An experimental investigation of molecular interactions between [Emim][triflate] ionic liquid & 2-alkoxyethanols and theoretical comparison by PFP theory

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Highlights

  • Molecular interaction studies were carried out for the binaries {[Emim][triflate] + 2-ethoxyethanol/2-propoxyethanol}.

  • Density and speed of sound are measured at 298.15, 303.15, 308.15, 313.15 and 318.15 K.

  • The interactions in both the systems are analyzed in terms of excess parameters.

  • Observed interactions are well explained in terms of PFP theory.

Abstract

Physical properties, such as density (ρ) and speed of sound (u) of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate, 2-ethoxyethanol/2-propoxyethanol and their binary mixtures are measured over the whole composition range as a function of temperature between 298.15 K and 318.15 K at atmospheric pressure. Experimental values were used to calculate the excess molar volumes (VmE), excess values of partial molar volumes (V¯mE), excess values of isentropic compressibility (κsE), free length (LfE), speeds of sound (uE) and isobaric thermal expansion coefficient (αPE) for the binary mixtures. These excess properties were fitted to the Redlich-Kister type equation to obtain the binary coefficients and the standard deviations. A qualitative analysis of these parameters indicates the interaction decreases with the increase in chain length. This was further supported by Prigogine–Flory–Patterson (PFP) theory of excess molar volume.

Introduction

Ionic liquids (ILs) are considered as green solvents with a lot of flexibility in mixing with industrial organic solvents [1], [2], [3], [4], [5], [6]. Many technological processes involving ILs require the knowledge of thermophysical properties of their mixtures with molecular organic solvents. To understand the interaction of their constituting cation and anions with the molecular solvents, the behaviour of ILs when mixed with molecular organic solvents is of extreme importance. The IL in the investigation is 1-ethyl-3-methylimidazolium trifluoromethanesulfonate, {[Emim][triflate]}, which is widely used as solvent in organic synthesis [7], [8], in biotechnology [9], in chromatography [10], etc. On the other hand, alkoxyethanols, which are amphiphilic organic solvents, have been used in numerous chemical processes. The mixtures containing 2-alkoxyethanols are very significant from theoretical viewpoint, not only because of their self-association, but also due to the strong intermolecular effects created by the presence of –OH and –O– groups in the same molecule. It is of great importance to understand the mixing behaviour of ILs in alkoxyethanols; some investigators have studied the thermodynamic behaviour of the mixtures of ILs with 2-alkoxyethanols [11], [12], [13], [14].

Systematic investigation of the physicochemical properties of [Emim][triflate] with molecular organic solvents including water, acetone, 1-pentanol, 1-butanol, methyl acetate, ethyl acetate, 2-propanol, tetra hydro furon (THF), triflic acid, methanol, ethanol, and 1-propanol have been reported. Vercher et al. [15] reported density, speed of sound, and refractive index data of [Emim][triflate] binary mixture with acetone, methyl acetate, and ethyl acetate, whereas, Reddy et al. [16], [17] reported density, speed of sound, and refractive Index data of [Emim][triflate] with 1-butanol and 1-pentanol. On the other hand, Vercher et al. [18] reported ultrasonic and volumetric properties of [Emim][triflate] with 2-propanol or THF at different temperatures. Christopher et al. [19] reported hydrogen-bonding and ion-ion interactions in solutions of triflic acid, and [Emim][triflate]. Bahadur et al. [20] reported excess molar volumes of binary mixtures an ionic (liquid + water). Vercher et al. [21] reported volumetric and ultrasonic studies of [Emim][triflate] with methanol, ethanol, 1-propanol, and water at several temperatures. On the other hand, Garcia-Miaja et al. [22] reported excess enthalpy, density, and heat capacity for binary systems of [Emim][triflate] ionic liquid with water. Basing on the above and in continuation But, the volumetric and thermoacoustic data of [Emim] [triflate] with 2-ethoxyethanol/2-propoxyethanol have not been reported. This paper extends our earlier work of volumetric and acoustic studies of [Emim][triflate] with 2-methoxyethanol/N-methyl pyrrolidone [23].

A preliminary examination on the miscibility of 1-ethyl-3-methylimidazolium trifluromethane sulfonate {[Emim][triflate]}with 2-ethoxyethanol (2-EE) or 2-propoxyethanol (2-PE) proved the complete miscibility of both in all proportions. Hence, in the present study, it is proposed to measure the physical properties (density, ρ, speed of sound, u) of the binary mixture [Emim][triflate] with 2- EE/2-PE in the temperature range from (298.15 to 318.15) K and over the whole composition range and to estimate their excess properties for their potential application in industrial processes. The determined excess properties for binary mixtures were fitted in Redlich-Klister polynomial equation.

Section snippets

Materials

The ionic liquid, [Emim][triflate] (CAS 145022-44-2) with purity 0.99 in mass fraction used in this work. It was purchased from Iolitec GmBH (Germany), while 2-ethoxyethanol (CAS 110-80-5) and 2-propoxyethanol (CAS 2807-30-9) with purity 0.995 in mass fraction was supplied by Sigma Aldrich. The chemicals used in the present investigation were purified by the methods described in literature [32], [33]. The water content in investigated IL and 2-EE/2-PE were determined using a Karl Fisher

Theory

The excess molar volume is given byVmE=x1M1+x2M2ρ-x1M1ρ1+x2M2ρ2

The M1&M2 refer to molar mass, x1&x2 are the mole fractions of [Emim][triflate] and NOP respectively, while ρ1, ρ2&ρ are the densities of [Emim][triflate], 2-EE/2-PE and their mixtures respectively.

The isentropic compressibility, κS, is computed directly from the measured values of speed of sound and density using the Newton-Laplace equationκs=-1VmVmPs=1ρu2=VmMu2

Excess isentropic compressibility is given byκsE=κs-κsidwhere, κS, is

Results and discussion

The values of the experimental density, ρ and speed of sound, u, for the binary mixtures of [Emim][triflate] with 2-EE/2-PE as a function of mole fraction, x1 of [Emim][triflate] at investigated temperatures are given in Table S1 of the Supplementary information. All the three liquids in the study, [Emim][triflate], 2-EE and 2-PE are considered as polar leading to complete miscibility of 2-EE/2-PE in [Emim][triflate] at all compositions and all temperatures. 2-alkoxyethanols are associated

Conclusions

The experimental results of densities and ultrasonic speed of sounds of binary liquids of [Emim][triflate] with 2-EE or 2-PE in the entire composition range at T = (298.15/303.15/308.15/313.15 and 318.15) K were utilized for calculation of excess parameters VmE, κsE, LfE, uE and αPE. The excess and deviation properties were fitted to Redlich-Kister type polynomial and corresponding standard deviations were calculated. In the present binary liquid systems of [Emim][triflate] with

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