THERMOPHYSICAL PROPERTIES OF 1-ETHYL-3-METHYLIMIDAZOLIUM CHLORIDE SOLUTION FROM 293.15 TO 323.15 K

ABSTRACT

Aqueous systems composed of ionic liquid make up a new alternative for use in processes involving the separation of biomolecules. The objective of this experiment was to obtain the thermo-physical properties of density, refractive index, electrical conductivity, molar volume, thermal expansion coefficient and apparent specific volume of the ionic liquid 1-ethyl-3-methylimidazolium chloride. The thermo-physical properties of aqueous solutions of this ionic liquid were measured as a function of the mass fraction w = (0.05, 0.125, 0.2, 0.275 and 0.35), temperature T = (293.15, 303.15, 313.15 and 323.15) K and pH = (7.5, 8.0 and 8.5). Models representing the combined effects between variables were fit since they are required for industrial applications where the physical parameters must be accurately calculated. Models representing the combined effects of the variables temperature, mass fraction and pH values of ionic liquid were adjusted and presented good fit.

Key words:
Thermodynamic properties; Binary mixtures; Ionic liquid; Models

INTRODUCTION

Ionic liquids (ILs) are organic salts which have a low melting point. They have unique physical-chemical properties, with negligible vapor pressure, non-flammable, non-explosive, electrochemically and thermally stable, and can be easily recycled. These salts present extremely low vapor pressures, and this feature has attracted attention given their potential as solvents to replace volatile organic solvents for a large variety of chemical reactions, separation processes and other applications. Such compounds have many favorable properties which make them attractive for many applications (Ficke et al. 2010; França et al. 2009; Muhammad et al. 2012).

Due to these characteristics, the ILs have been extensively studied in two-phase aqueous systems (ATPS). These systems consist of two aqueous phases rich in two structurally different compounds which are immiscible, where they separate into two phases above certain concentration values. The ATPS may be formed from polymer, surfactant and salt, which may be strategically combined to achieve high selectivity and efficiency for the extraction and purification of biomolecules (Vicente et al., 2014). However, most polymers used to form the phases have high viscosity and form turbid solutions, interfering in determination of the analytes. In recent years it has been demonstrated that ionic liquids are a viable alternative for ATPS composed of common polymers since they present unique advantages, such as low viscosity, formation of emulsion, absence of volatile organic solvents, rapid phase separation and high extraction efficiency (Yan et al., 2014).

The ATPS formed by ionic liquid have been successfully applied to separate, concentrate, isolate and purify biocompounds. Several studies with ILs have been published in recent years, including their application in complete removal of dyes, recovery of antibiotics, ethanol and butanol from fermentation broth, acetone, removal of organic contaminants from aqueous waste streams and protein partitioning (Ferreira et al., 2014; Huddleston et al., 1998; Fadeev and Meagher, 2001; Du et al., 2007; Dreyer et al., 2009). For these reasons, predicting the basic thermophysical properties of the phase forming systems at various concentrations and temperatures is also an indispensable requirement for the design and scale up of a wide range of separation process.

Given the above, the present study sought to evaluate the thermophysical properties of density, thermal expansion coefficient, molar volume, specific volume, refractive index and electrical conductivity of binary mixtures of water + 1-ethyl-3-methylimidazolium chloride in various conditions of pH, temperature and mass fraction.

EXPERIMENTAL SECTION

Materials

The 1-ethyl-3-methylimidazolium chloride (CAS: 65039-09-0) was purchased from Sigma Aldrich. Potassium hydroxide (CAS: 1310-58-3) and hydrochloric acid (CAS: 7647-01-0) were purchased from Vetec Fine Chemicals (Brazil) and Synth, respectively. All reagents were of analytical grade and used without further purification. The entire experiment was conducted at the Process Engineering Laboratory of the State University of Southwest Bahia (UESB), Itapetinga campus - Bahia, Brazil.

