33–35]. Similar to the observation for density and viscosity, the AAILs exhibit higher surface tensions than those of [C2mim] homologues and lower amino acid homologues. The temperature decrease for the surface tension of all four AAILs is consistent with previous reports. Increasing temperature is expected to decrease the strength of the hydrogen bonds and increase thermal agitation. Polarity is thus expected to decrease, yielding to a lower surface tension. Generally, the surface tension of the AAILs decreases when the temperature increases; both parameters are correlated by using the following equation [36]: γ mN:m−1 ¼ Es −Ss T

ð6Þ

where Es is the surface excess energy, and Ss is the surface excess entropy. Es and Ss are obtained from the graph of the surface tension versus temperature in Fig. 4. Es and Ss values are presented in Table 6. The surface excess energies of the studied AAILs are less than that of the fused salts (for example, the Es value for NaNO3 is 146 mJ·m− 2

Table 5 Experimental surface tension (γ) for the AAILs at various temperatures and 0.1 MPaa. T/K

[C2OHmim][Gly]

[C2OHmim][Ala]

[C2OHmim][Ser]

[C2OHmim][Pro]

293.15 303.15 313.15 323.15 333.15 343.15 353.15

62.59 60.32 58.27 55.17 53.27 51.01 48.83

53.13 51.88 50.82 49.50 47.92 46.63 45.18

43.25 41.95 40.10 37.41 35.68 33.75 31.21

52.34 50.61 48.42 46.36 44.25 43.67 41.95

a

Standard uncertainties are u(σ) = ±0.04 mN·m−1 and u(T) = ±0.01 K.

Fig. 4. Surface tension as a function of temperature: ■, [C2OHmim][Gly]; , [C8mim][Ala]; , [C2OHmim][Ser]; , [C2OHmim][Pro].


357

Table 6 Surface excess energy, Es surface excess entropy, Ss, decomposition temperature, Ts and glass transition, Tg of AAILs.

Es (mN·m−1) Ss (mN·m−1·K−1) Ts/K Tg/K

[C2OHmim][Gly]

[C2OHmim][Ala]

[C2OHmim][Ser]

[C2OHmim][Pro]

130.5 0.2318 493.11 208.06

92.3 0.1330 497.11 211.92

103.3 0.2034 489.97 218.63

103.6 0.1758 508.78 215.66

[37]), but higher than those of the common organic liquids (Es = 67 and 51.1 mJ·m−2 for benzene and n-octane, respectively). This finding suggests that the interaction energy between the pairing ions of the AAILs is much lower compared with the inorganic fused salts, but higher than that for organic solvents. Moreover, surface excess energies of the studied AAILs are higher than for other amino acid-based ILs without the hydroxyl functional group attached to the imidazolium cation [28]. The surface excess entropies of the studied AAILs are lower than that of the ILs without hydroxyl functional group [28]. This finding suggests a higher degree of organization in the AAILs that contain a hydroxyl group appended to the imidazolium cation. 3.4. Thermal decomposition Thermal decomposition of the AAILs is reported in terms of onset temperature, as shown in Table 6. Ts values for the AAILs are within 490–509 K, which are comparable to other AAILs reported earlier [26, 38]. All the AAILs in this work have good thermal stability N 473.15 K at a scan rate of 10 °C · min−1. The thermal stability of the AAILs is comparable with that of the ILs without a hydroxyl functional group in the imidazole alkyl chain. 3.5. Melting point and glass transition temperature For consistency with reported data for methylimidazolium amino acids based ILs [27,28,39], no melting points have been observed for the AAILs, which indicates non-crystallization characteristic and good supercool tendency. The glass transition of the AAILs is within 208.06 K to 218.63 K, which is comparable to the glass transition values reported for [C8mim][AA] [27]. Table 6 shows the glass transition of the AAILs that follow the decreasing trend [C2OHmim][Ser] N [C2OHmim] [Pro] N [C2OHmim][Ala] N [C2OHmim][Gly]. 3.6. Heat capacity (Cp) The heat capacity data of the AAILs within 293.15 K to 358.15 K are presented in Table 7 and Fig. 5. Cp of the AAILs is lower than the values reported for ammonium- and phosphonium-based AAILs [40].

Furthermore, C p for the AAILs is lower than that reported for 1octyl-3-methylimidazolium amino acid-based IL, where C p is 504.06–654.00 J·K− 1 mol− 1 at 293.15 K. By contrast, the highest Cp value in the studied AAILs was observed in [C2OHmim][Ala], which is 400.80 J·mol−1.K at the same temperature [27]. Moreover, the heat capacity of the AAILs is higher compared with that of traditional organic solvents such as ethanol and toluene.

