Hildebrand solubility parameters of ionic liquids: Effects of ionic liquid type, temperature and DMA fraction in ionic liquid

Hildebrand solubility parameters of ionic liquids: Effects of ionic liquid type, temperature and DMA fraction in ionic liquid

Chemical Engineering Journal 213 (2012) 356–362 Contents lists available at SciVerse ScienceDirect Chemical Engineering Journal journal homepage: ww...

223KB Sizes 1 Downloads 209 Views

Chemical Engineering Journal 213 (2012) 356–362

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Hildebrand solubility parameters of ionic liquids: Effects of ionic liquid type, temperature and DMA fraction in ionic liquid Piyarat Weerachanchai, Zhengjian Chen, Susanna Su Jan Leong, Matthew Wook Chang, Jong-Min Lee ⇑ School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore

h i g h l i g h t s " The type of ionic liquid presents the important effect on the solubility parameter. " The influence of anion: [PF6] > [Tf2N] > [Cl]. " The Hildebrand solubility parameters of the mixtures do not agree with the mixing rule. " The increase of temperature from 25 to 60 °C results in the decreases of the parameters.

a r t i c l e

i n f o

Article history: Received 21 July 2012 Received in revised form 16 October 2012 Accepted 19 October 2012 Available online 25 October 2012 Keywords: Ionic liquid Hildebrand solubility parameter Intrinsic viscosity Cohesive energy densities Molar internal energy Enthalpy of dissolution

a b s t r a c t The Hildebrand solubility parameters of 10 ionic liquids were investigated using the approach of intrinsic viscosity. Effects of dimethylacetamide (DMA) fraction in ionic liquids and dissolution temperature on the Hildebrand solubility parameter were also studied. Moreover, cohesive energy density, molar internal energy and enthalpy of dissolution were calculated. Cation and anion of an ionic liquid shows a significant influence on the solubility parameter. The values of ionic liquids containing BMIM cations are in the following order: [PF6] > [Tf2N] > [Cl], while those of ionic liquids containing EMIM cations are in the order: [BF4] > [DEPO4] > [AC]. Ionic liquid containing HOEMIM exerts the highest solubility parameter than that containing MBPYRRO or BMIM. Considering effect of varying DMA fraction, the Hildebrand solubility parameters of the mixtures do not correspond to the mixing rule. Their values tend to be closer to those of the ionic liquids than that of DMA for 40–90 vol% DMA added in the ionic liquids (1-Ethyl-3methylimidazolium acetate (EMIM-AC) and 1-Butyl-3-methylimidazolium chloride (BMIM-Cl)). With increasing temperature from 25 to 60 °C, decreases of Hildebrand solubility parameters from 25.2 to 24.7 for EMIM-AC and the mixture of EMIM-AC/DMA (60–40 v/v) were obtained. The cohesive energy densities of ionic liquids at the different conditions are proportional directly with the Hildebrand solubility parameter. The molar internal energy and the enthalpy of dissolution decrease with increasing DMA fraction in ionic liquids, while they are almost constant or show slight decreases with the increase of dissolution temperature for EMIM-AC and the mixture of EMIM-AC/DMA (60–40 v/v), respectively. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Solubility parameter is a numeric value to indicate the strength of molecular interaction between solvent molecules [1]. It has been shown as a useful factor in determining and predicting the fundamental properties of material such as solubility and swelling of polymers by solvents, and/or being an effective screening tool for selecting a favorable solvent for many applications, such as film coating, tablet coating and wet granulation binders, drug perme-

⇑ Corresponding author. Tel./fax: +65 6513 8129. E-mail address: [email protected] (J.-M. Lee). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.10.012

ation to skin, supported liquid membranes, and gas capture from mixture gases [2–8]. A solute is soluble in solvents which have a similar solubility parameter [2]. The concept of the solubility parameter which is related to the internal energy of a solvent and a solute was first proposed by Hildebrand. The Hildebrand solubility parameter (dH ) is defined as the square root of the cohesive energy density (CED), the energy required to break the interactions between molecules (DU) per molar volume (V) which is equal to the square root of the difference of enthalpy of dissolution (DH) and ideal gas constant (R) timing with temperature (T) per molar volume (V) [2,9,10]: 1

