**1. Introduction**

Ionic liquid is a green solvent. It is composed of organic cations and inorganic or organic anions; they can have liquid state near ambient temperature. Since this green solvent has unique properties when compared to conventional solvents such as larger temperature range of liquid state [1], high thermal stability, high ionic conductivity negligible vapor pressures, nonflammability and high solvating capacity (i.e., solubility), for polar or nonpolar organic, inorganic and organometallic compounds [2, 3]. It is well known that the solvation capacity of green solvent is influenced by the hydrogen bonded structure and interaction between the individual ions (cation or anion) with other substances. On the other hand, the green solvent is an organic salt and its microscopic structure is usually composed of a large cation with low order of molecular symmetry. Hence, the unstable lattice structure lowers the melting point to well below the room temperature [4]. Therefore, the green solvent has the capabilities as environmental-friendly solvent in many green chemical processes [5] such as, biocatalytical transformation, isomerization, used in multiphase homogeneous catalysis [6], synthesis, catalysis, liquid-liquid extraction and supercritical extraction, and also used as thermal fluids, lubricants, and working fluids in electrochemical devices such as batteries, capacitors and solar cells [7].

**2.2. Sample preparation**

5 K interval.

oscillator [8]:

erence oscillator. f1

**3. Theory**

**2.3. Density measurement**

*ρ* = *KA*

and f2

molar volume and apparent molar volume (**Figure 1**).

pressure. Hence the isobaric expansivity is defined as;

The binary mixture was prepared by transferring a known amount of the pure liquids via syringe into stoppered bottles and was properly sealed with parafilm tape to prevent evaporation and addition of moisture to the mixtures, using a Mettler AX-205 Delta Range balance with a precision of ±10−5 g. The estimated uncertainty on the composition measurement was ±10−4 g mole fraction. The stoppered bottles were placed inside a water-shaker bath set at atmospheric pressure, and allowed to shake for more than 6 h at 300 rpm in thermostatic shaker bath. Spring clamps were used to hold the flasks on the tray. The binary mixture was then allowed to settle for minimum of 12 h so that equilibrium is attained. The sample is taken from vial with a syringe to measure the density at temperature from 293.15 to 343.15 K with

Evaluation of Solution Thermodynamic Properties of Mixed Ionic Liquids at Different…

Density was measured using an Anton Paar DMA 4100 M with the oscillating U-tube method. In this method, the sample is introduced into a U-shaped borosilicate glass tube that is being excited to vibrate at its characteristic frequency. The characteristic frequency changes depending on the density of the sample. Through a precise determination of the characteristic frequency a mathematical conversion, the density of the sample can be measured. The density is calculated from the quotient of the period of oscillations of the U-tube and the reference

<sup>∗</sup> *Q*<sup>2</sup><sup>∗</sup> *f*

where; KA and KB are apparatus constants, respectively, Q is the oscillation period of the ref-

When two liquid chemical species mixed with each other, the total weight of the mixture is equal to the sum of masses of the individual chemical species. But it is not true in case of volume. When two miscible liquids are mixed with each other, volume of the mixtures may not equal to the sum of the volume of the individual chemical species [9]. Ideal binary liquid mixture does not have volume changes. Hence, the binary liquid mixture has deviation from ideality due to the molecular interactions between solute-solvent or two components in the mixtures. In addition, the binary liquid mixture volume either increase or decrease as the function of composition of component i in the mixture. This difference in the volume of the mixture can be taken as a criterion and measure of molecular interactions at molecular level by means of isobaric expansivity, excess molar volume, partial molar volume, excess partial

*Isobaric expansivity* is inversely proportional to the volume of component "i" in the mixture and its product with the rate of change of volume with respect to temperature at constant

<sup>1</sup> − *KB* ∗ *f*

are correction factors for temperature, viscosity, and nonlinearity.

<sup>2</sup> (1)

http://dx.doi.org/10.5772/intechopen.77016

125

But there is no systematic study on application of green solvent at different temperature and compositions. On the other hand, the solution thermodynamic properties of pure ionic liquids and its mixtures are of interest from the point of both basic and applied research [7]. Also, a detailed knowledge of the solution thermodynamic properties of mixed green solvents are important in relating the microscopic and macroscopic behavior. In this context, There is no data generated by experimental or theoretical approach. Moreover, the complete design of new green chemical processes and new green products based on green solvents and mixed green solvents can only be achieved when their solution thermodynamic properties such as molar volume, excess molar volume, partial molar volume, excess partial molar volume, and apparent molar volume are adequately characterized. But there is no data on solution thermodynamic properties of mixed green solvents at different temperature for an entire mole fractions range. Therefore, it is very important to accumulate a sufficiently large data bank not only for green processes and product design but also for the development of correlation for these properties. In addition, a better understanding of the behavior of mixed green solvent demands the knowledge of density and its temperature and composition dependence. Obtaining knowledge on the solution thermodynamic properties is extremely important to improve their selection and performance.
