Thermophysical, Volumetric, and Excess Properties of Aqueous Solutions of 1-Hexyl-3-methyl Imidazolium Bromide at 298.15 K and 0.1 MPa

Abstract

Room temperature ionic liquids (RTILs) are the environment-friendly alternatives for organic volatile solvents in a host of synthetic, catalytic, and electrochemical applications. These are also being used for various R&D works in nuclear fuel cycle research such as the recovery and purification of nuclides of interest from spent nuclear fuel matrices. In this work, density, refractive index (𝑛𝐷²⁵) at sodium 𝐷 line, and thermodynamic water activity of the aqueous solution of 1-hexyl-3-methyl imidazolium bromide were determined in dilute aqueous solutions at 298.15 K. These results were used to calculate the apparent molal volumes of each solute over various concentration ranges. The measurements were performed as per ASTM procedures. Vapour pressure was derived using water activity values. Information on excess properties and structural interaction was also reported.

1. Introduction

Room temperature ionic liquids are considered as young chemicals having variety a of applications in all types of areas in chemical industry due to their unique properties. They are referred to as designer solvents as we can design them for different reactions by changing the cation and anion. They can act as both polar and nonpolar solvents which minimizes the use of lot of chemicals making our environment neat and clean and thus referred to as “green solvent” [1]. Researchers have found that ionic liquids are more than a solvent and have several applications in electrochemistry, biological media, catalysis, organic synthesis, and so on [2–7]. Low vapour pressure, thermal stability, high thermal conductivity, large electrochemical window, and nonflammability are some of their properties which make them an innovative solvent. As a solvent, ionic liquids possesses several advantages over conventional organic solvents making it environmentally compatible [8]. The potential advantage of ionic liquids depends upon their thermophysical properties like density, refractive index, activity, vapour pressure, conductivities, viscosities, and so forth. Various research groups have studied the thermophysical properties of various ionic liquids to extract important information about them [9–16]. RTILs are extensively used for the extraction of metal ions in solvent extraction [17–19]. In nuclear industry these are used as green solvent for the extraction of various fission products in liquid-liquid extraction [20–22]. Currently, variants of 1,3 dialkyl imidazolium salt are being tried in processing of materials. In this paper one of such 1,3-dialkyl imidazolium salts that is, 1-hexyl-3-methyl imidazolium bromide (hereafter referred as [HMIm]Br), is studied extensively whose properties are not available yet. Here, apparent molal volume, refractive index, water activity, vapour pressure, and other derived properties of aqueous solutions of [HMIm]Br at 298.15 K and 0.1 MPa are reported.

2. Experimental Work

2.1. General Procedure

ASTM Grade-1 water as per ASTM D-1193 [23] with a resistivity of 18.2 MΩ·cm at 298.15 K and TOC < 15 ppb from a MILLIPORE Simplicity system was used in the experiments for reagent/solution preparation and as water standard. Water for density standard was drawn from the water purifier immediately before the experiments and it was degassed in an ultrasonic bath (TEC-110 H) at 35 KHz for 60 seconds. Solutes are weighed in a precision Shimadzu AUW220D balance (220 g, 0.01 mg resolution). When not in use, the solutions were kept in air-tight closed condition in a JULABO SW-22 Shaker Bath at 298.15±0.1 K. JULABO SW-22 Shaker Bath was coupled to JULABO FP-40 chiller for cooling. To ensure the ripple-free power supply to all the equipments, AC supply was regulated with an uninterrupted power supply (APC), set at high sensitivity. The ambient conditions were monitored with a Cole-Parmer hygrometer (0.1%) consisting of a thermometer (0.1°C) and pressure sensor (0.1 kPa). The compound [HMIm]Br (M/S io-li-tec, GmbH, structure shown in Figure 1) was examined by IR spectra shown in Figure 2 (3079.8 cm⁻¹, 2931.5 cm⁻¹, 2860.2 cm⁻¹, 1642.9 cm⁻¹, 1465.7 cm⁻¹, 1166.8 cm⁻¹, and 1627.8 cm⁻¹) and used without further purification.

