Studies on the Physicochemical Properties of Ionic Liquids Based On

Article pubs.acs.org/jced

Studies on the Physicochemical Properties of Ionic Liquids Based On 1‑Octyl-3-methylimidazolium Amino Acids Ouahid Ben Ghanem,*,† M.I. Abdul Mutalib,† Jean-Marc Lévêque,‡ Girma Gonfa,† Chong Fai Kait,‡ and Mohanad El-Harbawi§ †

Faculty of Chemical Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia Fundamental & Applied Sciences Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia § Chemical Engineering Department, King Saud University, Riyadh 11421, Kingdom of Saudi Arabia ‡

S Supporting Information *

ABSTRACT: A series of new synthesized ionic liquids based on 1-octyl-3methylimidazolium with glycinate, alaninate, serinate, prolinate, and asparaginate anions were prepared. Their thermophysical properties (density, viscosity, surface tension, and heat capacity) were measured at various temperatures and atmospheric pressures. Physicochemical properties such as thermal expansion coefficient values, molecular volume, standard molar entropy, and lattice energy standard were also determined using empirical methods. The effects of anions on the studied properties were further analyzed.

1. INTRODUCTION Ionic liquids (ILs) have garnered interest as alternative solvents in green processes because of their remarkable properties, such as chemical and thermal stability, nonflammability, and negligible vapor pressure.1,2 Moreover, considering the high number of cations and anions available, the properties of the ILs can be finely tuned. In addition, functional groups can be incorporated into the ion structure, further tuning the properties of ILs. Recently, a series of ILs containing amino acids exhibiting lower toxicity than commercially available first and second generation analogs was reported.3−5 Given the fact that amino acid ionic liquids (AAILs) contain both amino group and carboxylic acid residue in a single molecule, they have intrinsic properties such as strong hydrogen-bonding ability, which is valuable for dissolving biomaterials like cellulose and other carbohydrates. Furthermore, AAILs can be obtained at a low cost by using naturally derived amino acids.3−10 AAILs have high biodegradability11 and low toxicity12 and can be utilized in various applications, such as gas separation,5−10 reaction medium,13 and biomass dissolution.14 AAILs were first prepared by Fukumoto et al.3 in 2005 by combining 1-ethyl-3-methylimidazolium cation ([C2mim]) with 20 different amino acids. Tao et al.4 then prepared AAILs with nitrate anions using amino acids as cations. Since then, various AAILs have been prepared including dualfunctionalized AAILs, whose amino functional groups are used as both cation and anion.5 However, the effective use of these AAILs is constrained by their low thermal stability and © XXXX American Chemical Society

high viscosity. For instance, short-alkyl-chain imidazoliumbased AAILs such as 1-ethyl-3-methylimidazolium-based AAILs have low thermal stability.3,8,15 Phosphonium-based AAILs show good thermal7,16 and electrochemical stabilities17 but are relatively more viscous than 1-ethyl-3-methylimidazolium analogs. The thermophysical properties of AAILs strongly rely on both the side-chain structure of the amino acid3−8 and the cation alkyl chain length of AAILs.6 Although several studies have reported on certain physiochemical and thermal properties of AAILs with short to medium alkyl chains (C2 to C6), none have been provided for long-alkyl chain AAILs. In the present work, five long-alkyl-chain AAILs, namely, 1octyl-3-methylimidazolium glycinate [C8mim][Gly], 1-octyl-3methylimidazolium alaninate [C8mim][Ala], 1-octyl-3-methylimidazolium serinate [C8mim][Ser], 1-octyl-3-methylimidazolium prolinate [C8mim][Pro], and 1-octyl-3-methylimidazolium asparaginate [C8mim][Asn] were prepared and characterized. The general route to the synthesis and structure of these ILs is depicted in Figure 1. The density, viscosity, surface tension, and heat capacity of the ILs were measured at different temperatures and atmospheric pressure. Certain significant physicochemical and thermal properties were also determined from the experimental data. The thermal behavior of the AAILs Received: December 26, 2014 Accepted: May 7, 2015

A

DOI: 10.1021/je501162f J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Figure 1. General route to the synthesis and structure of the studied ionic liquids.