dH ¼ CED2 ¼

 12  1 DU DH  RT 2 ¼ V V

ð1Þ

P. Weerachanchai et al. / Chemical Engineering Journal 213 (2012) 356–362

The Hildebrand solubility parameter (dH ) can be evaluated by various methods: heat vaporization (DHV )-temperature data, group contribution, solubility measurement, osmotic pressure, swelling, intrinsic viscosity, etc. [9,11]. The intrinsic viscosity has been widely applied as an effective approach for measuring the extremely low vapour pressure of non-volatile compounds such as polymers [3,9], which are impossible to determine directly from DHV . The solubility parameter derived from this method is obtained by measuring the intrinsic viscosity of solute in a series of solvents. The Hildebrand solubility parameter of the solvent which gives maximum value of the solute’s intrinsic viscosity is the Hildebrand solubility parameter of solute [3,9,12]. The maximum of intrinsic viscosity implies maximum mutual interaction between solvent and solute [12]. Ionic liquids (ILs) are environmentally friendly molten salts, which represent a new class of solvents having high polarities, low melting points, non-volatility and designability [13–15]. They have gained overwhelming interest over the past years in a variety of industrial applications, like solvents, catalysts and electrically conducting fluids [13,16,17]. The use of ionic liquids as a solvent has played a vital role in improving a number of conventional processes, such as biomass pretreatment, drug delivery, gas storage, and handling applications [2,18,19]. However, the understanding of how the physicochemical properties of ionic liquids are used to design or select a promising solvent for an individual application is still relatively unknown. The solubility parameter of ionic liquid is a key to realizing their potentials. The certain data of Hildebrand solubility of ionic liquids are derived from several methods: solvent dependence on bimolecular rate constant of Diels–Alder reactions, computationally-based technique, inverse gas chromatography, melting temperature, activation energy of viscosity, enthalpy of vaporization, surface tension, Kamlet–Taft Equation [1,9,20,21]. The Hildebrand solubility parameter of ionic liquid obtained from intrinsic viscosity has been marked as an accurate method and it gives good agreement with various methods, like the solvent dependence on bimolecular rate constant of Diels–Alder reactions, computationally-based technique and activation energy of viscosity [9,20]. The ionic liquid structure is a critical factor in influencing the value of the Hildebrand solubility parameter. Ionic liquids which have different types of anion and cation have the different polarities and give different molecular interaction forces [9,20]. Generally, the solubility parameter decreases with increasing alkyl chain length [20]. However, it was found that there is no relationship between solubility parameter and alkyl chain length for 1alkyl derivatives [9]. The nature of anion also affects to the solubility parameter. The anion of ionic liquid which has a high polarity tends to have a high Hildebrand solubility [1]. In addition, some parameters, for instance; temperature of dissolution or reaction and fraction of solvent dissolved in ionic liquid, which are important parameters for their applications, may influence the Hildebrand solubility parameter. This work aims to study the effect of the important factors of ionic liquid type, dissolution temperature, and DMA fraction in ionic liquid on the Hildebrand solubility parameter. Moreover, the cohesive energy density, molar internal energy and enthalpy of dissolution, which were calculated from the Hildebrand solubility parameter and density of solutes, were also investigated. Ten potential ionic liquids were selected to determine the Hildebrand solubility parameter in this work. EMIM-AC, which has been known as an effective ionic liquid for biomass pre-treatment [18] and its mixture with DMA (60/40 v/ v), was used to examine the effect of dissolution temperature on the solubility parameter. Furthermore, DMA, which is an ordinary solvent mixed with ionic liquid for use in the bioconversion process, was used to study the effect of DMA fraction in ionic liquids (EMIM-AC and BMIM-Cl) on the solubility parameter.

357

2. Methods 2.1. Chemicals Almost all ionic liquids including 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4, P98.0%), 1-Butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6, P98.0%), 1-Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide (MBPYRRO-Tf2N, P98.0%), 1-Butyl-1-methylpyrrolidinium dicyanamide (MBPYRRO-N(CN)2, P98.0%), 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIM-Tf2N, P98.0%), 1-(2-Hydroxyethyl)-3methylimidazolium bis(trifluoromethylsulfonyl)imide (HOEMIMTf2N, P98.0%) were purchased from Merck. 1,3-Dimethylimidazolium methylsulfate (MMIM-MeSO4, P97.0%), 1-Ethyl-3-methylimidazolium acetate (EMIM-AC, P96.5%), 1-Butyl-3-methylimidazolium chloride (BMIM-Cl, P98.0%) and 1-Ethyl-3-methylimidazolium diethyl phosphate (EMIM-DEPO4, P98.0%) were acquired from Sigma–Aldrich. The analytical grade of solvents used possessing different Hildebrand solubility parameters including 2-butanol (22.2 MPa1/2), 1-butanol (23.1 MPa1/2), 2-propanol (23.5 MPa1/2), 1-propanol (24.5 MPa1/2), dimethylformamide (DMF, 24.8 MPa1/2), nitromethane (25.1 MPa1/2), allyl alcohol (25.7 MPa1/2), ethanol (26.5 MPa1/2), dimethyl sulfoxide (DMSO, 26.7 MPa1/2), propylene carbonate (27.3 MPa1/2), 2-pyrrolidone (28.4 MPa1/2), methanol (29.6 MPa1/2), diethylene glycol (29.9 MPa1/2), ethanolamine (31.3 MPa1/2), and water (47.9 MPa1/2) were also obtained from Sigma–Aldrich.