Figure 1 

Structure of [HMIm]Br.

Figure 2 

IR data of aqueous solutions of pure [HMIm]Br.

2.2. Measurement of Density and Refractive Index

For density and refractive index measurement, a precise vibrating-tube densitometer (Anton Paar DMA-5000), coupled with Anton Paar ABBEMAT RXA-156 refractometer and a 30-station autosampler SP-3 m, was used. The densitometer and refractometer had Peltier thermostats for maintaining the desired sample temperature. Density measurements were made in complete confirmation of ASTM-D4052 [24].

2.3. Measurement of Water Activity

Water activity was measured by a thermodynamic water activity meter (NOVASINA AW-Master) with a built-in thermoelectric Peltier element module for maintaining temperature of sample. Samples were loaded in polystyrene cups and placed into the activity meter. The activity meter was standardized using a two-step procedure. In the first step, purified and degassed Millipore water was used as a standard (𝑎_𝑤=1.000). In the second step, five immobilized electrolyte solutions, in sealed tablets supplied by M/S Novasina, were used as standards. These solutions were ~25% LiCl (𝑎_𝑤=0.113), ~25%MgCl₂·6H₂O (𝑎_𝑤=0.328), ~25% NaCl (𝑎_𝑤=0.753), 25% BaCl₂·2H₂O (𝑎_𝑤=0.901), and ~20%K₂SO₄ (𝑎_𝑤=0.973). The uncertainty estimated was ±0.1 K for temperature and ± 0.003 for 𝑎_𝑤 values.

The aqueous solutions of [HMIm]Br were made (~0.0005 m to 0.04 m) by the authors. The density, refractive index, water activity, and other derived properties for 1-hexyl-3-methyl imidazolium bromide solutions were determined and reported.

3. Results and Discussion

3.1. Theory

Partial molar volume is an extensive property and can be expressed by 𝑉_𝑖=𝜕𝑉𝜕𝑛_{𝑖𝑇,𝑃,𝑛𝑗≠𝑖}. (1)

For electrolytes, this partial molar quantity is composition dependent. In such cases apparent molal volume of component 2 in a solution (solvent is defined as component 1) is given by 𝑉_2,𝜙=𝑉−𝑛₁𝑉⁰₁𝑛₂. (2)

The apparent molar volume of a solute (designated as 2) in a solvent in a solution may be written as 𝑉_2,𝜙=1000𝑚𝜌𝜌₀𝜌₀+𝑀−𝜌²𝜌, (3) where 𝑉_2,𝜙 is the apparent molar volume, 𝑚 is the molality, 𝑀₂ is the molecular weight of the solute, and 𝜌₀ and 𝜌 are the densities of water and of the solution, respectively.

There are several empirical correlations to correlate the apparent molar volume of a solute in the solution with the solute concentration. The following equation lists the correlation proposed by Masson [25]: 𝑉^298.15K_2,𝜙=𝑉⁰_2,𝜙+𝑎_𝑣√𝑚, (4) where 𝑉⁰_2,𝜙 is the limiting apparent molal volume, 𝑚 is the molality, and 𝑎_𝑣 is an empirical constant determined from experimental results. The slope of the plot between 𝑉_2,𝜙 and “𝑚” will give the value of 𝑎_𝑣 and the intercept will give the value of 𝑉⁰_2,𝜙.

On equating (3) and (4) and rearranging, following form of equation for density of solution is obtained: 𝜌𝜌₀=1.00+𝑎₀𝑚𝑚₀+𝑏₀𝑚𝑚₀^1.5, (5) where 𝑎₀=𝑀₂−𝜌₀×𝑉⁰_2,𝜙10³,𝑏₀𝑎=−⁰10³. (6)

Similarly, the experimentally determined refractive index values of aqueous solutions of ionic liquid were correlated to the following equation: 𝑛𝐷=𝑛𝐷_𝑤+𝑎𝑚+𝑏𝑚², (7) where 𝑛𝐷 is the predicted refractive index of solution and 𝑛𝐷_𝑤 is the refractive index of water used in solution preparation. 𝑚 is molality of ionic liquid in water. 𝑎 and 𝑏 are adjustable coefficients.