Table 1. Chemical Formula, Purity, Molar Mass, and Water Content of AAILs [C8mim][Gly]

[C8mim][Ala]

[C8mim][Ser]

[C8mim][Pro]

[C8mim][Asp]

chemical formula

C14H27N3O2

C15H29N3O2

C15H29N3O3

C17H31N3O2

C16H30N4O3

purity (%) molar mass (g mol−1) 106 × w

≥99 269.4 304

≥99 283.4 138

≥99 299.4 315

≥99 309.4 350

≥99 326.4 500

2.3. Characterization. 1H NMR spectra were taken in deuterated solvent and recorded on a Bruker Avance 500 spectrometer. Before measuring the physicochemical and thermal properties, all ILs were dried under low pressure by placing in a vacuum oven for a few hours at 353.15 K. The water content of each AAIL was then measured using a coulometric Karl Fischer titrator (DL 39, Mettler Toledo) using Hydranal coulomat AG reagent. Three measurements were taken for each AAIL, and the average values are reported in Table 1. 2.4. Density and Viscosity Measurements. The density and viscosity of each AAIL was measured using Anton Paar densitometer (SVM 3000). The instrument was calibrated with Millipore-grade water, for which the data was established. 2.5. Surface Tension Measurements. Surface tension measurement was conducted using a pendant drop method at 293.15 K to 353.15 K. The drop was generated using a syringe and photographed using a camera (OCA 20). The shape of the drop and the surface tension were evaluated using (SCA 22) software 2.6. Thermal Analysis and Heat Capacity. 2.6.1. Thermal Decomposition. Thermal decomposition temperatures were measured using a PerkinElmer Pyris V-3.81 thermal gravimetric analyzer. IL samples were placed in sealed aluminum pans under nitrogen atmosphere at a heating rate of 10 K/min with ± 1 K temperature accuracy. 2.6.2. Glass Transition. Glass transition temperatures were determined using a differential scanning calorimeter (Mettler Toledo, MODEL (DSC1/500)). The samples were kept in sealed aluminum pans and heated at 10 K/min from room

was studied using thermogravimetric and differential scanning calorimetric analyses.

2. EXPERIMENTAL SECTION 2.1. Materials. All starting materials were used as received without any further treatment. These materials included 1methylimidazole (Merck, ≥ 99 %), 1-bromooctane (Merck, ≥ 98%), L-serine (Merck, ≥ 99.2 %), L-alanine (Merck, ≥ 99 %), L-proline (Merck, ≥ 99 %), L-asparagine (Merck, ≥ 99 %), Lglycine (Merck, ≥ 99 %), ethyl acetate (Fisher Scientific U.K., ≥ 99.99 %), ethanol (Merck, ≥ 99.9 %), and ion-exchange resin Amberlite IRA-402 (OH) (Alfa Aesar). 2.2. Synthesis of AAILs. The studied AAILs were prepared according to a previously reported method as follows.3,6 First, equimolar amounts of 1-methylimidazole and 1-bromooctane were mixed, heated, and continually stirred at 333.15 K for 24 h under nitrogen atmosphere. The resulting product (1-octyl-3methylimidazolium bromide) was then washed several times with ethyl acetate to remove unreacted materials. Then, 1-octyl3-methylimidazolium bromide was dissolved in distilled water and passed through an anion-exchange column filled with Amberlite IRA-402. A dilute solution of 1-octyl-3-methylimidazolium hydroxide was collected from the resin. The 1-octyl-3methylimidazolium hydroxide aqueous solution was added to excess amino acid (L-glycine, L-alanine, L-serine, L-proline, and L-asparagine) and gently mixed for 12 h. Water was removed by slow evaporation in a vacuum. The resulting IL was then dissolved in ethanol to precipitate unreacted amino acids, and the solution was filtered. Finally, ethanol was evaporated, and the IL was dried in a vacuum for 72 h. B



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Table 2. Experimental Density, ρ, Values of AAILs at Various Temperatures and 0.1 MPaa [C8mim][Gly]

[C8mim][Ser] ρ

K

g·cm−3

293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 a

[C8mim][Ala]

T 1.0417 1.0353 1.0291 1.023 1.0168 1.0106 1.0044 0.9986 0.9928

0.9853 0.9821 0.9786 0.9758 0.9729 0.9693 0.9663 0.9626 0.9597

1.0756 1.0693 1.0627 1.0561 1.0497 1.0433 1.0369 1.0305 1.0242

[C8mim][Pro]

[C8mim][Asn]

1.0645 1.0582 1.0518 1.0455 1.0391 1.0328 1.0264 1.0201 1.0139

1.0617 1.0572 1.0522 1.0486 1.0441 1.0390 1.0344 1.0301

Standard uncertainties are u(T) = 0.01 K and u(ρ) = 0.001 g/cm3.