2.2. Determination of intrinsic viscosity The Hildebrand solubility parameters of ionic liquids and mixtures of ionic liquid and DMA at different temperatures were determined by measuring their intrinsic viscosities using an Ubbelohde viscometer. The solutions of solute (ionic liquid or the mixtures of ionic liquids and DMA) in different solvents were prepared for five concentrations varying 0.5–5% (v/v). The viscosities of solutions were measured by controlling temperatures. The efflux times were measured at least 5 times (variation of efflux time being within 0.1 s). The intrinsic viscosity (g; dL/g) was determined from the common intercept of Huggins and Kraemer relationships as shown in Eqs. (2) and (3), respectively by fitting of specific viscosity t solvent (gsp ¼ tsolution ) per concentration and natural logarithm of relat solvent

tive viscosity (gr ¼ ttsolution ) per concentration vs concentration (C; g/ solvent

dL). kH and kK are Huggins, and Kraemer constants, respectively. tsolution and t solvent are the efflux times of solution and solvent, respectively.

gsp C

¼ g þ kH g2 C

ln gr ¼ g þ kK g2 C C

ð2Þ

ð3Þ

The intrinsic viscosities vs the Hildebrand solubility parameters (dH ) of solvents were plotted and fitted by the Mangaraj (Eq. (4)) to determine the Hildebrand solubility parameters of ionic liquid or mixtures of ionic liquid and DMA at different temperatures:

g ¼ gmax exp½Aðdsolvent  dIL Þ2 

ð4Þ

where A is a constant, dsolvent and dIL are the Hildebrand solubility parameters of the solvent and the ionic liquid or the mixtures of ionic liquid and DMA, respectively. dIL , A and gmax were obtained from curve fitting with OriginPro 8 program.

358

P. Weerachanchai et al. / Chemical Engineering Journal 213 (2012) 356–362

2.3. Determination of density Densities of ionic liquids and mixtures of ionic liquid and DMA at different temperatures were measured by a density meter (Anton Paar, DMA35 version3). 3. Results and discussion 3.1. Effect of ionic liquid type

Hildebrand solubility parameter ( δ H, MPa

1/2

)

The Hildebrand solubilities of ten potential ionic liquids were determined at 25 °C to study the effect of ionic liquid type. The effect of anion and cation on the Hildebrand solubility parameter is shown in Fig. 1. Fig. 1a shows the significant influence of the anions in the ionic liquids on the Hildebrand solubility parameter. BMIMPF6 provides the highest Hildebrand solubility (dH = 28.09), while BMIM-Cl presents the lowest value (dH = 24.14). The values of ionic liquids containing BMIM cations are in the following order: [PF6] > [Tf2N] > [Cl], while those of ionic liquids containing EMIM cations are in the order: [BF4] > [DEPO4] > [AC]. When considering different ionic liquids containing [Tf2N] anions, the type of cations also exerts an effect on the Hildebrand solubility parameter (Fig. 1a and b). The highest value was obtained from HOEMIM-Tf2N. The lower values were attained from MBPYRRO-Tf2N and BMIM-Tf2N, respectively. The Hildebrand solubility parameter of HOEMIM-Tf2N presents much higher than that of the others, possibly due to the higher polarity of HOEMIM containing a hydroxyl group. MBPYRRO-Tf2N and BMIM-Tf2N offer relatively similar Hildebrand solubility parameter. It may be

29

1-Butyl-3-methylimidazolium 1-Ethyl-3-methylimidazolium 1,3-Dimethylimidazolium 1-(2-Hydroxyethyl)-3-methylimidazolium 1-Butyl-1-methyl pyrrolidium

28 27 26 25 24 23

PF6 MESO4 BF4 Tf 2N N(CN)2 DEPO4 AC

Cl

Anion type

1/2 Hildebrand solubility parameter (δH, MPa )

(a) 28 Methylimidazolium 1-(2-Hydroxyethyl)-3-methylimidazolium Methylpyrrolidinium

27

2

1

3

26

6 7 8

4 5

25

9

24

23 .5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Alkyl chain length of cation

(b) Fig. 1. The effects of (a) anion type and (b) alkyl chain length of cation on the Hildebrand solubility parameter measured at 25 °C. Note: The numbers represent individual ionic liquid according to Table 1.

