Elsevier

Thermochimica Acta

Volume 600, 20 January 2015, Pages 95-101
Thermochimica Acta

Densities of aqueous mixtures of (choline chloride + ethylene glycol) and (choline chloride + malonic acid) deep eutectic solvents in temperature range 283.15–363.15 K

https://doi.org/10.1016/j.tca.2014.11.028Get rights and content

Highlights

  • For (DES + water) mixture, density decreases with increasing T within 283.15–363.15 K with quadratic dependence on T.

  • Excess molar volume of aqueous mixtures of DES is significant and negative at all T and compositions.

  • Density dependence on DES mole fraction follows exponential-rise-to-maxima.

  • The density data imply presence of strong interactions between water and DES.

  • Interstitial accommodation of water within H-bonded DES network is also strongly suggested.

Abstract

Deep eutectic solvents (DESs) are emerging as a new class of biodegradable green solvents; a cost-effective alternative to the conventional room temperature ionic liquids and organic solvents. Hydrophilic nature of DES finds its applications in many industrial and chemical processes. Aqueous mixtures of DESs have potential to afford modified properties for specific applications. A comparative study of densities of two well-known DESs named as ethaline (mixture of choline chloride and ethylene glycol in 1:2 molar ratio) and maline (mixture of choline chloride and malonic acid in 1:2 molar ratio) and their aqueous mixtures in the temperature range 283.15–363.15 K is presented. Decrease in density with increasing temperature is found to follow a quadratic expression. Excess molar volumes of the aqueous mixtures of both ethaline and maline are found to be negative and significant at all temperatures and compositions. Absolute excess molar volume is found to decrease as the temperature is increased from 283.15 K to 323.15 K. For temperature above 323.15 K, the excess molar volume does not change much with further increase in temperature till 363.15 K. The outcomes hint at the presence of relatively stronger interactions, preferably H-bonding type, between water and ethaline/maline, as compared to those among water and among ethaline/maline molecules, respectively. The excess molar volumes at higher temperatures strongly indicate facile interstitial accommodation of water within H-bonded ethaline/maline network to be also present within these aqueous DES mixtures.

Introduction

With the introduction of green chemistry in early 1990s, room temperature ionic liquids (RTILs) have emerged as ‘green solvents’ owing to their advantageous physical and chemical properties, e.g., extremely low vapor pressure, high thermal stability, high solvation capacity, high thermal conductivity, and wide liquid range [1], [2], [3], [4]. However, the main disadvantages to their use are their high cost and issues such as non-biodegradability and toxicity [5], [6], [7]. As greener alternative to RTILs, a new family of solvents, known as deep eutectic solvents (DESs) has emerged as novel media in chemistry. DES is a fluid generally composed of two or three cheap components that are capable of self-association, often through H-bonding interactions, to form a eutectic mixture with much lower melting point than either of the individual components [8], [9]. DESs possess similar physicochemical properties to the traditionally used RTILs, while possessing several advantages over the later. Many of the reported DESs are made from biodegradable components and are non-toxic. So, they are considered as better substitute to conventional volatile organic solvents and RTILs. DESs find major applications in the field of organic synthesis [10], electrochemistry [11], [12], [13], nanomaterial [14], biochemistry [15], [16] and other chemical and industrial processes. A DES can be prepared by forming a complex between a H-bond acceptor (HBA) such as an ammonium salt and a H-bond donor (HBD) such as an acid, alcohol or an amide. DESs are easier to prepare with high purity at a relatively cheaper cost. The first DES was investigated by Abbott et al. in 2003 containing choline chloride and urea in 1:2 mole ratio [17].

Unlike certain PF6 and (CF3SO2)2N RTILs, among others, the most popular class of DESs constituted of choline chloride, exhibit complete water miscibility. Subsequently, water, an obvious choice as environmentally-benign substance, can be used as a cosolvent to effectively and favorably modify properties of a choline chloride DES. In this paper, we have chosen two common and popular DESs, named ethaline and maline, that are composed of (choline chloride + ethylene glycol) and (choline chloride + malonic acid), respectively, combined in the mole ratio 1:2. Their melting points are significantly lower than the room temperature (ethaline: Tf = 213.15 K and maline: Tf = 283.15 K) [18]. Both the DESs are non-flammable and non-toxic and exhibit complete water miscibility. To assess and design the possible industrial-scale applications of DESs, it is essential to know the thermophysical properties of their aqueous mixtures. Among the key physicochemical properties, density has enormous importance as far as various industrial applications are concerned.