anion, the densities of the studied AAILs were also relatively lower than those of the analogs with short alkyl chains. For example, the densities of 1-alkyl-3-methylimidazolium alaninate [Cnmim][Ala] at 303.15 K were 1.1209 g·cm−3, 1.0978 g·cm−3, 1.0794 g·cm−3, 1.0610 g·cm−3, and 1.0426 g·cm−3 for n = 2, 3, 4, 5, and 6, respectively.6 As observed in Table 2, the density of [C8mim][Ala] at 303.15 K was 0.9821. This noticeable decrease in density with increased alkyl chain length has been reported earlier for other ILs with different cations and anions.22−24 The data also clearly indicated that the nature of the anion significantly affected the densities of the AAILs. Furthermore, the densities decreased in the order [C8mim][Ser] > [C8mim][Pro] > [C8mim][Gly] > [C8mim][Ala]. The density of [C8mim][Asn] was less sensitive to temperature change than the other AAILs. In general, similar trends have been reported for other AAILs with short imidazolium cation and dual amino-functionalized phosphonium AAILs.5−9 As expected, the densities of the studied AAILs linearly decreased with increased temperature as shown in Figure 2. Varying decrements of 2.60 % to 4.79 % in densities were observed at 293.15 K to 373.15 K, which was consistent with values reported for ammonium- and phosphonium-based AAILs.25 The experimental densities (ρ) were fitted using the leastsquares method based on the following equation:

temperature to 393.15 K. The samples were cooled to 193.15 K and subsequently reheated (10 K/min) to 393.15 K. The glass transition temperatures were measured with ± 1 K temperature accuracy. 2.6.3. Heat Capacity Measurements. Given its high accuracy, the sapphire method was used to determine the heat capacities of the compounds.18 The samples were placed in sealed aluminum pans, and the temperature was gradually increased at a constant heating rate (5 K/min) from 258.15 K to 348.15 K. Each temperature increment of 5 K was maintained for 15 min.

3. RESULTS AND DISCUSSION The structures of the AAILs prepared for this study were characterized using 1H NMR; the results confirmed the desired structures: [C8mim][Gly]: 1H NMR (500 MHz, DMSO) δ = 0.86 (t, 3H), 1.26 (dd, 10H), 1.8 (m, 2H), 2.5 (m, 1H), 2.71 (m, 2H), 3.21 (s, 1H), 3.88 (s, 3H), 4.18 (m, 2H), 7.80 (m, 2H), 9.69 (s, 1H). [C8mim][Ala]: 1H NMR (500 MHz, DMSO) δ = 0.87 (t, 3H), 1.03 (m, 3H), 1.23 (m, 10H), 1.76 (m, 2H), 2.92 (q, 1H), 3.3 (s, 2H), 3.89 (s, 3H), 4.19 (t, 2H), 7.82 (m, 2H), 9.84 (s, 1H). [C8mim][Ser]:1H NMR (500 MHz, DMSO) δ = 0.71 (t, 3H), 1.12 (m, 10H), 1.68 (m, 2H), 3.19 (t, 1H), 3.36 (s, 2H), 3.88 (s, 3H), 4.03 (m, 2H), 4.31 (s, 1H), 4.43 (m, 2H), 7.33 (m, 2H), 9.95 (s, 1H). [C8mim][Pro]: 1H NMR (500 MHz, CDCl3) δ = 0.86 (t, 3H), 1.27 (m, 10H), 1.50 (m, 2H), 1.72 (m, 2H), 1.79 (m, 2H), 2.77 (m, 2H), 3.17 (t, 1H), 3.96 (s, 3H), 4.20 (t, 2H), 7.79 (m, 2H), 9.58 (s, 1H). [C8mim][Asn]:1H NMR (500 MHz, DMSO) δ = 0.85 (t, 3H), 1.28 (m, 10H), 1.75 (tt, 2H), 1.99 (dd, 1H), 2.46 (dd, 1H), 3.37 (m, 6H), 3.87 (s, 3H), 4.18 (t, 1H), 7.75 (m, 2H), 9.46 (s, 1H). 3.1. Density. Densities of the five AAILs were studied in the temperature range 293.15 K to 373.15 K, as shown in Table 2. With a same cation, the densities were found to be lower for AAILs than for perfluorinate and other inorganic anions. For instance, the densities of 1-octyl-3-methylimidazolium with bis[(trifluoromethyl)sulfonyl]imide [Tf2N], hexafluorophosphate [PF6], tetrachloroaluminate [AlCl4], and tetrafluoroborate [BF4] at 303.15 K were 1.3200 g·cm−3, 1.2207 g·cm−3, 1.1187 g·cm−3, and 1.0887 g·cm−3, respectively, which were all higher than those for the studied AAILs.19−21 For a given