attributed to the similar alkyl chain length. However, pyrrolidinium provides a slightly higher value than imidazolium, in agreement with results from work of Marciniak [22] for anions of trifluoroacetate and trifluoromethanesulfonate. When compare the values of Hildebrand solubility parameter from intrinsic viscosity with those derived from other methods, the value for BMIM-Tf2N from this experiment (dH = 25.69) is close to that derived from the methods of Kamlet–Taft Equation (dH = 25.5) [1], activation energy of viscosity (dH = 26.5) [23] and intrinsic viscosity from work of Lee, S.H. and Lee, S.B (dH = 26.7) [9]. These three works provided the similar values of BMIM-PF6 with this work as well, their Hildebrand solubility parameter varies between 28.09 and 30.2 [1,9,21]. The Hildebrand solubility parameter derived from this experiment also corresponds with that computed through molecular dynamic simulation for MBPYRRO-Tf2N [2]. However, the Hildebrand solubility parameters for BMIMTf2N derived from methods of enthalpy of vaporization [9], surface tension [24], inverse gas chromatography [22,25] are different compared with this experiment, in the range of 19.8–22.9. For the methods of surface tension [24] and inverse gas chromatography [17], they indicated also that BMIM-PF6 gives dissimilar values (dH = 25.1 and 21.99, respectively) compared with this works (dH = 28.09). Furthermore, the Hildebrand solubility parameters of HOEMIM-Tf2N and MBPYRRO-Tf2N derived from surface tension are different from the value derived from this work (22.0 [24] and 21.1 [24] vs 26.49 and 25.81, respectively). The physicochemical properties of the ionic liquids including density, molar volume, Hildebrand solubility parameter, cohesive energy density, molar internal energy and enthalpy of dissolution are summarized and shown in Table 1. The cohesive energy density, molar internal energy and heat of dissolution were calculated using Eq. (1) with Hildebrand solubility parameter and density. It was found that the cohesive energy densities of ionic liquids in the range of 582.7–789.1 J/cm3 are higher than those of ordinary organic solvents (such as 515.3 J/cm3 for DMA) (Table 1). The cohesive energy densities of different ionic liquids are directly proportional to the Hildebrand solubility parameter. On the other hand, their molar internal energies are not only dependent on Hildebrand solubilities but they are also dependent on their molar volumes (see Eq. (1)). For example, BMIM-PF6, which provides the highest Hildebrand solubility parameter (28.09), has a lower molar internal energy (BMIM-PF6: 162.5 vs MBPYRRO-Tf2N: 201.0 kJ/ mol) than MBPYRRO-Tf2N, which possesses a lower Hildebrand solubility parameter (25.81) but a higher molar volume (BMIMPF6: 205.9 vs MBPYRRO-Tf2N: 301.7 cm3/mol). However, the molar internal energies of the different ionic liquids measured at 25 °C show linear trends with the enthalpies of dissolution (Table 1). This is due to the fact that the enthalpy of dissolution has a linear relationship with the molar internal energy when an ionic liquid is measured at a constant temperature (see Eq. (1)). The molar internal energy and the enthalpy of dissolution for the different ionic liquid vary in the range of 93.8–201.0 and 96.3–203.5 kJ/mol, respectively. In comparison with the values obtained from other methods, it was found that the cohesive energy densities of BMIM-Tf2N and BMIM-PF6 estimated from the works of Swiderski et al. [1] and Jiang et al. [26], respectively agree with those from this work. However, the other ionic liquids have the distinct values of cohesive energy densities according to their methods. For example, the value of EMIM-BF4 from this work is 681.7 J/cm3, while 802.5 and 1,030.0 J/cm3 were acquired from computed from different works of Singh and Kumar [27] and Jiang et al. [26], respectively. Considering the molar internal energy and heat of dissolution, the molecular dynamic simulation from work of Shimizu et al. [28] provides slight different internal energy for BMIM-Tf2N (178.0 [28] vs 192.2 kJ/mol) and BMIM-PF6 (184.0 [28] vs 162.4 kJ/mol). On the

Table 1 The solubility properties of ionic liquids and the mixtures of ionic liquids and DMA at different temperatures. MW (g/mol)

Molar volume (cm3/mol)

dH (MPa1/2)

CED (J/cm3)

DU (kJ/mol)

DH (kJ/mol)

liquid type 1,3-Dimethylimidazolium methylsulfate (MMIM-MeSO4) 1-(2-Hydroxyethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (HOEMIM-Tf2N) 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4) 1-Ethyl-3-methylimidazolium diethyl phosphate (EMIM-DEPO4) 1-Ethyl-3-methylimidazolium acetate (EMIM-AC) 1-Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide (MBPYRRO-Tf2N) 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIM-Tf2N) 1-Butyl-1-methylpyrrolidinium dicyanamide (MBPYRRO-N(CN)2) 1-Butyl-3-methylimidazolium chloride (BMIM-Cl) 1-Butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6)