Excess molar volume of solution is a thermodynamic property and is defined as the amount of volume which is in excess to that of an ideal solution at the same conditions of composition, temperature, and pressure and is given as 𝑉^𝐸_𝑚=𝑉_𝑚−𝑉^ideal_𝑚, (8) where 𝑉_𝑚 and 𝑉^ideal_𝑚 are the molar volumes of solution and pure components, respectively. Excess molar volumes of aqueous [HMIm]Br solution were calculated using (9) which was obtained after rearranging (8) using mole fractions, molar masses, and densities to represent the volumes of solution and pure components: 𝑉^𝐸_𝑚/cm³⋅mol⁻¹=𝑥₁𝑀₁1𝜌−1𝜌₀+𝑥₂𝑀₂1𝜌−1𝜌₁, (9) where 𝑉^𝐸_𝑚 is the excess molar volume; 𝑀₁ and 𝑀₂ are molar masses of water and [HMIm]Br, respectively; 𝑥₂ is the mole fraction of [HMIm]Br; 𝜌 is the density of aqueous [HMIm]Br solutions; and 𝜌₀ and 𝜌₁ are densities of Millipore water and pure [HMIm]Br, respectively.

The excess molar volumes were correlated with a Redlich-Kister-type equation by least-squares fittings given by 𝑉^𝐸_𝑚/cm³⋅mol⁻¹=𝑥₁𝑥₂^𝑛_𝑖=0𝐴_𝑖2𝑥_𝑖−1^𝑖, (10) where 𝐴_𝑖 are the adjustable coefficients.

Excess refractive index was also calculated in the same manner using the Redlich-Kister polynomial.

3.2. Correlation of Density of Aqueous Solutions [HMIm]Br

The experimental data (listed in Table 1) could be correlated by the following equation: 𝜌𝜌⁰𝑚=1.00+0.049𝑚₀𝑚−0.009𝑚₀^1.5. (11) The density of ASTM Grade-I water at 298.15 K was taken as 0.997043 g/cm³ [24]. The mean deviation and standard deviations were 0.9063×10⁻³% and −0.3958×10⁻³%, respectively. A good match between experimental and predicted values was observed. The deviation in density values was given in Figure 3.

Table 1 

Density values of aqueous solution of [HMIm]Br at 298.15 K.

Figure 3 

Plot of deviations for density of aqueous solutions of [HMIm]Br in water at 298.15 K and 0.1 MPa.

3.3. Correlation of Apparent Molal Volume

The apparent molal volumes for [HMIm]Br in aqueous solution at 298.15 were calculated with (3) (listed in Table 2) and correlated with (4). The first few values were not used in regression. The following correlation is obtained: 𝑉^298.15K_2,𝜙=195.39+31.74√𝑚. (12) The correlated and experimentally determined values are shown in Figure 4.

Table 2 

Apparent molal volume values of [HMIm]Br as a function of concentration.

Figure 4 

Apparent molal volume of [HMIm]Br in water at 298.15 K.

3.4. Correlation of Refractive Index

Experimental refractive index values have been shown in Table 3. The correlated equation is given as 𝑛𝐷=1.3324+0.0386𝑚−0.0021𝑚². (13) The variation of refractive index for aqueous solutions of [HMIm]Br at 298.15 K and 0.1 MPa with concentration is given in Figure 5.

Table 3 

Experimental data for refractive index, water activity, and vapour pressure of aqueous solution of [HMIm]Br.

Figure 5 

Variation in the refractive index of aqueous solutions of [HMIm]Br with concentration at 298.15 K and 0.1 MPa.

Experimental water activity values have been shown in Table 3. From determined water activity values, vapour pressures were estimated as 𝑃=𝑎_𝑤×𝑃⁰, (14) where 𝑃⁰=3.1686 kPa at 298.15 K. Figures 6 and 7 show variation of water activity and vapour pressure for aqueous solutions of [HMIm]Br at 298.15 K and 0.1 MPa with concentration.