Figure 2. Density as a function of temperature: ■, [C8mim][Gly]; red ●, [C8mim][Ala]; blue ▲, [C8mim][Ser]; green ▼, [C8mim][Pro]; and lime ◆, [C8mim][Asn]. C



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Table 3. Fitting parameter values A0, A1, and R2 for densities, standard deviation SD, molar volume Vm, molecular volume V, standard entropy S0, and crystal energy UPOT of AAILs at 298.15 K and 0.1 MPaa A0 A1 × 104 R2 SDb 105 Vm (cm3·mol−1) V (nm3) S0 (J·K−1·mol−1) UPOT (kJ·mol−1)

[C8mim][Gly]

[C8mim][Ala]

[C8mim][Ser]

[C8mim][Pro]

[C8mim][Asn]

1.221 −6.1250 0.9998 1.9946 259.428 0.4308 566.45 414.44

1.0792 −3.2028 0.9994 2.0768 288.098 0.4784 625.79 403.77

1.2644 −6.4400 1 1.0159 279.209 0.4636 607.39 406.92

1.2503 −6.3374 1 0.4851 291.569 0.48414 632.98 402.58

1.1988 −4.5200 0.9988 2.9326 306.795 0.5094 664.49 397.55

Standard deviation values were calculated using. bSD = (Σ(Zexp − Zcal)/n)1/2 where SD, n, Zexp, and Zcal are standard deviation, number of experimental points, and experimental and calculated values, respectively. a

ρ(g·cm−3) = A 0 + A1T

The standard entropy values are shown in Table 3. The standard entropy values of the AAILs studied were relatively higher than those of the AAILs with short alkyl chains. For example, the standard entropy values (at 298.15 K) reported for [Cnmim][Ala] (for n = 2, 3, 4, 5, 6) are 396.9 J·K·mol−1, 431.1 J·K·mol−1, 464.8 J·K·mol−1, 499.7 J·K·mol−1, and 535.8 J· K·mol−1, respectively,6 and the value increased to 566.45 J·K· mol−1 for [C8mim][Ala]. This increase suggested less organization in the AAILs as the chain length increases. Therefore, the molar volume and standard entropy values followed a similar trend. 3.4. Crystal Energy. In accordance with Glasser’s theory,26 the crystal energy (UPOT) of the AAILs can be calculated using the following equation:

(1)

where ρ is the density of the AAILs, A0 and A1 are the correlation coefficients, and T is the temperature (in Kelvin). The correlation coefficients were estimated by least-squares fitting method using eq 1. The estimated values of correlation coefficients, together with the SDs, are presented in Table 3. The experimental densities were used to determine other thermophysical properties, such as molar volume (Vm), standard entropy (S0), crystal energy (UPOT), and thermal expansion coefficients (α). 3.2. Molar Volume (Vm). The molar volumes of the AAILs were calculated at room temperature and atmospheric pressure using the following equation:

Vm(cm 3·mol−1) =

M ρ

UPOT(kJ·mol−1) = 1981.2(ρ /M )1/3 + 103.8

(2)

where M is the molecular weight (g·mol−1) and ρ is the density (g·cm−3). The values of Vm are listed in Table 3. The Vm values increased in the order of [C8mim][Gly] < [C8mim][Ser] < [C8mim][Ala] < [C8mim][Pro] < [C8mim][Asn]. These results showed that no direct relation existed between the molar volume and the density of the AAILs. The molecular volumes (V) of the AAILs were calculated from molar volume and Avogadro’s constant (NA) using eq 3 V=

