Elsevier

Fluid Phase Equilibria

Volume 445, 15 August 2017, Pages 35-44
Fluid Phase Equilibria

Understanding ion-ion and ion-solvent interactions in aqueous solutions of NMP based protic ionic liquids through partial molar properties and DFT calculations

https://doi.org/10.1016/j.fluid.2017.05.005Get rights and content

Highlights

  • Apparent molar properties of aqueous protic ionic liquids have been studied.

  • Infinite dilution partial molar properties calculated using Redlich-Mayer type of equation.

  • Ion-solvent interactions weakening with increase in hydrophobic anionic chain length.

  • Observed intra-ionic and inter-ionic interactions were supported by DFT.

Abstract

The precise and accurate thermophysical properties determination of ionic liquid (IL)-solvent binary system is needed for understanding the molecular interactions occurring between these components. The nature of ion-ion and ion-solvent interactions can be studied either experimentally on the basis of apparent and partial molar properties and corresponding infinite dilution properties or by computational studies. In this regard, we have measured the thermodynamic properties of binary aqueous solutions containing newly synthesized N-methylpyrrolidone based protic ionic liquids (PILs) in the temperature range (288.15–328.15) K and at ambient pressure. Experimentally measured density and speed of sound data have been used to calculate the infinite dilution partial molar properties and the corresponding empirical parameters through the Redlich-Mayer type of equation. It is observed that the studied PILs interacts strongly with water, however the ion-solvent interactions decreases with increase in anionic chain length and increase in temperature. Observed intra-ionic and inter-ionic interactions in studied aqueous ionic liquid mixtures were also supported by Density Functional Theory (DFT) calculations.

Graphical abstract

Ion-ion and ion-solvent interactions in aqueous solutions of NMP based protic ionic liquids.

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Introduction

Ionic liquids (ILs) have received increasing attention for their potential scientific and industrial applications [1], [2], [3]. Because of their unique physicochemical properties such as wide electrochemical window, negligible vapor pressures, high thermal stability, broad liquid temperature ranges, and specific solvent abilities, ILs are being employed as ‘designer solvent’ for targeted application [2], [4], [5], [6].

ILs can be classified as protic ionic liquid (PIL) and aprotic ionic liquid (AIL), the former involves the proton transfer between a Bronsted acid and a base, whereas the latter is obtained from non-reversible alkylation of the heteroatom thus inducing a formal charge center on the cation. The unitary charge is created on cation by the net addition of R+ or H+ to a heteroatom lone pair and this heteroatom is an integral part of the incipient cation [7], [8]. Most of the research work have been focused on AIL but recently PILs have gain increasing attention [9], [10], [11] due to interesting physical properties [12], [13]. Since, PILs participate in strong hydrogen bonding, which become advantageous for applications such as utilizing PILs for protein stabilization, fuel cell, etc. [14], [15]. Among PILs [16], [17], [18], [19], the new class of ILs based on N-methylpyrrolidone (NMP) containing organic anionic moiety have been recently synthesized and studied so as to overcome limitations such as toxicity, high viscosity and cost associated with other PILs [20], [21].

Water as a solvent or as an impurity is ubiquitous within ILs, so studies on the water-IL interaction have gained extensive interest [22], [23]. The IL-solvent (including water) combinations have shown better performance in CO2 capturing, cellulose dissolution, electrochemistry and catalysis [24], [25], [26], [27]. Results suggests the involvement of both anionic and cationic part of IL in modulating the IL-water interactions [22], [23]. However, most of the studies are conducted on imidazolium based AILs or ammonium based PILs [22], [28], [29], [30], [31], [32]. Recently [33], [34], [35] ILs have also been introduced as new absorbents so as to overcome problems such as thermal instability, high volatility and low system performance shown by organic working fluids in absorption cooling systems. The absolute water miscibility, lowering of partial pressure of water as compared to aqueous LiBr solution and absence of crystal formation are crucial absorbent features for optimizing the coefficient of performance (COP) for IL containing working pair [36].

So, the thermodynamic properties of pure and aqueous/nonaqueous ILs solutions with various molecular solvents (water, methanol, ethanol etc.) needed [19], [37], [38] to be studied for optimizing the suitable IL-solvent combination for targeted applications. For instance, accurate thermodynamic properties of pure and mixture fluids are necessary for the suitable design and optimization of the absorption cooling and/or heating systems [33].

In this regard, we have studied the interactions occurring between pyrolidinium based protic ionic liquids and water employing experimental and computational techniques. The density and speed of sound of three protic ionic liquid (PIL), viz. N-methyl-2-pyrrolidonium formate [NMP][For], N-methyl-2-pyrrolidonium propionate [NMP][Pro] and N-methyl-2-pyrrolidonium pentanoate [NMP][Pen] in water at (288.15, 293.15, 298.15, 303.15, 308.15, 313.15, 318.15, 323.15 and 328.15) K and at atmospheric pressure have been measured. These experimental data were used to calculate apparent molar volume, Vϕ, and apparent molar isentropic compression, Ks,ϕ. The density functional level of theory (DFT) was also employed to understand the nature of inter- and intra-molecular interactions occurring between ions of PIL with water at different temperatures.

Section snippets

Materials

The provenance of the chemicals used along with their purity and purification method have been mentioned in Table 1. All the chemicals were used as such without further purifications.

Synthesis and characterization of PILs

The N-methylpyrrolidone based PILs namely N-methyl-2-pyrrolidonium formate {[NMP][For]}, N-methyl-2-pyrrolidonium propionate {[NMP][Pro]} and N-methyl-2-pyrrolidonium pentanoate {[NMP][Pen]} have been synthesized by exothermic neutralization of base (N-methylpyrrolidone) with acid (formic or propionic or pentanoic

Volumetric properties

Thermodynamic properties studied by evaluating the partial molar quantities are supportive in describing the structure and properties of binary or ternary solutions, thus these quantities can delivers important information about solute-solvent, solute-solute and solvent-solvent interactions [17]. Moreover, these thermodynamic properties of binary or ternary solution containing ionic liquids and various solvents (including water) are required for chemical process design and optimization, for

Conclusions

Apparent molar volumes, Vϕ and apparent molar isentropic compressibilities, Ks,ϕ of three newly synthesized NMP based PILs as function of concentration and temperature has been calculated from experimentally measured density and speed of sound. Infinite dilution partial molar volumes (Vϕ and Kϕ) have also been calculated by employing Redlich-Mayer type of equation. Both (Vϕ and Kϕ) values increase with increase in temperature as well as with anionic chain length of PILs. Empirical Sv

Acknowledgements

The authors are grateful to IIT Madras for financial support, through Institute Research and Development Award (IRDA): CHY/15-16/833/RFIR/RAME.

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