Figure 6 

Variation of water activity with the concentration in the aqueous solutions of [HMIm]Br at 298.15 K at 0.1 MPa.

Figure 7 

Vapour pressure of aqueous solutions of [HMIm]Br at 298.15 K.

At infinity dilution there will be no solute-solute interaction. Here the positive values of 𝑉⁰_2,𝜙 are a result of strong ion-solvent interaction. In this case as the solvent volume is decreasing due to strong ion-solvent interaction, the apparent molal volume is increasing with the addition of the electrolyte.

Excess volume is a resultant contribution of physical, chemical, and structural effects. The physical contribution involving nonspecific interactions between the real species of mixture contribute a positive value to 𝑉^𝐸. The chemical interactions involving charge-transfer-type forces result in a decrease in volume and contribute a negative value to 𝑉^𝐸. The structural contributions are mostly negative which arises from changes of free volume and other various effects. In this experiment, excess molar volume is found to be negative as is common in completely miscible solvents. The negative excess molar volume generally indicates contraction in volumes upon mixing. It can be due to the presence of hydrogen bonding. Figures 8 and 9 show the plot of excess molar volumes and excess refractive index versus concentrations, respectively. The values of excess volume and excess refractive index were reported in Table 4.

Table 4 

Excess volume and excess refractive index values of aqueous solution of [HMIm]Br.

Figure 8 

Plot of excess molar volume versus mole fraction concentration of aqueous solution of [HMIm]Br solutions at 298.15 K and 0.1 MPa.

Figure 9 

Plot of excess refractive index versus mole fraction concentration of aqueous solution of [HMIm]Br solutions at 298.15 K and 0.1 MPa.

4. Conclusion

In the present work density, refractive index, and water activity of [HMIm]Br in aqueous solutions at 298.15 K and 0.1 MPa were determined experimentally at Reprocessing R&D Division. Based on the experimental results, apparent molal volume, vapour pressure, and excess properties have been estimated and reported. At infinity dilution there will be no solute-solute interaction. Therefore 𝑉⁰_2,𝜙 gives information about solute-solvent interaction while 𝑎_𝑣 will provide information about solute-solute interaction. In the present study, 𝑉⁰_2,𝜙 and 𝑎_𝑣 both are positive, which indicates that both ion-solvent and ion-ion interactions are there, but as we have taken dilute solution,𝑉⁰_2,𝜙 value is higher than 𝑎_𝑣 value indicating strong ion-solvent interaction. As ion-solvent interaction is high in comparison to ion-ion interaction, the salts will act as structure breakers in that concentration range. The negative values of excess molal volume show the significant deviations from ideality. The negative excess molar volumes of ([HMIm]Br + water) reflect the compactness of solution, that is, existence of net attractive interaction between the dissimilar molecules which could be due to the hydrogen bonding interactions.

Nomenclature

𝜌0:	Density of water
𝜌:	Density of aqueous solution
𝜌1:	Density of pure [HMIm]Br
𝑚:	Molality
𝑉:	Total volume of the solution
𝑛1:	Number of moles of water
𝑛2:	Number of moles of the solute
𝑉01:	Partial molal volume of pure component
𝑉2,𝜙:	Apparent molal volume
𝑉02,𝜙:	The limiting apparent molal volume
𝑎𝑣:	Empirical constant
𝑛𝐷:	Predicted refractive index of solution
𝑛𝐷𝑤:	Refractive index of water at sodium d-line
𝑎:	Coefficient of refractive index correlation
𝑏:	Coefficient of refractive index correlation
𝛼:	Cubic expansion coefficient  (thermal isobaric expansibility)
𝑉𝐸𝑚:	Excess molar volume
𝑉𝑚:	Molar volume of solution
𝑉ideal𝑚:	Molar volume of pure components
𝑀1:	Molar mass of water
𝑀2:	Molar masses of [HMIm]Br
𝑥1:	Mole fraction of water
𝑥2:	Mole fraction of [HMIm]Br
𝐴𝑖:	Adjusted coefficients for excess molar volume
Subscript 2:	Solute
Subscript 1:	Solvent.