Methods

Preparation of solutions

Aqueous solutions of 1-ethyl-3-methylimidazolium chloride ([C₂mim]Cl) were prepared using an analytical balance M254A (Bel Engineering) with an accuracy of ± 0.0001 g. Stock solutions of 1-ethyl-3-methylimidazolium chloride (w = 0.50) were prepared for each pH value (7.5, 8.0 and 8.5) by addition of hydrochloric acid or potassium hydroxide when required. These pH values were chosen because they are commonly used in ATPS for separation of biomolecules (Sampaio et al., 2017). Appropriate quantities of the stock solution were diluted in tubes and shaken manually to obtain the desired mass fraction w = (0.05, 0.125, 0.2, 0.275 and 0.35). The analyses were performed at temperatures of T = (293.15, 303.15, 313.15 and 323.15) K, in all prepared solutions. Distilled water was used in all the experiments.

Density

The densities of the solutions were determined using a Bench Digital densimeter DMA 5000M (Anton Paar) with accuracy of ±5 x 10^-6 g·cm^-3 and repeatability of ±1 x 10^-6 g·cm^-3 in the operating range of 0 to 3 g·cm^-3. The temperature range of the equipment is 273.15 K to 363.15 K with an accuracy of ±0.01 K and repeatability of ±0.001 K.

Refractive Index

For the refractive index, the digital refractometer Q767BD (Quimis) was used with accuracy of ±0.0002 and wavelength of 589 nanometers, as is conventionally done. This device was connected to a thermostatic bath (Tecnal, Te-184), allowing for temperature control with an accuracy of 0.1 K. The equipment was calibrated with distilled water at the studied temperature, and the sample was then placed in the prism of the refractometer for direct reading.

Electrical conductivity

Electrical conductivity was determined using a bench digital conductivity meter Q795m (Quimis) with a precision of 0.5 %. The equipment was calibrated with a standard 0.01 M KCl solution. The temperature of the samples was controlled in a thermostatic bath (Tecnal, Te-184), allowing for temperature control to within 0.1 K.

Molar Volume

The molar volume (V _m ) is the volume occupied by one mole of a substance at a fixed temperature and pressure. The V _m of the binary mixtures of water + 1-ethyl-3-methylimidazolium chloride was calculated using the following equation:

$$V_m=\frac{M}{\rho }$$ (1)

where: M is the molecular mass in g·mol^-1, ρ is the density in g·cm^-3 and V_m is the molar volume cm³·mol^-1.

Thermal Expansion Coefficient

The density calculated for the binary mixtures of water + 1-ethyl-3-methylimidazolium chloride was used to calculate the thermal expansion coefficient (α_p ), using the following equation:

$${\alpha }_p=-\frac{1}{\rho }{\left(\frac{\partial \rho }{\partial T}\right)}_p$$ (2)

where: ρ is the density obtained in g·cm^-3.

Apparent specific volume

The apparent specific volume (v_2Ø) was calculated from the density data using equation 3:

$$v_{2\varnothing }=\frac{1}{\rho }\left[1+\frac{{\rho }_0-\rho }{w{\rho }_0}\right]$$ (3)

where: ρ (g·cm^-3) and ρ₀ (g·cm^-3) are the densities of the ionic solution and pure water, respectively.

Statistical analysis

All statistical analyses were performed using SAEG v.9.1 (Ribeiro Júnior, 2001). The experiment was conducted in a completely randomized design (CRD) with two replications and in triplicate. The experimental data obtained was fitted to polynomial models, where the correlation coefficient was calculated for the treatment means. Correlation coefficients between the predicted and real values were calculated for all models. The standard deviation for each property was also calculated. All analyses were performed considering a 5% significance level. The expanded uncertainties in density, refractive index, electrical conductivity, molar volume, apparent specific volume and thermal expansion coefficient were calculated as combined uncertainties multiplied by 2. The coverage factor of 2 yields a 95% confidence interval.

RESULTS AND DISCUSSION

Density, refractive index and electrical conductivity of the binary mixtures

The density, refractive index and electrical conductivity of ionic liquid solutions were measured at different temperatures T = (293.15, 303.15, 313.15 and 323.15) K, pH values (7.5, 8.0 and 8.5) and mass fraction w = (0.05, 0.12, 0.2, 0.275 and 0.35). Table 1 shows the mean and standard deviation of these properties in all conditions studied.