1.330 1.580 1.294 1.157 1.104 1.400 1.440 0.950 1.085 1.380

208.24 407.31 197.97 264.26 170.21 422.41 419.37 208.31 174.67 284.18

156.57 257.79 152.99 228.36 154.16 301.72 291.23 219.27 160.99 205.93

26.36 26.49 26.11 25.41 25.16 25.81 25.69 25.54 24.14 28.09

694.85 701.72 681.73 645.67 633.03 666.16 659.98 652.29 582.74 789.05

108.79 180.90 104.30 147.45 97.59 200.99 192.20 143.03 93.81 162.49

111.27 183.37 106.78 149.92 100.07 203.47 194.68 145.51 96.29 164.96

DMA 11 12 13 14 15 16 17 18 19

fraction in ionic liquid 1-Ethyl-3-methylimidazolium acetate 1-Ethyl-3-methylimidazolium acetate-DMA(60–40 v/v) 1-Ethyl-3-methylimidazolium acetate-DMA(40–60 v/v) 1-Ethyl-3-methylimidazolium acetate-DMA(10–90 v/v) 1-Butyl-3-methylimidazolium chloride 1-Butyl-3-methylimidazolium chloride-DMA(60–40 v/v) 1-Butyl-3-methylimidazolium chloride-DMA(40–60 v/v) 1-Butyl-3-methylimidazolium chloride-DMA(10–90 v/v) DMAa

1.104 1.054 1.016 0.981 1.085 1.039 1.006 0.963 0.947

170.21 126.37 110.77 92.29 174.67 127.53 111.27 92.35 87.12

154.16 119.85 109.02 94.08 160.99 122.78 110.65 95.90 92.00

25.16 25.07 25.49 24.90 24.14 24.35 24.78 24.57 22.70

633.03 628.50 649.74 620.01 582.74 592.92 614.05 603.68 515.29

97.59 75.32 70.84 58.33 93.81 72.80 67.95 57.89 47.40

100.07 77.80 73.31 60.81 96.20 75.28 70.42 60.37 49.88

1.104 1.096 1.086 1.054 1.045 1.035

170.21 170.21 170.21 126.37 126.37 126.37

154.16 155.32 156.74 119.85 120.92 122.13

25.16 24.79 24.70 25.07 24.93 24.79

633.03 614.54 610.09 628.50 621.50 614.54

97.59 95.45 95.62 75.32 75.16 75.05

100.07 98.05 98.39 77.80 77.76 77.82

Chemical

Dissolution temperature 20 1-Ethyl-3-methylimidazolium 21 1-Ethyl-3-methylimidazolium 22 1-Ethyl-3-methylimidazolium 23 1-Ethyl-3-methylimidazolium 24 1-Ethyl-3-methylimidazolium 25 1-Ethyl-3-methylimidazolium a

acetate at 25 °C acetate at 40 °C acetate at 60 °C acetate-DMA(60–40) at 25 °C acetate-DMA(60–40) at 40 °C acetate-DMA(60–40) at 60 °C

P. Weerachanchai et al. / Chemical Engineering Journal 213 (2012) 356–362

Density (g/cm3)

Ionic 1 2 3 4 5 6 7 8 9 10

No.

Ref. [3].

359

360

P. Weerachanchai et al. / Chemical Engineering Journal 213 (2012) 356–362

other hand, the molecular dynamic simulation from work of Jiang et al. [26] shows greatly different values of heat of dissolution for EMIM-BF4 (161.3 [26] vs 106.8 kJ/mol) and BMIM-Cl (162.0 [26] vs 96.3 kJ/mol) but show relatively close value for BMIM-PF6 (172.0 [26] vs 165.0 kJ/mol). 3.2. Effect of DMA fraction in ionic liquid

Hildebrand solubility parameter (δ H, MPa1/2 )

Four fractions of DMA in EMIM-AC and BMIM-Cl were prepared for purposes of studying the effect of DMA on the solubility parameter at 25 °C. The addition of DMA from 0 to 60 vol% into an ionic liquid tends to increase the Hildebrand solubility parameter (Fig. 2a). However, the increase of DMA from 0 to 40 vol% does not have much influence on the Hildebrand solubility parameter for the mixtures of EMIM-AC and DMA (25.16 vs 25.07), indicating that the Hildebrand solubility parameter of the mixtures of ionic liquid and DMA does not correspond to the Kay’s mixing rule [3,11,12,29]; Hildebrand solubilities of mixtures does not fall in the range of that of ionic liquid and DMA. The Hildebrand solubilities of the mixtures tend to be closer to those of ionic liquid than that of DMA. This could be an advantage for using a mixture of ionic liquid and DMA as a solvent to reduce its viscosity to facilitate solute dissolution and ease of handling and operations [30,31]. The mixtures possess lower viscosities than the ionic liquids but their solubilities are still close to those of the ionic liquids. Nevertheless, 90 vol% of DMA added in ionic liquid leads to the decrease of the Hildebrand solubility parameter compared with that of 60% DMA