Vm NA

The estimated UPOT values are listed in Table 3. The crystal energies of the present AAILs were found to be much lower than those of inorganic fused salts. The crystal energy for cesium iodide, which has the lowest crystal energy among alkali chlorides, is 613 kJ·mol−1.27 The low crystal energy of the AAILs explained the formation of liquid salts at room temperature. Moreover, the UPOT of [C8mim][Ala] and [C8mim][Gly] were lower than those reported for short-alkyl chain AAILs. The UPOT values reported for [Cnmim][Ala] (for n = 2, 3, 4, 5, 6) are 456 kJ·mol−1, 446 kJ·mol−1, 437 kJ·mol−1, 428 kJ·mol−1, and 421 kJ·mol−1,6 whereas the values for [Cnmim][Gly] are 469 kJ·mol−1, 457 kJ·mol−1, 446 kJ·mol−1, 437 kJ·mol−1, and 429 kJ·mol−1,28 respectively. The UPOT values of the present AAILs decreased in the order [C8mim][Gly] > [C8mim][Ser] > [C8mim][Ala] > [C8mim][Pro] > [C8mim][Asn], which was opposite to the trend of molar volume. This finding suggested that more compact AAILs had higher lattice energy. 3.5. Isobaric Thermal Expansion Coefficient (α). The experimental density values were used to calculate thermal expansion coefficients using the following equation:

(3)

The molecular volumes of the AAILs are also listed in Table 3. The AAILs analyzed in this study exhibited higher Vm values than those reported for short alkyl chains with the same anions. For example, the molecular volume of [Cnmim][Ala] (for n = 2, 3, 4, 5, 6) at 298.15 K were 0.2948 nm3, 0.3222 nm3, 0.3492 nm3, 0.3772 nm3, and 0.4062 nm3, respectively,6 whereas for [C8mim][Ala], the value increased to 0.4784 nm3. The increase in molar volume of the AAILs was due to the expanded alkyl chain length, which increased the molecular size of the AAILs. The result was consistent with the mean contribution of the molecular volume of each methylene (CH2) group (0.0278 nm3 per CH2) reported by Fang et al.6 for AAILs with [Ala] anion. 3.3. Standard Entropy. The values of standard entropy (S0) were calculated from molar volume based on the relationship established by Glasser26 using the following equation: S 0(J ·K−1mol−1) = 1246.5 × V (nm 3) + 29.5

(5)

α(K−1) =

1 ⎛ ∂Vm ⎞ 1 ⎛ ∂ρ ⎞ ⎜ ⎟ =− ⎜ ⎟ Vm ⎝ ∂T ⎠ P ρ ⎝ ∂T ⎠ P

(6)

The values of the isobaric thermal expansion coefficients are shown in Table 4. The α values of the AAILs were found to be higher than those of fused salts but less than those of molecular organic compounds. The effect of temperature on the coefficients of thermal expansion of the present AAILs was insignificant. The thermal expansion coefficients of [C8mim]-

(4)

where V is the molecular volume of the AAILs. D



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Table 4. Isobaric Thermal Expansion Coefficient, α, and Dynamic Viscosity, η, of AAILs as a Function of Temperature and 0.1 MPaa [C8mim] [Gly]

[C8mim] [Ala]

T

α·104

K

K−1

293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 T

5.88 5.92 5.95 5.99 6.02 6.06 6.1 6.13 6.17

3.25 3.26 3.27 3.28 3.29 3.3 3.32 3.33 3.34

K 293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 a

[C8mim] [Ser]

5.99 6.02 6.06 6.1 6.13 6.17 6.21 6.25 6.29 η

[C8mim] [Pro]

[C8mim] [Asn]

5.95 5.99 6.03 6.06 6.1 6.14 6.17 6.21 6.25

4.26 4.28 4.29 4.31 4.33 4.35 4.37 4.39

anions. This might be explained by the presence of hydrogen bonding from amino acid anions and from van der Waals interactions caused by long alkyl chains.9,29 Furthermore, the viscosity of ILs reportedly increases with increased molecular weight of anion.29,30 For comparison, the dynamic viscosities of [C2mim][Gly], [C2mim][Ala], [C2mim][Ser], and [C2mim][Pro] at 293.15 K were 80.37 mPa·s, 235.36 mPa·s, 611.87 mPa·s, and 626.95 mPa·s, respectively.9 When the cation was switched to [C8mim], these values increased to 486.4 mPa·s, 705.1 mPa·s, 2853.0 mPa·s, and 1969.8 mPa·s for [Gly], [Ala], [Ser], and [Pro] anions, but decreased to 115.10 mPa·s, 93.3 mPa·s, and 473.0 mPa·s with [Tf2N], [AlCl4], and [BF4], respectively.21,31 As expected, the viscosity of the AAILs markedly decreased with increased temperature. Experimental viscosities (η) were fitted by Vogel−Fulcher−Tammann (VFT) equation log η(mPa·s) = A +

mPa·s 486.4 248.2 137.9 82.4 52.3 34.9 24.4 17.7 13.3

705.1 347.3 184.6 106.9 65.7 42.9 29.1 20.7 15.2

2853 1178.7 536.2 271.2 149.6 88.9 56.2 37.5 26.2

1969.8 839.9 401.2 211 120.3 73.2 47.2 31.9 22.4

B (T − T0)

(7)

where η denotes the viscosity of the ILs and A, B, and T0 are the adjustable parameters. The correlation coefficients were estimated by least-squares fitting. The values of the different parameters for viscosity, together with correlation coefficients, are presented in Table 5.