3.7. Acute toxicity to green algae and V. fischeri The effective concentrations at 20%, 50%, and 80% were determined for S. quadricauda (green algae) and V. fischeri. The effective concentration values are summarized in Table 8. For green algae, the values of EC50 of the AAILs decreased in the order [C2OHmim][Pro] N [C2OHmim][Ser] N [C2OHmim][Gly] N [C2OHmim][Ala]. The result indicated that all the AAILs are practically harmless. The results show that the growth of green algae is inhibited by only 20% at a concentration of more than 1500 mg L−1 for the most toxic AAIL. Based on EC50 concentration, the most toxic AAIL is [C2OHmim][Ala] with a concentration of as high as 3141.31 mg·L− 1. In addition, concentrations as high as 5000 mg·L−1 are required to inhibit growth by 80%. The toxicity results indicate that the sensitivities of both aquatic organisms to AAILs differ and their toxicities do not follow the same trend. The same behavior was reported in literature where the effect of the same ILs on different organisms was completely different [41]. V. fischeri was found to be less sensitive to the AAILs than green microalgae, with EC50 values typically ranging from approximately 8.103 to 14.103 mg·L−1. The measured toxicities are similar to that of 1-methylimidazole starting reactant material for the synthesis of such IL, which is between the values known for acetone or phenol [42]. The short alkyl chain and the presence of a hydroxyl group lower the toxicities of the four AAILs to V. ficheri than the reported toxicity for bromide anion and butyl side chain-based ionic liquids with 1butylpyridinium, 1-methyl-1butylpiperidinium, or even 1-methyl-1butylpyrrolidinium cations, the latter exhibiting the lowest toxicity of these butyl-based ILs [42].

Table 7 Heat capacity, Cp, of the AAILs as a function of temperature and 0.1 MPaa. Cp/J·K−1 mol−1 T/K

[C2OHmim][Gly]

[C2OHmim][Ala]

[C2OHmim][Ser]

[C2OHmim][Pro]

293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15 358.15

325.70 328.38 330.96 332.44 333.55 335.40 337.49 339.30 341.18 342.99 345.21 347.70 349.58 351.59

400.80 402.24 405.52 408.02 409.93 412.80 415.30 418.57 421.22 424.02 427.50 431.17 433.88 436.66

379.75 381.82 384.01 386.18 387.55 389.89 392.30 394.93 396.80 399.89 403.26 406.46 408.84 411.30

387.00 389.93 393.21 396.43 398.13 400.92 404.09 407.41 410.64 413.91 417.92 421.89 424.73 427.84

a

Standard uncertainties are u(T) = ±1 K and u(Cp) = 5%.

Fig. 5. Experimental heat capacities, Cp, as a function of temperature for the AAILs: ■, [C2OHmim][Gly]; , [C8mim][Ala]; , [C2OHmim][Ser]; , [C2OHmim][Pro].

358


Table 8 Toxicity of AAILs expressed at different effective concentrations in mg/L on green algae and Vibrio fischeria. Effective concentrations Green algae EC20 EC50 EC80 Vibrio fischeri EC20 EC50 EC80 a

[C2OHmim][Gly]

[C2OHmim][Ala]

[C2OHmim][Ser]

[C2OHmim][Pro]

2003.28 ± 0.0 3455.37 ± 0.035 5960.01 ± 0.0

1611.98 ± 0.0 3141.31 ± 0.028 6121.56 ± 0.0

2084.71 ± 0.0 4090.33 ± 0.022 8025.48 ± 0.0

2436.17 ± 0.0 4580.87 ± 0.031 8613.70 ± 0.0

7540.70 ± 0.0 11649.23 ± 0.003 17992.19 ± 0.0

4857.17 ± 0.0 8123.27 ± 0.005 13585.43 ± 0.0

7285.95 ± 0.0 10526.46 ± 0.004 15205.66 ± 0.0

9043.92 ± 0.0 14509.36 ± 0.005 23270.97 ± 0.0

±Standard deviation at 95% interval confidence.

No other data for the toxicity of amino acid ionic liquids based on imidazolium cations toward V. fischeri are available in literature thus far. A recent paper reported the toxicities of cholinium-based ionic liquids in combination with various anions, but not with amino acids. Although cholinium-based ionic liquids are generally known to be less toxic than their imidazolium homologues, the EC50 values of all cholinium-based ionic liquids are a magnitude lower than those reported in this study, with the exception of cholinium bicarbonate. Based on the results in Table 8, AAILs can be considered nontoxic compounds mainly because of the presence of a short hydroxyl functional group and the environmentally friendly nature of amino acid anions. 4. Conclusions Four new amino acid-based ILs have been successfully synthesized and characterized by incorporating 1-(2-hydroxylethyl-3methylimidazolium) as cation with four amino acid anions, namely, glycinate, alaninate, serinate, and prolinate. The thermo-physical properties, including density, viscosity, and surface tension, significantly decreased with increased temperature, whereas heat capacity increased slightly. Property variation indicated the influence of the anion structure on the presented ILs, which could be a result of different internal interaction parameters between the cations and anions. Through the appropriate selection of the cation and anions, these AAILs were found to be nontoxic to green algae and V. fischeri, which indicates that these AAILs are environment friendly and have a potential in different applications. Funding This work was funded by the YAYASAN UTP Project No. 0153AA-A20 and the Centre of Research in Ionic Liquid. The authors also extend their appreciation to the Deanship of Scientific Research at King Saud University for supporting this work through research group no. RGP-VPP-303. Prime novelty statement This work presents the synthesis, characterization and thermophysical study of a new set of Room Temperature Ionic Liquids (RTILs), composed of 1-(2-hydroxyethyl-3-methylimidazolium) cationic unit with four different amino acid as anions in a bid to design ILs with desirable environmentally friendly properties. Indeed, ionic liquids can be more toxic than organic solvents if not well designed. To confirm the green character of these new RTILs, their toxicity was assessed against most studied aquatic organisms S. Quadricauda (green algae) and V. Fischeri. The measured effective concentrations confirmed the green character of these newly designed RTILs. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.molliq.2015.09.017.

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