27 EMIM-AC/DMA BMIM-Cl/DMA

26

25

24

23

22 0

20

40

60

80

100

DMA volume%

110

110

Δ U of EMIM-AC/DMA Δ U of BMIM-Cl/DMA Δ H of EMIM-AC/DMA Δ H of BMIM-Cl/DMA

100 90

100 90

80

80

70

70

60

60

50

50

40 0

20

40

60

80

40 100

Enthalpy of dissolution (Δ H, kJ/mol)

Molar internal energy (Δ U, kJ/mol)

(a)

DMA volume%

(b) Fig. 2. The effect of DMA vol% dissolved in EMIM-AC or BMIM-Cl on Hildebrand solubility parameter and (b) molar internal energy and enthalpy of dissolution measured at 25 °C.

and it has a lower value than that of the pure ionic liquid. It may be possible that DMA has a stronger effect on the Hildebrand solubility parameter than ionic liquid: the solubilities of mixtures being closer to that of DMA (dH = 22.7). Considering the cohesive energy density (CED), it was noted that the addition of DMA in an ionic liquid alters the required energy to break the interactions between molecules, represented in the form of CED (Table 1). The cohesive energy densities of the mixtures of EMIM-AC/DMA and BMIM-Cl/DMA from 0 to 90 vol% are following ranges of 620.0–649.7 and 582.7–614.1 J/cm3, respectively. It signifies that the cohesive energy densities of the mixtures containing 40–90 vol% DMA are closer to those of the ionic liquids (633.0 J/cm3 for EMIM-AC and 582.7 J/cm3 for BMIM-Cl), and higher than that of DMA (515.3 J/cm3). Moreover, the molar volumes of the mixtures decrease with increasing a DMA amount causing drastic changes in the molar internal energy. The molar internal energy declines linearly as the DMA fraction increases (Fig. 2b). It should be noted that the decreases of internal energy and heat of dissolution are almost consistent in range of 47.4–100.1 kJ/mol with increasing DMA in both ionic liquids (EMIM-AC and BMIM-Cl) from 0 to 100 vol% at 25 °C. 3.3. Effect of dissolution temperature The dissolution temperature is an important parameter to affect the solubility properties of ionic liquid. The slight decrease of Hildebrand solubility parameter with increasing temperature has been mostly found in previous works [11,25,32]. For this work, the EMIM-Ac and the mixture of EMIM-AC/DMA (60–40 v/v) were used to study the effect of dissolution temperature on the solubility parameter. It should be noted that the increase of temperature from 25 to 60 °C shows the decrease of the Hildebrand solubility parameter, varying in the range of 24.7–25.2 (Fig. 3a). EMIM-AC/ DMA (60–40 v/v) gives a linear decrease with increasing temperature, whereas, EMIM-AC exhibits greater decrease of Hildebrand solubility parameter than that of EMIM-AC/DMA (60–40 v/v) when the dissolution temperature increases from 25 to 40 °C. But it gives a slight decrease of Hildebrand solubility parameter with increasing temperature from 40 to 60 °C. The increase of dissolution temperature influences the cohesive energy densities of ionic liquid and the mixture (Table 1): a decrease of cohesive energy density with increasing temperature. The cohesive energy densities alter in the range of 610.1–633.1 J/cm3 for EMIM-AC and 614.5– 628.5 J/cm3 for EMIM-AC/DMA (60–40 v/v). It was found that there are almost no effects of temperature on molar internal energy and enthalpy of dissolution for EMIM-AC/DMA (60–40 v/v), giving average values of 75.18 and 77.79 kJ/mol, respectively (Fig. 3b). On the other hand, the molar internal energy and enthalpy of dissolution for EMIM-AC show a slight decrease when the temperature increases from 25 to 40 °C but they show almost constant values with temperature increase from 40 to 60 °C. They are in the range of 95.4–97.6 and 98.1–100.1 kJ/mol, respectively. 4. Conclusions This study has shown that the ionic liquid type, DMA fraction in ionic liquid and dissolution temperature are the significant parameters affecting the solubility properties. The nature of anion influences strongly the Hildebrand solubility parameter. BMIM-PF6 provides the highest value of the Hildebrand solubility parameter, while BMIM-Cl presents the lowest value. The cation type also shows an important effect on the solubility parameter. HOEMIM cation contributes the highest value of Hildebrand solubility parameter among the other cations which have the same anion. The increase of DMA fraction from 0 to 60 vol% presents the

Hildebrand solubility parameter ( δ H, MPa1/2)

P. Weerachanchai et al. / Chemical Engineering Journal 213 (2012) 356–362

References 25.4

EMIM-AC EMIM-AC/DMA(60/40 vol%)

25.2

25.0

24.8

24.6

20

30

40

50

60

70

o

Temperature ( C)