3519.9 1436.8 659.1 333.3 182.6 107.1 66.6 43.5

Table 5. Fitting Parameter Values for Viscosities with Correlation Coefficient R2

Standard uncertainties are u(T) = 0.01 K and u(η) = 1 % mPa·s.

[Gly] and [C8mim][Ala] were lower than the values reported for short-alkyl-chain AAILs with the same anions.6 3.6. Viscosity. The experimental dynamic viscosities of the present AAILs over various temperatures are shown in Table 4. The viscosities of AAILs increased in the order [C8mim][Asn] > [C 8mim][Ser] > [C 8 mim][Pro] > [C 8mim][Ala] > [C8mim][Gly], as shown in Figure 3. However, the viscosities of these AAILs were higher than those of short-alkyl chain analogs and those bearing [C8mim] cation with inorganic

IL

A

B

T0

R2

[C8mim][Gly] [C8mim][Ala] [C8mim][Ser] [C8mim][Pro] [C8mim][Asn]

−3.1811 −3.5578 −3.7056 −3.6667 −4.3159

1198.58 1320.59 1378.72 1361.10 1607.26

165.26 162.67 175.04 172.23 174.40

0.9999 0.9999 0.9999 1.0000 1.0000

3.7. Surface Tension. The measured surface tension (γ) values of the AAILs over various temperatures are reported in Table 6, and the temperature dependence of the surface tension Table 6. Surface Tension, γ, Values of AAILs at Various Temperatures and 0.1 MPaa [C8mim] [Gly]

[C8mim] [Ser]

T

γ

K

mJ·m−2

293.15 303.15 313.15 323.15 333.15 343.15 353.15 a

[C8mim] [Ala]

35.37 33.98 32.81 31.35 30.07 28.75 27.42

28.24 27.90 27.23 26.61 26.105 25.55 24.99

34.29 32.80 32.2 31.6 31.0 30.01 29.23

[C8mim] [Pro]

[C8mim] [Asn]

29.28 28.88 28.46 28.17 27.76 27.62 27.14

34.32 33.70 32.96 31.99 31.73 30.88 30.19

Standard uncertainties, u, are u(T) = 0.1 K and u(γ) = 0.25 mJ·m−2.

is shown in Figure 4. The surface tension values of the studied AAILs were found to be lower than the reported values for the same anion group in AAILs with short alkyl chains. For example, the surface tension reported for [Cnmim][Ala] (for n = 2, 3, 4, 5, 6) at 303.15 K are 52.1 mJ·m−2, 49.5 mJ·m−2, 47.2 mJ·m−2, 45.4 mJ·m−2, and 42.8 mJ·m−2, respectively.6 In the present study, the surface tension value of [C8mim][Ala] at

Figure 3. Viscosity as a function of temperature: ■, [C8mim][Gly]; red ●, [C8mim][Ala]; blue ▲, [C8mim][Ser]; green ▼, [C8mim][Pro]; and lime ◆, [C8mim][Asn]. E



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γ(mJ·m−2) = Es − TSs

(8)

The values of Es and Ss values were obtained from the plot of surface tension versus temperature shown in Figure 4. The calculated values are presented in Table 7. The surface excess energy of the AAILs were significantly lower than those of fused salts (e.g., Es = 146 mJ·m−2 for NaNO3)37 but close to that of organic liquids (Es = 67 mJ·m−2 and 51.1 mJ·m−2 for benzene and n-octane, respectively). This result suggested that the interaction energy between the ions in the AAILs is much lower than that in inorganic fused salts. By contrast, the surface excess entropies of the AAILs were lower than that of molecular solvents, such as water (0.138 mJ·m−2·K−1) and benzene (0.13 mJ·m−2·K−1).37 This finding indicated a relatively higher degree of organization in the AAILs than in molecular solvents. 3.8. Enthalpy of Vaporization (Δg1Hm°). Determining the molar enthalpy of vaporization of ILs is extremely difficult because of their negligible vapor pressure. However, estimating them indirectly can be performed using the empirical equation proposed by Kabo and Verevkin38

Figure 4. Surface tension as a function of temperature: ■, [C8mim][Gly]; red ●, [C8mim][Ala]; blue ▲, [C8mim][Ser]; green ▼, [C8mim][Pro]; and lime ◆, [C8mim][Asn].