Table 1
Density (ρ), refractive index (n_D) and conductivity (κ) of the binary mixtures of water + 1-ethyl-3-methylimidazolium chloride at the different mass fraction (0.05, 0.12, 0.2, 0.275 and 0.35), temperatures T = (293.15, 303.15, 313.15 and 323.15) K and pH (7.5, 8.0 and 8.5)^a.

T/K		w		ρ/g·cm-3		nD		κ/10-3mS·cm-1
pH 7.5
293.15		0.050		1.0039±0.0001		1.3416±0.0001		17.5446±0.7642
		0.125		1.0130±0.0001		1.3537±0.0001		40.6085±0.3863
		0.200		1.0242±0.0001		1.3678±0.0001		65.0967±1.5680
		0.275		1.0355±0.0001		1.3826±0.0009		87.4540±1.8243
		0.350		1.0476±0.0004		1.3962±0.0005		107.8054±3.0910
303.15		0.050		1.0000±0.0001		1.3406±0.0003		17.6565±0.5265
		0.125		1.0099±0.0001		1.3527±0.0003		40.5151±0.5184
		0.200		1.0205±0.0001		1.3664±0.0001		64.8170±1.4372
		0.275		1.0313±0.0001		1.3812±0.0005		87.2676±2.3521
		0.350		1.0430±0.0004		1.3948±0.0002		107.3345±3.4826
313.15		0.050		0.9966±0.0001		1.3391±0.0001		17.4303±1.2536
		0.125		1.0057±0.0001		1.3518±0.0001		38.1956±0.1174
		0.200		1.0162±0.0001		1.3648±0.0003		61.6610±1.2508
		0.275		1.0266±0.0002		1.3797±0.0011		83.1728±2.1267
		0.350		1.0380±0.0004		1.3927±0.0005		102.9411±3.4685
323.15		0.050		0.9926±0.0001		1.3374±0.0004		17.9765±0.9637
		0.125		1.0002±0.0001		1.3504±0.0001		39.6268±1.1170
		0.200		1.0111±0.0005		1.3630±0.0005		63.7163±1.3768
		0.275		1.0214±0.0002		1.3778±0.0008		85.8561±2.6360
		0.350		1.0326±0.0004		1.3908±0.0005		105.9541±3.6763
pH 8.0
293.15		0.050		1.0043±0.0001		1.3414±0.0001		16.6523±0.4220
		0.125		1.0137±0.0001		1.3535±0.0001		39.7316±0.6737
		0.200		1.0239±0.0001		1.3676±0.0001		62.7498±0.2775
		0.275		1.0350±0.0001		1.3812±0.0001		83.5054±4.5756
		0.350		1.0469±0.0001		1.3954±0.0001		103.3806±1.7279
303.15		0.050		1.0015±0.0001		1.3404±0.0001		16.6523±0.4485
		0.125		1.0104±0.0001		1.3522±0.0001		39.8251±0.2771
		0.200		1.0202±0.0001		1.3663±0.0001		63.2166±0.3827
		0.275		1.0308±0.0001		1.3797±0.0001		83.6921±4.3116
		0.350		1.0423±0.0001		1.3938±0.0001		102.9166±2.6478
313.15		0.050		0.9979±0.0001		1.3388±0.0002		16.0550±0.5440
		0.125		1.0065±0.0001		1.3513±0.0001		38.4293±0.6457
		0.200		1.0159±0.0001		1.3647±0.0001		60.4849±0.0129
		0.275		1.0262±0.0001		1.3780±0.0003		79.3894±3.7525
		0.350		1.0373±0.0001		1.3919±0.0005		98.6143±2.0328
323.15		0.050		0.9934±0.0001		1.3375±0.0004		16.7187±0.3161
		0.125		1.0020±0.0001		1.3498±0.0001		39.8625±0.6461
		0.200		1.0111±0.0001		1.3631±0.0004		62.4534±0.5195
		0.275		1.0210±0.0002		1.3761±0.0006		82.2503±3.7510
		0.350		1.0320±0.0001		1.3893±0.0001		101.9130±1.9156
pH 8.5
293.15		0.050		1.0048±0.0002		1.3414±0.0001		16.6693±0.1053
		0.125		1.0143±0.0001		1.3542±0.0001		40.3545±0.4767
		0.200		1.0252±0.0001		1.3688±0.0001		59.9509±0.7293
		0.275		1.0367±0.0002		1.3830±0.0001		81.6592±0.0442
		0.350		1.0494±0.0001		1.3981±0.0001		102.4296±0.0442
303.15		0.050		1.0019±0.0002		1.3404±0.0001		16.6959±1.6150
		0.125		1.0110±0.0001		1.3529±0.0001		40.5327±0.0427
		0.200		1.0215±0.0001		1.3675±0.0001		60.4862±0.4783
		0.275		1.0325±0.0002		1.3813±0.0001		81.8447±0.7336
		0.350		1.0447±0.0001		1.3963±0.0001		102.2526±0.2182
313.15		0.050		0.9983±0.0002		1.3391±0.0002		17.1746±1.8654
		0.125		1.0070±0.0001		1.3520±0.0003		41.2442±0.3189
		0.200		1.0171±0.0001		1.3658±0.0001		61.2896±0.2325
		0.275		1.0278±0.0002		1.3800±0.0001		82.9370±0.8661
		0.350		1.0397±0.0001		1.3950±0.0002		103.7576±0.5100
323.15		0.050		0.9939±0.0002		1.3379±0.0002		18.0786±1.7400
		0.125		1.0025±0.0001		1.3507±0.0002		42.6684±0.5454
		0.200		1.0122±0.0002		1.3644±0.0001		63.3420±1.0086
		0.275		1.0228±0.0002		1.3783±0.0001		86.0201±0.6099
		0.350		1.0343±0.0001		1.3930±0.0001		107.4759±1.7396