Densities of aqueous mixtures of ethaline have been measured by other researchers. In a recent study, Leron et al. reported the densities of (choline chloride + ethylene glycol) and their aqueous mixtures; however, the temperature range (298.15–333.15 K) of this report was significantly narrower [19]. In another study, the same group reported high pressure volumetric properties of choline chloride–ethylene glycol based deep eutectic solvent and its mixtures with water covering the temperature range of 298.15–323.15 K and pressure range of 0.1–50 mPa [20]. The temperature range in this study again is rather restricted. In a recent contribution towards temperature-dependent density measurement of ammonium- and phosphonium-based DESs was from Shahbaz et al. in which the densities were predicted using artificial intelligence and group contribution techniques [21]. In another study, Shahbaz et al. reported predicted densities of ethaline using atomic contributions at 298.15 K [22]. The same group also reported the predicted densities of ionic liquids analogues by using Eotvos and Guggenheim empirical rules in the temperature range 298.15–328.15 K [23]. Siongco et al. reported the densities, viscosities, and refractive indices of (N,N-diethylethanol ammonium chloride + glycerol) and (N,N-diethylethanol ammonium chloride + ethylene glycol) and their aqueous mixtures from 298.15 to 343.15 K [24]. In a recent study, Hsieh et al. reported the diffusivity, density and viscosity of aqueous solutions of choline chloride/ethylene glycol and choline chloride/malonic acid; however, the temperature range (303.15–323.15 K) of this report was significantly narrower [25]. Lin et al. also reported Henry’s constant of carbon dioxide-aqueous DES (choline chloride/ethylene glycol, choline chloride/malonic acid) systems at the pressure 101.00 kPa in the temperatures range 303.15–323.15 K [26]. The temperature ranges of this report is also significantly narrower. We present densities of ethaline, maline and their aqueous mixtures (over the entire composition range) and covering the temperature range from 283.15 to 363.15 K.

Section snippets

Materials

Ethaline 197 (mol wt. 88.03 g/mol) and maline 200 (mol wt. 115.80 g/mol), a mixture of (choline chloride + ethylene glycol) and (choline chloride + malonic acid) in 1:2 mole ratio, respectively, were purchased from Scionix Ltd., and used as received. Alternatively, ethaline and maline were also prepared by mixing choline chloride (≥99%, from Sigma–Aldrich) with ethylene glycol (≥99.8%, from Sigma–Aldrich) and choline chloride (≥99%, from Sigma–Aldrich) with malonic acid (≥99%, from Sigma–Aldrich) in

Temperature dependence of densities of ethaline and maline and their aqueous mixtures

Experimentally measured densities of ethaline and maline, respectively, and their water mixtures as a function of temperature in the range 283.15–363.15 K over the entire composition range are reported in Table 3, Table 4, respectively. As expected, densities of ethaline, maline, water, and aqueous mixtures of DESs, respectively, are found to decrease with increase in temperature. Thermal expansion usually results in decreased density of a substance as the temperature is increased. The

Conclusions

Densities of aqueous mixtures of DESs ethaline and maline and their temperature and composition dependence in the temperature range 283.15–363.15 K reveal interesting information on interactions present within these systems. Variation of density with temperature for these systems is found to be quadratic in nature. Negative and significant values of excess molar volume at all temperature and composition hint at the presence of stronger interactions between water and ethaline/maline as compared

Acknowledgements

This work is generously supported by the Department of Science and Technology (DST), Government of India through a grant to SP [grant number SB/S1/PC-80/2012]. AY would like to thank CSIR, Government of India for her fellowship. MV and SN would like to thank Summer Faculty Research Fellow Programme 2014 at IIT Delhi.

References (33)

  • A. Yadav et al.

    Densities and dynamic viscosities of (choline chloride + glycerol) deep eutectic solvent and its aqueous mixtures in the temperature range 293.15 K to 363.15 K

    Fluid phase Equilibr.

    (2014)
  • R.D. Rogers et al.

    Ionic liquids–solvents of the future?

    Science

    (2003)
  • L.E. Ficke et al.

    Interactions of ionic liquids and water

    J. Phys. Chem. B

    (2010)
  • G.A. Baker et al.

    An analytical view of ionic liquids

    Analyst

    (2005)
  • Ionic Liquids as Green Solvents: Progress and Prospects; ACS Symposium Series 856

  • R.J. Bernot et al.

    Effects of ionic liquids on the survival, movement, and feeding behavior of the freshwater snail, Physa acuta

    Environ. Toxicol. Chem.

    (2005)
  • Cited by (0)

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