Δ1g Hm°(kJ·mol−1) = A(γVm 2/3NA1/3) + B

(9)

where γ, Vm, and NA are the surface tension (J·m−2), molar volume (m3·mol−1), and Avogadro’s constant, respectively. The values of the empirical constants A and B at 298.15 K are 0.01121 kJ·mol−1 and 2.4 kJ·mol−1, respectively. The calculated values of the molar enthalpy of vaporization of the AAILs are given in Table 7. 3.9. Thermal Decomposition. The thermal decomposition of the studied AAILs was determined in terms of onset temperature (Ts), as shown in Table 7. Figure 5 shows the thermogravimetric analysis (TGA) profiles for the AAILs. The Ts values of AAILs ranged within 474 K to 497 K, consistent with earlier reports on other AAILs.9,39 Thermal decomposition decreased in the order [C8mim][Pro] > [C8mim][Ser] > [C8mim][Ala] > [C8mim][Asn] > [C8mim][Gly]. This trend was similar to Ts values recorded for [C2mim][Pro], [C2mim][Ser], [C2mim][Ala], and [C2mim][Gly], that is, 475 K, 482 K, 489 K, and 528 K, respectively.9 A similar trend was also reported by Tao et al.39 for cholinium-based AAILs. 3.10. Glass Transition Temperature. The glass transition temperature (Tg) is measured as the midpoint of a small heat capacity change upon heating from the amorphous glass state to liquid state. The thermal transition temperatures of the studied AAILs are presented in Table 7. The order of the AAILs based on decreasing Tg values was [C8mim][Gly] < [C8mim][Ala] < [C8mim][Pro] < [C8mim][Ser] < [C8mim][Asn]. In agreement with reported data for [Cnmim][AA]based ILs,6,15,28 no melting point could be observed for our studied AAILs, highlighting their strong tendency to supercool without crystallization. The studied AAILs also displayed a

303.15 K was 27.9 mJ·m−2, which was consistent with the decreasing trend reported. A decrease of about 7 mJ·m−2 in surface tension values was observed when switching the anion from [Gly] to [Ala], even though the latter had additional methyl group compared to the former. In fact, different decreasing trends have already been observed in recent studies. For instance, Carvalho et al.32 showed that by increasing the alkyl chain length without H-bonding capacity, the surface tension of some ILs suffered a slight decrease. Most interestingly and very similarly to our work, Hossain et al.33 showed that the presence of H-bonding linked to an increase of side alkyl chain could lead to a more substantial decrease in surface tension values. They indeed reported for 1-(6hydroxylhexyl)-3-methylimidazolium chloride, [6OHimC][Cl], 1-(6-hydroxylhexyl)-3-ethylimidazolium chloride, [6OHimC2][Cl] and 1-(6-hydroxylhexyl)-3-butylimidazolium chloride, [6OHimC4][Cl] respective surface tensions of 48.8 mJ·m−2, 41.3 mJ·m−2, and 30.8 mJ·m−2 at 303.15 K. Table 6 shows that the effect of amino acid anions on surface tension is insignificant. The same conclusion can be drawn by observing the effect of halide or perfluorinated anions, as previously reported. For example, the surface tension of [C8mim][PF6], [C8mim][Cl], [C8mim][Tf2N], and [C8mim][BF4] at 303.15 K were 34.6 mJ·m−2, 31.7 mJ·m−2, 31.3 mJ· m−2, and 30.4 mJ·m−2, respectively.32,34,35 The surface excess energy (Es) and surface excess entropy (Ss) of the AAILs were calculated using the measured surface tension data through eq 836

Table 7. Surface Excess Energy Es, Surface Excess Entropy Ss, and Enthalpy of Vaporization Δg1Hm° at 298.15 K Decomposition Temperature in Terms of Onset Temperature Ts and Glass Transition Temperature Tg of AAILsa −2

Es (mJ·m ) Ss (mJ·m−2·K−1) Δg1Hm° (kJ·mol−1) Ts (K) Tg (K) a

[C8mim][Gly]

[C8mim][Ala]

[C8mim][Ser]

[C8mim][Pro]

[C8mim][Asn]

74.146 0.1323 136.02 474 204

44.628 0.0556 118.23 485 207

57.025 0.0787 138.12 490 219

39.313 0.0344 123.35 497 211

54.484 0.0688 148.69 481 229

Standard uncertainties are u(Ts) = 1 K and u(Tg)= 1 K. F



Article

Figure 5. Thermogravimetric profiles of AAILs: black --, [C8mim][Gly]; red --, [C8mim][Ala]; blue --,[C8mim][Ser]; green --, [C8mim][Pro]; and lime --, [C8mim][Asn].