105

100

100

95

95 Δ U of EMIM-AC Δ U of EMIM-AC/DMA(60/40 v/v) Δ H of EMIM-AC Δ H of EMIM-AC/DMA(60/40 v/v)

90 85

90 85

80

80

75

75

70

70

20

30

40

50

60

Enthalpy of dissolution (

105

Δ H, kJ/mol)

(a) Molar internal energy (Δ U, kJ/mol)

361

70

o

Temperature ( C)

(b) Fig. 3. The effect of dissolution temperature on (a) Hildebrand solubility parameter and (b) molar internal energy and enthalpy of dissolution of EMIM-AC and the mixture of EMIM-AC/DMA (60/40 vol%).

maximum Hildebrand solubility parameter for the mixtures of EMIM-AC/DMA and BMIM-Cl/DMA followed by the decrease of the solubility parameter at 90 vol% DMA added. It should be noted that the mixtures of ionic liquid and DMA could provide a benefit for using solvent due to their lower viscosity but still having similar solubility with that of its ionic liquid. The studies of effect of dissolution temperature on solubility parameters indicate that the increase of temperature from 25 to 60 °C displays linear decrease of the Hildebrand solubility parameter, varying in the range of 24.7–25.2 for the EMIM-Ac and the mixture of EMIM-AC/DMA (60–40 v/v). When considering the effects of solubility conditions on cohesive energy density, molar internal energy and enthalpy of dissolution, the cohesive energy densities of different ionic liquids are directly proportional to the Hildebrand solubility parameter. The different ionic liquid types have different values of molar internal energy depending on their Hildebrand solubility parameter and molar volume. The increase of DMA fraction in ionic liquids gives the lower values of molar internal energy and enthalpy of dissolution, but the increase of dissolution temperature contributes to the almost constant/slight decrease for EMIM-AC and the mixture of EMIM-AC/DMA (60–40 v/v), respectively. Acknowledgment This work is supported by Startup Grant of Nanyang Technological University and Competitive Research Programme (NRFCRP5-2009-03) of National Research Foundation in Singapore.