• ^a For densities the standard uncertainty for temperature is u(T) = 0.001 K, for refractive index and electrical conductivity the standard uncertainty for temperature is u(T) = 0.11 K. For densities, refractive index and electrical conductivity the standard uncertainty for mass fraction is u(w) = 0.0001. Expanded uncertainties are: for density: Uc(ρ) = 0.0004 g·cm^-3, for refractive index: Uc (_D) = 0.0011 and electrical conductivity: Uc(κ) = 1.72 mS·cm^-1 (0.95 level of confidence).

Figures 1, 2 and 3 shows the mean of these properties in all conditions studied. The density, refractive index and electrical conductivity of the binary mixtures of water + 1-ethyl-3-methylimidazolium chloride increased with increasing mass fraction of the ionic liquid and decreased with the increase in temperature at each pH condition studied. Muray et al. (2013) observed similar behavior, studying thermo-physical properties of ionic liquid imidazolium base. Similar behavior for density was found by Rafie et al. (2016) studying the volumetric properties of ionic liquids in sucrose aqueous solution at different temperatures and ambient pressure.

Figure 1
Density ρ of binary mixtures of water + chloride, 1-ethyl-3-methyl imidazolium as a function of the mass fraction at different temperatures, T/K: ●, 293.15; ○, 303.15; ▼, 313.15; ∆, 323,15 at each value pH: (A) 7.5, (B) 8.0 and (C) 8.5.

Figure 2
Refractive index n_D of binary mixtures of water + chloride, 1-ethyl-3-methyl imidazolium as a function of the mass fraction at different temperatures, T/K: ●, 293.15; ○, 303.15; ▼, 313.15; ∆, 323,15 at each value pH: (A) 7.5, (B) 8.0 and (C) 8.5.

Figure 3
Electrical conductivity κ of binary mixtures of water + chloride, 1-ethyl-3-methyl imidazolium as a function of the mass fraction at different temperatures, T/K: ●, 293.15; ○, 303.15; ▼, 313.15; ∆, 323.15 at each value pH: (A) 7.5, (B) 8.0 and (C) 8.5.binary mixtures of water + chloride, 1-ethyl-3-methyl imidazolium.