Figure 6. Experimental heat capacity Cp as a function of temperature of AAILs: ■, [C8mim][Gly]; red ●, [C8mim][Ala]; blue ▲, [C8mim][Ser]; green ▼, [C8mim][Pro]; and lime ◆, [C8mim][Asn].

linear relationship between viscosity and glass transition temperature, as previously reported for tetrabutylphosphonium-based amino acid ILs.7 3.11. Heat Capacity (Cp). The heat capacity data over the 293.15 K to 353.15 K temperature rang are presented in Table 8 and Figure 6. The heat capacities of the studied AAILs fall

[Gly] > [C8mim][Ala], and their values were also comparatively lower than published data for AAILs with short alkyl chain length. By contrast, the viscosities of the AAILs increased in the order [C8mim][Gly] < [C8mim][Ala] < [C8mim][Pro] < [C8mim][Ser] < [C8mim][Asn]. In addition, the viscosities of the AAILs were higher than those of short-alkyl chain AAILs with the same anions. The surface tensions of the AAILs were lower than those of other AAILs with short alkyl chains. As expected, density, viscosity, and surface tension markedly decreased with increased temperature. Thermal decomposition decreased in the order [C8mim][Pro] > [C8mim][Ser] > [C8mim][Ala] > [C8mim][Asn] > [C8mim][Gly]. The heat capacities of [C8mim][Ser] and [C8mim][Ala] were closely similar. Furthermore, variations in heat capacity for each studied AAIL were almost analogous. These results confirmed that amino acid anions significantly affected the thermosphysical properties of AAILs, the differences in the studied properties of the AAILs could be due to the differences internal interactions (hydrogen bonding, π−π interaction, van der Waals interactions, etc.) in each ionic liquid molecule.

Table 8. Heat Capacity Cp of AAILs as a Function of Temperature and 0.1 MPaa [C8mim] [Gly]

[C8mim] [Ser]

T

Cp

K

J·K−1 mol−1

293.15 303.15 313.15 323.15 333.15 343.15 353.15 a

[C8mim] [Ala]

504.06 511.67 518.04 526.8 536.66 546.99 555.85

592.06 600.95 609.08 620.49 632.82 645.14 657.43

599.21 607.97 614.69 624.21 634.19 644.72 655.67

[C8mim] [Pro]

[C8mim] [Asn]

561.68 569.53 578.89 590.88 603.42 615.85 627.51

654 662.93 670.78 682.99 694.16 705.81 717.01

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

within the same range of values reported for other imidazoliumbased ILs.18 However, the Cp values of the AAILs were relatively lower than those reported for ammonium- and phosphonium-based AAILs. For example, the Cp of tributylmethylammonium serinate ([N4441][Ser]) and tetrabutylphosphonium serinate ([N4441][Ser]) at 298.15 K are 635 J· mol−1·K and 749 J·mol−1·K, respectively,25 whereas the Cp value of [C8mim][Ser] was 604 J·mol−1·K. The heat capacities of the studied AAILs were also comparatively higher than those of conventional organic solvents, such as ethanol (112 J·mol−1· K) and toluene (158 J·mol−1·K).

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

* Supporting Information S

Table and figures showing the heat capacities of studied AAILs as a function of temperature. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/je501162f. Corresponding Author

*E-mail: [email protected]. Phone: +601126430805. Funding

This work was funded by YAYASAN UTP Project No. 0153AA-A20 and the PETRONAS Ionic Liquid Research Centre. 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-VPP303′.

4. CONCLUSIONS In this work, the physicochemical and thermal properties of AAILs with 1-octyl-3-methylimidazolium ([C8mim]) cation and glycinate ([Gly]), alaninate ([Ala]), serinate ([Ser]), prolinate ([Pro]), and asparaginate ([Asn]) anions were studied. The densities of the AAILs were found to decrease in the order of [C8mim][Ser] > [C8mim][Pro] > [C8mim]-

Notes

The authors declare no competing financial interest. G



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Article

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