[1] K. Swiderski, A. McLean, C.M. Gordon, D.H. Vaughan, Estimates of internal energies of vaporisation of some room temperature ionic liquids, Chem. Commun. 10 (2004) 2178–2179. [2] Y.S. Sistla, L. Jain, A. Khanna, Validation and prediction of solubility parameters of ionic liquids for CO2 capture, Sep. Purif. Technol. 97 (2012) 51–64. [3] P. Bustamante, J. Navarro-Lupión, B. Escalera, A new method to determine the partial solubility parameters of polymers from intrinsic viscosity, Eur. J. Pharm. Sci. 24 (2005) 229–237. [4] W.L. Archer, Hansen solubility parameters for selected cellulose ether derivatives and their use in the pharmaceutical industry, Drug Dev. Ind. Pharm. 18 (1992) 599–616. [5] S. Dünnebeil, G. Sadowski, W. Arlt, Comparison of two predictive gE models for vapour—liquid equilibrium calculations, Chem. Eng. J. 61 (1996) 21–26. [6] J. Blath, M. Christ, N. Deubler, T. Hirth, T. Schiestel, Gas solubilities in room temperature ionic liquids – correlation between RTIL-molar mass and Henry’s law constant, Chem. Eng. J. 172 (2011) 167–176. [7] S. Dinda, A.V. Patwardhan, S.R. Panda, N.C. Pradhan, Kinetics of reactive absorption of carbon dioxide with solutions of aniline in carbon tetrachloride and chloroform, Chem. Eng. J. 136 (2008) 349–357. [8] A.P.d.l. Ríos, F.J. Hernández-Fernández, H. Presa, D. Gómez, G. Víllora, Tailoring supported ionic liquid membranes for the selective separation of transesterification reaction compounds, J. Membr. Sci. 328 (2009) 81–85. [9] S.H. Lee, S.B. Lee, The Hildebrand solubility parameters, cohesive energy densities and internal energies of 1-alkyl-3-methylimidazolium-based room temperature ionic liquids, Chem. Commun. (2005) 3469–3471. [10] A. Vetere, An improved method for predicting the vapor—liquid equilibria of subcritical mixtures, Chem. Eng. J. 55 (1994) 115–124. [11] A.F.M. Barton, Solubility parameters, Chem. Rev. 75 (1975) 731–753. [12] V. Malpani, P.A. Ganeshpure, P. Munshi, Determination of solubility parameters for the p-xylene oxidation products, Ind. Eng. Chem. Res. 50 (2011) 2467–2472. [13] J.-M. Lee, J.M. Prausnitz, Polarity and hydrogen-bond-donor strength for some ionic liquids: effect of alkyl chain length on the pyrrolidinium cation, Chem. Phys. Lett. 492 (2010) 55–59. [14] J.-M. Lee, S. Ruckes, J.M. Prausnitz, Solvent polarities and Kamlet–Taft parameters for ionic liquids containing a pyridinium cation, J. Phys. Chem. B 112 (2008) 1473–1476. [15] J.-M. Lee, Solvent properties of piperidinium ionic liquids, Chem. Eng. J. 172 (2011) 1066–1071. [16] N. Galonde, K. Nott, A. Debuigne, M. Deleu, C. Jerôme, M. Paquot, J.P. Wathelet, Use of ionic liquids for biocatalytic synthesis of sugar derivatives, J. Chem. Technol. Biotechnol. 87 (2012) 451–471. [17] N.V. Plechkova, K.R. Seddon, Applications of ionic liquids in the chemical industry, Chem. Soc. Rev. 37 (2008) 123–150. [18] P. Weerachanchai, S.S.J. Leong, M.W. Chang, C.B. Ching, J.-M. Lee, Improvement of biomass properties by pretreatment with ionic liquids for bioconversion process, Bioresour. Technol. 111 (2012) 453–459. [19] M. Moniruzzaman, N. Kamiya, M. Goto, Ionic liquid based microemulsion with pharmaceutically accepted components: Formulation and potential applications, J. Colloid Interface Sci. 352 (2010) 136–142. [20] A. Marciniak, The Hildebrand solubility parameters of ionic liquids – Part 2, Int. J. Mol. Sci. 12 (2011) 3553–3575. [21] P.K. Kilaru, R.A. Condemarin, P. Scovazzo, Correlations of low-pressure carbon dioxide and hydrocarbon solubilities in imidazolium-, phosphonium-, and ammonium-based room-temperature ionic liquids. Part 1. Using surface tension, Ind. Eng. Chem. Res. 47 (2007) 900–909. [22] A. Marciniak, The solubility parameters of ionic liquids, Int. J. Mol. Sci. 11 (2010) 1973–1990. [23] P.K. Kilaru, P. Scovazzo, Correlations of low-pressure carbon dioxide and hydrocarbon solubilities in imidazolium-, phosphonium-, and ammoniumbased room-temperature ionic liquids. Part 2. Using activation energy of viscosity, Ind. Eng. Chem. Res. 47 (2007) 910–919. [24] H. Jin, B. O’Hare, J. Dong, S. Arzhantsev, G.A. Baker, J.F. Wishart, A.J. Benesi, M. Maroncelli, Physical properties of ionic liquids consisting of the 1-butyl-3methylimidazolium cation with various anions and the bis(trifluoromethylsulfonyl)imide anion with various cations, J. Phys. Chem. B 112 (2007) 81–92. [25] F. Mutelet, V. Butet, J.-N. Jaubert, Application of inverse gas chromatography and regular solution theory for characterization of ionic liquids, Ind. Eng. Chem. Res. 44 (2005) 4120–4127. [26] H. Jiang, F. Zhao, J. Wang, Z. Liu, J. Ren, R. Liu, J. Shang, Y. Hu, Molecular simulations of imidazolium-based ionic liquids with [L-lactate]  anion and the binary mixture of [bmim][L-lactate] and 1-octanol, J. Mol. Liq. 165 (2012) 63–70. [27] T. Singh, A. Kumar, Static dielectric constant of room temperature ionic liquids: internal pressure and cohesive energy density approach, J. Phys. Chem. B 112 (2008) 12968–12972. [28] K. Shimizu, M. Tariq, M.F.C. Gomes, L.s.P.N. Rebelo, J.N.C. Lopes, Assessing the dispersive and electrostatic components of the cohesive energy of ionic liquids using molecular dynamics simulations and molar refraction data, J. Phys. Chem. B 114 (2010) 5831–5834. [29] P. Benjumea, J. Agudelo, A. Agudelo, Basic properties of palm oil biodiesel– diesel blends, Fuel 87 (2008) 2069–2075.

362

P. Weerachanchai et al. / Chemical Engineering Journal 213 (2012) 356–362

[30] H. Tadesse, R. Luque, Advances on biomass pretreatment using ionic liquids: an overview, Energy Env. Sci. 4 (2011) 3913–3929. [31] P. Mäki-Arvela, I. Anugwom, P. Virtanen, R. Sjöholm, J.P. Mikkola, Dissolution of lignocellulosic materials and its constituents using ionic liquids – a review, Ind. Crops Prod. 32 (2010) 175–201.

[32] S.S. Moganty, R.E. Baltus, Regular solution theory for low pressure carbon dioxide solubility in room temperature ionic liquids: ionic liquid solubility parameter from activation energy of viscosity, Ind. Eng. Chem. Res. 49 (2010) 5846–5853.