The decrease in density with increasing temperature can be due to the increased mobility of liquid molecules from the increase in thermal energy, making the interactions within the system weaker and causing the volume expansion and reducing the density (Siongco et al., 2013). Figure 4 shows the comparison of measured densities for the (1-ethyl-3-methylimidazolium chloride + water) binary system in this work at different pH values and the literature at T= 303.15 K. This discrepancy in slopes may come from differences in values of pH and also small differences in the densities due to pressure.

Figure 4
Comparison of measured densities for (water + 1-ethyl-3-methyl imidazolium chloride) binary system with the literature at T=303.15 K: ● (Rafie et al. (2015); Rafie et al. (2016)); this work ○, pH 7.5; ▼, pH 8.0; ∆, pH 8.5.

Similar behavior can be observed for the index of refraction for all mass fractions of 1-ethyl-3-methylimidazolium chloride, temperature and pH values. Regarding the temperature, at constant mass fraction of 1-ethyl-3-methylimidazolium chloride, an increase in temperature causes an expansion of the liquid volume, reducing the concentration and decreasing in the refractive index of the aqueous solutions. It was also noted that the refractive index increased with increasing mass fraction of 1-ethyl-3-methylimidazolium chloride. Similar behavior was observed by Tang et al. (2014), who observed that the index of refraction increased with the mass fraction of the ionic liquid.

It can be seen that the electrical conductivity increased with increasing mass fraction of 1-ethyl-3-methylimidazolium chloride due to the increase in the amount of ions present in the aqueous solution (Xu et al., 2015).

From the experimental data, polynomial models were adjusted to the thermophysical properties as a function of temperature, mass fraction and pH of the ionic liquid by fitting the experimental data to the general model (Eq. 4), and thereby obtaining the combined effect of these properties with regards to the three variables. Non-significant parameters were eliminated based on the Student’s t-test and p-value less than 0.05.

$${\displaystyle \begin{array}{lll}\psi & =& {\beta }_1+{\beta }_{2}w+{\beta }_{3}T+{\beta }_{4}p+{\beta }_5{w}^2+\\ & +& {\beta }_6{T}^2+{\beta }_7{p}^2+{\beta }_8{w}^3+{\beta }_9{T}^3+{\beta }_{10}{w}^4+\\ & +& {\beta }_{11}wT+{\beta }_{12}wp+{\beta }_{13}Tp+{\beta }_{14}wTp\end{array}}$$ (4)

where: ψ is the thermodynamic property and β₁, β₂, β₃, β₄, β₅, β₆, β₇, β₈, β₉, β₁₀, β₁₁, β₁₂, β₁₃ and β₁₄ are constants determined from the experimental data.

Table 2 shows the coefficients obtained from the polynomial regression (Eq. 4) for density (ρ), refractive index (n_D ) and electrical conductivity (κ). The fit between experimental and predicted data by the model was satisfactory, with R² values and correlation coefficients greater than 0.94.

Table 2
Fixed parameters for the models (Eq. 4) for density (ρ), refractive index (n_D) and electrical conductivity (κ) for binary mixtures of water + 1-ethyl-3-methylimidazolium chloride.

Parameters	Properties
	ρ/g.cm-3	nD	κ/10-3 mS.cm-1
β1	1.0866	1.3739	1300.4804
β2	0.2746	0.0011	4.0230
β3	-0.0003	-0.0001	-4.5354
β4	0.0014	ns	-151.3819
β5	ns	0.00001	-0.0086
β6	ns	ns	0.0052
β7	ns	ns	6.2508
β11	-0.0004	ns	Ns
β12	ns	ns	-0.0980
β13	ns	ns	0.1690
R²	0.9966	0.9449	0.9984
Correlation coefficient	0.9987	0.9996	0.9995

• ns: not significant. The parameters β₈, β₉, β₁₀ and β₁₄ are not in the table because they were not significant for any property.

Table 2 points to an inverse relationship for the temperature behavior for the density and refractive index. This indicates that the increase in temperature causes a decrease in these thermodynamic properties in question. The decrease of these properties with temperature is due to the increased mobility of the molecules of binary mixtures, causing a volume expansion and the decrease of intermolecular interactions (Siongco et al. 2013).

Moreover, binary mixtures had positive coefficients for the mass fraction of 1-ethyl-3-methylimidazolium chloride, confirming that the presence of the ionic liquid contributed to increasing the density and refractive index.

It can also be seen in Table 2 that the electrical conductivity was more affected by the mass fraction of 1-ethyl-3-methylimidazolium chloride than by temperature, confirming that the increase in the mass fraction of 1-ethyl-3-methylimidazolium chloride increases the electrical conductivity.

Figures 5, 6 and 7 show the data predicted by the model with regards to density, refractive index and electrical conductivity as a function of temperature and mass fraction obtained from Eq. 4 when the pH values were fixed. It was observed that for all thermodynamic properties the term related to temperature was negative, indicating that the three previously mentioned properties were negatively affected by the increase in this variable. In contrast, the parameter related to mass fraction was positive, indicating that there is an increase in these properties with the increase of this variable.

Figure 5
Density ρ of binary mixtures of water + 1-ethyl-3-methyl imidazolium chloride predicted by the model at different values of pH (A) 7.5 (B) 8.0 and (C) 8.5.

Figure 6
Refractive index n_D of binary mixtures of water + 1-ethyl-3-methyl imidazolium chloride predicted by the model.

Figure 7
Electrical conductivity κ binary mixtures of water + 1-ethyl-3-methyl imidazolium chloride predicted by the model at different values of pH (A) 7.5 (B) 8.0 and (C) 8.5.

Apparent specific volume, molar volume and thermal expansion coefficient of the binary mixtures

From the experimental data obtained for density and from equations 1 to 3, the properties of molar volume, thermal expansion coefficient and apparent specific volume were calculated for the binary mixtures of water + 1-ethyl-3-methylimidazolium chloride at different temperatures T = (293.15, 303.15, 313.15 and 323.15) K, pH values (7.5, 8.0 and 8.5) and mass fraction w = (0.05, 0.12, 0.2, 0.275 and 0.35). The results are shown in Table 3.

Table 3
Apparent specific volume (v_2Ø), molar volume (V_m) and thermal expansion coefficient (α_p) of the binary mixtures of water + 1-ethyl-3-methylimidazolium chloride at different mass fractions (0.05, 0.12, 0.2, 0.275 and 0.35), temperatures T = (293.15, 303.15, 313.15 and 323.15) K and pH (7.5, 8.0 and 8.5).

T/K		w		v2f/g·cm-3		Vm/cm3·mol-1		αp/K-1
pH 7.5
293.15		0.050		0.8871±0.0009		146.0508±0.0061		0.3444±0.0001
		0.125		0.8722±0.0007		144.7430±0.0116		0.3416±0.0001
		0.200		0.8503±0.0003		143.1545±0.0079		0.3382±0.0001
		0.275		0.8354±0.0006		141.5942±0.0194		0.3348±0.0001
		0.350		0.8203±0.0014		139.9591±0.0525		0.3313±0.0001
303.15		0.050		0.9163±0.0003		146.6156±0.0021		0.3457±0.0001
		0.125		0.8785±0.0003		145.1814±0.0051		0.3427±0.0001
		0.200		0.8587±0.0003		143.6738±0.0083		0.3394±0.0001
		0.275		0.8441±0.0006		142.1659±0.0185		0.3362±0.0001
		0.350		0.8292±0.0013		140.5791±0.0508		0.3327±0.0001
313.15		0.050		0.9199±0.0025		147.1254±0.0177		0.3469±0.0001
		0.125		0.8878±0.0009		145.7863±0.0149		0.3441±0.0001
		0.200		0.8661±0.0004		144.2804±0.0102		0.3409±0.0001
		0.275		0.8521±0.0007		142.8196±0.0228		0.3377±0.0001
		0.350		0.8371±0.0013		141.2564±0.0479		0.3343±0.0001
323.15		0.050		0.9189±0.0003		147.7131±0.0023		0.3483±0.0001
		0.125		0.9035±0.0007		146.5963±0.0117		0.3460±0.0001
		0.200		0.8744±0.0027		145.0039±0.0656		0.3425±0.0001
		0.275		0.8596±0.0010		143.5481±0.0321		0.3394±0.0001
		0.350		0.8444±0.0009		141.9960±0.0335		0.3361±0.0001
pH 8.0
293.15		0.050		0.8784±0.0006		145.9903±0.0040		0.3442±0.0001
		0.125		0.8661±0.0001		144.6432±0.0019		0.3414±0.0001
		0.200		0.8522±0.0003		143.1995±0.0079		0.3383±0.0001
		0.275		0.8376±0.0002		141.6642±0.0074		0.3350±0.0001
		0.350		0.8228±0.0004		140.0552±0.0153		0.3315±0.0001
303.15		0.050		0.8857±0.0005		146.4019±0.0033		0.3452±0.0001
		0.125		0.8739±0.0001		145.1070±0.0023		0.3425±0.0001
		0.200		0.8605±0.0004		143.7187±0.0099		0.3395±0.0001
		0.275		0.8463±0.0004		142.2377±0.0118		0.3363±0.0001
		0.350		0.8317±0.0004		140.6726±0.0149		0.3330±0.0001
313.15		0.050		0.8926±0.0008		146.9351±0.0054		0.3465±0.0001
		0.125		0.8809±0.0002		145.6737±0.0027		0.3438±0.0001
		0.200		0.8679±0.0004		144.3225±0.0093		0.3410±0.0001
		0.275		0.8540±0.0005		142.8804±0.0156		0.3379±0.0001
		0.350		0.8395±0.0004		141.3466±0.0140		0.3346±0.0001
323.15		0.050		0.9020±0.0020		147.5952±0.0142		0.3480±0.0001
		0.125		0.8873±0.0002		146.3333±0.0025		0.3454±0.0001
		0.200		0.8746±0.0003		145.0083±0.0080		0.3426±0.0001
		0.275		0.8613±0.0110		143.6009±0.0343		0.3396±0.0001
		0.350		0.8465±0.0003		142.0755±0.0126		0.3363±0.0001
pH 8.5
293.15		0.050		0.8688±0.0040		145.9903±0.0040		0.3442±0.0001
		0.125		0.8604±0.0008		144.6432±0.0019		0.3414±0.0001
		0.200		0.8446±0.0001		143.1995±0.0079		0.3383±0.0001
		0.275		0.8300±0.0008		141.6642±0.0074		0.3350±0.0001
		0.350		0.8140±0.0001		140.0552±0.0153		0.3315±0.0001
303.15		0.050		0.8765±0.0039		146.4019±0.0033		0.3452±0.0001
		0.125		0.8686±0.0005		145.1070±0.0023		0.3425±0.0001
		0.200		0.8532±0.0001		143.7187±0.0099		0.3395±0.0001
		0.275		0.8389±0.0008		142.2377±0.0118		0.3363±0.0001
		0.350		0.8231±0.0001		140.6726±0.0149		0.3330±0.0001
313.15		0.050		0.8836±0.0045		146.9352±0.0054		0.3465±0.0001
		0.125		0.8760±0.0001		145.6737±0.0027		0.3438±0.0001
		0.200		0.8609±0.0002		144.3225±0.0093		0.3410±0.0001
		0.275		0.8468±0.0009		142.8804±0.0156		0.3379±0.0001
		0.350		0.8311±0.0001		141.3466±0.0140		0.3346±0.0001
323.15		0.050		0.8904±0.0048		147.5952±0.0142		0.3480±0.0001
		0.125		0.8824±0.0004		146.3333±0.0025		0.3454±0.0001
		0.200		0.8684±0.0009		145.0083±0.0080		0.3426±0.0001
		0.275		0.8536±0.0007		143.6009±0.0343		0.3396±0.0001
		0.350		0.8382±0.0001		142.0755±0.0126		0.3363±0.0001

• ^a For apparent specific volume, molar volume and thermal expansion coefficient the standard uncertainty for temperature is u(T) = 0.001 K and for mass fraction is u(w) = 0.0001. Expanded uncertainties are for apparent specific volume: Uc(v_2Ø) = 0.002 g·cm^-3, for molar volume: Uc(_m) = 0.6508 cm³·mol^-1 and for thermal expansion coefficient: Uc(α_p) 0.0020 = K^-1 (0.95 level of confidence).

The molar volume, thermal expansion coefficient and apparent specific volume of the binary mixtures of water + 1-ethyl-3-methylimidazolium chloride decreased with an increase in mass fraction and increased with an increase in temperature.

Similar behavior for the apparent molar volume was found by Rafie et al. (2015) studying the ionic liquid, 1-ethyl-3-methyl imidazolium chloride in aqueous lithium nitrate, lithium bromide, and lithium chloride solutions at temperatures 298.15 to 318.15 K.

Using the same procedures as above, polynomial models were fit to the properties of molar volume, thermal expansion coefficient and apparent specific volume of the binary mixtures as a function of temperature, mass fraction and pH of the ionic liquids, by fitting the experimental data to the general model (Eq. (4)). Table 4 shows the coefficients obtained from the polynomial regression (Eq. 4) for the properties of apparent specific volume (v_2Ø), molar volume (V_m) and thermal expansion coefficient (α_p ).

Table 4
Fixed parameters of the models (eq. 4) for apparent specific volume (v_2Ø), molar volume (V_m) and thermal expansion coefficient (α_p) for the binary mixtures of water + 1-ethyl-3-methylimidazolium chloride.

Constants	Properties
	v2Ø/g.cm-3	Vm/cm3.mol-1	αp/K-1
β1	0.8764	131.8090	0.3040
β2	-0.0082	-0.3314	-0.0004
β3	0.0008	0.0520	0.0001
β4	-0.0276	ns	ns
β11	ns	0.0005	ns
β12	0.0008	ns	ns
R²	0.9751	0.9978	0.9950
Correlation coefficient	0.9907	0.9993	0.9991

• ns: not significant. The parameters β₅, β₆, β₇, β₈, β₉, β₁₀, β₁₃ and β₁₄ are not in the table because they were not significant for any property.

Figures 8, 9 and 10 show the data predicted by the model with respect to specific volume, molar volume and thermal expansion coefficient as a function of the temperature and mass fraction obtained from eq. 4, in which the pH was fixed for each value. It can be observed in the figures that the values of these 3 properties decreased as the mass fraction increased and increased with increases in temperature.

Figure 8
Specific volume v_2Ø of binary mixtures of water + 1-ethyl-3-methyl imidazolium chloride predicted by the model at different values of pH (A) 7.5 (B) 8.0 and (C) 8.5.

Figure 9
Molar volume V_m of binary mixtures of water + 1-ethyl-3-methyl imidazolium chloride predicted by the model.

Figure 10
Coefficient of thermal expansion α_p of binary mixtures of water + 1-ethyl-3-methyl imidazolium chloride predicted by the model.

CONCLUSION

It was possible to obtain experimental data for the properties density (ρ), electrical conductivity (κ) and refractive index (n_D ), under various conditions of temperature, mass fraction and pH of aqueous solutions of the ionic liquid. From the experimental data for the density it was possible calculate the apparent specific volume (v_2Ø), the coefficient of thermal expansion (α_p ) and molar volume (V_m ). Polynomial models for the properties presented satisfactory fits to the experimental data. The results were satisfactory and can be used for the design and scale up of separation process under such experimental conditions.

NOMENCLATURE
• W  - mass fraction
• T  - temperature, K
• ILs  - ionic liquids
• ATPS  - two-phase aqueous systems
• ([C₂mim]Cl)  - 1-ethyl-3-methylimidazolium chloride
• P  - density, g∙cm^-3
• V_m  - molar volume, cm³∙mol^-1
• α_p  - thermal expansion coefficient, K^-1
• v_2Ø  - apparent specific volume, cm³∙g^-1
• n_D  - refractive index
• Κ  - electrical conductivity, mS∙cm^-1
• CRD  - completely randomized design
• ψ  - thermophysical property
• β  - adjusted parameter
• R²  - correlation coefficient

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Publication Dates

• Publication in this collection
  15 July 2019
• Date of issue
  Jan-Mar 2019

History

• Received
  20 Mar 2018
• Reviewed
  21 May 2018
• Accepted
  16 June 2018